GB2435839A

GB2435839A - Diesel engine particulate filter regeneration

Info

Publication number: GB2435839A
Application number: GB0604320A
Authority: GB
Inventors: William David Lamb; David Arthur Ketcher; Mike James Watts
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2006-03-06
Filing date: 2006-03-06
Publication date: 2007-09-12
Anticipated expiration: 2026-03-06
Also published as: GB2435839B; GB0604320D0; JP2007239743A

Abstract

A method of operating a variable displacement diesel engine for promoting an exothermic reaction in the after-treatment system of the engine in order to regenerate a diesel particulate filter, which comprises injecting 10, fuel into at least one disabled cylinder 20, while varying the timing of the operation of at least one valve 12, 14, of the cylinder to reduce the maximum pressure in the combustion chamber of the cylinder and thereby preventing the ignition of the injected fuel. Advantageously cylinder pressure is reduced and combustion temperatures are not reached thus allowing fuel to be injected at or near the top, centre of the cylinder. Preferably fuel is sprayed into a piston bowl 18, and does not wet the cylinder bore. Advantageously engine oil contamination is reduced thus reducing wear on the engine.

Description

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DIESEL ENGINE PARTICULATE FILTER REGENERATION

This invention relates to a method of operating a variable displacement diesel (compression ignition) engine for promoting an exothermic reaction in the after-treatment system of the engine in order to regenerate a the diesel particulate filter.

In order to improve economy and reduce emissions, some engines are able to disable operation of individual cylinders. To disable a cylinder, fuel supply to the cylinder is terminated and the inlet and exhaust valves no longer operate. The cylinder then functions as a spring storing and releasing energy to and from the air trapped inside it. Such engines due to their ability to deactivate individual cylinders, are known as variable displacement engines (VDE5) The present invention is concerned with variable displacement compression ignition engines. Diesel engines utilise emissions control devices known as particulate filters which trap soot particles in the exhaust gases.

These filters require periodic cleaning or regeneration.

To regenerate the particulate filter, it is current practice to inject fuel into a cylinder of a diesel engine after the combustion has finished in order to increase the levels of hydrocarbons (HC) in the exhaust gases. This creates an exothermic reaction in the aftertreatment system to aid in the regeneration of a diesel particulate filter.

A major drawback to this practice is that a proportion of the post-combustion injected fuel is washed by the cylinder bores into the engine oil and causes dilution of the oil. If the fuel in the oil reaches an excessive level, then serious engine wear can occur.

The mechanism for this is thought to relate to impingement of the fuel spray directly onto the bore wall.

This is caused by the necessity to inject the fuel after combustion has finished, by which point the piston has moved down the cylinder. The fuel then sprays above the top of the piston and hits the bore wall.

With a view to mitigating the foregoing disadvantage, the present invention provides a method of operating a variable displacement diesel engine for promoting an exothermic reaction in the after-treatment system of the engine in order to regenerate a diesel particulate filter, which comprises injecting fuel into at least one disabled cylinder while varying the timing of the operation of at least one valve of the cylinder to reduce the maximum pressure in the combustion chamber of the cylinder and thereby prevent the ignition of the injected fuel.

Because fuel is injected in the present invention into a disabled cylinder, the valve timing may be altered with a view to reducing peak cylinder pressure. The reduced cylinder pressure means the temperatures required for combustion are not reached allowing the fuel to be injected at or near top dead centre. Diesel engines are often designed with a bowl in the piston crown and if injecting occurs at or near top dead centre the fuel is only sprayed into the bowl and does not wet the cylinder bore.

To maintain a sufficiently low pressure in the combustion chamber to prevent the fuel from igniting when it is injected, the mass of air retained in the combustion chamber needs to be reduced. Conventionally, diesel engines operate unthrottled, or even turbocharged, to maximise the mass of air trapped in the cylinder before compression. In the present invention, valve timing is used to reduce the mass of trapped air and this may be achieved by either admitting less air into the cylinder or expelling air from the cylinder after it has been admitted.

Admitting less air can be achieved by reducing the duration of the air intake event while the piston is moving down the cylinder (from TDC towards BDC) and expelling air can be achieved by maintaining the intake valve open, or opening the exhaust valve, while the piston is moving up in the cylinder. Though it would be undesirable to reintroduce fuel laden air back into the intake manifold.

In addition to pumping fuel through an inactive cylinder, the preferred embodiment allows the valve timing of at least one of the intake and exhaust valves to be controlled in order regulate the volume of air pumped through the inactive cylinder.

Reference to an "expansion stroke" and a "compression stroke" have intentionally been avoided because the cylinder at this time is not firing and is acting only as a pump delivering air and unburned fuel into the after-treatment system where it can react exothermically to burn off soot collecting in the particulate filter.

Fuel can be injected into a cylinder during every revolution of the crankshaft or potentially on any stroke desired. This is regardless of whether the diesel engine is a two stroke or a four stroke engine.

It is preferred to disable different cylinders of the engine, selected on a random or cyclic basis, in order to reduce the wear on any individual cylinder and to maintain cylinder temperature both for reactivation and fuel atomisation purposes.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 is a flow chart showing the control strategy of an electronic control unit (ECU) implementing the method of the present invention, Figure 2 is a five stage diagram showing a section through an engine cylinder operating according to the present invention to inject fuel into the after-treatment system during each revolution of the crankshaft, and Figure 3 is a seven stage diagram showing a section through an engine cylinder operating according to the present invention to inject fuel into the after-treatment system during every second revolution of the crankshaft.

In order to operate as a variable displacement engine, an engine requires variable valve actuation that allows the timing of the intake and exhaust valves of at least one cylinder to be controlled independently of the other valves.

The present invention makes use of such a variable valve actuation system to enable fuel to be pumped into the after-treatment system through a disabled engine cylinder without it igniting until it reaches the aftertreatment device.

Since it is compression that generates the heat within the cylinder that causes ignition of the air and fuel mixture, maintaining a low pressure in the combustion chamber by reducing the mass of air that is compressed prevents the cylinder from firing when fuel is injected.

Particulate filter regeneration is only effected when the engine is operating with one or more disabled cylinders.

The first decision taken is to determine if a cylinder is to be allowed to remain active or disabled. This is decided by he engine electronic control unit on the basis of the prevailing load and speed conditions.

If the cylinder is to remain active, then the left hand side of the flow chart is followed, regardless of the state of the particulate filter. In this case, standard valve timing is adopted and injection timing is adopted, which results in full combustion and normal HC levels in the exhaust gases. If the engine cylinder is to be disabled and the particulate filter needs to be regenerated, then the right had side of the flow chart is followed. Namely, the valve timing is altered to reduce cylinder pressure to a level at which fuel does not spontaneously ignite when it is injected. Fuel injection timing is modified, if necessary, so that the injected fuel does not burn in the combustion chamber but passes into the exhaust gases which then have a high HO content and air to burn the surplus fuel in order to generate an exotherm.

Figures 2 and 3 show in detail the method in which the ECU controls the engine, valves and injectors in order to achieve the desired effect. The diagrams of Figures 2A to E and 3A to G, all show a single cylinder at different crankshaft angles. This is a cylinder that is disabled and does not contribute to the power output of the engine. The purpose of the introduction of fuel and air into this engine cylinder is to pump unburned fuel and air oxygen into the diesel particulate filter, downstream of the exhaust valve.

Figure 2 refers to a operating mode which injects fuel into the cylinder 20 on every revolution of the crank shaft (not shown) whereas in figure 3, fuel is injected into the cylinder during every second revolution of the crankshaft, In Figure 2A, the piston 16 is at top dead centre. The inlet valve 12 and exhaust valve 14 are closed to prevent collision with the piston crown. At this point, fuel is injected by injector 10 into the piston bowl 18. By injecting at top dead centre, no fuel impinges on to the walls of the cylinder.

In Figure 2B, the piston 16 starts to move downwards.

In this context, the term "downwards" is being used to mean from TDC towards BDC. As soon as is possible to avoid it coming into contact with the piston, the inlet valve 12 opens to allow air into the cylinder. The shape of the inlet port is such that air entering the cylinder will be swirling. This aids in picking up and carrying the atomised fuel that was injected into the piston bowl 18 in Figure 2A.

In Figure 2C, the piston has reached bottom dead centre and is full of swirling air and fuel mixture. In the illustrated mode of operation, because it is not desired to retain a full air charge within the cylinder, by the time the piston reaches BDC, the inlet valve has not fully closed and the exhaust valve has commenced to open.

The piston 16 then begins to move upwards as shown in figure 2D and forces out the swirling air and fuel mixture into the exhaust stream along with the hot exhaust gases from the active cylinders.

The exhaust valve 14 remains open as late as possible to allow as much mixture to exit the cylinder as possible.

Eventually the exhaust valve is closes to prevent collision with the piston 16.

The piston continues to move towards top dead centre with both valves 12 and 14 closed. There is some compression of the remaining air and fuel mixture that has not had enough time to be forced out of the cylinder, but the compression is not enough to produce the pressure required to ignite the mixture.

Finally in Figure 2E, which is the same as Figure 2A, the cycle starts again and more fuel is injected into the piston bowl 18 as it reaches top dead centre.

Figures 3A to G show a method in which fuel is injected on alternate revolutions of the crankshaft. This mode is less effective at supplying fuel and air to the particulate filter than the mode described above.

In Figure 3A, fuel is injected into the combustion chamber with the piston 16 at top dead centre, in the same manner as in Figure 2A. Likewise, in Figure 3B, as in Figure 2B, the intake valve 12 is opened to admit air into the combustion chamber until the piston reaches BDC in Figure 30. In this embodiment, however, both valves 12 and 14 are closed when the cylinder reaches BDC.

Throughout Figures 30, 3D and 3E, the intake and exhaust valves 12 and 14 remain closed while the crankshaft performs a complete revolution. As the piston moves to TDC the gas is compressed but no fuel is injected when the piston reaches TDC. The air in the cylinder during this time acts as a spring to store energy and this energy is returned to the crankshaft during the expansion stroke as the piston returns to the BDC position, as shown in Figure 3E. As a result, during this complete revolution of the crankshaft, the cylinder does minimal net work.

In the next phase, shown in Figure 3F the piston moves up once again to expel the fuel and air trapped in the cylinder into the after-treatment system, this being analogous to Figure 2D. Late in this stroke the exhaust valve is closed, returning to the position of Figure 3G which is the same as Figure 3A and represents the start of a new cycle.

In piston stroke corresponding to the movement from Figure 30 to Figure 3D, there is air and fuel in the combustion and it is important to ensure that the pressure is not sufficient at TDC for the mixture to ignite. This is achieved in the embodiment of Figure 3 by limiting the time that air is admitted into the combustion by delaying the opening of the intake valve or closing it before the piston reaches BDC. An alternative to this would be to open the exhaust valve as the piston moves from Figure 30 to Figure 3D so that part of the intake air is exhausted during the first upwards stroke of the piston (Figure 3D) and the remainder in the second upward stroke (Figure 3F) In keeping with VDE technology, it is preferable for the disabled cylinder to be cycled between all the available cylinders of the engine. This promotes even engine wear across the cylinders. It is also beneficial when reactivation of the cylinder is required, as prolonged disabling of the same cylinder causes its temperature to fall. This yields undesirable effects when the cylinder is reactivated, such as poor emissions and noise, until the cylinder is up to normal running temperature.

In order to select which cylinder is to be deactivated, it is preferable for all the cylinders to have variable valve actuation. It is therefore possible for any cylinder so equipped to function according to the method of the present invention.

Claims

Claims 1. A method of operating a variable displacement diesel engine

for promoting an exothermic reaction in the after-treatment system of the engine in order to regenerate a diesel particulate filter, which comprises injecting fuel into at least one disabled cylinder while varying the timing of the operation of at least one valve of the cylinder to reduce the maximum pressure in the combustion chamber of the cylinder and thereby prevent the ignition of the injected fuel.

2. A method as claimed in claim 1, wherein fuel injection is timed to occur at or near top dead centre.

3. A method as claimed in claim 1, wherein maximum cylinder pressure is reduced by modifying the timing of the intake valve to reduce the mass of air admitted into the cylinder.

4. A method as claimed in claim 1, wherein maximum cylinder pressure is reduced by modifying the timing of at least one of the intake and exhaust valves to expel a proportion of the mass of air admitted into the cylinder prior to compression of the air.

5. A method as claimed in any preceding claim, wherein fuel is injected into the cylinder and discharged into the after-treatment system during each revolution of the crankshaft.

6. A method as claimed in any of claims 1 to 4, wherein fuel is injected into the cylinder and discharged into the after-treatment system once during each two revolutions of the crankshaft.

-10 - 7. A method as claimed in any preceding claim, wherein different cylinders of the engine are disabled, selected on a random or cyclic basis.

8. A method as claimed in any preceding claim, wherein the valve timing is at least one of the intake and exhaust valves is controlled in order regulate the volume of air pumped through the inactive cylinder.

9. A method of operating a variable displacement diesel engine for promoting an exothermic reaction in the after-treatment system of the engine in order to regenerate a diesel particulate filter, substantially as herein described with reference to and as illustrated in the accompanying drawings.