GB2133470A - I.C. engine carburetted mixture homogeniser - Google Patents

Abstract

A plate for fitting between a carburettor and an inlet manifold has a central circular opening (3) from the periphery of which extends a plurality of teeth (7) which defines a convergent main flow path for mixture. The teeth (7) are circumferentially spaced to provide slots (9) which at their radially outer ends are enlarged to provide subsidiary flow paths for mixture 3. After passing through the plate, the main and subsidiary flows mix, and the resulting turbulence reduces average droplet size and disturbs the boundary layer which tends to form on the manifold walls. <IMAGE>

Description

SPECIFICATION Fuel homogeniser for internal combustion engine This invention relates to a fuel homogeniser for an internal combustion engine, that is a device for promoting mixing of air and fuel supplied to an internal combustion engine. The invention aims to provide a device achieving an improvement of thermal efficiency and power output by extraction of energy from fuel that is otherwise not burnt efficiently.

Various devices are known that consist of a metal plate, shaped to fit the carburettor to inlet manifold flange with the necessary fixing holes.

An internal hole in the plate carries a series of projections or vanes upon which the fuel and air stream impinges as it flows through the manifold.

These devices claim to provide fuel economies by restricting the manifold, increasing the surface area of the manifold to increase the heat transfer into the fuel, impacting the fuel droplets onto vanes to break them up or inducing a helical swirling into the mixture of fuel and air to improve mixing.

The former devices unduly restrict engine performance by limiting the potential flow in the manifold. The last overlooks the fact that swirl while useful as a means of preventing detonation in the burning fuel inside the combustion chamber, is not desirable in the initial stages of fuel and air mixing.

Introduction of a helical swirl produces a velocity component at a right angle to the airflow.

This is in effect a centrifuge which would tend to throw the heavier fuel droplets against the walls of the manifold. Thus if such devices were successful, their effect would not be that intended.

The present invention differs from those described above not only in its intention, which is to make the engine more powerful and efficient, but also in the means by which it works.

According to the invention there is provided a device for promoting mixing of air and fuel supplied to an internal combustion engine, the device comprising a plate for fitting between a carburettor and an inlet manifold of the engine, the plate having an opening therethrough the periphery of which is formed with a plurality of teeth which project to one side of the plate and which are shaped so that the inner envelope of the teeth defines a convergent main flow path for air/ fuel passing through the opening in a direction towards said one side of the plate, adjacent teeth being separated by a corresponding slot which is enlarged at its radially outer end, the enlarged portions of the slots providing a subsidiary flow path for air/fuel which subsequently mixes with air/fuel following the main flow path.

The preferred device makes a partial venturi within the manifold. Air passing through this is reduced in pressure and increased in velocity. Air moving through slots in the edge of the device is slowed by friction. It is then reunited with the faster flow that has passed through the venturi.

The consequent turbulence has all its major velocity components parallel to the direction of flow and results in extra work being done on the fuel droplets. This reduces the average particle size with a consequent increase in surtace area and evaporation rate.

Air flow within a manifold is non-uniform because amongst other things of boundary layer flow along the walls. This can hamper engine efficiency under some conditions. For example in the steady state of running under constant throttle opening, the effect of the boundary layer is to make the air near the manifold wall lean in fuel compared to the central portion of the flow. A non-homogeneous charge is delivered to the combustion chamber. As a result the carburettor may have to be set to deliver more fuel than is necessary, to provide an average mixture rich enoughto prevent detonation or overheating in the combustion chamber.

Sudden throttle opening can produce the opposite condition, especially in a cold engine. An excess of fuel droplets may coalese on the manifold walls. The slower, smoother flow in the boundary layer gives poor conditions for the rapid evaporation of any liquid fuel film. This fuel is not therefore available for combustion when required, needlessly enriching the mixture later, when a steady speed has been achieved.

The shock produced by the mixing of faster and slower gas streams, as explained above is sufficient to break up the boundary layer downstream of the device. The effect depends upon the kinetic energy within the airstream and is therefore more pronounced at high engine speeds and airflow rates.

In a manifold fitted with the device, a more homogeneous air and fuel mixture is supplied to the combustion chamber. At steady speeds and fuel is merely more evenly distributed while under sudden acceleration fuel droplets that coalesce on the manifold wall can more readily evaporate.

A further effect of the device is that since the boundary layer is an efficient insulator, when it is broken up there is better heat transfer from the manifold into the airflow. This further increases the evaporation rate of the fuel. Use of the device ensures that distribution and vapour to liquid ratio of the fuel in the combustion chamber are better than normal. The mixture is thereby effectively enriched under conditions stated above. This enables more power to be developed, or resetting of the carburettor to allow for cleaner burning within the combustion chamber.

According to the present invention therefore, improved power and thermal efficiency can be obtained from an internal combustion engine by use of a suitable homogeniser between the carburettor and inlet manifold.

For minimum restriction to airflow a material of high stiffness is required so that the thinnest possible sheet may be used. For this reason stainless steel is preferred for manufacture of the device.

The external shape, central hole, holes for slots and fixing holes are formed from a 0.025inch sheet by punching. The central hole, concentric with the carburettor bore is of approximately 5/8 x D, where D is the diameter of the inlet manifold.

At the same time, or subsequently, a ring of smaller holes is formed in the annulus of metal remaining between the central hole and the periphery of the manifold bore. The locus of their centres is a circle concentric with the manifold bore, diameter approximately 0.9 x D.

Their diameter d is chosen such that their outside edges match the periphery of the manifold bore and that sufficient metal remains for adequate strength. The values of D and d are not critical for the operation of the device and may be varied for ease of layout or to accommodate standard tools. Typically, in a 1.75 inch unit, d=1 1/8 inch and d=1/8 inch.

A cut is made between the central hole and each of the outer holes which is radial to the first and tangential to the latter. The cut portions are formed into a conical surface by indenting with suitable press tools. They are then folded at an angle of 30 to 70 degrees to this conical surface forming a series of approximately radial slots and lobes. Part of the air and fuel mixture flows through these in use and is slowed by friction.

For convenience of tooling, the direction of the slots has been generally in one sense, but this is not material to the operation of the device.

A preferred embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which Figure 1 is a plan view of a device in accordance with the invention, Figure 2 is a section on the line A-A of Figure 1, Figure 3 is a plan view of a drilled blank from which the device is made, and Figure 4 is a sectional view illustrating gas flow through the device when fitted between a carburettor and an inlet manifold of an internal combustion engine.

In the embodiment, the device 1 is made from 0.025 inch stainless steel sheet. A stainless steel sheet 2 is provided with a central opening or hole 3 and a series of smaller holes 4 is drilled in the annulus of metal surrounding the hole 3. The circular hole 3 and its surrounding circular holes 4 are connected by radial cuts 5 which merge tangentially with the holes 4. Holes 6 for fixing studs are provided to match the manifold flange.

The lobes or teeth 7 formed between the cuts 5 are bent to one side of the plate so that their inner envelope defines a converging main flow path 10 (Fig. 4) for gases passing through the opening 3 in a direction which is downwards in Figures 2 and 4. The frusto-conical shape defined by the teeth 7 projects from the sheet at an angle of 30-60 degrees to the plane of the sheet. The shorter parts 8 of the teeth 7 are bent outwards from the conical surface to form slots 9 between adjacent teeth. This gives better results than the opposite orientation since a clean edge is presented to the airflow. The angle to which the parts 8 are bent is 30--70 degrees. The lower limit is set by the ability of the slot to sustain airflow: the upper by the loss of the venturi effect of the cone.The depth of each slot in the plane of the sheet is 1/6 or less than the manifold bore.

The base of each slot 9 is enlarged by virtue of the holes 4, and the enlarged portions of the slots 9 together provide a subsidiary flow path 12 for gases. In use the device 1 is mounted between the carburettor 14 and the inlet manifold 16 with the teeth 7 extending towards the latter. After passing through the butterfly valve 18 of the carburettor 14, the air/fuel mixture passes through the opening 3. Most of the air/fuel mixture follows the main path 10 and is accelerated through the venturi provided by the converging teeth 7. The radially outer portion of the air/fuel mixture follows the subsidiary flow path 12 and passes through the slots 9 (mainly through the enlarged bases of the slots 9), thereby being slowed by friction. Downstream of the device 1, the air/fuel in the main flow path 10 and the subsidiary flow path 1 2 mix, as indicated at 20.Air and fuel passing through the venturi section is accelerated. That flowing through the slots is slowed by tnctlon. I he shock produced by the recombination of these flows downstream of the device reduces the average particle size of fuel droplets, increasing their evaporation rate. It also breaks up the boundary layer flow on the manifold walls downstream. This effect also increases heat transfer into the fuel from the manifold. The overall result is that a more homogeneous air/fuel mix, with a higher than normal vapour to liquid ratio is delivered to the combustion chamber.

It is very difficult to measure a modest gain in engine performance by road testing a vehicle, since experimental errors may be large. To ensure reproducable conditions and eliminate bias the device was tested by a third party, the Department of Mechanical Engineering at Trent Polytechnic.

In a series of 72 tests on an engine with an unusually efficient manifold gains in power and in thermal efficiency were measured. As predicted by the theory of the invention the gains were directly proportional to engine speed, increasing from zero at idling to plus 12.2% thermal efficiency and plus 4.3% brake horsepower at full speed.

Claims (7)

1. A device for promoting mixing of air and fuel supplied to an internal combustion engine, the device comprising a plate for fitting between a carburettor and an inlet manifold of the engine, the plate having an opening therethrough the periphery of which is formed with a plurality of teeth which project to one side of the plate and which are shaped so that the inner envelope of the teeth defines a convergent main flow path for air/fuel passing through the opening in a direction towards said one side of the plate, adjacent teeth being separated by a corresponding slot which is enlarged at its radially outer end, the enlarged portions of the slots providing a subsidiary flow path for air/fuel which subsequently mixes with air/fuel following the main flow path.

2. A device according to claim 1, wherein the radially outer end of each slot has a curved shape.

3. A device according to claim 2, wherein the radially outer end of each slot is circular or partcircular.

4. A device according to claim 3, wherein one edge of each tooth merges substantially tangentially with the circular or part circular portion of a corresponding slot.

5. A device according to any of the preceding claims and made by cutting a circular opening in a metal plate, forming a plurality of holes surrounding the opening, forming radial cuts interconnecting the holes and the opening so as to provide a plurality of teeth which are then bent to one side of the plate, and twisting each tooth to introduce a degree of swirl into the air/fuel flow passing through the opening.

6. A device according to any of the preceding claims, wherein the inner envelope of the teeth provides a venturi forming a frustum of a cone having a base angle between 30 and 60 degrees.

7. A device for promoting mixing of air and fuel supplied to an internal combustion engine, constructed and arranged substantially as herein particularly described with reference to the accompanying drawings.