WO1999060259A1

WO1999060259A1 - Arrangement for exhaust gas recirculation and internal combustion engine

Info

Publication number: WO1999060259A1
Application number: PCT/SE1999/000727
Authority: WO
Inventors: Jörgen Mårdberg
Original assignee: Scania Cv Aktiebolag (Publ)
Priority date: 1998-05-04
Filing date: 1999-05-03
Publication date: 1999-11-25
Also published as: SE9801544D0; SE510155C2; SE9801544L

Abstract

An arrangement for exhaust gas feedback in a combustion engine (1) with an ordinary first exhaust manifold (4), while a transfer line (8) is arranged between the engine's exhaust side and its inlet side. The invention is distinguished by the fact that the transfer line (8) starts from a separate second exhaust manifold (3) for EGR gases which is arranged on the exhaust side of the engine and which is connected to at least certain cylinders, while from each of said cylinders an exhaust duct (2) runs to the second exhaust manifold, an EGR exhaust valve (7) is situated in the orifice of the exhaust duct (2) in each of said cylinders, and the EGR exhaust valves (7) are arranged to be controlled so that the pressure in the delivering cylinders during their expansion stroke is used for EGR feedback. The invention also relates to a diesel engine incorporating the arrangement.

Description

Arrangement for exhaust gas recirculation and internal combustion engine

The present invention relates to an arrangement in accordance with the preamble to patent claim 1. It relates also to a supercharged diesel engine incorporating such an arrangement.

STATE OF THE ART

A known practice in combustion engines is to feed exhaust gases back to the engine inlet in order to reduce the nitrogen oxides content of the exhaust. The exhaust gases fed back have the effect of lowering the combustion temperature and hence reducing the proportion of the sulphur in the inlet air which can be converted to nitrogen oxides. In Otto engines, this technique, usually called EGR (exhaust gas recirculation), has become widely used as a relatively simple way of reducing the content of harmful exhaust emissions. In diesel engines, however, this technique has not become so widely used, because diesel engines have particular problems which make it impossible to apply directly to them solutions which apply to Otto engines.

One of these particular problems is that combustion in diesel engines normally takes place with excess air. This results indirectly in relatively large volumes of exhaust gases having to be transferred for a relatively large proportion of engine operating range if the intended function is to be achieved. This problem is exacerbated in the case of engines of the supercharged type in that the pressure in their inlet system is greater than the pressure in their exhaust system for a large proportion of their operating range.

Among known solutions usable in supercharged engines, a distinction may be made between two main principles usually referred to respectively as "short route EGR" and "long route EGR". In short route EGR, the exhaust gases are taken from a point upstream from an exhaust turbine arranged in the exhaust system and are fed back at a point downstream from an inlet air compressor arranged in the inlet system. In long route EGR, the exhaust gases are taken from a point downstream from the exhaust turbine and are fed back at a point upstream from the inlet air compressor. Both principles have advantages and disadvantages. An advantage of the short route solution is the possibility of avoiding fouling of a charge air cooler for the inlet air. A disadvantage of the short route solution is that it requires some form of pressure-increasing arrangement because the exhaust gases are fed back at a point where the pressure is normally higher than the pressure prevailing in the exhaust manifold of the exhaust system.

A known practice is to use a separate supercharging unit to bring about this increase in the pressure of the exhaust gases, as described, for example, in WO96/ 18030 and WO96/ 18031. A disadvantage of those solutions is the need for the extra supercharging unit or some other pressure-increasing device, which makes those solutions both expensive and bulky. The power used for driving these arrangements increases the engine's fuel consumption.

OBJECT OF THE INVENTION

An object of the present invention is to eliminate the problems of the state of the art when using a solution for exhaust gas feedback according to the short route alternative in a supercharged diesel engine. The invention thus aims to make effective exhaust gas feedback possible without using a separate supercharging unit or other pressure- increasing device for the exhaust gases fed back.

According to the invention, this is achieved in an arrangement of the kind mentioned in the introduction by its being provided with the features indicated in the characterising part of patent claim 1.

The invention makes it possible for the pressure which normally prevails in a cylinder during the end of the expansion stroke to be used for transferring exhaust gases to the engine's inlet pipe, which, owing to supercharging, has a relatively high inlet pressure. The need to provide a charging compressor or the like for feeding the EGR gases in is thus eliminated, thereby simplifying and reducing the cost of the installation. Providing a separate exhaust gas duct from the respective cylinders with an EGR exhaust valve arranged in the cylinder makes it possible to use the portion of the expansion phase in which the pressure is sufficient to ensure transmission of EGR gases but the amount of working pressure drawn off is negligible. Simple pipe layout is facilitated by gathering the exhaust gases fed back into a separate exhaust manifold and leading them to the engine inlet via a single transfer line.

Advantageous embodiments of the invention result in further advantages. Arranging for the EGR exhaust valves to be independently controllable makes it possible for exhaust gas feedback to take place independently of the engine's ordinary valves, and for both the quantity and timing of the feedback to be optimised.

Installing the EGR cooler in the transfer line is a simple way of achieving good cooling of the EGR gases, as well as having the effect that no charge air comes into contact with the cooler, contact which would otherwise cause heating of the charge air.

Connecting the transfer line after the charge air cooler means that fouling, obstruction and corrosion can be avoided at that point.

Further features and advantages distinguishing the invention are indicated in the embodiment described below.

DESCRIPTION OF THE DRAWINGS

An embodiment exemplifying the invention will now be described in more detail with reference to the attached drawings, in which:

Fig. 1 depicts schematically an embodiment of the invention in a four-stroke six- cylinder diesel engine, and Fig. 2 depicts part of a pressure/volume diagram for a diesel engine to illustrate the function of the invention.

DESCRIPTION OF AN EMBODIMENT

Fig.1 depicts schematically a combustion engine 1 of piston engine type with six cylinders which are arranged in a straight line, i.e. it is a type of engine usually called six- cylinder in-line. It is a four-stroke diesel engine intended, for example, for a heavy-duty vehicle such a truck or a bus. The various cylinders are provided with separate cylinder heads 15 which are all identical and are provided, in a conventional manner, with at least one inlet valve 5 for supply of combustion air and at least one exhaust valve 6 for removal of exhaust gases arising from combustion. The inlet valves 5 and the exhaust valves 6 are controlled in a conventional manner by an undepicted camshaft. An ordinary first exhaust manifold 4 leads the exhaust gases from the cylinders to a turbine 20 in a turbocharger (supercharging unit) from which the exhaust gases are led on to a silencer and exhaust pipe (not depicted).

Also arranged in accordance with the present invention is a transfer line 8 designed to transfer exhaust gases (EGR gases) from the exhaust side of the cylinders to the inlet side of the engine. The transfer line 8 discharges into an inlet line 21 downstream, as seen in the directional flow of the inlet air, from a charge air cooler 23 and before the tappings to the various cylinders. In addition to the inlet and exhaust valves 5,6, an EGR exhaust valve 7 is also arranged in each cylinder head 15 and has associated with it an exhaust duct 2 which discharges into a separate second exhaust manifold 3 together with further exhaust ducts 2 from other cylinders. The second exhaust manifold 3 is itself connected to the inlet to an EGR valve 18, the outlet of which is connected to an EGR cooler 19 which is itself connected to the transfer line 8. The EGR control valve takes the form of a poppet valve which is controlled under the influence of a control system intended for the purpose on the basis of detected engine parameters. As this control system is of conventional design for EGR exhaust feedback purposes, no further description of it is given here.

The EGR exhaust manifold 3 may be arranged as a separate external manifold. In an alternative embodiment of the invention whereby the engine is provided with a cylinder head common to all the cylinders, the exhaust manifold 3 may be incorporated in the engine's cylinder head, which makes it easy in the latter case to machine the respective ducts and fit the valves.

In this embodiment, the EGR exhaust valves 7 take the form of poppet valves of the same type as are conventionally used for inlet and exhaust valves of combustion engines. These valves 7 are intended to be controlled by separate cams on the engine's ordinary camshaft, which also incorporates, in a conventional manner, cams for controlling the engine's inlet and exhaust valves. In alternative embodiments the valves 7 may instead be controllable separately, by hydraulic, electrical or other known means, under the influence of a control system not described here in more detail.

5 The function of the arrangement described above will now be described with reference to Fig.2, which shows a p-v (pressure- volume) diagram for a diesel engine. In the diagram, the y-axis represents the pressure P in one of the engine's cylinders and the x-axis the momentary cylinder volume V for that cylinder. The curve depicted represents, in an inherently conventional manner, the engine's four strokes or phases, i.e. the induction 10 stroke 13, the compression stroke 11, the expansion stroke 10 and the exhaust stroke 12.

The initial supposition is that combustion has recently taken place in the cylinder and the combustion gases are expanding, which corresponds to a working point moving downwards along the curve section 10. The cylinder's inlet and exhaust valves 5,6 are

15 supposed to be closed, as also the EGR exhaust valve 7 at the end of the respective exhaust duct 2. At the end of the expansion stroke 10, corresponding to a point 9 on the curve, the valve 7 opens to the exhaust duct 2 in the respective cylinder and at approximately the same time the exhaust valve 6 opens so that the exhaust gases can also be led into the exhaust system. The EGR exhaust valve 7 having opened results in

20 exhaust gases flowing into the EGR exhaust manifold 3, an inflow which, owing to the high pressure, may be likened to a pulse wave or a pressure wave.

At the end of the expansion stroke 10 or somewhat thereafter, the EGR exhaust valve 7 will close, represented by the point 16. The diagram shows that the pressure at the point 25 16 is greater than the pressure in the induction stroke 13, thereby consistently ensuring that exhaust gases can be fed back by their own pressure.

The opening of the exhaust valve 6 and the opening of the EGR exhaust valve 7, corresponding to the point 9, take place at a pressure pi which in a conventional modern 30 engine corresponds to a pressure somewhat higher than 9 bar absolute at full load. This pressure pi exceeds the pressure p₂ which prevails during the induction stroke 13 and which is about 3 bar absolute in a modern supercharged diesel engine. The pressure difference between pi and p₂ thus brings it about, with a good margin, that the exhaust gases contained in the EGR exhaust manifold 3 and the transfer line 8 can be transferred to the inlet side of the engine. For the exhaust gas feedback to take place, it is in addition necessary that the EGR valve 18 be in an open position, which it normally adopts when exhaust gas feedback is to take place, under the influence of the control system intended for the purpose.

The diagram also shows that the pressure during the exhaust stroke 12 is smaller than the pressure during the induction stroke 13, which is a consequence of the engine being supercharged. Hence the problem presented in the introduction, namely that the exhaust gases cannot be transferred to the inlet without pressure-increasing treatment.

To make it possible to set the quantity of exhaust gases fed back to the cylinders, the EGR valves 7 may be separately controlled. In cases where more exhaust gases are to be fed back, the EGR valves 7 are controlled so as to open earlier, and when smaller quantities of exhaust gases are to be fed back the EGR valves 7 open later, relative in either case to the situation described above whereby these valves 7 open at approximately the same time as the exhaust valves. It is advantageous, however, for the valves 7 to be controlled so that they close at approximately the same time, in the course of the cycle, independently of the quantity of exhaust gases transferred.

In addition, the control of the quantity of exhaust gases to be fed back is preferably brought about by control of the EGR control valve 18 and by means of an undepicted control system which in a manner known per se incorporates sensors for detecting engine parameters appropriate to the purpose. If the EGR exhaust valves are controlled in a manner dependent on the valves 5 and 6, an EGR control valve is necessary. It is preferable that no EGR transfer takes place during starting of the engine, as this would risk causing oxygen deficiency in the cylinders. Care is taken to ensure that the opening of the EGR control valves takes place successively to avoid causing unnecessary disturbance of the process. Large proportions of EGR gases can be transferred at very low loads and when idling. The possibility of the most effective conceivable engine braking requires the EGR control valves 18 or the EGR exhaust valves 7 being closed, the latter in cases where there are no separate EGR control valves 18. A certain pulsation in the region 22 of EGR gases entering the inlet duct promotes the mixing of EGR gases with pure air. This is achieved automatically if the volumes adopted for the lines 2-3-19-8 are not too large.

It is very important in this context that the EGR gases transferred be properly cooled, which is brought about by the cooler 19.

It is within the scope of the invention for only certain of the cylinders of an engine to contribute to the EGR feedback by being connected to the extra exhaust manifold, e.g. one bank of cylinders in a V-engine. It is also possible to arrange that only certain of the cylinders be supplied with EGR gases.

An important benefit of the invention is that although the EGR quantity drawn off reduces the power of the engine's turbine, this power is recovered in that the EGR gases are fed directly to the inlet side of the engine without being diverted via turbine and compressor. In overall terms, there is therefore no appreciable loss of charging pressure.

The invention works well in all four-stroke engines, not only diesel engines. It is thus considered particularly advantageous for supercharged diesel engines but may also be used with advantage both in unsupercharged engines and in other types of engines.

Claims

PATENT CLAIMS

1. Arrangement for exhaust gas feedback in a combustion engine (1) with a multiplicity of cylinders which on the exhaust side are connected to an ordinary first exhaust manifold (4), while a transfer line (8) is arranged between the exhaust side of the engine and its inlet side, characterised in that the transfer line (8) starts from a separate second exhaust manifold (3) which is arranged on the exhaust side of the engine and which is connected to at least certain cylinders, while from each of said cylinders an exhaust duct (2) runs to the second exhaust manifold (3), an EGR exhaust valve (7) is situated in the inlet of the exhaust duct (2) of each of said cylinders, and the EGR exhaust valve (7) is arranged to be controlled so that the pressure in delivering cylinders during their expansion stroke is used for EGR feedback.

2. Arrangement according to claim 1, characterised in that the EGR exhaust valves (7) are controllable by means of a separate cam on a camshaft, which camshaft is the same as that which controls the engine's ordinary inlet and outlet valves (5,6).

3. Arrangement according to claim 1, characterised in that the EGR exhaust valves (7) are controllable, preferably hydraulically controlled, independently of the engine's inlet and outlet valves (5,6).

4. Arrangement according to any one of claims 1-3, characterised in that an EGR cooler (19) is arranged in the transfer line (8).

5. Arrangement according to any one of claims 1-4, characterised in that the outlet from the transfer line (8) is connected downstream from a charge air cooler (23) arranged in the inlet line (21).

6. Arrangement according to any one of claims 1-5, characterised in that a settable EGR control valve (18) is arranged in the transfer line (8).

7. Arrangement according to claim 6, characterised in that the EGR control valve (18) is arranged to be controlled on the basis of the engine's operating state.

8. Arrangement according to any one of the foregoing claims, characterised in that transfer of EGR gases is limited to only certain of the engine's cylinders.

9. Arrangement according to any one of the foregoing claims, characterised in that the EGR exhaust manifold (2,3) takes the form of a separate manifold.

10. Supercharged diesel engine, characterised in that it incorporates an arrangement according to any one of the foregoing claims.