GB2276099A - Exhaust emission control - Google Patents

Abstract

In an engine with an exhaust gas ignition and catalyst temperature management system for raising the temperature of the catalytic converter during cold starts, air is added to the exhaust gases while the engine is still partly cold and operating in deceleration mode, even after a stoichiometric mixture is supplied to the engine combustion chambers, to force the catalyst to function as an oxidation catalyst rather than a three-way catalyst to prevent break-through of carbon monoxide and hydrocarbons. Air is supplied via pump 30, valve 32 and conduit 34 to afterburner chamber 16 between catalyst brick 10a, 10b. This avoids cooling brick 10a which is still being warmed to light off temperature and provides the excess oxygen necessary to oxidise the CO and hydrocarbons by means of brick 10b. <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 exhaust gas ignition (EGI) system for heating the catalytic converter during cold starts, the EGI 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 into 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, a catalyst temperature management (CTM) system operative after the EGI system has been disabled for increasing the proportion of the catalyst reaching the light off temperature, the CTM system including means for supplying an excess of fuel the combustion charge, and for introducing additional air into the exhaust gases before they enter the catalytic converter to promote the exothermic reaction between the additional air and the exhaust gases in the active part of the catalyst and a fuelling system operative after the CTM system has been disabled for maintaining a stoichiometric fuel to air ratio in the engine combustion chambers for normal operation of the engine to permit the catalytic converter to operate as a three-way catalyst. Such an engine will hereinafter be referred to as an engine of the type described.

Background of the invention Exhaust gas ignition 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.

Such 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 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 that has been started at the heated part of the catalyst.

From the points of view of reducing fuel consumption, and reducing NOx emissions, this imposed exothermic regime also cannot be maintained for too long and it is desirable to revert to a stoichiometric fuelling regime as quickly as possible even though not the whole volume of the catalytic converter is at its normal working temperature. By operating the catalytic converter as a three way catalyst as soon as possible, maximum efficiency for cleaning all three pollutants NOx, CO and HC can be achieved. This normally requires closed loop control of the engine fuel quantity using a feedback signal from an oxygen or X sensor in the exhaust system, in order to hold the air/fuel ratio within a narrow range about stoichiometry.

Despite all the steps described in the prior art for reducing the light off time of the catalytic converter, namely EGI followed by a limited period of lesser fuel enrichment (CTM), the experimental evidence indicates that on returning to a normal fuelling regime with a stoichiometric fuel to air ratio, the catalytic converter still cannot fully cope with all the CO and HC emissions and allows what is termed "break-through" of the higher concentration peaks that tend to occur during deceleration phases of the drive cycle when the engine is cold, because the catalytic converter is still only effective over part of its entire volume.

Object of the invention The present invention seeks to provide an improved system to control the exhaust emissions after the EGI and CTM systems have been disabled but before the engine and the entire catalytic converter have reached their normal operating temperatures.

Summary of the invention According to the present invention, an internal combustion engine of the type described, is characterised by means operative while a stoichiometric mixture is supplied to the combustion chambers of the engine and while only part of the volume of the catalyst in the catalytic converter has reached its light off temperature to add air to the exhaust gases upstream of the catalyst, at least when the engine is operating in deceleration mode, in order to make the catalytic converter operate as an oxidation catalyst instead of a three-way catalyst.

A three way catalyst carries out three different reactions in reducing emissions. First it performs two oxidation reactions to oxidise CO and HC to form CO2 and H2O. Second it performs a reduction reaction to reduce NOX back to nitrogen by reacting it with HC and CO. The oxidation reactions require an excess of oxygen while the reduction reaction requires a deficiency of oxygen. At stoichiometry, a compromise is reached between the requirements of the different reactions and they all take place simultaneously and it is for this reason that the fuelling is critical to the correct operation of the catalytic converter.

A steady stoichiometric ratio cannot readily be achieved and in practice fluctuations within a fairly broad band can occur,. especially during transients. To take this into account, the wash coat of known three-way catalytic converters contains cerium oxide, which acts as an oxygen store. Only small amounts of oxygen can be stored in this way but they are sufficient to iron out temporary excursions of HC and CO which would otherwise cause break-through.

During normal operation, the mixture strength does oscillate between being weak and rich. The oxygen stored in the cerium oxide while the mixture is weak is used to neutralise the HC and CO in the exhaust gases when the mixture is rich.

However, during deceleration modes, this stored oxygen is not sufficient to cope with the CO and HC emissions and in the invention, this fact is recognised and additional oxygen is introduced into the gases to force the three way converter to act temporarily as an oxidation catalyst only.

A consequence of this action is that the efficiency of the converter in coping with NOx will be reduced. However, inasmuch as NOx does not present a serious problem with a cold engine operating in deceleration mode, it is beneficial in reducing the overall emissions to assist the catalytic converter to function solely as an oxidation catalyst in order to neutralise the excessively high concentrations of HC and CO in the exhaust gases during those cold deceleration modes.

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 is a schematic representation of an engine embodying the invention, and Figure 2 is a similar view of any alternative engine embodying the invention.

In Figure 1, an engine 12 receives air for combustion from an intake manifold that incorporates an air filter 22, an intake throttle 24 and a fuel injector 20. The exhaust gases of the engine 12 flow through an exhaust manifold 14 to a three-way catalytic converter made up of two catalyst bricks 10a and lOb separated from one another by an afterburner chamber 16 containing a spark plug 18. A pump 30 is connected to the exhaust manifold upstream of the catalytic converter to introduce additional air into the exhaust system through a valve 32. An EGO (exhaust gas oxygen) sensor 38 in the exhaust pipe measure surplus oxygen to provide the engine control system with a signal to allow a stoichiometric fuel ratio to be supplied to the combustion chambers of the engine 12 when the engine warm to assure correct operation of the three-way catalytic converter 10a, lOb.

When the engine 12 is first fired from cold and the catalytic converter bricks 10a, lOb are cold, steps are taken by the engine management system to heat up the second brick 10b of the catalytic converter as quickly as possible.

To this end, the quantity of fuel injected into the engine is so great that the exhaust gases contain a significant proportion of hydrogen. This may require a mixture strength typically of the order of 1.7k - 2.0X. At the same time, the pump 30 is operated so that on reaching the chamber 16 sufficient oxygen will have been added to the exhaust gases to produce a mixture that is capable of being ignited when cold. In the afterburner chamber 16 the spark plug 18 is fired repetitively and this ignites the exhaust gases to produce a hot steady flame that heats up the front face of the second brick lOb of the catalytic converter.

The front face of the brick lOb reaches its light-off temperature in a very few seconds during this EGI phase of operation and continued operation of the afterburner could result in permanent damage. For this reason, after this initial period for lighting off the front face of the catalyst only, the mixture strength fed to the engine cylinder is reduced and the flame in the afterburner 16 is extinguished.

There is now commenced a CTM phase in which a richer than stoichiometric mixture is fed to the engine cylinders and the pump 30 continues to add air at a rate regulated by the valve 32 into the exhaust system. The richer than stoichiometric ratio ensures that the exhaust gases contain CO and HC components and these react exothermically with the additional air within the partially operative catalytic converter to generate more heat to heat the entire catalytic converter brick lOb to its light-off temperature.

At this point, the CTM phase is terminated and the engine reverts to normal operation in which the control of the fuelling is carried out by the engine management system which, using the signal from the EGO sensor 38, regulates the injection quantity to ensure that the engine cylinders receive a stoichiometric ratio.

At the time that the CTM phase is discontinued the catalytic converter is still not active over its entire length and though it can cope with the emissions during steady engine operation, it cannot neutralise the HC and CO content of the exhaust gases when the engine is operating in deceleration mode. During deceleration modes, the throttle 24 is closed and little air enters the combustion chambers. However fuel on the walls of the intake ports evaporates in the high vacuum and passes unburnt through the combustion chambers.

The amount of oxygen stored in the three-way catalyst cannot counteract all the HC and CO emissions.

To overcome this problem, the invention proposes injecting air into the exhaust system by means of the pump 30 and the valve 32. The oxygen entering the system disturbs the averaged out stoichiometry of the exhaust gases forcing the converter to operate as an oxidation catalyst only. There now being enough oxygen present to react with the unburnt HC and CO and with the catalytic converter functioning more efficiently as an oxidation catalyst rather than a three-way catalyst, the invention prevents the break-through of HC and CO.

In the embodiment of Figure 2, the additional air from the pump 30 is injected through a pipe 34 and a distribution stand pipe 36 into the afterburner chamber rather than upstream of the catalyst brick 10a. The advantage of this construction as compared with the embodiment in Figure 1 is that the cold additional air does not cool down the first catalyst brick 10a and interfere with its reaching its light off temperature. Thus the first catalyst brick continues to the see the hot undiluted exhaust gases and reaches its light-off temperature as rapidly as possible, whereas the second brick lOb, which at this time is the only one functioning correctly, receives an oxygen rich mixture to allow it to function as an oxidation catalyst in the manner previously described.

Claims (3)

1. An internal combustion engine of the type described, characterised by means operative while a stoichiometric mixture is supplied to the combustion chambers of the engine and while only part of the volume of the catalyst in the catalytic converter has reached its light off temperature to add air to the exhaust gases upstream of the catalyst, at least when the engine is operating in deceleration mode, in order to make the catalytic converter operate as an oxidation catalyst instead of a three-way catalyst.

2. An internal combustion engine as claimed in claim 1, wherein the catalytic converter has two bricks and an afterburner chamber disposed between the bricks, wherein the means for adding air to exhaust gases includes a pipe opening into the afterburner chamber of the catalytic converter.

3. An internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.