Repowering

Repowering can either involve converting the existing diesel engine to spark-ignition or replacing the engine entirely with a new engine. Westport Power is well known for CNG and LNG engines and they have both spark-ignition and diesel-CNG/LNG dual fuel engines. Other manufacturers make natural gas engines as well. This route is quite expensive, especially if engine replacement is not required. Converting a diesel engine to spark ignition involves the following:

• Replacement of the diesel injectors with spark plugs,
• Replacement of the diesel fuel tank with an LNG fuel tank(s)
• Addition of an ignition system
• Addition of a throttle,
• Addition of a natural gas fuel system,
• Addition of emission controls,
• Possible modifications to the intake manifold,
• Lowering of the compression ratio.

Diesel-Dual Fuel

Aftermarket diesel-LNG dual fuel conversions are generally much less costly than repowering. The LNG fuel system typically operates independently of the diesel fuel system and the engine reverts back to 100% diesel operation when the LNG system is shut down either from running out of LNG or from manual override from the driver. The better LNG fuel systems supply fuel to the engine with a load-based map (ie, pressure vs RPM) as well as throttle (accelerator pedal) position. The advantage with a diesel dual fuel is that if the vehicle runs out of fuel, the vehicle will run exactly as before on diesel fuel, with the exception that the maximum range may be reduced if an LNG tank was substituted for a diesel tank. Common diesel-LNG dual fuel systems available for heavy duty trucks are:

• Technocarb - EcoDiesel System
• American Power Group - Dual Fuel
• Bex America - "Smart" Blended Dual Fuel System
• Engine Control Technology - 2-Fuel

Light Duty Vehicles

As mentioned earlier, LNG conversions are only done commercially on heavy duty vehicles, like Class VIII trucks. Technically, there is no reason that smaller vehicles (such as delivery trucks, taxis, commuter cars, etc) could not be converted to LNG instead of CNG. The cost of the fuel tank is the main reason that it is not feasible because light duty vehicles do not use enough fuel to pay back the cost of the LNG conversion with current LNG tank prices. With economies of scale, the cost of LNG tanks should come down. A second reason is that LNG tanks are not available in sizes that would fit easily into a light duty vehicle. The exception being pickup trucks as a 20" tank can easily be mounted in the bed. However, if public LNG stations become commonplace, LNG tank manufacturers may consider LNG tanks better suited to a wider range of vehicle types.