Today, natural gas engines for stationary and vehicular applications are not only faced with stringent emission legislation, but also with increasing requirements for power density and efficient fuel consumption. For vehicular use, downsizing is an advantageous approach to lowering on-road fuel consumption and making gas engines more competitive with their diesel counterparts. In SI-engines, the power density at a given compression ratio is limited by knocking, or NOx emissions. A decrease in compression ratio, lowering both NOx emissions and the risk of knocking combustion, increases fuel consumption. An increase in air-fuel-ratio, required to avoid knocking at higher thermal loading, increases boost pressure, HC and CO emissions, and mechanical loading and causes the danger of misfiring. As a result, the performance of the latest production gas engines for vehicles remains at a BMEP of 18…20 bar with a NOx emission level of 2…5 g/kWh.
The key to increasing power density while keeping low NOx emission levels and high compression ratios is the combustion layout. The well-balanced relation between charge motion (both swirl and turbulence), temperature control via retarded ignition timing, and combustion velocity results in a specific power of BMEP = 30 bar, achieved in a prototype test engine, an increase by 50% compared with the baseline engine. In combination with downsizing, this can be used to reduce fuel consumption to levels thus far known only by diesel engines.