Method for using partial homogeneous charge compression ignition in a diesel internal combustion engine for NOx trap regeneration

Abstract

A method for operating a diesel IC-engine being able to run in a conventional mode and in a partial Homogeneous Charge Compression Ignition (pHCCI) mode. The method comprises the features of switching the pHCCI mode from lean pHCCI to rich pHCCI as a response to a control signal indicating that the NOx-absorbent shall be regenerated. In the rich pHCCI mode, the ratio of recirculated exhaust gases is from 20 to 55%. The rich pHCCI mode further comprises the feature of injecting at least 70% of the fuel to be injected in each working cycle in the main injection whereby the central axis of the fuel spray is directed towards the piston bowl cavity. The method will increase the operation window for rich pHCCI which in turn will have the benefits of less oil dilution and better fuel economy while regenerating the NOx-absorbent.

Description

TECHNICAL FIELD [0001] The invention relates to a method for controlling a diesel internal combustion engine being able to run in a conventional mode and in a partial Homogeneous Charge Compression Ignition mode (pHCCI). The invention further relates to a method for operating the engine in a rich pHCCI-mode. BACKGROUND AND SUMMARY OF THE INVENTION [0002] A major problem for engine manufacturers all over the world is to combine the features of low fuel consumption and low emissions of particulate matter (PM), unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). Generally speaking, there are two different ignition systems for Internal Combustion engines (IC-engines), Spark Ignition (SI) and Compression Ignition (CI) of which the latter is the one generally used for diesel engines. CI-engines, e.g. diesel engines, have an advantage in view of SI-engines regarding the fuel consumption. In order to further reduce the harmful emissions in CI-engines, it has been sug...

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Benefits of technology

[0007] It is further described that the EGR ratio used in the rich pHCCI mode is from 20 to 55%, i.e. 20 to 55% of the exhaust gases are recirculated mixed with air in order to form the gas admitted to the cylinder, the so called combustion gas. This interval of EGR will allow the rich pHCCI to be performed in a wider range than have been achieved before. Hence, this novel way of controlling the engine will increase the possibility to use rich pHCCI for the engine.

[0008] One of the great benefits of this rich pHCCI mode is the reduction of oil dilution associated with the rich regeneration of the NOx-absorbent. As is evident from the claims, the major part of the fuel to be injected is injected in the main injection for each working cycle wherein the central axis of the injected fuel is directed towards the piston bowl cavity. Hence, by using rich pHCCI instead of a rich, conventional mode is it possible to avoid injecting fuel in such a way that it hits the liner of the cylinder which leads to oil dilution problems due to fuel sticking to said liner. Usually when running a diesel engine in a rich mode, there is an early pilot injection and / or one or more late, large post injections. These kinds of injections are usually associated with oil dilution problems. The present idea does not use any large post or pilot injections and avoids or diminishes the problem with oil dilution. As a result of decreased oil dilution, wear of the piston and cylinder will diminish, losses of power due to increased friction between the piston and the cylinder can be avoided and less frequent change of oil will be needed.

[0009] Hence, using the method of the present invention provides the advantage of less oil dilution.

[0012] In another aspect of the invention, the temperature of said gas being admitted to the cylinder, i.e. the temperature of the combustion gas entering the cylinder, during said rich pHCCI mode is in the interval of 60 to 200 degrees Celsius, preferably in the interval of 60 to 130 degrees Celsius. In order to achieve a more homogeneous fuel mixture, it is suggested to use a relatively low temperature of the inlet gas in order to let the injected fuel have time to spread and mix with the combustion gas in the combustion chamber. A lowering of the temperature of the combustion gas will result in a reduction of soot formation which in turn will widen the operation window for rich pHCCI combustion.

[0013] In a further aspect of the invention, the rich pHCCI mode further comprises a pilot and / or a post injection in a working cycle wherein the fuel injected in the pilot and / or post injection vaporises before it reaches a liner of a cylinder wall, if it is directed towards the liner. The use of a small pilot and / or post injection may contribute to stabilise the combustion and widen the operating window in which it is possible to perform rich pHCCI. To avoid that the fuel reaches the liner it is important that these injections are relatively small. It may also help if the injected fuel is atomised so that the fuel may mix with the combustion gas and diffuse into the gas.

[0014] In yet another aspect of the invention there is only one single main injection injected in each working cycle in the rich pHCCI mode. Hence, all fuel injected in a working cycle in said rich pHCCI mode is injected in the main injection. By injecting all the fuel at one time, in the main injection, it can be assured that all the fuel will be directed towards the piston bowl cavity. For a large part of the operating area, this will be the optimal injection strategy for lowest fuel consumption and emissions during the regeneration.

Problems solved by technology

A major problem for engine manufacturers all over the world is to combine the features of low fuel consumption and low emissions of particulate matter (PM), unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx).

In the conventional mode, having heterogeneous charge compression ignition combustion, the burnt gas temperature will be highly heterogeneous with very high local temperatures values creating high NOx emissions.

However, there are still several problems associated with the regeneration according to U.S. Pat. No. 6,276,130.

One major problem is that the suggested mode of operating the engine will have a very small operating window in respect of engine load and speed for fulfilling all the required parameters as set in the patent.

Furthermore, in conventional rich mode, there is always a great risk of oil dilution, i.e. the part of the fuel being injected adheres to the liner of the cylinder and mixes with the oil, when using a rich A / F-mixture.

These kinds of injections are usually associated with oil dilution problems.

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