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 Ideally the chamber should be compact, so that the minimum of heat is lost to the cooling system, and shaped in such away that the mixture burns evenly, producing a progressive increase in pressure.

The dome-like, hem spheri- cal combustion chamber, with the valves inclined at 90ํ to each other and a centre spark plug, has long been recog- nised as the most effective design.

The layout leaves room for a large, free-flowing inlet tract from which the mixture swirls easily into the space above the piston. The exhaust valve is opposite the inlet in a cross- low layout. This gives the most efficient filling and emptying of the cylinder, while the angling of the valves permits large valve sizes to be used which admit a larger volume of mixture.

The generous power output from a hemispherical combus tion chamber means that it is often used in high-performance engines. Its drawback is that valves inclined at 90ํ need two overhead camshafts—or one overhead camshaft and a com- plex linkage—to operate them. The expense of this sort of valve gear makes it uneconom- ical to produce for cheaper cars.

Placing the valves more upright and turning the hemis- pherical chamber into a shal- low pent-roof shape reduces the efficiencya little, but allows the use of simpler valve gear, and a single overhead cam- shaft and rockers.

The designers of mass-pro- duced cars try to get the advantages of a hemispherical chamber more cheaply and usually opt for one of these alternatives:

Bowl in piston. The cylinder head is flat or nearly flat and the combustion chamber forms in the top of the piston. When used on an over-square engine (where the diameter of the pis- ton is greater than its stroke) large valves can be fitted.

At the top of the stroke, the rim of the piston very nearly touches the cylinder head, pro- viding a degree of squish’, a squeezing action that forces mixture at the edge of the cylin- der towards the centre. This assists burning and helps to compensate for the offset posi- tion of the spark plug.

Bathtub combustion

chamber. This is formed in the cylinder head and has two vertical valves and a side- mounted spark plug. Mixture in the squish’ area between the piston and the flat face of the head provides some turbu- lence within the mixture, which helps it burn smoothly. Some bathtub chambers have a shaped projection between the valves, giving the chamber a heart-shaped plan view. This swirls the incoming mixture, helping to atomise it further.

Wedge shape. Both valves are inclined at the same angle in the sloping roof, allowing a slightly smoother inlet tract compared with the vertical- valve bathtub shape. The ‘squish’ action is the same as in a bathtub chamber.

Future trends. If a weaker mixture (less fuel to air) can be burned in the combustion chamber, this reduces the amount of noxious fumes em it- ted from the exhaust. Normally a weak mixture causes over- heating, knocking or pinking and can be difficult to ignite.

Engine designers are solv- ing these problems by using more sophisticated combus- tion chambers. In Japan, where exhaust emission regulations are more stringent than in Europe, Honda uses a two- stage combustion chamber which is fed with mixture in two strengths. A rich mixture is introduced to a small primary chamber round the spark plug, while a separate stream of weak mixture is fed to the main chamber below it. When the plug fires, the rich mixture ignites, and the burning spreads smoothly to the weak mixture in the other chamber.

Honda says this CVCC (Compound Vortex Controlled Combustion) unit easily meets emission regulations without the need to use a complicated exhaust system to filter the exhaust further.