DE102014222464A1 - Method for operating an internal combustion engine - Google Patents

Abstract

The present invention relates to a method for operating an internal combustion engine, in which water is injected into the combustion air. In order to prevent the NOx conversion in the catalyst from decreasing, it is provided according to the invention that, depending on the amount of water supplied, the set air ratio is shifted towards a rich mixture.

Description

The invention relates to a method for operating an internal combustion engine, in which water is injected into the combustion air.

Such a method is known. The injected into the air intake tract water causes by the applied heat of vaporization an effective intercooler and thereby also achieves an internal cooling of the engine. Due to the colder and thus higher density of combustion air results in an increase in performance. Furthermore, the ignition angle can be set further toward pre-ignition because the cooler combustion air is less prone to knocking. This reduces the exhaust gas temperature. The required for high engine speeds and loads mixture enrichment for component protection can be reduced or eliminated.

However, the supplied water partially dissociates during combustion and thereby brings additional oxygen into the combustion chamber. In particular, the reduced mixture enrichment in the water injection leads to increasing NOx emissions before the catalyst. At the same time, the NOx conversion rate in the catalytic converter decreases and causes high NOx end emissions downstream of the catalytic converter.

It is therefore an object of the present invention to remedy this situation.

This object is achieved in a method of the type mentioned in the present invention that, depending on the amount of water supplied, the set air ratio is shifted towards a rich mixture.

As a result of the method according to the invention, the increase in the NOx emission can be limited while ensuring a high conversion rate in the catalyst.

The dependent claims contain advantageous developments and refinements of the invention.

According to an advantageous embodiment, the desired air ratio is set in a multi-dimensional map depending on the amount of water.

Advantageously, the parameters of the lambda control are influenced in a further characteristic field as a function of the amount of water.

With the proposed measures, concerning the maps, can reduce the required amount of water, limit the NOx raw emissions and ensure a high conversion rate in the catalyst.

Further details, features and advantages of the invention will become apparent from the following description with reference to the drawings. Show it:

1 a graph of HC and NOx mass emissions before the catalyst without water injection and with a 30% water injection,

2 a graph of HC and NOx mass emission after the catalyst without water injection and with a 30% -tigen injection of water,

3 a graph is a graph of HC and NOx emissions before the catalyst over the water rate, and

4 a graph is a graph of HC and NOx emissions downstream of the catalyst versus water rate.

The in the 1 and 2 The diagrams shown show the raw HC emission and the raw NOx emission before and after the catalyst.

The raw emissions before the catalyst are reduced by the water injection, whereas the emissions after the catalyst are higher due to the bad conversion rate with the water injection.

In both cases the same parameters were used for lambda control. At slightly above-stoichiometric air ratios, NOx conversion with water injection breaks down much earlier than without water injection. Accordingly, the control parameters of the lambda control in the case of water injection must be adjusted as a function of the amount of water injected (control constants and setpoint). For this purpose, the various parameters of the lambda control are stored in load- and speed-dependent maps as a function of the amount of water.

The in the 3 and 4 The diagrams shown show the HC emission and the NOx emission respectively before and after the catalyst via the water rate.

The water rate indicates the mass ratio of the injected water mass to the injected fuel mass. In the example shown, a rich mixture with approximately 20% enrichment must be set during operation without water injection in order to comply with the exhaust gas temperature limits. In this case, the NOx production is suppressed by the oxygen deficiency and the NOx emission before the catalyst is relatively low. As the water rate increases, the mixture enrichment is reduced and the NOx emission increases until the air ratio can be set to 1.0. The NOx conversion in the catalytic converter reaches high values in low air conditions and, in combination with the low NOx raw emissions, results in a low NOx end value. As the air ratio increases, the NOx conversion rate initially decreases slightly to drop to 0 on reaching stoichiometric combustion (λ = 1.0), resulting in excessive end-of-NOx emissions. The parameters of the lambda control are not optimized for this condition.

The foregoing description of the present invention is for illustrative purposes only, and not for the purpose of limiting the invention. Various changes and modifications are possible within the scope of the invention without departing from the scope of the invention and its equivalents.

Claims (3)

Method for operating an internal combustion engine, in which water is injected into the combustion air, characterized in that, depending on the amount of water supplied, the set air ratio is shifted towards a rich mixture. A method according to claim 1, characterized in that the desired air ratio is set in a multi-dimensional map depending on the amount of water. A method according to claim 2, characterized in that the parameters of the lambda control are influenced in a further map depending on the amount of water.

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