The sum of the development of the automo bile engine from its stationary prototype has been that of transforming an engine capable only of turning its crankshaft over with a cer tain power and at a certain speed into one whose power may be varied from 100 to 3 per cent of its maximum while its speed is varied from 100 to 10 per cent, all by the effortless shifting of air currents in a car buretor to change the rate of fuel and air feed and an accompanying regulation of the spark by which the fuel gas is ignited.
For further facts and details of automobile engines in their great diversity of design it is necessary to consult some of the numerous treatises and textbooks on this large and intri cate subject.
The improvements gradually introduced in automobile engines have reacted strongly upon stationary and marine engines, to the effect that engines of the automobile type are now frequently used for stationary and marine work. In the measure as manufacture of automobile engines on a large scale has reduced their cost, engines for motor boats are becoming very simi lar to those used for motor trucks in the sizes below 60 horse-powers, and engines for racing boats similar to those for racing cars. The larger boat motor engines are however influ enced by Diesel engine design. (See DIESEL ENGINE). To maintain a distinction between the automobile engine and small stationary and ma rine engines of the most advanced type, it may be said that for the latter the weight of the en gine is relatively unimportant while robustness, low cost and suitability for operating with kero sene as fuel are important, and their makers therefore usually follow the continuous develop ment of the automobile engine only to the point where weight reduction becomes costly and where the high engine speed becomes useless or harmful for their purposes. It is to be said,
however, that such are the recognized merits of the automobile type of engine and so greatly is its cost reduced by large manufacture and its operation simplified that means are found more and more for employing it without altera tion in stationary and marine service, especially by gearing down the speed and utilizing irre versible worm gearing to protect the light weight engine from injurous shocks incidental to their work, such as may be sustained in marine work, for example, by the impact of waves on propeller blades.
In the matter of operating automobile en gines with kerosene or other heavy fuels it is found that the lengthening of the piston stroke — the maximum being now about six inches and the heating of the carburetor and air intakes greatly facilitate the use of the heavy fuels, including alcohol mixtures, after the cold engine has been started on gasoline, but it still remains doubtful if the heavier fuels can ever be used exclusively and under all circumstances of climate and temperature so long as the fuel mixture is introduced by means of the suction stroke of the piston causing an air current that merely siphons the fuel from ajet. Develop ments for making the automobile engine one adapted for many sorts of fuel still belong in the main to the future.