GB2429074A - Engine valve control during cylinder reactivation - Google Patents

Abstract

A method of controlling an internal combustion engine having a plurality of cylinders, for switching between a first mode in which at least one cylinder is deactivated by closing its inlet and exhaust valves and a second mode in which a previously deactivated cylinder is active. The switching is effected by cycling the engine while operating the inlet valves to admit air into the deactivated cylinder so as to raise the temperature of the cylinder prior to introducing fuel to activate the cylinder. The number of cycles in which air but no fuel is admitted into the cylinder may be set in dependence upon input from a demand pedal. Both inlet and exhaust valves may be operated when switching from the first mode to the second mode prior to the introduction of fuel.

Description

Variable Displacement Engine Control This invention relates to controlling

an internal combustion engine having a cylinder that may be selectively disabled, and more particularly to smoothing the transition between a disabled and subsequently re-enabled cylinder operating mode.

Recent advances in engine development have lead to the production of variable displacement engines or VDEs. VDEs are internal combustion engines having the ability to selectively disable cylinders when power output does not demand use of the full engine. This creates a smaller capacity of working engine hence the term variable displacement engine.

The concept of VDE may be employed in both spark and compression ignition engines and leads to increased fuel economy predominantly when the vehicle is cruising, such as on motorways. Depending on the torque requirement and the hardware available, many different configurations of cylinders can be run. For example 4,6,8,10 and 12 cylinder engines with as many cylinders deactivated as is possible.

When switching between fully enabled and partially disabled operating modes there may be appreciable differences in noise and performance, which lead to driver dissatisfaction. it is desirable to be able to switch between operating modes with as little driver perception as possible. Further to this, it is highly desirable to reduce emissions wherever possible in order to comply with ever more stringent government legislation.

These problems are particularly emphasised on a compression ignition engine. Diesels are known for their severe combustion noise, which has been improved substantially in recent years primarily through the use of pilot injection. Diesels also benefit from increased temperature within the combustion chamber in order to reduce emissions when fuel is introduced for combustion.

When disabling a cylinder in a diesel VDE, the lack of combustion within that cylinder causes its temperature to drop. On reactivation of that cylinder the lower combustion temperatures result in increased emissions and also increased noise levels. Typically cold starting a cylinder results in high hydrocarbon and CO emissions levels during the first few cycles of engine use.

is PCT application WO99/49193 addresses this same problem by heating the deactivated cylinders with the glow plug normally used during engine start up. This solution provides a very low thermal heating capacity as glow plugs are designed to produced a hot spot purely to promote localised combustion rather than heat the entire cylinder. Modern day slim tip glow plugs make this solution even less viable due their even further reduced heating capacity and low wattage.

Further to these obvious problems, this proposed solution requires additional control apparatus as well as wasted electrical energy through having to heat the glow plugs.

With a view to mitigating at least some of these disadvantages, the present invention provides a method of controlling an internal combustion engine having a plurality of cylinders, for switching between a first mode in which at least one cylinder is deactivated by closing its intake and exhaust valves, and a second mode in which a previously deactivated cylinder is active, wherein the switching is effected by cycling the engine while operating the inlet valves to admit air into the deactivated cylinder so as to -3...

raise the temperature of the cylinder prior to introducing fuel to activate the cylinder.

Preferably, the number of cycles in which air but no fuel is admitted into the cylinder is set in dependence upon input from a demand pedal of the engine.

It is further preferable if both inlet and exhaust valves of the cylinder are operated when switching from the first to the second mode prior to the reintroduction of fuel.

In order to supply increased torque in response to a driver demand, additional fuel may be supplied to other, active cylinders of the engine during the time that air but no fuel is admitted into the deactivated cylinder.

Preferably during reintroduction of fuel to reactivate a cylinder, the quantity of fuel injected is increased progressively until the fuel quantity matches that provided to the other active cylinders.

A further advantage is provided when the rate of increase of injected fuel quantity is set in dependence upon input from a demand pedal of the engine.

This can be improved upon still further when the additional fuel supplied to the other, active cylinders is decreased progressively as the quantity of fuel injected into the previously deactivated cylinder is increased.

The invention will now be described further by way of example with reference to the accompanying drawings in which Figure 1 is a block diagram representing the operation of a conventional variable displacement engine, and Figure 2 is a graph highlighting the respective fuel input for active and deactivated cylinders according to a preferred embodiment.

The present invention provides a method of operating a conventional diesel engine such as that shown in figure 1. A conventional modern compression ignition engine 10, is controlled by an engine control unit (ECU) 12. Diesel engines are normally operated at wide open throttle, the output of the engine being determined by the fuel provided to the engine for combustion. When a certain torque level is required, the driver depresses a demand pedal or accelerator 14. The ECU 12 receives a signal from a demand pedal position sensor and controls the fuelling system 16 to inject more fuel into the engine 10.

In order to function as a variable displacement engine the engine 10 must be capable of disabling at least one of its cylinders.

Development of the variable displacement concept has led to closing the intake and exhaust valves of the disabled cylinder at the same time as discontinuing the supply of fuel. By closing the valves, the remaining air inside the cylinder is repeatedly compressed and expanded without the loss associated with pulling and pushing air through the inlet and exhaust manifolds respectively. Closing of the valves is represented by box 18 in figure 1.

When the valves are forced to remain closed, no new air is allowed into or out of the cylinder through the valves.

This causes the cylinder to behave as an air spring with much of the energy stored in the compressed air during compression being released during expansion. Any energy lost during this cycle is transmitted as heat in to the cylinder walls. However, as the pressure increases during compression, a small amount of air is lost through leakage past the piston rings on each upward stroke. Prolonged running in a deactivated mode results in an equilibrium level being reached wherein during cycling of the engine, the pressure in the cylinder oscillates about atmospheric pressure. With this reduced amount of air in the cylinder, the heat generated during compression is substantially reduced.

The loss of heat in the cylinder does not present a problem, as there is no need for the cylinder to be warm once combustion has ceased. On reactivation, this is no longer the case. Reactivation is usually required when there is a need for increased torque. This is usually determined by the driver's input to the demand pedal 14, though other factors may also be relevant. The level of demand can differ depending on the circumstances, for example when cruising on a motorway, it can be expected that cylinder deactivation will occur. A slight incline on the motorway will call for increased torque whereas the desire to overtake another vehicle quickly and safely will call for a greater increase in torque.

The difference in required output can be managed to some degree by the number of cylinders that can be reactivated due to the torque demand. For example, a t.ielve cylinder engine may be operated in a four cylinder mode when cruising, changing to a six, eight, ten or full twelve cylinder operating mode when the driver presses on the accelerator.

The problem created by reactivation of the cylinder occurs when the temperature within the reactivated cylinder drops excessively. Injecting fuel into a cold cylinder is disadvantageous in terms of noise, smoke, Carbon dioxide and hydrocarbon emissions. The greater the number of cold cylinders reactivated, the stronger these effects appear.

Ideally the transition between deactivated and activated running of any cylinder should be as seamless as possible. The preferred embodiment of the invention acts to control the engine in order to generate as much heat as possible within the deactivated cylinders prior to the cylinder being reactivated. Such control continues until the previously deactivated cylinders run in a way indistinguishable from the cylinders that had not been previously deactivated.

The input from the accelerator pedal 14 can be determined by both the pedal position and also the rate of change of pedal position. Once an increased torque request is indicated, the ECU 12 sends a control signal to the valve actuation control 18, which causes the inlet valves to resume normal operation and allow air into the cylinder. The valve deactivation mechanism need not be described herein in detail as it is well known to those skilled in the art.

Deactivation can in particular be achieved mechanically, electrically, hydraulically or electro-hydraulically By opening the cylinder to the inlet port once in every four strokes, the volume of air within the cylinder can be maintained to a point where useful heat can be lost from the air to the surroundings during the compression stroke. The high compression will force some volume of air past the piston rings. This will occur twice during a four stroke cycle as failure to open the exhaust valves will result in two compression strokes before the inlet valve is once again opened to the inlet port.

Should it be too complicated to provide a valve deactivation system capable of separately deactivating the engine's inlet and exhaust valves, deactivating and reactivating all the valves together will still allow the production of heat through compression of the air, albeit potentially less effectively. This is as a result of the exhaust stroke pushing out the remaining heated air that did not pass the piston rings.

To increase in-cylinder temperatures still further, a small amount of fuel may suitably be introduced, for its combustion to produce heat. Optimally, the amount of fuel injected into the cylinder is progressively increased as the engine continues to cycle until the cylinder is at normal operating temperature and the injected amount of fuel is the same as that supplied to the other cylinders that had not previously been deactivated.

In order to operate this way, the exhaust valves must be reactivated along with the intake valves to allow removal of the combustion gases. Because most of the heat in this operating mode comes from combustion rather than compression alone, the loss of efficiency compared with the earlier method of fuel free intake valve only operation is insignificant.

Temperature sensors within the deactivated cylinders may determine the number of cycles of the engine during which the fuelling level to the deactivated cylinders is increased. Similarly the rate of increase of the fuelling may be dependent on the torque demand by way of the accelerator pedal 14, as described above. It is important that the rate should not be too high, as this would equate to a step change in the fuelling. Such a step change does not solve the initial problem of noise, smoke and emissions and would be noticeable to the driver. It is therefore desirable to make the reintroduction of fuel as progressive as possible but this has a downside in that a lack of responsiveness is experienced by the driver.

To overcome this problem, it is preferable to increase the fuelling to the active cylinders in order that they provide more torque to satisfy the torque demand immediately.

Figure 2 explains the sequence of events when the demand pedal is depressed while the engine is operating with deactivated cylinders. In Figure 2, the torque demand is represented by a chain dotted line. Immediately after the step increase in torque demand, the fuelling to the active cylinders increases, as shown by the dotted line. This causes a subsequent rise in torque output as shown by the broken line.

Region 1 of the diagram defines the period during which deactivated cylinders are run with their intake valves operating as normal but without fuelling (either with or without functioning exhaust valves) . In region 2, the fuelling the deactivated cylinders, shown by the solid line, is reintroduced. The extra fuel causes the torque output to increase further. As the amount of fuel injected into the previously deactivated cylinders continues to rise, the previously deactivated cylinders start to yield useful work and contribute to the torque output. At this point the fuelling to the active cylinders (dotted line) reaches a maximum and then begins to fall. Eventually the fuelling level to all cylinders meets at the level required to produce the torque output dictated by the accelerator position.

Crucially as the fuelling rate in the deactivated cylinders is increased from zero to the final constant level across all cylinders, there are no sudden step changes in fuelling yet the torque output instantly reacts to and continues to meet the torque demand from the driver. Having no step changes as the deactivated cylinder gets hotter ensures that no excess smoke is produced from the exhaust pipe of the car, no sudden change in noise can be detected by the driver, rather a slow progression between engine notes that it not discernible, and crucially the emissions outputs remain at a manageable level for the after treatment devices in the exhaust system 20, downstream of the engine.

Change in noise is one of the problems that the present seeks to combat. Research has shown that a small pilot injection of fuel prior to the main injection has a significant impact on the sound of a compression ignition engine for a small cost in terms of fuel economy. With this in mind, on introduction of the initial fuelling when reactivating of the cylinder, it is preferable to provide a small pilot injection prior to the main injection of fuel.

Since the noise of the engine is determined by the amount and timing of the pilot injection, when increasing the amount of fuel provided to a deactivated cylinder, keeping the timing and volume of the pilot injection constant, but increasing the volume of the main injection will enable the cylinder to approach the fuelling of an active cylinder but again without a perceivable change in noise.

Alternatively, the volume and timing of the pilot injection can be moderated as a function of cumulative crank angle, combustion chamber temperature, pedal demand or input data from NVH (noise, vibration and harshness) sensors.

This can be further improved upon by gradually altering the timing and/or volume of the pilot injections of the other cylinders, prior to and after reintroduction of the fuelling to the deactivated cylinders. This will enable the ECU to gradually alter the sound of the entire engine to - 10 mask any detectable changes caused by activating the disabled cylinders.

While it is intended to ensure emissions stay low throughout the running of the engine, it is also possible to intentionally run excessively rich within the cylinders that need to be brought up to operating temperature. Such operation would produce extra undesirable emissions in the short term, but the increased speed of temperature recovery may in some circumstances, result in a reduction of the total emissions released from the deactivated cylinder during its reactivation.

Claims (10)

1. - 11 - Claims 1. A method of controlling an internal combustion engine

   having a plurality of cylinders, for switching between a first mode in which at least one cylinder is deactivated by closing its inlet and exhaust valves, and a second mode in which a previously deactivated cylinder is active, wherein the switching is effected by cycling the engine while operating the inlet valves to admit air into the deactivated cylinder so as to raise the temperature of the cylinder prior to introducing fuel to activate the cylinder.
2. 2. A method as claimed in claim 1, wherein the number of cycles in which air but no fuel is admitted into the cylinder is set in dependence upon input from a demand pedal of the engine.
3. 3. A method as claimed in claim 1 or 2, wherein both inlet and exhaust valves of the cylinder are operated when switching from the first to the second mode prior to the

   reintroduction of fuel.
4. 4. A method as claimed in any preceding claim, wherein additional fuel is supplied to other, active cylinders of the engine during the time that air is admitted into the deactivated cylinder.
5. 5. A method as claimed in claim 4, wherein during reintroduction of fuel to reactivate a cylinder, the quantity of fuel injected is increased progressively until the fuel quantity matches that provided to the other active cylinders.

   - 12 -
6. 6. A method as claimed in claim 4 or 5, wherein during reintroduction of fuel to reactivate a cylinder, fuel injection is effected by multiple injections, the timing and volume of which are controlled.
7. 7. A method as claimed in claim 5 or 6, wherein the injected fuel quantity and timing is set in dependence upon input from a demand pedal of the engine.
8. 8. A method as claimed in claim 5, 6 or 7, wherein the injected fuel quantity and timing is set in dependence upon any of crankshaft angle, combustion chamber temperature and NVH sensors.
9. 9. A method as claimed in claims 5 to 8, when dependent from claim 4, wherein the additional fuel supplied to the other, active cylinders is decreased as the quantity of fuel injected into the previously deactivated cylinder is increased.
10. 10. A method of controlling an internal combustion engine, substantially as herein described with reference to and as illustrated in the accompanying drawings.