EP1996808A1

EP1996808A1 - Heated fuel filter

Info

Publication number: EP1996808A1
Application number: EP07732014A
Authority: EP
Inventors: Lee Francis Ackrell
Original assignee: Regenatec Ltd
Current assignee: Regenatec Ltd
Priority date: 2006-03-13
Filing date: 2007-03-13
Publication date: 2008-12-03
Also published as: WO2007104968A1; CN101501316A; GB0605022D0

Abstract

A heated fuel filter assembly (5), particularly for use with engines operating using plant oil as a fuel. A filter housing (42) is connected to a filter head (39). A cylindrical filter screen (43) within the housing, separates the interior of the housing into internal (44) and external (45) portions. An inlet fuel conduit (40) communicates with the internal portion, and an outlet fuel conduit communicates with the external portion. An elongate heating member (46) projects axially into the internal portion so as to heat fuel within the filter assembly. The heating member may be a hollow member through which engine coolant circulates.

Description

Heated Fuel Filter

This invention relates to a heated fuel filter for an internal combustion engine. The invention is particularly concerned with a heated fuel filter for use in an environment in which plant oil is used as the fuel.

Plant oil causes problems in running internal combustion engines, foτ example an engine which has been converted from running on conventional diesel fufel. When operating an engine on plant oil, the viscosity of the cold fuel can cause a number of issues. These include fuel starvation caused by the excessive pressure required to pass a sufficient flow of fuel through the fuel filter. It is therefore known to heat the plant oil to reduce its viscosity. For example, an engine maybe adapted to start on conventional diesel fuel, and to continue with that fuel until plant oil has reached a sufficient temperature to run safely, at which point the engine is switched to run on the plant oil.

There have been various proposals to bring the plant oil up to a working temperature. Some of these have used heat exchange between the engine coolant and the plant oil, and this could be achieved, for example, by placing a coolant heated jacket around a fuel filter for the plant, oil. Existing method have various drawbacks such as complexity, cost, efficiency, ease of installation and so forth.

An object of the present invention is to provide an improved means for heating fuel whilst the fuel passes through a fuel filter.

Viewed from one aspect, the invention provides a fuel filter assembly comprising a generally cylindrical filter housing, a generally cylindrical filter screen within the housing, separating the interior of the housing into internal and external portions, a first fuel conduit communicating with the internal portion, a second fuel conduit communicating with the external portion, and an elongate heating member projecting longitudinally into the internal portion.

The heating member may extend generally parallel to the axis of the internal portion of the housing, and preferably extends along the axis of the housing.

By projecting into the fuel filter, preferably centrally - i.e. axially of the filter housing and filter screen - improved heating is provided as compared to using exterior heating means such as a jacket.

The heating member could be a solid rod metal provided with an electrical heating element, for example, or be in thermal contact with e.g. the engine coolant so that heat passes through the rod by conduction through the metal. Preferably, however, the heating member is hollow and heated by the engine coolant or another source of heated fluid passing through the member. There could be a number of passages in the heating member for carrying the heated fluid. In a preferred embodiment, however, the heating member comprises a sheath having a closed end and an end communicating with a conduit carrying engine coolant, and a hollow probe extending into the sheath to adjacent its closed end, the end of the probe which is adjacent the closed end of the sheath being provided with a first opening, and the other end extending into the engine coolant conduit and being provided with an opening which faces into the flow of coolant through the conduit.

Without the probe, coolant such as water would pass into the sheath, but would tend to be static once there, so that the temperature of the coolant in the sheath would gradually cool. The probe defines a second line of communication with the coolant, and it is anticipated that in general use the coolant will flow into the probe opening in the conduit, down through the probe, out of the opening, and then up the sheath, outside of the probe, and into the main flow of coolant through the conduit. The exact mechanism by which flow takes place is not material, and the fact is that there will be a constant flow of hot coolant through the heating member which will heat fuel within the filter assembly. In general the heating member will extend axially of the filter assembly and will pass through an opening defining the first fuel conduit, which will normally carry fuel into the filter assembly. The probe preferably extends axially of the sheath, and terminates, in the coolant conduit, in a portion which is bent so as to be parallel to the direction of coolant, flow, and terminates in the opening facing into the flow of coolant. In one preferred embodiment, a radially extending projection is provided around the portion which faces into the fluid flow. This projection, which could be provided by a grommet for example, increase fluid flow through the sheath.

Generally, the sheath will simply cominunicate directly with the coolant conduit, forming a branch off it. The terminating portion of the probe is preferably disposed approximately axially in the conduit.

In a preferred embodiment of the invention, a standard head housing for a screw on cartridge fuel filter is modified so that the fuel heating member can be inserted through the existing fuel input orifice. The preferred heating member is a coolant fed "pipe in pipe" heat exchanger, the pipes being constituted by the sheath and probe.

The arrangement has been found sufficient to force enough coolant through the heating member and to heat plant oil fuel to a sufficiently high temperature.

The invention extends to the fuel filter assembly, an engine incorporating such an assembly, a method of modifying a fuel filter assembly to be in accordance with the invention, and a method of operating an internal combustion engine using the improved filter assembly. The invention also extends to the heating member in the form of the sheath and probe, for use in modifying an existing fuel filter assembly.

Preferably the filter assembly is used in an engine operating on plant oil, and particularly one which operates initially on conventional diesel fuel, or another less viscous fuel, until the temperature of the plant oil has been increased sufficiently for - A -

it to be used. The filter assembly may, for example, be used in a system as disclosed in WO2006/005930, the contents of which are incorporated herein by way of reference. The following description of preferred embodiments of the invention is in the context of such a system but it will be appreciated that they may be used in many other contexts.

Embodiments of the invention will now be described by way of example and with reference to the accompany drawings, in which:

Figure 1 is a schematic diagram of a plant oil fuel system incorporating a fuel filter in accordance with the invention;

Figure 2 is a diagrammatic view of a filter assembly in accordance with one embodiment of the invention;

Figure 3 is a diagrammatic view of a modified type of heating member;

Figure 4 is a diagrammatic view of a modified type of heating member;

Figure 5 is a diagrammatic view of a modified type of heating member;

Figure 6 is a diagrammatic view of a modified type of heating member;

Figure 7 is a diagrammatic view of a modified type of heating member; and

Figure 8 is a diagrammatic view of a filter assembly incorporating the heating member of Figure 6.

Referring now to the drawings, the fuel system 1 comprises a primary fuel tank 2 containing a quantity of plant oil fuel, hi the arrangement shown, primary tank 2 is the conventional fuel tank for the vehicle i.e. the tank which would contain diesel fuel in a conventional installation. The fuel system also comprises an auxiliary tank 3 containing a quantity of conventional diesel fuel. The auxiliary tank 3 is generally of a smaller volume than the primary tank 2.

Primary fuel tank 2 is connected via fuel line 4 to a first plant fuel oil filter 5 which filters debris and prevents unwanted matter entering the remainder of the fuel delivery system.

From the fuel filter 5 the fuel passes to a fuel select valve 6 which is arranged to receive a control signal from an engine controller (not shown) and to switch fuel delivery to the remainder of the fuel delivery system either from the plant fuel supply in primary tank 2 or from the diesel fuel supply in auxiliary tank 3. The fuel select valve 6 receives fuel from the auxiliary tank 3 via fuel line 7 which includes an in-line cartridge filter 8 arranged to remove debris from the diesel fuel supply.

On receipt of the control signal the fuel select valve switches the fuel supply from or to the plant fuel source such that the respective tank (2 or 3) is ftuidly connected to a . T-ρiece 9.

T-piece 9 comprises a first input port 9a arranged to receive fuel from the fuel selection valve 6, and a second input port 9b arranged to receive fuel from a recirculation loop described below. The T-piece output port 9c communicates fuel received from the input ports to an injector pump 11, through a heat exchanger 10.

A first output 12 from the injector pump 11 is arranged to recirculate fuel, via fuel recirculation conduit 13, to a 3-port recirculation/flush valve 14.

The 3-port recirculation/flush valve 14 is provided with an input port 14a which receives fuel from the recirculation conduit 13 and two output ports. The first output port 15 is arranged to communicate the recirculated fuel back to the T-piece 9 such that the fuel recirculates around and through the heat exchanger 10. The second output port 16 of the 3-port recirculation/flush valve 14 is arranged to communicate fuel from the recirculation conduit 13 to the first fuel tank 2 via fuel conduit 17.

Fuel conduit 17 fluidly connects the 3 -port recirculation/flush valve 14 to a first input port 18a of a T-piece 18. T-piece 18 receives a second input through second input port 18b from an injector pump return line 19 which is described below.

The output from output port 18c of T-piece 18 communicates fuel to a first input 20a of a second T-piece 20 which outputs fuel via an output 20b directly into the first fuel tank 2. A second input 20c to T-piece 20 is connected to an injector bleed off line 21 which is described below.

The 3-port recirculation/flush valve 14 is arranged to receive a control signal from the fuel delivery system controller (not shown) which controls the valve such that fuel from the recirculation conduit 13 is either directed back to the heat exchanger 10 or to the first fuel tank 2. The operation of the valve will be described below.

Returning to the heat exchanger 10, the exchanger has a primary circuit connecting the T-piece 9 to the injector pump 11. The secondary circuit of the heat exchanger is provided with inlet port 22 and outlet port 33. In a conventional way, the secondary circuit is arranged to heat the primary circuit of the heat exchanger using heat communicated through the secondary circuit conduits. Heat is received directly from conduits connected to the interior heater of the vehicle which in turn receives heat from the engine which would otherwise be released to atmosphere. Receiving heat from the interior heating has an advantage over rec'eiving heat directly from the radiator because the radiator circuit has a thermostatic valve which does not open until the coolant has reached a particular temperature. This would not therefore allow the recirculation loop to be heated quickly,

The injector pump 11 is, in the arrangement shown, a combined lift pump arranged to circulate fuel around the recirculation circuit, and high pressure pump arranged to pass fuel at high pressure to the engine injectors. The injector pump 11 is also provided with an integral resistive heater (not shown) to provide supplementary heating before the engine has reached a satisfactory operating temperature as described below. This heater is controlled so that it only operates if the temperature is too cold and to the extent that there is sufficient electrical capacity in the electrical system from the engine.

The first output 12 from the injector pump is the output from the lift pump which circulates fuel around the recirculation loop as described above. A second output 23 from the injector pump is connected to the injector pump return line 19 which, as described above, connects to the input 18b of the T-piece 18. The injector pump return line 19 is arranged to communicate excess fuel and/or air which may leak into the pump (either from the pump housing, connections leading to the pump or from the conduits communicating fuel), safely and directly back to the fuel tank 2. This arrangement prevents air building-up in the recirculation conduit 13 since any air is diverted to the fuel tank 2 on each cycle.

In the arrangement shown in figure 1, a compression ignition engine 34 has four combustion chambers 35, 36, 37 and 38, with corresponding high pressure fuel injector output lines 24, 25, 26, 27 from the pump 11. Each of the high pressure output lines is connected to the high pressure portion of the injector pump 11 which pressurises the fuel for delivery to the respective injectors 28, 29, 30, 31 via the lines 24, 25, 26 and 27. The injector pump and injectors operate in a conventional way and will not therefore be described in detail.

Each of the injectors 28, 29 ,30, 31 is connected in series to the injector bleed off conduit 21. The injector bleed off conduit is arranged to communicate excess fuel, or air which may leak into the injector or bleed lines, safely and directly back to the fuel tank 2 via the T-piece 20 described above. This prevents any air build-up in this portion of the fuel delivery system which, in conventional systems, is diverted into the recirculation conduit described above. The fuel delivery control apparatus (not shown) is also connected to a thermistor 32 which is located immediately after the injector pump in the recirculation conduit 33 which is connected to the first output 12 of pump 11. The thermistor 32 is located at least partially within the fuel flow i.e. within the conduit 33, so as to provide an accurate indication of the fuel temperature immediately after leaving the fuel pump.

The individual components and interconnections of the fuel delivery system are described above. In operation the delivery system operates as follows.

In a start up state, the fuel select valve 6 is arranged to direct fuel from the auxiliary fuel tank 3 to the heat exchanger and thence to the injector pump 11. The 3-port recirculation/fJush valve 14 is set to direct fuel from the recirculation conduit 13 to the T-piece 9.

At start the fuel in the first tank 2 is at ambient conditions and has a high viscosity. The fuel in the auxiliary tank is, as described above, conventional diesel fuel.

The engine is started in a conventional way and receives diesel fuel from the auxiliary tank 3, through the fuel filter 8 and fuel select valve 6, through the heat exchanger 10 and into the injection pump 11. A portion of the fuel is pressurised in a conventional manner and directed to each of the injectors with the remainder of the fuel being recirculated around the recirculation conduit 13, 3-port recirculation/flush valve 14 and back to the T-piece 9.

In this 'start-up' sequence the engine operates in a conventional way with fuel being delivered to the fuel injectors 28 to 31.

As the engine continues to operate the temperature of the engine coolant will begin to increase. The resultant increase in engine coolant temperature causes a corresponding rise in the heat exchanger secondary circuit temperature (as a result of coolant being circulated from heat exchanger inlet 22 to outlet 33). The temperature of the fuel in the recirculation conduit is continuously measured by thermistor 32 and signals provided to a fuel delivery control apparatus or unit (not shown).

The temperature in the recirculation loop provides an accurate representation of the temperature of the fuel within the injector pump itself and provides the control unit "with a signal indicating a minimum temperature of fuel in the injector. The fuel temperature in the conduit will either be substantially the same as the temperature in the fuel pump or below the temperature in the fuel pump.

The engine continues to operate in a 'start-up' mode in which the fuel select valve 6 and valve 14 are arranged as described above. The engine continues to operate and the engine block temperature and coolant temperature rise. There is a corresponding rise in the heat exchanger primary circuit temperature which in turn heats the fuel passing through the primary circuit which, in this start-up mode, is diesel.

The control¹ unit is pre-programmed with a temperature at which the fuel can be switched over from conventional diesel to plant oil fuel i.e. from the auxiliary tank 3 to the first tank 2. This pre-programmed value is determined as a temperature at which there is sufficient thermal capacity in the heat exchanger 10 to raise the plant fuel oil to a temperature at which the viscosity is sufficiently low to avoid injection pump and/or injector damage. This temperature is typically 80 degrees C.

Thus, the engine continues to operate in a conventional way until the temperature of fuel leaving the fuel pump is at (or above) the pre-programmed temperature as measured by the thermistor 32. This indicates that the recirculation conduit and heat exchanger have reached a state at which the heat exchanger can heat fuel in the primary circuit to above 80 degrees C i.e. above the temperature required to sufficiently reduce the viscosity of the plant oil when the engine is switched over to plant fuel. It will be appreciated that the temperature within the fuel pump may be higher than the temperature in the conduit as measured at thermistor 32.

Once the pre-programmed temperature is reached the control unit sends a control signal to the fuel selection valve 6 which switches the fuel supply to the recirculation conduit from diesel (in tank 3) to plant oil (in tank 2). Plant fuel then passes from tank 2, via filter 5, to the T-piece 9 and into the heat exchanger 10.

At switch-over the recirculation circuit initially contains only diesel fuel into which the plant oil is now introduced. Thus, as the engine runs and the plant oil is recirculated the ratio of plant oil to diesel oil increases until the recirculation circuit only contains plant oil.

As plant oil is introduced it passes through the primary circuit of the heat exchanger 10 which raises the temperature to, or above, 80 degrees C. The plant oil fuel passes into the fuel injector high pressure pump where it is pressurised and pumped to the injectors 28 - 31 for injection into the combustion chambers 35 to 38.

Any fuel leaking from the injectors is directed via conduit 21 to the first tank 2. Since the conduits between the injectors are prone to leakage, any air leaking in is directed to the first tank 2, thereby reducing the build-up of air. Similarly, air leaking into the injector pump or fuel lines connected thereto is directed via conduit 19 to the first tank 2, again preventing a build-up of air.

The engine continues to operate using pure plant oil fuel from tank 2 with the combustion process corresponding to that of a conventional diesel cycle.

hi the situation where the ambient temperature drops it may be necessary to operate the engine using a blend of diesel and plant oil. In this arrangement the fuel select valve 6 may be a fuel blending valve arranged to blend a proportion of the first and second fuels in response to indications from the control unit which in turn receives - π -

input signals from the thermistor 32 indicating recirculation fuel temperature (and thereby viscosity).

At a time when the user wants to turn-off the engine, for example at the end of a journey, the fuel delivery system must be flushed of plant oil. The engine is therefore arranged to go through a 'flush cycle¹ before engine shut down to prevent viscous plant oil remaining in the engine as the engine cools. The flush cycle operates as follows :

Before engine shutdown the fuel supply control unit operates the 3-port recirculation/ftush valve 14 so as to direct fuel via conduit 17 and T-pieces 18 and 20 to the first tank 2. Thus, the recirculating fuel (which is pure plant oil) is directed out of the recirculation loop and back to the plant oil fuel tank. The engine continues to operate as normal. After a time lag of approximately 5 seconds the control unit switches the fuel select valve 6 from plant fuel to diesel fuel.

Diesel fuel then enters the heat exchanger and fuel pump and flushes the plant oil from these components and from the injectors and recirculation conduit 13. After a further preset time of 60 seconds the flush valve 14 is switched again thereby directing the fuel to the T-piece 9 and around the recirculation loop. An audible alarm is then given to the user after a further time period of 30 seconds indicating that the flush cycle is complete and that it is safe to shut-down the engine.

The flush cycle essentially comprises three stages : Stage one causes plant oil to move at a.higher than normal rate in the entire fuel line (by directing fuel to the first tank) thus preventing contaminants from collecting. Stage two rapidly purges plant oil from the recirculating loop (i.e. heat exchanger, valves and injection pump) using the pure diesel fuel. Stage three is timed, to allow the injectors to be purged of plant oil with pure diesel. This sequence enables the system to optimise the amount of diesel used for the flushing process. The flush cycle prevents high viscosity fuel remaining in engine components as the fuel begins to cool and become more viscous. Turning now to Figure 2, there is shown a first embodiment of the plant fuel oil filter 5. The filter includes a fixed filter head 39 having a central fuel inlet 40 of circular cross section and a fuel outlet 41. Releasably attached to the filter head 39 in a conventional manner is a conventional type of filter cartridge 42 of generally cylindrical shape. Within the filter cartridge is a cylindrical filter screen 43, dividing the filter cartridge into a central cylindrical space 44 which receives the unfiltered fuel through the inlet 40, and an outer annular space 45 which receives the filtered fuel which passes out through outlet 41. To this extent, the filter is conventional.

Extending axially into the central space 44 is an elongate, cylindrical, electrical heating element 46 connected via leads 47 to a source of power. There may be control circuitry, a thermostat and so forth. The heating element 46 heats the fuel in the filter from the central region.

The heating element 46 extends axially by a sufficient extent to provide heating of the fuel in the central space 44 of the filter cartridge. Thus, for example it may extend for substantially the entire axial extent of the central space, or for a major part, or at least halfway, or at least two thirds or at least three quarters. The same will apply to the heating elements in the other embodiments.

The diameter of the heating element, in this and the other embodiments, should be chosen so that it does not obstruct fuel flow through the inlet excessively. However, it may occupy a significant amount of the cross section of the fuel inlet and in one practical embodiment the inlet has a diameter of 10 mm and the heating element has a diameter of 8 mm. This reduces the flow cross section available for the fuel, but in the context of the system of Figure 1 thus is not a problem, because the volume of fuel that has to pass through the filter is less than in a conventional arrangement. In the system of Figure 1, excess fuel is circulated around the recirculation loop, without passing through the filter 5 again. In a conventional system, excess fuel would pass back to the tank and then pumped through the filter again. Figure 3 shows an alternative embodiment, the cartridge and the filter head being omitted for the purposes of clarity in this and subsequent figures. The heating element in this embodiment is an elongate rod 48 of steel or another suitable material. This is connected at its upper end to a metal plate 49, which in turn is connected to a metal component 50 of the engine, such as an exhaust manifold, engine block and so forth. Heat is transferred to the heating element by conduction through the plate.

In a modified arrangement as shown in Figure 4, the plate 49 is connected to a metal conduit 51 through which engine coolant passes. Heat is conducted to the heating element through the plate 49.

A further arrangement is shown in Figure 5. In this embodiment the heating element is an elongate cylindrical member 52 which is connected directly to the coolant conduit 51. The interior 53 of the tube member opens into the conduit 51 at a junction 54. In this embodiment, coolant passes into the member 52 to as to heat it.

It has been found that in some circumstances the water in the heating element tends not to circulate, meaning that it does not reach the same temperature as the coolant in the main conduit 51, or at least as quickly. In the alternative arrangement of

Figure 6, an elongate tube 55 extends into the interior 53 of member 52. At its upper end, the tube 55 terminates in a laterally extending portion 56 with an open end 57 which faces into the flow of coolant through conduit 51. At its lower end, the tube 55 has an opening the interior 53 of the member 52. This arrangement promotes the flow of coolant into the interior 53 of the member 52 and then back out again into the main conduit 51. It is believed that there is a flow path through the tube 55 with coolant exiting into the bottom of the member 52, and then flowing up the annular space around tube 55 to the junction 54 with the main coolant conduit 51.

Figure 7 shows a modification of the embodiment of Figure 5, in which a barrier, such as an O ring, is provided around the laterally extending portion 56. The barrier may extend wholly or partially to the wall of the conduit 51, and will encourage a greater flow of coolant into the tube 55.

In the embodiments of Figures 4 to 7, the engine coolant conduit 51 is preferably formed as a passage in the filter head 39.

Figure 8 shows a filter assembly 5' which has a filter head 39' which is a modification of the filter head 39 in Figure 2. In addition to fuel inlet 40' and outlet 41', the filter head incorporates an engine coolant conduit 51'. The heating element is as described with reference to Figure 6, but the tube 55 projects into coolant conduit 51' in the filter head 39'.

It will be appreciated that the embodiments are for illustrative purposes only and there may be many variations within the scope of the invention.

The invention may be vied from many different aspects. For example, viewed from a further aspect the invention provides a fuel filter assembly comprising a filter housing, a filter within the housing, separating the interior of the housing into ^■ internal and external portions, a first fuel conduit communicating with the internal portion, a second fuel conduit communicating with the external portion, and an elongate heating member projecting into the internal portion. Viewed from another aspect the invention provides a fuel filter assembly comprising a filter head, a filter cartridge releasably attached to the filter head, and an elongate heating member projecting centrally into the filter cartridge from the filter head.

Claims

1. A fuel filter assembly comprising a generally cylindrical filter housing, a generally cylindrical filter screen within the housing, separating the interior of the housing into internal and external portions, a first fuel conduit communicating with the internal portion, a second fuel conduit communicating with the external portion, and an elongate heating member projecting longitudinally into the internal portion.

2. A fuel filter assembly as claimed in claim 1 , wherein the first fuel conduit is ah inlet and the second fuel conduit is an outlet.

3. A fuel filter assembly as claimed in claim 1 or 2, wherein the heating member extends along the axis of the internal portion of the housing.

4. A fuel filter assembly as claimed in claim 1 , 2 or 3, wherein .the elongate heating member extends over a major part of the axial extent of the internal portion of the housing.

5. A fuel filter assembly as claimed in any preceding claim, wherein the heating member comprises an electrical element.

6. A fuel filter assembly as claimed in any of claims 1 to 4, wherein the heating member comprises an elongate solid member, connected to a heated component to receive heat by conduction.

7. A fuel filter assembly as claimed in claim 6, wherein the heating member is adapted to be connected to an engine component.

8. A fuel filter assembly as claimed in claim 6, wherein the heating member is adapted to be connected to an engine coolant conduit.

9. A fuel filter assembly as claimed in any of claims 1 to 4, wherein the heating member comprises an elongate hollow member adapted for connection to an engine coolant conduit to receive coolant from the conduit..

10. A fuel filter assembly as claimed in claim 9, wherein a tube extends into the elongate hollow member from the engine coolant conduit, the tube having a portion extending into the direction of coolant flow.

11. A fuel filter assembly as claimed in claim 10, wherein a flow restriction barrier is provided between the tube portion and the coolant conduit .

12. A fuel filter assembly as claimed in any preceding claim, wherein the filter housing is releasably attached to a filter head.

13. A fuel filter assembly as claimed in any of claims 8 to 11, wherein the filter ^"hnnsin^p is releasably attached to a filter head and the coolant conduit is provided in the filter head.

14. A fuel filter assembly as claimed in any preceding claim, incorporated in a ^■ fuel system of an internal combustion engine.

15. A fuel filter assembly as claimed in claim 14, wherein the internal combustion engine operates using plant oil as a fuel.

16. A fuel filter assembly as claimed in claim 15, wherein the first fuel conduit receives fuel from a plant oil fuel tank and the second fuel conduit supplies filtered plant oil to a recirculating circuit for supplying fuel to an injector pump and receiving excess fuel from the pump without the excess fuel passing to the plant oil fuel tank.