Petrol Engine - Explanation, Otto-Cycle and Efficiency | Physics Grade 11 Notes


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Petrol Engine - Explanation, Otto-Cycle and Efficiency | Physics Grade 11 Notes

Petrol Engine - Explanation, Otto-Cycle and Efficiency

Petrol Engine
The petrol engine is an internal combustion engine developed by Otto in 1876. It consists of a cylinder fitted with piston. The cylinder is provided with an inlet and an outlet valve I and O respectively. The opening and closing of these valves are controlled by the motion of the piston. It is a four-stroke engine and uses 98% air and 2% petrol vapor mixture as working substance. The working method is described as follows:

  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 and outlet valve are closed. The mixture undergoes adiabatic expansion and its volume is reduced to about 1/5 of the original volume as a result the temperature of the mixture is raised about 6000C.
  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 vapor and air ignites. The temperature rises about 20000C and a pressure of about 15 atmospheres are 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.
4 Stroke Petrol EngineOtto-Cycle on a PV-diagram
The portion AB represents the suction stroke in which a 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 an explosion of the mixture. Both temperature and pressure rise 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 quesi-static adiabatic expansion from volume V1 to volume V2. During this process, air suffers a drop in temperature and pressure. At E the exhaust valve is opened and the pressure falls to atmospheric pressure at B, at constant volume V2. The portion BA represents exhaust at constant pressure.
p-v diagram for petrol engine cycleEfficiency
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 is given by
Η = w/ Q1 = (Q1 – Q2­) / Q1
or, η = 1 – (V1 / V2)γ-1
or, η = 1 – (1/ϱ)γ-1

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