GB2028953A

GB2028953A - An Internal Combustion Engine with Exhaust Braking

Info

Publication number: GB2028953A
Application number: GB7925705A
Authority: GB
Original assignee: MAN Maschinenfabrik Augsburg Nuernberg AG
Current assignee: MAN AG
Priority date: 1978-07-25
Filing date: 1979-07-24
Publication date: 1980-03-12
Also published as: JPS6321003B2; JPS5517697A; IT1122284B; DE2832542A1; GB2028953B; SU1080751A3; IT7924584A0; HU182525B; SE7906342L; RO80198A; CH641873A5; IN151679B; SE438888B; DD145121A1; FR2433635A1

Abstract

An internal combustion engine comprising a spring-loaded engine valve (13) actuatable by means of a cam (2), a tappet (4) and a push rod (10), the cam (2) having a pre-lift ramp (5) for effecting exhaust braking of the engine. A single piston (6) is slidable in the tappet (4) between a bottom stop (8) and a top stop (7) therein and biased by a spring (9) against the top stop (7) and/or the push rod (10). The tappet (4) has an inlet (14) for hydraulic, control fluid to bias the piston (6) towards the top stop (7). There is also a valve (17) preventing escape of hydraulic fluid from the tappet (4) during exhaust braking. During normal operation of the engine when the pre-lift ramp (5) lifts the tappet (4) the valve (17) is held open and hydraulic control fluid flows from the tappet (4) so that the piston (6) contacts the bottom stop (8) and the engine valve (13) remains closed, whereas during exhaust braking of the engine when the pre-lift ramp (5) lifts the tappet (4) the valve (17) is closed and hydraulic control fluid cannot flow from the tappet (4) so that the pistons (6) remains against the stop (7) and the engine valve (13) opened to effect exhaust braking. <IMAGE>

Description

SPECIFICATION An Internal Combustion Engine with Exhaust Braking This invention relates to an internal combustion engine with exhaust braking.

An internal combustion engine incorporating an exhaust brake is disclosed in the German Preliminary Patent Specification 1 057 385. In that engine, the piston provided in the tappet of a valve train is pressed during normal operation of the engine by the force of a spring against a bottom stop, whereby the tappet attains its normal length which is necessary for opening and closing the associated gas-change valve of the engine. During exhaust brake action, the piston is acted upon by oil subjected to an increased pressure, so that the piston is displaced against the force of the spring upwards to a top stop. The tappet now has an extended length and the gas change valve is held continuously open by a predetermined amount which results in a reduction of the compression work in the associated cylinder of the engine.

Within the piston of the exhaust brake system there is a second piston supported slidably for a vaive-clearance-compensating device which is caused by another spring and by the admittance of oil to contact the push rod or the top stop respectively. While the oil for the clearancecompensating device can be tapped off the lubricating oil circuit of the engine, it is necessary for actuating the system of the exhaust brake system to have a much higher pressure available because this pressure has to overcome the force of the spring acting on the piston and the force of the valve spring.

As a result, there are generally two oil pipes, each provided with a non-return valve leading to the tappet, and also two pumps producing different pressures which, in addition to the complicated tappet configuration, call for a complete and complex control system. Naturally, such a tappet may be supplied with oil from only one pump, but this will involve an increased number of necessary control valves to produce the different pressures which also results in a complex control system.

Obviously the complexity of the tappet configuration and its control enhances its vulnerability. This is true especially for the hydraulic valve-clearance compensation where, due to the increasing leakage losses an accurately defined opening and closing of the valve as the engine speed increases is no longer guarenteed.

Furthermore, it is doubtful whether the valve can actually be held continuously open during exhaust brake operation since the fluid pressure is unlikely to be sufficient to counteract the forces arising to a constant degree over an extended period of time.

An object of the present invention is to provide an internal combustion engine with exhaust braking in which the above-mentioned disadvantages are at least reduced.

The invention provides an internal combustion engine comprising a spring-loaded engine valve actuatable by means of a cam via a tappet and a push rod, the cam having a pre-lift ramp for effecting exhaust braking of the engine, a single piston slidable in the tappet between a bottom stop and a top stop therein and biased by a spring against the top stop and/or the push rod, and inlet in the tappet for hydraulic control fluid to bias the piston towards the top stop, and means preventing escape of hydraulic fluid from the tappet during exhaust braking such that during normal operation of the engine when the pre-lift ramp lifts the tappet hydraulic control fluid flows from the tappet so that the piston contacts the bottom stop and the engine valve remains closed, whereas during exhaust braking of the engine when the pre-lift ramp lifts the tappet hydraulic control fluid cannot flow from the tappet so that the piston remains against the top stop and the engine valve is opened to effect exhaust braking.

Such a tappet is of straightforward design, easy to manufacture and does not call for any maintenance. Since the spring maintains the piston against its top stop or the push rod even without the admittance of a hydraulic control fluid, there is generally no backlash in the complete valve train.

Preferably, the pre-lift is arranged to lift the tappet during a compression stroke of the engine.

In contrast with the above-mentioned, the German Patent Publication 1057 385, the pre-lift ramp or pre-lift cam causes the exhaust valve to open only during the compression stroke during exhaust brake action. A degree of pumping effect causes leakage losses, which depend on the type of control system, to be compensated continually.

Separation of any air bubbles from the control fluid is assisted as is already known by having the piston slidable in a vertical direction. Finally, the damping effect on valve train oscillations should be mentioned which results from the arrangement of the spring and exists also during normal operation of the engine, because during every revolution of the cam the tappet fills with control fluid in the range of the basic cam circle, the control fluid being then displaced again into the circuit for the control fluid.

Preferably, the maximum stroke of the piston corresponds to the rise of the pre-lift ramp of the cam. The control fluid is preferably oil tapped off the engine lubricating circuit.

Preferably, the fluid escape-preventing means comprise a shut-off valve arranged in a controlfluid supply passage and comprising a valve spring and a valve member for preventing the return flow of the control fluid from the tappet, the valve member being holdable during normal operation in an open position by a pin extending into the control fluid supply passage so as to allow return flow of the control fluid. This pin may be formed as the piston rod of a pneumatic piston and cylinder and held in its working position by the force of a spring acting on the piston of the pneumatic cylinder.

In this arrangement, a lubricating oil pump os the engine may be employed to supply the hydraulic control fluid.

Instead of the snut-off valve a plurality of control-fluid supply ducts may be provided leading to respective tappets, the ducts communicating with a common header connected to a pump, preferably a high pressure pump, and incorporating a relief valve to maintain the pressure of the control fluid constant, and a changeover valve for the discharge of the control fluid. Preferably the changeover valve is a solenoid valve.

The latter arrangement affords a number of additional advantages. Thus, for instance, the pump ensures that on closing of the changeover valve the piston provided in the tappet very quickly bears firmly against its top stop and that any leakage losses occurring are immediately made up for so that a clearly defined valve lift is guaranteed. Since the actuating pressure for the piston is positively provided by the pump, the air content in the oil is of no consequence.

Furthermore, any dirt in the oil will not be deposited in high-precision parts but mostly swept out by the leak oil. Moreover, this arrangement necessitates only one pump and one changeover valve for the complete engine irrespective of the number of cylinders so that the equipment becomes straightforward, not susceptible to trouble, and is of low cost.

Considering the power requirement of the pump, this may well amount to some kilowatts, but will arise only during the braking when it is even welcome. During normal engine operation, the pump will deliver against zero pressure through the open changeover valve and no power will be consumed.

Two embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a longitudinal section through a tappet with a control valve, and Figure 2 shows the tappet of Fig. 1 with an alternative control arrangement shown schematically.

In Fig. 1, a tappet 4 operable by a cam 2 and formed with a bore or cavity 3 is in longitudinal sliding engagement in a casing 1. The cam 2 is formed additionally with a pre-lift or a pre-lift cam 5. A piston 6 is arranged in the cavity 3 of the tappet 4 and is slidable between a top stop 7 and a bottom stop 8. In its normal position, the piston 6 is urged by the force of a spring 9 against the top stop 7 or against the bottom end of a push rod 10 which actuates an exhaust valve 13 of an engine cylinder (not shown) via a rocker arm 11 which is shown schematically. The exhaust valve 13 is maintained in a closed position by the force of a valve spring 12.

The cavity 3 of the tappet 4 communicates through one or more ports 14 and an annular groove 1 5 with a control-fluid supply line 1 6 in which is arranged a shutoff valve comprising a ball-shaped valve member 17 and a spring 18. A pin 1 9 extends into the control-fluid supply line 1 6 and is formed as the piston rod of a pneumatic piston 21 and cylinder 20. During normal operation of the engine the pin 1 9 is urged against the valve member 1 7 by a spring 22 acting on the piston 21 of the pneumatic cylinder 20 so that the valve member 17 is held in an open position. The piston 21 can be acted upon against the force of the spring 22 by compressed air supplied by means of a passage 23.A further passage 24 is provided to discharge leakages so that no control fluid can penetrate into the pneumatic cylinder 20.

During normal operation of the engine, the valve member 1 7 is held in its open position by the pin 19, in order words, the control fluid can circulate. The force exerted upon the piston 6 by the control fluid and the spring 9 is considerably smaller than the force of the valve spring 12, so that in passing the pre-lift ramp 5 the piston 6 is forced against the bottom stop 8, i.e. the discharge valve 13 remains closed.

During exhaust brake action, the piston 21 is supplied with compressed air, so that the pin 1 9 moves so as to release the valve member 17, which is then pressed against its seat by the spring 18, thereby preventing return flow of the control fluid. When the pre-lift ramp 5 now reaches the tappet 4, the piston 6 cannot move in the cavity 3. The push rod 10 is therefore immediately lifted, and the exhaust valve 1 3 is opened by a predetermined partial lift. Loss of control fluid by leakage in the piston 6 itself and in the tappet are made up for again by the control fluid supply line 1 6 during the period when the tappet6 runs on the base circle of the cam 2.This is because when the tappet 4 descends so far that the piston 6 can no longer be pushed downwards by the push rod 10, the force of the spring 9 alone will be sufficient to press the piston 6 against the top stop 7. The valve member 17 is lifted off its seat and control fluid is drawn in.

In figure 2, the same parts are denoted by the same numerals as in figure 1 and the arrangement of the tappet is the same. With respect to the control system, the control-fluid supply line 16 of all the tappets 4 of an engine not shown communicate with a common header 25, which is connected through a passage 26 with a.

high-pressure pump 27. A relief valve 28 and a solenoid-operated changeover valve 29 are provided for the header 25.

During normal operation of the engine, the changeover valve 29 is in the position illustrated, and the control fluid can flow off continuously so that no pressure builds up in the passage 26 and in the header 25. As soon as the pre-lift ramp 5 moves under the tappet 4, the piston 6 decends downwards, relative to the tappet and the valve 1 3 does not open. If the changeover valve 29 is moved into its second position, exhaust brake action commences, and a pressure builds up in the header 25 and consequently in all the cavities 3 of the tappets 4. The pressure is determined by the relief valve 28 and is sufficient to hold the pistons 6 against the top stops 7, even when the pre-lift ramp 5 raises the tappet 4.

Generally, it should be noted that during exhaust brake operation, the valve lift is greater than the lift of the pre-lift ramp 5 which means that it may be necessary to provide the pistons of the engine with pockets on the exhaust side to prevent interference. Furthermore, it is necessary to take into account the increased valve lift in selecting the prestressing (Hertz stress at cam nose) and the solid length of the valve spring.

Claims

1. An internal combustion engine comprising a spring-loaded engine valve actuatable by means of a cam via a tappet and a push rod, the cam having a pre-lift ramp for effecting exhaust braking of the engine, a single piston slidable in the tappet between a bottom stop and a top stop therein and biased by a spring against the tip stop and/or the push rod, an inlet in the tappet for hydraulic control fluid to bias the piston towards the top stop, and means preventing escape of hydraulic fluid from the tappet during exhaust braking such that during normal operation of the engine when the pre-lift ramp lifts the tappet hydraulic control fluid flows, from the tappet so that the piston contacts the bottom stop and the engine valve remains closed, whereas during exhaust braking of the engine when the pre-lift ramp lifts the tappet hydraulic control fluid cannot flow from the tappet so that the piston remains against the top stop and the engine valve is opened to effect exhaust braking.

2. An internal combustion engine as claimed in claim 1 when the pre-lift ramp is arranged to lift the tappet during a compression stroke of the engine.

3. An internal combustion engine as claimed in claim 1, or 2, wherein the maximum stroke of the single piston corresponds to the lift of the pre-lift ramp.

4. An internal combustion engine as claimed in claim 1,2 or 3, wherein oil is used as the hydraulic control fluid.

5. An internal combustion engine with exhaust brake claimed in any of one of claims 1 to 4, wherein the said fluid escape preventing means comprise arranged in a control-fluid supply passage and a shut-off valve comprising a valve spring and a valve member for preventing the return flow of the control fluid, from the valve member being holdable during normal operation in an open position by a pin extending into the control fluid supply passage so as to aliow return flow of the control fluid.

6. An internal combustion engine as claimed in claim 5, wherein the pin is displaceable against the bias of a spring by compressed air.

7. An internal combustion engine as claimed in claim 6, wherein the pin is piston rod of a pneumatic piston and cylinder and during normal operation of the engine the return spring acting on the pneumatic piston causes the pin to hold the valve member in the open position.

8. An internal combustion engine as claimed in any of the preceding claims, wherein a lubricating oil pump of the engine is used to deliver the hydraulic control fluid.

9. An internal combustion engine as claimed in any of claims 1 to 4, wherein a plurality of control-fluid supply ducts leading to respective tappets of the engine communicate with a header connected to a pump and incorporating a relief valve maintaining the pressure of the control fluid constant, and a changeover valve for allowing escape of the control fluid during normal operation but preventing escape during exhaust braking.

10. An internal combustion engine as claimed in claim 9, wherein the changeover valve is a solenoid valve.

11. An internal combustion engine as claimed in claim 9 or 10, wherein a high-pressure pump is provided for supplyong of the hudraulic control fluid.

12. An internal combustion engine substantially as herein described with reference to figure 1 or figure 2 of the accompanying drawing.