Engine Types and Requirements for Preparation of Fuels 360122
THERE are two main requirements as to proper preparation of the fuel charge for rapid combustion in our present engines:
The fuel must be vaporized, or in a similarly small order of subdivision, before ignition.
The fuel and air must be intimately mixed.
Light fuels, such as our present aviation gasoline, may be vaporized in the carburetor and supercharger to quite a satisfactory degree, providing that the intake air is heated when flying at low temperatures. With this system, as we know, the air and fuel mixing is quite thorough.
Heavier fuels, if released in the carburetor, may not vaporize in the intake-air flow, but instead may puddle and trickle on the side walls. Under such conditions, not only does the fuel fail to reach the cylinders in metered charges but also, if and when it should do so and if it vaporizes in the cylinder rather than getting on the cylinder wall, there is usually inadequate means for mixing the vapor with the air charge.
Fuel injection takes care of metering and delivering the fuel charge but, in practice with any jets the writer has seen, the fuel is delivered directly to what really is only a small part of the volume occupied by the air charge. Not only is the fuel not mixed, but it is so concentrated that the heat transfer from the air charge, which is an essential part of the vaporization process, is unduly slow. To deal with these difficulties it has been necessary to provide special charge turbulence or swirl for best results with nearly every form of injection.
Even though such turbulence be applied, the matter of time for its effective operation is important. Fuel sufficiently volatile to vaporize in the cylinder on the intake stroke, if injected then, apparently can be mixed completely with the air charge before the spark occurs. In air-cooled engine practice this condition seems to be possible with fuels of maximum boiling point of around 450 deg. fahr. With higher volatility limits, the combustion-chamber and wall temperatures do not seem high enough to insure vaporization on the intake stroke, and adequate temperatures are not reached until late in the compression stroke. With spark ignition the proportion of the air that can be reached by fuel vapor is critically dependent upon the fuel volatility, accurate injection timing, and the degree of turbulence obtainable.