• The inlet valve closes just after BDC so the cylinder can be filled to its full capacity with air/fuel mixture.
• The exhaust valve is closed.
• As the piston descends, depression created in the cylinder allows the air/fuel mixture to enter via inlet valve.
• The quantity of air/fuel mixture entering is increased due to atmospheric pressure in the inlet system.
• The piston reaches BDC and starts to ascend, then, just after BDC the inlet valve closes and the induction stoke ends.

• As the piston ascends at the end of the induction stroke, the inlet and exhaust valves are closed. The air/fuel mixture is compressed within the cylinder to a ratio of approximately 8:1 - 10:1
• The piston continues to rise until just before TDC when a spark occurs from the spark plug.
• The compression stroke ends.

• As the spark occurs, the air/fuel mixture will burn rapidly. This causes a flame front to spread across the cylinder.
• A rapid increase in pressure will force the piston down the bore towards BDC.
• Just before BDC, the exhaust valve opens and the ignition stroke ends.

• The exhaust valve has opened just before BDC. This allows the exhaust gases to start escaping from the cylinder.
• The piston continues moving passed BDC travelling back up towards TDC, forcing exhaust gases out of the cylinder through the exhaust valve.
• Just before TDC, the inlet valve opens and the cycle starts again.

This cycle is known as the OTTO cycle. Eugenio Barsanti and Felice Matteucci first patented the OTTO Cycle engine in 1854; the first prototype was made in 1860. The cycle is said to be named after Nicolaus Otto, a German engineer who in 1876, is said to have had the final ideas for the design that is still used today.

It uses quite a few components, actuators and sensors, but they are all set up to work in accordance with each other. Everything works together to produce effective power output, performance and economy.

Petrol engine blocks are normally made from cast iron, although some performance engine blocks are aluminium (with cast iron cylinder sleeves).

The Engine runs at a Compression Ratio of 8:1 - 10:1. The Air/Fuel Mixture Ratio changes depending on running conditions;

Idle - 14.7:1

Acceleration - 12:1

Cruising - 16:1 - 18:1

Manufacturers use 3 different types of fuelling systems to deliver fuel to the engine; Single Point Injection, Multi Point Injection and Carburetor System Delivery. Single and Multi Point systems run at an injection pressure of approximately 3 bar.

The Multi Point Injection system delivers fuel into the inlet manifold near where the manifold connects to the cylinder head. This system uses one injector for each cylinder and is seen to be the most effective as it provides more precise fuelling, better fuel economy, better power output and lower exhaust emissions. Although the Multi point fuel injection system is more expensive to manufacture - and has more individual parts - if a problem arises, it can be cheaper to repair as you can replace individual components and not have to replace the whole system.

The Single Point Injection system uses one injector as part of the throttle body unit. Fuel is injected into the throat of the throttle body, it then travels through the inlet manifold to all the cylinders. This system is cheaper to manufacture. However, if a problem occurs, the whole system will have to be replaced.

In a Carburetor System, the air and fuel is mixed within the carburetor. The air is drawn through the carburetor (through a venturi covered and controlled by a butterfly) by vacuum pressure from the inlet manifold. The amount of air entering the carburetor is controlled by the butterfly, which is controlled by the throttle cable. The air/fuel mixture is then delivered to the cylinders through the inlet manifold. This is the general operation of a Carburetor system, however, there are several variations in the operation of carburetor system. This system can be expensive to run and is not as economical as the Multi Point Injection system.

There are two types of camshaft operating systems; Over Head Valve and Over Head Cam. The camshaft has a system of cams which operate the opening and closing of the valves by the shape of the cam’s lobe through a system of rockers. The camshaft runs at half - engine RPM (half crankshaft speed) and is connected to the crankshaft via a chain, belt or gears. Some engines will have more than one camshaft (twin camshaft).

Actuator – a device that receives output signals from the ECU (Electronic Control Unit) and converts it into an action or motion. For example, an injector receives electrical current from the ECU; this tells the injector when to deliver fuel.

Compression Ratio – this is the difference in the capacity of the cylinder when the piston is at TDC and BDC. At TDC, the capacity of the cylinder is 8 times smaller than when the piston is at BDC; this is a compression ratio of 8:1.

Throttle Body Unit - part of the air intake system that controls the amount of air flowing into the engine, controlled by the throttle cable. The throttle cable responds to the driver’s use of the accelerator pedal.

Venturi - part of the carburettor, cylindrical shaped, air will flow through and is controlled by a throttle butterfly. This restricts or increases the quantity of air that flows to the cylinder.

BDC - Bottom Dead Centre, this is when the piston is at the bottom of the cylinder and at its lowest position in the stroke.

TDC - Top Dead Centre, this is when the piston is at the top of the cylinder and at its highest position in the stroke.