Difference between 2 stroke and 4 stroke Engine

  • In the 2 stroke engine it consists of one cycle with one rotation
  • In the 4 stroke engine it consists of one cycle with two rotations.
  • The inlet and the exhaust valves of the 4 stroke engine are replaced by 3 ports in a 2 stroke engine.
  • In 2 stroke engine the lubrication oil must be mixed with the petrol
  • In case of 4 stroke engine the lubrication and the fueling must take place separately
  • In the 2 stroke engine due to the presence of the lubrication oil, some of the lubrication oil must be combusted during the time of heat addition. Hence there is more friction and loss of work in 2 Engine
  • The power available in the 2 stroke engine is only 70% of the calculated power
  • In 4 stroke engine the power available is 90% of the calculated power
  • In 2 stroke engine the transfer port and the exhaust port are opened at the same time. Hence some fresh air fuel mixture will be escaped out of the engine cylinder, due to which there is loss of fuel.
  • For single cylinder engine the size of the flywheel is bigger than the 4 stroke engine.
  • Due to the above reason the 2 stroke engine on the roads are replaced by the 4 stroke engine
  • The overall efficiency of a 4 stroke engine is much higher than that of a 2 stroke engine.
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Difference in between the petrol engine and diesel engine

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  • The temperature at the end of the compression is less for the petrol engine.
  • Thus a spark plug is needed for the compression. In case of the diesel engine the temperature at the end of the compression is above the self-ignition temperature of the fuel.
  • Hence the fuel ignited is allowed to enter the cylinder due to the high temperature.
  • No spark plug is required for the diesel engine.
  • For the separate entry of the fuel after the compression stroke, a nozzle is provided for the diesel engines.
  • In case of the petrol engine they do not have the nozzle.
  • In petrol engine the air and the fuel, are mix inside a carburetor before entering the engine cylinder. In diesel engine there is no mixing of air and fuel before suction stroke. Hence diesel engine does not have a carburetor.
  • Heat addition must take place at the constant volume for petrol engine. It takes place at the constant pressure for diesel engine
  • For the same compression ratio, the efficiency of a petrol engine is higher than that of the diesel engine. This is theoretical.
  • In an actual practice the compression ratio of a diesel engine is 1.5 times to 2 times more than that of a petrol engine.
  • Hence the diesel engine efficiency is much higher.
  • Under some conditions the air fuel ratio for the petrol engine must be 16 and for the diesel engine case the ratio of the air fuel mixture must be 25 to 35.
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Difference between the Theoretical and Actual p-v diagram of a 4 stroke petrol Engine

Actual suction:

During the suction stroke the atmospheric air must be entered into the engine cylinder. Then the suction pressure must be below the atmosphere air. At the start of the suction stroke the piston speed is less, hence less air will enter into the cylinder. Then the difference between the atmosphere pressure and the suction pressure will be less at the start of the suction stroke. Then the piston picks up the speed, where more air enters inside. The pressure difference is considered to be higher. Finally the piston slows down and come to rest at bottom dead center. Hence the pressure difference will also be less. Actual suction process will be given by like 0 –a^1  – 1

Actual compression:

Due to heat carried away by cold water, the actual compression process is Polytropic. In account of heat loss the work input for the actual compression process will be more. Hence area in the p-v diagram for actual compression will be lesser. Hence the slop for the actual compression process will be higher than the adiabatic index. Process is given by 1-c-2^1

Actual heat addition:

Heat addition takes place during the changing direction of the piston at the top dead center. The volume first decreases and later increases. Further, the pressure increases throughout heat addition. The pressure increases through heat addition. The actual heat addition process is given by  2^1– d –3^1

Actual Expression:

Due to heat carried away by cold water the work output from the polytrophic expansion process is less. The expansion process is given by the line is  3^1– e –4^1

Heat rejection:

The actual heat rejection is not a constant volume process. During heat rejection the piston under goes change in direction at bottom dead center. The volume initially increases and then decreases. The pressure decreases throughout heat rejection. It is given by the line  4^1– f – e^1.

Actual Exhaust:

During exhaust the leftover exhaust gas is pushed out by the upward motion of the piston depending on the piston speed, the actual exhaust process is given by 1- b^1 – 0

The area formed by the combustion of the suction and exhaust process is known as pumping of engine loss.

The following area must be taken as negative

(0 – a^1 – 1- 1^1 – b^1 – 0). The area available due to the other operation is treated as positive where the net area is determined.

h_m=\frac{net \:\: area}{length \:\:\: of \:\:\: diagram}

imep=h_m \times Spring \:\:\: scale

The ratio of net actual indicated work done to the ideal indicated work done is defined as diagram factor of engine.

df = IWD actual/ IWD ideal.

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