With the forthcoming passage of the NAT GAS Act (HR 1380) in the USA, LNG conversions are expected to increase within the next few years. This bill is essentially the implementation of part of the Pickens Plan and T Boone Pickens expects that this Plan can eliminate the consumption of OPEC oil in the USA with the substitution of natural gas for diesel fuel in heavy duty trucks.
LNG (Liquefied Natural Gas) conversions are similar in many ways to CNG (Compressed Natural Gas) conversions but are only really practical for heavy duty trucks for several reasons. The main difference between a CNG conversion and an LNG conversion is the fuel tank. CNG systems store natural gas as a very high pressure (typically 3000 or 3600 psi) gas while LNG systems store natural gas as a cryogenic liquid (-240°F is typically the lowest temperature in the tank). The internal pressure within an LNG tank is either due to the gas temperature (saturation pressure) or due to pressurization from backpressure during refuelling. In some installations, an internal cryogenic fuel pump is used to further increase fuel pressure to the engine. LNG's properties change as heat leaks into the tank(s) so that the pressure and density of the liquid changes over time. Ideally, the vehicles should be refueled daily and one week is about the longest the fuel should take to be consumed.
LNG conversions are generally only done for heavy duty trucks for the following reasons:
- Refuelling: Because of the relative scarcity of LNG stations, a trucking fleets often have central terminals where the vehicles regularly return and it makes sense to install an on-site LNG fuelling station there.
- Cost: Heavy Duty trucks use a lot of fuel and the fuel savings just from the volume of fuel can pay for the substantial cost of an LNG tank.
- Tanks: LNG tanks are most commonly available in 20" and 26" diameter sizes, which can directly replace diesel fuel tanks.
There are two ways of converting a heavy duty truck to LNG. The first is to repower the diesel engine with a spark-ignition engine. The second is to add a diesel-LNG dual fuel system. In either case, one or two LNG tanks must be added to the truck and, in the case of the spark-ignition engine, the diesel fuel tank must be removed. Of the two options, diesel dual fuel engines are more fuel efficient because diesel engines have a higher thermal efficiency due to their higher compression ratios and because they do not have pumping losses from a throttle.