An internal combustion engine (ICE) burns fuel in the combustion chamber formed by the piston and the cylinder. Fuel energy is converted to work to drive the piston down. Below are two types of internal combustion engines:
- Gasoline spark ignition (SI) engines require a spark from an electric spark plug to start the combustion process during the compression stroke.
- Diesel compression ignition (CI) engines do not require a spark. The diesel fuel auto-ignites when it sprayed into the cylinder during the compression stroke.
The Combustion Chamber
A combustion chamber is that part of the internal combustion engine in which the fuel/air mix is burned. It is where air is sucked in and combined with fuel, compressed, burnt and then released to exhaust – this is known as the four-stroke Otto cycle.
In gasoline engines, the spark plug initiates combustion giving a smooth burn that develops pressure, forcing the piston down. However, under high pressure and temperature some gasoline fuels can show a tendency to combust spontaneously. This can result in a ‘knocking’ or ‘pinking’ sound as small erratic bursts of combustion occur. Thus, an engine may suffer from depleted performance and in the most severe cases permanent damage can occur. Gasoline fuel’s octane number is a measure of how well the fuel can resist this knocking.
Timing of combustion is critical to the effective energy transfer from the burning fuel to the engine. Depending on engine design (e.g. octane responsive engines), use of a higher octane quality (than originally recommended by the vehicle manufacturer) can allow advanced ignition timing (i.e. earlier spark) that can result to improvement in engine performance.
For gasoline spark ignition engines, there are two types of technologies on how the fuel is delivered to the combustion chamber:
- Port fuel injection (PFI) engines – fuel is injected onto the back of the hot inlet valves and evaporates to mix with the fresh air before entering the cylinder during the intake stroke.
- Gasoline direct injection (GDI) engine – fuel is injected directly into the cylinder.
It is the job of the fuel injector to ensure that the fuel is dispersed and mixed into the air in the exact pattern and volume required. Some fuels can lead to fouling of the injector holes, which can disrupt the spray pattern, affecting the efficiency of the engine.
Inlet valves perform the job of opening and closing several hundred times a minute, depending on engine design, to allow the air, or the air/fuel mix, into the combustion chamber. In a conventional port fuel injected gasoline engine, inlet valves can be prone to deposit formation, which can potentially affect efficiency of engine and may result to poor responsiveness.
Piston and Piston Rings
Pistons transfer the force from combustion to the connecting rods while piston rings are designed to form a seal between the piston and the cylinder wall. Pistons slide up and down the cylinder bores several thousand times a minute.