Carburetion, Manifolding and Fuel Antiknock Value 380135
INCREASED performance at high speed, which is an important characteristic of cars built during the last few years, has required the use of intake manifolds of considerably greater cross-sectional area which, together with certain other changes in engine design, cause unequal distribution of the less volatile portions of the fuel not vaporized in the manifold. Consequently, the fuel which enters the cylinders varies in composition. If other conditions are equal, the relative knocking tendency of the cylinders is determined by the distribution of antiknock value through the fractions of the fuel and by the air-fuel ratios of the mixtures in the cylinders. Ordinarily the leanest cylinder knocks the most.
Fuels rated in the laboratory by conventional methods as having equal antiknock value may differ considerably in road performance because of differences in volatility and distribution of antiknock value through their fractions. The problem is complicated further by differences in the volatility of the commonly used secondary reference fuels.
Recently developed instrumentation permits the determination of the knocking tendency, air-fuel ratio, and spark advance for the individual cylinders of an automobile engine on the road. A modified fuel system and intake manifold for the C.F.R. engine make possible the duplication and measurement in the laboratory of the effects caused by imperfect fuel distribution within the car engine.