GB2280128A - Exhaust emission control - Google Patents

Abstract

A method of operating an internal combustion engine having a catalytic converter 10, 11 and an afterburner is described for heating the catalytic converter during cold starts. The method comprising the steps of cranking the engine until the engine fires. After the engine has fired, the air intake is regulated to maintain a constant and predetermined air flow rate independently of engine load, fuel is supplied at a predetermined rate to the engine cylinders, air from pump 30 is added at a predetermined to the engine, exhaust gases and the air and exhaust gas mixture is ignited by glow-plug 17 in the afterburner. <IMAGE>

Description

Exhaust Emission Control Field of the invention The present invention relates to an internal combustion engine of the type having an exhaust system, a three-way catalytic converter in the exhaust system, an EGI system for heating the catalytic converter during cold starts. The term EGI (exhaust gas ignition) system herein refers to a system comprising means for enriching the mixture strength supplied to the engine combustion chambers during cold starts to generate hydrogen within the exhaust gases, an afterburner chamber arranged upstream of at least one brick of the catalytic converter, means for introducing additional air to the exhaust gases before the gases enter the afterburner chamber, and an igniter within the afterburner chamber for igniting the air/exhaust gas mixture to produce a flame for heating the front face of the catalytic converter.

Background of the invention EGI systems have been proposed earlier by the present Applicants to assist in lighting off a three way catalyst.

In an engine fitted with an EGI system, the mixture burnt in an engine combustion chamber during cold starts has a mixture strength sufficiently high to generate hydrogen in the exhaust gases. Air is added directly to the exhaust gases to form with the engine exhaust gases a gaseous mixture capable of being ignited even when cold and this mixture is ignited in an afterburner chamber arranged upstream of the catalytic converter to raise the temperature of the catalyst rapidly after the engine has fired.

Hitherto, a complex control system has been required to maintain the correct amounts of air and fuel to achieve reliable ignition both within the combustion chambers of the engine and within the afterburner chamber. The strategy adopted previously relied upon the presence of a system for maintaining the idling speed of the engine constant regardless of the load on the engine, which can vary depending on what ancillary equipment within the vehicle is switched on and on cold engine friction which varies with ambient temperature. To maintain constant idling speed, it was necessary to regulate the intake air mass flow entering the combustion chambers. To maintain a required degree of fuel enrichment in the engine to achieve EGI, it was then necessary to regulate the quantity of fuel supplied to the engine during each engine cycle.Finally, to ensure an ignitable mixture and complete combustion of the exhaust gas and air mixture in the afterburner chamber, the additional air supplied directly into the exhaust system also had to be regulated accurately to match the amount of excess fuel in the engine exhaust gases.

The complexity of the system therefore was brought about by the need to regulate separately three individual parameters, namely the throttle by-pass intake air mass flow to maintain constant idling speed, the fuel quantity to match the variable air mass in the engine and the additional air mass to match the variable fuel mass in the afterburner.

Obiect of the invention The present invention seeks to provide an engine of the type described initially in which the control of the EGI system is considerably simplified in order to reduce manufacturing cost and improve operating reliability.

Summarv of the invention According to a first aspect of the present invention, there is provided an internal combustion engine having an EGI system and an idling speed control system for regulating the engine intake air mass flow during operation of the EGI system to maintain the mass flow constant without regard to engine load and engine speed, wherein during operation of the EGI system a predetermined quantity of fuel is supplied to the engine during each operating cycle matched to the engine intake air mass flow to ensure the presence in the exhaust gases of a proportion of hydrogen sufficient to support cold ignition in the exhaust afterburner and wherein the means for adding air into the exhaust system of the engine is operative to supply air at a predetermined rate matched to the predetermined fuel quantity supplied to the engine so as to achieve a predetermined fuel to air ratio in the gaseous mixture ignited in the afterburner.

According to a second aspect of the invention, there is provided a method of operating an internal combustion engine having a catalytic converter and an afterburner for heating the catalytic converter during cold starts, the method comprising the steps of cranking the engine until the engine fires, after the engine has fired, regulating the air intake to maintain a constant and predetermined air flow rate independently of engine load, supplying fuel at a predetermined rate to the engine cylinders, adding air at a predetermined rate to the engine exhaust gases and igniting the air and exhaust gas mixture in the afterburner.

Preferably, after at least part of the catalytic converter has reached its light-off temperature, the intake air is regulated to maintain a constant idling speed and the fuel metered to the engine is adjusted as function of the flow rate of the intake air to maintain a rich predetermined fuel to air ratio and the rate at which air is added to the exhaust system is reduced to a second flow rate sufficient to permit oxidation within the catalytic converter of the carbon monoxide and unburnt hydrocarbons created by the incomplete combustion within the engine combustion chambers.

The engine of the invention and its method of operation differ from the prior art in that the EGI system requires neither the fuelling schedule nor the flow rate of additional air to be varied from one engine start to another and both these are predetermined during EGI. dThe only special step that needs to be taken is to ensure that the engine intake air mass flow also remains constant when normally this parameter is allowed to vary to maintain the idling speed constant. A consequence of the present invention, therefore, is that the idling speed is allowed to vary with engine load during EGI, but because the EGI system is only operated for a very short time after the engine has fired, the engine is returned to normal idling speed control before the driver has had time to notice the difference.

The invention therefore dispenses with the need for accurate regulation of the additional air flow rate and the accurate monitoring of fuel metering during EGI. Instead, both these parameters revert to fixed predetermined values and do not require closed loop control throughout the time that the EGI system is in operation.

It is important to set the constant intake air mass flow to level sufficient to stop the engine from stalling under the worst starting conditions, i.e. cold starts with all the ancillary equipment switched on. For this reason, under normal starting conditions the idling speed will increase momentarily during EGI operation but this short spurt occurring immediately after the engine has first fired is not noticeable and in any event is an effect that many drivers attempt to achieve by depression of the gas pedal.

Whereas in the prior art it was proposed to operate under EGI conditions even during cranking, in the present invention the correct condition for exhaust gas ignition can only be achieved after the engine has first fired and the intake air mass flow has reached its predetermined desired value. This means that a normal fuelling regime is applied during cranking thereby avoiding possible problems of sooting and plug fouling that could be caused by excessive fuelling before the engine has fired.

An EGI system can raise the front face of the catalyst brick to its light off temperature very rapidly but if it is left operating for too long, it can overheat and damage the catalyst locally. Therefore the EGI system must be switched off after a few seconds and the partly active catalytic converter must be relied upon to clean the exhaust gases from the time that the EGI system is switched off until the entire catalytic converter has reached its light off temperature.

Though a small part of the catalytic converter is operational, experimental evidence indicates that it is not capable of coping with all the hydrocarbon and carbon monoxide emissions immediately after the EGI system is disabled. For this reason, it has also been previously proposed to use a catalyst temperature management (CTM) system to assist in the lighting off of the remainder of the catalyst by introducing extra fuel and additional air into the exhaust system to promote the exothermic catalytic reaction. The amount of additional air used during operation of the EGI system is excessive for CTM operation as it would over dilute the exhaust gases with air and may even cool down the partially active catalytic converter.

The amount of additional air in the preferred embodiment of the invention is reduced after the EGI system is switched off but under these conditions it still need not be accurately regulated. This is because the catalyst is partially functional and is at this time operating only as an oxidation catalyst, so that a variable proportion of excess air does not seriously impair its performance. It is possible therefore to reduce the pump speed electrically (by reducing the supply voltage or pulsing the power on and off with variable pulse width) or to operate a two position valve located between the pump and the exhaust system.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a block schematic diagram of an engine of the invention.

Description of the preferred embodiment The single figure shows an engine 12 having an intake throttle 24 and a by-pass passage 36, the entire air flow to the engine being measured by an air flow meter 22. Fuel is injected into the intake ports by fuel metering nozzles 20.

The exhaust gases pass through an exhaust down pipe 14 to a catalytic converter made up of two catalyst bricks 10 and 11 separated from one another by an afterburner chamber 16 within which a glow plug igniter 17 is mounted. The exhaust down pipe has an EGO sensor 38 mounted in it and additional air can be fed into it by an air pump 30 through a check valve 32.

When the engine is warm and running normally, the fuel injected by the nozzles 20 is determined by a closed loop control system that receives input signals from the air flow meter 22 and the EGO sensor. This mode of operation does not lie at the heart of the present invention and need not be described in detail in the present context.

The invention is concerned with engine operation during initial cold start up to bring the catalyst bricks 10 and 11 to their light-off temperature as quickly as possible. For this purpose, while the engine is cranked, a fuel rich mixture is supplied to the engine cylinders until it fires and begins to run. At this point, the EGI system comes into operation and, regardless of engine load aad speed, it regulates the amount of air flowing through the passage 36 so that a predetermined air flow rate is measured by the air flow meter 22. At the same time, the air pump 30 is switched on to supply air at a predetermined rate into the exhaust system through the check valve 32 and fuel is supplied at a predetermined rate through the nozzles 20.

During this phase, the only closed loop control employed is to maintain a constant intake air flow rate, all other parameters being preset. Though the idling speed may rise depending on engine load and friction, the reliable operation of the EGI system is ensured in that the mixture strengths both in engine combustion chambers and in the afterburner chamber 16 are set at their respective predetermined levels.

The quantity of fuel injected during the EGI phase is selected to ensure that the gases in the exhaust system contain sufficient hydrogen to be ignitable from cold, after oxygen has been added by the pump 30. On reaching the afterburner chamber, the gases are ignited by the glow plug 17 and burn as a flame upstream of the second catalyst brick 11. This heats the front face of the second catalyst brick 11 to its light-off temperature very rapidly and thereafter, to avoid overheating, the EGI system is switched off and the engine is run in CTM (catalyst temperature management) mode.

In CTM mode, a less-rich mixture is supplied to the engine cylinders and less air is added by the pump so that while the gaseous mixture in the catalytic converter is no longer ignitable, it does contain components that can react with one another exothermically within the partially active catalytic converter to raise the temperature of the remainder of the catalytic converter. During this mode of operation, it is not essential that the quantity of additional air supplied by the pump 30 should stoichiometrically match the carbon monoxide and unburnt hydrocarbons in the engine exhaust gases as the three-way catalytic converter will always act as an oxidation catalyst at this stage, as long as an excess of air is available. It is not therefore essential to regulate the supply of the air pump 30 even during this phase and it suffices to cut down the flow either by using a two position throttle valve in place of the check valve 32 or by pulse width modulating the voltage supply to the pump 30.

The invention can therefore be seen to provide an engine with a much simplified control system that can nevertheless achieve exhaust gas ignition and catalyst temperature management in order to minimise noxious exhaust emissions during cold starts.

Claims (3)

1. An internal combustion engine having an EGI system and an idling speed control system for regulating the engine intake air mass flow during operation of the EGI system to maintain the mass flow constant without regard to engine load and engine speed, wherein during operation of the EGI system a predetermined quantity of fuel is supplied to the engine during each operating cycle matched to the engine intake air mass flow to ensure the presence in the exhaust gases of a proportion of hydrogen sufficient to support cold ignition in the exhaust afterburner and wherein the means for adding air into the exhaust system of the engine is operative to supply air at a predetermined rate matched to the predetermined fuel quantity supplied to the engine so as to achieve a predetermined fuel to air ratio in the gaseous mixture ignited in the afterburner.

2. A method of operating an internal combustion engine having a catalytic converter and an afterburner for heating the catalytic converter during cold starts, the method comprising the steps of cranking the engine until the engine fires, after the engine has fired, regulating the air intake to maintain a constant and predetermined air flow rate independently of engine load, supplying fuel at a predetermined rate to the engine cylinders, adding air at a predetermined rate to the engine exhaust gases and igniting the air and exhaust gas mixture in the afterburner.

3. A method as claimed in claim 2, wherein, after at least part of the catalytic converter has reached its light-off temperature, the intake air is regulated to maintain a constant idling speed and the fuel metered to the engine is adjusted as function of the flow rate of the intake air to maintain a rich predetermined fuel to air ratio and the rate at which air is added to the exhaust system is reduced to a second flow rate sufficient to permit oxidation within the catalytic converter of the carbon monoxide and unburnt hydrocarbons created by the incomplete combustion within the engine combustion chambers.