SI VS CI ENGINES: DIESEL VS GASOLINE ENGINE

SI VS CI ENGINES: DIESEL VS GASOLINE ENGINE
March 6, 2020 No Comments Mechanical Engineering FAHADH V HASSAN

SI VS CI ENGINES: DIESEL VS GASOLINE ENGINE

An engine is a machine that converts some form of energy into mechanical work. Internal combustion engines can be divided according to several characteristics. Depending on the creating of the mixture and the ignition (the thermodynamic process), engines are divided into Otto (spark ignition) and Diesel (compression ignition) engines. \


WHAT IS SI ENGINE?

The four-stroke petrol engine performs the working cycle in four steps. During that time, the crankshaft makes two turns.
The first cycle is the intake – the clip moves from TDC (top dead center) to BDC (bottom dead center), the suction valve starts to open before the piston gets to TDC, and closes after the piston has passed the BDC position. A 0.7-0.9 bar pressure is generated in the cylinder, which, by means of an open valve, draws the fresh working mixture which is mixed in the cylinder with the combustion products remaining from the previous process.

During compression, the clip moves to TDC and compresses the gases in the cylinder. The pressure rises from 11 to 18 bar and temperature from 400 to 600 0C. The piston begins to reduce the cylinder chamber space. Both cylinder valves are closed and compression of fuel and air is carried out.

Combustion begins before TDC and lasts until BDC. Compression ratio values range from 6 to 11. During expansion, the compressed fuel and air mixture are ignited by electric high voltage spark plugs just before the piston came back to the TDC position. The combustion gas pressure at the beginning of the expansion stroke is 40-60 bar and the temperature 2000-2500 C.

Due to the high combustion pressure, the piston moves from TDC to BDC, where useful mechanical work is obtained. By increasing the volume of the cylinder and the expansion of the combustion gases, the pressure, and combustion gas temperature drop so that at the end, the combustion gas pressure is 3-5 bar, and the temperature is 700-1000 C. During the exhaustion the piston moves from BDC to TDC and then pushes out the cylinder gases through the open exhaust valve. Combustion gases are under pressure of 1.05 to 1.20 bar.

Depending on the cylinder charge, the pressure in front of and behind the exhaust can be above criticism and a good part of the gas comes out due to pressure difference. High pressures and temperatures in the engine cylinder represent an important precondition for achieving a high degree of efficiency.
At full engine power, the exhaust gas temperature is about 700-1000 C.


WHAT IS CI ENGINE?

The four-stroke diesel engine has the same cycle.
The first phase (intake) starts by opening the suction valve. The clip moves from the TDC to BDC, whereby due to volume increase above the piston, the pressure is generated in the cylinder so that the actual cylinder pressure during the suction time is 0.7 – 0.85 bar.
In the case of turbocharged engines is up to 2 bar. The second stroke (compression) begins by closing the suction valve and ends at TDC. The capacity of the injected working medium is reduced 14 to 24 times in the compression chamber.

The air pressure at the end of the compression is 30 – 60 bar and its temperature is 600 – 9000C. After compression of the air, expansion begins, and the fuel is injected into the cylinder with the pressure of 90 – 2000 bar, depending on the injection device. The fuel is scattered, heated, mixed with air and ignited.

During combustion, the temperature rises to 2000 – 25000C and the pressure to 60 – 120 bar. In this tact, the maximum mechanical and thermal stresses of the piston mechanism occur. During extension the clip is switched from BDC to TDC, thus draining the cylinder through the exhaust valve. The pressure at which the exhaust gases pass is 1.05 to 1.20 bar. The temperatures of the gases are 500 – 6000C.

DIFFERENCES BETWEEN THE SI AND CI ENGINES:

1) Type of cycle used: In the case of SI engines, the Otto cycle is used. In this cycle, the addition of heat or fuel combustion occurs at a constant volume. The basis of working of CI engines is the Diesel cycle. In this cycle, the addition of heat or fuel combustion occurs at constant pressure.

2) Introduction of fuel in the engine: In the 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. In CI engines, the quantity of fuel to be injected is controlled but the quantity of air to be injected is not controlled.

3) Ignition of fuel: By nature, 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 placed in the cylinder head of the engine. The voltage is provided to the spark plug either from the battery or from the magneto. 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.

4) Compression ratio for the fuel: In the 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.

5) 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.

6) 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 is burned heterogeneously, hence producing lower speeds.

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

SI VS CI ENGINE

Spark Ignition Engine

* SI engines are high-speed engines.
* SI engines have a low thermal efficiency
* Petrol is used as fuel, which has a high self-ignition temperature.
* Knocking takes place at the end of combustion.
* SI engine works on the otto cycle.
* Homogeneous mixture of fuel, hence high pressure is generated.
* Constant volume cycle.

Compression-Ignition Engine

* CI engines are low-speed engines.
* CI engines have high thermal efficiency.
* Diesel is used as fuel, it has a low self-ignition temperature.
* Knocking takes place at the beginning of combustion.
* CI engine works on the diesel cycle.
* Heterogeneous mixture of fuel, hence low pressure is generated.
* Constant pressure cycle.

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FAHADH V HASSAN Hi, I'm Fahadh V Hassan an aspiring blogger with an obsession for all things tech. this blog is dedicated to helping people learn about technology.

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