Difference Between Combustion in SI (Spark Ignition) and CI (Compression Ignition)Engine

Difference Between Combustion in SI (Spark Ignition) and CI (Compression Ignition) Engine:

Here, we are going to compare the difference between SI and CI Engines. Basically, SI Engine is a Spark Ignition Engine that uses Petrol as a fuel and works on Otto Cycle whereas CI engine is a Compression Ignition Engine that uses Diesel as a fuel and works on Diesel Cycle. However, both SI and CI Engines operates on four-stroke cycle, power can be obtained only in every two revolution of the crankshaft.

We know that both SI and CI engines have much in common, it is worthwhile to compare them based on important parameters like basic cycle of operation, fuel induction, compression ratio etc. The detailed comparison is given as below.

1) Basic Cycle:

SI Engine : Works on Otto cycle or constant volume heat addition cycle.

CI Engine : Works on Diesel cycle or constant pressure heat addition cycle.

2) Fuel:

SI Engine :Gasoline, a highly volatile fuel. Self-ignition temperature is high.

CI Engine : Diesel oil, a non-volatile fuel. Self-ignition temperature is comparatively low.

3) Fuel supply: 

SI Engine :  Fuel supply is either through carburetor or through Electronic Point Fuel Injection (MPFI) system.

CI Engine : A fuel pump and injector are necessary. Fuel supply system is any type of high pressure fuel injection system like Direct Injection (D.I.), Common Rail Diesel Injection (CRDI) etc.

4) Load control: 

SI Engine : Throttle controls the quantity of fuel-air mixture introduced.

CI Engine : The quantity of fuel is regulated. Air quantity is not controlled.

5) Ignition:

SI Engine : Requires an ignition system with spark plug in the combustion chamber. Primary voltage is provided by either a battery or a magneto.

CI Engine : Self-ignition occurs due to high temperature of air because of the high compression. Ignition system and spark plug are not necessary.

6) Compression ratio : 

SI Engine : 6 to 10. Upper limit is fixed by antiknock quality of the fuel.

CI Engine : 16 to 22. Upper limit is limited by weight increase of the engine.

7) Torque and Power :

SI Engine : SI engines produces less torque and has more power delivery.

CI Engine : CI engines produces more torque and has less power delivery.

8) Speed: 

SI Engine : Due to light weight and also due to homogeneous combustion, they are high speed engines ranges between 3000 to 6000 rpm

CI Engine : Due to heavy weight and also due to heterogeneous combustion, they are low speed engines range between 400 to 3500 rpm.

9) Pressure and Temperature :

SI Engine : Pressure ranges from 6 bars to 12 bars and temperature ranges from 250 degree Celsius to 300 degree Celsius.

CI Engine : Pressure ranges from 35 bars to 40 bars and temperature ranges from 600 degree Celsius to 700 degree Celsius.

10) Thermal efficiency:

SI Engine : Because of the lower CR, the maximum value of thermal efficiency that can be obtained is lower.

CI Engine : Because of higher CR, the maximum value of thermal efficiency that can be obtained is higher.

11) Starting Engine:

SI Engine : Due to spark ignition, there is no starting trouble of vehicle in cold weather as spark is provided by spark plug.

CI Engine : In compression ignition engine as only air is compressed in compression stroke which makes difficult for starting of vehicle in cold weather. Hence glow plugs are used to compensate.

12) Weight: 

SI Engine : Lighter due to lower peak pressures.

CI Engine : Heavier due to higher peak pressures.

13) Power Output:

SI Engine : Power output is governed by controlling the quantity of fresh charge supply to engine. So SI Engine uses quantity governing.

CI Engine : Power output is governed by controlling the quantity of fuel only to be injected while air quantity remains same. So, CI Engine uses quality governing.

14) Fuel Efficient :

SI Engine : Its running cost is high. (Less fuel efficiency and hence specific fuel consumption is more).

CI Engine : Its running cost is low. (More fuel efficiency and hence specific fuel consumption is low).

15) Initial Cost :

SI Engine : Its initial cost is low.

CI Engine : Its initial cost is high.

16) Exhaust Emission :

SI Engine: Exhaust Emission is lower.

CI Engine: Exhaust Emission is higher.

17) Based on Noise :

SI Engine : SI Engine works silently and hence noise production is lower.

CI Engine : CI Engine create more noise and hence noise production is higher.

18) Maintenance Cost :

SI Engine : Its maintenance cost is low.

CI Engine : Its maintenance cost is high.

19) Application :

SI Engine : Their use is limited to two wheeler automobiles and personal cars only. May also be used in some other small applications where light engine is primary requirement.

CI Engine : These are used in heavy duty vehicles, Electric Gen Sets and other heavy duty power applications where low running cost is primary requirement.

Suggestions and Advice :

As we know that SI Engine (Petrol Engine) is silent, lighter, has lower initial cost and has less starting trouble in morning. Whereas, CI Engine (Diesel Engine) is more powerful, has more pickup (torque), more mileage.

So it depends on how often you use your vehicle. If you drive your vehicle daily for 5-15kms than only you should buy diesel engine because you will gets advantage in diesel engine on daily driving or if you are not regular driving your vehicle and kept it parked than you should buy petrol vehicle as maintenance cost will have to be paid which is more in diesel engine than petrol engine.