Thermodynamics: Four-Stroke and Two-Stroke Engines

Thermodynamics 1
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Thermodynamics 2
P8
The Four-stroke-cycle spark-ignition engine
Spark ignition engine is an IC engine that basically operates on petrol, the mixture air and fuel is
ignited from a spark plug in this engine. The operation sequence of this type includes four
sequences as discussed below;
First sequence: ignition stroke: here the inlet valve will open therefore the piston movement
will develop a suction pressure which will induce fresh charge of air as well as atomized petrol.
The stroke is sometimes known as the intake stroke. This stroke can be illustrated using the
following diagram;
Figure 1: Showing ignition stroke (Blair, 2011).
Second sequence: Compression stroke: This is the second sequence immediately after the
induction stroke. In the stroke both the exhaust together with the inlets hence the piston in this
stroke will move upward. The charge is hence gradually compressed to a compression ratio to
1/8 to 1 /10 of the initial volume, increasing the charge temperature and pressure (Blair, 2011).
This stroke can be illustrated using the following diagram;

Thermodynamics 3
Figure 2: Showing Compression stroke (Blair, 2011).
Third Sequence: Power stroke: For this stroke, it is both the exhaust valve and the inlet valve
that is supposed to be kept closed In the stroke compressor just before the piston reaches the Top
Dead Center (TDC) that will be ignited by the spark plug by the charge (National Research
Council, 2011) . The charge will hence begin to burn immediately when the piston has attained
the TDC level which leads into a quick increase in temperature and hence increase in pressure
making the piston to move downwards. This stroke can be illustrated using the following
diagram;

Thermodynamics 4
Figure 3: Showing power stroke (Blair, 2011).
Fourth sequence: Exhaust stroke: The exhaust valve will open immediately the power stroke
ends. This is true because when the pressure of the cylinder is increased as compared to the
atmospheric pressure. The burnt gas remaining will hence be pushed by the upward movement of
the piston in this stroke. This can be illustrated in the following diagram:
Figure 4: Showing exhaust stroke (Blair, 2011).
The Four-stroke-cycle compression-ignition engine
Is a type of engine that uses diesel in its operation. This is an internal combustion engine where
the fuel ignite, it is pumped into the chamber of combustion and it is due to temperature
elevation of the air in the cylinder as a result of adiabatic compression (mechanical compression)
(Bell, 2018). For this type of engine, the first two strokes are the same to those of the ignition
engine discussed above.
Compression stroke: Both the exhaust and the inlet valves are closed. Hence the piston will
move upwards. The charge is then gradually compressed to ½ to 1/24 of its initial volume. This
will make the pressure to move to about 40 bars.
Power stroke: Before the compression stroke ends the fuel is injected, this fuel is vaporized
through the heated charge. The mixture of the fuel and air is burnt, burning of this mixture will
make the pressure to rise in the gas cylinder quickly and make the piston to move away (down)
from the head of the cylinder.
Graphically the above 4 strokes can be shown as below;