GB2223800A - I.C. engine valve gear - Google Patents

Abstract

Four valves 18 are opened substantially simultaneously to induce a gas charge and are also opened substantially simultaneously to allow efflux of the exhaust gases. A silicon nitride rotary valve 34 or a flap valve (50, Fig. 2) alternately connects an inlet duct 36 and an exhaust duct 38 to the valved passages 32. <IMAGE>

Description

Engines The present invention relates to internal combustion engines and particularly to a valve layout in the cylinder head of an internal combustion engine.

It is well known that internal combustion engines operating on the Otto cycle, produce power in proportion to the amount of combustable charge passed through their cylinders in a given time for any given type of working cycle. Thus, for any given size of engine cylinder and crankshaft speed the power may be augmented by increasing the amount of air induced into the engine. Such increases may be affected by adjustments to the induction pressure i.e. supercharging, or for naturally aspirated engines by altering the valve open time-area integral. In the latter case such alterations may include the provision of longer timing periods, increased overlap between exhaust valve closure and intake valve opening or the provision of more or larger valves.

Many high performance engines use 4 valves per cylinder with a relatively narrow included angle between intake and exhaust valve pairs. Such included angles may lie between 600 and 150. This configuration allows room in the centre of the combustion chamber for a spark plug and provides a "pent roof" shape which allows almost ideal flame propagation during combustion.

Whilst such 4 valve layouts and pent roof combustion chambers yield excellent results in general; improvements in torque have recently been secured by the use of 5 valves per cylinder. Such an arrangement generally comprises 3 intake valves and 2 exhaust valves. Engines have also been produced having 6 valves per cylinder, comprising 3 intake and 3 exhaust valves. Whilst such constructions provide greater power output, the mechanical complexity and therefore cost of manufacture is greatly increased.

The desired objective is to achieve the largest possible valve area to maximise breathing efficiency. Desmodromic valve operation, providing positive mechanical opening and closing of valves provides a high degree of mechanical safety whilst allowing very high of rates of valve opening and closing to be achieved Desmodromic operation however has the drawbacks of complexity, noise and high cost.

In systems using poppet valves the mass-flow limitations are usually those associated with partially choked flow between the valve head and valve seat.

Rotary valves, either of the Aspin or Cross types have high co-efficients of discharge permitting high flow rates. However, their sealing capability is less satisfactory and they frequently demand copious lubrication if seizure is to be avoided. Such copious lubrication usually results in high oil consumption.

The present invention has as its main objective to provide a reliable valve mechanism capable of operating at very high engine speeds and allowing high mass flow rates of charge.

According to a first aspect of the present invention an induction and exhaust system for a cylinder of an internal combustion engine comprises a combustion chamber having at least two gas ports, each gas port having primary valve means, driven substantially synchronously by primary valve drive means, said primary valve means being for opening and closing said gas ports to the flow of gas, the gas ports being connected by gas flow duct means to secondary valve means adapted to connect the gas flow duct means alternately with inlet passage means and exhaust passage means.

The induction and exhaust system of the present invention may be applied to either naturally aspirated or to pressure charged engines.

In a preferred embodiment there are provided at least four primary valves which may be of the poppet type.

The primary valve drive means may comprise conventional camshafts driven by any known method.

The secondary valve means may be a rotary valve of generally cylindrical form. Such a valve may be manufactured from a ceramic material, i.e. silicon nitride, and be supported by bearing means at each end.

Since this valve does not need to withstand compression and combustion forces, it may operate in its housing at a small positive clearance as there are no significant gas leakage problems.

Alternatively the secondary valve means may comprise an oscillating flap-type valve actuated indirectly from one of the primary valve drive systems in sequence with the desired valve events.

In an engine having the induction and exhaust system of the present invention all valves in the cylinder are opened substantially simultaneously to provide either the inlet function, or the exhaust function. The specific gas flow is controlled by the upstream or downstream secondary valve means. Thus, in a normally aspirated engine the mass flow of gas on the induction stroke of the piston is greatly increased thereby increasing the power output of the engine cylinder.

In an induction and exhaust system according to the present invention the initial exhaust pulse out of the cylinder during blowdown is very strong and favours the use of an exhaust gas driven turbo-charger or a strongly resonant extractor type of exhaust system. The exhaust phase proceeds under minimum back-pressure conditions but it is not necessary to fully close the open valves at the end of the exhaust event, instead, the rotary valve switches function and the 4 open valves then operate as intake valves. Since the intake phase commences with all primary valves open, flow resistance is minimal and thus the volumetric efficiency is very high. Micro turbulence in the inspired charge after passing the primary valves is intense and evenly distributed throughout the cylinder, enhancing combustion efficiency.

Because the primary valves are used for both the control of intake and exhaust gasses there is a considerable evening out of the valve head and valve seat temperatures since both hot and cool charges are being dealt with. The reduced peak valve head temperatures allow a higher compression ratio to be used whilst avoiding detonation problems.

Where poppet valves are used having conventional coil spring operation, the extended "valves open" phase reduces the stresses and surging tendencies. Cam and tappet contact stresses may be correspondingly reduced.

The primary valve drive means and the secondary valve drive are both driven at half crankshaft speed in a 4stroke engine.

The secondary valve deals with both the incoming atmospheric or boosted charge and also the exhaust gases.

Mixing of the two gas phases is minimal and unimportant in the plenum chamber which is formed between the primary and secondary valves outside the cylinder. The gas pressures are relatively very low compared with the combustion pressures and there are no significant leakage problems in a secondary valve of the rotary type. Such a valve may operate in its housing at a small positive clearance at engine speeds.

In order that the present invention may be more fully understood examples will now be described by way of illustration only with reference to the accompanying drawings of which: Figure 1 shows a cross-section through a schematic representation of an engine having an induction and exhaust system according to the present invention; Figure 2 shows an alternative arrangement of secondary valve means; and Figure 3 which shows exhaust and inlet valve lift plotted against crankshaft angle for a conventional engine in the upper graph and an engine according to the present invention in the lower graph.

Referring now to Figure 1 and where the induction and exhaust system of the engine is denoted generally at 10.

An engine having the induction and exhaust system includes at least one cylinder 12 having a piston 14. The engine cylinder head which is shown only in part and denoted at 16 comprises four primary valves 18 of the poppet type having the usual valve guides 20, springs 22, cam followers 24 and which are operated by cams 26. The heads of the valves 18 seal against conventional valve seat inserts 28 and control the flow of gas, denoted by the broad arrows 30, along the gas flow ducts 32. A secondary valve 34, of the rotary type connects the gas flow duct or plenum chamber 32 with either an inlet port 36 or an exhaust port 38 depending upon the rotational position of the valve 34. The secondary valve 34 may be manufactured from a ceramic material such as silicon nitride.The cams 26 are both driven synchronously such that the valves 18 are all. open, all closed or at any corresponding intermediate position therebetween. When the valves 18 are beginning to open and progress towards the fully open position the port 40 in the rotary valve 34 opens the inlet port 36 to the flow of incoming gas. Similarly, after combustion,when the piston 14 is rising and the valves 18 are opening to begin the exhaust phase the valve 34, by means of the port 42, connects the gas flow duct 32 with the exhaust port 38 to permit expulsion of the burnt gasses.

Figure 2 shows an alternative arrangement of secondary valve wherein the rotary valve 34 of Figure 1 is replaced by an oscillating flap valve 50 pivoted on a shaft drive 52. The flap valve is driven by a suitable drive (not shown) from the camshaft drive (not shown) and moves between seating positions 54, 56 depending upon the stage of the combustion cycle to allow entry of inlet gas or exhaust of the burnt charge as shown by the arrows 58.

Any form of fuel supply may be used such as carburettors, direct or indirect fuel injection, for example.

Figure 3 shows graphs of the exhaust and inlet valve lift plotted against crank angle. The chain-dotted line 60 represents incoming gas velocity and the shaded area defined by the dashed lines 62 and 64 represent the product of valve system discharge coefficient, Cd, and gas velocity. The upper graph shows the curve form for a conventional high performance engine whilst the lower graph shows comparable curves for an engine having an inlet an exhaust system according to the present invention. The greatly increased shaded area under the curve 64 denotes a significant increase in air-mass flow for the latter resulting in a considerable power increase.

Claims (9)

1. An induction and exhaust system for a cylinder of an internal combustion engine, the system comprising a combustion chamber having at least two gas ports, each gas port having primary valve means driven substantially synchronously by primary valve drive means, said primary valve means being for opening and closing said gas ports to the flow of gas, the gas ports being connected by gas flow duct means to secondary valve means adapted to connect the gas flow duct means alternately with inlet passage means and exhaust passage means.

2. A system according to Claim 1 wherein the primary valve means comprises poppet valves.

3. A system according to either Claim 1 or Claim 2 wherein there are four primary valves.

4. A system according to any one preceding claim wherein the primary valve drive means comprises cams and camshafts.

5. A system according to any one preceding claim wherein the secondary valve means comprises a substantially cylindrical rotary valve having gas flow ports formed in its outer surface.

6. A system according to Claim 5 wherein the secondary valve is made from a ceramic material.

7. A system according to Claim 6 wherein the ceramic is silicon nitride.

8. A system according to any one preceding claim from 1 to 4 wherein the secondary valve means comprises a flap-type valve.

9. An induction and exhaust system for an internal combustion engine substantially as hereinbefore described with reference to the accompanying specification and drawing.