GB2140556A - Determining fuel and/or air distribution in a multi-cylinder engine - Google Patents

Abstract

A method of determining mass flow rate of fuel supplied to one cylinder of a multi-cylinder internal combustion engine comprises interrupting the ignition of the cylinder during selected cycles so as to allow unburnt charges from the cylinder to enter into the exhaust system and monitoring the change in hydrocarbon level in the exhaust system caused by the interruption. The invention also proposes determining the mass flow rate of air by first setting a rich mixture to use up substantially all the oxygen and then determining the change in oxygen content of the exhaust gases as firing is periodically interrupted in the individual cylinders. The sparks to a cylinder are interrupted by means of a programmable divider 42 which acts on a gate 32 to prevent triggering pulses from a shaft encoder 30 from reaching the ignition circuit 34. The hydrocarbon or oxygen level is monitored by sampling at a point [16] in a cold part of the exhaust manifold and feeding the samples into a gas analyser. <IMAGE>

Description

SPECIFICATION Determining fuel and/or air distribution in a multicylinder engine The present invention relates to a method of determining the fuel distribution and/or air distribution between the cylinders of a multi-cylinder engine and is particularly applicable to a method for use in the design of inlet manifolds.

When designing an inlet manifold, it is important to ensure that all the cylinders of the engine are operating in a balanced manner receiving comparable charges. In the past, one method which has been used to assist in such design is the monitoring of the oxygen content in the exhaust gases in the individual branches of the exhaust manifold, this oxygen content being representative of the air to fuel ratio in the different cylinders. However, the fact that the cylinders all have an equal air to fuel ratio does not guarantee that the cylinders are receiving comparable charges since the amount of air entering the cylinders may be different for the different cylinders.

It has already been proposed in one method that an anemometer be used to determine the air flow rate in each branch of the induction manifold. This method is not entirely satisfactory, however, for several reasons. A hot wire or hot film anemometer is a delicate instrument and its introduction into the inlet manifold presents problems. Furthermore, it does not suffice to use the anemometer to obtain spot readings and instead detailed traverses across each tract of the manifold must be made and the readings averaged. The mechanical construction of the test manifold to permit such monitoring of the air speed causes excessive complications. A still further disadvantage is that the mere presence of the anemometer to some extent distorts the readings which are to be taken.

Because of the difficulties of the method described above, it has also been proposed in the past that the quantity of fuel in the individual branches of the manifolds be perturbed by the injection of a predetermined quantity of fuel into the branch of the manifold. By determining the change in fuel to air ratio caused by the addition of the given quantity of fuel it is possible, by solving simple simultaneous equations, to determine the air quantity and the fuel quantity.

This method also is unreliable because it assumes that the fuel injected only reaches one cylinder and in practice this is not the case. Apart from the difficulty of determining the quantity of fuel which has caused the perturbation of the air to fuel ratio, the manifold must be altered in order to permit the fuel to be introduced.

The present invention seeks to provide a method of determining the fuel and/or distribution which mitigates at least some of the foregoing disadvantages.

In accordance with a first aspect of the present invention, there is provided a method of determining mass flow rate of fuel to one cylinder of a multicylinder internal combustion engine, which comprises interrupting the ignition of the cylinder of the engine during selected cycles so as to allow unburnt charges from the cylinder to enter into the exhaust system and monitoring the change in hydro-carbon level in the exhaust system caused by the interruption.

Conveniently, the hydrocarbon level is monitored in a section of the exhaust manifold which is common to all the cylinders. This offers the first advantage of simplicity and also enables the exhaust gases to be sampled for analysis from a cold part of the exhaust system.

In order to determine the quantity of fuel entering any given cylinder, the engine is first allowed to run under given load and speed conditions and the hydrocarbon level in the exhaust is measured. The sparks at the spark plug of one cylinder are then omitted at regular intervals, whereby unburnt charges are periodically allowed to enter into the exhaust system resulting in a rise in the hydrocarbon level. The increase in the hydrocarbon level is clearly related both to the quantity of fuel entering that cylinder and to the proportion of unburnt charges, that is to say the proportion of omitted sparks in that cylinder.

In order to obtain comparative measurements regarding the fuel burnt in each cylinder, the fuel entering each cylinder may be measured by omitting the sparks reaching the different cylinders in turn and in this way the fuel distribution between the cylinders may be monitored thus enabling the efficiency of the manifold to be tested without interfering with the manifold.

It is possible to monitor the oxygen content of the exhaust gases in the individual branches of the exhaust manifold in order to provide a measure of the fuel to air ratio in the individual cylinders and from this the mass flow rate may be determined not only of the fuel reaching each cylinder but also the air.

The above method of measuring the rate of air flow to each cylinder is however inconvenient because of the need to sample exhaust gases in the individual branches of the exhaust manifold and it is preferred to use a technique analogous to that proposed above for the measurement of the fuel quantity to enable the mass of air to be computed directly.

It is not possible merely to run the engine under normal conditions and to monitor increases in the oxygen content of the exhaust gases when individual cylinders are periodically prevented from firing since the change in the oxygen level will not necessarily be caused exclusively by the air entering the exhaust system from unburnt charges but will also be dependent on the level of fuel reaching the same cylinder. This ambiguity as to the cause of the increase in the air in the exhaust system cannot readily be resolved.

However, if the engine is first re-set so as to run with a rich mixture then it can be ensured that when all cylinders are firing the oxygen content of the exhaust gases is substantially zero or negligible and if under such conditions the cylinders are periodically prevented from firing then the change in the oxygen level in the exhaust is determined solely by the mass rate of flow of air to the cylinder to which sparks have been omitted.

Thus, in accordance with a second aspect of the present invention, there is provided a method of determining the mass flow rate of air to one cylinder of a multi-cylinder combustion engine which comprises running the internal combustion engine with a mixture sufficiently rich to ensure that the oxygen content in the exhaust gases is negligible, interrupting the ignition of the cylinder of the engine during selected cycles so as to allow unburnt charges from the cylinder to enter the exhaust system and monitoring the change in the oxygen level in the exhaust system caused by the interruption.

Preferably, the oxygen level is monitored in a section of the exhaust pipe which is common to all cylinders both in the interest of simplicity and to permit the gases to be sampled in the cold part of the exhaust system.

After the engine has been run on a rich mixture to establish the air quantity reaching each cylinder, it may be returned to the correct mixture setting and once again readings may be taken to establish the increase in oxygen level in the exhaust system when individual cylinders are prevented periodically from firing. If the relative increases in the oxygen content are substantially the same when the engine is running with an over-rich mixture are the same as for a normal mixture, it may be assumed that the cylinders are all receiving the same air to fuel ratio under both operating conditions. If, however, the relative increases in oxygen content for the different cylinders differ for the rich mixture from the normal mixture, then it may be deduced that the cylinders are not all receiving an equal fuel to air ratio.

Consequently, it is possible to test the design of an inlet manifold by comparing the increases in oxygen level in the exhaust system when the engine is running with a weak mixture with the values obtained when the engine is running at a normal mixture without the need for direct measurement of the fuel quantity by the hydrocarbon monitoring proposed above.

It is of course important when comparing the oxygen level changes with a rich mixture and a normal mixture to ensure that the other operating parameters should not be allowed to vary, in particular the position of the throttle, the engine speed and the engine temperature should remain constant.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the inlet and exhaust manifolds of an internal combustion engine, and Figure 2 shows a block circuit diagram of an ignition system for implementing the method of the invention.

In Figure 1,there is shown an engine 10 having an inlet manifold 12 and an exhaust manifold 14. The inlet manifold 12 is designed to match the charge to each cylinder as closely as possible.

In the exhaust manifold, there is provided a sampling point 16 and four oxygen sensors 18. The sampling point is common to all four cylinders, in the example illustrated wheres each cylinder has its own oxygen sensor. Each oxygen sensor provides a reading which is representative to the fuel to air ratio while the sampling point is for taking samples for hydrocarbon hydrocarbon analysis to indicate the proportion of unburnt hydrocarbon in the exhaust gases.

The ignition system is shown schematically in Figure 2 and comprises a shaft encoder 30, the output pulses of which are fed by way of a gate 32 to an electronic lg nition circuit 34. The electronic ignition circuit 34 applies a low tension signal to a high tension coil 36 the output from which is fed by way of a distributor 38 to the individual spark plugs 40 associated with the engine cylinders. The output from the shaft encoder 30 is further connected to a divider 42 which controls the gate 32, the divider 34 being preferably programmable to allowthe proportion of omitted sparks to be varied.

If the gate 32 is maintained permanently open, then the ignition system is conventional. The shaft encoder 30 produces pulses when the engine crankshaft is in predetermined positions, for example top dead centre for the individual cylinders, and these pulses are applied to the electronic ignition circuit 34 which applies a signal to the coil 36 so as to produce a spark correctly times in relation to the shaft encoder pulse. As the operation of the ignition system itself is not relevant to the present application, it is felt that no further detailed description of it is necessary.

The programmable divider 42 serves to produce an output pulse for each predetermined number of pulses from the shaft encoder. If one pulse in every thirty two pulses acts on the gate 32 to prevent the shaft encoder pulse from being passed onto the electronic ignition circuit 34, then one cylinder of the engine will be prevented from firing firing once in every eight cycles. By virtue of the programmable nature of the divider, it is possible to interrupt the pulses to anyone of the cylinders and to vary the rate of interruption of the sparks to that cylinder.

In order to determine the fuel distribution, the divider 42 is first programmed so that one of the cylinders is prevented from firing once in a predetermined number of cycles. The rise in the average hydrocarbon level at the sampling point 16 is monitored, this change being occasioned by the fuel from that cylinder entering the exhaust system unburnt. Consequently, the rise is proportional to the fuel in each charge of the engine. Because the spark is interrupted infrequently, the engine does not run erratically and the quantity of fuel entering the exhaust system during the unburnt cycle is the same as the quantity entering, and being burnt, during the remaining cycles.

After a reading has been obtained in one of the cylinders, the operation is repeated for the remaining cylinders by reprogramming the divider 42. The differences in monitored hydrocarbon levels will now be indicative of the relative amounts of fuel entering each cylinder during each cycle.

In one mode of operation to derive an absolute measurement of the quantity of fuel entering any given cylinder, the engine is first allowed to run under given load and speed conditions and the hydrocarbon level of the exhaust is measured. With the engine correctly firing the fuel to air ratio is also measured using the respective oxygen sensors 18 in the individual tracts of the exhaust manifold. The divider 42 is then programmed to interrupt the sparks and the change in hydrocarbon level in the exhaust is measured by analysing samples taken from the sampling point 16 and during this time the fuel to air ratio is not longer monitored. From a knowledge of the fuel to air ratio and the change in hydrocarbon level when sparks to that cylinder are interrupted, it is possible to determine both the air mass flow rate and the quantity of fuel in each cycle.

In an alternative method of measuring the absolute quantity of fuel entering each given cylinder, after the rises in hydrocarbon have been measured a measure of the air quantity entering with the fuel is derived by resetting the mixture of the engine such that all the cylinders run at a rich mixture. The mixture should be sufficiently rich to ensure that substantially all of the oxygen content is used up in combustion leaving only a negligible proportion of oxygen in the exhaust gases. After the mixture strength has been altered, the engine is once again run and individual cylinders are periodically prevented from firing. The increase in the oxygen content of the exhaust gases is now monitored by sampling the exhaust gases at the point 16 and analysing for oxygen content. For each cylinder the oxygen rise will be dependent upon the charge of air entering that cylinder per cycle.From a knowledge of the absolute quantity of the air and fuel entering the engine during each cycle, it is then also possible to determine if the air to fuel ratio is constant for all the cylinders of the engine.

The latter method of measuring oxygen content during interruptions of the firing of the cylinders of the engine may be used as an alternative to the monitoring of the rise in the hydrocarbon level in order to measure the air fuel distribution to the individual cylinders. If the engine is re-set to run with a stoichiometric mixture and the cylinders are again prevented from firing at regular intervals the oxygen level will rise by different amounts for the different cylinders. If the ratios of these amounts to one another are the same as the ratios of the corresponding amounts when the engine is running with a rich mixture, then it may be assumed that the fuel to air ratio reaching all the cylinders is equal. If on the other hand the ratios of the amounts to one another varies with the mixture stength then it may be assumed that the cause is that the air quantity is the exhaust gases is being altered not only by the fact that some of the charges are entering unburnt into the exhaust system but that the air to fuel ratio is also varying between cylinders accounting for some oxygen being present in the exhaust system as a result of incomplete combustion.

The invention therefore provides an efficient method of monitoring the performance of an inlet manifold 12 without in any way interfering with the flow within the inlet manifold. Furthermore, the method may be implemented simply and inexpensively employing conventional exhaust analysis equipment.

Claims (9)

1. A method of determining mass flow rate of fuel to one cylinder of a multi-cylinder internal combustion engine, which comprises interrupting the ignition of the cylinder of the engine during selected cycles so as to allow unburnt charges from the cylinder to enter into the exhaust system and monitoring the change in hydrocarbon level in the exhaust system caused by the interruption.

2. A method as claimed in claim 1, wherein the hydrocarbon level is monitored by means of a hydrocarbon sensor arranged in a section of the exhaust manifold which is common to all the cylinders.

3. A method as claimed in claim 1 for performing a comparison between the the mass flow rate of fuel to the different cylinders of an engine, which comprises interrupting the sparks periodically to each cylinder in turn and comparing the changes in hydrocarbon level caused by the different cylinders.

4. A method as claimed in claim 1 for deriving an absolute measurement of the fuel mass flow rate to the cylinder which additionally comprises monitoring the oxygen content in the tract of the exhaust manifold from the cylinder to determine the fuel to air ratio for the cylinder when the engine is operating without spark interruption.

5. A method as claimed in claim 4, wherein the oxygen content is monitored by means of a plurality of sensors each associated with only a respective one of the tracts of the exhaust manifold.

6. A method of determining the mass flow rate of air to one cylinder of a multi-cylinder combustion engine which comprises running the internal combustion engine with a mixture sufficiently rich to ensure that the oxygen content in the exhaust gases is negligible, interrupting the ignition of the cylinder of the engine during selected cycles so as to allow unburnt charges from the cylinder to enter the exhaust system and monitoring the change in the oxygen level in the exhaust system caused by the interruption.

7. A method as claimed in claim 6, wherein the oxygen level is monitored in a section of the exhaust pipe which is common to all cylinders.

8. A method as claimed in claim 6 or 7, wherein the increase in the average oxygen content of the exhaust gases is additionally measured while interrupting the firing of individual cylinders periodically with engine running with a stoichiometric mixture, the ratios of the increases caused by individual cylinders to one another being compared for the two different mixture settings to determine the relative fuel to air ratio supplied to the cylinders.

9. A method of determining mass flow bate of fuel and/or air to one cylinder of a multi-cylinder internal combustion engine, substantially as herein described with reference to and as illustrated in the accompanying drawings.