** Comparison of Spark Ignition and Compression Ignition Internal Combustion Engines

Comparison of Spark Ignition and Compression Ignition Internal Combustion Engines


Item/Description SI ENGINE CI ENGINE
Thermodynamic cycle used Otto Cycle Diesel Cycle
Heat addition At constant volume At constant pressure
Fuel used Petrol/Gasoline Diesel
Ignition temperature Self Ignition temperature is high and needs a spark plug Self ignition temperature is lower and is achieved by compression
Carburetor/fuel pump Carburetor and spark plug are required. Voltage required is supplied by the battery Fuel pump and injector are required.
Fuel air mixture Fuel air mixture enters the engine during the suction stroke Air is compressed alone and then the fuel is injected directly into the combustion chamber
Fuel regulation Fuel air mixture is controlled as per load on the engine Controls the fuel as per load but cannot control the flow of air as per load
Fuel ignition Fuel ignition is by spark plug Fuel ignition is self ignition because of high temperature of compressed air
Compression ratio 7 to 10 15 to 20
Weight of vehicle Light weight vehicle because of low pressure ratio requiring less thick parts    Heavy weight vehicles because of high pressures requiring thick parts
Speed achieved High speed because of Light weight vehicles Low speed because of Heavy weight vehicles
Thermal efficiency Low because of low compression ratio and less power produced High because of high compression ratio and more power produced


1. Air and fuel are premixed. Air alone is compressed first andthen fuel is admitted.
2.  Mixture is compressed and then ignites with the help of a spark plug. Fuel ignites because of high temperature of compressed air
3. These have been found to be more useful in small power engines where power/weight and power/volume is high or in light compact vehicles. These have been found to be more useful in large power engines, (truck, buses etc.) where high power/weight and power/volume is not important rather high thermal efficiency is more important.
4. In these engines, premixed charge is throttled in spite of losses during throttling since full power is not always required. In these engines, no throttling is done but control on power produced is achieved through control of fuel flow in the fixed mass of air.
5. If the equivalence ratio (φ) (fuel air ratio) falls below 0.7, the mixture may misfire and may stop altogether. In these engines equivalence ratio is changing from zero in pure air to 1 for complete combustion. The fuel will ignite when required equivalence ratio is achieved.  
6. Burning of fuel is slow only for lean mixtures since here there is no delay period. In these engines, there is always a delay period because the fuel will mix first with compressed air to the required equivalence ratio and only then it will ignite. 
7. Knocking chances are more since mixture can explode and may cause severe damage to the engine because of intense pressure waves.    Knocking chances are in case of large delay period.
8.  Petrol engines have lower efficiency at all loads because of throttling losses.   Diesel engines have higher efficiency at part load since throttling losses are not there and high compression ratio also increases efficiency at all loads on the engine. 
9. There is no such situation in petrol engines Under heavy load condition, it is possible that no air left for the fuel admitted and it will give heavy smoke because of incomplete burning under more power requirements