Energy Fuels 2010, 24, 355–364 Published on Web 12/01/2009
: DOI:10.1021/ef900832c
Study on Low Temperature Combustion for Light-Duty Diesel Engines Jes us Benajes, Santiago Molina,* Ricardo Novella, and Rogerio Amorim CMT-Motores T ermicos Universidad Polit ecnica de Valencia Camino de Vera s/n, 46022, Valencia, Spain Received July 31, 2009. Revised Manuscript Received October 8, 2009
This paper presents a study on the feasibility of obtaining low temperature mixing-controlled combustion (mixing-controlled LTC) in a small HSDI engine with the objective of avoiding simultaneous NOx and soot formation. This mixing-controlled LTC strategy is based on reducing the equivalence ratio at the liftoff cross section and also the local combustion temperatures, but maintaining the conventional diesel spray structure. A parametric study has been carried out to evaluate the effects of in-cylinder gas density, temperature, and oxygen concentration on the characteristics of the mixing-controlled LTC characteristics. Low NOx and low soot mixing-controlled diesel combustion has been attained by combining low incylinder gas temperatures together with high air densities and low oxygen concentrations. However, the mixing-controlled LTC concept also presents an important drawback related to the engine efficiency deterioration.
1. Introduction The development of diesel engine technologies was mainly responsible for the increment in the share of the vehicles equipped with diesel engines in the European market, from around 12% in 1995 to 50% in 2005. However, additional efforts in the frame of diesel engine research are still required due to the stricter future emission standards and the increasing demand for high fuel economy engines. Besides, advanced gasoline concepts and hybrid vehicles are progressively becoming more affordable technologies, being suitable future alternatives for conventional diesel engines. After evaluating the situation, researchers and manufacturers have reacted using ultimate technology for reducing pollutant emissions and improving the performance of diesel engines, such as advanced injection systems, EGR control, etc. Thus, one of the most important challenges of diesel engine research consists of developing internal measures for avoiding the pollutant emissions formation inside the cylinder during the combustion process. However, the diesel combustion process in a direct injection engine is a complex phenomenon. In order to better understand how it proceeds, some conceptual models of reactive free diesel sprays have been developed. These conceptual models have been continuously improved due to the use of more modern optical techniques applied to the diesel spray visualization.1-5 Additionally, other authors
Figure 1. Schematic view of the conceptual model developed by Dec.
have extended these combustion models considering the influence of wall-spray and spray-spray interactions.6,7 Although they were of vital importance, it is interesting to point out that most of these investigations were carried out in a constant-volume vessel, with the aim of analyzing the conventional diesel combustion independently of the engine dynamics. Initially, the development of this work is first based on the conceptual model of a conventional diesel combustion introduced by Dec, which has been widely accepted by the research community.1 As it is shown in Figure 1, in this model he presented a spatial sketch of conventional diesel combustion, including the zones where NOx and soot were initially formed. According to this model, from the injector nozzle the fuel entrains some of the air that surrounds the spray. Then, at a given distance from the nozzle known as the lift-off length, a fuel-rich premixed reaction zone develops in the internal region of the spray. Initial soot precursors are formed as products of these fuel-rich premixed reactions. These soot precursors travel toward the front of the spray in a high temperature and oxygen deficient environment, creating suitable conditions for soot particles formation. Thus, these soot particles trend to enlarge as they go toward the flame vortex, where the largest particles are found. At the surroundings of the flame, the remaining fuel burns in contact with the oxygen of the external air. This mixing-controlled diffusion flame at the spray periphery significantly increases the flame temperature and enhances the thermal NOx formation. Also, the high local temperatures attained in the diffusion flame promote the
*To whom correspondence should be addressed. E-mail: samolina@ mot.upv.es. Telephone: þ34 963 877 650. Fax: þ34 963 877 659. (1) Dec, J. E. A Conceptual Model of DI Diesel Combustion Based on Laser-Sheet Imaging. SAE Paper 970873, 1997. (2) Naber, J. D.; Siebers, D. L. Scaling Liquid-Phase Fuel Penetration in Diesel Sprays Based on Mixing-Limited Vaporization. SAE Paper 1999-01-0528, 1999. (3) Higgins, B.; Sibers, D. L. Diesel-Spray Ignition and PremixedBurn Behavior. SAE Paper 2000-01-0940, 2000. (4) Payri, F.; Pastor, J. V.; Garcı´ a, J. M.; Pastor, J. M. Measure. Sci. Technol. 2007, 18 (8), 2579-2598. (5) Desantes, J. M.; Pastor, J. V.; Pastor, J. M.; Julia, E. Fuel 2005, 84, 2301-2315. (6) Bruneaux, G. Int. J. Engine Res. 2008, 9, 249-265. (7) Pickett, L. M.; L opez, J. J. Spray-Wall Interaction Effects on Diesel Combustion and Soot Formation. SAE Paper 2005-01-0921, 2005. r 2009 American Chemical Society
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: DOI:10.1021/ef900832c
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oxidation of the soot particles previously formed inside the spray. For this conventional diesel combustion conditions, the maximum local temperature at the diffusion flame zone is typically in the range of 2500-3000 K. Therefore, NOx mostly forms following the thermal mechanism, also called as the extended Zeldovich mechanism, in the lean side of the flame where oxygen is available. However, the thermal NOx formation is considered insignificant for temperatures lower than 2000 K.8,9 Engine-out soot is the final result of a competition between soot formation and soot oxidation. Soot formation starts with a chemical process called pyrolysis in which the molecular structure of organic compounds are altered forming soot precursors, PAH (polycyclic aromatic hydrocarbons), and acetylenes. Then, nucleation, or soot particle inception, takes place in a temperature range from 1400 to 1600 K. This process consists of radical additions of small hydrocarbons to larger aromatic molecules. Surface growth, the next step in soot formation, is the mass addition to the surface of a nucleated soot particle. Finally, two processes of particle association happen simultaneously: coalescence and agglomeration. Coalescence occurs when two particles collide and form a new particle, that is, two spherically shaped particles form a bigger single spherical particle. Agglomeration happens when individual particles stick together to form a chainlike structure. The fact is that soot is really formed during the nucleation; the other processes can be described only as enlarging effects defining the size of the final soot particle. Soot oxidation is the conversion of carbon or hydrocarbons to combustion products. Once the carbon is partially involved in oxidation to CO, this carbon will not participate in soot formation. Soot oxidation is avoided for temperatures below 1300 K, whereas its formation starts when the temperature is above 1400 K. In diffusion flames, soot formation and soot oxidation rates increase with temperature. Thus, combustion temperature is the most influent parameter for both processes.10 On the one hand, soot formation in a diesel spray depends fundamentally on three factors: equivalence ratio at the lift-off length (ΦH), residence time required for going from the lift-off length to the flame sheet, and local temperature inside the flame. On the other hand, soot oxidation depends on the flame temperature along the combustion process. Therefore, local combustion temperatures must be reduced to avoid thermal NOx formation. Initially, this reduction in flame temperature worsens the soot oxidation process, leading to a substantial engine-out soot increase. Nevertheless, if temperature is further decreased soot formation is finally avoided and a simultaneous reduction in NOx and soot emissions is attained. The use of premixed combustion strategies, such as premixed compression ignition (PCI), is an alternative to reduce the formation of both pollutants. The basic concept is the separation of the injection event and the combustion process. Thus, in these concepts the combustion starts after the end of injection and the diesel spray structure is lost. As a result, the fuel is partially premixed with the surrounding air, reducing both local equivalence ratios and combustion temperatures. However, the PCI-like strategies usually present problems
related to ignition control, load limits, and combustioninduced noise. Another promising alternative to reduce NOx and soot formation in the frame of diesel engines is the mixing-controlled LTC concept. The concept is based on preserving the mixing-controlled diesel spray structure, but introducing a significant reduction in the equivalence ratio at the lift-off length cross section and also in the combustion temperature. The mixing-controlled LTC concept presents additional advantages compared to the PCI strategies, such as its low combustion noise and the combustion phasing directly controlled by the injection. This combustion concept has been successfully implemented using a free-spray configuration in a constant-volume vessel.11 However, implementing the mixing-controlled LTC concept in engines is still a challenge because of the nonsteady evolution of the in-cylinder gas thermodynamic conditions. Moreover, the lift-off length and the averaged equivalence ratio at this spray section can be affected by spray-spray and spray-wall interactions. For low in-cylinder gas temperature, the fuel evaporation rate is slowed down and the spray liquid length increases. Depending on the operating conditions, the liquid fuel could reach the bowl surface. This liquid fuel impingement onto the bowl wall is a well-known source of HC and CO emissions. Finally, mixing-controlled LTC also requires very high intake boost pressures to achieve very low oxygen concentrations (very high EGR rates) with suitable values for A/F ratio. 2. Objectives Most of the investigations related to the mixing-controlled LTC concept have been carried out in constant-volume vessels and heavy-duty optical engines. However, typical in-cylinder gas flows (swirl, squish) and spray-wall and spray-spray interactions have not been reproduced in these constantvolume vessels. Optical engines do not generate complete realistic environments because of their maximum pressure and temperature limits, which are generally lower than those found in production engines. Finally, the geometric limitations are more restrictive in small engines, not allowing long liquid and lift-off lengths due to the short distances from the injection nozzle to bowl surfaces. The main objective of the present investigation is to evaluate the potential of the mixing-controlled LTC concept in a HSDI diesel engine for avoiding both NOx and soot formation. The mixing-controlled LTC has been generated by reducing the in-cylinder gas temperature and its oxygen concentration at the same time. This concept has been implemented in a HSDI engine not only to confirm the possibility of achieving a low temperature mixing-controlled combustion, but also to analyze the influence of different parameters separately and their interactions, and finally to identify any restriction for applying this combustion concept. 3. Experimental Setup and Methodology This section describes the experimental facility and the methodology followed to generate the test plan using a previously generated engine map.
(8) Heywood, J. Internal Combustion Engines Fundamental. 1st ed.; McGraw-Hill Inc.: 1988. (9) Turns, S. R. Prog. Energy Combust. Sci. 1995, 7, 361-385. (10) Tree, D. R.; Svensson, K. I. Prog. Energy Combust. Sci. 2007, 6, 272-309.
(11) Pickett, L. M.; Siebers D. L. Non-Sooting, Low Flame Temperature Mixing-Controlled DI Diesel Combustion. SAE Paper 2004-01-1399, 2004.
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Table 1. Engine Characteristics characteristic
description
CR displacement bore stroke bowl diameter nozzle
15.4:1 0.353 dm3 73.7 82.0 mm 42 mm 6 0.129 mm
3.1. Experimental Facility and Tools. The engine used in this study was a single-cylinder, HSDI diesel engine. The use of a single-cylinder engine in this investigation is justified by the high capability of controlling engine parameters and the precision of the measurements. For this type of engine, boost pressure is controlled independently of the engine operational condition, thus high intake boost pressures can be combined with very high EGR rates. The engine main characteristics are shown in Table 1. This engine was installed in a fully equipped test cell, with all the auxiliary devices required for its operation and control. The test cell was designed to provide high rates of cooled EGR (up to 70%) without water condensation in the intake. Also, the intake temperature after mixing fresh air and EGR can be reduced down to 40 °C. The in-cylinder instantaneous pressure was measured by means of a piezoelectric transducer. A total of 50 consecutive engine cycles were recorded in order to avoid cycle-bycycle variations during engine operation. In-cylinder pressure trace was then averaged, filtered, and postprocessed with inhouse combustion diagnostic software, which calculates the heat release law based on the first law of thermodynamics.12,13 This zero-dimensional model assumes uniform pressure and temperature along the combustion chamber. However, unburned and burned gas temperatures were calculated considering that the gases inside the cylinder could be divided in two zones.14 The actual fuel injection rate profile was measured with an IAV-EVI injection analyzer and following the Bosch method.15,16 The nonreactive diesel spray development inside the cylinder was modeled with a 1D simulation software, in order to evaluate the nonreactive diesel spray development inside the cylinder.17 The main inputs of this model are the injection rate, engine geometry, and in-cylinder gas thermodynamic conditions. Exhaust gases were analyzed by a HORIBA 7100D, which is able to measure HC, CO, CO2, NOx, and O2. Each gas sample was collected during 60 s in order to increase the quality of data. Soot emissions were measured with an AVL 415 smokemeter, providing results directly in FSN (filter smoke number) averaged from 3 consecutive measurements under the same operating conditions. The obtained FSN value was converted into mg/m3 by means of the correlation presented by Christian et al.18 3.2. Engine Map. The engine map relates different engine parameters to the A/F ratio and EGR rate. This map was
Figure 2. Engine map developed for intake air temperature of 75 °C. Light blue lines are volumetric oxygen concentration, green lines are intake air pressure, pink lines represent gas density, and red lines corresponds to air-fuel ratio. Note that gas density and intake air pressure are parallel lines. The green triangle is the target area for defining the test plan.
designed considering the possibility of controlling the combustion process by adjusting the gas oxygen concentration.19,20 The first step consists of defining the load condition of the engine test in terms of fuel consumption per cylinder-cycle (mf) because it is linked directly to intake air mass flow (mint = mair þ mEGR) in function of A/F ratio and EGR ratio by the eq 1: A=F ¼ ð1 - EGRÞmint =mf
ð1Þ
where mint is intake total air mass flow and mf is fuel mass flow. From the hypothesis of complete combustion, it is possible to calculate the isoline oxygen volumetric concentration (XO2_int) according to the eq 2: XO2
int
¼ XO2
air ð1 -EGRðA=FÞSTOICH =ðA=FÞÞ
ð2Þ
where XO2_int and XO2_air are intake and ambient air volumetric O2 concentrations, respectively, and (A/F)STOICH is the stoichiometric air-fuel ratio. Then, as a second hypothesis it is assumed that, for a given values of intake pressure and temperature, volumetric efficiency and gas constant are both independent of the EGR ratio. Once a suitable engine map is defined for a determined engine load, engine geometry, and operating conditions;such as clearance volume, volumetric efficiency, engine speed, and intake air temperature (Tint);are associated to intake total air mass flow to obtain values for intake air pressure (pint) and in-cylinder gas density (Fg). The engine maps have been very useful for designing the test plan because they relate various engine operating parameters. From these maps, it is possible to know the values of the most important parameters as a function of operating attributes. As a reference, the engine map calculated for Tint = 75 °C used in this investigation is shown in Figure 2. It is noticeable that gas density and intake air pressure are parallel. In other words, gas density is easily controlled by changing intake air pressure. 3.3. Engine Test Plan. Siebers investigated the influence of various parameters on liquid length.21 Moreover, Garcı´ a proposed an empirical equation to estimate this liquid length:
(12) Lapuerta, M.; Armas, O.; Hernandez, J. Appl. Thermal Eng. 1999, 5, 513-519. (13) Lapuerta, M.; Armas, O.; Berm udez, V. Appl. Thermal Eng. 2000, 20, 843-861. (14) Desantes, J. M.; Arregle, J.; Molina, S.; Lejeune, M. Influence of the Egr Rate, Oxygen Concentration and Equivalent Fuel/Air Ratio on the Combustion Behavior and Pollutant Emissions of a Heavy-Duty Diesel Engine. SAE Paper 2000-01-1813, 2000. (15) Bosch, W. Fuel rate indicator is new measuring instrument for display of the characteristics of individual injections. SAE International, SAE Paper 660749, 1966. (16) Payri, R.; Salvador, F. J.; Gimeno, J.; Bracho, G. A new methodology for correcting the signal cumulative phenomenon on injection rate measurements. Exp. Tech. 2008, 32, 46-49. (17) Pastor, J. V.; Encabo, E.; Ruiz, S. New Modelling Approach for Fast Online Calculations in Sprays. SAE International, SAE Paper 2000-01-0287, 2000. (18) Christian, R.; Knopf, F.; Jasmek, A.; Schindler, W. MTZ Motortech. Z. 1993, 54, 16-22.
LL ¼ ðdnozzle =150Þ 7:2455 105 Tg-1:269 Fg-0:53
ð3Þ
where dnozzle is the nozzle orifice diameter, Tg is gas temperature, (19) Nakayama, S.; Fukuma, T.; Matsunaga, A.; Miyake, T. A New Dynamic Combustion Control Method Based on Charge Oxygen Concentration for Diesel Engines. SAE International, SAE Paper 2003-01-3181, 2003. (20) Benajes, J. V.; L opez, J. J.; Novella, R.; Garcı´ a, A. Exp. Tech. 2008, 32, 41-47. (21) Siebers, D. Liquid-Phase Fuel Penetration in Diesel Sprays. SAE Paper 980809, 1998.
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and Fg is gas density. Siebers and Higgins observed how the reduction in ambient temperature extends the lift-off length, producing a significant increment in the amount of air entrained in the fuel spray before reaching this lift-off length.23 Also, an equivalence ratio value at the lift-off cross section below 2 was affirmed to reduce soot formation to negligible amounts.24 In more recent studies, the effects of ambient conditions, nozzle orifice diameter, injection pressure, and oxygenated fuels on liftoff length and air entrainment were investigated. Based on those researches, Siebers, Higgins, and Picket developed two equations to estimate lift-off length (H) and equivalence ratio at the lift-off (ΦH), respectively: 22
H ¼ 7:04 108 Tg -3:74 Fg -0, 85 dnozzle 0:34 UZst -1 0 0 111=2 B B C A B H B CC C B1 þ 16Br ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiC C φH ¼ 2 p ffiffiffiffi @ @ AA F st Ff Ca dnozzle
Table 2. Engine Test Plan parameter mf IP SoE Tint (Tg) XO2 (YO2) Fg
17 mg/cm 1200 bar -10 CAD 40 °C (900K) - 55 °C (950K) - 75 °C (1000K) 9% (10%) - 10% (11.1%) - 11% (12.2%) - 12% (13.3%) 26 kg/m3 - 30 kg/m3 - 35 kg/m3 - 40 kg/m3
which is directly correlated to NOx formation and soot formation and oxidation rates. The massive use of EGR combined with very high intake boost pressures was studied by Noehre26 and Colban.27 However, they did not have the compromise to obtain strictly a mixing-controlled LTC combustion. Their results confirmed the potential of reducing intake oxygen concentration down to very low levels together with high air densities for simultaneously decreasing NOx and soot emissions. The reduction in combustion temperatures caused by low oxygen concentration avoids soot and NOx formation. On the contrary, important drawbacks in CO, HC, and FC were reported. The engine test plan was specifically designed to generate a mixing-controlled, low temperature combustion (LTC) process with the lowest possible value of the equivalence ratio at the liftoff length cross section. Based on the Sandia National Laboratories studies, the limits for A/F, oxygen concentration, and ambient gas density were determined from the following premises: (1) Engine operating conditions was IP (injection pressure) of 1200 bar midload, with constant mf of 17 mg/cm3 and IMEP around 9.0 bar and a single injection. Fuel injection SoE was -10 CAD aTDC that, in this case, is equivalent to a SoI at -5 CAD aTDC. (2) A/F was kept over 18 to avoid excessive HC and CO emissions. (3) Gas density was kept below 40 kg/m3 because of the maximum intake pressure allowed for the equipment. (4) The lowest range provided by the experimental facility was selected for oxygen concentration to reduce the combustion temperature as much as possible since it hardly affects the equivalence ratio at the lift-off length. Other investigations have obtained very good results with oxygen concentrations from 12% until 8%. However, as it is illustrated by the engine map shown in Figure 2, it was not possible to obtain 8% of oxygen concentration because of A/F restrictions and/or intake pressure limitations. Reducing XO2_int down to 8% requires an intake air pressure as high as approximately 4.0 bar, which is not suitable for the experimental facility. Finally, oxygen concentrations higher than 12% were not expected to generate too high combustion temperatures.3-24 (5) Initially, the range for the in-cylinder gas temperature range (Tg) was initially defined from 850 to 1000 K, similar than that investigated at Sandia National Laboratories. Also, good results were obtained using a Tg value of 800 K, but in the engine used in this work with conventional CR equal to 15.4:1 this temperature can only be attained by reducing the intake air temperature down to around 0 °C. This range indicated an intake air temperature range (air þ EGR) from 20 to 75 °C. Summing up, the chosen intake air temperatures was from 20 °C (Tg = 850 K) to 75 °C (Tg = 1000 K). It is important to highlight that each intake air temperature requires the development of a new engine map because the relationship between Fg and pint changes when varying intake air temperature. (6) Based on the previous observations, the engine test plan range was finally defined
ð4Þ
ð5Þ
Fair 0:75 tanðθ=2Þ
where U is injection velocity, Zst is stoichiometric fuel mixture fraction, Ff is fuel density, Ca is the nozzle hole area contraction coefficient, and θ is the spray spreading angle.8-24 Siebers also evaluated the relationship between ignition processes and the lift-off. They observed how the lift-off length is generally longer for fuels with longer ignition delays (τig). Based on their results, they affirmed that ignition delay trends to increase by decreasing temperature, gas density and oxygen concentration: τig µ eð1=TAMBÞ Fg n Zst m
value 3
ð6Þ
where values of n ≈ -1.3 and m ≈ -1.0 are usual for conventional diesel fuels.25 According to several studies carried out at Sandia National Laboratories in a constant-volume vessel with controlled gas conditions, each parameter presents a different effect on the diesel spray morphology and also on the air-fuel mixing characteristics. Increasing gas density simultaneously causes a reduction in spray liquid and lift-off lengths. Also, air entrainment rate increases due to the higher mass of air for the same volume entrained. The global net result is a slight increment in ΦH, thus gas density could be used as a control factor to avoid a possible liquid wall impingement. Decreasing gas temperature extends the liquid and lift-off lengths, but it hardly affects the amount of air entrained into the fuel spray. Thus, the amount of air entrained into the spray at the lift-off length cross section increases when reducing the gas temperature. However, a reduction in gas temperature decreases the combustion temperatures, leading to a reduction in NOx formation and also having an influence on the rates of soot formation and oxidation. A reduction in intake oxygen concentration increases the liftoff length without affecting the rate of air entrainment, but the air entrained into the spray contains a lower mass of oxygen. The final net effect is a constant amount of air entrained into the spray at the lift-off length cross section independently of intake oxygen concentration. The dilution slows the O2-fuel mixture formation and also has great influence in the flame temperature, (22) Garcı´ a, J. M. Aportaciones al estudio del proceso de combusti on turbulenta de chorros en motores diesel de inyecci on directa; Ph.D. Thesis. Universidad Politecnica de Valencia, Departamento de Maquinas y Motores Termicos: Valencia, 2004. (23) Siebers, D.; Higgins, B. Flame Lift-Off on Direct Injection Diesel Under Quiescent Conditions. SAE Paper 2001-01-0530, 2001. (24) Siebers, D.; Higgins, B.; Pickett, L. Flame Lift-off on DirectInjection Diesel Fuel Sprays: Oxygen Concentration Effects. SAE Paper 2002-02-0890, 2002. (25) Pickett, L.; Siebers, D.; Idicheria, C. Relationship Between Ignition Processes and the Lift-Off Length of Diesel Fuel Sprays. SAE Paper 2005-01-3843, 2005.
(26) Noehre, C.; Andersson, M.; Johansson, B.; Hultqvist, A. Characterization of Partially Premixed Combustion. SAE Paper 2006-01-3412, 2007. (27) Colban, W. F.; Miles, P. C.; Oh, S. Effect of Intake Pressure on Performance and Emissions in an Automotive Diesel Engine Operating in Low Temperature Combustion Regimes. SAE Paper 2007-01-4063, 2007.
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Figure 3. NOx-soot trade-off curves.
Figure 4. Rate of heat release for 40 and 75 °C intake air temperature and gas density of 40 kg/m3 engine tests.
and is shown in Figure 2 (green triangle), where the black points inside the shaded area are the testing points. They are exactly the intersection between gas density and oxygen concentration isolines and, using this engine map, it is possible to read their respective values for intake air pressure and EGR rate, which are the actual controlled engine parameters. In a small-bored diesel engine, the available free length for the flame development is very short and the possibility of wall impingement must be carefully considered. Thus, both liquid and lift-off lengths were calculated for each point through the eqs 3 and 4 and compared to the distance from the injector tip to the piston bowl wall. Eventually, Tint = 20 °C (Tg = 850 K) was removed from the test plan because the lift-off length for those points was much longer than the limit. The final test plan is shown in Table 2. The ranges selected for oxygen concentration and gas density are also shown in Figure 2, providing a total of 10 points per intake air temperature. The total number of test points in this parametric study was 30, so that for each engine tested point, only one engine operating parameter was varied following a parametrical research approach. The equivalence ratio at the lift-off (ΦH) cross section was predicted according to eq 5.
Figure 5. τig/tinj vs intake air oxygen concentration for 40 °C intake air temperature.
The lowest predicted ΦH was around 3.5, which was still far from the proposed value of 2.0 required to avoid soot precursors formation.24 359
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Figure 6. Adiabatic flame temperature for 75 °C of intake temperature. The left graph shows the variation over oxygen concentration and the right is in function of in-cylinder gas density.
Figure 7. NOx vs maximum adiabatic flame temperature and soot vs Tad90 graphs.
4. Results and Analysis In this section, the experimental results obtained combining different oxygen concentration; gas density and intake air temperature are presented and discussed. This analysis is focused on the influence of each parameter in terms of pollutant emissions and combustion characteristics. 4.1. Oxygen Concentration. As shown in Figure 3, the first effect of decreasing oxygen concentration consists of a significant decrease in NOx formation. Also, the NOx level in all tests is very low and never exceeds 1 g/kgfuel. However, engine-out soot emissions trend to increase until they reach its peak close to 7 g/kgfuel in the worst cases, then these soot emissions start to steeply decrease. This behavior was observed in the entire temperature range, indicating a general behavior that was also previously observed in other studies.10,26-28 It is attributed to the decrease in soot precursors formation rate due to the lower adiabatic flame temperatures. The results comparing the different oxygen concentrations keeping the same intake air temperature and density are presented in Figure 4 in terms of RoHR. From this figure, it was observed how the premixed combustion peak and the slope of this premixed combustion decreased, which indicates a reduction in the chemical reaction rates caused by the lower oxygen concentration. Moreover, the maximum RoHR value during the mixing-controlled combustion stage is also reduced, indicating an overall deceleration of the entire process. Figure 5 shows the τig/tinj versus intake air oxygen concentration for 40 °C intake air temperature. Although the
Figure 8. Soot vs intake air oxygen concentration for 40 kg/m3 gas density.
slope of the RoHR curve is largely affected by intake oxygen concentration, the ignition delay remains almost constant even for the lowest oxygen concentration. For the most interesting gas density of 40 kg/m3, the combustion process started before the end of injection. Even at the lowest oxygen concentration a complete premixed combustion is not attained. Thus, focusing on the operational condition with gas density of 40 kg/m3, intake air temperature of 40 °C, and oxygen concentration of 9%, both NOx and soot emissions were negligible, thus confirming the possibility of implementing the mixing-controlled LTC concept in this HSDI engine. From Figure 6 (left), adiabatic flame temperature decreased with oxygen concentration. Thus, the high temperatures responsible for NOx formation were avoided, then NOx emissions were reduced from low to insignificant values. This combustion temperature reduction also affected
(28) Idicheria, C.; Pickett, L. Soot Formation in Diesel Combustion under High-EGR Conditions. SAE Paper 2005-01-3834, 2005.
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Figure 9. (a) RoHR vs normalized HRL graph for the case of 75 °C and 12% of oxygen concentration showing the effect of gas density variation. (b) Rate of heat release curves for Tint = 55 °C and 12% of oxygen concentration for various air densities. The red circle denotes the increasing presence of cool flame reactions when gas density was decreased.
the engine-out soot emissions. Changing oxygen concentration from 12 to 10%, the soot oxidation rate was slowed down and soot formation rate was less affected, leading to the very high values of soot observed. Reducing further the oxygen concentration to 9%, the soot formation rate was largely slowed down and, as a result, engine-out soot emissions were steeply reduced. Figure 7 (left) presents a NOx versus maximum adiabatic flame temperature (TadMax) plot for all the tests included in this investigation, where the tests corresponding to the highest gas density of 40 kg/m3 have been highlighted. As expected, an important decline of NOx emissions with the reduction of TadMax was observed. Thus, NOx is expected to be mainly formed following the thermal mechanism, which is strongly affected by the combustion temperature. An additional plot relating soot emissions with the estimated adiabatic temperature when the 90% of the fuel mass is burnt (Tad90) has been included in Figure 7. This characteristic temperature is considered representative of the soot oxidation process because, generally, when this temperature is very high, all formed soot is oxidized before the end of combustion. In this case, Tad90 was always reduced when decreasing oxygen concentration, whereas soot emissions initially increased because the soot oxidation process was more relevant than the soot formation process. Afterward, soot emissions largely decreased, although the oxidation process was still being deteriorated, confirming how soot formation is more relevant under the LTC conditions than soot oxidation. Thus, these results also corroborate how, for this mixing-controlled LTC concept, soot formation is significantly affected by the combustion temperature trending to decline very steeply for Tad90 below 1900 K. Finally, these results were consistent, and for the three intake temperatures the behavior of soot emissions was very similar. Figure 8 relates soot emissions with intake air oxygen concentration for the three different intake temperatures. The curves have a similar shape as that observed in Figure 7 (right), since the adiabatic flame temperature during the cycle is mostly affected by oxygen concentration. As the oxygen concentration does not have significant effects on ΦH, the reduction in engine-out soot was expected to be caused by the decrease of combustion temperatures. 4.2. Gas Density. In this investigation, gas density represents a key parameter since it controls the mixing process and thus the liquid length, and also the lift-off length, without
Figure 10. Soot-NOx trade-off graph comparing different intake air temperatures for 40 kg/m3 of gas density.
significant effects on the equivalence ratio at the lift-off length cross section. Theoretically, increasing gas density trends avoid wall-impingement since this leads to a reduction in combustion delay and liquid length. Observing NOx-soot curves in Figure 3, increasing gas density NOx emissions are kept approximately constant, while soot slightly decreases. Furthermore, NOx formation was controlled mainly by oxygen concentration because gas density has a low influence in the adiabatic flame temperature and, therefore, also in NOx formation. Soot emissions were slightly reduced whereas gas density was increased. As the maximum adiabatic flame temperature is hardly affected by changes in density, it is supposed that it was soot oxidation instead of soot formation that was mainly responsible for the observed soot reduction. Figure 9a relates the RoHR with the normalized HRL for the test with intake air temperature of 75 °C and oxygen concentration of 12%. The fact that the RoHR remained higher for higher air densities until the end of the combustion corroborates how the mixing process is more effective for higher densities, thus an improvement in soot oxidation is also expected. In Figure 9b, RoHR curves for different air densities are presented for an intake air temperature of 55 °C and oxygen concentration of 12%. Analyzing this plot, the ignition delay increased when decreasing gas density as was previously observed in Figure 5. Additionally, the energy released by the cool flame reactions was more evident in low gas density conditions (red ellipse in Figure 9b). This is the 361
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Figure 11. HC, CO, and IMEP vs XO2 for 40 °C intake air temperature engine tests. HC and CO increased steeply whereas IMEP reduced with oxygen concentration reduction.
result of the longer time available to mix fuel and air before the start of this cool flame heat release due to the extended ignition delay, which implies more quantity of fuel participating in these reactions. In Figure 3 soot emissions increased for the lower gas density conditions. Considering the RoHR evolutions shown in Figure 9b (XO2 = 12% and Tint = 55 °C), in those cases the combustion process was highly premixed instead of mixing-controlled. However, this complete premixed, highsooting combustion is not discussed as it is out of the scope of the present investigation, which is focused on the mixingcontrolled combustion process. It is important to re-cover the idea that only with high gas density it is possible to reduce oxygen concentrations down to the suitable levels to achieve the complete extinction of soot. 4.3. Air Temperature. Intake air temperature affects directly unburned gas temperature at the beginning of the combustion process. The effects of air temperature on the soot-NOx trade-off curve for gas density of 40 kg/m3 are shown in Figure 10. NOx emissions were not significantly influenced by intake air temperature in the LTC range. However, reducing intake air temperature shifts down soot-NOx curves in this plot. In fact, soot emissions also decreased, and this trend is not supposed to be caused only by the reduction in the adiabatic flame temperatures because, as can be observed in Figure 7, this temperature was only slightly decreased. Also, soot oxidation is hardly affected, as can also be confirmed by the Tad90 values shown in Figure 7 (right). A suitable explanation for the reduction in soot emissions is based on the reduction of ΦH due to the longer lift-off length caused by the reduced unburned gas temperature. This lower ΦH is expected to avoid the formation of soot precursors inside the flame. As a result of combining all these effects, the mixingcontrolled LTC has been successfully attained, showing the
Figure 12. Fuel spray penetration for three different operational conditions obtained from a simulation code and SoC angle.
potential of achieve a near-zero NOx and soot combustion process. It is important to point out how, in some cases, the cumulative effect of reducing intake air temperature and gas density at the same time produces a highly premixed and rich combustion process, which generates high levels of soot. 4.4. Effects on HC, CO, and IMEP. Figure 11 present the results related to HC and CO emissions together with IMEP values for the engine tests with intake air temperature of 40 °C. The other intake air temperature results were not shown here because the trends are similar. The steeply increase in HC and CO, together with IMEP reduction, observed when reducing oxygen concentration was the result of an important degradation of the combustion process. As expected, the indicated efficiency was worsened due to the use of excessive EGR. Moreover, CO and HC emissions reached unacceptable levels for the lowest values of oxygen concentration. Similar results were observed by Noehre26 and Colban.27 It was suspected that the very high CO and HC levels were mainly caused by liquid fuel wall impingement. For 362
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: DOI:10.1021/ef900832c
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Figure 13. CO and HC vs Tad90 graphs.
evaluating this possibility, the nonreactive fuel spray behavior was modeled for 3 different cases with intake air temperature of 40 °C. Figure 12 shows fuel spray penetration (solid lines) until it impinges against the wall or until the start of combustion angle (dotted-lines). The simulation time was limited to the ignition delay because the model is not able to reproduce reactive conditions. Note that fuel spray penetration is affected by gas density, but not by oxygen concentration. Furthermore, increasing gas density is a very effective strategy for reducing the liquid length, so it is effective against excessive wall impingement. Although wall impingement was evident for oxygen concentration of 9% and gas density of 40 kg/m3, it was much more intense in the case of gas density 26 kg/m3. HC and CO were really very high for this lowest density case, but they were even higher for the oxygen concentration of 9%, which indicates that combustion deterioration is an important source of these two pollutants. Observing the plots included in Figure 13, CO and HC increases considerably for Tad90 lower than 2000 K, corresponding to the same temperature at which soot emissions start to decrease due to the reduction in soot formation (see Figure 7). Thus, in the LTC range, the deterioration of the combustion process caused by an extremely low oxygen concentration affects significantly CO and HC emissions. 4.5. Discussion. This investigation was carried out with the objective of evaluating the feasibility of implementing a mixing-controlled LTC concept in a small HSDI engine, transferring the knowledge from constant-volume vessel studies to an actual production engine. The mixing-controlled LTC with zero NOx and zero soot was achieved for the lowest levels of both oxygen concentration and intake air temperature, together with the highest gas density. On the other hand, this combustion concept was successfully achieved in only one of the operating conditions under investigation, but HC and CO emission levels were extremely high and the engine efficiency was worsened. Some opportunities to improve and/or widen the mixing-controlled LTC concept operating range have been identified: (1) As the engine used was a small HSDI, the quantity of injected fuel per cycle at midload operating condition was small; consequently, injection duration was also relatively short. The engine operational conditions required to generate the mixing-controlled LTC, low air temperature and oxygen concentration, extend also the ignition delay. As a result, the combustion process is shifted from being mixing-controlled to highly premixed conditions. Thus, reducing fuel injection rate and extending the injection duration will be interesting to obtain a stabilized mixing-controlled combustion after the
long ignition delay. (2) Wall impingement was observed in different engine operating conditions caused also by the long ignition delays. According to literature, both liquid and liftoff lengths are reduced through the use of smaller-diameter nozzles. Also, a lower equivalence ratio at the lift-off length cross section is achieved. These combined effects are expected to extend the mixing-controlled LTC concept to a broader range of operational conditions. 5. Conclusions An experimental study on the feasibility of obtaining a mixing-controlled LTC in a HSDI diesel engine has been performed. Initially, an operating engine map was developed for each intake air temperature and the most suitable conditions for producing a mixing-controlled LTC were selected to be investigated. Based on the results obtained on this investigation, some conclusions have been obtained: (1) It is possible to generate sootless and zero-NOx mixing-controlled LTC in a small diesel engine. The engine operating condition requirements corresponds to very low volumetric oxygen concentration of 9%, high gas density of 40 kg/m3, and cooled intake air temperature (air þ EGR) of 40 °C. However, HC and CO emission levels sharply increases and IMEP decreases by around 5%. (2) Reducing oxygen concentration results in adiabatic flame temperatures low enough to avoid NOx formation. The engine-out soot emissions increases with oxygen concentrations in the range between 10 and 11%, and afterward they decreases for lower oxygen concentrations. Also, decreasing the oxygen concentration slows the premixed combustion process down to a level at which the characteristic premixed combustion peak is not observed separately from the mixing-controlled combustion. (3) Reducing gas density increases the ignition delay and thus promotes the change toward a highly premixed combustion. On the contrary, increasing gas density avoids wall impingement and increases combustion efficiency. Also, the low level of oxygen concentration necessary to achieve a mixing-controlled LTC requires high trapped mass to keep lean air-fuel ratios, thus implying high air densities inside the cylinder. Thus, gas density is more than a control parameter for reducing liftoff length with no significant losses in ΦH when the mixingcontrolled LTC concept is implemented in an actual production engine. (4) Sootless conditions have been generated by reducing soot formation. This is attained by simultaneously reducing intake air temperature and oxygen concentration. The longer lift-off length associated to low air temperatures allows more air entrainment in the inert part of the spray and reduces the mean equivalence ratio at the lift-off length cross 363
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section, and this decreases the formation of soot precursors. Both lower intake air temperature and oxygen concentration reduce local temperatures along combustion, which also inhibits intermediate soot formation processes. (5) In this work, fuel injection duration was short due to the high injection rate provided by the injection system configuration. Reducing injection rate allows longer injection durations so the transition toward a highly premixed combustion observed in some operating conditions is avoided. Besides, this action will promote longer mixing-controlled, low temperature combustion processes in those cases where this has been already attained.
HRL = Heat Release Law HSDI = High-Speed Direct Injection IMEP = Indicated Mean Effective Pressure IP = Injection Pressure LL = Liquid Length LTC = Low Temperature Combustion mair = Intake air mass flow rate mEGR = EGR mass flow rate mf = Fuel mass per cycle mint = Intake total air mass flow rate NOx = Nitrogen Oxides O2 = Oxygen pint = Intake air pressure RoHR = Rate of Heat Release SoE = Start of Energizing SoI = Start of Injection Tg = In-cylinder gas temperature Tad90 = Adiabatic Flame Temperature at 90% of Burned Gas TadMax = Maximum Adiabatic Flame Temperature TDC = Top Dead Center tinj = Injection Duration Tint = Intake air temperature U = Injection Velocity XO2_int = Intake gas oxygen concentration XO2_air = Air oxygen concentration XO2 = Oxygen concentration YO2 = Oxygen concentration (mass) Zst-1 = Stoichiometric fuel mixture fraction ΦH = Equivalence ratio in the lift-off length cross-section τig = Autoignition Delay θ = Jet spreading angle Fg = In-cylinder gas density Ff = Fuel density
Acknowledgment. The authors wish to acknowledge the Spanish Ministry of Innovation and Science for the financial support through the project OBTICOMB (reference code: TRA2007-67961-C03-01).
Definitions, Acronyms, Abbreviations A/F = Air fuel ratio ATDC = After Top Dead Center Ca = Nozzle orifice area contraction coefficient CAD = Crankshaft Angle Degree CO = Carbon Monoxide CO2 = Carbon Dioxide CR = Compression ratio DIES = Direct Injection Engine Simulation software dnozzle = Nozzle orifice diameter EGR = Exhaust Gas Recirculation FC = Fuel consumption FSN = Filter Smoke Number H = Lift-off length HC = Hydrocarbons
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