WO2010046826A1

WO2010046826A1 - Method of starting an internal combustion engine

Info

Publication number: WO2010046826A1
Application number: PCT/IB2009/054575
Authority: WO
Inventors: Thomas Tsoi-Hei Ma; Hua Zhao
Original assignee: Brunel University
Priority date: 2008-10-23
Filing date: 2009-10-16
Publication date: 2010-04-29
Also published as: GB201106402D0; GB2476435A; GB2476435B

Abstract

A method is disclosed for starting an internal combustion engine having a compressed air supply. The engine is provided with cam actuated poppet valves adapted to operate selectively with a first valve timing schedule that causes the engine to act as a compressed air driven motor and with a second schedule that causes the engine to perform as an internal combustion engine. In the invention, prior to each engine shut down, the engine is operated with the first valve timing schedule for a short period of time with the fuel supply discontinued, and during subsequent restarting of the engine, the engine is operated with the first valve timing schedule and connected to the compressed air supply to crank the engine. On reaching a desired cranking speed, the compressed air supply is disconnected switching the valve timing is switched to the second schedule and normal engine aspiration and fuelling are resumed.

Description

METHOD OF STARTING AN INTERNAL COMBUSTION ENGINE

Field of the invention

The present invention relates to starting of an internal combustion engine connected to a compressed air supply .

Background of the invention

It is desirable, in the interest of fuel economy and the reduction of carbon dioxide and noxious emissions, to switch off an engine when a vehicle that it drives is at a standstill. However, restarting the engine requires power and one must achieve a balance between the energy saving achieved by switching off the engine and the energy cost of restarting it.

Storing braking energy in a battery and using an electric motor to restart the engine provides an adequate solution for smaller vehicles. However, in a public service vehicle, such as a bus with very frequent stops and a large engine, the electrical solution is not an attractive one because of the size of battery and motor/generator that would be required.

It has already been proposed by the Applicants in EP 1747351 to recover the kinetic energy of a vehicle during braking by operating the four stroke engine as an air compressor and to store the compressed air thus generated. This provides a convenient supply of compressed air and the present invention seeks to provide an effective method of restarting an internal combustion engine using energy stored in the form of compressed air. Summary of the invention

In accordance with the present invention, there is provided a method of starting an internal combustion engine having a compressed air supply, the engine being provided with cam actuated intake and exhaust valves adapted to operate selectively with a first valve timing schedule that causes the engine to act as a compressed air driven motor and with a second schedule that causes the engine to perform as an internal combustion engine, the method comprising the steps of, prior to each engine shut down, operating the engine with the first valve timing schedule for a short period of time with the fuel supply discontinued, and, during subsequent restarting of the engine, operating the engine with the first valve timing schedule and connecting the engine to the compressed air supply to crank the engine, and, on reaching a desired cranking speed, disconnecting the compressed air supply and switching the valve timing to the second schedule and resuming normal engine aspiration and fuelling.

In the same way as an engine can be made to act as a compressor by changing its valve timing, it can also be made to operate as a compressed air driven motor by changing the valve timing. Thus in principle one could operate an engine as an air driven motor for cranking purposes and switch to normal operation as a combustion engine once it has reached a sufficient speed.

However, in an engine with cam actuated valves, the switching between valve timing schedules requires power in the form of hydraulic pressure that is not available when the engine has been stopped. Furthermore, the engine when stopped may have cylinders in which both the intake and the exhaust valves are open simultaneously (as a result of normal valve overlap) and connecting a compressed air supply to a cylinder under such conditions would result in an uncontrolled loss of compressed air without any power being derived from it.

The present invention is thus predicated on the realisation that steps need to be taken to precondition the engine for a restart at the end of its previous run. The preconditioning is to ensure that the valve timing is correctly set for compressed air motor operation with negligible valve overlap and with disconnection of the fuel supply.

For compressed air driven motor mode, an engine must have some strokes in which compressed air is admitted into a combustion chamber and allowed to expand and other strokes in which air that has already expanded and released its energy to be exhausted to the ambient atmosphere.

In the case of a two stroke engine, this can be achieved using a dedicated port at the top of the cylinder controlled by a poppet valve and leading to a compressed air supply. Torque is applied to the engine crankshaft during each and every down stroke of the piston by opening the poppet valve and the air is discharged either through the scavenging port at the bottom of the cylinder or an exhaust port at the top of the cylinder controlled by a second poppet valve.

In the case of a four stroke engine, different strategies may be adopted to enable it to perform as an air motor. The four strokes of the piston will herein be termed the intake, compression, power/expansion and exhaust strokes, as in a conventional combustion cycle, even though during air motor operation these terms are not appropriate.

Simply connecting the intake port to the compressed air supply for the entire duration of the intake stroke would apply high torque to crank the engine. However, the resistance that the piston would then encounter during the compression stroke (in attempting to increase the pressure of the already compressed air by the compression ratio) could bring the crankshaft to a standstill. This can be avoided in several ways. One would be to open the exhaust valve during the compression stroke and maintain it open for the remainder of the four stroke cycle.

Another way is to maintain the intake valve open during the compression stroke so as to return the compressed air back to the air tank. In the latter case, the contribution from the first two strokes would be self-cancelling but the following expansion and exhaust strokes can then perform as an air motor. Compressed air remaining in or introduced into the combustion chamber at the commencement of the expansion stroke will expand to crank the engine and be discharged during the normal exhaust stroke.

It will be noted from the above examples that regardless of the type of engine or the valve timing schedules adopted to switch between internal combustion mode and air motor mode, it is essential to ensure that the appropriate cam profiles are engaged prior to connecting the compressed air supply to restart the engine. Because of the absence of any hydraulic power to effect the cam profile switching when the engine is already stopped, the present invention teaches effecting the necessary cam profile switching before the engine is switched off. Though the valve timing is changed during engine shut down to the timing required for air motor operation, the engine will not motor because at this time the compressed air supply is not connected to it. However, after the engine has stopped, it can immediately be cranked for restarting by connecting the compressed air supply because the valve timing has been correctly preset. 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 valve timing diagram showing typical intake and exhaust valve events of an engine operating as a four stroke internal combustion engine, this being referred to herein as a second valve timing schedule,

Figures 2 to 7 show different valve timing diagrams for intake and exhaust valve events when the same four stroke engine is operating as a compressed air driven motor, each of these being referred to herein as a first valve timing schedule,

Figure 8 is a valve timing diagram showing typical intake and exhaust valve events of an engine operating as a two stroke internal combustion engine, this being referred to herein as a second valve timing schedule, and Figures 9 and 10 show different valve timing diagrams for intake and exhaust events when the two stroke engine of Figure 8 is operating as a compressed air driven motor, this being referred to herein as a first valve timing schedule.

Detailed description of the preferred embodiment (s)

In all the diagrams, the intake events are shown in solid lines and the exhaust events are shown in dotted lines so that they may readily be distinguished from one another. In the usual manner, the valve opening is plotted on the Y- axis and the crankshaft angle is plotted on the X-axis. Instead of crankshaft degrees, the legends along the X-axis indicate the positions of the piston when at top and bottom dead centres.

Figure 1 is a conventional timing diagram for a four stroke engine. The intake valve is opened shortly before top dead centre (TDC) at the end of an exhaust stroke and remains open throughout the intake stroke. Both the intake and the exhaust valves then remain closed during the following compression and power/expansion strokes, ignition taking place near TDC at the end of the compression stroke. Finally, the exhaust valve is opened during the exhaust stroke to discharge the combustion gases.

The invention applies equally to spark ignition and compression ignition engines. In one case, the fuel is introduced during the intake stroke and spark ignited at the end of the compression stroke whereas in the other the air is compressed and fuel is injected near TDC at the end of the compression stroke and is spontaneously ignited by the gases that have been heated by compression. In both cases, the fuel supply needs to be discontinued when the engine is operated as a compressed air driven motor but it is not essential in a spark ignition engine to discontinue the spark.

To allow for efficient breathing, each of the intake and exhaust valve events lasts longer than 180° of crank angle and is typically nearer to 250° in duration. Furthermore, it will be noted that there is an overlap period at the end of the exhaust stroke when both the intake and exhaust valves are opened at the same time.

When switching the engine to operation as a compressed air driven motor, it is possible as shown in Figures 2 to 4 for the intake valve event to remain generally unaltered but during this time the intake manifold is connected to a compressed air supply rather than the ambient atmosphere. A throttle valve in the intake manifold, which may be the intake butterfly valve in the case of a spark ignition engine, is closed and the compressed air is introduced downstream of the throttle valve. In the case of a diesel engine, a dedicated valve needs to be provided for this purpose. The latter valve may be a non-return valve which will automatically prevent reverse flow of the compressed air in the manifold. When the intake valve is opened at TDC in the usual manner, compressed air will be admitted into the cylinder forcing the piston down to create a motor power stroke that cranks the engine. For the engine to act as an effective compressed air driven motor, it now remains only to discharge the compressed air from the cylinder while absorbing a minimum amount of energy from the flywheel.

If one were to retain the exhaust valve timing shown in Figure 1 when operating in the air motor mode, this objective would not be met as the resistance offered by the compression stroke which follows the intake stroke would bring the crankshaft to a halt, especially since the air filling the cylinder at the bottom dead centre (BDC) position of the piston is already under high pressure. A first requirement therefore is to provide an exhaust stroke immediately following the intake stroke.

Simply phase shifting the exhaust valve event in Figure 1 to advance the opening time of the exhaust valve by nearly 360° is difficult to achieve. Furthermore, one has to be careful during the phase shifting to avoid the exhaust valve ever being fully open while the piston is near TDC because in some engines there would be no clearance between the head of the valve and the piston crown. The preferred embodiment of the invention therefore relies on a cam profile switching system that does not involve a gradual sweep but provides an instant changeover between timing schedules. Cam profile switching (CPS) systems are generally well known in the art and need not therefore be described in detail in the present context. It suffices to describe just the valve timing schedules that the different cam profiles will produce on switching. Figure 2 shows a valve timing schedule suitable to an engine having a single exhaust valve or two exhaust valves operated in synchronism. The exhaust valve event shown in Figure 2 would therefore apply to both exhaust valves if more than one valve is present.

The profile of the switched cam in Figure 2 provides an event with a lower valve lift to ensure clearance at TDC between the valve head and the piston crown, and a duration that lasts over substantially all the time that the intake valve is closed. In other words, at all times other than the air motor power stroke, air can enter and leave the cylinder freely through the exhaust valve while offering minimal resistance to the crankshaft. No exhaust valve is ever open at the same time as the intake valve to make sure that compressed air in the intake manifold cannot escape directly to atmosphere.

Figure 3 is applicable to an engine with two exhaust valves per cylinder in which a CPS system is only used on one of the exhaust valves to produce the exhaust event labelled I in Figure 3. The other exhaust valve continues to operate as normal apart from an additional phase shifting that is required to avoid valve overlap. For that purpose, the whole of the exhaust valve event may be advanced or else the event of the intake valve may be retarded.

In some CPS systems, such as the switchable bucket tappet referred to above, the cam switching is used to modify an existing valve event only by adding (i.e. not subtracting) to its duration or lift. If such a system is used, the resulting exhaust valve event is shown in Figure 4, which may once again be the event of a single exhaust valve or that of two exhaust valves operated synchronously. Because the entire exhaust event as used during internal combustion operation is retained, it is once again necessary to fine tune the timing by phase shifting to avoid valve overlap.

CPS systems are normally used in engines only to switch between profiles to optimise valve timing while the engine is running. For example, some systems have a first set of cams optimised for low speed operation and another set for high speed operation. CPS systems have also been proposed to switch between normal internal combustion operation and an air compressor mode in which the engine acts as a brake and produces compressed air that may be stored in a reservoir for later use. In these cases, the CPS system can always rely on a hydraulic supply for activating the switching that is available while the engine is running.

Furthermore, the CPS system can only perform the switching sequence when the engine is running because the profile switching cannot occur in all positions of the valve train. A latching pin may be urged in a direction to switch profiles but that switch will not occur until the valve train has moved to an unloaded position and the pin is free to move and engage a different cam.

Because of the above limitations of CPS systems, one cannot simply choose to operate the engine in compressed air driven motor mode at the time that it is to be restarted. Not only is there no hydraulic power to effect a switching but there is no cranking power to turn the engine to a position where the CPS system can instigate a cam profile change. As explained previously, with the valve timing schedule set as shown in Figure 1, supplying compressed air to the cylinder will at best crank the engine through half a turn before it is brought to a stop.

To avoid this problem, the present invention proposes preconditioning the engine for compressed air cranking at the time that the engine is being shut down. For this purpose, the cam profile switching is not carried out during restarting of the engine but earlier during shut down. When the fuelling, and optionally also the ignition system, is cut off to initiate engine shut down, the CPS system is operated to switch cam profiles from the second valve timing schedule to the first valve timing schedule but the compressed air supply is not connected to the engine. This change in schedule will only require two rotations of the engine crankshaft for all the cylinders and as the flywheel will ensure that the engine will rotate for many more revolutions before the engine comes to a stop, the changeover will always have been completed before the engine stops .

On restarting the engine, the compressed air is switched on to crank the engine in the manner described above and on reaching a sufficient speed, there will sufficient time and hydraulic power to switch the engine back to internal combustion engine mode so that the engine will start when fuelling and ignition are resumed.

Figures 5 to 7 show alternative first valve timing schedules in which instead of retaining the same intake event as in four stroke combustion mode and changing the exhaust event, the exhaust event is retained and the intake event is changed. The strategies of Figures 5 to 7 are generally analogous to those of Figures 2 to 4.

In Figure 5, an intake event commences shortly after TDC at the commencement of the intake stroke after completion of the exhaust event of the previous cycle. However, the intake valve in this case remains open throughout the time that the exhaust valve is closed. The energy transferred to the crankshaft during the intake stroke is returned by the engine flywheel to the compressed air supply during the compression stroke and no net work is done by the compressed air until the crankshaft passes TDC at the start of the power/expansion stroke. The resulting motor power stroke commences at this instant and continues until the commencement of the exhaust event.

From the analogy with Figure 3, it will be understood how Figure 6 shows the first timing schedule using the same strategy as in Figure 5 implemented in an engine with two intake valves. Similarly, Figure 7 shows the timing schedule if a CPS system is used that only allows addition to existing events.

The invention is also applicable to a two-stroke engine having a dedicated compressed air poppet valve at the top of the cylinder controlling connection to a compressed air supply.

Figure 8 shows the valve timing for a uniflow two stroke engine when operating in a two stroke combustion mode. The engine is assumed to have a poppet exhaust valve at the top of the cylinder and an intake port at the bottom of the cylinder that is covered and uncovered by the piston skirt .

As the piston approaches BDC in each engine cycle, the intake port is uncovered as represented by the solid line in Figure 8 and the exhaust port at the top of the cylinder is opened. Air is blown in through the intake port and help push the exhaust gases out of the exhaust port, this being referred to as scavenging. Both the intake and exhaust ports are closed shortly after BDC and the remainder of the upstroke compresses the trapped charge. Near TDC, combustion is initiated either by a spark or by diesel fuel injection and the power stroke then continues until the exhaust valve is opened and a fresh charge is admitted into the cylinder.

When operating in an air motor mode, as shown in Figure 9, the intake event controlled by the piston skirt remains unaltered. The compressed air poppet valve is open during the down stroke after TDC to crank the engine and closed before the exhaust valve opens. The spent air is then allowed to escape through the exhaust port to the ambient atmosphere.

It is possible, as shown in Figure 10 to disable the exhaust valve completely when operating in the air motor mode and to allow the spent air to escape through the intake port at the bottom of the cylinder.

The timing schedules for two stroke engine can be implemented either by switching to a zero profile cam or by cam deactivation. The compressed air poppet valve is deactivated in the second timing schedule (combustion mode) and activate with a predetermined profile in the first timing schedule (air motor mode) . Similarly, the exhaust valve may either be activated if spent air is to be allowed to escape to the ambient atmosphere through the exhaust system or deactivated if the spent air is to be discharged through intake port.

The stop/start procedure described above is intended primarily for the frequent stops that occur within a journey and not for the first start of the day which will ordinarily be carried out using a starter motor. An engine stop can be instigated by a control system when, for example, it determines that the engine is hot, is in neutral and has been idling for more than a predetermined length of time. An engine restart by compressed air can be instigated by depression of the accelerator pedal. Such stops and starts may be accompanied by changes in the valve timing schedules as proposed by the present invention.

Engine stops instigated by the vehicle operator turning the ignition key should not involve a change in the valve timing schedule as the engine will need to be in firing mode when an attempt is made to start it from cold using the starter motor.

Claims

1. A method of starting an internal combustion engine having a compressed air supply, the engine being provided with cam actuated poppet valves adapted to operate selectively with a first valve timing schedule that causes the engine to act as a compressed air driven motor and with a second schedule that causes the engine to perform as an internal combustion engine, the method comprising the steps of:

(a) prior to each engine shut down, operating the engine with the first valve timing schedule for a short period of time with the fuel supply discontinued, and

(b) during subsequent restarting of the engine, operating the engine with the first valve timing schedule and connecting the engine to the compressed air supply to crank the engine, and, on reaching a desired cranking speed, disconnecting the compressed air supply and switching the valve timing to the second schedule and resuming normal engine aspiration and fuelling.

2. A method as claimed in claim 1, wherein the engine is a four stroke engine and has at least one exhaust valve controlling an exhaust port through which gases are discharged into the ambient atmosphere, wherein the intake event is substantially the same in both the first and the second schedule and in the first schedule the exhaust valve is opened immediately after closure of the intake valve and closed before reopening of the intake valve.

3. A method as claimed in claim 2, wherein the engine has two exhaust valves which, in the first timing schedule, are opened immediately after closure of the intake valve and closed before reopening of the intake valve.

4. A method as claimed in claim 2, wherein the engine has two exhaust valves and wherein, in the first timing schedule, one of the exhaust valves is operated with substantially the same timing as in the second valve timing schedule and the other exhaust valve is operated in accordance with the first valve timing schedule.

5. A method as claimed in claim 4, wherein the closing timing of the first exhaust valve is phase shifted to avoid overlap with the event of the intake valve.

6. A method as claimed in claim 4, wherein the opening timing of the intake valve is phase shifted to avoid overlap with the closing timing of the first exhaust valve.

7. A method as claimed in claim 1, wherein the engine is a four stroke engine and has at least one intake valve controlling an intake port through which gases are admitted into the engine cylinder, wherein the exhaust event is substantially the same in both the first and the second schedule and in the first schedule the intake valve is opened immediately after closure of the exhaust valve and closed before reopening of the exhaust valve.

8. A method as claimed in claim 7, wherein the engine has two intake valves which, in the first timing schedule, are opened at the commencement of the intake stroke and closed before or after top dead centre at the end of the compression stroke.

9. A method as claimed in claim 7, wherein the engine has two intake valves and wherein, in the first timing schedule, one of the intake valves is operated with substantially the same timing as in the second valve timing schedule and the other intake valve is operated in accordance with the first valve timing schedule.

10. A method as claimed in claim 9, wherein the opening timing of the first intake valve is phase shifted to avoid overlap with the event of the exhaust valve.

11. A method as claimed in claim 9, wherein the opening timing of the exhaust valve is phase shifted to avoid overlap with the event of the first intake valve.

12. A method as claimed in claim 1, wherein the engine is a two stroke engine having a dedicated compressed air port connected to a compressed air tank and a compressed air poppet valve controlling the compressed air port, wherein in the first timing schedule the compressed air poppet valve is opened only during parts of the piston down stroke during which the combustion chamber is isolated from the intake and exhaust ports and in the second timing schedule, the compressed air control valve is deactivated.

13. A method as claimed in claim 12, wherein the engine has an exhaust port at the top of the cylinder controlled by an exhaust poppet valve, and wherein in the first timing schedule the exhaust poppet valve is deactivated.