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.

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.

LNG Storage

A crucial aspect of a LNG conversion is the storage of LNG in the vehicle and the refilling of the storage tank. Most LNG storage systems (typically originating from the industrial gas industry) are promoted as single line fill systems. However, a vent line must be connected to the tank at some point to relieve the pressure in tank to facilitate filling. Therefore, all systems are actually two line (fill & vent) fill systems, whether they are advertised that way or not. There are often LNG issues with single line fill systems and these issues are eliminated with a 2-line fill system.

LNG Cost

It is straightforward to put LNG in terms of Diesel Gallon Equivalent (DGE). According to the US DoE Fuel Energy Spreadsheet, LNG contains 74,720 BTU/gallon (LHV) and low sulfur diesel contains 129,488 BTU/gallon (LHV). If natural gas is selling for $4/MCF and natural gas contains 983 BTU/ft³ (LHV), the DGE price of LNG works out to

• Natural Gas Cost per BTU = (4/1000 $/ft³) / (983 BTU/ft³) = $4.069 x 10E-6 / BTU
• LNG cost = (4.069 x 10E-6 $/BTU) x (74,720 BTU/gallon) = $0.304/gallon
• LNG DGE cost = (4.069 x 10E-6 $/BTU) x (129,488 BTU/gallon) = $0.527/DGE

One MCF of natural gas contains the equivalent energy of about 7.59 gallons of low sulfur diesel (983 x 1000 / 129,488) so it's actually better than the 1 MCF = 7 gallons of diesel T Boone Pickens has been suggesting in several interviews.

Besides the commodity price for natural gas, operating and maintenance (O&M) costs of the on-site refueling equipment can substantially add to the price of LNG and extend the payback time of the conversion.