Difference between a CI engine and an SI engine
1.Basically SI engines work on Otto Cycle and CI engines on Diesel cycle
2. SI engines use gasoline as fuel and CI engines use diesel as fuel.
3. SI engines use spark plugs and CI engines don't this is because the compression ratio of SI engines is small and the temperature developed is not high enough to start the combustion and so spark plug is used to initiate the combustion once the initial spark is generated the flame front takes care of the rest ,whereas in a CI engine the temperature generated is higher than the ignition temperature of diesel due to high compression ratio and so a spark plug is not needed.
4.The torque generated by CI engines are very high when compared to their counterpart SI engines this is because the CI engines at high compression ratio and so the piston travels far more distance into the cylinder and so the radius of crank also increases ,since torque is cross product of force and radius therefore this increase in radius causes an increase in torque ,the above is one of the many factors contributing to high torque of CI engines,and this is the reason why almost all the trucks and heavy vehicles use CI engines and passenger vehicles and bikes use SI engines.
5.Vibrations produced by CI engines is very high when compared with SI engines this is because of the high compression ratio used.
6. CI engines are heavy when compared to SI engines because of the need to withstand the high pressure developed ,the walls of the engine are made thicker to increase their strength and this one of the reason why Diesel engines are not usually preferred in bikes as they increase the weight of the bike and the vibrations make the ride wearisome.
7.Thermal efficiency of CI engines are higher when compared with SI engines because of the difference in compression ratio adding to this ,poor oxidation of fuel and few other factors make Diesel engines more fuel efficient than petrol engines.
8. Carburettors are used in SI to prepare the air - fuel mixture which is combusted and in case of CI engines fuel at high pressure is injected at the end of compression stroke.
9.Volume to power ratio of SI engine is low when compared to CI engine and that is one of the reason why CI engines are costlier because the volume of combustion chamber of a Diesel engine that will produce the same power as a petrol engine is large and so more material is consumed to make it and this adds to the overall cost.
10.Starting a SI engine is much easier than starting a CI engine because of high compression ratio.If you had done your bachelors in Mechanical or Automobile stream you would have felt this by the way you had started the engines by using an hand crank.
11.Maintenance cost of SI engines are low when compared to CI engines because of the high fatigue stress that is caused in CI engines.
12.SI engines always operate at high speed whereas CI engines operate at low speed,one of the main reason for this is the increase in crank radius in case of Diesel engines.
2. SI engines use gasoline as fuel and CI engines use diesel as fuel.
3. SI engines use spark plugs and CI engines don't this is because the compression ratio of SI engines is small and the temperature developed is not high enough to start the combustion and so spark plug is used to initiate the combustion once the initial spark is generated the flame front takes care of the rest ,whereas in a CI engine the temperature generated is higher than the ignition temperature of diesel due to high compression ratio and so a spark plug is not needed.
4.The torque generated by CI engines are very high when compared to their counterpart SI engines this is because the CI engines at high compression ratio and so the piston travels far more distance into the cylinder and so the radius of crank also increases ,since torque is cross product of force and radius therefore this increase in radius causes an increase in torque ,the above is one of the many factors contributing to high torque of CI engines,and this is the reason why almost all the trucks and heavy vehicles use CI engines and passenger vehicles and bikes use SI engines.
5.Vibrations produced by CI engines is very high when compared with SI engines this is because of the high compression ratio used.
6. CI engines are heavy when compared to SI engines because of the need to withstand the high pressure developed ,the walls of the engine are made thicker to increase their strength and this one of the reason why Diesel engines are not usually preferred in bikes as they increase the weight of the bike and the vibrations make the ride wearisome.
7.Thermal efficiency of CI engines are higher when compared with SI engines because of the difference in compression ratio adding to this ,poor oxidation of fuel and few other factors make Diesel engines more fuel efficient than petrol engines.
8. Carburettors are used in SI to prepare the air - fuel mixture which is combusted and in case of CI engines fuel at high pressure is injected at the end of compression stroke.
9.Volume to power ratio of SI engine is low when compared to CI engine and that is one of the reason why CI engines are costlier because the volume of combustion chamber of a Diesel engine that will produce the same power as a petrol engine is large and so more material is consumed to make it and this adds to the overall cost.
10.Starting a SI engine is much easier than starting a CI engine because of high compression ratio.If you had done your bachelors in Mechanical or Automobile stream you would have felt this by the way you had started the engines by using an hand crank.
11.Maintenance cost of SI engines are low when compared to CI engines because of the high fatigue stress that is caused in CI engines.
12.SI engines always operate at high speed whereas CI engines operate at low speed,one of the main reason for this is the increase in crank radius in case of Diesel engines.
FIRST GENERATION
Diesel engine
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
Petrol engine
1)Petrol ,kersosene,CNG,LPG can be used a fuel ,as all these fuels have same vapourising behaviour
2)follows otto cyle
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
Second generation ( forced induction)
diesel engine
1)Turbochargers and super chargers ,twin chargers were introduced , this allowed diesel engine to suck in more quantities of compressed air .
volkswagen named them TDI , honda called it IDtec......
2) intoduction of multicylinder engines were boon which lead to common rail diesel injection (CRDi ) here a single fuel pump regulates the pressurized fuel ,controlled by soleniod injection.
3)VVTi variable valve timing had been introduced (first announced by fiat )
petrol engine
1) intoduction of turbochargers and super chargers had certian limits petrol engines as it result in preingition (still same problem air fuel mixture is present during compression) (petrol engine could not use compression igniton as its auto igniton temperature is higher than diesle engine
2)Direct injection in petrol enignes were used ,carburetors were eliminated (carburetors doesnt suit quicker change in response ,of drivers needs)petrol was injected into the cylinder during suction stroke
3)VVTi variable valve timing had been introduced (first announced by fiat )
THIRD GENERATION(present)
Petrol engines(major boon to petrol engines )(TSI)
-Petrol engine were adpoted to use stratified injection (fuel injected into the cylinder after compression stroke)
-this removed limtations on petrol engines like compression ratio limit , turbo and superchargers limit
volkswagen named them TSI
Diesel engine
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
Petrol engine
1)Petrol ,kersosene,CNG,LPG can be used a fuel ,as all these fuels have same vapourising behaviour
2)follows otto cyle
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
Second generation ( forced induction)
diesel engine
1)Turbochargers and super chargers ,twin chargers were introduced , this allowed diesel engine to suck in more quantities of compressed air .
volkswagen named them TDI , honda called it IDtec......
2) intoduction of multicylinder engines were boon which lead to common rail diesel injection (CRDi ) here a single fuel pump regulates the pressurized fuel ,controlled by soleniod injection.
3)VVTi variable valve timing had been introduced (first announced by fiat )
petrol engine
1) intoduction of turbochargers and super chargers had certian limits petrol engines as it result in preingition (still same problem air fuel mixture is present during compression) (petrol engine could not use compression igniton as its auto igniton temperature is higher than diesle engine
2)Direct injection in petrol enignes were used ,carburetors were eliminated (carburetors doesnt suit quicker change in response ,of drivers needs)petrol was injected into the cylinder during suction stroke
3)VVTi variable valve timing had been introduced (first announced by fiat )
THIRD GENERATION(present)
Petrol engines(major boon to petrol engines )(TSI)
-Petrol engine were adpoted to use stratified injection (fuel injected into the cylinder after compression stroke)
-this removed limtations on petrol engines like compression ratio limit , turbo and superchargers limit
volkswagen named them TSI
Here are the major differences
- Spark ignition uses petrol as the fuel, but Compression ignition uses diesel.
- Spark Ignition works on otto cycle while Compression Ignition works on diesel cycle.
- Spark Ignition is used in petrol engines while Compression Ignition is used in diesel engines.
- Compression Ignition has more efficient than Spark Ignition.
- Compression Ignition produces more noise than Spark Ignition when it works.
- Compression Ignition produces more hydrocarbons at the exhaust stroke of the engine than Spark Ignition engines.
- Spark Ignition engine has a spark plug, but Compression Ignition does not have one.
- Spark Ignition air-fuel mixture enters into the combustion chamber, but in Compression Ignition, air and fuel enter separately in to the combustion chamber.
- Compression Ignition has higher compression ratio than Spark Ignition.
- Spark Ignition is more harmful due to the pre-detonation things than Compression Ignition.
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