I.C.Engine air is essentially required to burn the fuel. The oxygen in the air helps for the proper burning of fuel.
To obtain excellent combustion results from fuel, the air and the fuel must be mixed in a proper ratio.
For complete combustion, the air-fuel ratio is approximately 15: 1 by weight.
This ratio is known as a chemically correct air-fuel ratio in automobile engineering. This is the ideal ratio in an internal combustion engine.
However, the air-fuel ratio can range from 20: 1 to 8: 1; in this range also combustion of fuel can occur.
Any ratio outside of this range is either too rich or too lean to sustain flame propagation.
As per the requirement of the engine, the carburetor provides an air-fuel ratio, which must be within the combustion range.
An engine is cold at the time of starting so, a very rich mixture is required.
A rich mixture is also required at the time of idling and producing maximum power.
During the normal running, a comparatively lean mixture can be used.
For petrol engines, different air-fuel ratios are required under various conditions of load. These are discussed below
A Very rich mixture (10: 1) is required at starting of the engine.
During starting very small amount of fuel vaporizes and the rest of it stays in the liquid state so as to give an ignitable mixture.
An idling, engine demands a rich mixture, which can be made leaner as the throttle is gradually opened.
During idling, the pressure in the inlet manifold is about 20 to 25% of atmospheric pressure.
At a suction stroke, the inlet valve opens and the product of combustion trapped in the clearance volume expands in the inlet manifold.
Later when the piston moves downwards, the gases along with the fresh charges go into the cylinder.
A rich mixture must be supplied during idling, counteract the tendency of dilution and get an ignitable mixture.
Most of the time, an engine is running in medium load condition, therefore, it is desirable that the running should be most economical in this condition.
So a lean mixture can be supplied, as the engine has low fuel consumption at medium load.
For a multi-cylinder engine, slightly more fuel is required due to maldistribution of fuel.
When maximum power is required, the engine must be supplied with a rich mixture as the economy is no consideration.
As the engine enters the power range, the spark must be retarded otherwise knocking would occur.
A lean mixture burns at the latter part of the working stroke.
As the exhaust valve exposes high-temperature gases and has very less time to cool down.
Moreover, the excess air in the lean mixture may cause an oxidizing action on the hot exhaust valve and leads to failure.
Even during normal running, sometimes more power is required for a short period such as to accelerate the vehicle for overtaking, etc. During this period rich mixture is required.
I hope this will help you to understand something about the combustion of fuel under different running conditions of the engine.
To get the more details about the topic, I further recommended reading
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