What is petrol engine

The first practical petrol engine was built in 1876 in Germany by Nikolaus August Otto.
The first petrol combustion engine (one cylinder, 121.6 cm3 displacement) was prototyped in 1882 in Italy by Enrico Bernardi.

 In petrol engine mainly these process included :
  1. Suction of air
  2. Mixing of fuel with air after breaking the liquid fuel into highly automised
  3. Ignition of air-fuel mixture either with a spark or by self ignition after raising the temperature of air by compressing it mainly this self ignition used in diesel engine.
  4. Burning of highly automised fuel particles which result in releasing heat energy.
  5. Heat energy converted into kinetic energy in form of reciprocating motion. the expansion of heated gas and their forces act on the engine piston, pushing them downwards result in reciprocating motion of piston.
  6. The reciprocating motion of piston enables the crank-shaft to rotate and finally gets converted into the rotary motion and passed it motion on the wheel.

Working principle of petrol engine (Otto Engine) explained below :

 The conventional Internal Combustion Engine operates on two basic principle :
  1. Otto Cycle
  2. Diesel Cycle
Petrol engine works on principle of Otto cycle so now let we know what is Otto cycle how and how it works? What is Otto Cycle?

Otto cycle is also known as four stroke spark-ignition cycle. It was named after German engineer Nikolaus Otto who invented first four stroke engine.
Otto cycle constructed in four stroke cycle :
  • Suction Stroke : 
In suction stroke piston moving downward direction and opening of inlet valve that you see in above fig stroke 1. that opening of inlet valve enter fuel and creates suction of air and fuel mixture. this stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.)
  • Compression Stroke :
In compression stroke begins at (B.D.C) or end of suction stroke and  piston moves upward and inlet valve closed. In this stroke piston compresses the air-fuel mixture in preparation for ignition during the power stroke. Both inlet and exhaust valve are closed in this stroke.
  • Combustion Stroke : 
Combustion stroke also known as power or ignition stroke. while piston is at (T.D.C) at the end of compression stroke the compressed fuel-air mixture is ignited by spark plug and returning piston to the (B.D.C).This stroke produces mechanical work from the engine to turn the crankshaft. before start of combustion stroke the crankshaft has completed a full 360 degree revolution.
  • Exhaust Stroke :
Exhaust stroke means outlet. during this stroke piston returns (B.D.C) to (T.D.C) while exhaust valve is open and the piston push the exhaust gases above them through the exhaust valve which opens during this stroke.


The Otto cycle is constructed from:
  • Top and bottom of the loop : a pair of quasi-parallel and isentropic processes ( Adiabatic reversible )
  • Left and right side of loop : a pair of parallel isochoric process ( constant volume )
The process are described by :




An Otto cycle is an idealized thermodynamics cycle that describes the functioning of a typical 

Spark ignition piston engine

  1. Process 0–1 a mass of air is drawn into piston/cylinder arrangement at constant pressure.
  2. Process 1–2 is an adiabatic (isentropic) compression of the air as the piston moves from bottom (BDC) to (TDC).
  3. Process 2–3 is a constant-volume heat transfer to the working gas from an external source while the piston is at (TDC). This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning.
  4. Process 3–4 is an adiabatic (isentropic) expansion (power stroke).
  5. Process 4–1 completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is at (BDC)
  6. Process 1–0 the mass of air is released to the atmosphere in a constant pressure process.

Efficiency of Otto Cycle :

The starting point is the general expression for the thermal efficiency of a cycle:
$\displaystyle \eta = \frac{\textrm{work}}{\textrm{heat input}} =\frac{Q_H +Q_L}{Q_H} = 1+\frac{Q_L}{Q_H}.$
The convention, as previously, is that heat exchange is positive if heat is flowing into the system or engine, so $ Q_L$ is negative. The heat absorbed occurs during combustion when the spark occurs, roughly at constant volume. The heat absorbed can be related to the temperature change from state 2 to state 3 as:
$\displaystyle Q_H$$\displaystyle =Q_{23} =\Delta U_{23} \qquad (W_{23} =0)$   
$\displaystyle =\int_{T_2}^{T_3} C_v dT =C_v (T_3 -T_2).$   
The heat rejected is given by (for a perfect gas with constant specific heats)
$\displaystyle Q_L =Q_{41} =\Delta U_{41} =C_v (T_1 -T_4).$
Substituting the expressions for the heat absorbed and rejected in the expression for thermal efficiency yields
$\displaystyle \eta =1-\frac{T_4 -T_1}{T_3-T_2}.$
We can simplify the above expression using the fact that the processes from 1 to 2 and from 3 to 4 are isentropic:
$\displaystyle T_4 V_1^{\gamma-1} =T_3 V_2^{\gamma-1}, \qquad T_1 V_1^{\gamma-1} =T_2 V_2^{\gamma-1}$
$\displaystyle (T_4 - T_1) V_1^{\gamma-1} = (T_3-T_2)V_2^{\gamma-1}$
$\displaystyle \frac{T_4-T_1}{T_3-T_2} =\left(\frac{V_2}{V_1}\right)^{\gamma-1}.$
The quantity $ V_1/V_2 =r$ is called the compression ratio. In terms of compression ratio, the efficiency of an ideal Otto cycle is:
$\displaystyle \eta_\textrm{Otto} = 1-\frac{1}{\left(V_1/V_2\right)^{\gamma-1}}=1-\frac{1}{r^{\gamma-1}}.$

Ideal Otto cycle thermal efficiency
Image fig5OttoEfficiencyVSCompressionRatio_web



The ideal Otto cycle efficiency is shown as a function of the compression ratio in Figure 3.11. As the compression ratio, $ r$ , increases, $ \eta_\textrm{Otto}$ increases, but so does $ T_2$ . If $ T_2$ is too high, the mixture will ignite without a spark (at the wrong location in the cycle).

You can also know :

Advantages of petrol engine

 Disadvantages of petrol engine

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