FORD VAN POINTERS.
Page 28
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By R. T. Nicholson (Author of "The Book of the Ford").
THE cylinder head at present in use on the Ford engine differs considerably from that used some years ago. It is much higher. The change was made mainly to "take care of" the inferior motor spirit sold to us of late years as petrol. Modern petrol, as compared with the spirit we formerly used, contains a. great deal of paraffin, and paraffin produces a great deal of carbon. Hence, of course, a greater tendency towards knocking.
411.—The Ford Cylinder Head.
To fit the Ford engine to the new conditions, the cylinder head was enlarged in 1917, so as to give a larger combustion chamber. This means that the compression was lowered, and the tendency towards "pinking," through early formation of carbon, was thereby reduced. At first sight it might seem as if this larger combustion chamber would mean more power ; but, in point of fact, the power of the modern Ford engine, in a clean state, is slightly leas than of old, because of the reduction of compression., As the engine carbonizes, however—and with modern fuel it does not take long to do that—compression increases, and, up to a certain point, power increases, too. Not only has more space been given to the fuel, but more room has been given to the simulation water in the engine. This again was an effort to suit the engine to a lower-grade fuel, which tends to produce more heat than the old. It therefore became necessary to have a greater body of water circulating.
Compression and Power.
When we talk of high and low compression, we must be quite clear what we mean. High compression arises when you force a large volume of "gas" into a comparatively small space ; low compression, on the other hand, arises when you force a relatively small volume of " gas " into that same space. High compression is quite a, different thing from good compression, just as low compression is quite a different thing from bad coluaression. True; if you had very bad compression, you would also have very low compression; but you get high compression and low compression from forcing a large or a small volume of gas into a given space, whereas you get good or bad _compression if that space is, or is not. gastight. If your pistons and valves are a bad fit you get poor compression, whatever the size of your combustion chamber ; and if your pistons and valves are a good fit: you get high or low compression according to the size of your combustion chamber.
Now, when the Ford Co. increased the size of. their combustion chamber they lowered the compression that is, they gave more space to the explosive mixture, which thus fired with less power than formerly, because the strength of any explosion depends largely upon the size of the space in which it takes place. If, for instance, you explode a pound of gunpowder "in the open," it will go off with comparatively small pep," whereas if you touch it off in a closed can, it will "get right down to business." So, when the engine is clean, we get less power with the modern Ford engine than with that formerly standard, simply because there is less pressure on the gas at the moment of explosion. As carbon forms, however, the space available for the explosion is reduced, and then power gradually increases—up to a point.
Why the Presence of Carbon is a:Disadvantage.
When formation of carbon reduces the combustion chamber beyond a certain point, you begin to lose power instead of gaining it. The compression has become too high. Compression becomes too high when the explosion occurs with such suddenness that the piston cannot yield to it readily enough.
Compression, as it becomesbigher and higher, tends to increase the suddenness of the explosion ; indeed, given high enough compression, you will get the "mixture" exploding "on its own" without the aid of a spark. With a spark the effect of great increase in compression is to render the explosion very sudden —to produce such an explosion as spreads very rapidly through the whole bulk of the mixture. Then the piston cannot yield to it quickly enough, and so resists the explosion. That means loss of power, not increase of power.
So you see that compression is, in fact, a sprat to catch a whale. Up to a certain point suddenness of explosion gives more power, because of the increased strength of the blow on the piston but, after that point, suddenness of explosion gives less power 'Because the piston begins to resist the blow.
Temporary Loss of Power after Decarboni zation.
That is why, after an engine has been decarbonized, it always seems for a time to give poor pulling power ; in fact, it does, for a time, pull poorly. As "compression is increased by formation of carbon, so pulling power is increased, but only up to a certain point. When carbonization has gone too far and pinking begins, power falls off.
Some 13 years ago, when motor engines were still more or less in the experimental stage, a good many mistakes were made in the effort to provide very high compression, with the result that an engine began to " pink " almost as soon as it began to work—as soon, at all events, as a thin film of carbon had formed. We have since learnt that if we want to make an engine that will give good average pulling power for a long time on end without decarbonization, we must be content to begin with less than full power, so that we may pick it up later on. But it has been poor spirit that has, in the case of the Ford engine, brought about the change. From which, please, do not understand that the Ford Co. have adopted a poor spirited policy. Not by any manner of means.