Petrol Engine

Types of Heat Engine

Heat engines can be divided into two classes:

1. External combustion engine
2. Internal combustion engine
3. External combustion engine
   In this engine, the combustion of the fuel takes place outside the engine. The steam engine is an example of this type.
4. Internal combustion engine
   In this, the combustion of the fuel takes place inside the engine. For example, petrol engine, diesel engine.

Petrol Engine

The petrol engine is called Otto engine because it was developed by Otto in 1876. This engine consists of a cylinder fitted with the piston. The cylinder is provided with the inlet valve I and outlet valve O. the opening and closing of these valves are controlled by the motion of the piston. It is four strokes internal combustion engines and uses a mixture of petrol and air (98% air and 2% petrol vapour) as the working substance. The working of the petrol engine may describe in the following steps:

1. Suction stroke
   In this stroke, the inlet valve I is opened and the air petrol mixture is sucked into the cylinder by the outward motion of the piston as shown in the figure.
2. Compression stroke
   In this stroke both the inlet valve I is opened and outlet valve O are closed. The mixture undergoes adiabatic compression and its volume is reduced to about 1/5^th of the original volume. As a result, the temperature of the mixture is raised to about 600^o This stroke is shown in the figure.
   
3. Working stroke
   During this stroke, both inlet and outlet valves remain closed. A spark is produced at the sparking plug and the compressed mixture of petrol vapour and air ignites. The temperature rises to about 2000^o C and a pressure of about 15 atmospheres is developed inside the cylinder. The piston is pushed down and the shaft rotates as shown in the figure. Only in this stroke, work is done by the piston.
4. Exhaust stroke
   The outlet valve opens and the burnt out gases are exhausted out to the atmosphere as shown in the figure. After exhaustion is completed the cylinder is again ready for the fresh cycle of four strokes.

Otto-cycle on a P-V Diagram

The P-V diagram of the Otto cycle is shown in the figure. The portion AS represents the suction stroke in which mixture of air and fuel is sucked in at atmospheric pressure. The portion BC represents the adiabatic compression stroke, the volume decreasing from V₂ to V₁. The portion CD represents the result of the explosion of the mixture. Both temperature and pressure at rising at constant volume. During this part, the heat energy from combustion is supplied to the engine. The portion DE represents the working stroke which indicates an adiabatic expansion from volume V₁ to volume V₂. During this process the suffers drop in temperature and pressure. At E the exhaust valve is opened and pressure falls to atmospheric pressure at B, at constant volume V₂. The portion BA represents exhaust at constant pressure. Then the cycle again starts to work.

Efficiency

Suppose Q₁ be the amount of heat supplied to the engine during the ignition stage CD and Q₂ be the amount of heat rejected during the release of heat EB. Then the work done W during each cycle is equal to Q₁ – Q₂. Hence the efficiency of the engine is given by

\begin{align*} \eta = \frac {W}{Q_1} = \frac {Q_1 – Q_2}{Q_1} = 1 - \frac {Q_1}{Q_2} \dots (i) \\ \text {The calculations shows that} \: \eta = 1 - \left (\frac {V_1}{V_2} \right )^{\gamma -1} \\ \text {or,} \: \eta = = 1 - \left (\frac {1}{\gamma } \right )^{\rho -1} \\ \text {where} \rho = V_2/V_1 \end{align*}is called compression ratio and\( \rho\)is the ratio of two specific heats of gas.

Merits and Demerits of Petrol Engine

Petrol engine is very suitable for use in cars, scooters and aero planes because its size is small and light in weight. Its efficiency is quite high as compared to that of the steam engine. This engine is practically smokeless.

Petrol is expensive and so does the running of this engine. Due to the explosion of petrol vapour, there is always a risk of fire and petrol engine is unreliable due to occasional failure of the spark plug.