Question

Compression ratio is the ratio of:
A) Swept volume to total volume
B) Total volume to the swept volume
C) Total volume to clearance volume
D) None of the above

Answer 1

Hint: We all know that an automobile engine's performance depends upon how much the air-fuel mixture is squeezed in the combustion cylinder. The more we press the air-fuel mix, the more we will get the torque in the crankshaft.

Complete step by step answer:
There are three volume terms generally defined in automotive engines. They are:
${V_c}$ which is clearance volume, ${V_s}$ which is swept volume, and $r$ which is the compression ratio. The clearance volume is the space left inside the cylinder untouched by the piston since to stop the piston striking the bottom dead center.
Swept volume is the volume that is covered by the cylinder during reciprocation.
The compression ratio r is defined as:
$r = 1 + \dfrac{{{V_s}}}{{{V_c}}}$
$ \Rightarrow r = \dfrac{{{V_s} + {V_c}}}{{{V_c}}}$
Here ${V_s} + {V_c}$ is the total volume, i.e., the sum of clearance volume and swept volume.

Therefore, the compression ratio is the ratio of Total volume to clearance volume, and the correct option is (C).

Additional Information: In the diesel engine, the fuel is mixed with the air inside the cylinder, but in the petrol engine, the air and the fuel are combined in a carburetor. The ignition is achieved with the help of the hot and the compressed air in the diesel engine case, whereas the fuel is ignited with an electric spark in the case of the petrol engine.

Note: In the case of the diesel engine; the compression ratio is very high, whereas in the case of the petrol engine, the compression ratio is meager and that is why the value of the power production rate is relatively higher in the case of the diesel engine, and here in the case of the petrol engine the amount of power production is low.