Important factors in the choice of a liquid fuel for use in portable internal combustion engines are: (1) low cost; (2) ease and safety of transportation or storage; (3) high volatility, i.e., readily convertible into vapour; (4) non-corrosive action on metals; (5) high heat efficiency; (6) ability to give satisfactory results in existing types of internal combustion engine.

Petrol is a liquid fuel composed of carbon (C) and hydrogen (H) in chemical combination. The principal method of producing petrol is by distillation of crude petroleum. The best mixture to use in a petrol engine is one composed of 2 cubic feet of petrol vapour to every 98 cubic feet of air. Petrol does not require any heat to vaporize it under ordinary atmospheric conditions. Pre-ignition of the charge is liable to occur if the compression pressure exceeds 100 lbs. per square inch. It does not corrode or deteriorate metal parts, but leaves a black carbon deposit if not properly burned. Its volatility is high and its specific gravity is low, being about 0·71. An average figure for the calorific value of petrol would be 20,000 B. Th. U. per lb. Petrol is very expensive and also needs care in handling. Private motorists are not allowed to store petrol or benzol.

Benzol is a liquid fuel containing more carbon (C) and less hydrogen (H) than petrol. The principal method of obtaining benzol is by distillation of coal tar. The strength of the mixture should be such that a little more air is supplied in proportion to the quantity of fuel used than is required for petrol. Generally, it may be said that when an engine has been running on petrol and is changed over to benzol the size of the carburettor jet orifice should be slightly reduced and the weight of the float increased—no other changes need be made anywhere. Benzol is very volatile and also highly dangerous to handle, on account of its low flash-point. It often contains impurities which attack the metal parts of the engine and gum up the valves. It is more liable to deposit carbon than petrol. Benzol attacks rubber, and paint on coachwork. It is as expensive as petrol at the present time. The specific gravity of benzol may be taken as 0·88 and its calorific value as 19,000 B. Th. U. per lb. It may be compressed above 100 lbs. per square inch without pre-igniting.

Alcohol is a liquid fuel composed of carbon (C), hydrogen (H), and oxygen (O). The principal method of obtaining alcohol is from the fermentation of vegetable matter, such as potatoes, beetroot, etc. About 6 cubic feet of vaporized alcohol to every 94 cubic feet of air should be used. The volatility of alcohol is very poor compared with petrol or benzol, and it generally contains some water in suspension. It will stand double the compression pressure of petrol without pre-igniting. Alcohol is not so liable to deposit carbon as petrol or benzol, but is very liable to cause rust. It is not obtainable as a fuel in Great Britain at present, owing to the high duty on it. Engines for use with alcohol ought really to be specially constructed for the purpose. Its calorific value is only 12,000 B. Th. U. per lb., and its specific gravity is 0·82. Alcohol requires to be heated before it will vaporize, this heat generally being obtained from the exhaust gases after the engine has been first started up. Alcohol is fairly safe to handle or store.

Paraffin is obtained during the distillation of petrol from crude petroleum, and consists of carbon (C) and hydrogen (H) in almost the same proportions as petrol. Its volatility is low, and it requires heat to vaporize it. The heat required for vaporization is usually obtained from the exhaust gases after the engine has been got running. In starting up a lamp must be used for heating the vaporizer of the carburettor. Paraffin will stand a little higher compression than petrol before pre-igniting. The specific gravity of paraffin may be taken as 0·80 and its calorific value as 18,000 B. Th. U. per lb. It is much cheaper than either petrol or benzol, being only about one-third of the cost. The chief objections to its use are its smell and the greasy character of the stain left by it on coachwork or clothes; also the difficulty of having to heat the vaporizing chamber of the carburettor. It is much safer to handle and store than either petrol or benzol, and requires about the same proportion of air to form an explosive mixture as that given for petrol. The range of variation of strength in the mixture which is permissible with paraffin is much less than with either petrol, benzol, or alcohol. Alcohol has the greatest range of variation in mixture strength. Paraffin is also very liable to deposit carbon, owing to the small range of variation permissible in the strength of the mixture.

Thermal Efficiency.—In the foregoing notes we have used certain terms which have not previously been explained, and therefore it is necessary to give one or two definitions.

The Specific Gravity of a fuel is the ratio of the weight of one gallon of the fuel to the weight of one gallon of water. As a gallon of water weighs 10 lbs., it will be evident from the above notes that a gallon of petrol only weighs 7·1 lbs., whereas a gallon of benzol will weigh 8·8 lbs. (approx.), hence it is not surprising to learn that more mileage per gallon is obtained with benzol than with petrol, even though the calorific value of benzol, per lb., is less than that of petrol. Sometimes the specific gravity is referred to as the density of the fuel, but this is only correct when grammes and centimetres are being used. The density of any fuel is the weight of 1 cubic foot expressed in pounds or, in general terms, the mass of unit volume of the fuel. The density of petrol in English units would be about 44 lbs. per cubic foot.

One British Thermal Unit is the quantity of heat required to raise the temperature of 1 lb. of water by 1 degree (Fahrenheit scale) when the temperature of the water is about 60°F.

The Calorific Value of any fuel (reckoned on the British system of units) is the amount of heat (expressed in British Thermal Units) which will be given out by 1 lb. of the fuel when it is completely burned. The liquid fuels we have to deal with are hydrocarbon compounds, and when completely burned the whole of the carbon is burned to carbon dioxide (CO₂) and the hydrogen to steam (H₂O), leaving no residue. By means of a calorimeter we can experimentally determine the calorific value of any fuel.

It has long been known that work can be turned into heat, and the petrol engine is a good example of the reverse process which consists in turning heat into work. In a steam engine and boiler plant the heat of the fuel is liberated under the boiler, and then a portion of it gets transferred to the water in the boiler and forms steam, which is then taken to the engine and does work in the cylinder, the whole being a wasteful process. The petrol engine is an internal combustion engine, or one in which the fuel is burnt inside the engine cylinder itself and converted directly into work. From every British Thermal Unit of heat liberated by the combustion of the fuel in the cylinder we should be able to get 778 foot-pounds of work if the thermal (or heat) efficiency of the engine was 100 per cent. The thermal efficiency (?) of any engine may be defined as the ratio which the heat equivalent of the work done per minute by the engine bears to the heat which would be liberated by the complete combustion of the quantity of fuel admitted to the cylinder per minute. Thus—

? = ((Horse-power of the Engine × 33,000)/778)/((Number of pounds of fuel consumed per minute) × (Calorific Value of the fuel))

Example:—An engine developing 30 horse-power uses 0·50 lb. of benzol per minute. What is its thermal efficiency? The calorific value of benzol may be taken as 19,000 B. Th. U. per lb.

? = (30 × 33,000/778)/(0·50 × 19,000) = 0·134, or 13·4 per cent.