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Today gas engines for stationary and vehicular applications are not only faced with stringent emission legislation but also with increasing requirements to power density, while reducing operating and investment cost. The dual fuel engine is very beneficial in terms of power density and efficiency and solves the problem of reduced spark plug life. For smaller engines, however, this concept is economically unattractive due to inevitable SCR (Selective Catalytic Reduction) exhaust gas aftertreatment. The key to low NOx-production is the application of modern injection systems with maximum flexibility concerning the injection parameters. A time-controlled pilot injection system offers the best potential for combining environmental-friendly, cost-saving operation, thus making gas engines even more competitive to their diesel counterparts in many applications.

With regard to increasingly stringent emission legislation natural gas is gaining interest as an alternative fuel. Concerning mobile application natural gas is often considered to produce potentially lower exhaust emissions compared to diesel and gasoline fuel. Nevertheless, also the exhaust gas quality of diesel and gasoline fuelled vehicles will be improved by applying advanced technical solutions. The paper reveals the state-of-the-art in exhaust emission behaviour of diesel, gasoline and natural gas fuelled vehicles. Passenger cars and light-duty trucks will be considered as well as HD-trucks. Emissions include NOx, THC, NMHC, CO, Aldehydes and PAH. In additon CH4 and CO2 emissions are discussed with respect to increasing concern about the greenhouse effect. Special emphasis is laid on the role of mixture formation systems, catalysts and gas composition for natural gas engines.