Particle emissions from compressed natural gas engines

Abstract

This paper presents the results of measurements conducted to determine particle and gas emissions from two large compressed natural gas (CNG) spark ignition (SI) engines. Particle size distributions in the range from 0.01}30 km, and gas composition were measured for five power
settings of the engines: 35, 50, 65, 80 and 100% of full power. Particle emissions in the size range
between 0.5 and 30 km, measured by the aerodynamic particle sizer (APS), were very low at a level
below two particles cm~3. These concentrations were comparable with average ambient concentration,
and were not considered in the succeeding analysis. Both engines produce signifcant amounts
of particles in the size range between 0.015 and 0.7 km, measured by the scanning mobility particle
sizer (SMPS). Maximum number of concentrations of about 1]107 particles cm~3 were very similar
for both engines. The CMDs were in the range between 0.020 and 0.060 km. The observed levels of
particulate emission are in terms of number of the same order as emissions from heavy duty diesel
engines (Morawska et al., Environ. Sci. Tech. 32, 2033}2042). On the other hand, emissions of CO
and NOx of 5.53 and 3.33 g kWh~1, respectively, for one of the tested engines, were considerably
lower than set by the standards. According to the specifications for the gas emissions, provided by
the US EPA (US EPA, 1997), this engine can be considered as a "low-emission" engine, although emissions of submicrometer particles are of the same order as heavy-duty diesel vehicles.