Monitoring fuel consumption of internal combustion engines

The present invention relates to an arrangement for monitoring the fuel consumption of internal combusion (i.c) engines, and in particular of such engines provided with fuel injection systems.

-It has previously been proposed to provide a flow sensor in the fuel line from the fuel tank to the carburettor(s) of a conventional i.c. engine. Such a sensor will provide an indication of the fuel flow between tank and engine, and may therefore be used to provide a measure of the fuel consumption of the engine, for example by generating flow related signals to suitable processing equipment such as that described in co-pending European Patent Application No. 80303783.7 (Publication No. 0029308)

A difficulty arises when it is desired to use a similar arrangement for monitoring the fuel consumption of a fuel injection engine. The design of the fuel system in such engines is such that a return path is always provided back from the fuel regulation/ distribution apparatus in the engine to the fuel tank. This is due to the fact that fuel injection systems are arranged to pump an excess of fuel so as to ensure a constant pressure under all operating conditions, the excess fuel being returned to the tank. It will be seen that simply providing a flow sensor in the feed path from the fuel tank would produce grossly incorrect fuel consumption figures, as much of the fuel already considered to have been:consumed would be continually returning to the tank for re-use.

One solution to this difficulty would be to provide a second flow sensor in the return path, and processing equipment capable of subtracting the return flow measure from the feed flow measure. This considerably increases the cost of the monitoring system, both in view of the need to provide two flow sensors and due to the increased complexity demanded of the processing equipment.

Furthermore, if an instantaneous fuel consumption reading is to be provided by this arrangement utilising two flow sensors, under certain circumstances an overall zero reading will be provided if the return path flow at the moment of monitoring equals the feed flow. In fact even a negative reading can sometimes result if the. return path flow should momentarily exceed the feed flow. It is thus not readily possible to provide an instantaneous reading using this arrangement.

The present invention provides a fuel injection system for an internal combustion engine provided with means for monitoring the fuel consumption thereof, comprising a fuel tank for storing fuel, at least one fuel injector for injecting fuel into a respective cylinder of the engine, a distributing means for supplying fuel to the fuel injector at a predetermined point in the cycle of the engine, a fuel pump for pumping fuel along a fuel feed path from the fuel tank to the distributing means, and a fuel return path for returning excess fuel from at least the distributing means,characterised in that a flow sensor is provided in the fuel feed path between the fuel tank and the fuel pump for providing an indication of fuel flow, in that the fuel return path rejoins the fuel feed path between the flow sensor and the fuel pump, and in that a cooling means is provided in the fuel return path, whereby cooled excess fuel is returned to the fuel feed path downstream of the flow sensor.

Accordingly, the re-routing of the fuel return path, causing it to return excess fuel into the feed path between sensor and pump, ensures that all fuel passing through the sensor will be used by the engine and therefore. meaningful readings can be obtained from a single sensor.

If a direct connection is made from the return outlets of the distributing means and/or injectors by means of conventional fuel piping into the fuel feed path by means of a T-piece or the like, it is found that, under certain conditions such as idling of the engine, fuel vapour lock will occur. This arises because under these conditions, a large proportion of the fuel is being continuously circulated around the feed path, return path and feed path again, only a small proportion actually being consumed by the engine. The temperature of the fuel is then found to rise until vapour lock occurs. This problem is overcome by the provision of the cooling means, preferably in the form of a radiator or heat exchanger, being included in the fuel return path.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:-

• Figure is a schematic diagram of a fuel system in accordance with one embodiment of the invention; and
• Figure 2 is a schematic diagram similar to that of Figure 1 but showing another embodiment.

Referring to Figure 1, the part of the system similar to that of a conventional fuel injection system will first be described. Referring to the drawing, a fuel pump 10 draws fuel from a supply line 11 to a fuel distributor or pressure regulator device 12 and thence to the fuel injectors 13 of the engine. The injectors 13 may be solenoid operated, in which case device 12 will be a fuel pressure regulator. Alternatively, device 12 may be a fuel distributor, and each injector 13 will include a one-way valve opening at a presot pressure. A fuel return line 14 provides a return path for excess fuel not distributed by device 12. In a diesel injection system, further components would be provided, such as a lift or pre-supply pump to feed fuel pump _;10, and a fuel return line as shown from each injector communicating with the main return line 14. In a conventional system, the fuel supply line 11 would be connected to the fuel tank of the vehicle or other apparatus and likewise the return line 14 would be arranged to feed excess fuel back into the tank.

However, as shown, the present arrangement has the return line 14 connected by means of a T-piece 15 to the supply line 11, the connection being made downstream of the pump 10. A main tank 18 supplies fuel to the fuel pump 10 by way of a fuel line 19 which includes a flow sensor 20 and may include a fuel filter 21. The flow sensor 20 is shown as including conductors 22 for transmitting flow-responsive signals to suitable processing equipment.

So as to cool the excess fuel and prevent the above- mentioned vapour lock problem, the return line 14 includes a cooling means in the form of a radiator 30. The radiator should be mounted in a position where it is subjected to a throughflow of cooling air, e.g. in a motor vehicle it may be positioned in front of the coolant radiator, the radiator fan drawing cooling air through the radiator.

In operation, fuel is drawn from the main tank 18 by way of fuel line 19 and via the flow sensor 20. A lift or pre-supply pump may be provided for this purpose, e.g. atthe outlet of the main tank 18; alternatively suction from the supply line 11 as a result of operation of the fuel pump 10 will generally be sufficient to cause flow of fuel from the main tank 18. The remainder of the system operates in the conventional manner except that fuel is returned to the T-piece 15 in the supply line 11 by way of the radiator 30 rather than to the main tank. It will be seen that fuel flow measured by the sensor 20 accurately reflects the true consumption of the engine.

Since the fuel is cooled each time it completes a circuit through the return line 14 and hence radiator 30, the temperature of the circulating fuel is kept low and hence vapour lock should not occur in the supply line 11.

Figure 2 shows another embodiment of the invention, parts similar to those shown in Figure 1 having the same references. In this version instead of the direct connection between return line 14 and supply line 11, there is provided an intermediate container 16. The main tank 18 thus supplies fuel to the intermediate container 16" by way of fuel line 19 including the flow sensor 20. A vent pipe 24 incorporating a suitable valve 25 may be provided so that the intermediate container 16 may share a common venting arrangement (not shown) of the main tank 18. The system operates in similar manner to that of Figure 1 except that cooled fuel is return to the intermediate container 16 rather than to the T-piece 15 of Figure 1. Again it will be seen.that fuel flow measured by the sensor 20. accurately reflects the true consumption of the engine.

The intermediate container 16 can itself act to cool the fuel returned via return line 14. It is envisaged that the radiator 30 and intermediate container 16 could be combined to provide a single cooling means.

For efficient operation, the size of the intermediate container shall be related to the specific fuel consumption of the engine. It is believed that a capacity of one litre is suitable for engine sizes generally fitted to motor vehicles. Generally, the intermediate container should be large enough to cope with maximum fuel flow demanded by the engine, but small enough to ensure quick response of the sensor 20.

Where a vent pipe is fitted as shown at 24, the valve 25 should be suitable for outflow of vapour from the intermediate container 16. However, it should neither interefere with any vacuum effect in the container which may be necessary for suction of fuel from the main tank, nor should it allow fuel to return to the main tank, otherwise the measurements obtained by the flow sensor 20 will be inaccurate.

Although fuel line 19 is shown as entering intermediate tank 16 at the top, a side entry may alternatively be provided.

Any suitable arrangement may be used as the flow sensor 20, but the generally preferred device is a flow transducer as disclosed in co-pending European Patent Application No. 81301889.2.