Operation of the four-stroke petrol engine:
The petrol engine was invented in 1876 by Nikolaus Otto and is therefore also referred to as an “Otto engine”. In this spark-ignition engine, chemical energy is converted into mechanical energy. For this, air, petrol and a spark are required. There are various techniques to obtain as much air as possible and a controlled quantity of fuel in the cylinder. With the help of variable valve timing or forced induction, a high volumetric efficiency is achieved. Fuel injection can be realised by two different injection systems: direct and indirect injection. More about this later.
The operation of the petrol engine, despite all innovative techniques, always comes down to the same principle. In a complete working cycle, the combustion of the petrol results in a rotation of the crankshaft. The crankshaft is connected to the drivetrain. The different steps of the working cycle are divided into four strokes: the intake, compression, power and exhaust strokes.
Intake stroke: the piston moves from top dead centre (TDC) to bottom dead centre (BDC). The intake valve opens simultaneously with the downward movement of the piston. This causes the piston to draw air into the cylinder. The air comes from the intake manifold and the air filter. Depending on the type of engine, fuel is also injected by an injector. After the piston has reached BDC, the intake valve closes.
Compression stroke: the intake and exhaust valves are closed and the piston moves towards TDC. The mixture of air and fuel is compressed.
Power stroke: a number of degrees before the piston reaches TDC, the spark plug generates a spark. Because petrol is highly explosive and there is sufficient oxygen present, combustion takes place. The force released pushes the piston downwards.
Exhaust stroke: after the power stroke, the piston has reached BDC. The exhaust valve opens and the piston moves up again; the burnt gases (exhaust gas) are pushed out.
As soon as the piston reaches TDC, the exhaust valve closes and the intake valve opens. In this situation both valves are opened slightly; the speed at which the exhaust gases flow out of the cylinder affects the incoming air along the intake valve. Air is already being drawn in while the piston is not yet moving towards BDC. This is also called “valve overlap”.
The cyclic process is described on the page Seiliger process. The animation below shows the four-stroke process of a petrol engine.
The animation shows the four-stroke process of only one cylinder. In automotive engineering, engines are often equipped with four cylinders. Three, five, six and eight cylinders are also used frequently. Some manufacturers also use ten, twelve or even sixteen cylinders. The power strokes of the cylinders follow each other in sequence: in a four-cylinder engine, two power strokes take place during each rotation of the crankshaft. The order is important; this is described in the next paragraph.
Intake stroke (1)
Compression stroke (2)
Power stroke (3)
Exhaust stroke (4)
Firing order (work diagram):
Engines always have a fixed firing order. With each power stroke, the combustion force is transferred via the piston to the crankshaft. The working forces must be distributed optimally when the crankshaft rotates, otherwise irregular movements can occur (resulting in extra vibrations and uneven running).
In a four-cylinder engine (both petrol and diesel) the firing order is 1-3-4-2. That means the power stroke first takes place in cylinder 1, half a crankshaft rotation later in cylinder 3, another half rotation later in cylinder 4 and another half rotation later in cylinder 2. The crankshaft has then rotated 2 full turns (720 degrees). This constitutes a complete combustion cycle.
In the work diagram below, it is shown which cylinders are in which stroke; at the moment cylinder 1 is in the power stroke, cylinder 4 is in the exhaust stroke. For information: the red arrows indicate the timing of the spark plug spark.
The image shows a four-stroke engine where the first cylinder (which is determined from the timing drive side) starts its intake stroke. The piston then moves from top to bottom.
In the above work diagram you can see that cylinder 2 has to start the compression stroke. That is correct, because it is still at BDC (bottom dead centre). Cylinder 3 starts the exhaust stroke and cylinder 4 starts the power stroke (at this moment the spark from the spark plug occurs, causing the piston to be pushed downwards by the force of the combustion of the petrol-air mixture).
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