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.
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