Effect of Several Oxygenates on Regulated Emissions from Heavy

Environ. Sci. Technol. 1997, 31, 1144-1150

Effect of Several Oxygenates on Regulated Emissions from Heavy-Duty Diesel Engines ROBERT L. MCCORMICK,* JEFFREY D. ROSS, AND MICHAEL S. GRABOSKI Colorado Institute for Fuels and High Altitude Engine Research and Department of Chemical Engineering and Petroleum Refining, Colorado School of Mines, Golden, Colorado 80401-1887

Oxygenates produce a significant reduction in emissions of particulate matter (PM-10) from diesel engines but in most cases also cause the nitrogen oxide emissions to increase. In this work, several oxygenates having a wide range of properties were blended with no. 2 diesel at the 1 and 2 wt % oxygen level. Emissions were measured using the hot start portion of the U.S. Heavy-Duty Transient Test (40 CFR, Part 86, Subpart N) in both a 2-stroke and a 4-stroke engine. It was found that at this oxygen level PM reductions on the order of 10-15% were obtained regardless of oxygenate chemical structure. The oxygenates affected the integrated NOx emissions differently. Methyl esters of soybean oil increased NOx 2-3%, decanoic acid had no effect, and octanol may have slightly decreased NOx. Examination of real time NOx concentration data for octanol and soy esters indicates that both oxygenates increase NOx during portions of the cycle where the engine is generating high torque at low speed with little or no turbo boost. For octanol, there is a compensating reduction in NOx at high speed and load. Several hypotheses regarding the effect of oxygenates on diesel NOx emissions are discussed.

Introduction The purpose of this study was to investigate the effect of several oxygenated fuel additives, having a range of boiling points and other properties, on emissions from diesel engines. To this end, oxygenates were blended with no. 2 diesel fuel at 1 and 2 wt % oxygen, and the effect on emissions was measured by the hot start portion of the heavy-duty transient test (40 CFR, Part 86, Subpart N). Testing was performed in both 2-stroke and 4-stroke diesel engines. The hardware is typical of current on-road engine technology and has been extensively used for fuel emission studies (1). Oxygenated fuels have a history of reducing exhaust emissions from motor vehicles. Additions of methyl tertbutyl ether (MTBE) and ethanol have been successful in reducing CO and non-evaporative hydrocarbon emissions from gasoline engines (2). Oxygenates are now mandated year-round as part of Federal reformulated gasoline in the nine serious and extreme ozone non-attainment areas and during winter months only in 39 CO non-attainment areas. The success of oxygenated gasoline has sparked interest in the use of oxygenated compounds as particulate matter (PM) emissions reducing additives in diesel fuel. Bennethum and Winsor (3) examined the oxygenated compound diglyme * Corresponding author telephone: (303) 273-3967; fax: (303) 2733730; e-mail: [email protected].

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(diethylene glycol dimethyl ether) in a 2-stroke engine. Heavyduty transient test results with the treated fuel showed a 6% CO and PM reduction at 0.5 vol % diglyme. At 5% diglyme, a 13% CO reduction and a 20% PM reduction were observed while HC and NOx remained unchanged. Winsor (4) reported further work in which dimethyl carbonate was added to diesel at 3.5 wt % oxygen. PM and CO reductions were approximately 15%. NOx emissions showed a 1.8% increase while HC emissions were unchanged. The neat methyl esters of rapeseed oil and soybean oil were also tested. HC, CO, and PM were decreased approximately 75%, 50%, and 30%, respectively, for both methyl esters. NOx emissions increased by approximately 18%. Graboski and co-workers (5) have recently measured emissions from 100% methyl esters of soy bean oil and blends of this material with no. 2 diesel in a 4-stroke engine. Substantial particulate emissions reductions were observed at all oxygenate levels. They observed a statistically significant 1% increase in NOx at 2 wt % oxygen in the fuel and higher NOx increases at higher oxygen levels. Liotta and Montalvo (6) investigated the emissions effects of three glycol ethers, an aromatic alcohol, an aliphatic alcohol, and a polyether polyol using a 4-stroke engine. The actual structures of these compounds were not revealed. Methyl soy ester and diglyme were also included for comparison to previous results. Based on heavy-duty transient testing, CO was generally reduced and NOx showed an increase with all oxygenates studied. The PM reduction experienced appeared to be related to the amount of oxygen in the fuel. Unregulated emissions of aldehydes and ketones were reported to decrease. Nikanjam (7) examined ethylene glycol monobutyl ether acetate as a diesel additive based on cost, fuel blending properties, and toxicity concerns. Emissions results from a 4-stroke engine showed CO and PM reductions of approximately 18% and a NOx increase of 3% for 3 wt % oxygen in the fuel. More recently, Ullman and co-workers (8) reported the effect of oxygenates and other fuel properties on emissions from a 1994 Model Detroit Diesel Series 60 4-stroke engine. Monoglyme and diglyme were used as oxygenates at the 2% and 4% oxygen levels. The engine was run with 5 and 4 g/bhp‚h NOx calibrations (1994 and 1998 emissions standards, respectively). Statistical models of the emissions dependency on fuel composition were developed with weight percent oxygen as the oxygenate variable. At 2% oxygen and constant aromatic content and cetane number, the model predicts a 1.5% increase in NOx for the 5-gram calibration and a 0.9% increase for the 4-gram calibration. FEV Engine Technology (9) investigated various soy ester blends with diesel in comparison to diesel using a 13-mode steady-state test with the Navistar 7.3 L engine. FEV found that NOx increased with biodiesel under all conditions of speed and load. Soy ester blends reduced particulate at high speed and all loads. At lower speeds, particulate was reduced at light and heavy loads but was increased at intermediate loads. McDonald and co-workers (10) examined emissions from blends of diesel and soy bean oil methyl esters for off-road mining applications using the ISO 8178-C1 8 mode offhighway test. Mutagenicity with soy ester and a diesel oxidation catalyst was reduced to half that of conventional diesel. The reduction in mutagenicity is due to a reduction in PAH. Formaldehyde emissions were not significantly increased using soy ester blends (with the diesel oxidation catalyst). Rantanen and Mikkonen (11) report similar results. These studies, combined with results on aldehyde and ketone emissions reported by Liotta and Montalvo (6), suggest that the use of oxygenates in diesel does not lead to an increase in emissions of reactive aldehyde and similar species. A number of more recent studies, however, indicate substantial

S0013-936X(96)00643-8 CCC: $14.00

 1997 American Chemical Society

increases in engine out aldehyde emissions (12). Clearly, aldehyde emissions are a major concern for diesel oxygenates and should be measured in all future studies. The studies cited above clearly indicate that substantial reduction in particulate emissions can be obtained through the addition of oxygenates to diesel fuel. Because diesel particulate is essentially all smaller than 1 µm (13) and some studies relate ambient fine particulate levels to adverse health effects (14-17), the addition of oxygenates to diesel fuel could have significant public health benefits. However, many oxygenates also appear to increase emissions of NOx. Nitrogen oxides can be a limiting reactant for ozone formation in some areas (18) and, in other areas, are an important source of fine secondary particulate (19). Therefore, fuels producing an increase in emissions of NOx will be unacceptable under the to be-proposed Substantially Similar Rule for diesel fuels (20). Many of the oxygenates tested to date, especially vegetable oil methyl esters, have a number of properties not typical of diesel fuels such as higher boiling point, viscosity, and surface tension that may contribute to the increased NOx emissions. In the current investigation, we tested several oxygenates with a range of properties as part of an effort to discover oxygenates or properties of oxygenates that produce a particulate reduction without increasing NOx. The testing was performed in Denver, CO, at an altitude of 1609 m (5280 ft) above sea level. Previous work at this laboratory (21) as well as high-altitude simulation studies reported by Southwest Research Institute (22, 23) indicate that heavy-duty transient PM emissions can be much higher at altitude than observed at sea level. For turbo-charged engines, the PM increase noted in all three of the cited studies was in the range of 50-100%. CO emissions are also observed to increase. However, altitude has not been observed to effect NOx emissions from turbocharged engines. Also, work in this laboratory has found that the effect of diesel fuel cetane number and aromatic content on NOx and PM emissions is proportionally the same at altitude as at sea level (21) although, as noted, the level of PM emissions is much higher. Given this effect of altitude on PM emissions, the PM reductions reported in this paper for oxygenated diesel fuels may not be in quantitative agreement with similar observations made at sea level. It is expected that the oxygenate effect on NOx emissions will be very similar to what might be observed at lower elevation.

Experimental Section Emissions testing was performed via the hot start portion of the heavy-duty transient test (CFR 40, Part 86, Subpart N) on two engines: a Detroit Diesel 6V92 and a Detroit Diesel Series 60. The 6V92 is widely used in urban bus applications while the Series 60 is used in heavy-duty diesel trucks. The following sections describe the engine test cell and procedures. Hot tests were performed comparing no. 2 diesel with the oxygenate blends. We have found the hot transient test to be a reasonable screening test for the performance of fuels and fuel additives in the complete cold plus hot start EPA protocol, and only hot start results are reported here. A detailed description of our laboratory and testing procedures is available (5), and they are only briefly described. Engine Descriptions. The Detroit Diesel 6V92 is a 1989 calibration, six cylinder, 9.0-L 2-stroke, electronically controlled (DDEC-II), direct injected, turbo-charged, and intercooled engine. The engine is nominally rated at 275 bhp at 2200 rpm. Engine specifications and wide open throttle (WOT) settings are shown in Table 1. The Detroit Diesel Series 60 engine is a 1989 calibration, six cylinder, 4-stroke engine, nominally rated at 345 bhp at 1800 rpm, electronically controlled (DDEC-II), direct injected, turbo-charged, and intercooled. Engine specifications and WOT settings are also listed in Table 1. Both engines are required to meet the 1988

TABLE 1. Engine Manufacturer’s Specifications DDC 6V92 model no. 6VF-183607-807-3B21 displacement 9.0 L rated speed/horsepower 2200 rpm/277 bhp max torque speed/torque 1200 rpm/833 ft‚lb idle speed/torque 600 rpm/130 ft‚lb intake depression -16.1 ( 1 in H2O backpressure 36.5 ( 3 in H2O DDC Series 60 model no. 6R-544 displacement 11.1 L rated speed/horsepower 1940 rpm/345 bhp max torque speed/torque 1800 rpm/1335 ft‚lb idle speed/torque 600 rpm/0 ft‚lb intake depression -16.0 ( 1 in H2O backpressure 32.6 ( 3 in H2O

heavy-duty engine emissions standards of 0.6 g/bhp‚h for PM and 6.0 g/bhp‚h for NOx (1 g/bhp‚h ) 0.1035 g/kW‚h). Engine Testing. The test matrix for each engine was carried out against the torque map for the non-oxygenated diesel reference fuel. The map is used to generate the transient cycle commands. By using a single map, all measurements were made against an identical test cycle. Because of the greater energy density of this fuel relative to the oxygenates, the engine is capable of generating both the greatest torque and greatest horsepower on the reference diesel at wide open throttle. Running other fuels on the diesel map forces them to perform from a load perspective as close to diesel as possible. The engines were mapped according to parameters provided by the manufacturer. Prior to running each fuel, the engine was fully warmed and the map conditions were checked to ensure that the engine operated properly on the fuel blend. No adjustments were made in the mapping settings even though the reduced power generated on the oxygenated additive containing fuels results in lower pressures and temperatures. For real-world application of oxygenated diesel fuels, operators would be prohibited from making adjustments to the engine as this would constitute tampering with the emissions control system. For each fuel or oxygenate blend, typically four replicate hot transients were performed. First, several tests with the base diesel fuel were run to establish an emissions base line. This was followed by several replicate tests for each oxygenate blend. The order of oxygenate testing was random. For the DDC Series 60, a final series of tests on the base diesel fuel was performed to quantify engine drift. Fuel Analysis and Blending. The no. 2 diesel fuel employed in these tests was obtained locally, and analysis is reported in Table 2. Oxygenates were selected to span the boiling range of diesel. Furthermore, octanol and methyl soy ester were selected because equal volumes of these oxygenates are required to obtain the same fuel oxygen level, but molecular weight, boiling point, and other properties are quite different. It was assumed that for PM reduction the chemical form of the oxygen was a much less significant factor than the amount of oxygen blended in the fuel. The oxygenates selected were ethanol, 1-octanol, decanoic (also called capric) acid, and soybean oil methyl ester. Ethanol, 1-octanol, and decanoic acid were obtained from Aldrich in 99.9%, 99%, and 96% purities, respectively. Methyl soy ester was manufactured by Procter and Gamble and obtained from Interchem. The material contained greater than 97% ester. Relevant properties of these oxygenates are listed in Table 3, along with typical no. 2 diesel properties for comparison. These materials cover the boiling range of diesel and have a wide range of cetane numbers. Viscosity and surface tension are also somewhat different than is typical of diesel.

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TABLE 2. Diesel Fuel Composition test elemental analysis, wt % carbon hydrogen oxygen nitrogen sulfur API gravity at 16 °C specific gravity FIA, wt % aromatics olefins saturates distillation, °C at 1 atm 10% 50% 90% cetane index

ASTM No. 86.4 13.3 0.11 0.02 0.029 36.3 0.843

D3178-89 D3178-89 by difference D3179-89 D2622-92 D1298-85 D1319-89

33.1 1.6 65.3 D86-90 213 256 317 47.3

D4737-90

Oxygenates were blended gravimetrically at the 1 and 2 wt % oxygen levels. Octanol, decanoic acid, and methyl soy ester were tested in the 6V92 engine at the 1 wt % level. Ethanol, octanol, and methyl soy ester were tested in the Series 60 engine at the 2 wt % level. Properties of these blends and the volume percent blended are listed in Table 4. For methyl soy ester, it has been shown that the neat cetane number can be used as the blending cetane number (5, 24). This means that the volume average cetane number of the blend components will very closely approximate the actual cetane number. This has also been assumed for the other oxygenates to calculate the cetane numbers in Table 4 (and using the cetane index for the no. 2 diesel). Other estimated blend properties are reviewed in the Discussion. Emissions Measurements. Dilution tunnel flow rate is measured with a critical flow venturi (CFV). The inlet temperature and pressure of the CFV are constantly monitored so that the instantaneous flow rate of diluted exhaust is known. The mass emissions are determined by integration of the instantaneous mass rates. Diluted exhaust is sampled from the tunnel at several locations were the exhaust is well mixed. NOx, CO, and CO2 are sampled through a line heated to well above the diluted exhaust dew point while hydrocarbon is sampled through a second line heated to 190 °C. Particulate is sampled through a secondary tunnel that ensures a filtered gas temperature below 52 °C. The gas emissions bench performs both continuous analysis and sampling to a bag for batch analysis at the end of the test. The bench contains a heated FID for THC, chemiluminescence NOx meter, and diluted exhaust infrared meters for CO and CO2. The bag sampler is equipped with four bags and two sample pumps. During a test, two bags are filled. One is a constant rate sampled background bag taken from the engine and dilution tunnel supply air. The second is a proportional sample collected at the gas sampling point through an independent sample line. Proportional sampling is managed through a critical flow orifice and bellows type vacuum sampling pump. Water is removed after the pump with a coalescing filter. In a given test sequence, the analyzer zero and span readings are checked prior to and after each test in accordance with the CFR. Real time emission data are time shifted for sampling time lag, and flow was compensated and integrated using real time dilution tunnel exhaust flow data. The background bag emissions are converted to mass using the integrated tunnel flow, and the net emissions are calculated. Finally, these total emissions are corrected using the indicated zero and span measurements and the true span gas concentrations. For particulate sampling, two independent mass flow controllers are used to regulate the total filtered gas sample

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and the secondary air rate. The computer determines the total sample volume by integrating the instantaneous flow difference. Flow is made proportional by sending a varying secondary air flow set point from the test manager computer, which is based upon the CFV flow rate. The net sample flow accuracy is calibrated to (2% or better. Particulate is collected on 70-mm filters. For each test, approximately 20 SCF of exhaust gas was passed over the filters. A parallel background sample was not collected. Instead the intake air is filtered to 95% ASHRAE efficiency, and periodic background checks are made. The mass collected in the background check made during this program was below the sensitivity of the instruments. The Code of Federal Regulations requires that the NOx emission from diesel and alternative fueled engines be corrected for humidity deviations from the EPA reference level of 75 grains/lb of dry air. In the work reported here, humidity was controlled within 4% of this level. Fuel Consumption and Carbon Balance. The fuel consumption was measured as the average of two direct measurements made using the difference from a weigh cell and totalized mass determined from a Micromotion mass flow meter. The weigh cell accuracy is known to be better than 0.5% of the fuel weight difference for a given transient test. The Micromotion meter is accurate to 0.35% of mass flow except at idle where errors of 4% are possible. From these two measurements, the maximum expected fuel error is thus under 1%. For each transient test, a carbon balance is performed using as inputs the fuel analytical data and the measured fuel consumption and using as outputs the emissions for CO2, CO, THC, and PM. PM is assumed to be 100% carbon, and total hydrocarbon is assumed to have the same H/C ratio as the fuel. PM is almost certainly not all carbon, but this should have no measurable affect on the carbon balance. The carbon balance closure is generally within (2%. This substantiates the fuel consumption estimates and demonstrates that no substantial systematic errors exist in the emission measurements.

Results Results of emissions testing in the 6V92, 2-stroke engine are shown in Table 5. Oxygenates were tested at 1 wt % oxygen in the fuel. Percentage change in the measured emissions versus the base no. 2 diesel are shown in Figure 1 and summarized in Table 7. Reported error bars are based on propagation of one standard deviation in the measured emission, from both the diesel and oxygenate blend runs, through the percent change calculation. All oxygenates tested produced a significant PM reduction in the range of 12-17%. Methyl soy ester produced a more than 2% increase in NOx emissions, significant at the 95% confidence level (utilizing a pooled variance for all runs reported in Table 5). The lower molecular weight oxygenates (decanoic acid and octanol) did not significantly change NOx. Hydrocarbon emissions were increased 4% for octanol, a statistically significant increase. Emissions of CO were significantly reduced for methyl soy ester and octanol. Series 60, 4-stroke engine emissions testing results are shown in Table 6. Base diesel runs performed at the beginning and end of the campaign indicate that drift in NOx emissions was very small (less than 2%). PM drift was higher at around 15%. Oxygenates were tested at 2 wt % oxygen in the fuel. Percentage change in measured emissions versus the base no. 2 diesel are shown in Figure 2 and summarized in Table 7. Ethanol generally produced very poor engine operation. The engine would crank for several seconds before firing whereas for all other fuels firing was immediate. Hydrocarbon emissions also increased dramatically for ethanol. Thus, the apparent large decreases in CO and PM emissions for the ethanol blend are not meaningful in that the engine could not operate normally on this fuel. Methyl soy ester and

TABLE 3. Properties of Oxygenates Tested oxygenate

boiling point, °C

no. 2 diesel ethanol 1-octanol decanoic acid methyl soy ester a

Ref 33.

b

cetane number

135-345 78 196 269 332

40-50 5-15 39 ∼45 51

density, g/mL at 20 °C

viscosity, cs at 40 °C

surface tension, dyn/cm at 20 °C

0.84 0.789 0.826 0.893 0.885

3.5a

22.5a

1.2b 10.6b

21.9b 27.5b

4.34c

34.9c

LHV, BTU/lb 19 000 11 550 16 180 14 350 16 380

Ref 34. c Ref 35.

TABLE 4. Estimated Properties of No. 2 Diesel/Oxygenate Blends oxygenate

oxygen, wt %

vol % oxygenate

cetane no.

aromatic, wt %

density, g/mL at 20 °C

viscosity, cs at 40 °C

surface tension, dyn/cm at 20 °C

ethanol octanol octanol decanoic acid methyl soy ester methyl soy ester

2 1 2 1 1 2

6.5 8.5 16.6 5.2 8.9 17.7

44.9 46.8 45.9 47.2 47.6 48.0

31.2 30.4 27.7 33.3 30.1 27.0

0.837 0.839 0.838 0.843 0.844 0.848

2.9 4.0 4.6

22.4 23.1 23.7

3.6 3.6

23.4 24.1

TABLE 5. DDC 6V92 Emissions Testing Results (g/bhp‚h)a fuel

THC

CO

NOx

PM

no. 2 diesel no. 2 diesel av 1-octanol 1-octanol 1-octanol 1-octanol av decanoic acid decanoic acid decanoic acid decanoic acid av methyl soyester methyl soyester methyl soyester methyl soyester av

0.528 0.525 0.526 0.545 0.549 0.551 0.547 0.548 0.494 0.533 0.523 0.528 0.520 0.519 0.535 0.535 0.531 0.530

2.483 2.515 2.499 2.457 2.435 2.400 2.422 2.429 2.341 2.463 2.490 2.639 2.483 2.302 2.314 2.326 2.362 2.326

4.85 4.86 4.855 4.796 4.851 4.797 4.757 4.800 4.893 4.864 4.869 4.863 4.877 5.008 4.980 4.961 4.915 4.966

0.334 0.3414 0.338 0.279 0.270 0.287 0.283 0.280 0.274 0.298 0.303 0.305 0.295 0.273 0.287 0.287 0.295 0.286

a

Oxygenate level at 1 wt % oxygen in the fuel.

octanol produced 20 and 12% reductions in PM, respectively. As observed for the 6V92 engine, methyl soy ester produced a 2-3% increase in NOx while octanol yielded a nearly 3% NOx reduction (both observations significant at the 95% confidence level). Hydrocarbon emissions decreased 10% for methyl soy ester and increased nearly 25% for octanol but are well below current standards in both cases. CO emissions were also decreased by both oxygenates. During the transient tests, gaseous emissions data were acquired continuously and nitrogen oxide concentration data with a time resolution of 1 Hz were used to generate the plots shown in Figure 3. The data in Figure 3 were obtained from the DDC Series 60 engine. The ratio of humidity-corrected NOx concentration with an oxygenate (octanol or soy ester) to the NOx concentration using no. 2 diesel is plotted as a function of torque and speed. This ratio is equal to 1 if there is no change in NOx with the addition of the oxygenate. For the data reported here, the ratio must be regarded as qualitative given differences in instantaneous engine power output and small differences in the NOx analyzer calibration between the various runs which have not been accounted for. Nevertheless, the data clearly show that for oxygenated diesel the NOx emission is not changed uniformly at all speeds and loads. For soy ester, the greatest NOx increase is found

FIGURE 1. Percent change in emissions for oxygenates tested in the DDC 6V92; 1 wt % fuel oxygen. under lugging conditions (