Diesel Engine Combustion
The preferred drive engines for motor vehicles are based on combustion engines. They utilize the oxygen in the combustion air to convert the fuel-based chemical energy that predominantly consists of hydrocarbons into heat, which in turn is transferred to
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Diesel Engine Combustion Klaus B. Binder
3.1
Mixture Formation and Combustion
3.1.1
Process Characteristics
The preferred drive engines for motor vehicles are based on combustion engines. They utilize the oxygen in the combustion air to convert the fuel-based chemical energy that predominantly consists of hydrocarbons into heat, which in turn is transferred to the engine’s working medium. The pressure in the working medium rises and, by exploiting the expansion, can be converted into piston motion and thus into mechanical work. The replacement of the working medium, also designated as a working gas, after expansion and combustion takes place inside the engine’s combustion chamber is referred to as ‘‘open process control with internal combustion’’ [3-1]. It applies to both gasoline engines and diesel engines. By contrast, a Stirling engine, for example, is described as an engine with closed process control and external combustion. In conventional gasoline engines, the air/fuel mixture forms in the intake manifold. A predominantly homogeneous mixture forms during the intake and compression cycle, which is ignited by a spark plug. This combustion system is also characterized by ‘‘external mixture formation’’, a homogeneous mixture and spark ignition. Starting at the spark plug, energy is released as the flame propagates and is therefore proportional to the surface area of the flame front. The flame speed depends on the fuel, the mixture temperature and the air/fuel ratio. The rate of combustion is additionally influenced by the surface area of the flame front. ‘‘Flame folding’’ induced by turbulences in the mixture causes it to increase with the engine speed. Flows of the mixture caused by the intake process and compression as well as combustion itself are a significant factor influencing flame folding. The fuel must be ignition resistant (detonation resistant) to prevent auto-ignition or premature ignition. The compression ratio is limited by ‘‘knocking’’ combustion or premature
K.B. Binder (*) Deizisau, Germany e-mail: [email protected]
ignition. In knocking combustion, conditions for ignition are obtained in the entire mixture not yet reached by the flame, the so-called ‘‘end gas’’. The highly compressed and therefore energy rich mixture burns nearly isochronously without controlled flame propagation. This produces steep pressure gradients with characteristic pressure oscillations and causes very high local thermal and mechanical stress of components. Longer operation during knocking combustion results in complete engine failure and must therefore be strictly avoided. Limited compression, necessary load control systems (quantity or throttle control) and limited supercharging capability diminish the efficiency of the process with external mixture formation and spark ignition. However, this process does not have any fuel induced particulate emission since no regions with a rich mixture appear in the combustion chamber because operation is homogeneous when l = 1. Modern gasoline engines also operate wi
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