Mixture formation

Four-stroke cycle of a gasoline engine with external mixture formation : In stroke 1, the piston sucks the mixture generated by the carburetor (left) into the combustion chamber

Mixture formation describes the way in which the fuel-air mixture for an internal combustion engine is generated.

A fundamental distinction is made between the external mixture formation , in which the mixture already takes place outside the combustion chamber , and the internal mixture formation, in which the combustion air is only mixed with fuel inside the combustion chamber ( direct injection ).

External mixture formation

With external mixture formation, the mixture is generated outside the combustion chamber and has time to distribute itself evenly before it is ignited. Gasoline engines with external mixture formation have either a carburetor or an intake manifold injection ; a special mixer replaces the carburettor in gas engines .

For model airplanes very small are diesel engines (a few cm ³ displacement ) with external mixture formation and compression ignition.

Internal mixture formation

Four-stroke cycle for a diesel engine with direct injection (schematic)

With internal mixture formation, only pure air is fed to the engine during the intake stroke and the mixture is only created by direct injection of fuel inside the combustion chamber ( direct injection ) or by means of pre-chamber injection into a swirl chamber with a glow head . In this way, the fuel can be metered very precisely and flushing losses due to valve overlap in the four-stroke engine and generally with two-stroke engines can be avoided. In principle, gas engines with internal mixture formation can also be implemented by supplying the fuel gas via their own inlet valves .

In Otto engines, gasoline direct injection at low pressure takes place at the beginning of compression and then, as usual, electrical spark ignition with spark plug at top dead center . In contrast, the air is heated so much by highly compressing the combustion chamber that the high pressure in the diesel engine with a first injection finely atomized fuel after a short time ( ignition delay ) ignited by itself. With a dynamically controlled fuel supply, the chronological course of the combustion can be controlled very precisely, which allows a constant pressure process with very high compression pressure and thus achieves the best efficiency .

In addition to diesel engines and gasoline engines with direct injection, gas turbines and burners in boiler systems also work primarily with internal mixture formation.

See also

• Stoichiometric fuel ratio
• Quantitative mixture regulation
• Qualitative mixture regulation

literature

• Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden 2005, ISBN 3-528-23933-6