GB2426470A

GB2426470A - Purging a particulate trap of a diesel engine

Info

Publication number: GB2426470A
Application number: GB0608713A
Authority: GB
Inventors: Kim Ford; James Bromham
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2005-05-26
Filing date: 2006-05-04
Publication date: 2006-11-29
Anticipated expiration: 2026-05-04
Also published as: DE102006023674A1; GB0608713D0; GB2426470B; GB0510702D0; DE102006023674B4

Abstract

The invention relates to a method of injecting fuel into the cylinders of a diesel engine having a particulate trap. The injection of fuel into each cylinder during each combustion cycle is divided in a plurality of discreet events together forming an injection state. To purge the particulate trap, the injection state is switched in a sequence between a first injection state that produces exhaust gases having a high temperature and a second injection state that produces exhaust gases with a high hydrocarbon content at a lower temperature The switching frequency is sufficiently high for the exhaust gases produced during combustion events with differing injection states to mix with one another in the exhaust system upstream of the particulate trap. The excess hydrocarbons combust to burn off the particles in the trap. Additionally different cylinders are sequentially selected to operate with a different injection state.

Description

Field of the invention Purging a particulate trap of a diesel engine The present invention relates to the purging of a particulate trap of a diesel engine, also termed a diesel particulate filter (DPF).

Background of the invention Diesel engines, when operating under certain conditions, generate soot particles in their exhaust gases. While the combustion process can be controlled to reduce the production of soot particles, they cannot be eliminated to the extent necessary to comply with emission regulations. It is therefore common to incorporate a particulate trap in the exhaust system to filter out soot particles and prevent them from being discharged into the ambient atmosphere. As soot accumulates in a soot trap, it becomes blocked and increases the exhaust back pressure. When the accumulation of soot in the trap is excessive, it has to be purged. Purging is effected by passing through the trap exhaust gases containing an excess of oxygen at a temperature sufficiently high for the soot to burn in the oxygen and to be converted to carbon dioxide that can be discharged from the exhaust system. As with spark ignition engines, diesel engines also produce hydrocarbons and oxides of nitrogen. To reduce the emission of such pollutants, it is common to provide a catalytic converter in the exhaust system positioned upstream of the particulate trap. It is also common to improve the efficiency and performance of a diesel engine by the use of a turbocharger mounted between the engine and the catalytic converter. As is well known, a turbocharger has a turbine driven by the exhaust gases which drives a compressor connected to pressurise the air in the intake system. This allows the mass of air drawn into the cylinders to be increased and hence increases the output power of the engine. As earlier mentioned, in order to purge the particulate trap it is necessary for the exhaust gases reaching the trap to have a very high temperature and a high hydrocarbon content. It is difficult, however, for reasons that will now be explained, to inject fuel into the cylinders of a diesel engine in such a manner as to satisfy both these objectives at the same time. When it is not possible to reach the desired engine exhaust temperature, the engine can be operated to contain sufficient hydro-carbon in the exhaust to cause an exotherm across the catalyst to reach the required particulate trap inlet temperature. With the lower engine out temperature and the high temperature required for the coated particulate trap this exotherm may be too great and may damage or deteriorate the catalyst. It has previously been proposed, for example in EP 1035314, to use multiple injection events in a single combustion cycle. The fuelling regime formed by a set of separate injection events in the same cycle is referred to herein as an injection state. With multiple injection events, one can introduce fuel at different times into the cylinder in order to achieve different results. The latter patent specification proposes up to six different injection events within each state. As can best be seen from Figure 2 of the latter specification, these injection events include a very early "pilot" injection, an early "pre" injection, two "main" injections, a late "after" injection and a very late "post" injection. The terms "early" and "late" are used with reference to the main injections which occur near top dead centre (TDC) at the end of the compression stroke. If an injection state includes an "after" injection event, i.e. one occurring shortly after the main charge has been ignited, then the injected fuel will burn in the cylinder. However, the generated heat will not increase the engine power output significantly but will simply raise the temperature of the exhaust gases. This mode of fuelling can for example be used when it is desired to reduce the lightoff time of the catalytic converter. If an injection state has a late "post" injection event at the end of the power stroke or during the exhaust stroke, the fuel does not burn but passes out of the exhaust system and can be used, for example, to purge the particulate trap by producing an exothermic reaction in the catalyst. One might therefore assume that to achieve both a high exhaust gas temperature and a high hydrocarbon content, all that is needed is to have both an "after" and a "post" injection within the same injection state. However, this does not in practice provide a solution to the problem as the "after" injection prolongs the elevated charge temperatures to such an extent that the fuel of the later "post" injection simply burns before reaching the particulate trap. US 6,666,020 discusses fuelling regimes which use different injection states in order to achieve the right conditions to purge or regenerate a particulate trap. The complexity of the fuelling regimes proposed in the latter patent specification serves to demonstrate the difficulty encountered in calibrating the quantities of fuel in the separate injection events to achieve the desired conditions of high exhaust gas temperature and high hydrocarbon content. Some of the factors that need to be considered are that increasing the "after" injection can raise the temperature of the exhaust gases to such a point as to risk damage to the turbocharger which lies upstream of the particulate trap.Increasing "post" injection on the other hand requires a reduction of the temperature of the exhaust gases to prevent the hydrocarbon burning in the turbocharger instead of increasing it, and additional fuel must then be reacted in the catalytic converter to restore the exhaust gases to their original temperature. Because of the difficulty in achieving high exhaust gas temperatures at the same time as high hydrocarbon content by appropriate setting of the injection state of the cylinders, it has been proposed to introduce hydrocarbons into hot exhaust gases by injecting fuel into the exhaust gas stream downstream of the turbocharger and/or the catalytic converter. This approach, which is referred to as underfloor injection, adds considerably to the cost of implementation and special precautions need to be taken on account of the potential fire hazard. It has also been proposed to resort to fuel additives to lower the temperature at which trap regeneration can be initiated but this solution has its own problems which are discussed at length in the '020 patent mentioned above. Object of the invention The present invention seeks therefore to provide a method of injecting fuel into the cylinders of a diesel engine that succeeds in achieving the conditions necessary to purge a particulate trap.

Summary of the invention According to the present invention, there is provided a method of injecting fuel into the cylinders of a diesel engine having a particulate trap, which comprises dividing the fuel injection into each cylinder during each combustion cycle in a plurality of discreet events together forming an injection state, and switching the injection state in a sequence between a first injection state that produces exhaust gases having a high temperature and a second injection state that produces exhaust gases with a lower temperature but a higher hydrocarbon content than the first injection state, the switching frequency being sufficiently high for the exhaust gases produced during combustion events with differing injection states to mix with one another in the exhaust system upstream of the particulate trap. The principle underlying the present invention is that the high temperature and the high hydrocarbon concentration as not generated by the same combustion cycle of a cylinder. The hydrocarbons introduced by the second injection state are in this way prevented from burning until they approach the particulate trap. The high temperature can be produced by one cylinder while the hydrocarbons are produced by another or the same cylinder may produce hot exhaust gases in one combustion cycle and hydrocarbon rich exhaust gases in a subsequent combustion cycle. 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 schematic representation of a conventional diesel engine in which the method of the present invention can be used to purge the particulate trap.

Description of the preferred embodiment As the engine layout shown in the accompanying drawing is conventional, it will be described only in sufficient detail to permit an understanding of the present invention. The drawing shows a diesel engine 10 having four cylinders 12. Air admitted into the cylinders 12 flows through an intake pipe 30, past an intake throttle 36, and through an intake manifold 15. A high pressure pump 20 draws diesel fuel from a tank 22 and pressurises a fuel rail 18. Electrically controlled injectors 16 introduce fuel under pressure from the fuel rail 18 into the individual cylinders, which results in the fuel igniting spontaneously on account of the elevated temperature and pressure. The exhaust gases resulting from the combustion flow into an exhaust manifold 14 and are eventually discharged to the ambient atmosphere through an exhaust pipe 50. A turbocharger 32 is arranged in the path of the exhaust gases as they leave the exhaust manifold 14. The turbocharger has a turbine wheel which is driven by the exhaust gases and in turn drives a compressor wheel arranged in the intake pipe 30. In this way, waste energy in the exhaust gases is used to compress the intake air and thereby increase the engine output power. An intercooler 34 upstream of the intake throttle 36 reduces the temperature of the intake air before it is admitted into the engine. To assist in reducing emissions, the illustrated engine also uses exhaust gas recirculation. Under certain engine operating conditions, an EGR valve 38 returns some exhaust gases to the intake system through an EGR pipe 40 which contains a cooler 42 for cooling the EGR gases. After leaving the turbocharger, the exhaust gases flow through a downpipe 44, a catalytic converter 46 and a particulate trap 48, the purpose of these last two being to clean the exhaust gases before they are discharged to the ambient atmosphere. The catalytic converter 46 is used to store and subsequently neutralise such pollutants as hydrocarbons and oxides of nitrogen (NOx) and may also store sulphur (SOx) which may be present in the exhaust gases. The particulate trap 48 is used to filter out soot and other any particles that may be produced when the diesel fuel is burnt in the cylinders. It is possible and quite common for the catalytic converter 46 and the particulate trap 48 to be formed as a single unit sharing a common housing. When the catalytic converter is functioning correctly, the chemical reactions taking place within it increase the exhaust gas temperature. Temperature sensors 52 and 54 arranged respectively upstream and downstream of the catalytic converter 46 enable the performance of the catalytic converter 46 to be monitored. The state of the particulate trap 48 is monitored by a differential pressure sensor 56. The signal from the sensor 56 is used to initiate a purge cycle of the particulate trap 48 when it determines that the trap is full from the resulting high back pressure. Alternative regeneration triggers may be used, based on distance, duty, sensors or other principles. The injectors 16 are electrically controlled in the manner proposed in EP 1035314 so that within each engine operating cycle multiple differently timed fuel injection events take place, which events together form an injection state. The quantity of fuel injected at different times affects, amongst other things, the exhaust gas temperature and the hydrocarbon content.

Prior art attempts to achieve the correct conditions for particulate trap regeneration, involved calibrating the quantities of fuel within the individual events of the injection state. While operating with a fuelling regime suitable for purging the particulate trap, all the cylinders are operated with the same injection state for a time long enough to initiate the burning off of the particles in the trap. The present invention, on the other hand, recognises the fact that it is difficult at the end of a single combustion cycle to produce exhaust gases of a composition and a temperature suitable for purging a particulate trap. In the present invention, therefore, the injection states adopted in the cylinders of the engine are sequenced so that different combustion cycles produce exhaust gases having different temperatures and chemical composition. The switching between different injection states is sufficiently rapid for the resultant exhaust gases to mix with one another before they reach the catalytic converter 46 and the filter trap 48. The invention avoids the problems of calibration by using different combustion cycles to produce the high temperature and the high hydrocarbon content. Thus, it is possible for example to switch from a high temperature injection state to a high hydrocarbon injection state every, say, nine engine combustion cycles. If the temperature is found to be high and the hydrocarbon content low, then the switching frequency can be increased to one cycle in eight instead of nine. Conversely, the temperature can be increased and the hydrocarbon content lowered by reducing the switching frequency to one combustion cycle in ten. It is preferred to avoid running one engine cylinder constantly with an injection state different from the other cylinders. If the same cylinder is always used to produce a high hydrocarbon content, then it will run at a different temperature from the others. This could cause problems when reverting to a different fuelling regime after completion of the trap regeneration. It is therefore preferred to use asynchronous sequencing so that different cylinders are sequentially selected to operate with a different injection state.

Claims

1. A method of injecting fuel into the cylinders of a diesel engine having a particulate trap, which comprises dividing the fuel injection into each cylinder during each combustion cycle in a plurality of discreet events together forming an injection state, and switching the injection state in a sequence between a first injection state that produces exhaust gases having a high temperature and a second injection state that produces exhaust gases with a lower temperature but a higher hydrocarbon content than the first injection state, the switching frequency being sufficiently high for the exhaust gases produced during combustion events with differing injection states to mix with one another in the exhaust system upstream of the particulate trap.

2. A method as claimed in claim 1, in which the sequencing is such that different cylinders are sequentially selected to operate with a different injection state from the remaining cylinders.