GB2248275A - Tuned i.c. engine intake and exhaust systems - Google Patents

Abstract

In a system having respective cylinder pipes leading to a common junction the pipes are of substantially of equal diameter with equality of flow division at the pipe junctions and variation of effective pipe length is achieved by opening valves 6 and 15 or by progressive movement of a pipe dividing wall (45, Fig. 5). A discontinuity (34, Fig. 4) in the pipe cross- section may also affect the effective pipe length due to flow separation at higher flow rates. The pipes may be arranged in compact serpentine form (Fig. 6). <IMAGE>

Description

AIR INTAKE SYSTEMS This invention relates to air intake systems for multicylinder internal combustion engines and methods for achieving broad range tuning therein.

It is known in the art relating to internal combustion engines that cylinder charging systems may be enhanced over one or more portions of their speed range by providing accoustically tuned intake or exhaust systems. The speed range enhanced by such charging systems is dependent on a number of generally known factors relating to the volumes and lengths of various components in the intake system.

It is known to tune the length of an inlet manifold in order to set up a standing wave in the branch leading to the cylinder at certain speeds so as to create a high pressure node at the inlet valve while it is closing so that the density of the intake air, and therefore the total charge of air, is increased. Similarly, it is known to tune exhaust manifolds to create a drop in pressure at the instant of valve closing to assist with scavenging.

Interference is a phenomenon which can occur in engines with overlapping induction phases such as four cylinder engines; such interference is speed dependent. Below a certain speed, dependent upon pipe length and induction phase overlap, a cylinder at the start of its induction phase is at a lower pressure than one at the end of its induction phase. Hence, inter-cylinder charge robbery occurs. This is termed interference.

Packaging constraints and peak power requirements normally result in relatively short primary pipe lengths for optimum high speed performance. Publication W089/03473 Al illustrates the extension of primary pipe tuning to the low speed range using a cluster valve to modulate the effective length of long individual pipes to each cylinder. However for spark ignition engines expensive multiple throttles are required and these have high Leakage rates which contribute to poor idling fuel economy and emissions.

An intake system in which manifold pipes cascade from a single duct, or multiples thereof, facilitates a lower cost solution to the problem.

The term 'uniflow' herein refers to intake and exhaust systems in which pipes cascade from predominantly a single junction and branch into pipes of substantially equal diameter so that the flow is divided equally at each stage.

Non-uniflow pipe systems by contrast use pipe runners or plenums with downstream branches leaving at intervals so that the runner carries progressively less flow.

The overall length of the pipe plus the duct predominantly determines system tuning in the lower speed ranges. Long systems provide flat broad band torque. As the overall length decreases the torque band narrows but its amplitude increases. A system comprising a throttled duct and uniflow manifold is termed a ducted uniflow intake system.

The duct length may be varied to effect a change in the overall length of the system.

The duct acts to create an inertial column with each cylinder sustaining the kinetic energy of the stream for the benefit of its neighbour. It is also beneficial in reducing interference effects.

Orthodox intake systems employing a manifold plenum configured as a pipe runner suffer from maldistribution as a result of differing overall length to each cylinder and effects of flow reversals within the plenum.

According to the present invention there is provided an internal combustion engine having a plurality of cylinders and an intake or exhaust system comprising a ducted uniflow, as herein defined, duct and cluster manifold combination, characterised in that the effective duct length is variable by valve means. The effective pipe length may comprise a cascade of pipes.

The engine tuning in the low speed range may be optimised by modulation of the effective duct length in order to vary the effective overall length. The plurality of duct entries may be accomodated within a single air filter housing or multiples thereof. In order to optimise engine tuning over the complete speed range, manifold modulation may be combined with duct modulation. A cluster valve may be used as described in W089/03473 Al to modulate the effective length of individual manifold pipes. Publication W089/03473 Al shows a four cylinder engine with modulation of a decoupling chamber using a 4:2 cascade of pipes.

In an embodiment of the cluster valve principle applied to cascade systems the moveable valve forms a common wall of variable length between the secndary pipes. The valve movement may effect incremental or progressive change in the length of the common wall.

In a further embodiment, a change in the effective length of a pipe or duct may be achieved by the use of a stepped bore automatically without recourse to valves and actuators. At low speeds the system tunes to the total length but at a higher speed flow detachment takes place at the step and system retunes to the shorter length.

Finally, the system may be further tuned according to the bore of the duct. Larger bore sizes increase the inertial head and raise the amplitude of the low speed torque peak in particular. The duct flow velocity (U) is defined by the equation VN/120a for a four stroke engine and VN/60a for a two stroke engine in which (V) is the combined capacity of the cylinders drawing from the duct measured in cubic metres, (N) is the engine speed in revolutions per minute and (a) is the internal cross section of the duct in square metres.

Inertial effects in a ducted uniflow intake system are most noticeable when the flow velocity in the duct does not exceed 26 metres per second at peak power.

In order that the invention may be clearly understood it will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic view of a cluster manifold and duct combination termed the ducted uniflow intake system, Figure 2 is a graph showing the relationship between engine speed, torque and brake mean effective pressure (bmep) for an engine using the system shown in Figure 1, Figure 3 is a schematic view of a valve mechanism to effect changes in effective duct or pipe length, Figure 4 is a schematic view of the stepped duct principle, Figure 5 is a schematic view of a modulated secondary pipe using a variable length common wall, Figure 6 is a schematic view of a practical ducted uniflow intake system compacted into the typical space available.

An air intake system according to the invention, see figure 1, comprises a first air filter consisting of a circular housing 1 having a filter element 2 fitted around the periphery. The outlet from the housing 1 is carried by a periphery. The outlet from the housing 1 is carried by a central duct 3 to a second air filter. The second filter consists of a circular housing 4 having a filter element 5 fitted around the periphery. The upper surface of the housing 4 has a connection 6 adapted to receive the duct 3.

The lower surface of the housing has an outlet pipe 7 connected to a throttle assembly 8 containing a throttle valve 9. The housing 4 contains a central valve means 10 operated by an actuator 11. The valve 9, shown in its open position, controls the passage of air through the filter element 5 into the outlet pipe 7.

The throttle assembly 8 is connected to a pair of secondary inlet pipes 12 and 13 separated by a common wall 14. The wall contains a port 15. The area of the port 15 can be varied by valve modulating means, not shown, so that when fully open the pipes 12 and 13 have an effective common cross sectional area. The pipes 12 and 13 divide at a junction 16 into four primary pipes 17, 18, 19 and 20 which are gathered together in cluster formation and lead to four engine cylinders, not shown.

The lines Lp1 and Lp2 show the effective lengths of the inlet pipes to the cylinders according to when the port 15 is in its open and closed positions respectively. Port 15 is shown in its open position which provides an effective primary pipe length from the inlet valve of Lpl. When the port 15 is closed the effective primary pipe length becomes Lp2.

The lines Ld1 and Ld2 show the effective lengths of the duct according to when the valve modulating means in the housing 4 is in its open and closed positions respectively.

The valve 10 is shown in its open mode enabling air to be drawn through the filter 5 resulting in an overall system length Ld1.

In an alternative embodiment the valve 10 may take the form of flaps which can be actuated to close off the redundant passage.

The effect on the engine characteristics of modulating the effective duct length and effective pipe length is shown in Figure 2. Duct modulation is the dominant parameter and determines the low speed bmep characteristics. Primary pipe modulation determines the high speed bmep characteristics. There is an inherent mid-range discontinuity between the low speed mode and the high speed mode which may be determined from an inertia parameter. This discontinuity is also the point of peak bmep potential but is normally seen as a trough in orthodox systems. To achieve this peak potential the effective primary pipe length is required to switch from short to long at the discontinuity. A second discontinuity is also apparent between the low speed mode and an ultra-low speed mode.

Here a short duct performs equally well as a very long duct.

The combination of peaks prescribes the optimum bmep curve using variable geometry induction and demonstrates significant gains over an orthodox ram manifold using a pipe runner system. Valve timing remained fixed.

Figure 3 shows the wall of a duct 21 containing ports 22, 23, and 24 and a valve cover 25 containing a port 26. The three positions of the valve cover illustrate how large variations in effective duct length can be achieved using a small valve movement. A similar principle may be applied to alternative valve mechanisms in order to achieve multiple increments using the minimum number of actuators.

Figure 4 shows the a bellmouth 31 as part of the an air filter housing 32 feeding a throttle duct 33. A step 34 is added to the duct 33 to provide a plurality of effective tuned duct Lengths, dependent on flow velocity, and hence engine speed. At high flow velocities detachment appears to occur at the step 34, the system tuning to effective duct length Ld1. At low flow velocities the system tunes to the entry point Ld2. The result is to broaden the width of the tuning peak as shown in Figure 2.

The stepped duct effect may be used alnng the length of the duct to effect tuning at different segments of the speed range without the addition of valves and actuators. The stepped duct example shown demonstrates a means of extending performance in the low speed range. When a step of sufficient differential area is introduced further along the duct, the tuning effect of the step may be extended to the high speed mode.

Figure 5 shows a 4:2 cluster manifold in which the effective length of a common wall 41 of secondary pipes 42 and 43 is variable in length by valve means. By way of example a motorised spool 44 extends or retracts flexible wall 45 between the effective lengths Lp1 and Lp2 according to engine speed. The action shown is progressive.

In further embodiment the action may be restricted to provision of a port at Lp1, the remaining common wall being fixed.

Figure 6 shows the compact ducted uniflow intake system packaged into the typical space available above or behind the drive shafts ( transverse installation ) and between the starter motor and alternator/engine mount on a four cylinder engine. The example shown is fully engine mounted including the air filter. This enables the complete engine to be run up and tested prior to installation and avoids the intake system being disturbed to facilitate assembly.

Internal walls 51, 52, 53, 54 form a continuous duct passage 55 within a housing 56 which may also contain an air filter element 57. Air enters through an air intake 58. The use of internal walls is thought to minimise the transmission of engine noise, vibration and harshness (NVH). A blanket fold arrangement of the duct passage is shown by way of example but it could equally be a snail or helical geometry. Because the duct cross section is generous, NVH, is low and the flow loss due to tight bend radii minimal. The duct is divided into two sections by a wall 59 to provide intermediate pipes 60, 61 feeding twin throttles 62, 63 and then on to primary pipes 64, 65, 66 and 67.

Duct ports 68, 69 and 70 provide incremental effective duct lengths in addition to an open duct end 71 when uncovered by valve means (not shown) using an actuator 72. In principle duct modulation is effected by uncovering one of the ports according to engine speed. The flow path to the air filter plenum is short circuited by a cavity 73 which also contains the valve means. A valve 74, shown in the closed position, modulates the effective length of the intermediate pipes 60, 61 via a port 75.

Claims (11)

1. An internal combustion engine having a plurality of cylinders and an intake or exhaust system comprising a ducted uniflow, as herein defined, duct and cluster manifold combination, characterised in that the effective duct length is variable by valve means.

2. An internal combustion engine as claimed in claim 1, characterised in that the effective duct length is progressively variable by valve means.

3. An internal combustion engine as claimed in claim 1, characterised in that the effective duct length is incrementally variable by multiple valve means.

4. An internal combustion engine as claimed in claim 3, characterised in that the effective duct length is incrementally variable by valve means in which the valve movement is smaller than the change in duct length effected.

5. An internal combustion engine as claimed in claim 1, characterised in that internal walls of variable length in a cqmpact duct housing are used to vary duct length.

6. An internal combustion engine having a plurality of cylinders and a ducted uniflow, as herein defined, intake/ exhaust system, characterised in that the duct has a stepped diameter.

7. An internal combustion engine as claimed in claim 1, characterised in that variable geometry modulation is effected on the intermediate pipes by one or more interconnecting ports in the common wall with associated valve means.

8. An internal combustion engine as claimed in claim 7 characterised in that the common wall is adapted to be moved to change the area of the interconnecting ports.

9. An internal combustion engine having a plurality of cylinders and a ducted uniflow intake system, characterised in that it has an inertial duct of sufficient cross sectional area whereby the flow velocity in the duct at peak engine power does not exceed 26 metres per second as defined by the equation VNZ120a for a four stroke engine and VN/60a for a two stroke engine in which V is the total swept volume of the cylinders drawing from the duct in cubic metres, N is the engine speed in revolutions per minute and (a) is the internal cross section of the duct in square metres.

10. An internal combustion engine as claimed in claim 1 and as herein described.

11. An internal combustion engine as herein described with reference to, and as shown in, Figures 1, 3, 4, 5 and 6 of the accompanying drawings.