REPLACING DIESEL ENGINES WITH CNG ENGINES – IMPACT ON EMISSIONS, BASED ON CHASSIS DYNAMOMETER TESTS OF MEDIUM AND HEAVY-DUTY BUSES

Abstract This paper compares carbon dioxide, carbon monoxide, methane and nitrogen oxide emissions from medium and heavy-duty buses using diesel, diesel-hybrid, and CNG powertrains. Comparisons are made using results from chassis dynamometer-based tests with driving cycles intended to simulate a wide range of operating conditions. Tail pipe emissions are measured by diluting the vehicle exhaust in a full-scale dilution tunnel by mixing with conditioned air. Samples are drawn through probes of raw exhaust, diluted exhaust and measured using laboratory grade emission analyzers. Emissions measurements include carbon dioxide, carbon monoxide, nitrogen oxides, and hydrocarbons including methane. Fuel consumption for liquid fuels is measured using weighing scales, while a gas flow meter is used for measuring CNG. Experimental data from 20 buses tested on a chassis dynamometer over the last 8 years is analyzed and a comparison of results from similar buses but with the differently fueled powertrains is presented. It is shown that replacing diesel engines with CNG engines does not significantly reduce the emissions of carbon dioxide, while it increases carbon monoxide and methane emissions, but reduces NOx emissions.