FR2877048A1 - Internal combustion engine e.g. direct injection diesel engine, control method for vehicle, involves controlling closing of exhaust valve before piston reaches top dead center, and opening inlet valve after reaching center - Google Patents

Abstract

The method involves controlling a closing of an exhaust valve such that the closing is completed before that a piston associated to the valve reaches a top dead center. An inlet valve associated to the piston is started to open after reaching the top dead center. The closing and opening are controlled only when a motor load is less than a predetermined load. An independent claim is also included for an engine of a vehicle.

Description

The invention relates to a method for controlling vehicle engines.

  In internal combustion engines of vehicles, it is conventional to implement a recirculation of the exhaust gas or EGR. This is the case, for example, in compression-ignition diesel engines for which, at certain operating points, recirculation of the unburned gases is carried out. The use of EGR gases in larger or smaller quantities in diesel engines significantly reduces the nitrogen oxides, or NOx, emitted into the atmosphere.

  The temperature of the recirculated exhaust gas also has a strong influence on pollutant emissions, especially on unburnt low engine loads. In particular, the aim is to reduce unburnt emissions at low loads. As a result, diesel engines are increasingly using an EGR gas cooling system to limit NOx emissions. However, this cooling has the effect of increasing the emissions of unburnt at low engine loads when the engine is cold and the oxidation catalyst is not primed.

  In order to limit the emissions of unburnt at points of low load or when the engine is cold, it is possible to provide a bypass of the heat exchanger recirculated exhaust gas. This allows for warmer EGR gas, which is favorable for reducing unburnt emissions. This solution compatible with the Euro 4 standard is however limited. The future Euro 5 (or Sulev) standard will be much more severe in terms of smoke, NOx and unburnt emissions. In order to achieve NOx emissions targets, it is planned to introduce a much larger amount of EGR gas into the engines, which will significantly increase unburnt emissions. Under these conditions, the diversion of the EGR gas cooler will no longer be sufficient to reach unburnt emissions at engine output compatible with the future Euro 5 standard.

  An object of the invention is to further improve the performance of engines with respect to anti-pollution standards.

  For this purpose, a method of controlling a vehicle engine is provided according to the invention in which: - a closure of at least one exhaust valve is commanded so that the closure is completed before an associated piston the valve has reached a top dead center; and - an opening of at least one intake valve associated with the piston is started after reaching top dead center.

  The invention applies to all types of engine and not only to diesel engines.

  The method according to the invention may furthermore have at least one of the following characteristics: said closure and opening is controlled only when the load of the motor is less than a predetermined value; - An opening of the exhaust valve is started at a time of the cycle of the prior piston at a time when it starts, if not, its opening for the closure is completed when the piston reaches the top dead center; the inlet valve is closed at a time of the piston cycle at which its closure is completed if it has begun to open when the piston reaches top dead center; the closing of the exhaust valve is begun at a point on a trajectory of the valve located below a point at which it starts, otherwise it is closed so that the closure is completed when the piston reaches the point dead high; and - a closing of the intake valve at a point of a trajectory of the valve located below a point at which it starts, otherwise closing when it has begun opening when the piston reaches the top dead center.

  Also provided according to the invention a vehicle engine comprising: at least one piston; and - exhaust and intake valves associated with the piston, the engine comprising control means arranged to: - control a closing of the exhaust valve so that the closure is completed before the piston has reached a point dead high; and - begin an opening of the intake valve after reaching the top dead center by the piston.

  The engine according to the invention may also have at least one of any of the following characteristics: it comprises an exhaust gas recirculation circuit; and - it is a diesel engine with direct injection and compression ignition.

  Other features and advantages of the invention will become apparent in the following description of several embodiments of the invention given by way of non-limiting example with reference to the accompanying drawings in which: - Figure 1 is a schematic view of an engine according to a preferred embodiment of the invention; FIG. 2 shows two curves illustrating valve lift laws in two motors, the second of which constitutes a preferred embodiment of the invention; - Figure 3 is a view similar to Figure 2 corresponding to two other engines, the second corresponds to another preferred embodiment of the invention; FIGS. 4A and 4B are diagrams illustrating the evolution of the pressure in the cylinders of the motors of FIG. 2 as a function of the volume, the diagram of FIG. 4B being a view on a larger scale of a part of the FIG. Figure 4A; FIG. 5 is a diagram showing the performance of the engines corresponding to FIGS. 2 and 3 with regard to hydrocarbon emission and fuel consumption; and FIG. 6 is a diagram similar to FIG. 5 showing the performance in terms of hydrocarbon emission and exhaust gas temperature.

  FIG. 1 schematically illustrates a motor 2 according to a preferred embodiment of the invention. This engine comprises an air filter 4 communicating with a compressor 6 of a turbocharger 8. A line 10 indirectly communicates the compressor with an intake distributor 12 controlling the admission of gas into cylinders formed in a cylinder head 14 of the engine. In each cylinder is a piston not shown. In addition, each cylinder is associated with at least one intake valve and at least one exhaust valve, and preferably two of each. The movement of the exhaust valves is controlled by an exhaust manifold 16. The engine comprises an external exhaust gas recirculation circuit 18 taking a fraction of the exhaust gas at the outlet of the cylinder head for reinjecting them into the circuit The amount of exhaust gas recirculated by this circuit 18 can be controlled by means of a valve 20 in a manner known per se. The circuit 18 includes in particular a cooler and a bypass of the latter which have not been illustrated. The fraction of the non-recirculated exhaust gas rotates a turbine 22 of the turbocharger 8 and is conveyed to an exhaust device 24 comprising in particular an oxidation catalyst 26.

  At the operating points corresponding to the medium and high loads of the engine, it is the external EGR circuit 18 which feeds the intake circuit with highly cooled burnt gases. Indeed, at these operating points, it is mainly the NOx emissions that must be reduced. The unburnt emissions are relatively low and the oxidation catalyst is already primed.

  In the present case, it is sought to increase the amount of recirculated exhaust gas without excessively increasing the fuel consumption at the operating points of the engine corresponding to the low loads or those at which the engine is cold. For this purpose, an internal recirculation of the exhaust gases is carried out without passing through the circuit 18 and thanks to a suitable control of the intake and exhaust valves by means of the distributors 12 and 16.

  Thus, to increase the amount of internal EGR gas, the valve lift laws are modified through the distributors 12 and 16.

  In order to increase the amount of internal EGR gas, the exhaust valves are prematurely closed in order to trap a portion of the flue gases in each of the cylinders. This premature closure of each exhaust valve of each cylinder has the effect of causing a compression of the burnt gases in the cylinder at the end of the ascent of the piston. This negative work has the effect of increasing fuel consumption. In order to benefit from the positive work of expansion of the burnt gases present in the cylinder, the opening of the or each intake valve of the cylinder is furthermore delayed. This recovery of the positive work of relaxation has the effect of reducing fuel consumption.

  FIGS. 4A and 4B illustrate the evolution of the pressure in the cylinder as a function of the volume of the chamber. It can be seen from this PV diagram that the later opening of the intake makes it possible to reduce the area of the negative loop corresponding to the negative work. This reduction in negative work is reflected directly in overconsumption of more than 10% of fuel. This premature closure of the exhaust makes it possible to reduce the hydrocarbon emissions by about 50% in the present example as will be seen below.

  Different technical solutions are possible to combine the premature closure of the exhaust and the delayed opening of the intake. Two of these solutions are presented with reference to FIGS. 2 and 3.

  In a motor configuration 1, as illustrated in FIG. 2, a premature closure of the exhaust valve is obtained by introducing a simple phase shift corresponding to an advance in the control law of the valve. Thus, the opening of the exhaust valve according to the curve (2) in dashed lines at an instant of the cycle of the prior piston is begun at an instant at which it starts otherwise its opening for the closure to be completed when the piston reaches the top dead center as indicated by the curve (1) in solid lines. Therefore, the exhaust valve is closed so that the closure is completed before the piston has reached top dead center. In this example, only the exhaust valve has a phase advance with respect to the other operating circumstances of the engine. In particular, the valve lift law regarding admission is unchanged.

  In configuration 2, the same phase shift is maintained in advance for the control law of the exhaust valves and a lag phase shift is introduced for the control law of the intake valves. In other words, the opening of each intake valve is started after reaching the top dead center, that is to say at the end of a predetermined period. It follows that the closing of the intake valve at an instant of the cycle of the rear piston begins at a time when it starts or else its closure when it began its opening when the piston reaches the top dead center. The engine according to this configuration 2 constitutes a preferred embodiment of the invention.

  With reference to the diagram of FIG. 3, the exhaust advance and the admission delay are obtained this time either by a simple phase shift in the valve lift laws but by a simple spreading in these laws. This is in fact to reduce the trajectory of the valve compared to normal circumstances, without changing its speed.

  Thus, in the configuration 3, the opening of the exhaust valve (curve 5) begins at the same time as in the other circumstances (curve 4). In addition, it starts closing the exhaust valve at a point on its path (curve 5) located below a point at which one would start otherwise its closure (curve 4) so that the closure ends when the piston reaches the top dead center. It follows that this closure is obtained here again before the piston has reached the top dead center. In this configuration, only the law of the exhaust valve has been modified in relation to normal circumstances, the law of emergence of the intake valve being unchanged.

  Finally, in the configuration 4, the same exhaust spread is carried out as in the configuration 3 and a similar spreading is added for the intake valve. Specifically, it begins opening the inlet valve (curve 6) after a predetermined period after the piston has reached the top dead center. The closing of the intake valve is begun at a point on its trajectory located below the point where it would start if not closed (curve 4) when it has begun to open when the piston reaches the top dead center. As a result, the trajectory of the valve is reduced. In addition, the closing is completed at the same time as if it ended under normal circumstances, namely if the opening had been controlled while the piston was at the top dead center.

  Configuration 4 is also a preferred embodiment of the invention.

  In configurations 1 to 4, the advance of the exhaust closure will for example be equal to a value between 30 and 40 crankshaft angle. In configurations 2 and 4, the delay in opening the intake will for example be between 20 and 30.

  In configurations 1 and 2, the velocities and amplitudes of the valve movements are unchanged compared to the cases of medium or high load operation.

  In configurations 3 and 4, the valve speeds are unchanged from normal circumstances. s

  Distributors 12 and 16 for implementing these control laws can be easily made from distributors of the prior art.

  In the following, for the configurations 1 to 4 and for a conventional motor, test results are presented at the operating point corresponding to 1500 rpm and 105 Pa of PME.

  Figure 5 illustrates the influence of the variable distribution on hydrocarbon emissions (HCI) and on fuel consumption (CSI) in these operating circumstances. The quantities of oohydrocarbons emitted correspond to the columns and must be read with reference to the ordinate axis on the left. Fuel consumption is shown on the graph and should be read with reference to the right y-axis.

  The influence of the variable distribution on the hydrocarbon emissions as well as on the exhaust temperature (TAVT) is similarly illustrated in FIG.

  It should be noted that, in configuration 2, the setting used makes it possible to reduce the hydrocarbon emissions by 50% and to obtain an exhaust temperature gain of 45 C (FIG 6) for an overconsumption of 10% (FIG. Fig. 5).

  In configuration 4, the setting adopted makes it possible to reduce the hydrocarbon emissions by 55% as illustrated in FIG. 5 and to obtain an exhaust temperature gain of 30 C as shown in FIG. 6 for overconsumption of 4% as shown in Figure 5.

  It is also noted that, whatever the technical solution chosen among the configurations 2 and 4, the delayed opening of the intake makes it possible to limit the overconsumption of fuel as illustrated in FIG. 5. When the opening is not delayed as in configurations 1 and 3, the exhaust temperature is higher. These configurations can nevertheless be retained in cases where it is desired to prime the oxidation catalyst very rapidly even if the overconsumption is important.

  It is also noted that the technical solution constituted by the configurations 3 and 4 makes it possible to strongly limit the overconsumption. This solution is however more complex and more expensive but may be used in cases where consumption is the number one priority.

  The technical solution of configurations 1 and 2 has the advantage of being simple and cheap. It allows a reduction in unburnt emissions about the same as in the solution of configurations 3 and 4. On the other hand, overconsumption is higher. This solution can be used in cases where the cost price is 1 o priority.

  It is important to note that these two embodiments are just two examples among others. The results obtained are only given as examples. This invention can be implemented on all types of internal combustion engines irrespective of the number of valves per cylinder.

  The advantages of the invention, particularly as illustrated by these preferred embodiments, are numerous with respect to the motors of the prior art.

  Thus, GB-2 327 711 discloses introducing an exhaust delay by means of a variable distribution system. This solution appears to be a replacement for an external EGR circuit. However, this solution has the defect of not allowing to cool the EGR gas on points of high load. In the absence of this cooling, it is impossible to reach low levels of NOx emissions. In addition, the engine will be penalized with air filling because the hot internal EGR gas will take much more room in the cylinder. In contrast to this document, the advance at the exhaust and the opening delay implemented in the context of the invention are only for the low loads of the engine. On moderately and heavily loaded operating points, it is a conventional external EGR gas circuit with a high performance cooler which supplies the engine with highly cooled flue gas. Indeed, on these operating points, it is mainly the NOx emissions that must be reduced. The unburnt emissions are lower and the oxidation catalyst is initiated.

  In the document EP-0 560 476, it is provided in particular to phase out both the exhaust law and the intake law to the right, that is to say to open and close the exhaust later. and admission. The exhaust offset is 40 crankshaft and the offset at intake is 24. This difference in offset makes it possible to increase the crossover and thus increase the flow of flue gas from the exhaust to the intake. This increase in internal EGR gases can reduce hydrocarbon emissions. However, this solution is not at all suitable for a direct injection diesel engine. Delaying the closing of the exhaust can cause contact between the valves and the piston. Indeed, in a diesel engine, the distance between the top of the piston at the top dead center and the valves is very small, so that are usually planned in the piston to avoid contact. It is therefore impossible to complete the closing of the exhaust after the top dead center. Contrary to the content of this document, the invention provides for a premature closing of the exhaust valves (30 to 40 crankshaft before the top dead center) to trap the burnt gases in the cylinder. This solution, besides being able to be used on a diesel engine because it avoids the valve / piston contact, also makes it possible to obtain warmer internal EGRs. In addition, in this document, the internal EGR gases are obtained by transferring burnt gases into the intake during the crossing of the valves (simultaneous opening exhaust and intake). This solution generates more heat transfer and thus colder internal EGR gases.

  Finally, in EP-1 387 074, which is dedicated to the direct injection gasoline engine, it is expected to close the exhaust earlier by shifting the exhaust law in order to increase the amount of internal EGR and so reduce NOx and hydrocarbon emissions. Unlike the 2877048 it contains the content of this document, the invention is applicable to direct injection diesel engine for which the problem of reduction of unburnt emissions is very linked to the achievement of future pollution control standards. On the other hand, this document discloses prematurely closing the escapement by dehasing it. The phase shift of the single exhaust without changing the intake leads to significant overconsumption due to the compression of unburned gases (premature closure of the exhaust). Unlike the content of this document, in addition to the premature closure of the exhaust, the invention provides a delayed opening of the intake, which allows to recover a portion of the work of compression of the burnt gases. Indeed, by delaying the opening of the inlet, it allows the expansion of the burnt gases trapped in the cylinder. The recovery of the work of relaxation makes it possible to reduce in a big way the overconsumption. The second advantage of the delay at this opening is to limit the back-flow of unburnt gases to the inlet: The pressure in the cylinder being lower at the time of opening of the intake, a smaller amount of gas burned engulfs in admission.

  Of course, we can bring to the invention many changes without departing from the scope thereof.

  The invention is applicable to engines in which each cylinder comprises a single intake valve and a single exhaust valve. It is also applicable to engines where each cylinder comprises two valves of each kind.

Claims (9)

  A method of controlling a vehicle engine (2) characterized in that: - a closure of at least one exhaust valve is controlled so that the closure is completed before a piston associated with the valve has reaches a top dead center; and - an opening of at least one intake valve associated with the piston is started after reaching top dead center.   2. Method according to the preceding claim, characterized in that it controls said closing and opening only when the engine load is less than a predetermined value.   3. Method according to any one of the preceding claims, characterized in that one starts an opening of the exhaust valve at a time of the cycle of the piston before an earlier time at which it starts, otherwise, its opening for the closure s finishes when the piston reaches the top dead center.   4. Method according to any one of the preceding claims, characterized in that completes a closure of the intake valve at a time of the piston cycle which is completed its closure if it began its opening when the piston reaches the top dead center.   5. Method according to any one of the preceding claims, characterized in that the closing of the exhaust valve begins at a point of a trajectory of the valve located below a point at which it starts, otherwise , closing it so that the closing is completed when the piston reaches top dead center.   6. Method according to any one of the preceding claims, characterized in that it starts closing the inlet valve at a point of a trajectory of the valve located below a point at which it starts, otherwise , its closure when it began opening when the piston reaches the top dead center.   7. Vehicle engine (2) comprising: - at least one piston; and - exhaust and intake valves associated with the piston, characterized in that it comprises control means arranged to: - control a closing of the exhaust valve so that the closure is completed before the piston has reached a top dead center; and - begin an opening of the intake valve after reaching the top dead center by the piston.   8. Motor according to the preceding claim, characterized in that it comprises a circuit (18) of recirculation of the exhaust gas.   9. Engine according to any one of claims 7 to 8, characterized in that it is a diesel engine with direct injection and compression ignition.