Liquid Methane as an Engine Fuel
Page 51
If you've noticed an error in this article please click here to report it so we can fix it.
-p OSSIBIL1TIES of using methane
in liquid form as an 'engine fuel are dealt with by M". Pearce, .Ph.D., A.M.1.Mech.E., in a paper published by the Institution of Mechanical Engineers (Automobile Division/. Methane is the main constituent of natural gas, and of sewage gas produced in the sludge digestion process. In the gaseous state, it has a calorific value, by volume, of about twice that Of coal gas. Its critical temperature is about -80 degrees C.. and it cannot, therefore. be liquefied by pressure alone.
Its boiling point at atmospheric pressure is about -160 degrees C., and the calorific value of the liquid stored at atmospheric pressure is about twothirds that of petrol. Methane in its g iseous form is being used most successfully at the present time, notably by the fleet of vehicles operated by Croydon Corporation.
Dispensing with Cylinders
One of the main drawbacks to the use of methane in a gaseous form in a road vehicle is that a number of highpressure cylinders has to be accommodated. By liquefyint the methane it becomes possible to reduce the bulk and weight of the container.
Liquefaction of methane and its separation, if necessary, from a gaseous mixture, do not present any new fundamental problem to the gas separation industry; the outstanding questions are related to safety and economics.
With due precautions, the danger should be no greater than with petrol, but the economic position, says Mr. Pearce, is certainly unfavourable in Britain at the present time.
One of the most interesting features of the paper relates to the evaporation of the liquid, the development of vehicle fuel tanks being one of the most important and difficult parts of the research work. What are known as Dewar vessels are in common use for the storage of liquefied gases, such as air. oxygen and nitrogen. The evaporatiori loss is caused by the entry of heat by radiation acrOss :the vacuum space forfned in the wall's, and by conduction down the neck and any other support of the inner vessel. Radiation, is usnally minimized by making the facing surfaces of a metal of high reflectivitY and by giving the surfaces a high polish.
A high vacuum is necessary to minimize gas conduction. and to achieve this the vessel must be leakproof and must be baked luring evacuation in order to drive off any gas absorbed on the surfaces.. Even then, with metal vessels, a high vaauum after sealing off can be ensured only with the help of activated charcoa in contact with the inner vessel.
The normal type of metal Dewar, however, does not conform either in shape or robustness with the requirements of a container suitable for use on a motor vehicle, so that a special type of vessel had to be evolved. Copper is used for the shell and austenitic. stainless steel for the neck, which is made much shorter than on a normal Dewar. Charcoal is carried in a re-entrant tube in the Inner shell.
Mr. Pearce describes the fuel system finally developed., Liquid methane is poured into the, ank. and any,.Vapour generated in th pisoceis is, vented , through a pipe :hich would in-Com
1 ,, . _ . .
mereial service, be connected to a gasholder.
Methane which evaporates' inthe tank passes out through-a relief valve to the evaporator.. This relief valve is adjusted to blow Off at a pressure difference •of about. 1 lb. per sq.in.: which , is sufficient to-force • liquid . up a. dip pipe and through a lagged pipe to a liquid regulator,valve. . The latter is controlled by a diaphragm the ..upper side of which communicates with .the atmosphere. .
The diaphragm is .spring-loaded, so that if the pressure in the evaporator chamber fans below about 0.5 lb._ per sq. in.; the liquid regulator valve opens to admit more liquid which falls on an exhaust-heated plate and is vaporized. Gas is fed from the' evaporator to the gas regulator, mounted under the bonnet, from which it is drawn to the gas mixer. The diaphragm-controlled gas regulator valve opens at a suction of about 1 in. of water. When the engine is stopped this valve is closed, and gas entering the evaporator through the relief valve causes the pressure to rise until a second relief valve blows off, discharging to atmosphere through a pipe carried above the vehicle roof.
The mixer finally ' used was an American product marketed under the name, Algas. It was modified to deal with 'methane and is of the constantdepression' type. li consists essentially of an air valve and gas valve working in tandem.
Dealing with trials conducted on a Leyland Lion bus which ran for 25.000 miles on liquid methane. a fuelconsumption figure of 5.95 m.p.g., based on net consumption. was obtained, and this was considered to be exceptionally good, as compared with the 7.2 m.P.g. averaged over 3,000 miles' running on petrol, as a gallon of petrol is thermally equivalent to 1.515 gallons of methane.
Clean Burning Raising the compression ratio from the normal 5.5 to 1 to 8 to 1 by fitting special pistons, much improved the power output when running on methane, making it about equal to that on petrol when Using the lower compression ratio. Fuel economy, however, fell from 5.95 m.p.g. to 5.15 m.p.g., based on net consumption.
After the trials the engine was stripped for examination and it was found to be exceptionally clean throughout. Further, the indications. were that the rate of wear when using methane, with a compression ratio of 8 to 1, was likely to be less than when using petrol with a compression ratio of 5.5 to 1.
During the trials, when a normal compression ratio was employed, the fuel consumed by the engine was 1.203 gallons. whilst the quantity put into the tank was 1,308 gallons. showing a loss by evaporation at the rate of 3 gallons per day over a total period of 842 hours. Basic loss and tank-cooling loss accounted for a further 32 gallons. the respective percentage losses on net consumption being 8.7 and 2.7.