Particle Characterisation of Modern CNG, Gasoline and Diesel Passenger Cars

Tailpipe particle measurements have been performed in Euro-4 passenger cars. The investigation included compressed natural gas fuelled (bifuel vehicle concepts, commercially available) spark-ignition vehicles, gasoline port-injection spark-ignition vehicles, gasoline direct injection spark-ignition vehicles as well as compression ignited, diesel vehicles with and without particle filters. Particle number and soot mass was measured. In addition particle number size distributions have been recorded at constant vehicle speed operation.

In the current study, the particle emissions of Compressed Natural Gas (CNG) vehicles are significantly lower than the corresponding ones of the other spark-ignition vehicles. While direct injection spark-ignition vehicles exhibited high particle emission, only topped by the diesel vehicles without a particle filter, diesel vehicles with particle filters had lowest particulate emission, provided the diesel particulate filter was not regenerated shortly prior to the measurement cycle. CNG vehicles had almost as low particulate emissions in all cycle parts with low and moderate vehicle speeds. At typical highway driving speeds CNG vehicle particle emissions were significantly higher than the corresponding ones of the diesel particulate filter equipped vehicles. The enrichment of fuel-air mixture was identified as the main reason for the particle emission of the CNG vehicles at high vehicle speeds. We believe this enrichment to be a remnant from the gasoline operation the engine was originally designed for. In gasoline engines enrichment of the mixture reduces process and exhaust gas temperatures mainly due to fuel evaporation during and following the intake, protecting the catalyst from high temperatures. In parallel it improves engine performance and torque due to increased flame velocity. Since CNG-rich mixtures only slightly reduce process or exhaust temperatures and the maximal flame speed is at nearly stoichiometric air-fuel ratios we identify there a relatively easy to capture potential which, if reaped, may lead to the cleanest possible vehicle propulsion system.