Each movement of the cylinder up or down the cylinder is one stroke of the four stroke combustion cycle or Otto cycle. Most modern internal combustion engines use the four stroke cycle. The four stroke cycle consists of an induction stroke where air and fuel are taken into the cylinder as the piston moves downwards, a compression stroke where the air and fuel are compressed by the upstroke of the cylinder, the ignition or power stroke where the compressed mixture is ignited and the expansion forces the cylinder downwards, and an exhaust stroke where the waste gases are forced out of the cylinder. The intake and outlet ports open and close to allow air to be drawn into the cylinder and exhaust gases to be expelled.
During the intake stroke the inlet valve opens at the top of the cylinder, as the piston moves down air and fuel are drawn into the cylinder. As the piston reaches its lowest position the inlet valve closes and the piston travels upwards compressing the air-fuel mixture. As the piston reaches its highest position at maximum compression a spark ignites the mixture causing a rapid expansion of gas raising the pressure in the cylinder and forcing the piston downwards. Once the cylinder has reached its lowest position the outlet port opens and as the piston rises up the cylinder the exhaust gases are forced out. The valves which open and close the port are sprung to make them naturally close. The valves are opened by a system of rotating cams and pushrods driven by a camshaft which in turn is timed and driven from the crankshaft. The valve timings vary between engines depending on the setup, generally there is some overlap to speed the flow of gases
The intake stroke of the combustion cycle is when the piston travels down the cylinder with the intake port/ports open. A mixture of air and explosive fuel are drawn into the cylinder, the proportions of which are called the air-fuel ratio. Both the air-fuel ratio and the quality of the mixture (dispersion, droplet size etc.) is important for an efficient combustion process. There are two methods of mixing air and fuel in a combustion engine, using a carburettor or fuel injection system.
In a carburetted engine, during the intake stroke of the piston a vacuum is created in the inlet manifold. With a multi cylinder engine the vacuum is almost constant. The carburettor is located at the top of the manifold and air is drawn through it by the vacuum created in the manifold. The carburettor has a small fuel chamber supplied from the fuel tank by a pump, fuel passes through the carburettor to small fuel jets positioned in the air flow. The flow of air past the jets creates a pressure difference causing the fuel to be drawn out. The fuel vaporises in the air flow and passes through the manifold and into cylinders on their intake stroke. The diagram below shows the basic operation of a fixed jet carburettor.
Electronic fuel injection systems spray fuel at high pressure either directly into the combustion chamber or into the intake port of the cylinder during the intake stroke. Using fuel injection enables improved control over the air-fuel mixture and reduces the power required to draw fuel from the jets. The diagram below shows a typical electronic fuel injection system.
Diesel engines typically use direct injection which injects fuel directly into the combustion chamber during the compression stroke. The intake stroke on a diesel engine only draws air into the cylinder.
The compression stroke is the upwards movement of the piston in the cylinder with the valves closed following the intake stroke. This upwards motion compresses the fuel air mixture inside the combustion chamber raising the pressure. The difference between the initial volume of the cylinder and the final volume at the top of the compression stroke is known as the compression ratio. Typically this is approximately 9:1 in spark ignition engines and 15:1 for diesel engines. The compression ratio is particularly important in compression fired engines such as diesel engines. The fuel-air mix and compression ratio is critical to avoid pre-ignition which is the abnormal ignition of fuel in the combustion chamber before the combustion stroke. In diesel engines the fuel is injected under high pressure towards the top of the compression stroke. The distribution of fuel before combustion is also of interest because it affects the efficiency of combustion.
Spark plugs are used to generate the spark which ignites the compressed fuel and air mixture in the spark ignition engine. To generate the spark a high voltage of around 20,000 Volts is applied. Low voltage current is fed through the primary winding of an inductor coil generating a magnetic field. The high voltage is generated when the low voltage supply is interrupted and the magnetic field breaks down generating a high voltage in the secondary winding which has a much larger number of coils. The low voltage supply to the coil is controlled by the distributor which also controls the spark plug that the high voltage surge is sent to. The distributor timing is critical and usually is timed mechanically from the engine. The diagram below shows the typical set-up of an ignition system for a spark ignition engine.
Compression ignition engines such as the diesel engine do not use spark plugs to ignite the fuel-air mix. When the piston reaches the top of the compression stroke the temperature and pressure in the combustion chamber is sufficient to ignite the mixture. Controlled ignition in both spark ignition and diesel engines is essential for efficient combustion and avoid uncontrolled combustion effects such as pre-ignition, auto-ignition and engine knock.
Exhaust gases are pushed out of the cylinder by the upwards motion of the piston following the ignition stroke. The exhaust gases are passed into the exhaust manifold and channelled into the exhaust pipe where they are released into the atmosphere. The exhaust system may contain a smoke box to trap the larger soot particles, it may also be fitted with a catalytic converter which removes some of the harmful components from the exhaust gases. On newer cars some of the exhaust gases are recycled back into the inlet system (typically at the manifold or air filter), this is known as exhaust gas re-circulation EGR.
The efficiency of the combustion process and the design of the engine determine the exhaust constituents. Typically exhaust gases contain oxygen, nitrogen, water vapour, carbon dioxide, carbon monoxide, hydrogen, nitrous oxides, particulates and unburned hydrocarbons.
Exhaust and Inlet Valve Overlap
Exhaust and inlet valve overlap is the transition between the exhaust and inlet strokes and is a practical necessity for the efficient running of any internal combustion engine. Given the constraints imposed by the operation of mechanical valves and the inertia of the air in the inlet manifold, it is necessary to begin opening the inlet valve before the piston reaches Top Dead Centre (TDC) on the exhaust stroke. Likewise, in order to effectively remove all of the combustion gases, the exhaust valve remains open until after TDC. Thus, there is a point in each full cycle when both exhaust and inlet valves are open. The number of degrees over which this occurs and the proportional split across TDC is very much dependent on the engine design and the speed at which it operates.