Unit 61: Engineering Thermodynamics Lesson 12: Combustion Engines

Objective The purpose of this lesson is to examine a number of Heat Engine Cycles.

Reciprocating piston internal combustion engine working cycle The reciprocating piston engine works on the four-stroke or two-stroke cycle. In the spark ignition petrol engine the power is generated by the fuel / air mixture being drawn into the cylinder during a suction stroke and being ignited by means of a spark towards the end of the compression stroke.

Reciprocating piston internal combustion engine working cycle In the case of the compression-ignition oil or diesel engine air only is drawn in during the suction stroke and towards the end of the compression stroke diesel fuel is injected. The high temperature of the compression air induces the ignition of the fuel / air mix

The Practical Four-stroke cycle The sequence of operations by which the four stroke engine converts heat energy into mechanical energy is known as the four-stroke cycle. A mixture of petrol and air is introduced into the cylinder during the induction stroke and compressed during the compression stroke. At this point the fuel is ignited and the pressure wave produced by ignited fuel drives the piston down on its power stroke

The Practical Four-stroke cycle Finally the waste products of combustion are ejected during the exhaust stroke. Handout 1

The Practical Four-stroke cycle Stage 1: (Suction or induction stroke). The inlet valve is open and the piston moves down the cylinder, sucking in fuel / air mixture (charge) point 1 to 2 Stage 2: (compression stroke). With inlet and exhaust values closed, the charge is compressed as the piston moves up the cylinder (points 2 to 3). At the end of the stroke ignition occurs as the cylinder rises and there is a sudden rise in temperature and pressure of the charge which is complete at point 4.

The Practical Four-stroke cycle Stage 3: (Power or expansion stroke). Both valves remain closed, the piston moves down the cylinder on the power stroke, work is done by the charge gases on the piston, the exhaust value opens (point 5) and the pressure decreases to near atmospheric (point 6) Stage 4: (Exhaust stroke). Spent charge gases are exhausted through the open exhaust valve as the piston rises, the pressure on this stroke is slightly above atmospheric.

Energy Losses During the cycle losses will occur For example, during the expansion and compression processes heat will be transferred from the cylinder walls to the cooling system. The ignition of the charge (heating) takes a finite time and therefore cannot occur at constant volume. The net work done by the engine is therefore less than the ideal case. i.e. reduced area of the power loop in diagram on handout 1 compared to the ideal Otto Cycle

Energy Losses Thus losses occur in real compressions and expansions due to heat transfer and friction.