EP1279802A1 - Combustion control device for an internal combustion engine - Google Patents

Abstract

Curves (A,A') show the pressure and energy release against crankshaft angle for a diesel/air/recirculated exhaust fuel mixture which has about 70% exhaust gas. If a smaller proportion (60%) of recirculated exhaust gas is used and 1-8 cubic mm of fuel is injected at about 15 degrees before top dead center the slow oxidation period is extended (OL2) and the pressure and energy curves (B,B') are almost identical to the previous results. An Independent claim is also included for equipment to control auto ignition by fuel injection during slow oxidation phase.

Description

The present invention relates to a method and a device for controlling combustion for an internal combustion engine.

It relates more particularly to the combustion of a homogeneous mixture of fuel with the intake air or with a mixture of air and recirculated exhaust gases (EGR).

The reduction of pollutant emissions generated by internal combustion engines combined with a reduction in consumption is a constant concern of developers of internal combustion engines.

In new generations of engines, provision is made, for low and medium loads, to replace the combustion of the fuel mixture, conventionally ignited either by compression or by a spark plug, by combustion of the ignition type by compression of a homogeneous charge.

This is known in particular for diesel engines under the English term "Homogeneous Charge Compression Ignition" (abbreviated to HCCI) and for gasoline engines for a combustion mode called "Controlled Auto Ignition" (in English "Controlled Auto Ignition") CAI).

This combustion is achieved when the homogeneous mixture of fuel, air and possibly recirculated exhaust gases has reached a certain temperature threshold which promotes its combustion by self-ignition (or self-ignition).

Generally, this type of combustion is characterized by a phase of slow oxidation of the fuel mixture preceding the combustion of this mixture.

Thanks to this kind of combustion, it is possible to greatly reduce the emissions of nitrogen oxides (NOx) and particles emitted by the engine.

However, this combustion has a number of significant drawbacks.

Indeed, it is necessary to obtain a good timing of the combustion, that is to say to determine the exact moment when the fuel mixture will self-ignite, so that this combustion takes place in particular when the position of the piston has reached its adequate position at the moment when the fuel mixture has all the physico-chemical characteristics required to self-ignite.

There are many ways to do this, such as admitting water vapor or using recirculated exhaust gases in the combustion chamber.
The most often used means to obtain the desired timing is to admit or retain in the combustion chamber of the exhaust gases from the engine, called recirculated exhaust gases, with a high rate to stall the combustion at the moment wish.
These recirculated exhaust gases make it possible to “calm” combustion, because they reduce the oxygen (O ₂ ) content of the fuel mixture.

However, these high rates of recirculated exhaust gases lead to high richnesses of the fuel mixtures, richnesses which approach stoichiometry, which further reduces the operating range of the engine. Indeed, beyond a richness greater than 0.96 the fuel mixture no longer contains enough oxygen to burn properly.
In addition, because of this high wealth, emissions of pollutants such as carbon monoxide (CO), unburnt hydrocarbons (HC) and smoke are of high amplitudes.

The present invention proposes to remedy the drawbacks mentioned above by proposing a method of controlling the combustion which makes it possible to obtain the desired timing of this combustion.

To this end, a method of controlling the combustion by self-ignition of a homogeneous fuel mixture for an internal combustion engine comprising a combustion chamber inside which is carried out a phase of slow oxidation of said mixture beforehand. of the combustion of this mixture, is characterized in that a specific quantity of fuel is introduced into the combustion chamber during the slow oxidation phase of the fuel mixture to lengthen the duration of this phase.

Advantageously, the determined quantity of fuel can be introduced during at least one fuel injection.

Preferably, a quantity of fuel less than or equal to 10 mm ³ can be introduced.

This quantity of fuel introduced can be between 1 and 8 mm ³ .

Preferably, a fuel mixture comprising fuel, air and recirculated exhaust gases can be produced in the combustion chamber.

A fuel mixture can be produced with a maximum rate of recirculated exhaust gases of the order of 0.7.

The fuel introduced can be of the petrol or diesel type.

The invention also relates to a device for controlling the combustion by self-ignition of a homogeneous fuel mixture for an internal combustion engine comprising a combustion chamber inside which is carried out a phase of slow oxidation of said mixture preceding combustion of the fuel mixture, characterized in that it comprises means for controlling the injection of a determined quantity of fuel during the slow oxidation phase of this mixture.

The control means can be the engine computer controlling the fuel injectors.

The other characteristics and advantages of the invention will emerge on reading the description which follows and to which is appended the single figure which shows the pressure and energy release curves present in the combustion chamber as a function of the angle of the crankshaft.

In this figure, curves A and B show the evolution of the pressure P (in bars) in a combustion chamber of an internal combustion engine of diesel type, as a function of the angle of the crankshaft V (in degrees) , and curves A ′ and B ′ the evolution of the release of energy E inside this chamber as a function of the crankshaft angle and this respectively without and with the method according to the invention.

The combustion chamber is generally delimited by the upper face of a piston, the wall of a cylinder inside which this piston slides, in a linear reciprocating movement, and the internal face of a cylinder head closing the cylinder.

For reasons of simplification, it is mentioned in the description of the angle of the crankshaft but it is clear that this angle corresponds to a precise position of the piston which moves in the cylinder under the effect of a rod-crank system that the crankshaft controls.

As can be seen in the single figure, curve A (dotted line) shows the evolution of the pressure prevailing in the combustion chamber with a fuel mixture with a richness of the order of 0.96 and comprising diesel fuel, air and a recirculated exhaust rate corresponding to approximately 70% of the total mixture.
Around 260 ° of crankshaft angle, the fuel is introduced into the combustion chamber by a first injection then, around 320 ° of crankshaft angle, by a second injection to obtain, at approximately 360 °, which corresponds to the high compression dead center (TDC), the almost homogeneous mixture between fuel, air and recirculated gases. At the same time, the pressure in the combustion chamber will increase substantially linearly to reach a maximum in the vicinity of the top dead center. From this position, this pressure will decrease to substantially reach atmospheric pressure around 420 ° of crankshaft angle.
Simultaneously and with reference to curve A '(dotted line), the release of energy in this combustion chamber will be substantially zero at the end of the second injection (at approximately 320 ° of crankshaft angle) up to around 340 ° crankshaft angle. Then, from this point, the homogeneous fuel mixture, under the effect in particular of compression, undergoes a slow oxidation phase which begins at point O1 and lasts for a certain period (indicated OL1 in the figure) while preceding the start of combustion of this fuel mixture (indicated by arrow C in the figure). This combustion results in a strong release of energy up to around 360 ° of crankshaft angle (TDC), then the energy stored in the combustion chamber will decrease until reaching an almost zero value around 400 ° crankshaft angle.

It is clear that if one wishes to offset the moment C when the combustion of the fuel mixture will start so as to improve the efficiency of the engine, the most used solution consists in increasing the rate of recirculated exhaust gases present in the chamber. combustion and therefore emissions of pollutants such as carbon oxides will increase rapidly.

To obtain a desired setting while reducing pollutant emissions, the applicant has planned to introduce into the combustion chamber, during the slow oxidation phase of the fuel mixture and at a determined time in this phase, a determined quantity of fuel .

This fuel is introduced either by a single injection or by a multitude of injections.

This late introduction of fuel thus makes it possible to absorb part of the energy released into the combustion chamber by the fuel mixture in the slow oxidation phase, this absorbed energy being used for vaporization of the fuel injected late.
This absorbed energy is then reconstituted during the continuation of the process of slow oxidation of the fuel mixture by causing an extension of the duration of this slow oxidation and this, until reaching the necessary characteristics (temperature in particular) to start the combustion of the mixture carbide.
Thanks to this, the start of the combustion of the fuel mixture will be offset by the lengthening of the duration of the slow oxidation.

By way of example, the applicant has carried out tests with a known fuel mixture combustion method (curves A and A 'in dotted lines) and with the method according to the invention (curves B and B' in solid lines on the figure).

As already mentioned, the fuel mixture of curves A and A ′ contains around 70% of recirculated exhaust gases and has a richness of the order of 0.96. After combustion of this mixture, the emissions of carbon oxides (CO) were of the order of 40g / kWh.

During these tests, it was planned to significantly reduce the rate of recirculated exhaust gases (rate in the vicinity of 60%) of the fuel mixture, and this resulted in the advantage of significantly decreasing consequent the richness of this mixture (of the order of 0.91) which, thanks to the process of the applicant, gave as results the curves B and B '.

Normally, due to the decrease in the rate of recirculated gases and consequently by the presence in greater quantity of oxygen, the slow oxidation phase of the fuel mixture begins earlier (around 335 ° of crankshaft angle) and the start of combustion generally occurs after an angular displacement of the crankshaft of about 15 °, which brings the start of combustion in the chamber to around 350 °. During the combustion process, the maximum peak of energy release is obtained when the piston has not reached its top dead center position, which causes a disturbance of the engine, in particular in terms of efficiency.

According to the process used by the applicant and with reference to curve B ', the start of the slow oxidation of the fuel mixture also takes place early (point O2) around 335 ° of crankshaft angle but, thanks at the introduction of a determined quantity of fuel into the combustion chamber, the duration of the slow oxidation period is lengthened (indicated OL2 in the figure) to end at point C of the start of combustion of the fuel mixture identical to that of curve A '.

Generally, this amount of fuel introduced late is less than or equal to 50% of the total amount of fuel introduced and preferably is between 10 to 40% of this total amount.
The total quantity as mentioned includes or the fuel injection carried out beforehand in the slow oxidation phase to obtain the homogeneous fuel mixture and the or the injection made during the slow oxidation phase.

More specifically, it is planned to introduce a quantity of fuel less than or equal to 10 mm ³ and preferably between 1 and 8 mm ³ .

For example, during tests carried out by the applicant, on a total quantity of carbide injected of 8.6 mm ³ , the quantity of fuel introduced (in two injections) during the slow oxidation phase is 3 mm ³ or approximately 37%.
For these tests, the applicant injected the fuel during the slow oxidation phase at a crankshaft angle corresponding to approximately -15 ° relative to the top dead center.

As visible on curve B 'in the figure, the quantity of fuel injected has the consequence of absorbing a quantity of energy (shown by an arrow D in the figure) generated by the slow oxidation of the fuel mixture and then the energy absorbed is reconstituted during the continuation of this phase until the fuel mixture has obtained all the characteristics so that its combustion takes place at point C.

From this point, the release of energy is almost identical to that of the fuel mixture described in relation to the curve A '.

In addition, as can be seen in the figure and with reference to curve B, the latter substantially follows the evolution of curve A and thus makes it possible to obtain the same pressure characteristics prevailing in the combustion chamber.

Thus, thanks to the invention, the same pressure and energy release characteristics can be obtained with the same crankshaft angles as a mixture with a high rate of recirculated gases and that with a fuel mixture containing a lower rate of gas. exhaust recirculated and with a lower richness of the mixture.
In addition, thanks to the applicant's process, carbon monoxide (CO) emissions have been greatly reduced with an emission of around 29g / kWh.

Of course, the present invention can be applied to fuel mixtures with a fuel of gasoline or diesel type.

Thus, for a mixture of petrol, air and recirculated gases, the start of combustion by self-ignition is preceded, as in the examples described above, by a slow oxidation phase of this mixture.

Therefore, one can introduce a determined amount of gasoline (in one or more injections) to lengthen this slow oxidation phase and thus stall the start of combustion at the desired time.

Preferably, this process applies for low or medium engine loads and, for heavy loads, conventional combustion processes will be used.

The invention as described above will be applied to an internal combustion engine which will include the control means necessary to contain the parameters of the late injection (time of the late fuel injection, quantity of fuel, number of injections, ...) and order the associated fuel injectors that this engine usually contains.

Advantageously, the engine computer which most internal combustion engines include will contain the parameters and control the fuel injectors accordingly.

Claims (10)

Method for controlling the combustion by self-ignition of a homogeneous fuel mixture for an internal combustion engine comprising a combustion chamber inside which a phase of slow oxidation of said mixture is carried out before the combustion of this mixture, characterized in that a specific quantity of fuel is introduced into the combustion chamber during the slow oxidation phase of the fuel mixture in order to lengthen the duration of this phase. Method according to claim 1, characterized in that the determined quantity of fuel is introduced during at least one fuel injection. Method according to claim 1 or 2, characterized in that a quantity of fuel less than or equal to 10 mm ^{3 is introduced} . Method according to claim 3, characterized in that a quantity of fuel between 1 and 8 mm ^{3 is introduced} . Method according to one of the preceding claims, characterized in that a fuel mixture comprising fuel, air and recirculated exhaust gases is produced in the combustion chamber. Method according to claim 5, characterized in that a fuel mixture is produced with a maximum rate of recirculated exhaust gases of the order of 0.7. Method according to one of the preceding claims, characterized in that gasoline-type fuel is introduced. Method according to one of claims 1 to 6, characterized in that diesel fuel is introduced. Device for controlling the combustion by self-ignition of a homogeneous fuel mixture for an internal combustion engine comprising a combustion chamber inside which is carried out a phase of slow oxidation of said mixture preceding the combustion of the fuel mixture, characterized in that it comprises means for controlling the injection of a determined quantity of fuel during the slow oxidation phase of this mixture. Device according to claim 9, characterized the control means are the engine computer controlling the fuel injectors.

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