GB2051227A

GB2051227A - Diesel Engines and Their Idling Operation

Info

Publication number: GB2051227A
Application number: GB7921331A
Authority: GB
Original assignee: Ricardo Consulting Engineers Ltd
Current assignee: Ricardo PLC
Priority date: 1979-06-19
Filing date: 1979-06-19
Publication date: 1981-01-14
Also published as: GB2051227B

Abstract

To reduce engine noise and roughness when the engine is idling the intake air may be throttled by a valve 10 and the cylinder heater plugs 2 switched-on. The valve 10 and plugs 2 may be controlled automatically in response to an engine speed detector 19, 20, the position of the fuel pump rack 14, an ambient temperature detector 46 and an engine coolant temperature detector 16. The engine speed and pump rack position may be transmitted hydraulically to control the valve and plugs, Fig. 6 (not shown). <IMAGE>

Description

SPECIFICATION Diesel Engines and Their Operation This invention relates to compression-ignition engines operating on the Diesel cycle, referred to as diesel engines, and to their operation and control, and is concerned with reducing the noise and roughness of such engines when running under idling conditions.

In a normal 4-cycle diesel engine, pure air is induced on the suction stroke into each cylinder and is compressed. Shortly before the end of the compression stroke a fuel injection nozzle opens and starts to spray in finely-atomised fuel. A slight delay occurs, depending upon the compression temperature and pressure and the self-ignition temperature of the fuel used, after which the fuel which was first injected self-ignites at one or more places depending upon the spray pattern used. After the fuel which was injected during the delay period has ignited it burns very quickly, leading to almost constant volume combustion; the fuel which continues to be injected then burns almost as it is injected, leading to almost constant-pressure combustion.Since this latter part of the fuel is injected under controlled conditions, pre-ignition such as limits the compression ratio usable in a petrol engine cannot occur. In a diesel engine the fuel is selected to have a low self-ignition temperature, and the compression ratio used is high, being chosen to be sufficient to permit starting in the lowest ambient operating temperatures.

Due to its ignition principles it is possible 'and usual to operate a diesel engine over its whole load range by merely varying the quantity of fuel injected per cycle, and with a full charge of air in the cylinder even at low loads. Thus, to a first approximation, the maximum cylinder pressure in a diesel engine is roughly constant over its load and speed range, and due to the use of a high compression ratio it is higher than that existing at full throttle in a comparable petrol engine.

As a consequence of the high maximum cylinder pressures and the high rate of rise of pressure, a diesel engine generates more "combustion noise" than a petrol engine especially under idling conditions. Moreover the higher gas pressures prevailing throughout the operating cycle result in greater instantaneous torque variations in a diesel engine than in a petrol engine. This cyclic irregularity at idling speeds causes oscillation of the layshaft of the gearbox about its mean rotation speed, even in neutral, and the resultant impact loading of the gear teeth causes additional noise. Furthermore, unless the engine mountings have been very carefully designed, there is increased engine movement which reacts on the vehicle suspension and produces vibrations usually known as "roughness".

Certain types of diesel engine, for example the so-called "pre-chamber" engines, are fitted with heater plugs to assist cold starting. These "pre-chamber" engines have upwards of 50% of the clearance volume in a cell within the cylinder head which is connected to the cylinder space immediately above the piston by one or more passages. The combustion fuel is injected into the cell volume, and on combustion the burning air-fuel mixture expands out of the cell through the passage(s) into the cylinder space, where further combustion may occur at the higher fuel injection levels. A disadvantage of these pre-chamber engines is that duririg cold starting compression heat is lost as the air is pushed through the cold passage throat(s).This heat loss makes cold starting more difficult, and accordingly this type of engine usually has a higher compression ratio in order to increase the compression temperatures. In addition a heater plug operated from the vehicle battery is usually provided in the combustion cell, positioned so that part of the fringe of the atomised fuel injected into the cell passes very close to the heater plug and will be heated by it to a temperature approaching its "cracking" point. The heater plugs are switched on about 30 seconds prior to switching on the engine starter motor when starting from cold to assist the starting. Once the engine is firing regularly the heater plugs are switched off, and the engine will continue to operate satisfactorily.

According to the present invention, a method of operating a diesel engine which is fitted with heater plugs comprises running the engine under idling conditions with the air intake throttled so as to reduce the maximum cylinder pressure, and with the heater plugs activated to ensure ignition of the injected fuel under these throttled idling conditions.

The applicant has found that a considerable reduction in the observed noise level of a diesel engine is achieved when running it in accordance with the method of the present invention. In addition, a reduction of "roughness" may be achieved as a result of reduced movement of the engine on its mountings.

During a series of tests to study noise problems existing in a particular diesel engine having a Ricardo Comet Mark V combustion chamber fitted with heater plugs, it has been found by the applicant that a marked reduction in idling noise level-about 9 dBA on an original value of 73 dBA at 1 metre- was obtained in an anechoic noise measuring cell when the intake air was fairly heavily throttled with the heater plugs switched on. A marked improvement in the subjectively assessed noise was also apparent, with the traditional high-frequency impulsive diesel "knock" virtually inaudible.

From another aspect the present invention comprises a diesel engine the or each of whose combustion chambers is fitted with a heater plug, and which is further provided with means for throttling the air intake of the engine when running under idling conditions, and means for activating the heater plug whilst running throttled under these conditions.

Preferably the engine is provided with a means for controlling automatically the operation of the heater plugs in dependence on the engine speed and load, or on engine speed alone, and means for controlling the operation of the throttle in dependence on both engine speed and load.

The invention may be carried into practice in various ways, but two specific embodiments will now be described by way of example only and with reference to the accompanying drawings, in which Figure 1 is a graph showing cylinder pressure-time indicator diagrams of a diesel engine taken both when idling with normal intake of air and no heater plugs in operation, and when idling with the air intake throttled and the heater plugs switched on, Figure 2 is a graph showing characteristics of engine torque with crank angle for a 4-cylinder 4stroke engine, respectively under the two specified idling conditions, Figure 3 is a diagrammatic elevation of a diesel engine showing its intake system, heater plugs system and fuel injection pump purely diagrammatically, Figure 4 is a sectional view of a combustion chamber of the engine of Figure 3, Figure 5 is a circuit diagram of an electronic control system for controlling the low-speed lowload running of the engine of Figures 3 and 4, and Figure 6 is a diagram of an alternative control system which is of mechanical-hydraulic type.

Referring to the drawings, Figure 1 shows cylinder pressure-time indicator diagrams taken during the testing of the diesel engine referred to above with its Ricardo Comet Mark V head fitted with heater plugs. The curve marked A was taken with the engine idling at 700 rpm and no external load, with normal air aspiration and with the heater plugs switched off. The curve marked B was taken under similar idling conditions but with the heater plugs switched on and with the air intake to the engine throttled to a value about 400 mmHg below atmospheric. The significantly lower maximum cylinder pressure on curve B will be noted, together with the reduced rate of rise of pressure when combustion occurs after top-dead-centre (portion X of curve) as compared with curve A.Combustion is late for both curves, due to the characteristics with speed of the particular fuel injection system used with this engine.

Figure 2 shows the result of multiplying the instantaneous cylinder pressures by the corresponding effective crank radii to produce the engine's instantaneous torque or turning moment diagram. This particular engine was a cylinder 4-cycle one with the cylinders in line. The cycle of events in each cylinder takes two engine revolutions or 7200 crankshaft before it is repeated. With a 4cylinder engine the cylinders fire sequentially at equal intervals of 1800 and the instantaneous torque diagram is obtained by vectorially adding the instantaneous torques for each cylinder. When this is done a combined torque diagram is obtained which repeats itself every 1800 crank Such diagrams for the throttled and unthrottled cases are compared in Figure 2.For convenience and generality the instantaneous torque has been expressed as the product of the cylinder pressure in Ibf/in2 and the instantaneous effective crank radius for a crank having a radius of 1 in. If the actual torque values are required for an engine having individual cylinder swept volumes of V in3 the values given for the ordinates in Figure 2 must be multiplied by 0.5 V.

In Figure 2, the curve for the unthrottled case with the heater plugs off is marked C and the curve for the throttled case with the heater plugs activated is marked D. The horizontal line E is the arithmetic mean torque for each of the two diagrams C and D considered separately. The value for the ordinate for this line E corresponds to the friction torque within this system when idling, i.e. no load, engine and gearbox. The lower instantaneous torque values in Curve D for the case when idling throttled and using heater plugs can readily be seen as well as the lower rates of torque change with crank angle.

Now the excess of the instantaneous torques over the mean value leads to an acceleration of the engine rotating system which continues until the curves drop below the mean torque line when the rotating system decelerates to arrive at the same speed as it started at when the mean torque line is crossed again.

The area under the positive loop of Curve C or Curve D representing the excess torque represents the excess energy added to the rotating system during that part of the torque cycle. Similarly the area between the mean torque line E and the negative torque values represents the work deficiency during the second period of the torque cycle.

If the minimum angular velocity of the crankshaft is w1 radians/s, the maximum crankshaft velocity is w2 radians/s, the mean (nominal) crankshaft velocity is w radians/s, and the polar inertia of the crankshaft plus flywheel and clutch is l Ibs-in-sec2, the change in crankshaft energy content is:

Since for a given engine I and w are constant, the cyclic variation of speed=w2-w1 ocE. The ratio of the area (23.2 cm2) under the unthrottled curve C in Figure 2 to that for the throttled inlet curve D (10.8 cm2) is 2.13. This means that the speed variation is reduced by 1/2.13=0.47 times its usual value with the throttling and the heater plugs activated.

It is this reduced cyclic variation in speed which reduces the noise from the impacting of gear teeth within both the engine timing gears and gearbox which represents part of the noise which is apparent when idling.

The reduction of the instantaneous torque reduces movement of the engine on its mountings, thus reducing vehicle "roughness". Simultaneously the reduced torques diminish piston side-thrust and deflections of the engine structure about the crankshaft axis, whilst the lower gas pressures result in lower deflections of the engine structure along the line of the cylinder bores. The lower absolute values combined with a much reduced harmonic content of both the gas pressure and the torque diagrams result in much less excitation of the whole engine structure, including the vibratory amplitudes of panels and covers One of the major noise sources in a diesel engine is piston "slap", which occurs around top-dead-centre as the piston traverses the bore.Piston slap is largely a direct function of the maximum cylinder pressure P max. thus by throttling, to achieve very low values of P max, piston slap (normally clearly audible at idle when piston-bore clearances are relatively high) is very significantly reduced. The mechanism of noise reduction using this invention is therefore one of reduced combustion noise (lower rates of pressure rise) and markedly reduced mechanical noise (piston slap and vibration-excited noise).

The above is the basic explanation for the observed reduction in noise level when throttled at idling and with the heater plugs in operation to reduce the ignition delay and ensure ignition of the diesel fuel when injected into the engine with lower than usual cylinder gas pressure prevailing.

The provision of an engine control system which automatically switches on the heater plugs and throttles the intake air to the appropriate degree as idling conditions are approached, will now be discussed with reference to Figures 3, 4 and 5.

Figure 3 shows diagrammatically a diesel engine 1 whose cylinder head has precombustion chambers of a type such as the Ricardo Comet type Mark V (British Patent No. 786,329), and has a heatec plug 2 fitted into each precombustion chamber 3 (Figure 4) which has in the past been used to provide a local source of high temperature in the precombustion chamber in proximity to the outer edges of the fuel injection nozzle spray 4 to assist starting from cold. The heater plugs 2 at one time consisted of single coils of thick resistance wire exposed to the combustion gases, but this arrangement was found to have a poor working life and nowadays the heater coils are contained in, and insulated from, outer sheaths of heat-resistant metal. The heater plugs 2 are heated by passing an electrical current through them.Usually the heater plugs are wired in parallel so that they all, in a multicylinder engine, have the full vehicle battery voltage across them. One connection is normally by a metallic (copper usually) strip 5 running the length of the engine, and the return connection is to earth via the engine structure. Figure 4 shows a part-section of a cylinder head 6 showing a typical disposition of the heater plug 2 within the prechamber 3 of a Comet V combustion chamber, and also showing the fuel injection nozzle 7 and the tangential passage 8 leading from the prechamber 3 into the cylinder space, the passage 7 being formed in an insert plug 8A of refractory material.

In Figure 4 a "Pintaux" type injection nozzle 7 is shown, but if desired a single-conical-spray pintle-type nozzle may be fitted instead. The fringe of the single spray of such a nozzle would also be arranged to contact the heater plug 2.

As seen in Figure 3 the engine is provided with an intake system normally consisting of a pipe 9 running the length of the cylinder head 6 with side branches 9A, substantially at right angles to the longitudinal pipe 9, connecting with the intake port to each cylinder cast into the cylinder head 6. A throttle 10 is fitted in, and towards the entry to, the longitudinal pipe 9 immediately downstream of the air-cleaner 11. The throttle 10 would normally be of the butterfly type, but may be of another type such as a cylindrical cock, which can be almost completely closed if required. The degree of throttling is controlled by an external control, such as a lever 12, whose movement is controlled by the output of a logic device to be described.An adjustable stop 1 2A is provided for engagement by the lever to determine the extent to which the throttle can be closed.

A fuel injection pump 1 3 is driven by the engine in a normal way and supplies each cylinder in the desired firing sequence with the required quantity of fuel under pressure via a separate pipe and fuel injector, not shown. Dependent on the type of fuel injection pump used the fuel quantity injected per cycle into each cylinder is controlled either by a linearly moving control rod shown at 14 or by the angular movement of a control fulfilling the same purpose. A transducer 1 5 is positioned to monitor the position of the fuel pump control. A temperature sensor 1 6 is fitted to the cylinder head to monitor the water jacket temperature since this can have a marked effect on the fuel ignition delay.

The engine is also provided with a speed indicator, to supply information to the logic section of the associated control mechanism. This may be an electronic frequency meter operated by one or more pulses per shaft revolution, by means of an inductive pick-up working in close association with, say, the gear teeth on the flywheel starter ring. A number of known alternatives exist for providing an electrical output proportional to engine speed. Alternatively, a mechanical device may be driven by the engine consisting of some form of weight system whose centrifugal force is controlled by a suitable spring.

The attitude adopted by the control spring and/or other component in the device, is indicative of the operating speed.

It may not always be necessary to activate the heater plugs during throttled-intake idling when the engine's water jacket temperature is up to its normal (thermostatted) operating value. If desired a temperature operated switch controlled by the sensor 1 6 may be inserted in the glow-plug activating circuit in such cases, to allow current to flow only when the coolant temperature is low.

The following Table 1 shows the operating permutations required of an automatic control system to operate the heater plugs and the throttle as required.

Table 1 Condition Heater Plugs Throttle Engine Speed (a) High Off Open (b) Upper (warning) limit On Open (c) Lower idling limit On Closed (d) Below intended idling speed On Open Fuel Pump Rack (a) High (engine on load) Off Open (b) Upper (warning) low limit On Open (c) Lower (operating) limit On Closed Water Temperature (a) Cold On Closed (b) Low (below operating value) On Closed (c) Normal running Off/On Closed The terms "closed" and "close" used in the Table and in the following description and claims mean the operative position or condition of the throttle in which it throttles the engine air intake, and putting the throttle in that condition or position; whilst the term "open" in this context means the inoperative position or condition of the throttle, and putting it in that position or condition.

Figure 5 shows the diagrammatic arrangement of an automatic electronic system to control the switching on and off of the heater plugs and the opening and closing of the engine's intake throttle valve, in dependence on signals representing engine load and speed and operating temperature, which provides the requirements of Table 1.

In Figure 5, the engine speed signal is shown as being derived from a speed sensor in the form of an electromagnetic pick-up 19 associated with a toothed wheel or disc 20 mounted on a suitable engine shaft 21 which rotates at crankshaft speed, normally either the crankshaft itself or the drive shaft (half engine speed) of the fuel injection pump 13. The toothed starter ring on the flywheel may be convenient for use on the wheel 20. The pick-up 1 9 is mounted in close proximity to the teeth of the wheel 20, and as the teeth pass by its pole piece a small voltage pulse is induced in the pick-up each time a tooth passes. This induced voltage signal is passed to an amplifier and Schmitt trigger circuit 22 whose output is a well-shaped series of constant amplitude pulses whose frequency is directly proportional to engine speed.The pulsed output from 22 is passed to a frequency-to-voltage converter 23, whose voltage output corresponds to engine speed and is supplied to one input 24, 24' or 24" of each of three differential amplifiers 25, 26 and 27, here used as voltage level comparators. The second input of each of the differential amplifiers can be set up with a fixed but adjustable input voltage proportional to a required engine speed by means of a variable resistor, 28, 28' or 28" whose voltage tapping is connected to the respective second input. The two inputs to each differential amplifier 25, 26 or 27 are compared by it, and the resultant output signal represents the difference between the actual instantaneous engine speed and the preset value set up by means of the variable resistor 28, 28' or 28".The preset engine speed set up by the resistor 28 represents the upper (warning) limit of speed below which the heater plugs are required to be switched on; that set up by the resistor 28' represents the upper (warning) speed below which the air intake throttle 10 is required to be closed; and that set up by the resistor 28" represents the low-load speed of the engine below which the throttle is required to be re-opened. It will be understood that the maintenance of engine idling speed is carried out by a conventional governor which is normally fitted as a part of the fuel pump assembly.

In Figure 5 the four heater plugs 2 are shown connected in parallel to the vehicle battery 29 in a circuit controlled by a relay 30. The output voltage from the differential amplifier 25 is supplied to an inverter 31 whose output is supplied to one input of an "AND" logic gate 32, the output of which controls the relay 30.

The coolant water temperature sensor 1 6 provides an output voltage signal which is supplied to one input of a differential amplifier 35, and a voltage corresponding to the required operating temperature is set up at the other input by a variable resistor 36. The output of the differential amplifier, representing the difference between actual and required operating temperatures of the engine, is supplied to a further inverter 37 whose output is supplied as a second input to the "AND" gate 32.

In many engines it will be found that it is essential to activate the heater plugs when idling with the air intake throttled, regardless of the engine coolant temperature. In these cases the water temperature sensor 16, differential amplifier 35 and inverter 37 become redundant, and in Table 1 the relevant entry in the column "Heater Plugs", in the last line relating to the water temperature in normal running, would be "On".

The rack bar 14 of the fuel injection pump 13 normally has a linear movement, although with certain types of pump it may move angularly. The output voltage from its position transducer 1 5, representing the instantaneous fuel quantity being injected per firing cycle, is supplied to one input of each of two further differential amplifiers 38 and 39 at whose second inputs preset voltages can be set up by means of variable resistors 40 and 40'. The resistor 40 is adjusted to set up a voltage corresponding to the upper (warning) limit of engine load below which the heater plugs are required to be switched on, whilst the resistor 40' is set to the lower (operating) limit of load below which the throttle is required to be closed. The output voltage of the differential amplifier 38 is supplied to a second inverter 41 whose output is supplied as the third input to the "AND" gate 32.The output of the differential amplifier 39 is supplied via a third inverter 42 to one input of a second "AND" gate 43 whose output controls the throttle 10.

Since at idling the fuel quantity injected is small, being only that required to overcome engine and gearbox friction, or even less if the engine is being overrun by the vehicle, the preset voltages fixed by the resistors 40 and 40' represent quite small fuel quantities. The output from the differential amplifier 38 represents a quantity smaller than that preset to warn that idling conditions are being approached.

The outputs of the differential amplifiers 26 and 27 associated with the speed sensor are supplied, respectively through a further inverter 44 and directly, to respective second and third inputs of the "AND" gate 43. An ambient air temperature sensor 46 is also provided, and its signal is supplied, via a further differential amplifier 47 provided with a preset resistor 48, directly to a further input to the "AND" gate 43.

The output signal from the second "AND" gate 43 is supplied to a relay 50 which when energised acts on the lever 1 2 to close the throttle 10. A mechanically adjustable stop 1 2A is provided to limit the extent to which the throttle will be closed by the relay. If it is found necessary to allow the heater plugs 2 time to reach their working temperatures after being switched on, before the relay 50 is energised to close the throttle, a monostable circuit 52 having an adjustable time delay is provided, connected to the output of the second "AND" gate 43, and its output is supplied via an inverter 53 to a second input of a further "AND" gate 54, whose output is delivered to the relay 50, the "AND" gate 43 being directly connected to the first input of the gate 54.

Thus the throttle 10 will be closed by the relay 50 only when the "AND" gate 43 is conducting, i.e. when all four of its inputs are in logic 1 state, giving a logic 1 output. This means that firstly the engine speed must be below the idling limit (c) of Table 1 set by the resistor 28', but above the lower threshold of idling speed (d) as set by resistor 28', so that the differential amplifier 26 provides a logic 0 signal to the invertor 44 whose output signal to "AND" gate 43 is logic 1, while the differential amplifier 27 provides a logic 1 output signal direct to the "AND" gate 43. Secondly the fuel quantity must be below the lower (operating) limit (c) of Table 1, as set up by the resistor 40' so that the differential amplifier 39 provides a logic 0 signal to the inverter 42 whose output to the "AND" gate 43 is thus logic 1.Thirdly the ambient air temperature as measured by the sensor 46 must be above the threshold value set by the resistor 48 so that the differential amplifier 47 provides a logic 1 signal direct to the "AND" gate 43. When all these conditions are fulfilled the throttle 10 will be closed by the relay 50, but whenever any one of the conditions is not fulfilled the relay will hold the throttle open.

As regards the heater plugs, their actuation is controlled by the "AND" gate 32 which has three inputs only. The arrangement shown is for use in cases where the heater plugs are required to be switched off when the engine coolant temperature is at its normal on-load operating value, as set up by the resistor 36. When the coolant temperature as sensed by the sensor 1 6 is at or above its normal operating temperature the output signal from the differential amplifier 35 is logic 1, resulting in a logic 0 output signal from the inverter 37 to the "AND" gate which will prevent it from activating the relay 30 to switch on the heater plugs, regardless of engine speed or load. With a coolant temperature below the operating value, the output of inverter 37 to the "AND" gate is logic 1. Subject to this overriding temperature control, where provided, the heater plugs will be switched on whenever the engine speed and fuel quantity settings both fall below the upper (warning) values of Table 1, i.e. amplifiers 25 and 38 provide logic 0 output signals which are inverted to logic 1 at the "AND" gate 32 by the inverters 31 and 41.

Figure 6 shows diagrammatically another control system for operating the throttle and heater plugs with a slightly simpler form of logic. It is a mechanical-hydraulic system. Here the engine shaft 21, probably in practice the fuel pump driveshaft, rotates a speed sensor 60 having centrifugal weights 61 with toes 62 which bear against a capped ballrace 63, the inner race of which is centred by the end of the spool 64 of a hydraulic spool valve 65. A spring carrier 66 locates a spring 67 whose purpose is to react the centrifugal forces generated by the weights 61. Thus the position of the spool 64 in the housing of the valve 65 is dependent on the speed of rotation of the engine shaft 21. The detailed geometry of the weights 61 and their contact toes 62 determines the speed-force displacement characteristics for the valve spool 64.This permits the use of control lands on the valve spool which overrun drilled or annular ports in the valve housing 68.

The fuel quantity signal is derived from the fuel injection pump 13, whose rack 14 is coupled to the spool 69 of a second hydraulic spool valve 70.

The coolant temperature signal is derived from a temperature sensor 16 coupled to a switch 71 in the battery circuit 72 of the heater plugs 2, giving overriding control of the actuation of the heater plugs in response to coolant temperature.

As already mentioned, with most engines it is necessary for the heater plugs to be activated whenever idling under throttle-closed conditions, and for use in such cases the switch 71 would be omitted, or held permanently closed.

In the system of Figure 6, operation of the throttle 10 is not temperature-controlled.

The hydraulic system comprised by the two control valves 65 and 70 and their hydraulic connections will now be described. The valve 65 has high-pressure inlet ports 75, 76 connected to a supply line 77 for pressure fluid and leading into the interior of the valve housing 68 on opposite sides of a central control land 78 on the valve spool 64, and a low-pressure return port 79 in the vicinity of a second control land 80 and connected to a return line 81 (to which the inner ends of the chambers in the housings of the two valves are also vented). Working ports 82 and 83 of the valve 65 are controlled by the lands 78 and 79 respectively. The working port 82 is connected to the fluid inlet port 84 of the second valve 70, and the working port 83 is connected to one end of a hydraulic cylinder 84 whose plunger is spring-biassed by a return spring 85.The plunger rod 86 of the plunger 84 operates a pressure switch 87 in the battery circuit 72 of the heater plugs 2.

The second valve 70, whose spool is operated by the fuel injection pump rack 14, has an outlet port 88 connected to the return line 81, the ports 84 and 88 being on opposite sides of a central land 89 on the valve spool 69, and has a single working port 91 controlled by the land 89, and connected to one end of a hydraulic cylinder 94 whose plunger rod 95 is spring-biassed by a return spring 96 and is coupled to the operating lever 12 of the throttle 10.

The operation of the system of Figure 6 will now be described. If the engine is running at a speed significantly higher than idling speed, the centrifugal force acting on the weights 61 will cause the spool 64 of the valve 65 to move to a position towards the left in Figure 6, thus causing the land 80 to move past the port 83 thus connecting the latter to the return port 79 and allowing fluid to escape from under the piston of the cylinder 84 to the return line 81. This permits the pressure switch 87 to be opened by the return spring 85 of the cylinder, interrupting the current supply to the heater plugs 2.

In this position of the valve spool 64, the land 78 covers the working port 12 and so prevents the supply pressure from reaching the inlet port 84 of the second valve 70 so that the cylinder 94 remains unpressurised.

If now the engine speed drops to its idling value, for example, when a vehicle being driven by the engine is stopped and the clutch withdrawn and neutral gear position selected, the valve spool 64 will be moved progressively to the right in Figure 6 by the spring 67 as the centrifugal force on the weights 61 drops with falling engine speed. This causes the land 80 to uncover the port 83 so as to connect the supply pressure to the cylinder 84 whose plunger rod 86 extends to close the pressure switch 87 and switch on the heater plugs, provided that the coolant-temperature switch 71 is closed, in those cases where coolant temperature override control of the heater plugs is practicable.As the engine speed drops further to idling speed, the land 78 uncovers the valve port 82 and thus connects the supply pressure to the inlet port 84 of the second control valve while the pressure switch 87 remains closed.

The fuel pump rack 14 moves towards the left in Figure 6 as the engine load is reduced, i.e. as the fuel quantity injected per cycle is reduced. When the rack 14 reaches a certain predetermined position, it moves the spool 69 of the second valve 70 into a position in which the land 89 uncovers the port 91, connecting supply pressure to the cylinder 94 via the speed-controlled valve 65 and move its plunger 95 to the right in Figure 6, against the spring 96, thereby closing the throttle 10 via the lever 12. The degree to which the throttle closes can be adjusted by the screw stop 12A.

When the engine accelerator pedal is depressed the fuel pump rack moves to the right in Figure 6 to increase the quantity of fuel being injected, thus moving the valve spool 69 towards the right. This causes the land 89 firstly to cover the port 91, cutting off the supply pressure from the cylinder 94, and on continued movement of the spool 69, to uncover the port 91 again and connect it to the return port 88, permitting oil to drain from the cylinder 94 to return so that the spring 96 moves the plunger of the cylinder towards the left and thus fully re-opens the throttle 10. With the increase in fuel quantity supplied to the engine, the engine speed rises above idling speed, causing the weights 61 to move the valve spool 64 towards the left again until, when an appropriate speed is reached, the cylinder 84 is de-pressurised and its spring 55 opens the pressure switch and switches off the heater plugs.

It will be appreciated that the control system of Figure 6 provides for control of the heater plugs in response to engine speed, subject to overriding control in response to coolant temperature, but the fuel quantity is not a factor in the control of the heater plugs. On the other hand the operation of the throttle 10 is subject to both engine speed and fuel quantity, so that the throttle will only be closed when both these parameters are below their preset values, but the coolant temperature is not a factor which affects the throttle control. Thus the arrangement of Figure 6 fulfills some but not all of the requirements of Table 1.

The present invention is applicable to any kind of diesel engine provided with heater plugs to aid cold starting. In practice, heater plugs are provided in engines of the "prechamber" type as previously discussed, but it is difficult to accommodate a heater plug in the combustion chamber of a "directinjection" (or open-chamber) engine on account of the space occupied by the inlet and exhaust valves plus that required for the fuel injector in a central or near-central position. For this reason it is unlikely that heater plug assistance will be provided for "direct-injection" engines, and it follows that the principal application of the present invention is to engines of the "prechamber" type.

Claims

1. A method of operating a diesel engine which is fitted with heater plugs, which method comprises running the engine under idling conditions with the air intake to the engine throttled, so as to reduce the maximum cylinder pressure per cycle, and with the heater plugs activated.

2. A method as claimed in Claim 1 which comprises running the engine under varying conditions of load and speed, activating the heater plugs when the engine speed and the fuel injection quantity are simultaneously below respective predeterm4ined first values, throttling the air intake when the quantity of fuel injected per cycle and the engine speed are simultaneously below respective predetermined second values, de-activating the heater plugs when either the engine speed or the fuel injection quantity exceeds the respective first value, and restoring unthrottled air aspiration when either the engine speed or the fuel quantity exceeds the respective second value.

3. A method as claimed in Claim 2, in which the said second values are respectively less than the said first values.

4. A method as claimed in Claim 1, which comprises running the engine under varying conditions of load and speed, activating the heater plugs when the engine speed falls below a predetermined first value"throttling the air intake when the quantity of fuel injected and the engine speed are both simultaneously below respective predetermined first and second values, de-activating the heater plugs when the engine speed rises above the first value, and restoring unthrottled air aspiration when either the fuel quantity or the engine speed exceeds the respective first or second value.

5. A method as claimed in Claim 4, in which the said second value of the engine speed is less than its first value.

6. A method as claimed in any one of Claims 2 to 5, in which the heater plugs are automatically de-activated, regardless of engine speed and fuel injection quantity, whenever the engine coolant temperature reaches a predetermined value in the region of its normal value for engine operation under load.

7. A diesel engine fitted with heater plugs and witli a selectively closable throttle operatively associated with the engine air intake passage for throttling the aspiration of air by the engine, and provided with means for closing the throttle and activating the heater plugs when the engine is running under idling conditions.

8. A diesel engine as claimed in Claim 7, provided with a control system for the heater plugs which automatically activates the heater plugs when both the quantity of fuel injected per cycle and the engine speed fall below respective predetermined first values, and which automatically deactivates the heater plugs in response to a rise in either the fuel quantity or the engine speed above the respective first value.

9. A diesel engine as claimed in Claim 7 or Claim 8 provided with a throttle control system which automatically closes the throttle when both the quantity of fuel injected per cycle and the engine speed are simultaneously below respective predetermined second values, and automatically re-opens the throttle in response to a rise in either the fuel quantity or the engine speed above its respective second value.

1 0. A diesel engine as claimed in Claim 8 and 9 in which the said second values are less than the said first values.

11. A diesel engine as claimed in Claim 9 or Claim 10, having means for subjecting the operation of closing the throttle to a predetermined delay period after the engine speed and fuel quantity have both fallen below the respective second predetermined values.

12. A diesel engine as claimed in any one of Claims 8 to 11 which includes an electronic control system incorporating logic circuitry and comprising both the throttle control system and the control system for the heater plugs.

13. A diesel engine as claimed in Claim 7, provided with a control system for the heater plugs which automatically activates the heater plugs when the engine speed falls below a predetermined first value, and which automatically de-activates the heater plugs in response to a rise in the engine speed above the said first value.

14. A diesel engine as claimed in Claim 13 provided with a throttle control system which automatically closes the throttle whenever both the quantity of fuel injected per cycle and the engine speed are simultaneously below respective predetermined first and second values, and automatically re-opens the throttle in response to a rise in either the fuel quantity or the engine speed above the respective first or second value.

1 5. A diesel engine as claimed in any one of Claim 13 or Claim 14 which includes a hydraulic and mechanical control system comprising both the throttle control system and the control system for the heater plugs.

1 6. A diesel engine as claimed in any one of Claims 8 to 1 5, which is liquid-cooled and is provided with an automatically-operating control responsive to the temperature of the engine coolant for de-activating the heater plugs when the said temperature reaches a predetermined value in the region of its normal value for engine operation under load, the said temperature-responsive control overriding the heater plug control system.

1 7. A method of operating a diesel engine under idling conditions, substantially as specifically described herein with reference to Figures 1 to 4, or to Figures 1 to 5, or to Figures 1 to 4 and 6 of the accompanying drawings.

1 8. A diesel engine provided with means for operating it under idling conditions, substantially as specifically described herein with reference to Figures 3 and 4 or to Figures 3 to 5, or to Figures 3, 4 and 6 of the accompanying drawings.