GB2076056A - Fuel heating device - Google Patents

Abstract

An electrically operated fuel heating device includes a body 11 in which is formed a passage 16 through which fuel can flow. The body is formed from metal and has an electrically operated heat generating means 17 mounted on an exterior surface thereof so that the heat generating means 17, conveniently comprising power transistors (23), Fig. 2 (not shown), does not come into contact with the fuel. Associated with the power transistors (23) is a circuit, (Fig. 2), for controlling the current supplied to the transistors in response to temperature. <IMAGE>

Description

SPECIFICATION Fuel heating device This invention relates to an electrically operated fuel heating device for use in a fuel supply system of a compression ignition engine.

It is well known that fractions of the fuel supplied to compression ignition engines can freeze in cold conditions. The frozen fractions of fuel can clog the filters which are employed in such systems and this can lead to the prevention of fuel flow to the engine. It is also known to provide electrically operated heating devices which heat the fuel and one form of such a device is shown in the specification of United States Patent 4091265. In this case the heating device is located within a chamber through which the fuel flows before it is passed through the filter.

The heating device described in the aforesaid specification comprises a heating unit mounted in the chamber, the heating unit carrying a pair of resistors in which most of the electrical power is dissipated, and a pair of power transistors which control the electric current flow in the resistors respectively. In addition, the unit carries control transistors and a temperature responsive device.

The various components are immersed in the fuel which flows through the chamber. The disadvantage with this type of device is the fact that the surface areas of the heated components are very low, and therefore in order to achieve rapid heating the temperature of the components must bye fairly high. This can cause a problem if the chamber should empty of fuel and in such a case, the risk arises of the surfaces of the components becoming dangerously heated. A further disadvantage is the fact that the fuel is in contact with the electrical components which therefore have to be chosen to withstand the effect of the fuel and also any contaminant in the fuel.

The object of the present invention is to provide a heating device for the purpose specified in a simple and convenient form.

According to the invention an electrically operated fuel heating device for use in a fuel supply system of a compression ignition comprises a body defining at least in part, a fuel flow passage through which fuel can flow, the body being formed from heat conductive material, electrically operated heat generating means mounted on said body in heat exchange relationship therewith but out of contact with fuel flowing in use in said passage, and means for controlling the electrical current flow to said heat generating means whereby the heat supplied to the body can be controlled.

An examDle of a heating device in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is an exploded view of the device, and Figure 2 is shown on two sheets and shows one example of a circuit diagram of the device.

Referring to Figure 1 of the drawings the device comprises a multi-part body the first part of which is indicated at 10 and the second part at 1 The body part 10 defines a stepped recess and both body parts are formed from metal for example a light alloy. The body part 10 has an inlet 12 and an outlet 13 which communicates by way of passages in the body part 10, with the innermost portion of the recess.

The body part 11 is shaped to fit into the innermost portion of the recess and it is provided with an apertured flange 14, the apertures receiving screws whereby the flange can be held adjacent the step 15 defined in the body part 10.

A gasket 22 is located between the flange and the step 1 5 during assembly, the gasket acting as a fuel seal but also acting to minimise heat transfer between the parts of the body. The body part 11 defines a passage or passages formed by drillings which lie in close proximity to the outer surface of the body part 11 from which the flange 14 extends, drillings are provided in the body part 11 so as to connect with the passages formed in the body part 10. The passage in the body part 10 which communicates with the outlet 1 3 is provided with a recess to accommodate an "O" ring to establish a seal with the body part 11 and whilst as illustrated, the body part 11 is provided with a single passage, there may be as many as are thought necessary to achieve satisfactory.heat exchange.

The body part 11 carries a plurality of power transistors 17, these being mounted in heat exchange relationship with the surface of the body part 11. The transistors form the heat generating means. Conveniently when the bodies have been assembled potting compound is poured into the recess up to the level of the step 1 9.

Also provided is a printed circuit board generally indicated at 1 8 and which is secured as by rivets, against the step 21 in the body part 10.

The printed circuit board carries a plurality of components which together form a circuit for controlling the current flow in the power transistors. Finally a cover 20 is provided which is secured to the rim of the body part 10. In use, fuel flows through the passages in the body parts 10 and 11 and becomes heated because the body part 11 is heated by the power dissipated in the power transistors. The surface area available for the transfer of heat is substantially larger than in the case of the device shown in the aforesaid United States Specification. The surfaces therefore do not have to be heated to such a high temperature to ensure adequate heating of the fuel. Moreover, the fuel does not come into contact with the electrical components and therefore there is no need to design or arrange these to withstand the effect of the fuel or any contaminant.The passage in the body 11 may be formed by a groove in the face of the body presented to the base wall of the recess, said base wall acting to close the groove.

Turning now to Figure 2, this shows one example of a circuit of the device. The aforesaid power transistors are indicated at 23 and it is in these transistors that substantially all the power is dissipated. The transistors are in fact of the type known as P channel Mosfets. The circuit includes a positive supply rail 24 which is connected in use, by way of a fuse to the positive terminal of the vehicle storage battery, the negative terminal of the battery being connected to ground, the circuit including a negative supply rail 25. The drain terminals of the transistors 23 are connected to the positive supply rail 24 whilst the source terminals are connected by way of a common source resistor 26 to the negative supply rail.

Moreover, connected between the supply rails 24 and 25 is a transient absorption device 27 and the gate terminals of the transistors are each connected to a supply rail 28 by way of respective resistors 29. A further positive supply rail 30 is provided and this is connected to the storage battery of the vehicle by way of a choke 31, a fuse and a switch, a further transient absorption device 32 being connected between the junction of the choke 31 and the fuse and the negative supply line 25. A pair of capacitors 32A, 33 are connected between the supply rails 25 and 30.

The source terminals of the transistors 23 are connected by way of a resistor 34 to the inverting input terminal 35 of one half 36 of a duai comparator integrated circuit, type CA3290, the other half of which is indicated at 37. Terminal 8 of the integrated circuit is connected by way of a resistor 38 to the supply rail 30 and terminal 4 is connected to the supply rail 25. A capacitor 39 is connected between the inverting input terminal 35 and the supply rail 25 and a resistor 40 is connected between the terminal 35 and the supply rail 30. The non-inverting input terminal 41 is connected to the supply rail 25 by way of a pair of resistors 42, 43 connected in series with the resistor 43 being bridged by a capacitor 44.The output terminal 45 of the comparator is connected to terminal 41 by way of a diode 47 and a resistor 48 and to the supply rail 30 by way of resistors 49, 50 connected in series. The aforesaid supply rail 28 is connected to the junction of the resistors 49 and 50. Moreover, the junction of the resistors 49 and 50 is connected to the supply rail 25 by way of a capacitor 51.

A field effect transistor 52 is provided having its source connected to the supply rail 25 and its drain to the output terminal 45 of the comparator 36. The gate of the transistor 52 is connected to the supply rail 30 by way of a resistor 53 which is bridged by a diode 54 and it is connected to the supply rail 25 by way of a capacitor 55. The output terminal 45 of the comparator 36 is connected to the output terminal 56 of the comparator 37. The supply terminal 8 of the integrated circuit is connected to the supply terminal 25 by way of a pair of capacitors 57, 58 and the non-inverting input terminal 59 is connected to the junction of a pair of resistors 60, 61 which are connected in series between the supply rails 30 and 25. The resistor 61 is bridged by a capacitor 62.The inverting input terminal 63 of the comparator 37 is connected to the inverting input terminal of the comparator 36 and by way of a resistor 64 to terminal 3 of a proprietary temperature controller integrated circuit 64. This integrated circuit is a National LM 3911 type.

Terminal 2 of the integrated circuit is connected to terminal 3 by way of a resistor 65 having a capacitor 66 connected in parallel therewith and in addition terminal 2 is connected by way of resistors 67, 68 connected in series to the supply line 25. The junction of the resistors 67 and 68 is connected by way of a resistor 69 to the source terminals of the transistors 23. Terminal 4 of the integrated circuit 64 is connected to the supply rail 25 and terminal 3 is additionally connected to the supply rail 25 by way of a current regulation diode 70 conveniently of type IN 5304.

The controlling voltage on the rail 28 of the transistors 23 is a function of four circuits which perform the duties of temperature control, power dissipation limit, initial low voltage drop out and final low voltage clamp. The temperature control circuit is based upon the integrated circuit 64. This circuit includes an internal component which sets the voltage appearing between terminals 3 and 4 and this voltage is applied to the potentiometer chain comprising resistors 65, 67 and 68. The integrated circuit also includes an operational amplifier connected as a comparator. The sensor which is contained in the integrated circuit is connected to the comparator the other terminal of which is connected to terminal 2 and as the temperature rises the sensor output will decrease and when it falls below the voltage at terminal 2, the operational amplifier output will switch to low.

The output of the operational amplifier is connected to terminal 1, and this is turn is connected to the point which is referenced in the circuit diagram VX. When this is effectively connected to the supply terminal 25, the transistors 23 will be turned off. A measure of proportional control is provided by the feed back through resistor 69. This means that with the device set by adjustment of resistor 65, to switch on at say + SOC, the maximum power will not be developed until the sensed temperature has fallen to say - 50C.

The control of the power dissipation by the transistors is effected by monitoring the current flowing through them and the voltage across them. The current is sensed by the comparator 36 the inverting input terminal of which is connected by the resistor 34 to the source terminals of the transistors. The battery voltage is sensed using resistor 40, the resistors 40 and 34 feed the inverting input terminal 35 whilst the noninverting input terminal is provided with the reference voltage obtained from the integrated circuit 64. When the power dissipation in the transistors 23 is low, pin 35 of the comparator 36 is less positive than pin 41 and the output of the comparator 36 is therefore high. As will be observed, the output terminal 45 is also connected to the point VX. This allows the transistors to be switched on. As the power dissipation increases further by reason of an increase in voltage and/or the current, pin 35 becomes more positive and may become more positive than pin 41 in which the output of the comparator goes low and as with the temperature control, the transistors 23 will be turned off. Single polarity positive feedback is provided by diode 47 and resistor 48 and this provides a positive switching.

It is necessary to provide initial low voltage dropout because the transistors 23 are controlled for power dissipation and hence if the battery voltage were very low there could be an excessive current flow through the transistors. Sensing of battery voltage is obtained using the comparator 37, the non-inverting input terminal 59 of which is provided with a proportion of the battery voltage by way of the potential divider constituted by the resistors 60 and 61. As with the comparator 36, the reference voltage is supplied by the integrated circuit 64. The circuit is arranged by adjustment of resistor 60, to switch off the transistors when the battery voltage falls to approximately 10 volts.

Additional protection is needed if the battery voltage should fall to a very low value, say 4 volts, at which the comparators 36 and 37 will not operate correctly. This protection is achieved using the transistor 52. This transistor is arranged to clamp the point VX to the supply rail 25 when the battery voltage on the supply rail 30 is below the gate voltage. Transistor 52 is selected so that for the expected temperature range the gate voltage lies between 5 and 9 volts. When the circuit is switched on capacitor 55 ensures that transistor 52 clamps until such time as the voitages applied to the comparators have stabilised. Moreover, diode 54 provides a rapid discharge path for the capacitor 55 to ensure that transistor 52 clamps when the battery voltage decays as for example when the aforesaid switch is opened.The choke 31 and the capacitors 32, 33 provide filtering of the supply to the comparators and the integrated circuit, and the supply to the comparators 36, 37 is further filtered by means of the capacitors 57, 58.

The voltage applied to the gates of the transistors 23 is that which appears across the capacitor 51 and this is the voltage which appears at the junction of resistors 50 and 49. When the gate voltage of the transistors 23 is above the threshold voltage, the current flowing in the transistors 23 is directly proportional to the voltage across the capacitor.

As shown in the circuit diagram a lamp 71 is provided and this is controlled by a transistor 72 which has its drain connected to the positive terminal of the battery by way of the lamp. The source and gate are connected to the sources and gates of the transistors 23. A diode 73 is provided to pass any voltage spikes on to the supply rail 24 where they can be absorbed by the device 27.

For full power operation, the supply voltage must be between 11 and 1 5 volts which therefore disables the comparator 37 and the transistor 52.

Moreover, the sensed temperature must be below the dynamic operation limits of integrated circuit 64. Under these conditions the voltage on capacitor 51 will rise thereby producing an increasing output current until comparator 36 is switched so that its output goes low. Capacitor 51 will rapidly discharge through resistor 49 and this will cause a reduction in the output current until the comparator 36 again switches so that its output goes high. This cycle is repeated giving a substantially constant mean power output.

As the sensed temperature increases the output of integrated circuit 64 will interrupt the full power cycle by taking current from capacitor 51. As a result the voltage across the capacitor will decrease and there will be a reduction in the current which by way of resistor 69, will cause the integrated circuit to revert to its former condition so that comparator 36 will temporarily again take control. As the temperature continues to increase the degree of control effected by the integrated circuit 64 will increase until a point is reached at which no power will be dissipated by the transistors.

Claims (10)

1. An electrically operated fuel heating device for use in a fuel supply system of a compression ignition engine comprising a body defining at least in part, a fuel flow passage through which fuel can flow, the body being formed from heat conductive material, electrically operated heat generating means mounted on said body in heat exchange relationship therewith but out of contact with fuel flowing in use in said passage, and means for controlling the electrical current flow to said heat generating means whereby the heat supplied to the body can be controlled.

2. A heating device according to Claim 1 in which said heat generating means is clamped on a surface of said body.

3. A heating device according to Claim 2 in which said body includes a peripheral flange, the device including a further body defining a stepped recess in the innermost portion of which said first mentioned body is located with said flange being positioned adjacent a first step defined by said further body, a sealing gasket located between said first step and said flange and means securing said bodies in assembled relationship, said sealing gasket acting to provide heat insulation between the two bodies.

4. A. heating device according to Claim 3 including a fuel inlet and a fuel outlet formed in the further body, said fuel inlet and fuel outlet communicating with the opposite ends of said passage.

5. A heating device according to Claim 4 including a circuit board mounting component forming the means for controlling the electric current flow, said circuit board being supported upon a second step defined by said further body.

6. A heating device according to Claim 5 including a cover secured to said further body to close said7recess.

7. A heating device according to Claim 4 in which said passage is defined by a driliing or drillings in said first mentioned body.

8. A heating device according to Claim 3 in which said passage means is defined by a groove formed in said first mentioned body, the groove being closed by said further body.

9. A heating device according to Claim 2 in which said heat generating means comprises a power transistor.

10. An electrically operated fuel heating device substantially as hereinbefore described with reference to the accompanying drawings.