GB2176955A - Starting control circuits for compression ignition engines - Google Patents

Abstract

A control circuit for controlling the operation of a starting aid 11 of a compression ignition engine includes a power circuit 16 which controls the power dissipated in the starting aid 11 and a timer circuit which controls the operation of the aid so that it remains in operation after the engine has started. The circuit includes a one terminal temperature responsive device 22 which is mounted on the engine structure so as to be responsive to the engine temperature, the device acting to determine the length of time the aid remains in operation once the engine has started. A switch 17 is closed when the engine starting motor is energised so that a capacitor 26 charges via this switch. The voltage on capacitor 26 when the switch 17 opens at the end of the engine cranking period depends on the temperature sensed by device 22 and a comparator 23 operator so that the aid 11 remains energised after opening of switch 17 for a period dependent on such capacitor voltage. A further comparator (not shown) may be connected to device 22 to prevent charging of capacitor 26, or discharge it, if the engine temperature is above, or rises above, a predetermined level. <IMAGE>

Description

SPECIFICATION Control circuit This invention relates two a control circuitforcontrolling the operation of a starting aid or aids of a compression ignition engine.

Such an aid may be ofthe type which is mounted within the cylinder of the engine and has an electric heating element which is directly exposed within a combustion chamber ofthe engine. In this case there are as many aids as there are engine cylinders.

Another type of aid is of the type which is mounted in the manifold of the engine and includes an electric heating elementwhich in operation, vaporises liquid fuel and causes ignition ofthe air/fuel mixture,the resulting flame heating the air flowing to the engine.

The heating elements in order to assurethe aid or aids reach theirworking temperature quickly are overrated so that it is essential to provide some form oftemperature control circuit so as to ensure that they do not exceed the safe working temperature.

When starting a compression ignition engine itis desirableto arrange for a short period of preheating before cranking ofthe engine takes place, to maintain the aid or aids in operation while cranking the engine and to maintain the aid or aids in operation once the engine has started to assure satisfactory combustion ofthefuel bythe engine during warm up. it is known to provide a timing device of fixed period for the latter purpose and the length ofthefixed period hasty be chosen so as to caterforthe coldest conditions and the slowest rate of engine warm up.As a result there may be many times during the life of the engine when the aid oraids are maintained in operation oncethe engine has started, for an unnecessary length oftime with a consequent heavy drain on the electrical sys tem ofthe engine and a shortening ofthe working life ofthe aids and the heavy duty relay which is normally provided to control the currentflowing in the aids.

The object of the present invention is to provide a control circuitforthe purpose specified in a simple and convenient form.

According to the invention a control circuitforcon trolling the operation of a starting aid oraidsfora compression ignition engine comprises a power control circuitforcontrolling currentflow in the aid or aids, and a timing means operable upon termination of cranking of the engine to maintain the powercircuit in an operative condition whereby the aid or aids remain energised, said timing means including a one terminal temperature responsive means whereby the on period of said timing means varies in accordance with the temperature.

An exampleofa control circuitin accordancewith the invention will now be described with reference to the accompanying circuit diagram which shows por tionsofthecircuitin blockform.

The circuit includes terminals 10 and 12forconnec- tion to the negative and positive terminals of the storage battery of the electrical system associated with the engine. The starting aids are collectively indicated at 11 being ofthetypewhich are mounted in the combustion chambers ofthe engine. Oneterminal of each aid is connected to the terminal 10conveniently by way of the engine structure and the other terminals can be connected to the terminal 12 byway of a normally open relay contact 13, the operating coil of the relay being indicated at 14. One end of the coil is connected to terminal 12 and the other end ofthe coil can be electrically connected to the terminal 10 by way ofthe collector emitter path of a transistor 15.

The conduction of the transistor is controlled buy a power control circuit 16 which when in operation, ensures that the heating elements of the aid reach their safe working temperature as quickly as possible and then maintainsthem substantiallyatthattemperature by switching the aids on and off byway ofthe transistor 15 and the relay. The power control circuit switches the aids on as soon astheterminals 10 and 12 are connected to the source of supply and it in cludes a so-called preheat timer circuit which in the eventthat no attempt is made to crankthe enginefor starting purposes, will cause the supply of current to the aids to be cut off after a predetermined time.In the event that cranking of the engine takes place, the preheattimercircuitisinhibitedsothattheaidswill be energised while the engine is being cranked. The indication ofthefactthatthe engine is being cranked is provided by a switch 17 one terminal of which is connected to the terminal 12. The switch is closed when the starting motor of the engine is energised.

The remaining portion ofthecircuitisforcontrolling the time during which the starting aids are maintained in an energised state following starting ofthe engine this being indicated by opening of the switch 17.

A reference voltage supply 18 is provided this being connected to terminal 12 byway of a resistor 19 and to the terminal 10 by way of a zenerdiode 20. Connected between the supply line 18 and terminal 110 is the series connection of a resistor 21 and a resistor 22, the latter having a negative temperature coefficient.

This resistor is mounted on the engine structure convenientlyso asto be responsive to the engine coolant temperature. The voltage intermediate the resistors varies in accordance with the engine coolanttemperature being low when the engine is hot and high i.e.

more positive, when the engine is cold.

Also provided is a comparator 23 one inputterminal of which is connected to a potentiometer chain comprising resistors 24,25 connected between the line 18 and the terminal 10. The other input of the comparator is connected to terminal 10 byway of a capacitor 26 which has a resistor 27 connected in parallel therewith. This same input of the comparator is connected to the cathode of a diode 28 the anode of which is connected by way of resistors 29,30 in series, to the switch 17. The junction of the resistors 29 and 30 is connected to the anode of a diode 31 the cathodeofwhich isconnectedtothelinel8andthe anode ofthe diode 28 is connected to the anode of a further diode 32 the cathode of which is connected to the junction ofthe resistors 21 and 22.

In operation, before an attemptismadeto startthe engine the capacitor 26 will be discharged by the resistor 27 and whentheterminals 10 and 12 are connected to the storage battery the capacitor 26 will remain in a discharged state. The output of the com paratorwill be such as would preventthe power control circuit switching the starting aids on. This however is overruled by the aforesaid preheat timer circuit so thatthe starting aids will beturned on. When the engine starting motor is operated the switch 17 is closed and the capacitor 26 is charged by way ofthe resistors30,29 and the diode 28 in series. The voltage intermediate the resistors 29 and 30 is clamped by the diode 31 so that it exceeds the voltage on the line 18 by the volt drop ofthe diode 31.Moreover the voltage at the anode ofthe diode 28 is clamped by the diode 32 so that it exceeds the voltage at the junction ofthe resistors 21 and 22 by the volt drop across the diode 32. The capacitor is therefore charged to a voltage which is less than the voltage on the anode ofthe diode 28 by the amount of the volt drop acrossthe diode 28. In other words it is substantially equal to the voltage at the junction of the resistors 21 and 22. This voltage is above the reference voltage provided by the resistors 24 and 25 and the comparator output switches to allow operation ofthe starting aids.

When the engine starts as indicated to the circuit by opening of the switch 17, the charging circuit for the capacitor is broken and the capacitor starts to discharge by way of the resistor 27. When the voltage across the capacitorfalls to the reference value determined by the resistors 24 and 25, the comparator changes state and the power control circuit is rendered inoperative so thatthe currentflowto the starting aids ceases. The time constant of the capacitor 26 and the resistor 27 is chosen so as to provide the required period of operation once the engine has started, in accordance with the temperature ofthe engine coolantwhen the starting motorisfirst brought into operation.The required time of continued operation ofthe starting aids once the engine has been started, can be determined by carrying out tests on an engine in a cold room, atdifferenttemper- atures and in one example it was found that when the temperature ofthe engine was -40" Celsius, the time required following starting of the engine was of the order of 36 seconds the time decreasing to approximately 6 seconds when the temperature was +20 Celsius.

With the circuit as described the negative tempera ture coefficient resistor is a one terminal device and can be mounted on the engine structure with a single connection into the control circuit, the structure of the engine providing the earth return connection. The mounting ofthe resistor is therefore greatlyfacilitated.

When the engine coolant temperature is above a predetermined valuetests have shown that it is not necessaryto continue to energisethe starting aids oncethe engine has started andthe circuit maybe modified by the addition of a further comparator one input of which is connected to the junction ofthe resistors 21 and 22 and the other inputofwhich is connected to a potentiometer chain providing a refer ence voltage of suitable magnitude. The outputterminal of the comparator is connected to the junction of the diode 28 and capacitor 26 whereby charging of the capacitor 26 in the manner described upon cranking the engine, is prevented when the engine coolant temperature is above a predetermined value.

Moreover, if the engine coolanttemperature does not rise above the predetermined value while the aids are energised, the capacitor 26 will be discharged and the aids de-energised.

Claims (6)

1. A control circuit for controlling the operation of a starting aid oraidsfora compression ignition engine comprising a power control circuitforcontrolling current flow in the aid or aids, and timing means operable upon termination of cranking of the engine to maintain the power control circuit in an operative condition whereby the aid or aids remain energised, said timing means including a oneterminaltempera- tureresponsivemeanswherebytheon period of said timing means varies in accordance with the temperature.

2. A control circuit according to Claim 1 in which said timing means includes a comparator, meansfor supplying a reference voltage to one input terminal of the comparator, a capacitor and a discharge circuit for the capacitor, the other input terminal of the com pa- rator being connected so as to receive the voltage across the capacitor, and further means operable when the engine is cranked to charge the capacitorto a voltage determined by said temperature responsive means, the arrangement being such that when cranking of the engine ceases the capacitor will discharge and when the voltage across said capacitor is less than said reference voltage the comparatorwill oper atetode-energisesaidaid oraids.

3. A control circuit according to claim 2 in which said temperature responsive means comprises a resistorwhich is connected in series with a further resistor across a constant voltage supply and diode means for clamping the charging voltage applied to said capacitor, to substantially the voltage level atthe junction of said resistors.

4. A control circuit according to Claim 2 or Claim 3 in which said further means includes a switch operable upon cranking oftheengine.

5. Acontrol circuitaccording toanyoneofthe preceding claims in which said oneterminal device is mounted on the engine structure which provides an earth return connection.

6. A control circuit for controlling the operation of a starting aid or aids for a compression ignition engine substantially as hereinbefore described with reference to the accompanying circuit diagram.