In the recent years, it has become obvious that one of the main fields of interest in alternate fuels is the public transportation sector. Natural Gas seems to be advantageous. It is available and environmentally friendly, even if the greenhouse effect of methane is considered. The operation range of vehicles running on CNG (Compressed Natural Gas) is poor due to the large pressure vessels, but in case of urban buses with low daily mileage this is acceptable. On the other hand, the use of an environmentally friendly fuel is favorable especially in urban areas.
Although there are some advantages of Natural Gas, diesel buses dominate the market. The reason is the better part-load fuel efficiency of the Diesel principle which is superior to the Otto-cycle due to the absence of engine throttling. The efficiency levels of Spark-Ignition (SI) -type, Lean Burn Natural Gas engines are quite comparable to diesel engines during full load conditions. The penalty in on-Road fuel consumption results from the 60…80% share of idle/low load engine operation.
To overcome this, throttling in Natural Gas Engines should be reduced or eliminated. In a supercharged Lean-Burn engine, load in the upper load range can be influenced to a certain degree via waste-gate or VNT control. The low load/idle operation requires a stratified charge instead of the homogenous charge at high loads. One way to do this is a Pilot Injection-type engine, if the injection equipment is optimized to small diesel amounts to replace the spark plug with 1…3% of the energy share at full load and 100% at idle conditions.
To demonstrate the potential of such an engine, FEV has set up a demonstration engine with a Common-Rail Injection System. The 6-cylinder HD engine is supercharged and intercooled. Unlike conventional Dual-Fuel Engines, the emission level in steady-state operation is below Euro V (valid 2008) except methane, comparable to SI engines. On the other hand, unlike conventional SI engines, the fuel consumption is similar to diesel engines.