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Which of the following statements is more relevant in the context of IC engines?

The value of the compression ratio for diesel engines lies between 16:1 to 22:1 whereas for the petrol engine it lies between 6:1 to 10:1.

Compression ratio (r): It is defined as the ratio of volume before compression (Total volume)  to the volume after compression ( Clearance Volume ).

\(r = \frac{{({V_c} + {V_s})}}{{{V_c}}}\:\)

where Vc and Vs are clearance and swept volume respectively.

  • The value of the compression ratio for diesel engines lies between 16:1 to 22:1 whereas for the petrol engine it lies between 6:1 to 10:1.
  • Therefore, diesel engines (C.I. Engines) are larger and also heavier than petrol engines (S.I. Engines) because of the large compression ratio in the case of diesel engines.
  • Hence, for the same power output, the power to weight ratio for diesel engines is lower than petrol engines.
  • The reason for the large compression ratio being compression ignition i.e. in diesel engines, ignition takes place by the high temperature developed by the compression since there is no spark plug like in diesel or C.I. Engine.
  • Also, the flashpoint at which fuel gets ignited is more in the case of diesel engines so the compression ratio needs to be sufficient enough to produce that self-ignition temperature of the fuel for the combustion to take place.
  • In Compression Ignition ( CI ) engine walls are thicker than Spark Ignition ( SI ) engine due to high compression ratio and internal pressure in the diesel engine as compared to the petrol engine.

Difference Between SI engine and CI engine

SI engine CI engine
Petrol is used as fuel. Diesel is used as fuel.
A petrol engine draws a mixture of petrol and air during the suction stroke. A diesel engine draws only air during the suction stroke.
The pressure at the end of the compression is about 10 bar. The pressure at the end of the compression is about 35 bar.
The charge (i.e. petrol and air mixture) is ignited with the help of a spark plug. The fuel is injected in the form of a fine spray. The temperature of the compressed air is about 600° C at a pressure of about 35 bars.
A carburetor is used to atomize, vaporize and meter the correct amount of fuel according to the requirement. Fuel injection pumps and atomizers are used to inject metered quantities of fuel at high pressure according to the requirement.
A petrol engine has a compression ratio of approximately from 6 to 10. A diesel engine has a compression ratio of approximately 16 to 22.
The combustion of fuel takes place approximately at a constant volume. In other words, it works on the Otto cycle. The combustion of fuel takes place approximately at constant pressure. in other words, it works on the Diesel cycle.
Less vibration, and hence, smooth running. More vibration, and hence, rough running and noisier.
Engine weight is less. Engine weight is more.
It emits carbon monoxide. (CO) It emits carbon dioxide. (CO2)