DIFFERENCE BETWEEN SPARK IGNITION ENGINESAND COMPRESSION IGNITION ENGINES

Presented by :- AMAN JAIN

A2305407009 B.Tech (Mech. &

Automation)

INTRODUCTION

Spark Ignition EngineThe term spark-ignition engine normally refers to internal combustion engines,specifically petrol engines, where the initiation of the combustion process of the air-fuel mixture is ignited within the combustion chamber by a

spark from a spark plug. Spark-ignition engines may be either two-stroke or four-stroke.A four-stroke spark-ignition engine is an Otto cycle engine.

Compression Ignition Engine

A diesel engine (also known as a compression-ignition engine) is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. The diesel engine is modeled on the Diesel cycle. The engine and thermodynamic cycle were both developed by Rudolph Diesel in 1897.Diesel engines are manufactured in two stroke and four stroke versions.

SOME OF THEDIFFERENCES BETWEEN SI AND CI ENGINE

1) Type of fuel used:  The most prominent difference between

Spark Ignition (SI) and Compression Ignition (CI) engines is the type of fuel used in each. In SI engines petrol or gasoline is used as fuel, hence these engines are also called petrol engines. In CI engines diesel is used as fuel, hence they are also called diesel engines.

2) Type of cycle used:

In case of SI engines, the Otto cycle is used. In this cycle, addition of heat or fuel combustion

occurs at a constant volumeThe basis of working of CI

engines is the Diesel cycle. In this cycle the addition of heat or fuel

combustion occurs at a constant pressure. 

3)Introduction of fuel in the engine In case of SI engines, during the piston's suction stroke, a mixture of air and fuel is injected from cylinder head portion of the cylinder. The air-fuel mixture is injected via the carburetor that controls the quantity and the quality of the injected mixture. In the case of CI engines, fuel is injected into the combustion chamber towards the end of the compression stroke. The fuel starts burning instantly due to the high pressure. To inject diesel in SI engines, a fuel pump and injector are required.

4) Ignition of fuel: Petrol is a highly volatile liquid, but its self-ignition temperature is high. Hence for the combustion of this fuel a spark is necessary to initiate its burning process. To generate this spark in SI engines, the spark plug is used.With diesel, the self-ignition temperature is comparatively lower. When diesel fuel is compressed to high pressures, its temperature also increases beyond the self-ignition temperature of the fuel. Hence in the case of CI engines, the ignition of fuel occurs due to compression of the air-fuel mixture and there is no need for spark plugs.

5)Compression ratio for the fuel: In case of SI engines, the compression ratio of the fuel is in the range of 6 to 10 depending on the size of the engine and the power to be produced. In CI engines, the compression ratio for air is 16 to 20. The high compression ratio of air creates high temperatures, which ensures the diesel fuel can self-ignite. 6)Weight of the engines: In CI engines the compression ratio is higher, which produces high-pressures inside the engine. Hence CI engines are heavier than SI engines.

7)Speed achieved by the engine: Petrol or SI engines are lightweight, and the fuel is homogeneously burned, hence achieving very high speeds. CI engines are heavier and the fuel isBurned heterogeneously, hence producing lower speeds. 8) Spark Plug: In case of CI engine,Spark Plug is not required but in case of SI Engine,Spark Plug is required for combustion of fuel.

9) Thermal efficiency of the engine: In case of CI engines,the value of compression ratio is higher; hence these engines have the potential to achieve higher thermal efficiency. In case of SI engines,the lower compression ratio reduces their potential to achieve higher thermal efficiency.

10. Air Fuel Ratio: The air-fuel mixture is homogeneous throughout in SI while in CI engines only air enters and mixture is heterogeneous in the cylinder.

11. Air-Fuel Mixture:

In SI engine, properly mixed air-fuel mixture in enters the cylinder. But in CI engine, only air enters the cylinder.

12. Size of Combustion Chamber: In SI engine, the combustion chamber is smaller because there is no smoke. But in CI engine, due to smoke, combustion chamber has to be larger. 13. Efficiency: CI engines are considered more efficient than SI engines.

14. Knocking: In SI engines, knocking is due to pre-ignition. It can result in detonation and can damage the engine altogether. But in CI engines, knocking is due to ignition delay, and hence it is least harmful. 15. Carburettor: Carburettor is needed in SI Engines whereas use of Carburettors is eliminated in CI Engines.

16. Top Dead Centre: The SI spark plug is timed to start combustion at the ideal moment, usually some degrees before the piston reaches the top (TDC or Top Dead Centre). The burning mixture then drives the piston down for the power stroke. The CI fuel is injected (sprayed) into the cylinder at the ideal moment - this is usually some degrees before TDC, and the red-hot air then starts the fuel burning. This drives the piston down for the power stroke.

17. Engine Weight: Petrol engines are lightweight and achieve higher speed. Diesel engines, on the other hand, are heavy engines and achieve lesser speed.

18.Engine Strength: Compression ignition engine has to be built much stronger than an spark ignition engine due to the fact that the Compression Ignition engine will "Detonate" the fuel and will cause it to explode . This is why diesel engines are so much louder than gas engines. Spark Ignition system ignites the fuel and it burns in a rapid but controlled manner that "Pushes" the piston down during the power stroke and puts less stress on the engine parts.

19. Revolutions per minute: In SI Engine,there is higher maximum rpm due to lighter weight and in case of CI Engine there is lower rpm due to higher weight. 20. Self IgnitionTeamperature : SI Engines have higher self ignition temperature

than CI Engines.

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