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Internal Combustion Engine Cycles

An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine or a petrol engine. It is the thermodynamic cycle most commonly found in automobile engines using petrol or gasoline fuel.

Process 0–1: Constant Pressure
A mass of air (working fluid) is drawn into the cylinder, from 0 to 1, at atmospheric pressure (constant pressure) through the open intake valve, while the exhaust valve is closed during this process. The intake valve closes at point 1.

Process 1-2: Isentropic compression
In this process, the piston moves from bottom dead centre(BDC) to top dead centre (TDC) position. Air undergoes reversible adiabatic isentropic compression.We know that compression is a process in which volume decreases and pressure increases. Hence, in this process,
volume of air decreases fromV1 to V2 and pressure increases from P1 to P2.

Process 2-3: Constant Volume Heat Addition
Process 2-3 is Isochoric (constant volume) heat addition process. Here, piston remains at top dead centre for a moment. Heat is added at constant volume (V2=V3) from an external heat source.

Process 3-4: Isentropic expansion
In this process, air undergoes isentropic (reversible adiabatic) expansion. The piston is pushed from top dead centre (TDC) to bottom dead centre (BDC) position. Here, pressure decrease from P3 to P4, volume rises from V3 to V4, the temperature fall and entropy remains constant.

Process 4-1: Constant Volume Heat Rejection
The piston rests at BDC for a moment and heat is rejected at constant volume (V4=V1). In this process, the pressure falls from P4 to P1, and temperature decreases.

The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated during the compression of air in the combustion chamber, into which fuel is then injected. This is in contrast to igniting the fuel-air mixture with a spark plug as in the Otto cycle (four-stroke/petrol) engine. Diesel engines are generally used in aircraft, automobiles, power generation, diesel-electric locomotives, and both surface ships and submarines.

Process 1 to 2: Isentropic compression of the fluid (blue)
  • This process is called isentropic as there is no heat transferred (adiabatic) to or from the system and it is a reversible process.
  • The gas inside the cylinder is compressed isentropically from a volume V1 to V2.
  • The ratio of V1 and V2 is referred to as the compression ratio.
  • Work is done by the piston on gases (negative work Win), which means external work has to be done to compress the gases.
  • This process is characterized by the compression stroke of the 4-stroke cycle.
Process 2 to 3:  Reversible constant pressure heating (red)
  • Isobaric means that the process carried out at constant pressure.
  • With the pressure being constant, heat is added externally until volume V3 is reached.
  • The ratio of V3 and V2 is referred to as the cut-off ratio.
  • Heat is added to the system (positive heat Qin), by combusting the air-fuel mixture.
  • This process is characterized by the initial part of the power stroke of the 4-stroke cycle until volume has expanded to V3.
Process 3 to 4: Isentropic expansion (yellow)
  • This process is also isentropic.
  • The gas inside the cylinder expands from V3 to V4 which is equal to V1.
  • The ratio of V4 (or V1) and V3 is known as the expansion ratio.
  • Work is done by the gases on the piston (positive work Wout), thus powering the engine by pushing the piston down.
  • This process is characterized by the final part of the power stroke of the 4-stroke cycle until volume has expanded to V4.
Process 4 to 1: Reversible constant volume cooling (green)
  • Isobaric means that the process carried out at constant volume.
  • With the volume being constant, heat is removed until pressure comes down to p1.
  • Heat is removed from the system (negative heat Qout), by flushing out the combusted gases.
  • This process is characterized by the exhaust and intake stroke of the 4-stroke cycle.

Diesel cycle vs Otto Cycle:

Otto Cycle

Diesel Cycle

Otto cycle is given by the Nicolas Otto in 1876.

It was given by Dr. Rudolph Diesel in 1897.

It is ideal cycle for petrol engine.

It is ideal cycle for diesel engine.

Otto cycle has higher thermal efficiency.

It has lower thermal efficiency.

This cycle has comparatively low compression ratio. It compresses the mixture up to 11:1 ratio.

Diesel cycle has high compression ratio. It compresses the mixture up to 22.1 ratios.

Otto cycle engine used spark plunge to ignite the air-fuel mixture.

Fuel automatically ignites due to high temperature of compressed gas.

Heat addition takes place at constant volume.

Heat addition takes place at constant pressure.

Air fuel mixture is drawn into the cylinder during intakestroke.

Once air is drawn into intake stroke. Fuel is injected after end or compression stroke by an injector.

Heat is added partly at constant volume and partly at constant pressure, the advantage of which is that more time is available for the fuel to burn completely. Because of lagging characteristics of fuel, this cycle is invariably used in compression ignition engines. It consists of two adiabatic and two constant volume and one constant pressure processes. Efficiency lies between Otto and diesel cycle. Since part of the combustion takes place at TDC (at constant vol.) and some after TDC when the piston is moving down increase in volume but due to the continuous addition of fuel the pressure maintained constant for some time. By varying the fuel injection timing with respect the engine speed the engine speed, the engine efficiency can be improved.
The dual cycle consists of following operations:
  • Process 1-2: Isentropic compression
  • Process 2-3: Addition of heat at constant volume.
  • Process 3-4: Addition of heat at constant pressure.
  • Process 4-5: Isentropic expansion.
  • Process 5-1: Rejection of heat at constant volume.


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