GB2203488A - Manifold tuning for I.C. engines - Google Patents

Abstract

In a manifold system an externally controlled source of sound or pressure waves (14) is disposed within a tract (10) and activated at a predetermined time preceding closing of an associated engine valve (12) such that the pressure pulse from the source (14) reaches the valve as it is closing during each engine cycle. The source may be a loudspeaker or a reservoir (16), Fig. 2 having an electrically controlled valve (18) or may be a normally open shutter in the tract (10) closable momentarily to thereby create a pressure pulse. Sound emitted by the source (14) may be counteracted by a source of anti-sound. <IMAGE>

Description

MANIFOLD TUNING The present invention relates to the tuning of inlet and exhaust manifolds of internal combustion engines.

It is well known that some degree of supercharging can be achieved by selecting the length of the intake system to take advantage of the pressure waves caused by the rapid opening of the intake valve and that propagates back and forth along the intake tract in a series of compression and rarefaction waves. By choosing the tract length such that the time taken for the compression wave to arrive back at the intake valve is approximately the same as the time interval of the intake valve opening period, the density of the intake charge as it is being trapped inside the engine cylinder can be momentarily increased thereby increasing the power output of the engine.

As the speed of sound is substantially constant (approx.

350 m/s in air), the time taken for the pressure wave to traverse an intake tract of fixed length for a fixed number of reflections is also constant. On the other hand, the duration of the intake event, if constant when expressed in terms of crankshaft angle, will vary inversely with engine speed. Thus, the time from the start of the pressure wave perturbation to the instant when the intake valve closes is not constant and progressively shortens with increased engine speed. It will be clear from this that for a fixed length of intake tract, there is only one speed at which the desired matching or tuning of the intake tract is achieved. The resulting torque curve for such an engine would exhibit a narrow high peak at the tuned engine speed and the torque would fall off rapidly on each side of this peak.

Variably tuned intake systems have attempted to achieve increased torque over a broader speed range by providing a control enabling the length of the intake tract to be varied in dependence upon engine speed in order to match the changing time delay requirements.

The tuning of engine manifolds to take advantage of pressure waves is not restricted to the intake system but can also be used in exhaust systems. In the case of exhaust manifolds, tuning is used to achieve improved scavenging, that is to say removal of residual exhaust gases from the combustion chamber. Here, a negative pulse is required at the exhaust valve as it is being closed.

Manifold tuning can furthermore be employed not only to increase engine power output but to derate an engine under part load operation. Thus, it is possible in the case of an intake system to arrange for a negative pulse to be present during intake valve closing as this reduces the mass of air without throttling, thereby reducing pumping losses.

Both the fixed length and variable length manifold systems previously proposed are passive systems in as much as the pressure wave perturbation used to alter the charge density is created by the engine itself and the various designs of the manifold systems have only been intended to optimize the phasing of a process which occurs naturally within the engine.

The present invention shares with the tuned manifold systems described above the fact that a shock or sound wave is employed to vary charge density at the instant of valve closing but seeks to provide greater control over the process so as to enable regulation of the breathing efficiency to be achieved over a wide range of engine speeds.

According to the present invention, there is provided a manifold system for an internal combustion engine comprising at least one externally controlled source of sound or pressure pulses disposed in a tract of the system and means for activating the or each source to create or emit a pressure pulse during each engine cycle such that the pressure pulse from the or each source reaches an associated valve, either directly or after reflection, as the valve is closing.

An important advantage of the use of an active system is that one is no longer tied to tuned lengths and one can instead vary the timing of the pressure pulse to suit the existing manifold tract length. Previously, to achieve tuning at low engine speeds required long tracts which were difficult to package within the engine compartment of road vehicles and the technique of intake manifold tuning was used mostly on racing cars where the tuned manifold lengths are shorter because of the higher engine speeds.

The most common requirement for such a system is to increase the volumetric efficiency under full load and the system will therefore most frequently be employed in the intake manifold to produce a positive pressure pulse for each intake valve as it is closing. It will be clear, however, that the system can equally be used to derate the engine under part load by transmission of a negative pressure pulse and can be used in an exhaust system to improve scavenging.

The source of pressure pulses may be a loudspeaker- of conventional design for example using a diaphragm. As an alternative source, a pressure reservoir may be disposed within the manifold and be fitted with an electrically operated valve which is opened and closed rapidly to send pressure waves down the manifold. The reservoir may be pressurized by means of a compressor but in the case of an intake system it is preferable to use the back-pressure in the exhaust system to pressurize the reservoir. In the case of an exhaust system, a vacuum reservoir may be connected to the intake manifold which will act as a vacuum source at part load when good scavenging is required.

It is alternatively possible to create a pressure pulse by means of a further shutter in the manifold which is externally controlled. Such a shutter differs from the first two proposals in that the power in the pulse is derived from the air flow itself. Shuttering of fluid streams is a known source of pressure pulses, as is well demonstrated by the effect of water hammer.

A loudspeaker or shutter will not make any contribution to the net air mass flow to the engine whereas the reservoir will make some difference. However, it is important to note that in both cases it is the pressure wave transmitted at the speed of sound that is employed to increase the charge density. In the case of a reservoir, the air mass may be small enough in relation to the total air mass in the charge to be disregarded.

If the source is a loudspeaker, it is preferred that it be disposed entirely within the manifold, rather than in a wall of the manifold, and that it face the valve so that the pressure pulse should suffer a minimum of attenuation. A pressurized reservoir, on the other hand may be secured within a wall of the manifold to face the opening of the intake tract.

Advantageously, the source may be fitted with a concentrator to amplify the pressure pulse reaching the valve. The source may be positioned upstream or downstream of the throttle in the case of the intake manifold and may also be arranged upstream or downstream of the fuelling point.

In a multi-cylinder engine, there may be provided a source for each cylinder or a source common to two or more cylinders. In each case the source should be arranged to send a wave to the associated cylinders but to no other cylinder. In the case of a fuel injection system with individual intake tracts connected to a plenum chamber, it is convenient to place one source per cylinder within the plenum chamber facing the associated the tract without restricting the air flow into it. For a carburetted engine, a similar solution may be adopted but it would be preferable to use a single source upstream of the carburettor. Having regard to the required dimensions of a loudspeaker, it may be convenient in this case to incorporate the source within the air filter housing.

Because of the total enclosure of the source within the intake manifold system, noise nuisance may be negligible but where the sound emitted is objectionable then it may be counteracted by a source of anti-sound as the source of the sound is localised and electrically generated.

Thus the source of anti-sound may itself employ the signal used to activate the source.

The activation of the source must be correctly timed relative to the valve closure to achieve the desired effect. The mechanism required for achieving this aim is no different in principle from the mechanism needed for achieving spark timing and for this reason the ignition timing signal may itself be employed if provided with a time offset. Alternatively, any regular timing pulses related to the valve timing may be employed to trigger a pulse generator having a pulse width which varies with engine speed and of which the trailing edge is employed to activate the source.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically a first embodiment of the invention employing a loudspeaker, and Figure 2 is a similar view of an embodiment using a source of pressure waves including a pressure reservoir.

Figures 1 and 2 both show one tract 10 of an intake manifold which leads to the intake valve 12 of one of the engine cylinders. The tract includes a chamber 20 within which there is arranged a source of pressure pulses. In the case of Figure 1, the source of pressure waves is a loudspeaker 14 whereas in Figure 2 it is a reservoir 16 connected to an external pressure supply by a pipe 22 passing out of the intake manifold and having an electrically controlled valve 18 designed to open and close very rapidly to create a pressure pulse. The pressure supply may conveniently be the back pressure in the exhaust manifold.

In a further alternative which is not illustrated, the source of pressure pulses consists of a normally open shutter in the tract which may be closed momentarily to interrupt the air flow and thereby create a pressure pulse. As the pulse may in this case be of the wrong sign, it is possible to time the pulse to reach the valve after reflection at an open end of the tract to cause it to be inverted.

The air flow and fuel metering are controlled in a conventional manner and the purpose of the pressure pulses sources in each embodiment are to vary the charge density within the cylinder just as the intake valve is closing. the throttling of the air flow and the fuel metering may be carried out upstream or downstream of the pressure source and there may be a single pressure source common to all cylinders or an individual source for each cylinder. Of course, other combinations are also possible such as two sources shared between four or six cylinders.

If increased power output is required, it is arranged for a positive pressure pulse to compress the charge while the inlet valve is closing. The wave propagating along the direction illustrated takes a finite predetermined time to travel from the source 14 or 16 to the inlet valve 12. The source must therefore be activated a fixed time before inlet valve closure and the control for the source may be derived from the ignition systems of from any signal in phase with the valve timing.

Because the duration of the valve event shortens with engine speed, it is necessary to take the engine speed into account when timing the activation of the pressure source.

The chamber 20 is conveniently shaped so as to act as a concentrator. This has the dual effect of increasing the amplitude of the pressure pulse at the inlet valve 12 and of preventing multiple reflections.

In a conventional or passive tuned manifold, it is believed that the opening of the inlet valve causes transmission of a positive pressure or compression pulse towards the open end of the tract. This pulse is inverted and reflected at the open end so that a negative pressure or rarefaction pulse is sent back to the valve. This negative pressure pulse is reflected without inversion from the closed combustion chamber back towards the open end and after a further reflection with inversion again reaches the inlet valve as a compression pulse to compress the charge as the valve is closing. The pressure pulse is thus attenuated by passing four times up and down the intake tract and by three imperfect reflections at the open and closed ends of the tract.

In the present invention, by contrast, the active generation of the pressure pulse enables a more powerful and better controlled pressure pulse to be created, the pulse need not be attenuated by long propagation paths and multiple reflections and can be concentrated by suitable design of the manifold. The extent to which the charge density can be modified may therefore be potentially greater than can be achieved by passive tuning and the control can be made to extend over a broad range of engine speeds without any alteration to the length of the intake manifold and without creating the packaging problems of large passive tuned manifolds.

In the case of a fuel injected engine, the chambers 20 of the different cylinder may be joined to form the plenum chamber connected by way of a common throttle to an air filter box. The tracts 10 in this case are the ducts leading from the plenum chamber to the individual engine cylinders. It is of course alternatively possible for the fuelling to be done by a carburettor but in this case it would not be desirable to have a large wet chamber and it would be preferable to provide a pressure source for each carburettor common to all the cylinders fuelled by that carburettor.

Claims (12)

1. A manifold system for an internal combustion engine comprising at least one externally controlled source of sound or pressure pulses disposed in a tract of the system and means for activating the or each source to create or emit a pressure pulse during each engine cycle such that the pressure pulse from the or each source reaches an associated valve, either directly or after reflection, as the valve is closing.

2. A system as claimed in claim 1, wherein the system is an intake system and the or each source is operative to produce a positive pressure pulse for each intake valve as it is closing, whereby to increase the density of the trapped charge.

3. A system as claimed in claim 1, wherein the system is an intake system and the or each source is operative to produce a negative pressure pulse for each intake valve as it is closing, whereby to derate the engine under part load conditions.

4. A system as claimed in claim 1, wherein the system is an exhaust system and the or each source is operative to produce a negative pressure pulse for each exhaust valve as it is closing, whereby to improve scavenging of residual exhaust gases.

5. A system as claimed in any preceding claim, wherein at least one source of pressure waves is a loudspeaker.

6. A system as claimed in preceding claim, wherein at east one source of pressure waves includes a reservoir connected to a pressure source and an electrically operated valve connecting the pressure reservoir to the interior of the manifold system.

7. A system as claimed in claim 6 when appended to claim 3, wherein the reservoir is pressurized from the back pressure in the exhaust manifold.

8. A system as claimed in claim 6 when appended to claim 4, wherein the reservoir is pressurized by means of the vacuum in the intake manifold.

9. A system as claimed in any preceding claim, wherein the source is fitted with a concentrator to amplify the pressure pulse reaching the valve.

10. A system as claimed in any of claims 1 to 4, wherein the source of pressure pulses is a shutter for interrupting the air flow in the tract.

11. A system as claimed in any preceding claim, wherein a source of anti-sound is provided outside the manifold system in close proximity to the pressures source in order to reduce noise.

12. A system constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.