Question

Consider the following statements pertaining to supercharging of engines:

1. The power output for a given engine increases

2. The loss of power due to altitude is compensated

3. The increase in supercharging pressure decreases the tendency to detonate in spark ignition engines

4. The mechanical efficiency of supercharged engines is quite high compared to naturally aspirated engines

Which of the above statements are correct?
1. 1 and 3
2. 3 and 2
3. 2 and 3
4. 1 and 2

Answer 1

Correct Answer - Option 4 : 1 and 2

The purpose of supercharging an engine is to raise the density of the air charge before it enters the cylinders.

Thus, the increased mass of air will be inducted which will then be compressed in each cylinder. This makes more oxygen available for combustion. Consequently, more air and fuel per cylinder will be forced into the cylinder, and this can be effectively burnt during the combustion process to raise the engine power output to a higher value.

Purpose of supercharging:

• Provides better mixing of the air-fuel mixture
• Increase the volumetric efficiency
• The mechanical efficiencies of supercharged engines are slightly better than the naturally aspirated engines.
• Increase the power output of the engine
• It tends to increase the possibility of detonation in a S.I. engine and lessen the possibility of knocking in a C.I. engine

 

Supercharging in SI and CI engine:

Supercharging increases the pressure and temperature of the charge at the end of compression. This reduces ignition delay in CI engine, thereby the combustion becomes smooth, and the tendency for knocking is avoided. In SI engine the short ignition delay promotes detonation.

Hence supercharging is preferred in diesel engine than in petrol engine.

Supercharging Limits:

The limit of supercharging for an SI engine is set by knock while that for a CI engine is set by thermal loading.