Combustion chambers

A combustion chamber is that part of an internal combustion engine (ICE) in which the fuel/air mix is burned.

ICEs typically comprise reciprocating piston engines, rotary engines, gas turbines and jet turbines.

The combustion process increases the internal energy of a gas, which translates into an increase in temperature, pressure, or volume depending on the configuration. In an enclosure, for example the cylinder of a reciprocating engine, the volume is controlled and the combustion creates an increase in pressure. In a continuous flow system, for example a jet engine combustor, the pressure is controlled and the combustion creates an increase in volume. This increase in pressure or volume can be used to do work, for example, to move a piston on a crankshaft or a turbine disc in a gas turbine. If the gas velocity changes, thrust is produced, such as in the nozzle of a rocket engine.

At top dead centre the pistons of a petrol engine are flush (or nearly flush) with the top of the cylinder block. The combustion chamber may be a recess either in the cylinder head, or in the top of the piston. A design with the combustion chamber in the piston is called a Heron head, where the head is machined flat but the pistons are dished. The Heron head has proved even more thermodynamically efficient than the hemispherical head. Intake valves permit the inflow of a fuel air mix; and exhaust valves allow burnt gases to be scavenged.

Various shapes of combustion chamber have been used, such as: L-head (or flathead) for side-valve engines; "bathtub", "hemispherical", and "wedge" for overhead valve engines; and "pent-roof" for engines having 3, 4 or 5 valves per cylinder. The shape of the chamber has a marked effect on power output, efficiency and emissions; the designer's objectives are to burn all of the mixture as completely as possible while avoiding excessive temperatures (which create NOx). This is best achieved with a compact rather than elongated chamber.

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