Critical Analysis of High Particle Number Emissions from Accelerating Compressed Natural Gas Buses

Abstract

Compressed natural gas (CNG) engines are thought to be less harmful to the environment than conventional diesel engines, especially in terms of particle emissions. Although, this is true with respect to particulate matter (PM) emissions, results of particle number (PN) emission comparisons have been inconclusive. In this study, results of on-road and dynamometer studies of buses were used to derive several important conclusions. We show that, although PN emissions from CNG buses are significantly lower than from diesel buses at low engine power, they become comparable at high power. For diesel buses, PN emissions are not significantly different between acceleration and operation at steady maximum power.However, the corresponding PN emissions from CNG buses when accelerating are an order of magnitude greater than when operating at steady maximum power. During acceleration under heavy load, PN emissions from CNG buses are an order of magnitude higher than from diesel buses. The particles emitted from CNG buses are too small to contribute to PM 10 emissions or contribute to a reduction of visibility, and may consist of semivolatile nanoparticles.

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

“…As discussed earlier, findings from a number of studies indicate that nanoparticle emissions (and total particle number emissions) are now comparable, if not lower, for DPF-equipped diesels versus CNG and GEE vehicles. [63][64][65]68,[76][77][78] Finally, Figure 9 compares the ratio of elemental carbon (EC) to total carbon (TC; where TC ¼ EC þ organic carbon [OC]) for different engine exhaust types, providing further evidence that NTDE particulate is more comparable to engine exhaust particulate from CNG-and gasoline-fueled engines than to TDE particulate. 63,103,104 As shown in Figure 9, EC/TC ratios for NTDE are substantially lower than those for both TDE and gasoline-engine particulate for both transient and steady-state test cycles, bearing the greatest resemblance to those for CNG particulate.…”

“…For additional perspective on the health significance of DEP nanoparticles in NTDE, it is also important to note that the presence of a nucleation mode is not unique to NTDE, as studies have reported similar and sometimes greater particle number emissions in gasoline engine exhaust (GEE) and the exhaust of CNG engines than in NTDE. [63][64][65]68,[76][77][78] PRELIMINARY TOXICOLOGICAL DATA FOR NTDE Although there has been a dramatic increase in the number of emissions characterization studies for NTDE, there still remain relatively few toxicological data for NTDE. The need for a large-scale toxicological evaluation of NTDE has been well recognized for a number of years, serving as a motivating factor for the design of the comprehensive health effects components of the ongoing Advanced Collaborative Emissions Study (ACES).…”

Comparison of primary and secondary particle formation from natural gas engine exhaust and of their volatility characteristics

“…Natural gas engines emit little primary particle mass and less CO 2 than engines fueled with conventional fuels (Anderson et al, 2015;Bielaczyc et al, 2014) but their particle number emission can be significant Jayaratne et al, 2010). In addition, the size of the majority of the particles emitted by natural gas engines can be below the detection limits of traditional exhaust particle measurement devices (Alanen et al, 2015).…”

Evolution of Soot Particle Number, Mass and Size Distribution along the Exhaust Line of a Heavy-Duty Engine Fueled with Compressed Natural Gas

“…The absence of large hydrocarbon chains and aromatics [31][32][33] makes natural gas a fuel cleaner than gasoline or diesel and able to guarantee lower gaseous emissions [32,34,35]. However, despite particle mass (PM) formation is small, the number concentrations of particles emitted by compressed natural gas (CNG) engines are not negligible when compared to those of Diesel engines [31,36,37]. Moreover, it has been pointed out that particles emitted from natural gas engine falls mainly within the ultrafine size range [37][38][39][40][41], highlighting a harmful potential that requires additional research [42,43].…”