Car Parts and Accessories

The different shapes of combustion chambers

The performance of an internal combustion engine depends to a large extent on the shape of the combustion chambers.

An efficient combustion chamber must be compact, so that the surface area of the walls through which heat can be lost to the cooling system is minimal.

It is generally accepted that the most efficient form of combustion chamber would be a sphere with the point of ignition at its centre. This would give even burning in all directions and the least heat loss through the walls. But such a design is impracticable for a car engine; the best compromise is a hemisphere. Combustion chamber shapes currently used in car engines can be divided into four main categories: hemispherical, bath-tub, wedge and chamber-in-piston, all with overhead valves (OHV). Two other types, now largely superseded, are the side-valve (or `L’ head), and the ‘F’ head.

A form of hemispherical head is used in most modern overhead camshaft engines to improve performance and emission levels. Variations arise but all use one of the four basic shapes designed to operate efficiently at high compression ratios.

The hemispherical chamber is usually the most efficient, mainly because of its compactness. Of the others, the chamberin-piston layout has manufacturing advantages, in terms of consistency of chamber size and shape.

The cheapest layout is the side-valve, used in many earlier designs. However, the chamber shape restricts the compression ratio to little more than 6:1, which is too low for good power output or petrol economy. The ‘F’ head uses a combination of side valves and overhead valves; it has exhaust valves in the cylinder block and inlet valves in the cylinder head. This design also restricts the compression ratio, without being cheap to produce.

Hemispherical head—the classic combustion-chamber shape

One of the most efficient, practical shapes of combustion chamber is the classic hemi-spherical form, in which the piston top forms the base of a hemisphere, with the valves inclined at 90° to each other, and the spark-plug centrally between them. This design, classic in its symmetry, gives short flame travel from the spark-plug to the piston head, and therefore good burning.

It is used on high-performance engines, though nowadays the angle between the valves is usually less than the classic 90°.

The hemispherical layout requires the use of one or two overhead camshafts—or one side camshaft associated with complicated rocker and push-rod gear—to operate the two rows of valves.

Its shape assists the crossflow of gases, which enter the cylinder from one side of the engine and are exhausted from the other. This gives room for large and free- flowing inlet tracts, arranged so that the mixture enters the chamber easily and with a swirling motion.

The good gas flow resulting from the unobstructed opening of its large valve gives the hemispherical head a high volumetric efficiency. This means it can ‘breathe deeply’, drawing in a large volume of gas for the space available, although this space is never completely filled.

With efficient combustion, the hemispherical head gives a high power output.

However, the modern tendency towards larger cylinder bores and shorter piston strokes enables valves on an ordinary in- line engine, without hemispherical combustion chambers, to be big enough for most normal requirements. Such valves do not need special camshafts or rocker arrangements, making the engine less costly to produce.

Wedge, bath-tub and stratified Charge

A combustion chamber shape which is widely used to give a short flame path is the wedge or, occasionally, the bath-tub.

The wedge-shaped combustion chamber has the valves in its sloping roof and the spark-plug inclined on its deeper side.

The bath-tub consists of an oval- shaped chamber with inlet and exhaust valves placed vertically in the top and the spark-plug inclined at the side.

Both allow the use of a single overhead camshaft, or a single side camshaft and push-rods, as the valves are all in a line.

One solution to the problem of igniting the lean air / mixture charge of modern, low-emission engines is to have a separate, small combustion chamber, as shown in the stratified charge system below.

Chamber-in-piston

If the combustion chamber is formed in the crown of the piston rather than the cylinder head, it is called a chamber-inpiston, bowl-in-piston, or Heron head and is suited to high compression ratios.

It is used mainly in oversquare engines where the diameter of the piston is greater than its stroke.

At the top of its compression stroke the edge of the piston causes some ’squish’ or squirting of gas from the circumference of the piston into the bowl. this promotes good combustion with less chance of pinking—over-rapid combustion of the petrol / air mixture.

The combustion chamber is literally bowl-shaped, and being in the piston, remains hot and so helps vaporisation.

Side valve

By the 1950s, most manufacturers had switched from side to overhead valves for improved combustion, but side-valve gear is comparatively simple and cheap to produce.

Valves are alongside the cylinder, and the combustion chamber is formed above them in the cylinder head. The roof of the chamber slopes downwards over the cylinder, finishing with a small clearance above it. This forms a ’squish’ area, causing mixture to be squirted at the spark-plug.

In this design the combustion space is in the head of the piston. In some cases small “cut-outs” in the top edge of the piston ensure sufficient valve clearance. This layout ‘breathes” well (that is, draws in a large volume of mixture)

The most effective design of side-valve chamber provides a narrow space between the piston and the cylinder head to reduce any tendency to detonation of the burning petrol/air mixture. Even with this improved layout, the maximum compression ratio is still lower than is attainable with OHV engines.

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