WO2000071881A1

WO2000071881A1 - Internal combustion engine

Info

Publication number: WO2000071881A1
Application number: PCT/GB2000/001693
Authority: WO
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Global Technologies, Inc.
Priority date: 1999-05-22
Filing date: 2000-05-03
Publication date: 2000-11-30
Also published as: GB9911886D0; GB2350400A

Abstract

A reciprocating internal combustion engine is described wherein each cylinder has an exhaust valve (104) and two intake valves (112), (122) connected to separate intake ports (111), (121). A variable valve timing system allows the overlap period between the intake and exhaust events to be extended, and means are provided for ensuring that the resulting internal EGR gases are stored significantly more in one of the intake ports than in the other. The EGR gases are directed preferentially into one (111) of the intake ports either by arranging for its intake valve (112) to open earlier than the other intake valve (122) and/or by placing an obstruction (131) to back flow in the other intake port (121).

Description

Internal Combustion Engine

Field of the invention

The present invention relates to an internal combustion engine that operates with exhaust gas recirculation and with a stratified charge.

Background of the invention

It is known to use exhaust gas recirculation (EGR) as a means of reducing engine load in an internal combustion engine thereby reducing the need to throttle the engine intake system. This reduces the pumping work associated with intake throttling and improves fuel economy.

EGR may be achieved externally by an EGR pipe connecting the exhaust and intake manifolds of the engine. Alternatively, EGR may be achieved internally by increasing the valve overlap period when the exhaust and intakes valves are open simultaneously such that more exhaust gases are retained in the combustion chamber. In either case, the EGR gases are mixed with the intake charge to form a substantially homogenous combustible mixture which burns more slowly with increased EGR concentration until an engine combustion stability threshold is reached.

In order that the engine may tolerate a higher concentration of EGR and achieve better fuel economy at low loads, it is desirable for the EGR gases to be introduced into the engine in a stratified manner without mixing too much with the intake charge. In this case the EGR gases can occupy a substantial proportion of the cylinder volume, displacing the intake charge without significantly diluting it, thereby resulting in further reduction in pumping work while maintaining good combustion stability. A known example of stratified EGR that relies upon external EGR is described in US-A-5 , 379, 743.

Object of the invention

The present invention seeks to achieve stratified EGR in an internal combustion engine while avoiding the need for an external EGR system, which adds to complexity and cost.

Summary of the invention

According to the present invention, there is provided a reciprocating internal combustion engine, wherein each cylinder has at least one exhaust valve and two intake valves connected to separate intake ports, the engine having a variable valve timing system for allowing the overlap period between the intake and exhaust events to be extended, and means for ensuring that the resulting internal EGR gases are stored significantly more in one of the intake ports than in the other.

In the invention, the internal EGR gases, that is to say the back-flow of exhaust gases from the engine cylinder into the intake ports during the extended valve overlap period, are temporarily preferentially stored in one of the intake ports and are drawn back into the engine cylinder during the immediately following intake stroke through the same intake port while fresh intake charge is drawn into the engine cylinder through the other intake port. As in US-A- 5,379,743, the intake ports are designed to ensure that the exhaust gases and the fresh intake charge enter the engine cylinder separately and are circulated within the cylinder in substantially separate layers thereby maintaining a stratified EGR until the instant of ignition.

Ensuring that the internal EGR gases flow preferentially into one of the intake ports can be effected by advancing the opening timing of only one of the intake valves by suitable operation of the valve timing system. In a variable event valve timing system, this can be accomplished by extending the duration of the event of only one of the intake valves so that it opens earlier than the other valve but closes at the same time. In a variable phase valve timing system that can only vary the phase of the cams relative to one another without changing the event duration, this can be accomplished by simultaneous advancing two intake cams that have different opening timings to one another or by advancing only one intake cam to extend the overlap period of its associated intake valve without modifying the valve overlap duration of the other intake valve .

In addition to or instead of varying the timing of one or more of the intake valves as described above, internal EGR may be directed preferentially to one of the intake ports by placing an obstruction to exhaust back-flow in the other intake port. Such an obstruction may be formed by a one-way valve that permits the intake air to reach the intake port but prevents exhaust gases flowing back up the intake port.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a schematic plan view of a cylinder head having two intake valves and an exhaust valve,

Figures 2A to 2D are valve timing diagrams of different embodiments of the invention, and

Figures 3 is a schematic plan view similar to that of Figure 1 showing an alternative embodiment of the invention in which a one-way valve is placed in one of the intake ports . Detailed description of the preferred embodiments

Figure 1 shows the plan view of a cylinder head 100 having two intake valves 112 and 122 connected to separate intake ports 111 and 121 respectively and an exhaust valve 104. The engine illustrated also has two spark plugs 114 124 per cylinder but it is alternatively possible to provide a single spark plug. It should also be mentioned that it is alternatively possible to provide two exhaust valves in each cylinder.

As will be explained by reference to Figure 2, steps are taken to fill one of the intake ports 111 with internal EGR gases at the end of the preceding cycle, these gases being shown as the shaded area in Figure 1. The drawing shows the gas flow during the subsequent intake stroke in which the internal EGR gases stored in the intake port 111 and the fresh intake charge entering through intake port 121 remain separate within the combustion chamber to provide two stratified regions 113 and 123. The spark plug 124 in the region 123 is arranged within the fresh charge the ignitability of which is not reduced by being mixed with EGR gases . In this way reliable combustion can be ensured despite high levels of internal exhaust gas recirculation.

The present invention is concerned with the preferential storage of EGR gases internally in one of the intake ports 111 to enable the above described charge stratification to be achieved. In the embodiment of Figure 1 such preferential storage of EGR gases in the intake port 111 is achieved by advancing the opening timing of the intake valve 112 in relation to the intake valve 122 so that only the opening event of the intake valve 112 overlaps that of the exhaust valve 104 to any significant extent. Various possible valve timings are shown in Figure 2. Figure 2A shows the valve timing of a conventional engine during normal operation. The exhaust valve closes shortly after TDC (top-dead-centre) and the two intake valves 112 and 1222 commence to open at the same time as one another shortly before TDC (the events of two valves are shown slightly staggered so that they can be distinguished from one another in Figure 2A) . Around TDC there is a short valve overlap period in which EGR gases may flow back into the intake ports and be stored there temporarily in equal proportion. Such operation does not achieve stratified EGR.

The manner in which internal EGR gases can be directed preferentially into one intake port will depend on the type of variable valve timing system in use and the type of valve train.

The timing diagram of Figure 2B can be achieved with two camshafts having cams of fixed profile rotating at constant speed. One of the camshafts carries cams that operate the exhaust valve 104 and the intake valve 122, while the other camshaft carries a cam that operates only the intake valve 112. By advancing the timing of the second camshaft using any known phase change mechanism, the opening event of the intake valve 112 can be advanced to increase the overlap with the exhaust valve 104 preferentially.

The timing diagram of Figure 2C can be achieved using a valve train having the same configuration as that described above but in which the second camshaft can be controlled not only to change the phase of the event of the intake valve 112 but also to prolong its duration. In this case the opening timing of the intake valve 112 is advanced without a corresponding advance in its closing timing.

The timing diagrams of Figures 2B and 2C can alternatively be achieved using valve trains in which the two intake valves are operated by cams on the same camshaft while the exhaust valve is operated by a cam on a separate camshaft. In this case the intake valve opening times are always staggered from one another but by advancing the two opening times together the extent of the valve overlap for the intake valve 112 will always be much greater than the overlap of the other intake valve 122 with the exhaust valve 104.

A further possibility is shown in Figure 2D in which the overlap period for both intake valves is increased (by mounting the cams of both the intake valves 112 and 122 on the same camshaft and advancing the timing of that camshaft in relation to the exhaust camshaft) . This will increase internal EGR but will not preferentially direct the EGR into one of the intake ports. The differentiation between the intake ports is instead achieved by an arrangement such as that shown in Figure 3. Figure 3 is essentially the same as Figure 1 and differs from it only in that a one-way valve 131 has been placed in the intake port 121 close to the intake valve 122 to act as an obstruction to reverse gas flow. The one-way valve 131 does not interfere significantly with the normal aspiration of the engine but closes to prevent internal EGR gases from entering the intake port 121. The one-way valve 131 can be constructed in a known manner as a reed valve acted upon by a weak spring and opened automatically by the incoming air flow while causing a minimal pressure drop.

Systems for varying the phase of cams and camshafts relative to one another are well known and documented in the art and it is now therefore necessary to describe such systems within the context of the present invention. The manner of controlling such known systems to implement the present invention will be fully understood from the timing diagrams shown in Figures 2A to 2D.

Claims

1. A reciprocating internal combustion engine, wherein each cylinder has at least one exhaust valve (104) and two intake valves (112, 122) connected to separate intake ports (111, 121), the engine having a variable valve timing system for allowing the overlap period between the intake and exhaust events to be extended, characterised by means for ensuring that the resulting internal EGR gases are stored significantly more in one of the intake ports (111) than in the other (121) .

2. An engine as claimed in claim 1, wherein in order to ensure that the internal EGR gases flow are stored preferentially in one of the intake ports, the variable valve timing system is operative to advance the opening timing of only one of the intake valves to prolong the overlap of its event with the exhaust event without advancing the opening timing of the other intake valve.

3. An engine as claimed in claim 1, wherein in order to ensure that the internal EGR gases flow are stored preferentially in one of the intake ports, the intake valves are phased to open one after the other and are operated by cams arranged on the same camshaft, the variable valve timing system acting on the latter camshaft to advance the opening events of both intake valves in relation to the exhaust event.

4. An engine as claimed in claim 1, wherein in order to ensure that the internal EGR gases flow are stored preferentially in one of the intake ports, the variable valve timing system is operative to prolong the event duration of only one of the intake valves by advancing the opening timing of the intake valve to prolong the overlap of its event with the exhaust event without correspondingly advancing the closing timing of the intake valve.

5. An engine as claimed in any preceding claim, wherein, in order to ensure that the internal EGR gases flow are stored preferentially in one of the intake ports, an obstruction (131) to exhaust back-flow is placed in the other intake port.

6. An engine as claimed in claim 5, wherein the obstruction to exhaust back-flow comprises a one-way valve (131) that permits the intake air to reach the intake port but prevents exhaust gases flowing back up the intake port.