Air-fuel Requirement in SI Engines
The spark-ignition automobile engines run on a mixture of gasoline and air. The amount of mixture the engine can take in depends upon following major factors:
(i) Engine displacement.
(ii) Maximum revolution per minute (rpm) of engine. (Hi) Carburettor air flow capacity.
(iu) Volumetric efficiency of engine.
There is a direct relationship between an engine’s air flow and it’s fuel requirement. This relationship is called the air-fuel ratio.
The air-fuel ratio is the proportions by weight of air and gasoline mixed by the carburettor as required for combustion by the engine. This ratio is extremely important for an engine because there are limits to how rich (with more fuel) or how lean (with less fuel) it can be, and still remain fully combustible for efficient firing. The mixtures with which the engine can operate range from 8:1 to 18.5:1 i.e. from 8 kg of air/kg of fuel to 18.5 kg of air/kg of fuel. Richer or leaner air-fuel ratio limit causes the engine to misfire, or simply refuse to run at all.
Stoichiometric Air-Fuel Ratio
The ideal mixture or ratio at which all the fuels blend with all of the oxygen in the air and be completely burned is called the stoichiometric ratio, a chemically perfect combination. In theory, an air fuel ratio of about 14.7:1 i.e. 14.7 kg of air/kg of gasoline produce this ratio, but the exact ratio at which perfect mixture and complete combustion take place depends on the molecular structure of gasoline, which can vary somewhat.
Engine Air-fuel Ratios
An automobile SI engine, as indicated above, works with the air-fuel mixture ranging from 8:1 to 18.5:1. But the ideal ratio would be one that provides both the maximum power and the best economy, while producing the least emissions. But such a ratio does not exist because the fuel requirements of an engine vary widely depending upon temperature, load, and speed conditions. The best fuel economy is obtained with a 15:1 to 16:1 ratio, while maximum power output is achieved with a 12.5:1 to 13.5:1 ratio. A rich mixture in the order of 11:1 is required for idle heavy load, and high-speed conditions. A lean mixture is required for normal cruising and light load conditions. Figure 9.36 represents the characteristic curves showing the effect of mixture ratio on efficiency and fuel consumption.
Fig. 9.36. Effect of air-fuel ratio on efficiency and fuel consumption.
Practically for complete combustion, through mixing of the fuel in excess air (to a limited extent above that of the ideal condition) is needed. Lean mixtures are used to obtain best economy through minimum fuel consumption whereas rich mixtures used to suppress combustion knock and to obtain maximum power from the engine. However, improper distribution of mixture to each cylinder and imperfect/incomplete vaporization of fuel in air necessitates the use of rich mixture to obtain maximum power output. A rich mixture is also required to overcome the effect of dilution of incoming mixture due to entrapped exhaust gases in the cylinder and of air leakage because of the high vacuum in the manifold, under idling or no-load condition. Maximum power is desired at full load while best economy is expected at part throttle conditions. Thus required air fuel ratios result from maximum economy to maximum power. The carburettor must be able to vary the air-fuel ratio quickly to provide the best possible mixture for the engine’s requirements at a given moment.
The best air-fuel ratio for one engine may not be the best ratio for another, even when the two engines are of the same size and design. To accurately determine the best mixture, the engine should be run on a dynamometer to measure speed, load and power requirements for all types of driving conditions.
With a slightly rich mixture, the combustion flame travels faster and conversely with a slightly weak mixture, the flame travel becomes slower. If a very rich mixture is used then some “neat” petrol enters cylinder, washes away lubricant from cylinder walls and gets past piston
to contaminate engine oil. A very sooty deposit occurs in the combustion chamber. On the other hand, if an engine runs on an excessively weak mixture, then overheating particularly of such parts as valves, pistons and spark plugs occurs. This causes detonation and pre-ignition together or separately.
The approximate proportions of air to petrol (by weight) suitable for the different operating conditions are indicated below:
|Starting||9 :||: 1|
|Idling||12 :||: 1|
|Acceleration||12 :||: 1|
|Full power||12 :||: 1|
It makes no difference if an engine is carburetted or fuel injected, the engine still needs the same air-fuel mixture ratios.