OK I said there was more than enough info on the web to explain the four stroke cycle for a petrol (spark ignition (SI)) engine however I will slightly edit the article added to the web by Motoman in his blog as I feel he describes it best and similar to how I would teach this to my students.
A four-stroke spark ignition internal combustion engine is exactly that, it creates power from four individual strokes each lasting 180 crankshaft rotational degrees if we ignore intake and exhaust lead and lag. The text book explanations are widely available to read but can be described as follows;
i) Induction - The intake valve opens as the piston goes down to draw in the fuel and air mixture.
ii) Compression - The intake valve closes and the piston goes up compressing the mixture.
iii) Power/Ignition - The spark plug ignites the mixture forcing the piston down.
iv) Exhaust - The exhaust valve opens as the piston goes up to expel the burned/spent gas.
Each step of the process overlaps the actual strokes of the piston, it is far more appropriate and accurate to think of the cycle in terms of 8 phases rather than four 180-degree strokes;
Two exhaust phases;
i) Exhaust Blow-down:
The spent gases must be cleared from the cylinder as completely as possible. The only way to accomplish this is to open the exhaust valves about 30-40 degrees (or greater in some extreme high performance engines) before the bottom of the power stroke (exhaust valve lead), so that the pressure of the still burning charge causes it to begin to escape out of the cylinder. If the power phase were allowed to continue to the bottom of the piston stroke, the piston would have to work hard to push against the high pressure created by the still burning (and still expanding) gas during the upward exhaust stroke. Instead, some of its own pressure is used to blow itself out of the cylinder while the piston is still on the downward stroke.
ii) Exhaust Return:
By the time the piston reverses direction in the exhaust return phase, the excess pressure is gone. For high performance engines the best time to open the exhaust valves is a compromise between extracting the most power from the power phase at low RPM, and losing the least power from the exhaust phase at high RPM.
Three intake phases;
There are three distinctly different ways the intake charge enters the engine.
iii) Intake Overlap:
The intake phase actually begins during the end of the exhaust return phase when a certain amount of degrees before the top of the piston stroke, the intake valves open (inlet valve lead). This is also called the camshaft overlap period because the intake and exhaust valves are both open a small amount at the same time (the exhaust valves are closing and the intake valves are opening), the exhaust valves will remain open for a certain amount of crankshaft degrees into the induction stroke (exhaust valve lag).
The low pressure from the exiting exhaust creates a flow pattern across the top of the cylinder (the laminar region) that draws fresh intake mixture into the cylinder to displace the last remaining spent gases. The flow of intake mixture into the cylinder has been started while the piston is still going up against the direction of the flow it is pumping.
iv) Intake Suction:
Now the piston has passed the top and is accelerating on the downward stroke. At the same time, the intake valves are opening rapidly to allow the intake charge to enter the cylinder with minimal resistance. Since the fuel/air mixture has a certain amount of mass, it tends to lag behind the piston, and this lag time becomes more pronounced as the engine revolutions increase. As a result, the piston first creates a low pressure condition in the cylinder, and the mixture rushes in to fill it.
v) Intake Charging:
This is the time when the piston has passed the bottom of the stroke, and begun to move up. Because of the charge momentum created by the intake suction phase, the fuel and air mixture is still rushing down the intake tract to fill the cylinder. This phenomenon increases with the engine speed, to the point that a progressively higher percentage of the cylinder filling occurs after the piston is no longer physically "sucking" the charge in. Because of this, it is necessary to extend the intake phase way past the physical 180 degree intake stroke. On average, the valves do not completely close until the piston has moved up about 55 degrees past the bottom of the 180 degree stroke.
As you can see, the length of these phases has to do with the speed of the engine, this is another compromise, because while the delayed valve closing improves high RPM cylinder filling, the charge velocity is not high enough at lower RPM, and the piston will push some of the fuel/air mixture back into the port.
Also, in order to extract the most power from the intake phase, the inducted charge must burn completely and since fuel is heavier than air, it is possible for some of the fuel to separate from the mixture as it moves through the ports and into the cylinder. This causes distinct lean and rich pockets in the cylinder, which will result in poor combustion efficiency.
The fuel/air charge should remain turbulent in the cylinder to maintain a uniform mixture throughout. One popular way to do this in a two valve engine is to curve the intake port to swirl (fig 1.1) the mixture into the cylinder. This doesn't work with a four or five valve head though, because too much turbulence is created in the port, which disrupts the volume of flow into the cylinder. A multi-valve pent-roof design utilises tumble (fig 1.2) to maintain a homogenous intake charge instead.
Fig 1.1 - Motion of swirl within an Internal Combustion Fig 1.2 - Motion of tumble within an Internal Combustion
Engine’s Cylinder (K Reeves 2009) Engine’s Cylinder (K Reeves 2009)
vi) Compression Phase
The moment the intake valves are closed during the upward compression stroke marks the end of the intake phase, and the beginning of the compression phase. Since it is the expansion of the burning charge that pushes the piston down, the more the fuel/air charge can be initially compressed, the greater the total expansion will be once it is burned. (Further reading is available within my PV diagram and Mean Effective Pressure posts)
The limit to the maximum possible compression ratio is detonation. The one factor that has the greatest effect on limiting the detonation is the combustion efficiency.
Two Burning Phases
vii) Delay period/ Rapid pressure rise period;
The two burning phases actually contain the three phases of combustion commonly discussed with regard to the internal combustion engine. The first two phases have been placed together as the delay period is so short it does not warrant a numerical phase within this eight phase cycle.
The delay period consists of the time that the spark passes across the electrodes of the spark plug, igniting the fuel/air mixture and releasing enough heat energy to create a burn (this is the reason for spark advance), the second period is the rapid pressure rise which consists of the time it takes the heat to spread into a flame all the way through the fuel commonly known as flame propagation.
Heisler describes this as;
‘When the spark produces ignition the fuel molecules which are then burning, raise by conduction and radiation the temperature of adjacent molecules until they also ignite’ [1]
If this time is used when the piston is going down, then some of the potential power of the fuel will be lost. So, the best moment to ignite the spark will be before the piston has reached the top of its compression stroke.
viii) Power Production Stroke/After burning period
The piston reaches the top, reverses direction, and only now is the engine finally making power. The piston is then forced down to the point of the exhaust blow-down phase, about 140 degrees down from top, and the cycle starts over. As the flame front reaches the cylinder walls during the power stroke approximately twenty-five percent of the mixture is still not completely burnt and thus the lack of remaining oxygen in the cylinder makes it hard to react with the flame, during this period the flame front loses heat to a point where the flame is terminated.
[1] Advanced Engine Technology - Heinz Heisler. Butterworth-Heinemann 1995 ISBN 0340568224 Page 158
