Spark Ignition Engines 
1. It works on Otto Cycle which is also called as constant volume cycle. 
2. Fuel used is petrol. 
3. During the suction stroke itself the petrol is first admitted in to the carburettor, where it gets mixed with the air and then mixture enters the cylinder. 
4. Air and petrol mixture is drawn during the suction stroke. 
5. Low compression ration ranging from 7:1 to 12:1. 
6. The compressed air and petrol is ignited by the spark plug. This type of ignition is called spark ignition. 

Spark Ignition Engines 
1. It works on Diesel Cycle which is also called as constant pressure cycle. 
2. Fuel used is diesel. 
3. The diesel oil is pressurised by the fuel pump and then injected into the engine cylinder by the fuel injector at the end of compression stroke. 
4. Only air is drawn during the suction stroke. 
5. High compression ratio ranging from 16:1 to 20:1. 
6. The ignition of the diesel is accomplished by the compressed air which will have been heated due to high compression, to the temperature higher than the ignition temperature of the diesel. This type of ignition is called compression ignition or auto ignition.

The combustion process in a Diesel engine is a constant pressure process.

The "Clearance volume" of the engine is the volume between the cylinder head and the piston when the piston is at top dead center.

The compression ratio is the ratio of total volume of cylinder (displacement volume + clearance volume) to the clearance volume.

If clearance volume of I.C engines is increased, the compression ratio will decrease.

The compression ratio in petrol engine is kept less than in Diesel engine because higher or equivalent compression ratio in petrol engines is not possible due to pre-ignition.
Pre-ignition happens when the fuel-air mixture in a spark ignition engine ignites before it is supposed to because of contact with a hot surface. Over-heated spark plugs/ exhaust valves are the main causes of pre-ignition.