The Effect of Compression Ratio, Fuel Octane Rating, and Ethanol

Aug 3, 2015 - Higher ethanol content is one available option for increasing the octane ratings of gasoline and would provide additional engine efficie...
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The Effect of Compression Ratio, Fuel Octane Rating, and Ethanol Content on Spark-Ignition Engine Efficiency Thomas G. Leone,† James E. Anderson,*,† Richard S. Davis,‡ Asim Iqbal,§ Ronald A. Reese, II,§ Michael H. Shelby,† and William M. Studzinski‡ †

Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121, United States General Motors Powertrain, 850 Glenwood, Pontiac, Michigan 48340, United States § FCA US LLC, 800 Chrysler Drive, Auburn Hills, Michigan 48326, United States ‡

S Supporting Information *

ABSTRACT: Light-duty vehicles (LDVs) in the United States and elsewhere are required to meet increasingly challenging regulations on fuel economy and greenhouse gas (GHG) emissions as well as criteria pollutant emissions. New vehicle trends to improve efficiency include higher compression ratio, downsizing, turbocharging, downspeeding, and hybridization, each involving greater operation of spark-ignited (SI) engines under higher-load, knock-limited conditions. Higher octane ratings for regular-grade gasoline (with greater knock resistance) are an enabler for these technologies. This literature review discusses both fuel and engine factors affecting knock resistance and their contribution to higher engine efficiency and lower tailpipe CO2 emissions. Increasing compression ratios for future SI engines would be the primary response to a significant increase in fuel octane ratings. Existing LDVs would see more advanced spark timing and more efficient combustion phasing. Higher ethanol content is one available option for increasing the octane ratings of gasoline and would provide additional engine efficiency benefits for part and full load operation. An empirical calculation method is provided that allows estimation of expected vehicle efficiency, volumetric fuel economy, and CO2 emission benefits for future LDVs through higher compression ratios for different assumptions on fuel properties and engine types. Accurate “tank-to-wheel” estimates of this type are necessary for “well-to-wheel” analyses of increased gasoline octane ratings in the context of light duty vehicle transportation.

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39% but average vehicle weight remained essentially the same.9 Another major trend is “downspeeding”, that is, operating the engine at lower RPM and higher torque which improves efficiency while providing equal power. From 1979 to 2013, downspeeding was achieved (in part) by increasing the average number of transmission gears from 3.3 to 5.9 and by increasing U.S. market share of continuously variable transmissions (CVTs) from 0 to 14%.9 Engine efficiency does not increase continuously with load; at the highest loads, efficiency is degraded due to spark retard and/or fuel enrichment.8 Spark retard is required at high loads to avoid knock, which would cause objectionable noise and (in severe cases) permanent engine damage. Knock results from autoignition of the unburned air-fuel mixture due to high temperatures and pressures. Spark retard prevents knock but causes significant degradation in high-load efficiency and torque capability.10 Spark retard also increases exhaust temperature, which can

INTRODUCTION Government agencies worldwide are mandating dramatic improvements in vehicle fuel economy (FE) and emissions of CO2 and other greenhouse gases (GHGs).1 In the United States, the industry average fleet-wide CO2 emissions levels will decrease to 163 g/mi in 2025, equivalent to 54.5 mpg.2 Achieving these improvements will be challenging; consider that nearly 40 years were required for automakers to nearly double average passenger car fuel economy from 15.9 mpg in 19753 through dramatic changes in every aspect of vehicle design including engines, transmissions, body structures, and aerodynamics. Future improvements in FE and CO2 emissions are expected to become increasingly expensive, although estimates vary widely.4−7 The engines in light duty vehicles (LDVs) typically operate at a small fraction of maximum power to provide adequate reserve power for transient performance. However, engines have reduced efficiency at these light loads, largely because parasitic losses are a larger fraction of total work.8 Therefore, many design changes have been implemented to operate engines at higher loads, where they are more efficient. One such change is “downsizing”, or using a smaller engine in a given vehicle. Between 1975 and 2013, average engine displacement decreased © 2015 American Chemical Society

Received: Revised: Accepted: Published: 10778

March 24, 2015 July 8, 2015 August 3, 2015 August 3, 2015 DOI: 10.1021/acs.est.5b01420 Environ. Sci. Technol. 2015, 49, 10778−10789

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naturally aspirated, carbureted, CFR engine is no longer representative of modern spark ignition (SI) engines that have significantly different charge preparation (port fuel injection [PFI] or direct injection [DI]), combustion characteristics, and thermal efficiencies. In the quest for achieving higher engine thermodynamic efficiencies and specific power outputs to meet increasingly stringent FE and emissions requirements, SI engine operation is continuously being pushed toward the knocklimited combustion regime. Consequently, in recent years there have been multiple investigative efforts37−41 to better understand the relationship between engine knock behavior and fuel properties, as well as to develop new metrics for defining an engine’s octane requirement42 and relating them to the wellestablished RON and MON ratings. In 2001, Kalghatgi43,44 introduced the concept of the octane index (OI) to represent the knock behavior of a fuel in an engine. Octane index is defined as

necessitate enrichment of the air−fuel mixture to avoid damage to engines and aftertreatment systems (catalysts). Another factor related to knock is the engine’s compression ratio, which plays a fundamental role in engine efficiency.8 Higher compression ratio improves efficiency in the absence of knock, but it also causes higher temperatures and pressures of the unburned air-fuel mixture which lead to more knock at high loads. Thus, compression ratio needs to be limited to avoid knock or severely compromised torque output.11 The compression ratio selected for a particular engine depends on the expected duty cycle and fuel octane. A higher compression ratio can be used if an engine will operate primarily at light loads, such that degraded efficiency at high loads is more than offset by improved efficiency at light loads. A higher compression ratio can also be used if an engine will operate on fuel with a higher octane rating. A fuel’s octane rating is a measure of its resistance to knock. Knock is fundamentally a chemical process initiated by preflame reactions leading to autoignition. The preflame reactions are a strong function of temperature, so evaporative cooling from the fuel can also play a significant role, which is particularly important for fuels containing alcohol.12−20 Knock and fuel octane rating are becoming increasingly important due to many powertrain design trends including downsizing, downspeeding, cylinder deactivation, and hybridization. In addition to the gradual engine downsizing trend already described, turbocharging enables more dramatic engine downsizing and much higher engine loads. From 1998 to 2013, the U.S. market share of turbocharged engines increased from less than 1% to 15%.21 Cylinder deactivation also increases the importance of knock and fuel octane rating, because at light loads (where the engine would not normally be knock-limited), some of the cylinders are deactivated, thus requiring the remaining cylinders to operate at higher knock-limited loads.22 From 2005 to 2013, the U.S. market share of engines with cylinder deactivation increased from less than 1% to 8% (19% for light duty trucks).21 Hybridization dramatically reduces engine operation time at light loads, so that the engine is knock-limited almost all the time.23 To achieve continued improvement in engine efficiency and CO2 emissions, it is critical to quantify the roles of fuel octane rating and alcohol content. Higher octane fuel improves the efficiency of today’s engines through reduced spark retard (from optimum) at high loads, and could enable even higher efficiency if engines were optimized for higher-octane fuel.11,24−29 Alcohol and gasoline-alcohol blends also offer efficiency benefits independent of their octane value.14,30−33 The remainder of this paper will primarily focus on how the fuel affects compression ratio selection, but clearly additional benefits are possible due to interactions with downsizing, downspeeding, cylinder deactivation, and hybridization.

OI = (1 − K ) × RON + K × MON

(1)

where K is a constant for a given condition but varies with changes in engine operating parameters such as speed, load, and intake air temperature and pressure. eq 1 can be rearranged as OI = RON − K × (RON − MON) = RON − K × S (2)

where S = RON − MON is the sensitivity of the fuel. For a value of K = 0.5, OI = (RON + MON)/2 = AKI. eq 2 also implies that for a value of K > 0.5, MON would have a greater influence on the knock behavior of the engine while RON would be more relevant for K < 0.5. As defined by Kalghatgi, the constant K is related to the in-cylinder unburnt gas temperature. For a specific in-cylinder pressure, the value of K decreases with decreasing unburnt gas temperature. By definition K = 0 for the RON test. Thus, for in-cylinder conditions that result in unburnt gas temperatures lower than the RON test, K would be negative. Consequently, knock resistance can be greater than RON when K is negative, especially for fuels with large sensitivities. As SI engines have evolved they have become more efficient, which implies more of the chemical energy of the fuel is converted to useful work and less is rejected as heat. In addition, combustion chamber cooling has become more effective and there is greater emphasis on reducing the temperature rise over ambient in the air induction system. These factors contribute to lowering the in-cylinder unburnt gas temperatures which indicates a trend in SI engine design evolution that would result in lower K values. Evidence of negative K values has been corroborated by several studies45−47 including a single cylinder engine investigation by Mittal and Heywood.48,49 Elevated intake air pressure, as is encountered in modern downsized boosted engines, also contributes to lowering the value of K.48,50 As the data presented by Kalghatgi was focused on smaller displacement small cylinder bore European engines, the Coordinating Research Council (CRC) conducted a scoping study51 to investigate whether larger displacement U.S. engines exhibited a similar trend in K values. The CRC investigation concluded, in agreement with other studies, that K is less than 0.5 (and often is negative) across most operating conditions for modern SI engines. However, the CRC study also noted that for larger bore engines operating at high engine speeds with elevated intake air temperatures, K > 0.5, suggesting MON may be more representative of an engine’s octane requirement for some operating conditions.

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FUEL EFFECTS ON KNOCK RESISTANCE Two key factors that contribute to the knock resistance of a fuel include octane ratings measured using standard test methods and fuel evaporative cooling, as discussed below. Relevance of Existing Octane Rating Measures. Since the advent of the Cooperative Fuels Research (CFR) engine in 1929,34−36 fuel knock resistance has been quantified in terms of the Research Octane Number (RON) and Motor Octane Number (MON), with the antiknock index (AKI), defined as AKI = (RON + MON)/2, representing the octane rating displayed on fuel dispensers in the United States. However, the 10779

DOI: 10.1021/acs.est.5b01420 Environ. Sci. Technol. 2015, 49, 10778−10789

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Figure 1. Distributions of RON of U.S. gasoline in 2013 categorized by grade displayed at the point of sale (regular-grade, n = 459; premium-grade, n = 372).58 Samples collected in both winter and summer from a total of 30 major metropolitan areas in the US.

blend meets applicable octane rating and volatility specifications. Ethanol content in gasoline greater than 10%v is one option to provide higher octane ratings for gasoline, while maintaining existing BOB octane ratings.54,61 Evaporative Cooling. As liquid fuel evaporates after injection, the resulting air-fuel mixture achieves a lower temperature in relation to the heat of vaporization (HoV) of the fuel. The ignition delay time for hydrocarbon fuels is generally modeled as decreasing exponentially with increasing temperature.62 The heat of vaporization (HoV) for liquid-fueled engines can significantly impact the end gas temperature and thus the knock tendency of an engine. The fuel HoV per unit mass combined with the mass ratio of fuel required for a stoichiometric mixture determines the upper limit of charge cooling.17 The HoV of a stoichiometric mixture of ethanol is approximately 4-fold greater than that of gasoline.63 The HoV of hydrocarbon gasoline has been reported to contribute 4 ON of knock resistance in a DI engine relative to a PFI engine.64 In a gasoline turbocharged direct injection (GTDI) engine, gasoline contributed 5 ON of cooling-related knock resistance while E85 contributed 18 ON.19 Stein et al.18 used an engine with multiple fuel injection systems to quantify the impact of charge cooling on knock resistance, and concluded that half of the observed knock benefit of E50 could be attributed to the charge cooling effect. Several recent studies suggest that the carbureted RON test largely includes the HoV effect in ethanol blends up to ∼E30E40.20,28,65 Foong et al.20 modified the RON test to maintain a constant intake mixture temperature regardless of ethanol content. Their experimental results, with supporting modeling results, indicated that the standard carbureted RON test is expected to properly quantify the knock behavior of ethanol blends up to ∼E30-E40 including the evaporative cooling benefit when these fuels are used in DI engines. This is supported by two engine dynamometer studies using fuels with matched RON, but having differing ethanol content and thus differing HoV. Leone et al.28 compared 91-RON E10, E20, and E30 fuels in a GTDI engine. Thewes et al.65 compared 95-RON E0 and E20 fuels in a GTDI engine. Both studies found no additional antiknock benefit associated with higher ethanol content for fuels with equal RON. For ethanol blends above ∼E30-E40, however, Foong et al.20 showed some liquid fuel leaving the intake port and entering the cylinder. The subsequent evaporation of this liquid fuel has minimal impact on the charge temperature because the

With the ongoing, industry-wide shift toward smaller displacement, high specific output engines equipped with improved combustion chamber cooling and enablers for reduced intake air temperature, the value of K has been steadily decreasing which suggests that RON is more relevant for representing the octane requirement of an engine than MON or AKI. Octane Ratings and Ethanol Content of U.S. Market Gasoline. Gasoline in the U.S. is marketed in terms of AKI, not RON. Most regular-grade gasoline in the U.S. is sold with a minimum octane rating of 87 AKI, corresponding to approximately 91−92 RON. However, as shown from U.S. fuel survey data in Figure 1, the RON of regular-grade gasoline can actually be considerably higher or lower. The primary gasoline specification in the U.S. is ASTM D4814,52 but this only lists octane ratings currently in use in an Appendix and does not provide minimum requirements. Some states have enacted octane rating requirements which generally follow D4814. Gasoline with lower octane ratings (as little as 85 AKI, corresponding to ∼89 RON) is sold in some high altitude regions in the western U.S. based on ASTM D4814 recommendations52 supported by pre-1984 testing on vehicles using carbureted engines without knock sensors. Overall, the range of regulargrade gasoline in the U.S. is 89 to 96 RON, however LDVs need to be designed to accommodate fuel with the lowest expected octane rating. Premium-grade and midgrade gasoline are also typically available, with the former usually listed as 91−93 AKI minimum, corresponding to 96−98 RON. The vast majority (87%) of gasoline sold in the U.S. in 2013 was regular grade, with 10% premium and 3% midgrade.53 Premium gasoline consumption has declined as a percentage of the total since it peaked in the late 1980s at approximately 30%. Octane ratings for regulargrade gasoline are higher in many countries, with much of Europe having a minimum 95 RON for gasoline. Refinery and well-to-wheel implications of higher octane ratings for regulargrade U.S. gasoline have been investigated recently.54,55 Similar studies have been conducted to identify optimum octane ratings for European gasoline.56,57 Nearly all gasoline in the U.S. now contains 10%v ethanol (E10). Ethanol has a high octane value when blended in gasoline.59,60 Consequently, refiners are now able to produce a blendstock for oxygenate blending (BOB) with octane ratings lower than gasoline produced prior to the use of ethanol as a blending agent.61 When 10%v ethanol is added, the resulting 10780

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actual on-road operation by customers. Of these factors, the compression ratio is the predominant design factor that would be adjusted in future engines in response to a significant change in fuel octane ratings. In all engines, including the existing on-road fleet, fuel with higher octane ratings can enable more advanced spark timing and combustion phasing under knocklimited conditions. The impacts of higher octane fuel on both current and future vehicles are discussed below. Effects of Higher Octane Fuel on Existing Vehicles. While the most significant efficiency improvement with higher octane fuels will result from a full, system-level optimization including increased engine CR, efficiency gains from legacy vehicles (existing vehicles not optimized for the higher-octane fuel) can also be realized, with or without recalibration of the engine and controls. In order to optimize efficiency in typical driving conditions, the CRs selected for passenger car engines result in those engines being knock-limited on the recommended fuel at higher loads. Higheroctane fuels will improve knock resistance and consequently facilitate reduction in spark retard at these high load conditions. If the minimum octane rating of the fuel available to customers was increased, it may be technically feasible to update (or “reflash”) the engine calibrations on existing vehicles to extract the most benefit from the improved fuel properties. Since the efficiency loss due to retarded spark timing is nonlinear (see Supporting Information (SI) Section 1, Figure SI-1), the largest engine efficiency gains will be at the highest engine loads and the greatest vehicle FE gains will be realized on heavily loaded drive cycles and on vehicles with higher load factors. To assess the potential FE gains for this scenario, vehicle FE was predicted analytically using a drive-cycle simulation for a naturally aspirated, DI V6 engine designed and calibrated for operation on regular-grade fuel (91 RON) for three different vehicle applications on the U.S. EPA city-highway composite drive cycle (see description in SI Section 2). The engine was then recalibrated for operation on 97 RON, premium-grade fuel. The predicted FE gains from switching to premium-grade fuel for these recalibrated vehicles ranged from approximately 0.6% for the lightest vehicle on the lightest loaded test cycle to 4.4% on the heaviest vehicle and most aggressive drive cycle as shown in Figure 2. These gains are consistent with work by Leone et al.28 which showed 1.1% EPA Composite CO2 benefit for 96 vs. 91 RON fuel at constant CR. FE gains are expected to vary primarily with drive cycle and vehicle load factor, but may also depend on powertrain technologies that influence the speeds and loads at which the engine operates on a given drive cycle such as cylinder deactivation, hybridization, and downsizing/downspeeding. Calibration reflashing would provide greater increases in FE with the higher octane fuel than without reflashing, but would require automaker testing for each vehicle model, possible emissions recertifications, and individual implementation on each vehicle (likely through dealerships). Given the effort and impact, policy changes should be considered to allow these actions to contribute to automaker fleet-wide CAFE compliance. The efficiency gains noted in Figure 2 are estimated assuming the engine was recalibrated to take full advantage of the higher fuel quality. A lesser gain would be realized on most, if not all, vehicles without a calibration change. Most modern engines incorporate knock sensors and spark control which retard spark timing if knock is detected, and in extreme cases, use fuel enrichment to protect the engine and aftertreatment system. Since the knock sensor and spark control system can protect the engine, many of these vehicles also incorporate a

energy generally comes from downstream metal surfaces (intake valve, cylinder head). Therefore, the RON test is not expected to properly quantify knock behavior (with evaporative cooling benefit) when these higher-ethanol content fuels are used in DI engines and thus a separate evaporative cooling factor is used in the “effective octane number” described below. Engine design and calibration factors also influence how much of the fuel HoV is extracted from the charge or transferred from engine component surfaces. For example, DI is expected to realize the full HoV benefit, while PFI engines could see a moderate benefit with open-valve injection and little benefit with closed-valve injection. Because the HoV effect appears to be included in RON for ethanol-gasoline blends up to E40, it is possible that PFI engines may actually see less knock resistance than implied by RON for high HoV fuels, particularly if using closed-valve injection where the heat from fuel vaporization is not realized as a reduction of the air-fuel mixture temperature. The MON test does not include HoV effects because the charge temperature is controlled to a fixed temperature after the carburetor. As a result, high HoV fuels tend to have high sensitivities. Higher sensitivity has been suggested to result in additional knock resistance, particularly for late combustion phasing in GTDI engines at high loads,18 but this has not been well studied. Effective Octane Number. In order to calculate the overall knock resistance of the fuel in modern engines, including the effects of charge cooling, the concept of an “effective octane number” has been utilized.19,61,66,67 Here, the effective octane number (ONeff) is defined as the sum of RON and a charge cooling term (ONcool) which depends on ethanol content and engine type: ONeff = RON + ONcool

(3)

where, for DI engines, ⎧ 0 for 0 ≤ Ex ≤ 40 ⎪ ONcool = ⎨ ⎪ ⎩ 0.16 × (Ex − 40) for Ex > 40

(4)

and Ex = ethanol content in vol % (value betwen 0 and 100). As explained earlier, this representation of ONeff excludes evaporative cooling for fuels with less than 40%v ethanol and thus differs from past representations.19,61,66 eq 4 is intended for application to DI engines. For PFI engines, it is likely that ONcool is less than given by eq 4, including possibly negative values. (Insufficient information is available for greater precision.)

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ENGINE EFFECTS ON KNOCK RESISTANCE, COMPRESSION RATIO, AND EFFICIENCY For a given fuel, the knock resistance of a given engine depends on a number of design and operating factors including compression ratio, fuel injection hardware and strategy, boosting, bore size, charge mixing, combustion speed, in-cylinder heat transfer, engine cooling system design, exhaust backpressure, spark plug location, combustion chamber design, and coolant and oil temperatures among others. The knock resistance can also be influenced by the engine calibration as well as ambient conditions such as temperature, barometric pressure, and humidity. In practice, these are developed as a system to provide the best combination of vehicle FE, performance, and emissions considering both the applicable certification fuel(s) and available market fuels under the wide range of operating conditions presented by certification testing (drive cycles) and 10781

DOI: 10.1021/acs.est.5b01420 Environ. Sci. Technol. 2015, 49, 10778−10789

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Figure 2. Estimated vehicle FE gains from regular fuel (91 RON) to premium fuel (97 RON) with optimized calibrations on each fuel, for three hypothetical vehicles using a naturally aspirated, DI V6 engine.

Figure 3. Compression ratio distribution of model year 2013 engines in North American market. Data from ref 69

vehicles for which premium-grade (high octane) fuel is required as stated in the owners’ manual. Compression Ratio Increase Enabled by Higher-Octane Fuel. The magnitude of the change in CRthe amount of CR increase enabled by a given increase in fuel octane rating while maintaining similar knock characteristicshas been investigated in many studies.11,18,28,55,64,66,70−76 Several factors contribute to a wide range of reported values. One complication is that the methods typically used to increase the CR in an experimental studysuch as raising the top dead center (TDC) position of the piston or adding a “dome” to the piston topcan influence the combustion characteristics and lower the knock tolerance of the system. Additionally, some studies are based on market studies of engine CR versus the recommended fuel octane rating. These results are convoluted as engines with a “recommended” octane rating are generally designed to tolerate lower octane fuels available in the field and the selected CR therefore does not take full advantage of the “recommended” octane rating. Figure 4 shows approximate values determined in 8 engine studies identified in the available literature. The studies covered

base spark advance map that allows the vehicle to take advantage of fuels with increased octane and provide improved performance and fuel economy. Because of differences in vehicle design and calibration among vehicle models, it is difficult to quantify fleet-wide benefits for a higher-octane fuel. The gains realized will fall somewhere between zero and the value for a recalibrated vehicle depending on the base spark calibration, the vehicle application (load factor) and the drive cycle. Note that current vehicle calibrations are constrained by EPA certification requirements. The octane rating of EPA certification fuel is similar to commercial U.S. premium-grade fuel. To avoid compromised “in use” fuel economy with regular-grade fuel, the EPA requires68 that FE on all EPA drive cycles be within 3% for regular fuel (91 RON) and certification fuel (96 RON) unless the vehicle manufacturer explicitly states that only premium fuel should be used. Increasing Compression Ratio Enabled by HigherOctane Fuels. The primary mechanism by which improved fuel knock resistance translates into improved engine efficiency and vehicle FE is through increasing the geometric compression ratio of the engine. Three factors determine this gain: the baseline CR of the engine, the increase in CR allowed by the greater fuel knock resistance, and the efficiency increase resulting from the increased CR. Each of these factors is discussed below. Compression Ratios in Current LDV Engines. The compression ratios selected by OEMs for production engines vary based on several factors including the technology implemented in the engine, the specific application for which the engine is used, and the fuel grade (minimum octane rating) recommended for the engine. Figure 3 shows the distribution of compression ratios for 2013 model year vehicles sold in the North American market.69 The data represent each unique engine offered based on displacement, technology, and CR independent of the number of vehicle applications and are not sales weighted. While there is a wide variation of compression ratio represented−spanning a range from 8.4:1 to 13:1−there are some evident trends. On average, DI engines employ a CR about one ratio higher than PFI engines. Compression ratios for naturally aspirated engines are, on average, approximately 1.5 ratios higher than those of boosted engines. The engines with CR ≥ 12:1 are predominately associated either with hybrid vehicles that operate over a relatively narrow operating range employing the Atkinson cycle or with luxury or performance

Figure 4. ON increase required per CR increase (ON/CR) from eight engine studies.

a broad range of technologies including port and direct injection as well as naturally aspirated and turbocharged applications. The values determined vary over a range from approximately 2.5 to 9 ON/CR. In general, the better controlled studies (those based 10782

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Environmental Science & Technology on modern engine technology and utilizing consistent design practices for each CR level) support utilizing a factor of approximately 3 ON/CR for estimation of the benefits of increased fuel octane rating; this value is used for the analysis described later this paper. This value, taken from the lower end of the range of literature values, should represent what is possible with a combustion system that has been fully developed and optimized for the increased CR. Engine Efficiency Increase from Higher Compression Ratio. Theoretically, the efficiency (η) of the Otto cycle in a traditional spark ignition engine is strictly a function of the engine’s geometric CR and follows the relationship η = 1 − (1/CR(γ−1)) where γ is the specific heat capacity ratio of the incylinder gas.8 In practice, efficiency is reduced by heat transfer losses, friction, gas exchange losses, incomplete combustion, and other factors, but CR is still of primary importance. As CR is increased in a cylinder, the surface-to-volume ratio of the combustion chamber increases and the crevice volume becomes a greater fraction of the clearance volume.77 Thus, both heat transfer losses and crevice losses increase with increasing CR, offsetting some of the theoretical efficiency improvement, causing a maximum in the curve of efficiency versus CR. There have been many studies to quantify efficiency versus CR empirically for practical automotive engines.10,25,70,71,75,78−84 A recent paper by Smith et al.85 (MIT) analyzed seven of the most relevant and credible data sets, including refs 66, 73, 75, 76, 82, and 84. Incremental efficiency gains derived from their average results and those of other studies (shown in Figure 5) indicate that efficiency continues to improve to 14:1 CR and beyond, although at a diminishing rate.

Figure 6. Efficiency gain from increased CR relative to three different baseline CRs using the average data reported in Smith et al.85 and summarized in eq 5.

paper, but automakers may in fact achieve better results at high CR via optimization of combustion chambers, lower bore/ stroke ratios, reduced crevice volumes, etc. Engine Efficiency Increase from Downsizing (Enabled by Higher Compression Ratio). Smith et al.85 noted that higher CR should enable further engine downsizing, which leads to further efficiency improvements. This assumes that knock behavior is improved, for example with higher-octane fuel, so that efficiency improves at all operating conditions including maximum torque and power. For naturally aspirated engines, torque and power are constrained by airflow and efficiency. If CR and fuel octane are increased simultaneously and the engine remains equally knock-limited, airflow does not change so the torque and power increase by the same percentage as the efficiency improvement. For turbocharged engines, similar logic applies in the lowspeed region where torque is constrained by available boost, but not in the wastegate-controlled flat portion of the torque curve. At high speeds, power is constrained by many factors including turbocharger speed, component temperatures, enrichment limits, cylinder pressure limits, etc. Higher CR (with higher octane fuel) should be beneficial for all of these factors except peak cylinder pressure, but the increase in power (if any) is not necessarily equal to the increase in efficiency. Smith et al. used previous work by Gerty and Heywood73 (MIT) to estimate that the downsizing enabled by higher CR would multiply the efficiency benefit by a factor of 1.6. Downsizing is well-known to improve efficiency.77,86−96 However, the vehicle fuel efficiency improvement from downsizing varies significantly depending on the details of engine size, vehicle weight, aerodynamics, etc. This is because engine efficiency is highly nonlinear with load.10,28 Gerty and Heywood73 assumed a baseline average load of 2.6 bar BMEP, where efficiency improves steeply with increasing load. Most modern engines now operate at higher average loads (due to the trends of engine downsizing and downspeeding), and efficiency improves more gradually at higher loads. For this study, a conservative estimate is used to avoid overestimating the benefit of downsizing enabled by higher CR. The estimate is based on downsizing studies performed by Ricardo97 for the U.S. EPA, which indicate that downsizing would multiply the efficiency benefit by a factor of approximately 1.3 rather than the factor of

Figure 5. Incremental efficiency gain from 1 CR increase: ideal Otto cycle, average reported by Smith et al.,85 and additional engine studies not included in Smith et al. average.

Average values for cumulative relative efficiency increases (ΔηCR) as a function of CR as developed by Smith et al.85 are redrawn in Figure 6 using a range of starting compression ratios. The underlying equation that very closely fits these data is provided in eq 5. ΔηCR = −0.207% × (CR new 2 − CR base 2) + 6.44% × (CR new − CR base)

(5)

where CRbase and CRnew are the compression ratios of the baseline engine and the new engine configuration, respectively, over the range of CR from 8:1 to 14:1 given in Smith et al.85 This relationship was used for the analysis described later in this 10783

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Table 1. Summary of Recommended Parameters for Engine Efficiency and CO2 Emissions with Higher CR Enabled by HigherOctane Fuel Component

Recommendation

effective octane number (eq 3)

ONeff

ONeff = RON + ONcool

cooling octane numbera (eq 4)

ONcool

⎧ 0 for 0 ≤ Ex ≤ 40 ⎪ ONcool = ⎨ ⎪ ⎩ 0.16 × (Ex − 40) for Ex > 40

CR increase enabled by fuel knock resistance increase

ΔCR/ ΔONeff

1 CR per 3 ONeff

efficiency increase from CR increase (eq 5)

ΔηCR

ΔηCR = − 0.207% × (CR new 2 − CR base 2) + 6.44% × (CR new − CR base)

efficiency increase multiplier from additional downsizing

Fdownsize

1.0 for no downsizing; 1.1 for turbocharged engines; 1.3 for naturally aspirated engines

efficiency increase from ethanol content

Δηethanol

0.5% per 10%v ethanol

total efficiency increase (eq 6)

Δηtotal

⎞ ⎛⎛ Fdownsize × ΔηCR ⎞⎛ Δηethanol ⎞ Δηtotal = 100% × ⎜⎜1 + ⎟ − 1⎟ ⎟⎜1 + 100% 100% ⎠ ⎠⎝ ⎠ ⎝⎝

a

eq 4 is proposed for DI engines. As discussed in the text, PFI engines capture less evaporative cooling than DI engines such that ONcool is less than given by eq 4, possibly including negative values for fuel with ethanol content less than 40%v.

and increased dilution tolerance. An efficiency comparison of E0 gasoline and E85 by Jung et al.33 showed 4% relative improvement in brake thermal efficiency for E85 at several operating conditions that were not knock-limited. A detailed analysis of the data, together with computer modeling, explained the benefit as a combination of reduced heat transfer losses in the engine and differences in evaporative cooling in the engine compared to the combustion bomb used for measuring fuel heating value. Based primarily on the careful study by Jung et al.,33 a thermal efficiency benefit of 0.5% per 10%v ethanol is used for the analysis described in the rest of this paper.

1.6 estimated by Gerty and Heywood. For turbocharged engines, an even more conservative factor of 1.1 is used for this study because in many cases downsizing will be constrained by other factors as described above. For example, if CR is increased from 10 to 12, Figure 6 gives an efficiency benefit of 3.78%. Improved efficiency at maximum torque and power should enable engine downsizing, so that the total benefit is assumed to be 1.3 × 3.78% or 4.91% for naturally aspirated engines. The total benefit is assumed to be 1.1 × 3.78% or 4.16% for turbocharged engines. Other Fuel-Related Efficiency Considerations. The presence of ethanol in gasoline (now widespread) imparts other changes in fuel properties that have been shown to influence engine efficiency at all operating conditions, including those not limited by knock. The following effects have been identified and to some extent quantified in the literature:33,63,98 • Charge cooling impact on heat transfer • Adiabatic flame temperature impact on heat transfer • Heat of vaporization impact on the measured heating value • Charge cooling impact on pumping work • Combustion efficiency (i.e., chemical energy remaining in the products of combustion) • Charge cooling and chemistry impacts on combustion stability and dilution tolerance • Burn rate differences influence on combustion time losses • Specific heat ratio effects on the unburned and burned charge Several studies have concluded that ethanol improves part load efficiency compared to gasoline, even at light loads where knock and octane are not important.14,30,32,33,63,98,99,100 Nakata et al.30 performed engine dynamometer testing on ethanolgasoline blends ranging from 0% to 100% ethanol, at wide-open throttle and at part load. At part load, engine efficiency was 3% better with ethanol than gasoline, which was attributed to reduced heat transfer losses. Engine dynamometer data from Marriott et al.14 showed 3% to 6% benefits for E85 compared to gasoline, due to a combination of reduced heat transfer losses

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DISCUSSION The considerations discussed above provide a method for estimating the improvement in efficiency, fuel economy, and tailpipe CO2 emissions resulting from fuel with higher octane ratings used in future LDVs with engines utilizing higher compression ratios. The recommended relationships are summarized in Table 1. As an example, the method is compared to data from a recent study28 of higher engine CR and efficiency enabled by higheroctane fuels. The study involved a 3.5-L GTDI engine with CRs of 10:1 (baseline), 11.9:1, and 13:1 run on an engine dynamometer. Modeling of a light-duty pickup truck application using this engine was conducted to estimate FE and tailpipe CO2 emissions on two regulatory drive cycles, the lightly loaded FTP metro-highway cycle and the more heavily loaded US06 highway cycle. Details of the relevant fuel properties, engine results, and vehicle modeling results as reported in ref 28 are provided in Table 2 and are compared to values estimated using the approach outlined herein. All of the fuels contained less than 40%v ethanol, thus no additional evaporative cooling contribution was included in the calculation of effective RON. Assuming 3 ON/CR, the 96-RON and 101-RON fuels are estimated to enable an increase in CR to 11.8:1 and 13.3:1 respectively. As shown in Figure 7 (top panel), these estimates compare well with the actual dynamometer testing, in which 96-RON E20 fuel enabled 11.9:1 CR with 10784

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Table 2. Estimation of Efficiency, Volumetric Fuel Economy, and CO2 Emissions for a GTDI Engine with Higher CR Enabled by Higher-Octane Fuel with Comparison to Ref 28 Engine Dynamometer and Vehicle Modeling Resultsa,b fuel

91-RON E10 (baseline) ref 28

96-RON E20 ref 28

RON ethanol (%v) energy content (MJ/L) energy-based carbon content (gC/MJ) effective RON CR efficiency gain from higher CR (% vs baseline) efficiency gain from higher ethanol content (% vs baseline)

90.8 10.2 30.8 72.7 90.8 10.0 baseline baseline

96.2 20.4 29.7 72.5 n/a 11.9 n/ac n/ac

Estimates without Downsizing: total efficiency gain (% thermal efficiency change vs baseline)

baseline

FE change (% MPG change vs baseline)

baseline

tailpipe CO2 change (% g CO2/mi change vs baseline)

baseline

4.7% M/H 4.8% US06 1.0% M/H 1.1% US06 −4.8% M/H −4.9% US06

Estimates with Downsizing: efficiency gain multiplier from downsizing (Fdownsize) efficiency gain from downsizing (% vs baseline) total efficiency gain (% thermal efficiency change vs baseline) FE change (% MPG change vs baseline) tailpipe CO2 change (% g CO2/mi change vs baseline)

n/a baseline baseline baseline baseline

n/ad n/ad n/ad n/ad n/ad

101-RON E30

estimate

96.2 11.8 3.48% 0.51%

4.0% 0.3% −4.1%

1.1 0.35% 4.4% 0.6% −4.5%

ref 28 100.7 31.5 28.5 72.4 n/a > 13.0 n/a n/a

6.0% M/H 9.6% US06 −2.1% M/H 1.2% US06 −6.0% M/H −9.1% US06

n/a n/a n/a n/a n/a

estimate

100.7 13.3 5.35% 1.07%

6.5% −1.7% −6.5%

1.1 0.54% 7.0% −1.2% −7.0%

All fuel property data taken directly from ref 28. b“M/H” indicates result for U.S. EPA metro-highway test cycle. “US06” indicates result for EPA US06 highway test cycle. cThe total efficiency gain was measured in ref 28. A breakdown of contributing factors was not reported. dThe calculated changes in vehicle efficiency, fuel economy, and CO2 emissions in ref 28 did not include incremental benefits from additional downsizing. a

these fuel/CR combinations in the vehicle application described in ref 28. which did not include additional downsizing. The estimated gains are based on the assumption that the fuel and CR are well-matched, so that knock performance is similar to the baseline fuel and CR. This is true for the 96-RON E20 fuel and 11.9:1 CR, thus the benefits are similar on the EPA M/H and US06 drive cycles, and the estimates match the results from ref 28. But the 101-RON E30 fuel with 13.0:1 CR exhibited better knock performance than the baseline, thus ref 28 shows significantly better gains on the more heavily loaded US06 cycle. The estimated efficiency gains are greater than measured on EPA M/H due to higher CR, but less than measured on US06 because they do not include benefits from reduced spark retard. Calculating volumetric fuel economy (FEvol; miles/gallon or km/L) and tailpipe CO2 emissions (gCO2/mile) requires inclusion of fuel properties such as carbon content (Cm; kg C/kg fuel), net heat of combustion or lower heating value (LHVm; MJ/kg), and density (ρ; kg/L). Changes in fuel composition that provide higher octane ratings, for example through greater content of aromatic hydrocarbons or alcohols, can change these properties.54 Increased content of aromatic hydrocarbons or ethanol tend to have opposite effectsaromatics typically yield an increase in volumetric energy content (LHVv = LHVm × ρ; MJ/L) and carbon intensity or energy-based carbon content (Ce = Cm/LHVm; kg C/MJ), while ethanol tends to result in a decrease in these properties. These effects detract from or add to the changes in FE and CO2 emissions from the CR-related engine efficiency improvements, as calculated in eq 7 and 8.

knock-limited combustion phasing that was very similar to the baseline 91-RON fuel in the baseline 10:1 CR engine. Likewise, 101-RON E30 fuel with 13:1 CR was less knock-limited than the baseline case, and thus the CR could have been greater than 13:1. These gains are conservative estimates, as they were achieved with a simple exchange of pistons with different bowl size. The resulting combustion chamber geometry was suboptimum as the engine was not specifically designed to utilize the higher CR. Thus, higher performance is expected when the engine is designed and optimized for higher CR. Next, thermal efficiency gains are calculated from two factors: (1) increased efficiency from higher CR (ΔηCR) as shown in Figure 6, and (2) an additional efficiency increase from higher ethanol content (Δηethanol) in the fuels (0.5% per 10%v ethanol). The total efficiency gain (Δηtotal) is the combination of the above two factors and any benefit associated with downsizing as shown in eq 6. ⎛⎛ Fdownsize × ΔηCR ⎞ Δηtotal = 100% × ⎜⎜1 + ⎟ 100% ⎠ ⎝⎝ ⎞ ⎛ Δηethanol ⎞ × ⎜1 + ⎟ − 1⎟ 100% ⎠ ⎝ ⎠

(6)

where Fdownsize = 1.3 for downsizing of naturally aspirated engines, 1.1 for downsizing of turbocharged engines, or 1.0 for no downsizing. For the 96-RON E20 case, the total increase in efficiency is estimated to be 4.0% without additional downsizing and 4.4% with additional downsizing of the assumed turbocharged engine. As shown in Table 2 and Figure 7, the calculated gains appear to provide reasonable estimates of the efficiency increases reported for

⎞ ⎛⎛ Δηtotal ⎞⎛ LHVv,HO ⎞ ⎟⎟ − 1⎟⎟ Δ%FE vol = 100% × ⎜⎜⎜1 + ⎟⎜⎜ 100% ⎠⎝ LHVv,ref ⎠ ⎠ ⎝⎝ 10785

(7)

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reduction in CO2 emissions as compared to the full vehicle model results reported in ref 28. For the cases shown here without downsizing, higher-octane fuels appropriately matched with higher CR yielded 4−5% and 6−9% reductions in CO2 emissions for fuel octane rating increases of 5 and 10 RON in an engine with CR increases of 2 and 3 ratios, respectively. The CO2 emission reductions in these cases are very similar to the improvement in total efficiency because the fuels have very similar energy-based carbon content, differing by 0.4% or less. In contrast, the volumetric FE changes for the E20 and E30 cases are small (and less than the CO2 emissions decrease) because the greater efficiency enabled by the fuels’ greater octane ratings approximately offset the reductions in fuel energy content due to the greater ethanol content. Overall, this estimation method conservatively reproduces the vehicle fuel economy and CO2 emissions changes reported in ref 28 that were obtained using a full vehicle model. The incremental efficiency gain from additional downsizing, estimated here to add 10% to the octane-related efficiency increase (Fdownsize = 1.1) in this turbocharged engine, is shown in Table 2 and Figure 7. This assumption increases the total efficiency gain for 96-RON E20 to 4.4% and the CO2 emissions reduction to 4.5%. The discussion above considers the effects of octane ratings and ethanol content on engine knock characteristics, efficiency, fuel economy, and CO2 emissions. Fuel composition changes to provide higher octane ratings could also affect other engine and vehicle attributes,63 including cost of ownership, tailpipe emissions, performance, driveability, and NVH (noise, vibration and harshness). Vehicle design involves managing trade-offs between these multiple design objectives. In addition, considerations for a new fuel should also include well-to-wheels emissions, influence on the existing vehicle fleet, economic impacts, compatibility with refueling infrastructure, and compatibility with off-road engines (e.g., lawn mowers, snowmobiles, snow blowers, boats). Light-duty vehicles in the United States and elsewhere are required to meet increasingly challenging regulations on fuel economy, GHG emissions, and criteria pollutant emissions. Increasing the octane rating of regular grade gasoline would provide greater knock resistance for SI engines during high load operation, enabling higher efficiency and lower CO2 emissions in existing light-duty vehicles through more optimal combustion phasing, and in future vehicles by enabling higher compression ratios. Higher ethanol content is one available option for increasing the octane ratings of gasoline and would provide additional engine efficiency benefits for part and full load operation. This paper provides a detailed review of how such benefits can be estimated for different engine types and fuel assumptions, calculations that are critical for well-to-wheels analysis of increased gasoline octane ratings. Considering vehicles and fuels as a system, such fuel changes would be supported if the net effect of light-duty vehicle efficiency improvements and fuel production impacts54 yielded reductions in fuel consumption, emissions, resource consumption, and/or cost. For example, a recent study55 showed that a transition to higher-RON (98 RON) gasoline in the U.S. with LDVs optimized for the fuel could yield significant net reductions in GHG emissions and a net cost savings. In addition to higher CR, future light-duty vehicle trends will also involve greater use of downsizing, turbocharging, downspeeding, and/or hybridization to improve efficiency. As with

Figure 7. Compression ratios (top panel), improvement in volumetric fuel economy (middle), and reduction in CO2 emissions (bottom) for a GTDI engine in a light duty truck application for 96-RON E20 and 101-RON E30 fuels having knock-limited combustion phasing equivalent to baseline 91-RON E10 fuel at baseline 10:1 CR. Values from engine dynamometer testing and vehicle modeling in ref 28 are compared to values estimated using the approach described herein, for cases with no downsizing (as assumed in ref 28) and with additional downsizing (not considered in ref 28).

where LHVv,HO and LHVv,ref are the volumetric energy content of the higher-octane test fuel and the baseline reference fuel, respectively. −1 ⎛⎛ ⎞ Δηtotal ⎞ ⎛ Ce,ref ⎞ ⎜ ⎟⎟ − 1⎟⎟ Δ%CO2 = 100% × ⎜⎜1 + ⎟ ⎜⎜ 100% ⎠ ⎝ Ce,HO ⎠ ⎝⎝ ⎠

(8)

where Ce,HO and Ce,ref are the energy-based carbon contents of the higher-octane test fuel and the baseline reference fuel, respectively. These fuel property effects should be included in well-to-wheel assessments of higher-octane gasoline given their impact on tailpipe CO2 emissions. The resulting estimated changes in FE and CO2 emissions for the two higher-octane fuel cases are given in Table 1. As shown in Figure 7, the estimation method reported here provides a somewhat conservative estimate of the change in FE and 10786

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(6) Transport, Energy and CO2, Moving Toward Sustainability; International Energy Agency: Paris, France, 2009; http://www.iea. org/publications/freepublications/publication/transport2009.pdf. (7) Moawad, A.; Rousseau, A. Light-Duty Vehicle Fuel Consumption Displacement Potential up to 2045; U.S. Department of Energy, Report No. ANL/ESD-14/4, April 2014. (8) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill, Inc.: New York, 1988. (9) Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975 Through 2013, Report No. EPA-420-R-13011; U.S. Environmental Protection Agency: Washington, DC, 2013; http://epa.gov/fueleconomy/fetrends/1975-2013/420r13011.pdf. (10) Ayala, F. A.; Gerty, M. D.; Heywood, J. B. Effects of Combustion Phasing, Relative Air-fuel Ratio, Compression Ratio, and Load on SI Engine Efficiency, SAE Technical Paper 2006-01-0229, 2006; DOI 10.4271/2006-01-0229. (11) Duleep, K. Review to Determine the Benefits of Increasing Octane Number on Gasoline Engine Efficiency: Analysis and Recommendations Tasks 2-5, CRC Project No. CM-137-11-1b; Coordinating Research Council, Inc., September 2012. (12) Bromberg, L.; Cohn, D. R.; Heywood, J. B. Calculations Of Knock Suppression In Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection; MIT Laboratory for Energy and the Environment, Report No. LFEE 2006-001, April 2005. (13) Kapus, P. E.; Fuerhapter, A.; Fuchs, H.; Fraidl, G. K. Ethanol Direct Injection on Turbocharged SI EnginesPotential and Challenges, SAE Technical Paper 2007-01-1408, 2007; DOI 10.4271/2007-011408. (14) Marriott, C. D.; Wiles, M. A.; Gwidt, J. M.; Parrish, S. E. Development of a Naturally Aspirated Spark Ignition Direct-Injection Flex-Fuel Engine, SAE Technical Paper 2008-01-0319, 2008; DOI 10.4271/2008-01-0319. (15) Blumberg, P. N.; Bromberg, L.; Kang, H.; Tai, C. Simulation of High Efficiency Heavy Duty SI Engines Using Direct Injection of Alcohol for Knock Avoidance. SAE Int. J. Engines 2009, 1 (1), 1186− 1195. (16) Milpied, J.; Jeuland, N.; Plassat, G.; Guichaous, S.; Dioc, N.; Marchal, A.; Schmelzle, P. Impact of Fuel Properties on the Performances and Knock Behaviour of a Downsized Turbocharged DI SI Engine - Focus on Octane Numbers and Latent Heat of Vaporization. SAE Int. J. Fuels Lubr. 2009, 2 (1), 118−126. (17) Kasseris, E.; Heywood, J. Charge Cooling Effects on Knock Limits in SI DI Engines Using Gasoline/Ethanol Blends: Part 1Quantifying Charge Cooling, SAE Technical Paper 2012-01-1275, 2012; DOI 10.4271/2012-01-1275. (18) Stein, R. A.; Polovina, D.; Roth, K.; Foster, M.; Lynskey, M.; Whiting, T.; Anderson, J. E.; Shelby, M. H.; Leone, T. G.; Vandergriend, S. Ef fect of Heat of Vaporization, Chemical Octane, and Sensitivity on Knock Limit for Ethanol - Gasoline Blends. SAE Int. J. Fuels Lubr. 2012, 5 (2), 823−843. (19) Kasseris, E.; Heywood, J. Charge Cooling Effects on Knock Limits in SI DI Engines Using Gasoline/Ethanol Blends: Part 2-Effective Octane Numbers, SAE Technical Paper 2012-01-1284, 2012; DOI 10.4271/ 2012-01-1284. (20) Foong, T. M. Morganti, K. J., Brear, M. J., da Silva, G., Yang, Y., Dryer, F. L. The Effect of Charge Cooling on the RON of Ethanol/ Gasoline Blends, SAE Technical Paper 2013-01-0886, 2013; DOI 10.4271/2013-01-0886. (21) Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975 Through 2013, Report No. EPA-420-R-13011; U.S. Environmental Protection Agency: Washington, DC, 2013; http://epa.gov/fueleconomy/fetrends/1975-2013/420r13011.pdf. (22) Leone, T. G.; Pozar, M. Fuel Economy Benefit of Cylinder Deactivation - Sensitivity to Vehicle Application and Operating Constraints, SAE Technical Paper 2001-01-3591, 2001; DOI 10.4271/2001-01-3591. (23) Nagasaka, A.; Nada, M.; Hamada, H.; Hiramatsu, S.; Kikuchi, Y.; Kato, H. Development of the Hybrid/Battery ECU for the Toyota Hybrid System, SAE Technical Paper 981122, 1998; DOI 10.4271/981122.

higher CR, each of these trends involve greater operation of engines under higher-load, knock-limited conditions and as such, also benefit from higher-octane fuel by expanding the engine map that is not subject to retarded combustion phasing to avoid knock. Higher octane fuel is a key enabler for improved efficiency based on current engine/vehicle design trends.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01420. Further details on spark retard effects on efficiency and basis for estimated vehicle FE gains for higher octanerated gasoline in existing vehicles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 313-248-6857; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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DISCLAIMER Whereas this article is believed to contain correct information, Ford Motor Company (Ford), General Motors Company (GM), and FCA US LLC (FCA US) do not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of their companies. This disclaimer may not be removed, altered, superseded or modified without prior Ford, GM, and FCA US permission.

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