Combustion engines with Natural gas or Biogas have the potential of low exhaust emissions and at the same time fulfil the demands of the customers for high power and low energy consumption. Natural gas is used in city buses because it is more environmental friendly than diesel. With natural gas the particle emissions are minimal, but the emissions of NOx may be a trouble.We want with this report to show that there still are many ways for developments of the combustion engine and that Natural gas is an interesting alternative fuel. A possible concept is improvements of the lean burn engine with homogenous charge combustion. Another concept is Direct Injection of gas as in a modern diesel engine.