GB2280759A

GB2280759A - Operation of electrical heating elements

Info

Publication number: GB2280759A
Application number: GB9317667A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1993-08-06
Filing date: 1993-08-25
Publication date: 1995-02-08
Also published as: GB9317667D0; GB9316402D0

Abstract

A method of driving a heating element such as a glow plug from an electrical power supply comprises measuring the supply voltage periodically, initially driving an unsustainably high current through the heating element for a length of time related to the value of the measured voltage to bring the heating element rapidly to its operating temperature and subsequently regulating the mean current supply to the heating element to maintain the heating element temperature between upper and lower threshold limits. The heating element temperature is predicted from the energy gain (based on the supply voltage) and the energy less through at least radiation. <IMAGE>

Description

OPERATION OF ELECTRICAL HEATING ELEMENTS Field of the invention The present invention is concerned with a method for driving electrical heating elements, such as glow plugs, to bring them to their operating temperature as quickly as possible.

Background of the invention.

There are several applications that require heating elements to be brought to their operating temperature in as short a time as possible. One such application is the glow plugs used in the combustion chambers of Diesel engines. Here, in winter, one has to wait until the glow plugs are hot before attempting to crank the engine and it is obviously desirable to minimise the waiting time before the engine can be cranked.

Other applications that present themselves are concerned with reducing the time taken for a catalytic converter to reach its light off temperature. For this purpose, it has been proposed to use a fuel burner in which air and fuel are injected into a chamber upstream of the catalytic converter and the mixture is ignited and burns to heat the catalyst.

It has also previously been proposed to use an EGI (exhaust gas ignition) system to heat a catalytic converter when an engine is started from cold. In such a system, during cold starts, the engine is run with a rich mixture to produce hydrogen in the exhaust gases. Air is added to the exhaust gases to make a mixture capable of being ignited when cold.

The exhaust gas and air mixture is ignited in an afterburner chamber located upstream of the catalytic converter and burns as a flame to heat the catalyst to its light off temperature.

Once again, if a glow plug is used as the igniter in these applications, one does not wish to wait a long time for the glow plugs to reach their operating temperature before attempting to ignite the mixture.

In the applications concerned with reducing the light off time of catalytic converters, one could use spark plugs to ignite the fuel/air or exhaust gas/air mixtures, but such systems are complex as they require a high tension coil and ignition circuits for controlling the current supply to the coil. Problems can arise in suppressing radio interference from such igniters as the entire exhaust pipe can act as an antenna and it is preferred to avoid the need for high tension leads under the vehicle for safety reasons.

Glow plugs are usually operated by a simple ON/OFF switch that connects them to the vehicle battery. For a given supply voltage V, the heat output of a glow plug is given by V2/R, where R is the resistance of the glow plug. One of the problems that are encountered is that the battery voltage can vary significantly, especially when the vehicle is being started from cold. Furthermore, the resistance value R is selected to be fairly high, to permit the glow plug to remain intact even when the maximum battery voltage applied across it for some time. Because the need for reliability must always take precedence over rapid operation, glow plugs have in the past always needed around twenty to thirty seconds to reach their operating temperature, which is not entirely satisfactory.

There are also applications that use heating elements other than glow plugs in the operation of an internal combustion engine. One such application is in fuel vaporisers that heat the fuel to improve combustion quality. The vaporisation of the fuel is especially desirable during cold starts, when the intake manifold is cold, and it is clearly desirable to have the vaporisation effective immediately the engine is cranked so that it can assist with the first firing.

Obiect of the invention The present invention therefore seeks to provide a method of driving a heating element, such as a glow plug, that reduces the time taken by the heating element to reach its operating temperature.

Summarv of the invention According to a first aspect of the present invention, there is provided a method of driving a heating element such as a glow plug from a power supply which comprises applying an unsustainably high current through the heating element, periodically deriving a measure of electric power consumed by the heating element of the glow plug, periodically estimating the cooling effect on the element by heat losses through at least radiation, predicting the temperature of the element from the difference between the energy gain and energy loss, decreasing or switching off the applied current in response to the predicted element temperature exceeding a first threshold value and increasing or switching on the applied current in response to the predicted element temperature dropping below a second threshold value.

According to a second aspect of the invention, there is provided a method of driving a heating element such as a glow plug from an electrical power supply which comprises measuring the supply voltage periodically, initially driving an unsustainably high current through the heating element for a length of time related to the value of the measured voltage to bring the heating element rapidly to its operating temperature, disconnecting or reducing the supply voltage to prevent damage to the heating element and subsequently alternately reconnecting and disconnecting the supply voltage periodically to maintain the heating element temperature between upper and lower threshold limits.

According to a further aspect of the invention, there is provided a method of driving a heating element such as a glow plug from an electrical power supply which comprises measuring the supply voltage periodically, initially driving an unsustainably high current through the heating element for a length of time related to the value of the measured voltage to bring the heating element rapidly to its operating temperature and subsequently regulating the mean current supply to the heating element to maintain the heating element temperature between upper and lower threshold limits.

A further application of the invention is in engines provided with electrically heated catalysts. In order to minimise the time taken by an electrically heated catalyst to reach its light off temperature, it has been proposed to drive the heating element using an unregulated supply derived directly from the engine alternator with its regulator switched off or by-passed. The danger encountered with such a system is that of burning out the heating element of the catalyst because the unregulated alternator is capable of overdriving the heating element.

In the invention, rapid heat rise is achieved by over-driving the heating element, in other words passing a current through it that would after some time destroy the heating element. This is achieved most simply by using a heating element rated for a lower supply voltage, for example, using a glow plug with a 1.7V rating in a vehicle with a 12V power supply battery. After the operating temperature is reached, steps are taken to reduce the mean current drawn by the heating element to prevent its damage and keep it at its operating temperature.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of a system embodying the invention, and Figure 2 is a flow chart to explain the operation of the heat management unit in Figure 1.

Description of the preferred embodiment The invention is concerned with heating a heating element 12 by controlling the current supplied to it from a power supply 10 through a switching unit 14 the latter being controlled by a heat management unit 16.

The power supply need not be stable and may for example be the battery voltage in a motor vehicle which can drop to as low as 9 volts while the starter motor is being used in cranking the engine and can rise to more than 13 volts when the alternator is charging the battery.

The heating element 12 may be a glow plug of a diesel engine, or a glow plug used as an igniter in a fuel burner or an exhaust gas ignition system. As a further alternative the heating element may form part of a fuel vaporiser. In all cases it is desired to raise the temperature of the heating element as quickly as possible to its operating temperature regardless of the variations in supply voltage.

To achieve this, a heating element having a rated voltage lower than the minimum supply voltage is employed. In other words even at the minimum supply voltage the current supplied to the heating element would be unsustainable and is sufficiently high to cause permanent damage to the heating element. This excess current can rapidly raise the temperature of the heating element to achieve the desired objective but a heat management unit 16 is now required to prevent overheating and burning out of the heating element.

Because the temperature is required to rise very rapidly, closed loop control systems cannot operate reliably to stabilise the temperature of the heating element. Any sensing element such as a thermocouple used to measure the temperature of the heating element would itself have a thermal capacity and will take time to reach the same temperature as the heating element. But during this time the heating element will have burnt out before the current to it could be reduced.

The heat management system of the preferred embodiment therefore operate on the basis of an open loop. Based upon the power supplied to the heating element it is possible to estimate the increase in temperature and based upon the temperature one can estimate heat losses from the heating element. It is therefore possible to estimate the instantaneous temperature of the heating element at all times, preferably by the use of suitable software and to switch the power supply to the heating element on and off or to modulate it so as to maintain the estimated temperature of the heating element within a permissible band.

Open loop control systems are not reliable over extended period of use because of cumulative errors, but because heating elements such as glow plugs are not activated for a prolonged period of time, this problem is of no consequence in the present invention. When applied to fuel vaporisers which are require to operate over extended period of time it is possible to use a slower closed-loop to compensate for drift in the estimated temperature at regular intervals.

It is a particular advantage of the present invention that it may be implemented in software and does not require high expenditure on dedicated components. The flow chart shown in Figure 2 will serve to explain the operation of a program which can be included within the engine management microprocessor to control the switching of the heating element at all times.

Before the flow chart of Figure 2 is described in detail, it should however be emphasised that there is nothing to preclude the invention being implemented in the form of a dedicated or hard-wired circuit.

In Figure 2, there is represented a program loop that can be included in the engine management computer. Certain elements are assumed to be already present in the computer, such as clock signal to set the timing of the program loop and means for providing a signal that is a measure or an estimate of the starting temperature of the heating element.

There will usually be present such a signal within an engine management computer. For example if the heating element is part of a fuel vaporiser the engine coolant temperature can serve as a good indication of the heating element temperature. In the case of a catalytic converter, the operating temperature of the catalyst needs to be sensed or estimated in order to avoid activating the EGI system or the fuel burner during warm starts.

The first step taken in the loop, in the block designated 30, is to measure the instantaneous value of the supply voltage. This allows the power drawn from the power supply to be estimated in the block 32 which then increments the estimated temperature TEST, but only during cycles in which the power supply is connected to the heating element.

Depending on the estimated temperature, and in the case of a fuel vaporiser on the latent heat of vaporisation of the injected fuel, the next block 34 estimates the heat losses from the heating element and decrements the value of TEST accordingly. The heat losses will mostly be through radiation in the case of a glow plug used as an igniter and these follow a well known mathematical relationship, but it is also possible to compensate for losses through convection and conduction, especially in the case of a fuel vaporiser.

Within the comparison block 36, the estimated temperature is compared with upper and lower threshold values TMAx and TMIN.

If the estimated temperature lies within the permitted band, the status of the power supply connection is not changed in that cycle. If the estimated temperature exceeds the maximum permitted temperature T , then the power supply connection is set to OFF in the block 38 whereas if it is less than the lower threshold TWIN' the status of the power supply connection is set to ON in the block 40. In all three cases, in other words whether the status of the power supply connection is changed or not, the program loops back to the block 30 where the voltage of the power supply is measured at the start of the next clock cycle.

When the engine management system determines that the heating element should be hot, the lines of code that implement the flow chart in Figure 2 become effective. In the first clock cycle, the estimated temperature TEST will be found to be below its minimum threshold TMIN and the status of the connection to the power supply will be set to ON. In the next cycle, the estimated temperature TEST will rise and will continue to be incremented in consecutive clock cycles until the maximum threshold T is reached when the status of the power supply connection will be set to OFF. The power supply will not be reconnected for several cycles until the estimated temperature TEST again drops below its minimum threshold. In this way, the heating element will at first be continuously connected to the power supply for a length of time determined by the voltage of the power supply and in the starting temperature. Thereafter, the power supply will be pulsed with a mark to space ratio matched to the heat losses from the heating element, so as to keep its temperature within the permissible band.

It is possible periodically to measure the temperature of the heating element directly and to modify the estimated temperature TEST to compensate for drift errors caused by cumulative errors in estimation.

It is alternatively possible, by measurement of the supply voltage and from knowledge of the operating temperature of the heating element, to set an initial actuation period and the mean heating current level thereafter to achieve a similar effect to that described above without the need to perform a calculation in each clock cycle. This reduces the task imposed on the engine management computer, which is sometimes already extended by the number of tasks that it needs to perform in real time. However, this method is not to be recommended when the power supply voltage is subject to large fluctuations, bearing in mind that the heating effect varies as the square of the supplied voltage.

The mean steady state heating current may be controlled by pulsing the power supply with an appropriate mark to space ratio, by reducing the supply voltage through a voltage divider or by a connecting a resistance in series with the heating element. It is furthermore possible to modulate the supply voltage instead of switching it totally ON and OFF, and this can assist in improving the expected life of the heating element.

Claims

1. A method of driving a heating element such as a glow plug from a power supply which comprises applying an unsustainably high current through the heating element, periodically deriving a measure of electric power consumed by the heating element of the glow plug, periodically estimating the cooling effect on the element by heat losses through at least radiation, predicting the temperature of the element from the difference between the energy gain and energy loss, decreasing or switching off the applied current in response to the predicted element temperature exceeding a first threshold value and increasing or switching on the applied current in response to the predicted element temperature dropping below a second threshold value.

2. A method of driving a heating element such as a glow plug from an electrical power supply which comprises measuring the supply voltage periodically, initially driving an unsustainably high current through the heating element for a length of time related to the value of the measured voltage to bring the heating element rapidly to its operating temperature, disconnecting or reducing the supply voltage to prevent damage to the heating element and subsequently alternately reconnecting and disconnecting the supply voltage periodically to maintain the heating element temperature between upper and lower threshold limits.

3. A method of driving a heating element such as a glow plug from an electrical power supply which comprises measuring the supply voltage periodically, initially driving an unsustainably high current through the heating element for a length of time related to the value of the measured voltage to bring the heating element rapidly to its operating temperature and subsequently regulating the mean current supply to the heating element to maintain the heating element temperature between upper and lower threshold limits.

4. A method of driving a heating element from a power supply, substantially as herein described with reference to and as illustrated in the accompanying drawings.