Wednesday, May 11, 2011

Combustion chambers

Combustion chambers
The combustion chamber is the space between the top of the piston and Cylinder-Head when the piston is on the top dead-center (TDC). This is where the air-fuel mixture is compressed and burnt. In Petrol engines most of the combustion chamber is formed in the Cylinder-Head, but head (top) of the piston can also be shaped.
In diesel engines, the Cylinder-Head is usually flat and the combustion chamber is formed in the piston head, although some diesel engines have a pre-combustion chamber in the Cylinder-Head.

§         TDC is when the piston is at the top of its stroke and BDC is when it is at the bottom of its stroke.

Fig 2.4
The Cylinder-Head, valve ports and the combustion chambers of petrol engines are designed so of that the air-fuel mixture will be subjected to swirl, or turbulence. This movement occurs while the mixture is being taken into the cylinder (Figure 2.4) and also when it is being compressed in the combustion chamber.
Turbulence mixes the air and fuel and prevents fuel droplets from settling on the surfaces of the combustion chamber and Cylinder walls. The turbulence also helps to prevent local high pressure and high temperature areas during combustion, which could cause detonation.

Combustion chamber designs
For Petrol engines, there are three basic designs of combustion chambers, although there are variations. The names generally given to these three designs relate to their shapes, as shown in figure 2.5. These are:
1. Hemispherical
2. Bathtub
3. Wedge
As well as the combustion chamber formed in the Cylinder-Head, the top of the piston can be crowned or hollowed. In some cases the pistons have the depressions for the valves.

§         These designs of combustion chambers do not apply to diesel engines, which have different Cylinder-Heads and different pistons.

Hemispherical combustion chamber
In this design combustion chamber is approximately the shape of hemisphere. An intake valve is on one side of the combustion chamber and an exhaust valve is on the other. This provides a cross flow the air-fuel mixture enters the chamber on one side and combustion gases on the other. Because of this Cylinder heads of this design are also referred to as cross flow Cylinder-Heads. The position of the valves and ports to be used. Two intake and two exhaust valves are used on some engines. These arrangements assist the engines breathing.
This design is also referred to as a pent roof combustion chamber and is used in many passenger car-engines.
As soon as cross flow, hemispherical combustion chambers have an advantage because the spark plug is able to be located at the center of the chamber. Also with the spark located and the center, the flame travel distance is reduced and this provides rapid and effective combustion.
Burning of the fuel starts and the spark plug and travel rapidly outwards in all directions. This is known as flame propagation. With this designed of combustion chamber of flame front of burning fuel has less distance,  to travel than in some other designs.

Bathtub combustion chamber
This is the somewhat oval-shaped chamber in the cylinder head, with the valves side by side. The name has been derived from its shape, which has been likened to an inverted bathtub. The sparkplug is located on one side. This arrangement provides a short flame path from the spark-plug. The valves are usually vertical and the cylinder head and are in-line. A Relatively simple valve-operating mechanism is able to be used.
Turbulence in the combustion chamber is assisted by the shape of chamber and fact that it has a smaller cross section than the cylinder. This produces a squish effect when the air fuel mixture is compressed between the piston and the flat part of the cylinder head.

§    Squish is the term used to describe the squeezing effect of the gases that increases their velocity and turbulence.

The wedge shaped combustion chamber
In this design, the combustion chamber is shaped like a wedge, tapering away from the spark plug, which is located at the thick end of the wedge. The valves are inclined from the vertical, but all the valves are in-line.
Wedge shaped combustion chamber tend to have a smaller surface area than other designs and so have less area on which droplets of you can condense. This assists in reducing the amount of fuel that remains unburnt after combustion and so reduces the hydrocarbon emissions and the engine exhaust.

Diesel cylinder heads
Fig 2.6
With diesel engines, the face of the cylinder head is usually flat and the combustion chamber formed in the top of the piston instead of the cylinder head (figure 2.6). In some designs, the rim the piston provides squish that forces the air towards the center of the piston and into the combustion chamber. This causes turbulence as the fuel is being injected into a cylinder.
Combustion chambers of the diesel engines are specially designed to promote turbulence, so that the compressed air and injected fuel are properly mixed. There are a number of ways in which this is done, including the use of pre-combustion chambers.

§         Combustion chambers for diesel engines are discussed in later posts.

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