FR2835881A1 - I.c. engine with two-phase fuel injection has first phase with insufficient fuel to cause combustion prior to injecting second phase - Google Patents

Abstract

The i.c. engine, especially for a motor vehicle, has fuel injected into its combustion chamber (20) in two phases - the first when the piston (30) is at tdc on the inlet stroke and the second at the end of the compression stroke. The quantity of fuel injected in the first phase is less than in the second, and is insufficient to produce combustion before the second quantity is injected. The first injection takes place within plus or minus 20 degrees of tdc with the exhaust valves (21) open and represents no more than 20 per cent of the overall quantity of the two phases.

Description

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 The invention generally relates to engines and fuel injection strategies for optimizing the operation of these engines.

 More specifically, the invention relates to an engine, in particular for a motor vehicle, comprising at least one combustion chamber delimited by a cylinder head, a piston and a cylinder, a device for injecting fuel into the combustion chamber, and means for control of the injection device, the piston alternately moving inside the cylinder by varying the volume of the combustion chamber, the combustion chamber comprising exhaust valves for combustion gases and intake valves d fresh air, the movements of the piston in the cylinder defining at least one exhaust phase during which the volume of the combustion chamber decreases and the exhaust and intake valves are respectively opened and closed, the piston being in end of exhaust phase at top intake neutral, a fresh air intake phase during which the volume of the combustion chamber increases and the exhaust and intake valves are respectively closed and open, and a compression phase during which the volume of the combustion chamber decreases and the intake and exhaust valves are closed, the compression phase ending with the combustion of the fuel, the control means controlling the injection of a first quantity of fuel substantially at top intake neutral, and the injection of a second quantity of fuel at the end of the compression phase.

 Engines of this type are known from the prior art, in particular from European patent EP0767303, which discloses an engine in which the first and second amounts of fuel each represent 50% of the total amount of fuel to be injected.

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 The first quantity is injected at the start of the intake phase and forms a so-called depleted mixture with fresh air. This depleted mixture is homogenized and compressed during the compression phase and is consumed when the pressure exceeds a threshold, when the pressure exceeds a threshold, before the injection of the second quantity of fuel.

 The hot gases resulting from the combustion of the first quantity of fuel heats the second quantity of fuel with the latter in turn being consumed.

 This fuel injection strategy makes it possible to reduce the formation of soot and NOx because the air fuel mixture is more homogeneous during combustion than if all the fuel was injected at the end of the compression phase. It is known in fact that high local wealth in fuel at the time of the self-ignition of the mixture leads to the formation of soot and Nox.

 The first quantity of fuel, injected in the intake phase, forms with the air a very homogeneous mixture during the compression phase.

 Furthermore, the high temperature gases resulting from the combustion of the first quantity of fuel heat the second quantity of fuel at the time of its injection, thus allowing better dispersion of this fuel in the chamber. This phenomenon makes it possible to limit the amount of soot and NOx formed by the combustion of the second amount of fuel.

 This engine nevertheless has faults. Firstly because the two quantities of fuel undergo successive combustions, which is less favorable than simultaneous combustions. Indeed, the first combustion taking place before the dead center high combustion, produces a negative work which decreases the energy efficiency of the engine.

 Furthermore, it is difficult to optimize both the conditions for combustion of the first quantity

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 of fuel and of the second quantity of fuel, optimum combustion conditions for the first quantity of fuel not necessarily leading, after combustion, to optimum conditions for the second quantity. The fuel efficiency of the engine is therefore less good.

 The present invention aims to overcome the above problems.

 To this end, the engine of the invention, furthermore in accordance with the generic definition given in the preamble above, is essentially characterized in that the first quantity of fuel is less than the second quantity, and in that the first quantity is not sufficient to cause combustion of the fuel before injection of the second quantity.

 In a possible embodiment of the invention, the first quantity of fuel is injected in an interval between minus twenty crankshaft degrees and plus twenty crankshaft degrees relative to the top admission neutral point.

 Advantageously, the exhaust valves can be in the open position at the time of the injection of the first quantity of fuel.

 Preferably, the exhaust valves can be closed after top intake neutral to allow the re-aspiration of part of the combustion gases.

 For example, the first quantity of fuel can be injected into the combustion chamber in several successive charges.

 Advantageously, the second quantity of fuel can be injected into the combustion chamber in several successive charges.

 Preferably, the first quantity of fuel can represent at most 20% of the sum of the first and second quantities.

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 Other characteristics and advantages of the invention will emerge clearly from the description which is given below thereof, by way of indication and in no way limiting, with reference to the appended figures, among which: - Figure 1 is a schematic representation of the engine of the invention, - Figure 2 is a graphical representation of the position of the valves during the four operating phases of the cylinder for a first embodiment of the invention, the abscissa being graduated in crankshaft degrees and divided into four zones X , E, A and C respectively corresponding to the expansion, exhaust, intake and compression phases, the ordinate representing the percentage of opening of the exhaust (curve 1) and intake (curve 2) valves ) between 0 and the maximum value, the small and large syringes respectively marking the injection points of the first and second quantity of fuel.

 The invention relates to an engine, in particular for a motor vehicle, comprising in known manner at least one combustion chamber 20 delimited by a cylinder head 25, a cylinder 10 and a piston 30, a device 40 for injecting fuel into the combustion chamber 20, and control means 50 of the injection device 40. The piston 30 moves alternately inside the cylinder 10 by varying the volume of the combustion chamber 20.

 The engine typically comprises four cylinders, but the invention can be applied to engines with more or less than four cylinders.

 The cylinder head 25 includes exhaust valves 21 for fuel combustion gases and intake valves 22 for fresh air.

 The piston 30 can indifferently understand or not understand a bowl.

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 The movements of the piston 30 in the cylinder 10 define four successive phases.

During a first intake phase A, the fresh air coming from outside the engine enters the combustion chamber 20 through the open intake valves 22. The piston 30 moves in a first direction causing the volume of the combustion chamber 20 to increase, creating a call for air to this chamber when the intake valves 22 are open. The exhaust valves 21 are closed
During a second compression phase C, the piston 30 makes a movement in a second direction opposite to the first, and the intake 22 and exhaust 21 valves are closed. The volume of the combustion chamber 20 decreases and the pressure and the temperature therein increases.

 The compression phase C ends with the combustion of the fuel, which has been injected beforehand into the combustion chamber 20 by the injection device 40, as will be seen below.

 The fuel is consumed with the oxygen in the air, this operation producing combustion gases.

 A third expansion phase X follows the compression phase C during which the piston 30 starts again in the first direction under the effect of the increase in the pressure in the combustion chamber 20 resulting from the combustion of the fuel.

 The volume of the combustion chamber 20 therefore increases, while the intake 22 and exhaust 21 valves remain closed.

 Finally, during a fourth exhaust phase E, the exhaust valves 21 are open while the piston 30 moves in the second direction. The combustion gases are then expelled from the combustion chamber 20, the volume of which decreases. The inlet valves 22 are closed.

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 The piston 30 is at the end of the exhaust phase E at a top admission dead center PMHA.

 The control means 50 command the injection of a first quantity 61 of fuel substantially at the top admission dead center PMHA, and the injection of a second quantity 62 of fuel at the end of the compression phase C.

 The injection device 40 typically injects the fuel directly into the combustion chamber 20.

 According to the invention, the first quantity 61 of fuel is less than the second quantity 62, and this first quantity 61 is not sufficient to cause combustion of the fuel before injection of the second quantity 62.

 This first quantity 61 represents at most 20% of the total quantity of fuel injected.

 The first quantity of fuel 61 is injected in a time interval comprised between minus twenty degrees crankshaft and plus twenty degrees crankshaft relative to the top admission dead center PMHA.

 This first quantity of fuel 61 is injected into the combustion chamber 20 while hot combustion gases are still there. The first quantity 61 therefore heats up on contact with these gases and will therefore disperse and partially vaporize on contact with the residual combustion gases.

 As shown in FIG. 2, the exhaust valves 21 are in the open position when the first quantity of fuel is injected.

 The exhaust valves 21 close after the top admission dead center PMHA, typically thirty degrees crankshaft after the top admission dead center PMHA.

 The inlet valves 22 typically open fifteen degrees crankshaft before the PMHA top intake dead center.

 Fuel vaporized on contact with the combustion gases can therefore escape from the

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 combustion 20 through the exhaust 21 and intake 22 valves.

 When the piston 30 leaves the PMHA to start its movement in the first direction, at the start of the intake phase A, these fuel vapors are drawn back into the combustion chamber 20.

 As can be seen in FIG. 2, the exhaust and intake valves 21 and 22 are opened simultaneously for a time interval, which makes it possible to satisfy the air requirements of the engine under conditions of full load.

 If the first quantity of fuel 61 was injected before minus twenty degrees crankshaft relative to the top intake dead center PMHA, a significant quantity of fuel vapor could be entrained through the exhaust valves 21.

 Conversely, if the first amount of fuel 61 was injected after more than twenty degrees crankshaft relative to the top dead center intake PMHA, the amount of residual hot combustion gas in the combustion chamber would be too low, the heating of the first quantity 61 of fuel would not be sufficient and the injected fuel would risk impacting the walls of the cylinder. This is the case in particular for bowl pistons, the fuel not being injected into the bowl if the injection is too delayed.

 The interval from minus twenty degrees crankshaft to plus twenty degrees crankshaft relative to the top admission dead center PMHA is therefore an optimum for the injection of the first quantity of fuel 61.

 In this embodiment, it is therefore imperative that the exhaust valves 21 close after the top admission dead center PMHA to allow the fuel vapors to be sucked back into the combustion chamber 20.

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 The fuel is injected when the piston is close to top intake neutral. As a result, the injected fuel does not impact the walls of the cylinder. In the case of a bowl piston, the injected fuel remains confined in the bowl of the piston.

 This aspect is particularly important because it is known that the impact of the liquid fuel on the walls of the cylinder causes mechanical problems, such as for example the gumming of the segments.

 Furthermore, the mixture between the fuel and the fresh air introduced during the intake phase is favored because the fuel is vaporized. The mixture of air and fuel is very homogeneous, thus promoting combustion and reducing the amount of soot and NOx formed.

 In the embodiment of the invention described here, the first quantity 61 of fuel can be injected into the combustion chamber 20 at once or in several successive charges.

 Likewise, the second quantity 62 of fuel can be injected into the combustion chamber 20 at once or in several successive charges.

 It is therefore understandable that the invention makes it possible to obtain a very homogeneous fuel / air mixture, without exhibiting the faults of the prior engines. The first quantity of fuel has time to become homogenized with the air since it is injected substantially with PMHA, but it does not self-ignite due to its low richness.

 The prior injection of the first quantity of fuel is particularly advantageous under operating conditions where the speed of injection of the fuel into the combustion chamber is the factor limiting the performance of the engine, such as for example when the engine is running at full load. . We know that in this case the optimum stoichiometric conditions cannot be reached if the fuel is

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 effect in this case optimum stoichiometric conditions can not be reached if the fuel is injected only at the end of the compression phase, the amount of fuel injected being too small compared to air.

 The use of air present in the combustion chamber and the performance of the engine itself are therefore optimized at all speeds, as well as.

 Finally, it should be emphasized that the present invention applies to a diesel engine, but can just as easily be applied to other types of engines, such as direct injection petrol engines.

Claims (7)

1. Engine, in particular for a motor vehicle, comprising at least one combustion chamber (20) delimited by a cylinder head (25), a cylinder (10) and a piston (30), a fuel injection device (40) in the combustion chamber (20), and control means (50) of the injection device (40), the piston (30) alternately moving inside the cylinder (10) by varying the volume of the chamber combustion chamber (20), the combustion chamber (20) comprising exhaust gas exhaust valves (21) and fresh air intake valves (22), the movements of the piston (30) in the cylinder (10) defining at least one exhaust phase (E) during which the volume of the combustion chamber (20) decreases and the exhaust and intake valves (21, 22) are respectively opened and closed, the piston (30) being at the end of the exhaust phase (E) at a top intake dead center (PMHA), a fresh air intake phase (A ) during which the volume of the combustion chamber (20) increases and the exhaust and intake valves (21,22) are respectively closed and open, and a compression phase (C) during which the volume of the chamber combustion (20) decreases and the intake and exhaust valves (22, 21) are closed, the compression phase (C) ending with the combustion of the fuel, the control means (50) controlling the injection of a first quantity (61) of fuel substantially at top intake neutral point (PMHA), and the injection of a second quantity (62) of fuel at the end of the compression phase (C), characterized in that the first quantity (61) of fuel is less than the second quantity (62), and in that the first quantity (61) <Desc / Clms Page number 11>  is not sufficient to cause combustion of the fuel before injection of the second quantity (62).  2. Engine according to claim 1, characterized in that the first quantity (61) of fuel is injected in an interval between minus twenty crankshaft degrees and plus twenty crankshaft degrees relative to the top intake dead center (PMHA).  3. Engine according to claim 1 or 2, characterized in that the exhaust valves (21) are in the open position at the time of injection of the first quantity (61) of fuel.  4. Engine according to claim 3, characterized in that the exhaust valves (21) are closed after top intake neutral (PMHA) to allow the re-aspiration of part of the combustion gases.  5. Engine according to any one of claims 1 to 4, characterized in that the first quantity (61) of fuel is injected into the combustion chamber (20) in several successive charges.  6. Engine according to any one of claims 1 to 5, characterized in that the second quantity (62) of fuel is injected into the combustion chamber (20) in several successive charges.  7. Engine according to any one of claims 1 to 6, characterized in that the first quantity (61) of fuel represents at most 20% of the sum of the first and second quantities (61,62).