SI Engine (Spark Ignition Engine)
Spark Ignition (SI) Engine is a type of engine in which the combustion takes place by the spark generated by the spark plug. It uses petrol as fuel and works on Otto cycle. In the spark ignition engine the air fuel mixture is inserted into the cylinder with help of carburetor. The compression of the fuel takes place but it has low compression ratio. The fuel is ignited by the spark generated by the spark plug. SI engine produces less noise and vibration and their starting is very easy. They are light in weight and have less maintenance cost. They are mostly used in light commercial vehicles such as scooters, motorcycles cars etc.
Main Points about S.I .( Spark Ignition) Cycle:-
1)Petrol ,kerosene,CNG,LPG can be used a fuel ,as all these fuels have same vapourising behaviour .
2)follows Otto cycle .
3)both 4 stroke and 2 stroke variants are available.
4)air fuel mixture which is preblended by carburetor ,enter the cylinder while intake stroke.
5)the compression ratio is limited to 7-11 ,as while compression stroke air fuel mixture is present in the cylinder , if compression ratio Rc is higher ,it will result in preignition of fuel (knocking) (we know the trend that temperture increases ,as pressure is incresed),hence petrol engine have lower compression ratio.
6) petrol engines suffer lower torque as compression ratio is relativley less also petrol burns faster giving out all energy faster than diesel (as its vapourized).
7) petrol engine can gain higher RPM s easily as energy is relesed faster .
8)petrol engine are lighter in weight .
9)cost is less.
10)carburetors are used to blend the fuel mixture with air 11).
11)Ignition is by a spark ,at the end of compression stroke.
CI Engine (Compression Ignition Engine)
Compression Ignition (CI) Engine is an engine in which the combustion of fuel takes place by the heat of the compressed air. It uses diesel as fuel and works on Diesel cycle. In the compressed ignition engine, only air enters into the cylinder during suction stroke. It has high compression ratio because of the high ignition temperature of the diesel fuel. The fuel is ignited by the heat of the compressed air. Due to high compression ratio it produces more power. Due to incomplete combustion of the fuel, it produces more hydrocarbons which lead to air pollution. The noise and vibration problem is there in the CI engines. The maintenance cost of the CI engine is more as compared with the SI engines. They are mostly used in heavy duty vehicles such as busses, trucks, railways, ships etc.
Main Points about C.I Cycle:-
1)Only diesel can be used as a fuel.
2)follows diesel cycle.
3)both 4 stroke and 2 stroke variants are available.
4)only air enters the cylinder during intake stroke.
5)compression ratio is high (16-20) ,because diesel is little viscous than petrol , also diesel does not vapourize at atmospheric temperature,so it requires very high temperature to sustain the fire .
6)Generally diesel engines have higher torques ,bcoz of higher compression ratio and diesel as burns for more time (The calorific value of diesel fuel is roughly 45.5 MJ/kg (megajoules per kilogram), slightly lower than petrol which is 45.8 MJ/kg. However, diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 36.9 MJ/litre compared to 33.7 MJ/litre).
7)diesel engine takes time to build up rpms ,due to its larger stroke length .
8)diesel engines are built robust to take up all vibrations (due to higher compression ratio)(vibrations are produced as complete balancing of reciprocting engine is tedious).hence diesel engines are heavy .
9)Cost of diesle engine is relativle larger due to robust construction required.
10)Fuel injectors atomize the fuel by spraying fine droplets of fuel,into the cylinder ,whihc causes turblence leading to complete mixing of fuel (stratified injection).
11)auto igntion ,at end of compression stroke ,aided by injection of fuel .
|S.no||Aspect||Spark Ignition Engine||Compression Ignition Engine|
|1||Engine speed||SI engines are high speed engines.||CI engines are low speed engines.|
|2||Cycle efficiency||SI engines have low thermal efficiency||CI engines have high thermal efficiency.|
|3||Fuel used||Petrol is used as fuel, which has high self-ignition temperature.||Diesel is used as fuel, it has low self-ignition temperature.|
|4||Time of knocking||Knocking takes place at the end of combustion.||Knocking takes place at the beginning of combustion.|
|5||Cycle operation||SI engine works on otto cycle.||CI engine works on diesel cycle.|
|6||Pressure generated||Homogeneous mixture of fuel, hence high pressure is generated.||Heterogeneous mixture of fuel, hence low pressure is generated.|
|7||Constant parameter during cycle||Constant volume cycle.||Constant pressure cycle.|
Difference Between SI Engine and CI Engine in Tabular Form
|S.no||Parameter||SI Engine||CI Engine|
|1.||Definition||It is an engine in which the spark is used to burn the fuel.||It is and engine in which heat of compressed air is used to burn the fuel.|
|2.||Fuel used||Petrol is used as fuel.||Diesel is used as fuel.|
|3.||Operating cycle||It operates on Otto cycle.||It operates on Diesel cycle.|
|4.||Compression ratio||Low compression ratio.||High compression ratio.|
|5.||Thermal efficiency||High thermal efficiency.||Less thermal efficiency.|
|6.||Method of ignition||Spark plug is used to produce spark for the ignition.||Heat of compressed air is used for the ignition.|
|7.||Engine Speed||High speed engines.||Low speed engines.|
|8.||Pressure generated||Low pressure is generated after combustion.||High pressure is generated after combustion.|
|9.||Constant parameter during cycle||Constant volume cycle.||Constant pressure cycle.|
|10.||Intake||Air + fuel.||Only air.|
|Weight of engine||Si engine has less weight.||CI engine are heavier.|
|12.||Noise production||It produces less noise.||It produces more noise.|
|13.||Production of hydrocarbon||Less Hydrocarbon is produced.||More hydrocarbon is produced.|
|14.||Starting||Starting of SI engine is easy.||Starting of CI engine is difficult.|
|16.||Vibration problem||Less||Very High|
|17.||Cost of engine||Less cost||High cost|
|18.||Volume to power ratio||Less||High|
|20.||application||It is used in light commercial vehicles like motorcycle, cars etc.||It is used in heavy duty vehicles likes bus, trucks, ships etc.|