Question
Download Solution PDFThe compression ratio for petrol engines is
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFConcept:
Compression ratio (r): It is defined as the ratio of volume before compression to the volume after compression.
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 12:1.
- 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 for 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.
Important Points
Difference between C.I.Engine and S.I.Engine:
| C.I.Engine | S.I.Engine |
|
Works on the Diesel cycle. |
Works on the Otto cycle. |
| The working fuel is diesel. | Working fuel is petrol or gasoline. |
| The injector is used for injecting fuel into the combustion chamber. | A carburetor is used to form a homogenous air-fuel mixture to feed to the combustion chamber. |
| Ignition takes place as a result of high temperatures developed as a result of compression after the compression stroke. | Ignition takes place with the help of a spark plug. |
| compression ratio: 16:1 to 22:1 | compression ratio: 6:1 to 12:1 |
| Thermal efficiency is high. | Thermal efficiency is less. |
| Weight per unit power is more. | Weight per unit power is less. |
| Produce high torque and operating speed is less. | Produce less torque but operating speed is quite high. |
Last updated on Jun 24, 2025
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