A Comparison of Emissions from Vehicles Fueled with Diesel or Compressed Natural Gas

Abstract

A comprehensive comparison of emissions from vehicles fueled with diesel or compressed natural gas (CNG) was developed from 25 reports on transit buses, school buses, refuse trucks, and passenger cars. Emissions for most compounds were highest for untreated exhaust emissions and lowest for treated exhaust CNG buses without after-treatment had the highest emissions of carbon monoxide, hydrocarbons, nonmethane hydrocarbons (NMHC), volatile organic compounds (VOCs; e.g., benzene, butadiene, ethylene, etc.), and carbonyl compounds (e.g., formaldehyde, acetaldehyde, acrolein). Diesel buses without after-treatment had the highest emissions of particulate matter and polycyclic aromatic hydrocarbons (PAHs). Exhaust after-treatments reduced most emissions to similar levels in diesel and CNG buses. Nitrogen oxides (NO(x)) and carbon dioxide (CO2) emissions were similar for most vehicle types, fuels, and exhaust after-treatments with some exceptions. Diesel school buses had higher CO2 emissions than the CNG bus. CNG transit buses and passenger cars equipped with three-way catalysts had lower NO(x) emissions. Diesel buses equipped with traps had higher nitrogen dioxide emissions. Fuel economy was best in the diesel buses not equipped with exhaust after-treatment.

Particulate Matter in New Technology Diesel Exhaust (NTDE) is Quantitatively and Qualitatively Very Different from that Found in Traditional Diesel Exhaust (TDE)

“…Hesterberg et al 1 previously reported the highest emissions of total 2-, 3-, and !4-ring PAH compounds in diesel transit and school buses lacking aftertreatment. With use of aftertreatment, Hesterberg et al 1 reported that PAH emissions for transit buses were significantly reduced (on average 60-97% reductions, depending on the aftertreatment type and the number of PAH rings) and of a similar magnitude, and sometimes lower, than buses operating with compressed natural gas (CNG) fuel with and without aftertreatment. Several recent studies have characterized emissions of a suite of individual PAH species in NTDE, including those suspected to be carcinogenic.…”

“…1,14,17,19,25,26 For example, for the recent CARB-USC testing of four heavy-duty and medium-duty diesel vehicles operated using six different aftertreatment configurations (Table 1) and multiple test cycles, Biswas et al 17 reported consistent PM mass reductions of >90%, observing in all cases PM emission rates of less than 0.01 g/mile (Figure 2). Given the role of chemical composition as a key factor affecting DEP toxicity, recent studies have further investigated how emissions of specific DEP constituents are impacted by the use of different aftertreatment technologies.…”

“…1,11,12 We documented substantial reductions in emissions of a variety of regulated and unregulated species (e.g., PM, CO, nonmethane hydrocarbons, formaldehyde, benzene, acetaldehyde, and PAHs) in NTDE versus either TDE or "transitional" DE in a prior review of emissions data from transit buses, school buses, refuse trucks, and passenger cars. 1 Since the 2008 publication of this review, results from numerous characterizations of NTDE emissions have been published, including Phase 1 findings from the Advanced Collaborative Emissions Study (ACES), 13,14 a series of findings from a collaborative study of the California Air Resources Board (CARB) and the University of Southern California (USC), [15][16][17][18][19][20][21] and other detailed analyses. [22][23][24][25][26][27][28][29] Importantly, these new studies have substantially improved our understanding of how the DEP in NTDE differs from the DEP in TDE in terms of chemical and physical properties that can affect toxicity.…”

“…These advances resulted in the emergence of what has been termed "New Technology Diesel Exhaust", or NTDE. 1,7,8 As discussed further in the next section, NTDE has been defined as emissions from post-2006 diesel engines and earlier-model diesel vehicles retrofitted with aftertreatment devices. It is a product of the innovative development of integrated, multicomponent emissions reduction systems (engines, fuel injection systems, ultra-low-sulfur fuels, lubricants, and exhaust aftertreatment devices) to meet the tightened U.S. EPA emissions standards.…”

Determination of Urine 3-HPMA, a Stable Acrolein Metabolite in a Rat Model of Spinal Cord Injury

“…Taken together, measurement of urine 3-HPMA may represent a novel theranostic approach for SCI. Since acrolein may potentially be linked to many other conditions, such as cancer, 2,[25][26][27][28] aging, 29 pollution, 30,31 and smoking, 32,33 the benefits of this study has the potential to significantly enhance human health.…”