Why is the amount of fuel increasing

Fuel injection

Pre-chamber injection and vortex chamber injection

The tried and tested principle of pre-chamber injection results in relatively smooth engine running, but it is not optimal in terms of energy.

In the case of diesel engines, intake manifold injection is out of the question, as injection may only take place when combustion is to begin. Instead, the so-called Pre-chamber injection where the diesel fuel is injected at moderate pressure into an antechamber connected to the combustion chamber. (This is also where the glow plug required for a cold start is located.) Due to the lack of oxygen, only partial combustion takes place in the antechamber, which presses the resulting gases into the actual combustion chamber. Further combustion then takes place there.

The disadvantage of the antechamber principle is that the gases lose a relatively large amount of heat through contact with the wall of the antechamber. This reduces the efficiency of the engine. In addition, the slowed combustion when operating at high speeds is disadvantageous.

A variant of the prechamber injection is the vortex chamber injection, where the air vortices that arise are used in a targeted manner.

Pre-chamber injection and similar concepts are rarely used in car engines; they have been replaced by direct injection.

Direct injection

Modern common rail diesel engines are more efficient and powerful than traditional pre-chamber engines, and their smoothness can be quite similar.

Compared to the pre-chamber injection is at least in terms of the efficiency Direct injection cheaper, where the fuel is injected directly into the actual combustion chamber (often into a recess in the piston). Here, a significantly higher injection pressure (today often over 1000 bar or even over 2000 bar) is used in conjunction with very small nozzle diameters in order to atomize the fuel into very small droplets and thus the mixture formation in the very short time available (order of magnitude of about one millisecond at high speeds). At least in the case of simpler systems, the combustion tends to take place faster and thus harder, which can result in increased engine noise (“nailing”). However, this can be greatly reduced with a multi-stage injection, as is possible, for example, with the common rail system. For example, a small amount of fuel can first be introduced as part of the pre-injection so that the combustion begins gently. The main injection takes place shortly afterwards. In addition, a post-injection can take place, in particular if the exhaust gas temperature is to be increased briefly for the purpose of regenerating a particle filter.

One challenge is, on the one hand, to achieve a very fine atomization and precise metering even with low fuel consumption when idling, and, on the other hand, to inject the required high amount of fuel within a sufficiently short time at full load. For this reason, injection nozzles with a variable injection pattern through a variable number of active nozzle holes are now being developed.

A high exhaust gas quality in the diesel engine still requires extra-engine measures.

Compared to the previously common pre-chamber engines, the introduction of direct injection initially worsened the exhaust gas quality, particularly with regard to nitrogen oxide emissions. However, through the systematic optimization of the injection, significant improvements were then achieved, and at the same time there are advances in exhaust gas aftertreatment, which again reduce the importance of raw emissions. An attempt was initially made to reduce particle emissions and nitrogen oxide emissions by optimizing diesel injection and other so-called internal engine measures to such an extent that particle filters and catalytic converters are no longer required. However, it turned out to be impossible to comply with the most modern emission limit values ​​(e.g. according to Euro 6) for road vehicles in this way. Since external measures are essential anyway, the fuel injection can also be optimized primarily with regard to the energy efficiency of the engine and the exhaust gas quality can be ensured with the aid of external measures. However, the raw emissions regarding particles should not be too high either, in order not to require frequent regeneration of the particle filter.

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See also: fuel, carburetor, combustion engine, gasoline engine, lean-burn engine, stratified charge, diesel engine, cold start
as well as other articles in the categories of vehicles, engines and power plants