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.