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Most issues with LNG stem from the nature of the fuel, which is a cryogenic liquefied gas. Natural gas varies in composition from well to well and often contains small but significant quantities of hydrocarbon gases (ethane, propane, butane, etc) in addition to methane. LNG motor fuel is often considered to be nearly 100% methane but stripping out the other constituent gases can add significantly to its cost and lowers its energy content. The fuel system should be designed to make use of the local LNG composition to keep fuel cost low and tank energy content high . However, for the purpose of discussion, we shall assume that LNG contains only methane, which has the lowest boiling point of the gases commonly found in natural gas.

Because LNG is a cryogenic liquid, it only remains in a liquefied state when it is stored below -161.6 °C ( -258.9 °F) at atmospheric pressure.. At -240°F (~ -151°C), methane has a vapor pressure of 18 psig (~124 kPa). The Critical Point of Methane is -82.7°C & 4596 kPa (-116.9°F & 666.6 psi), above which methane becomes supercritical (ie, impossible to remain liquid no matter what the pressure). As with propane (aka LPG - Liquefied Petroleum Gas) vapor motor fuel systems, the internal pressure within the tank is required to move the liquid fuel to the engine. See Boiling Point. With LPG, the pressure within a motor fuel tank is the saturation pressure, which depends upon the ambient temperature because the tank is uninsulated. Some LNG systems use a cryogenic fuel pump inside the LNG tank to boost fuel pressure up to the level required by the fuel injection system.

With LNG tanks, the pressure within the tank depends upon the temperature of the fluid, how much heat has leaked into it through the insulation, and whether the tank was pressurized during filling. LNG must be maintained at as cold a temperature as possible to maximize fuel storage (colder is denser) while still providing enough pressure within the tank to adequately supply the engine.

Often, the LNG equipment used for automotive applications is a modification of cryogenic equipment used in stationary industrial applications. The problem with not developing equipment specifically for automotive applications is that fixes that account for vehicle motion and consistent fuel delivery appear to be afterthoughts rather than primary design considerations.

There are two ways of using LNG in a heavy duty vehicle: LNG mono-fuel operation in a spark ignition engine or diesel-LNG dual-fuel in a compression ignition engine. In either case, maintenance costs are higher for LNG-fueled engines because a LNG fuel system is more complex than a diesel fuel system. For LNG mono-fuel engines, the thermal efficiency of a spark ignition engine is lower than a compression ignition engine because of a lower compression ratio must be used and because of pumping losses from the throttle.


Refueling

LNG stations have refrigeration equipment to maintain the LNG at a constant temperature. When a vehicle is refueled, the LNG in the vehicle's tank will have gained some temperature and will be at a warmer temperature than the LNG supply temperature. When fuel is added to the vehicle's tank, the fresh LNG sinks to the bottom of the tank, which results in stratified fuel. The warmer LNG remains on top and provides the tank pressure while the newly added colder LNG sinks to the bottom. However, once the vehicle returns to the road, the LNG within the tank mixes and the resulting colder vapor temperature causes a collapse in tank pressure. The lower tank pressure affects the operation of the vehicle because the fuel system requires a minimum supply pressure. Too low a supply pressure can cause the fuel mixture to lean out.

Heat Leakage

Because the heat leaking into the LNG tank causes the pressure to rise, industrial cryogenic LNG tanks are commonly equipped with an economizer valve (aka, tank pressure control regulator). The economizer valve is basically a control valve that draws methane vapor from the tank and blends it into the liquid line to the LNG vaporizer. This affects the operation of the engine in three ways.

  1. The immediate composition of fuel supplied to the engine changes from the characteristic composition of local LNG to nearly 100% methane, which has immediate effect on the air-fuel ratio.
  2. The vapor from the economizer can cause the primary regulator's output pressure to drop because the upstream LNG vaporizer is receiving blend of vapor and liquid rather than a constant flow of liquid fuel.
  3. The composition of the liquid LNG remaining in the tank becomes less predominately methane and the other constituent gases become slowly more predominant, with a subtler, longer term effect on the air-fuel ratio. The change in LNG fuel composition due to the operation of the economizer is known as weathering.

Venting

Although "industrial gas" based tanks are commonly described as having a single line fill valve, they often cannot be filled without venting the vapor from the tank. To prevent natural gas from becoming a fire hazard in the vicinity of LNG refueling stations and to prevent greenhouse gas emissions as well, vent gases must be recovered. The vent gas recovery requires a second line so all LNG refueling operations are 2-line systems whether they are described that way or not. Venting is necessary because systems that operate at higher pressures must vented so as to allow the LNG transfer pump to be able to fill the tank.

Unless there is a mass flow meter on the vent line, the vehicle operator is giving the vented fuel back to the LNG station for free. For on-site refueling operations, the vented fuel goes back to the fleet operator. However, when refueling at public LNG stations, the loss of vented fuel can be a measurable cost to the vehicle operator.


There have been several organizations that have evaluated LNG in their fleets. Their experiences are likely very similar to those of other organizations.

Liquid Carbonic

References:

When Houston Metro began the evaluation of the LNG in 1992, they awarded Liquid Carbonic Inc (LCI) a 7-year LNG fuel supply contract. LCI ordered 5 LNG tractor for LNG distribution fleet, which used the prototype 12.7L spark-ignited DDC S60G engine. Due to Houston Metro's reduced demand for LNG, only 1 of the 5 tractors was used significantly to supply Houston Metro. Three of five LNG tractors eventually ended up in the Jack B. Kelley, Inc. fleet for demonstration purposes in Southern California.

Houston Metro

References:

The Metropolitan Transit Authority of Harris County, Texas, (Houston Metro) started converting their entire transit fleet to LNG in 1992 to 1996 and by 2002 switched back their entire fleet to clean diesel and diesel hybrid buses. Houston Metro used the DDC 6V92TA PING in their bus fleet, which is a 9.0L two-stroke, V6 diesel-LNG dual fuel engine. There were 10 LNG buses and 5 diesel powered control buses. Tank pressure collapse was one of the problems they encountered due to single line filling. The work-around they came up with was a cryogenic fuel pump to boost fuel pressure. However, the additional equipment inside the fuel tank became another problematic component to maintain. HM also found that fuel loss due to storage over time and during transfer could be as significant as 25%.

DART - Dallas Area Rapid Transit

References:

DART experience with LNG started with buses powered by the 10L, spark-ignition Cummins L10-280G engine in January of 1998. DART later included Cummins M11-280 powered buses in the evaluation. Although DART seemed to have a strong commitment to LNG with an order of 200 LNG vehicles, by the time the fleet reached a total of 139 by mid-1999, they subsequently cancelled the remaining orders for LNG buses and returned to diesel-powered buses because of poor performance and reliability. Although not directly mentioned in the reports, tank venting could have contributed somewhat to the poorer than expected fuel economy as it would be difficult to measure this blow-off. The Final Results did mention the possibility that the fueler would have to manually open tank vent valve in event of an LNG tank not taking on fuel.

OCTA - Orange County Transportation Authority

References:

OCTA operates a large fleet of LNG vehicles and is in the process of replacing its entire bus fleet with CNG-powered vehicles or other available or required cleaner-burning fuel. They have had issues with venting of the vehicle LNG tanks because the tanks could not maintain insulating vacuum. Because of the greatly increased heat gain due to vacuum loss, there OCTA buses had much greater than expected venting from the LNG tanks. Rather than trying to resolve heat gain problems, OCTA instead chose to switch the fleet to CNG. At the pumping station, submersible cryogenic pumps contributed additional heat to the fuel, which exacerbated venting problems and caused significant fuel losses.


A significant performance issue for LNG vehicles is the drop in power and fuel economy due to the use of spark-ignition engines. The use of diesel-LNG dual fuel allows a heavy duty vehicle to retain the high fuel efficiency and low maintenance of a compression ignition engine while benefiting from the substitution of higher-priced diesel fuel with much lower-priced LNG. The EcoDiesel System the most advanced diesel dual fuel system on the market today and is fully customizable for maximum performance and fuel economy of any diesel engine. Continuing with compression-ignition eliminates the need to maintain the ignition system of a new or repower spark-ignition LNG engine. Supplying a homogeneous natural gas fuel mixture post-turbo also minimizes natural gas injection maintenance costs, especially as compared to the cost of maintaining a direct LNG injection system into the combustion chamber.

Many of problems encountered by the previous cases appear to have originated with the LNG fuel storage system. The automotive LNG fuel systems must be designed specifically for mobile applications and not just repackaging industrial gas systems to fit onto vehicles. Things to look for an automotive system are:

Keeping the fuel temperature as cold as possible at all times is the key to having an properly functioning automotive LNG fuel system. Cold fuel keeps fuel costs low, ensures consistent fuel mixtures for the engine, and maximizes the range from an LNG tank.

Only Cryogenic Fuels Inc appears to provide an LNG system that incorporates the above considerations at this time.


LNG References