Question

At the time of starting, idling and low speed operation, the carburetor supplies a mixture which can be termed as
1. lean
2. slightly leaner than stociometric
3. stochiometric
4. rich

Answer 1

Correct Answer - Option 4 : rich

Explanation:

Air-fuel ratio:

Complete combustion takes place when all the fuel is burned, in the exhaust gas there will be no quantities of unburned fuel. 

Air-fuel ratio (AFR) is the ratio between the mass of air m_a and mass fuel m_f, used by the engine when running.

AFR =   \(\frac{{{m_a}}}{{{m_f}}}\)

The ideal (theoretical) air-fuel ratio, for complete combustion, is called the stoichiometric air-fuel ratio. 

For a gasoline (petrol) engine, the stoichiometric air-fuel ratio is around 14.7:1.

When the air-fuel ratio is higher than the stoichiometric ratio, the air-fuel mixture is called a lean mixture. For cruising condition, a lean mixture is used. 

An AFR of 16.5:1 is a lean mixture.

When the air-fuel ratio is lower than the stoichiometric ratio, the air-fuel mixture is called a rich mixture.

At the time of starting, idling, and low-speed operation, the carburetor supplies a rich mixture because while starting from cold the speeds as well as engine temperatures are low, hence much of the heavy ends supplied by the carburetor do not vaporize and remain in liquid form.

Also, the vapourised fuel may re-condense on coming in contact with cold cylinder walls and piston head.

Therefore even when the fuel-air ratio at the carburetor is well within the normal combustion limits, a rich mixture is supplied to improve the probability of combustion.