GB2263941A

GB2263941A - Four-stroke i.c. engine charge intake control.

Info

Publication number: GB2263941A
Application number: GB9302000A
Authority: GB
Inventors: Dieter Scherenberg
Original assignee: Daimler Benz AG; Mercedes Benz AG
Current assignee: Daimler Benz AG
Priority date: 1992-02-06
Filing date: 1993-02-02
Publication date: 1993-08-11
Also published as: ITRM930025A0; ITRM930025A1; IT1261745B; DE4203365C1; GB9302000D0; FR2687190A1; FR2687190B1

Description

2263941 1 Method for controlling the load on a four-stroke internal

combustion engine The invention concerns a method for controlling the load on a mixture- compressing four-stroke internal combustion engine.

A method of this type is known from "Fortschrittsberichte, VDI Reihe 12, No. 74., 1986, Page 192". In this method the internal combustion engine load is no longer controlled by a throttle butterfly located in the induction conduit of the internal combustion engine but by means of a variable valve control system and, in fact, in such a way that the mixture quantity to be introduced into the combustion space is matched to suit the particular load specified by a corresponding change to the lift curve of the inlet valve. Although this makes it possible to reduce the work which has to be performed by the internal combustion engine for the change of charge (there are no longer any throttling losses in the induction conduit), particularly in the low and medium-load range, an increased condensation of fuel in the intake pipe occurs in these load ranges because there is no longer a high vacuum (which promotes fuel evaporation) downstream of a positioned throttle butterfly and this increased condensation has a negative effect on the pollutant emission from the internal combustion engine.

From "SAE Paper 89000111, it is known art to open the exhaust valve briefly for a second time towards the end of the induction stroke in order, by this means, to achieve internal exhaust gas recirculation with the intention of heating the contents of the combustion space in order, for example, to improve the cold-starting behaviour. Because, however, the inlet valve is also open during the period of time when the exhaust valve is open towards the end of the induction stroke (and this occurs over the whole of the load range of the internal combustion engine), there is only a minimum pressure difference between the combustion space and 2 the exhaust gas duct - and this decreases even further with increasing load so that the proportion of exhaust gas which can be recirculated is relatively small. The result of this is that complete evaporation of the fuel cannot be ensured, particularly when the internal combustion engine has not yet reached its operating temperature. The mixture formation is consequently inadequate so that there is a relatively high pollutant emission in this case also. On the other hand, an increase in the second exhaust gas lift in order to increase the quantity of exhaust gas recirculated in the lower part-load range would lead to an excessive quantity of exhaust gas recirculated at higher part load and at full load and this increases the tendency to knock and reduces the power.

The present invention seeks therefore to further develop a method of the type described in the first paragraph in such a way that a reduction in pollutant emission is achieved, particularly in the low and medium partload range, without the disadvantages quoted occurring at upper part load and full load.

According to the invention there is provided a method for controlling the load on a mixture -compressing fourstroke internal combustion engine whose gas exchange is controlled by means of at least one inlet valve and one exhaust valve, in which method the time when the inlet valve closes is advanced in the direction of smaller crank angles with decreasing load, wherein in the low and medium partload range, the exhaust valve is opened briefly when a specified time span has elapsed after the closing of the inlet valve.

Preferably, the time span between the closing of the inlet valve and the brief opening of the exhaust valve is reduced with increasing internal combustion engine load.

Because, at part load, the piston of the internal combustion engine continues to move in the direction of bottom dead centre (BDC) after the closing of the inlet valve, a relatively high vacuum occurs within the combustion 3 space and this becomes larger as the internal combustion engine load becomes smaller. This occurs because the closing time is advanced in the direction of smaller crank angles a (displacement in the "early" direction) with falling load. In accordance with the invention, the exhaust valve is opened briefly during this relatively high vacuum in the combustion space and a relatively large quantity of exhaust gases, which are still hot, can therefore flow back from 1--he exhaust passage into the combustion space. This relatively large quantity of hot exhaust gas causes heating of the contents of the combustion space and also, therefore, of the fuel still present as droplets (which can therefore evaporate very rapidly) so that optimum mixture formation is provided in the combustion chamber. This substantially reduces the ejection of pollutants, in particular the emission of hydrocarbons. The fact that the vacuum in the combustion space occurring in the period between the closing of the inlet valve and the opening of the exhaust valve due to the motion of the piston in the direction of bottom dead centre additionally assists the evaporation of the fuel has an additional positive effect on the mixture formation and, therefore, on the pollutant emission. When the engine has not yet reached its operating temperature, additional fuel consumption advantages arise because better mixture formation makes a reduced mixture enrichment possible.

Embodiments of the invention are now explained in more detail, by way of example, with reference to the drawing, in which:- Fig. 1 shows a diagrammatic representation of the cylinder of a mixture- compressing four-stroke internal combustion engine, Fig. 2 shows the method according to the invention by means of an h=f(a) diagram and Fig. 3 shows a further embodiment example of the method according to the invention by means of an h=f (a) diagram.

Fig. 1 shows a mixture -compressing four-stroke internal 4 combustion engine 1 whose gas exchange is controlled by means of an inlet valve 3 located in an inlet passage 2 and an exhaust valve 5 located in an exhaust passage 4. In this arrangement, the load on the internal combustion engine 1 is not controlled by means of a throttle butterfly located in the induction conduit of the internal combustion engine (as it is in a conventional internal combustion engine) but is controlled by means of an appropriate control of the lift curve of the inlet valve 3. The modification to the lift curve of both the inlet valve 3 and the exhaust valve 5 takes place by means of a respective appropriate apparatus, not shown explicitly in the drawing, by means of which any given lift curve can be achieved for each valve.

Fig. 2 shows an h=f (a) diagram 6 which demonstrates the relationship between the lift h E of the inlet valve 3 and the lift h_,,,. of the exhaust valve 5, on the one hand, and the crank angle a (see also Fig. 1), on the other, over the range covered by the induction stroke. It nay be seen that at full load on the internal combustion engine 1, the inlet valve 3 is controlled in accordance with the lift curve IL, at medium part load in accordance with the lift curve II and in the lower part-load range in accordance with the lift curve III. The time when the inlet valve 3 closes is therefore displaced in the direction of smaller crank angles a (displacement in the "early" direction) with decreasing load. The area respectively enclosed between each of the three curves I, II and III and the abscissa 7 is a measure of the quantity of mixture induced per engine cycle and therefore of the respective internal combustion engine load. The top dead centre position of the piston 9 at the changeover from the exhaust stroke to the induction stroke (overlap TDC) is indicated in Fig. 2 by TDC. BDC indicates the bottom dead centre position of the piston 9 on transition from the induction stroke to the compression stroke. The invention now makes provision for the exhaust valve 5 to open briefly (in the lower and medium part-load range) even during the induction stroke at a specified interval 6 a after the time when the inlet valve 3 closes. This is represented in diagram 6 of Fig. 2 by the dash-line lift curve 8. The interval between the time aj_ and a., when the inlet valve 3 closes, on the one hand, and the point a3 at which the. exhaust valve 5 begins to open is indicated by 6 a-, at low part load and by,6 a 2 at medium part load. In the full-load range (curve I), the brief opening of the exhaust valve 5 (curve 8) occurs in a range in which the inlet valve 3 has not yet closed.

At low part load (curve III) and at medium part load (curve II), a vacuum occurs in the combustion space 10 due to the downwards motion of the piston 9 from a2_ or a2 to a. because there is no valve open between the time a-, or 02 when the inlet valve 3 closes and the time a,, when the exhaust valve 5 opens. If the exhaust valve 5 is now opened at the time a:3, there is a relatively high pressure difference between the combustion space 10 and the exhaust duct- 4; this causes a reverse flow of exhaust gas which is still hot from the exhaust gas duct 4. This hot exhaust gas causes heating of the previously induced mixture and, therefore, evaporation of the fuel still present in droplet form in this phase; this has a positive effect on the mixture formation within the combustion space 10. With increasing load, the intervalA a between the time when the inlet valve 3 closes and the time when the exhaust valve 5 opens becomes continually smaller, so that the vacuum occurring due to the downwards motion of the piston 9 before the exhaust valve 5 opens (and therefore the pressure difference between the exhaust gas duct 4 and the combustion space 10) also becomes continually smaller. The result of this is that the quantity of exhaust gas flowing back into the combustion space 10 during the brief opening of the exhaust valve 5 also becomes smaller. At full load (curve III), finally, the inlet valve 3 is also in an open position during the whole of the period within which the exhaust valve 5 is open (curve 8) so that there is only a comparatively minimal pressure difference still present between the combustion 6 space 10 and the exhaust gas duct 4. The quantity of exhaust gas recirculated during the brief opening of the exhaust valve 5 is therefore negligible in the full-load range. No significant loss of power or sensitivity to pinking is, therefore, to be expected at full load either.

The recirculated quantity of exhaust gas can also be produced by a phase displacement, dependent on load and rotational speed, of the exhaust valve and/or the inlet valve lift- h,2,,, and h-F, respectively. In a further embodiment example, therefore, Fig. 3 represents the lift curve of the exhaust valve 5 during the exhaust stroke by the curve IV, the lift curve of the inlet valve 3 during the induction stroke by the curve V and the lif t curve according to the invention of the exhaust valve 5 during the induction stroke (and following, after an interval, the time a- E:_. when the inlet valve 3 closes) by the curve 11. In this Fig. 3, the bottom dead centre position of the piston 9 on transition from the working stroke to the exhaust stroke is shown by BDC -1, the top dead centre position of the piston 9 on change-over from the exhaust stroke to the induction stroke (overlap TDC) is shown by TDC and the bottom dead centre position of the piston 9 on transition from the induction stroke to the compression stroke is shown by BDC2 The exhaust gas quantities which are recirculated into the combustion space 10 during an engine cycle are determined both by the overlap between the two valve lift curves IV and V (hatched area 12 plus the cross-hatched area 13) and by the interval between the time when the inlet valve 3 closes and the time a_,, ,5 when the exhaust valve 5 opens. If the two lift curves IV and 11 are jointly displaced in phase in the "early" direction (arrow 14 and dash- line curves IV' and 111), it may be seen that both the overlap area of the two curves IV' and V (cross-hatched area 13) and the interval between the time a:E:_,, when the inlet valve 3 closes and the time a,,,e5, when the exhaust valve 5 opens become smaller. There is, therefore, less recirculation of exhaust gas. In the reverse case, not shown explicitly in 7 the drawing for reasons of clarity, i.e. in the case of a phase displacement of the two curves IV and 11 in the "late" direction (arrow 15), both the overlap area of the two curves IV and V and the interval between the time aiE:,, when the inlet valve 3 closes and the time ao,,o when the exhaust valve 5 opens (and in consequence the quantity o-j' exhaust gas recirculated) become larger.

Instead of a phase displacement of the two curves IV and 11, the lift curve V of the inlet valve can also be displaced, a displacement in the "late" direction (arrow 15) leading to a reduction in the overlap area of the two curves IV and V and to a reduction of the interval between the tin, e aE:,-- when the inlet valve 3 closes and the time a2,, -,5 when the exhaust valve 5 opens; this again causes a reduction in the quantity of exhaust gas recirculated. In the reverse case, finally, i.e. in the case of a displacement of the lift curve V of the inlet valve 3 in the "early" direction (arrow 14), there is an increase in the overlap area between the two curves IV and V and an increase in the interval between the time aiE:,-, when the inlet valve 3 closes and the time a_A,5 when the exhaust valve 5 opens. Both therefore cause an increase in the rate of exhaust gas recirculation.

For certain operating ranges, the method according to the invention can also be combined with external exhaust gas recirculation (recirculation via separate exhaust gas recirculation conduit).

8 claims.

1. A method for controlling the load on a mixturecompressing f ourstroke. internal combustion engine whose gas exchange is controlled by means of at least one inlet valve and one exhaust valve, in which method the time when the inlet valve closes is advanced in the direction of smaller crank angles with decreasing load, wherein in the low and medium part-load range, the exhaust valve is opened briefly when a specified time span has elapsed after the closing of the inlet valve.

Claims

2. A method according to Claim 1, wherein the time span between the

closing of the inlet valve and the brief opening of the exhaust valve is reduced with increasing internal combustion engine load.

3. A method f or controlling the load on a mixturecompressing four-stroke internal combustion engine substantially as described herein with reference to and as illustrated in the accompanying drawings.