This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
As a source of power, alcohol has been applied in a manner similar to that in which petrol is used for the driving of motor engines. The vapour of the alcohol mixed with a due proportion of air is drawn into the cylinder of the engine. It is there compressed and ignited; explosive combustion follows; and the expansion of the resulting gases propels the piston. Absence of disagreeable odours, with lower volatility and therefore less danger of fire and smaller loss by evaporation, are important advantages claimed for alcohol as compared with petrol for motors. It has a higher flash-point (about 17° C. or 65° F.) than petrol, and more of its vapour is needed to form an explosive mixture with air, so that alcohol is the less dangerous. In hot countries, where there is likely to be much waste by evaporation, the advantage of lower volatility is specially noteworthy.
1 U.S. P., 1277149. 2 1913, 107, 257.
A greater " efficiency," or ratio of work done to heat expended, is also claimed for alcohol. True, its calorific value is only about five-eighths of that of petrol. Nevertheless its efficiency per brake horse power is from 28 to 31 per cent. compared with from 16 to 22 per cent. for petrol. This higher efficiency largely compensates for the lower calorific value: the product of calorific value and efficiency is approximately the same for the two fuels.
The following remarks by Prof. Lewes are instructive in connection with the question of using alcohol as a motor fuel.1
If alcohol and petrol be compared, it is seen that the calorific value of the former is not much more than one-half that of the latter, so that from this point of view alcohol would appear to be out of the question as a competitor with petrol: -
Calories per kilo.
Brit. Therm. Units per lb.
Petrol, sp. gr. 0.684............................
In practice, however, the difference almost disappears. A test made with these fuels in two 8-h.p. engines, designed for their use, gave the consumption as follows: -
Per h.p. hour.
Methylated spirit ...............................
The efficiency obtained with the petrol was calculated from these results to be only 165 per cent., as against 28 per cent. with the alcohol.
There is incomplete combustion of the petrol owing to secondary reactions. According to Sorel, in many cases as much as 82 per cent. of the hydrogen and 42 per cent. of the carbon present in petrol is wasted from this cause. Theoretically, one volume of petrol vapour requires 45 5 volumes of air for combustion, as calculated from the equation: requires theoretically only 143 volumes of air - or roughly about one-third of the quantity needed by petrol. Moreover, double the amount of compression possible with petrol can be employed.
C6H14 + 190 = 6CO2 + 7H1O
(where the petrol is taken as being equivalent to hexane, C6H14). In practice, however, the volume of air admitted is generally about 1 1/2- times the theoretical proportion, and is thus about 68 volumes of air to one of petrol vapour. Four-fifths of the air is inert nitrogen, which tends to hinder the combustion.
With alcohol there is not the same difficulty in getting approximately complete combustion, since one volume of alcohol vapour
1 V. B. Lewes, "Liquid and Gaseous Fuels." 302.
Summing up, we may say that the greater efficiency of alcohol is due to several factors. The volume of air required for complete combustion is only about one-third of that required by petrol, thus reducing the waste of heat in the exhaust. This smaller dilution with air ensures more perfect admixture before the explosion, and consequently favours complete combustion. The mixture can be subjected to a pressure of 200 lb. per square inch in the cylinder without spontaneous ignition, whereas the safety limit with petrol is 80 lb. Mixtures of alcohol vapour and air, containing any proportions between 4 and 13.6 per cent. of alcohol, are all explosive, whereas the explosive range with petrol and air is only from 2 to 5 per cent., thus requiring more exact adjustment of petrol and air in the cylinder.
The exhaust from the alcohol engine is smokeless and nearly odourless, and the products of combustion do not clog the cylinder valves.
On the other hand, should the combustion be imperfect), acetic acid and other corrosive products are liable to be formed from the alcohol. These, when they condense to the liquid form on cooling, are prone to attack the metal surfaces with which they come in contact, causing deterioration of the engine. More important still, the lower volatility of alcohol has the disadvantage that it makes the engine more difficult to start, especially in cold weather. Unless the vapour tension of the fuel in the cylinder is above a certain limit, it is not possible to obtain an explosive mixture. With petrol, there is no difficulty in securing this. Using a light petrol, it has been found that an explosive mixture can be obtained at 0° and at a pressure of 40 lb. per square inch, whereas air saturated with alcohol vapour at all temperatures below 20° is incombustible even at atmospheric pressure.1
Nevertheless, the advantages of alcohol remain, and it is possible to remove the disadvantages. One method is to use a more volatile fuel, e.g., petrol or benzol, as starter; and when this is done, it is found that running a few revolutions with the petrol or benzol before stopping the engine will also overcome the trouble of corrosion, as acid vapours are thus swept out before they condense and attack the metal. Another method is to mix a proportion of more volatile liquid with the alcohol itself. Thus one proposed fuel ("natalite") is stated to contain about 40 per cent., of ether, together with a trace of ammonia to neutralise any tendency to acidity that may develop during the combustion. In trials of this mixed fuel the engine is described as starting easily either " from cold" or when warm, and the fuel consumption was almost identical with that of petrol. The only alteration of the engine found necessary was a slight reduction of the supply of air to the carburettor, and there was no tendency to corrosion. Other mixtures, with petrol, benzol, acetone, etc., are described later on.