STRATIFIED EXHAUST GAS RECIRCULATION ENGINE

Abstract: A method and a system are described for operating a four-stroke reciprocating internal combustion engine, the system having exhaust and intake valve timings such that the exhaust valve is closed at or after 30 DEG after tdc, and the intake valve is opened at or after 30 DEG after tdc and closed at or after 70 DEG after bdc. The system further includes a bi-directional intake port throttle (26) in the intake port (24) for segregating a predetermined volume of air in the intake port immediately upstream of the intake valve (22) such that, when the intake valve opens, the air pressure in the segregated volume is substantially equal to the gas pressure in the cylinder and that, during the remaining intake stroke, air enters the cylinder substantially by bulk expansion of the air in the segregated volume from an initial pressure when the intake valve opens to a lower pressure at the end of the intake stroke.

Description: STRATIFIED EXHAUST GAS RECIRCULATION ENGINE Field of the Invention The present invention relates to a four-stroke reciprocating internal combustion engine which runs during part-load operation with stratified charge and a high proportion of internal exhaust gas recirculation (EGR) . Background of the invention It is known that two-stroke engines can run smoothly during part load conditions with a large stratified volume of exhaust gases in the cylinder and can also operate m a homogeneous charge compression ignition (HCCI) or controlled autoigmtion combustion mode without relying on a spark for ignition. A large stratified volume of exhaust gases is possible because of the inherent ability m the two-stroke engine to vary the scavenging efficiency over a wide range by delaying the start of the exhaust phase to a later part of the engine stroke and shortening the exhaust phase within the stroke. It is however difficult to achieve a similar condition in a four-stroke engine because the scavenging process in this case takes place throughout the entire stroke of the engine. EP-A-0 863 101, describes a method of controlling auto- ignition m a four stroke engine that relies on variable intake and exhaust valve timing. During part load, the time that burnt gases remain m the combustion chamber is increased and this is followed by a fresh intake period to achieve a stratified charge with a hign percentage of EGR. In one embodiment of the described embodiments of tne invention, the exhaust event; is prolonged to terminate during the intake stroke and tne intake event commences just after the terrr_na icn cf the a ^ s event and terminates during the compression stroκe . To avoid back-flow into the intake manifold during the compression stroke, a non-return valve is provided m the intake port but this can be dispensed with if one can arrange for the intake valve itself to close at the end of the induction stroke. Object of the invention The present invention aims to achieve a large stratified volume of exhaust gases m the cylinder of a four-stroke reciprocating internal combustion engine similar to that found m a two-stroke engine. Such an engine can be expected to yield low fuel consumption during part load operation because of reduced pumping work, and to be capable of operating m the HCCI combustion mode producing ultra low NOx emissions. Summary of the invention According to a first aspect of the invention, there is provided a method of operating a four-stroke reciprocating internal combustion engine, the method comprising the steps of drawing only exhaust gases from an engine exhaust port into the engine cylinder during at least the first 30° period of the intake stroke, drawing intake air from an engine intake port into the engine cylinder during the remaining period of the intake stroke, expelling a substantial proportion of the intake air back into the intake port during at least the first 70° period of the compression stroke, and trapping and compressing the remaining cylinder charge during the remaining period of the compression stroke, and further comprising the step of mounting a bi-directional flow obstruction m the intake port to segregate a predetermined volume of the intake port from the remainder of the intake manifold such that, when the intake valve opens, the air pressure m the segregated volume is substantially equal to the gas pressure m the cylinder and such that, during the remaining intake stroke, air enters the cylinder predominantly by bulk expansion of the air in the segregated volume from an initial pressure when the intake valve opens to a lower pressure at the end of the intake stroke. In EP-A-0 863 301, the provision of the non-return valve in the intake port does not achieve the purpose of the bi-directional obstruction of the present invention, which is to regulate the flow so as minimise the mixing between the intake charge and the exhaust gases already present m the combustion chamber. The action of a non-return valve is to compress the gas in the intake port between the nonreturn valve and intake valve and when the intake valve is next open there will be a surge flow to disturb the stratification within the combustion chamber. By contrast, in the present invention, equalisation in the intake manifold takes place while the intake valve is closed thereby eliminating the initial surge of air into the combustion chamber and regulating subsequent intake flow to reduce mixing in the combustion chamber and thereby preserve charge stratification. The terms "intake", "compression", "expansion" and "exhaust" strokes are used herein in their normal sense to refer to the four movements of the piston between its top (tdc) and bottom (bdc) dead centre positions during one complete engine cycle corresponding to 720° of crankshaft rotation. All measurements of angle used herein refer to angles of rotation of the crankshaft (as opposed to angles of rotation of the camshaft which rotates at half the speed) . According to a second aspect of the present invention, there is provided a system for operating a four-stroke reciprocating internal combustion engines according to the method set forth above, wherein the exhaust and intake valve timings are set such that the exhaust valve is closed at or after 30° after tdc, and the intake valve is opened at or after 30° after tdc and closed at or after 70° after bdc , and wherein a bi-directional obstruction is provided m the intake port for segregating a predetermined volume of air m the intake port immediately upstream of the intake valve. Preferably, the segregated volume is less than one half of the swept volume of the engine cylinder. Conveniently, the intake port obstruction is a two- position perforated butterfly throttle plate having a perforation ratio of less than 5% and movable between a closed position m which it lies across the intake port and an open position m which it lies parallel to the direction of air flow through the intake port . By following through the gas exchange process according to the said valve timings, it will be clear that the invention can enable a large volume of exhaust gases to be trapped m the cylinder. This volume may constitute between 30% and 50% of the overall volume of the trapped charge. Preferably, a variable valve timing system is provided to set the said valve timings when desired. Conveniently, the variable valve timing system is a variable camshaft phasing system for varying the phase of a single camshaft operating both the exhaust and intake valves . Alternatively, the variable valve timing system is a variable camshaft phasing system for varying separately the phases of separate camshafts operating separately the exhaust and intake valves respectively. In this case, the variable camshaft phasing system may be a dual -equal phasing system m which the phases of the exhaust and intake camshafts are varied by the same amount and m the same direction . In embodiments of the invention, fuel may be introduced into the intake air to form a homogeneous charge upstream of the intake port, within the intake port or directly into the cylinder. Because the stratified exhaust gases m the cylinder is significantly hotter than the combustible charge, the latter may be ignited by compression ignition at the stratification interface without relying on a spark, thus achieving HCCI or controlled autoignition . Where a spark is used to ignite the charge, the flame propagation time will also be short because of the compactness of the charge bounded by the stratified exhaust gases. Thus ultra low NOx emissions and low fuel consumption are achieved. 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 section through a combustion chamber of an internal combustion engine embodying the invention, Figure 2 is a valve timing diagram, and Figures 3 and 4 are sections similar to the section of Figure 1 at different times in the operating cycle showing the distribution of EGR gases and intake air within the combustion chamber. Detailed description of the preferred embodiments Figure 1 shows a variable volume combustion chamber defined between a cylinder 16 and a piston 14. The piston reciprocates between a top dead centre position (tdc) and a bottom dead centre position (bdc) as illustrated m Figure 1. The combustion chamber has a clearance volume 10, this being the minimum volume of the combustion chamber when the piston is at tdc. An intake port 24 controlled by an intake valve 22 supplies a fuel and air mixture to the combustion chamber while an exhaust port 18 controlled by an exhaust valve 20 allows burnt gases to be discharged to atmosphere at the end of the combustion cycle. The intake port 24 is obstructed by an butterfly throttle 26 the plate of which is perforated to allow some air through even when the butterfly plate is fully closed. The volume of air trapped between the plate 26 and the closed intake valve 22 is herein referred as the segregated volume. When operating under high load conditions, the butterfly plate 26 is moved to a position at right angles to the position illustrated m Figure 1 and presents no obstruction to the air intake. Also during high load conditions, the engine is operated with conventional valve timing. When operating under part load conditions, on the other hand, the butterfly plate 26 is moved to the closed position shown m Figure 1 and the valve timing of the intake and exhaust valves is as shown m Figure 2. In this Figure, the intake valve opening IVO occurs 30° after tdc during the intake stroke and the intake valve closing IVC occurs 90° after bdc m the compression stroke. Of course compression cannot take place while the intake valve is still open and the term compression stroke is being used m its normal sense, as previously mentioned, to indicate the movement of the piston from bdc to tdc. The exhaust valve opening EVO, on the other hand, occurs at about 20° before bdc at the end of the expansion stroke and the exhaust valve closing EVC occurs shortly after IVO. The combined effect of the valve timing shown m Figure 2 and the presence of the butterfly plate 26 m the intake port is to enable a stratified charge with a high proportion of exhaust gases to be achieved. Starting with the piston at tdc at the commencement of the intake stroke, the exhaust valve 20 at this time is open but the intake valve 22 is closed. Therefore as the piston moves down the cylinder, the expansion of the combustion chamber will draw into it exhaust gases from the exhaust port 18. At about 30° after tdc, the intake valve 22 opens and the exhaust valve 20 closes. At the instant of opening of the intake valve 22, both the pressure m the combustion chamber and the pressure m the segregated volume of the intake port 24 will be a little below atmospheric pressure. In view of this pressure balance, no sudden surge of intake air into the combustion chamber nor exhaust gases out of the combustion chamber occurs when the intake valve 22 opens. This is the piston position shown m Figure 1 where the combustion chamber is totally filled with exhaust gases shown by the shading 12 while the fresh intake air remains in the intake port . As the piston 14 continues to move towards bdc m the intake stroke, the butterfly plate 26 allows only a trickle of air to enter the segregated volume of the intake port 24 Nevertheless as the pressure is reduced m the combustion chamber by the downward movement of the piston 14, the gases m the combustion chamber and the segregated volume of the intake port will stretch or expand without major gas movement to occupy the increased space. This will result m some air entering the cylinder to produce the stratified distribution at bdc shown m Figure 3. During the first part of the ensuing compression stroke, the intake valve 22 is still open and the pressure m the cylinder is below atmospheric. Consequently, a net force acts on the piston 14 to pull it upwards into the combustion chamber while the gases contract. During this contraction, some of the air m the cylinder will be retracted into the segregated volume of the intake port 24 When the intake valve 22 is finally about to close, the distribution of gases will be as illustrated m Figure 4, that is to say, there will be two layers, one formed predominantly of exhaust gases and the other of intake air, and a high proportion of the trapped charge will comprise exhaust gases. This distribution of stratification is maintained during the next part of the compression stroke because there is no net gas movement tending to promote mixing, be it swirl or tumble. The large area interface between the layers of the stratified charge allows the intake gases to be heated by the exhaust gases creating the ideal conditions for autoignition. The function of the intake port throttle in the described preferred embodiment is two-fold. First, during the period between intake strokes of successive engine cycles, the segregated volume downstream of the port throttle will be refilled with ambient air flowing past the throttle so that the air pressure m the volume will have reached substantially ambient pressure before the intake valve next opens . Thus when the intake valve opens m the next cycle, the air pressure m the intake port will be substantially equal to the gas pressure m the cylinder and the exhaust gases already m the cylinder will remain m the cylinder . Second, as the piston moves progressively away from tdc towards bdc, the air in the segregated volume will enter the cylinder substantially by bulk expansion as the whole charge m the connected volume of the cylinder and the segregated intake port, being almost completely blocked at one end, is stretched uniformly to a decreasing pressure and reaches tne lowest density at bdc. In this case, the air velocity entering the cylinder will be relatively low and will not significantly disturb the correspondingly stretched volume of exhaust gases already m the cylinder thus preserving the EGR stratification m the cylinder. Furthermore, by designing the throttle as a perforated plate, the air drawn past the plate will be broken up into micro-jets, dissipating rapidly to a uniform low velocity with little turbulence, thus further reducing the penetration and mixing effect of the intake air entering the cylinder. During the first part of the compression stroke when the intake valve is still open, the initial low pressure m the cylinder will cause the piston to move away from bdc towards tdc, and some air will contract into the intake port as the whole charge in the connected volume of the cylinder and the segregated intake port shrinks uniformly to a higher sub-ambient pressure until the intake valve closes and compression begins. Because the exhaust gases are stratified, the content of the charge contracted into the intake port will be mainly intake air with most of the exhaust gases still retained m the cylinder, thus increasing the relative exhaust gas proportion m the cylinder. On the other hand, if mixing occurs to some degree, a quantity of exhaust gases will be carried with the contracted air and stored in the segregated volume. This quantity will be drawn back into the cylinder m the next engine cycle, adding to the exhaust gases already m the cylinder so that the absolute exhaust gas proportion in the cylinder will also increase from cycle to cycle, until it reaches a stable value after many cycles . To enable the valve timing to be varied m dependence upon whether the engine is operating under part load or hign load, a variable valve timing system is used to set the valve timings as desired. It should be noticed that the relative timing and the valve event durations used during part load operation can be the same as those used under high load and it is only necessary to phase shift both the intake valve and exhaust valve events by equal amounts relative to the engine crankshaft. Hence the variable valve timing system may in the simplest case comprise a single phase shifter acting on a single camshaft acting on both the intake and exhaust valves . Alternatively, the variable valve timing system may be a variable camshaft phasing system for varying separately the phases of separate camshafts operating on the exhaust and intake valves respectively. In this case, the variable camshaft phasing system may be a dual -equal phasing system in which the phases of the exhaust and intake camshafts are varied by the same amount and in the same direction. Fuel may be introduced into the intake air to form a homogeneous charge upstream of the intake port, within the intake port or directly into the cylinder. Because the stratified exhaust gases in the cylinder is significantly hotter than the combustible charge, the latter may be ignited by compression ignition at the stratification interface without relying on a spark, thus achieving HCCI or controlled autoignition. Where a spark is used to ignite the charge, the flame propagation time will also be short because of the compactness of the charge bounded by the stratified exhaust gases. Thus ultra low NOx emissions and low fuel consumption are achieved. It should be mentioned that while intended primarily for a naturally-aspirated engine, turbo-charging and supercharging would not interfere with the charge stratification, it being only necessary to ensure that the pressures on both sides of the intake valve are balanced at the time that the intake valve opens .

Claims: CLAIMS 1. A method of operating a four-stroke reciprocating internal combustion engine, the method comprising the steps of drawing only exhaust gases from an engine exhaust port into the engine cylinder during at least the first 30° period of the intake stroke, drawing intake air from an engine intake port into the engine cylinder during the remaining period of the intake stroke, expelling a substantial proportion of the intake air back into the intake port during at least the first 70° period of the compression stroke, and trapping and compressing the remaining cylinder charge during the remaining period of the compression stroke, and further comprising the step of mounting a bi-directional flow obstruction in the intake port to segregate a predetermined volume of the intake port from the remainder of the intake manifold such that, when the intake valve opens, the air pressure in the segregated volume is substantially equal to the gas pressure in the cylinder and such that, during the remaining intake stroke, air enters the cylinder predominantly by bulk expansion of the air in the segregated volume from an initial pressure when the intake valve opens to a lower pressure at the end of the intake stroke. 2. A system for operating a four-stroke reciprocating internal combustion engines according to the method set forth in Claim 1, wherein the exhaust and intake valve timings are set such that the exhaust valve (20) is closed at or after 30° after tdc, and the intake valve (22) is opened at or after 30° after tdc and closed at or after 70° after bdc, and wherein a bi-directional obstruction (26) is provided in the intake port for segregating a predetermined volume of air in the intake port (24) immediately upstream of the intake valve (22) . 3. A system as claimed in claim 2, wherein the port obstruction (26) comprises an intake port throttle. 4. A system as claimed m claim 3, wherein the intake port throttle (26) is a two-position butterfly throttle having a perforated plate movable between a closed position m which it lies across the intake port and an open position in which it l es parallel to the direction of air flow through the intake port . 5. A system as claimed m claim 4, wherein the perforated plate has a perforation ratio of less than 5%. 6. A system as claimed in any of claims 2 to 5 , wherein the segregated volume is less than one half of the swept volume of the engine cylinder. 7. A system as claimed m any of claims 2 to 6, wherein a variable valve timing system is provide to set the said valve timings when desired. 8. A system as claimed in claim 7, wherein the variable valve timing system is a variable camshaft phasing system for varying the phase of a single camshaft operating both the exhaust and intake valves. 9. A system as claimed m claim 7, wherein the variable valve timing system is a variable camshaft phasing system for varying separately the phases of separate camshafts operating separately the exhaust and intake valves respectively. 10. A system as claimed m claim 10, wherein the variable camshaft phasing system is a dual-equal phasing system m which the phases of the exhaust and intake camshafts are varied by the same amount and m the same direction .

Inventors: MA THOMAS TSOI-HEI, MA, THOMAS, TSOI-HEI
Applicants: FORD GLOBAL TECH INC, MA THOMAS TSOI HEI, FORD GLOBAL TECHNOLOGIES, INC., MA, THOMAS, TSOI-HEI
Application year: 2000
Main classification: F02B17
IPC codes: F02B1, F02B17, F02B47, F02B75, F02D9, F02D13, F02M25