While designed to reduce emissions and comply with emission limits, the use of biodiesel and other fuels in conjunction with emission controls can change the composition and toxicity of exhaust emissions. Engine performance with biodiesel blends and conventional diesel fuels is generally similar except that brake-specific fuel consumption increases using biodiesel due to its lower energy content. Biodiesel also reduces smoke opacity, PM, CO, and NMHC emissions, although NO_x may increase, depending on operating conditions.

Diesel engine exhaust also includes toxic unregulated pollutants, such as volatile organic compounds (VOCs) like benzene and formaldehyde and polycyclic aromatic hydrocarbons (PAHs) like benzo(a)pyrene. Unregulated pollutants also include ozone precursors and bioaccumulative and toxic compounds. Information pertaining to these emissions in diesel engine exhaust is much less complete than that for the regulated pollutants.

While lower emissions of unregulated pollutants might be expected for biodiesel blends given results observed for PM and NMHC, the literature is inconsistent. The US EPA (2002) has identified 11 toxics in diesel exhaust (acetaldehyde, acrolein, benzene, 1,3-butadiene, ethyl benzene, formaldehyde, n-hexane, naphthalene, styrene, toluene, and xylene) and stated that emissions could “increase or decrease when biodiesel is blended with diesel fuel, and of those ⟨species⟩ that decrease, the magnitude of that decrease will vary from one toxic to another”. Tests of diesel engine emissions of carbonyl compounds, e.g., acetaldehyde and formaldehyde, have shown both increases and decreases. Information pertaining to unregulated pollutants is especially scarce for biodiesel fuels that meet current ULSD requirements, for modern engines with EGR and aftertreatment systems, and for various engine loads.

This study investigates exhaust emissions of both regulated and unregulated pollutants from diesel engines and specifically compares emissions obtained using ULSD and a biodiesel blend. Results are presented for two engines at idle and three load conditions, both with and without aftertreatment systems, and with calibrations similar to the stock settings. A range of pollutants are measured, including several toxic and diesel marker compounds. The information presented can be used for developing emission inventories, apportioning air pollutant sources, and estimating exposures and risks.

—Chin et al.

For the study, they used a GM 2002 1.7 L engine manufactured by Isuzu, which is used in passenger cars in Europe and Asia. The engine is equipped with EGR and a platinum-based DOC. The second engine was a Ford 2008 6.4 L “Power Stroke” engine manufactured by International, used in pick-up trucks, school buses, tow trucks, and other vehicles. This engine also has an EGR system and is further equipped with a DOC and a catalyzed DPF that meets 2007 emissions regulations.

Fuels were a mid-cetane US specification ultralow sulfur diesel (ULSD, sulfur content <15 ppm) certification fuel and a custom-blended B20, which contained 20% soy methyl ester biodiesel (Peter Cremer North America, Cincinnati, OH) and 80% ULSD mixed by volume.

The experiments used 26 test conditions. The 1.7 L engine was operated at conditions similar to its 2002 emission calibration in all tests, which included idle and three load conditions expressed in brake mean effective pressure (BMEP): 200 kPa BMEP at 1500 rpm, 600 kPa BMEP at 1500 rpm, and 900 kPa BMEP at 2500 rpm, each with and without the DOC, and each with ULSD and B20 fuels. The fuel injection strategy for this engine used an advanced single injection.

The 6.4 L engine was operated under both 2004 and 2007 calibrations. For the 2004 calibration, the engine was operated under idle and two load conditions (600 kPa BMEP at 1500 rpm and 900 kPa BMEP at 2500 rpm), without the DOC and catalyzed DPF, each with the two fuels. For the 2007 calibration, the engine was operated under idle and one load condition (900 kPa BMEP at 2500 rpm) with the DOC and catalyzed DPF and with the two fuels. Engine speed and BMEP were the only parameters adjusted.

Among the findings:

• For the 1.7 L engine under load without a DOC, B20 reduced brake-specific emissions of particulate matter (PM), elemental carbon (EC), non-methane hydrocarbons (NMHCs), and most volatile organic compounds (VOCs) compared to ULSD; however, formaldehyde brake-specific emissions increased. With a DOC and high load, B20 increased brake-specific emissions of NMHC, nitrogen oxides (NO_x), formaldehyde, naphthalene, and several other VOCs.

• For the 6.4 L engine under load, B20 reduced brake-specific emissions of PM_2.5, EC, formaldehyde, and most VOCs; however, NO^x brake-specific emissions increased.

• When idling, the effects of fuel type were different: B20 increased NMHC, PM_2.5, EC, formaldehyde, benzene, and other VOC emission rates from both engines, and changes were sometimes large, e.g., PM_2.5 increased by 60% for the 6.4 L/2004 calibration engine, and benzene by 40% for the 1.7 L engine with the DOC, possibly reflecting incomplete combustion and unburned fuel.

• Diesel exhaust emissions depended on the fuel type and engine load (idle versus loaded).

• Comparing the two engines on the basis of brake mean effective pressure (BMEP), the 6.4 L engine had lower brake-specific emissions of all pollutants except NO_x.

The higher emissions found when using B20 are especially important given the recent attention to exposures from idling vehicles and the health significance of PM_2.5, the authors noted.

The study measurements show that the new engine calibrations and aftertreatment systems significantly reduce emissions of most pollutants. However, emission reductions under load do not necessarily portray trends while idling if the DOC does not attain its operating temperature. Although B20 decreased emissions of most pollutants, emissions of unregulated pollutants, including formaldehyde and benzene, increased with the 1.7 L DOC-equipped engine. Ideally, engine and control systems would reduce both regulated and unregulated emissions during both idle and load conditions and with both regulated and biodiesel fuels.

While unregulated pollutants comprise a small fraction of the total emissions, pollutants such as benzene, formaldehyde, and PAHs are important due to their toxicity. These emissions strongly depend on fuel and exhaust gas treatments and perhaps more so than the regulated pollutants. Future studies might be designed to evaluate the origin of these emissions.

This work demonstrates how emissions of regulated and unregulated pollutants in diesel exhaust depend on fuel, load, engine calibration, and exhaust aftertreatment technology. In addition to providing new information on emissions during idling and using B20, the results include a number of toxic species that can be used in emission inventories and as emission profiles in receptor models that are used to apportion air pollutant sources.

—Chin et al.

Resources

• Jo-Yu Chin, Stuart A. Batterman, William F. Northrop, Stanislav V. Bohac, and Dennis N. Assanis (2012) Gaseous and Particulate Emissions from Diesel Engines at Idle and under Load: Comparison of Biodiesel Blend and Ultralow Sulfur Diesel Fuels. Energy & Fuels doi: 10.1021/ef300421h