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TEXT OF THE DESCRIPTION

Field of the Invention

The present invention relates to internal combustion engines . More speci fically, the invention was developed with reference to hydrogen spark-ignition internal combustion engines .

Known Art

Hydrogen spark-ignition internal combustion engines generally operate with a relative air-to- fuel ratio (X, defined as a ratio between the actual air-to- fuel ratio and the stoichiometric air-to- fuel ratio ) which is definitely higher than a unit .

Especially in the case of internal combustion engines for heavy-duty applications , such as , e . g . , the engines for trucks or commercial vehicles , and of fixed-point engines , the engine mapping is calibrated in such a way as to define an optimum operating range as regards combustion stability and pollutant emission, especially the emission of nitrogen oxides . As regards the relative air-to- fuel ratio , in said operating range the reference value corresponds to relative air-to- fuel ratios of about 2 or higher than 2 . With such a relative air-to- fuel ratio , the generation of polluting substances during operation is nearly negligible . Unburnt hydrocarbons , particulate matter and carbon oxides are below the limits of detectability .

Such a ratio is easily met while operating in stationary or nearly stationary conditions , while it is very di f ficult to meet during a transient , particularly a transient with a transition from lower to higher loads , essentially due to the dif ferent increase rate of the fuel mass being introduced into the combustion chamber with respect to the increase ratio of combustion air. In other words, while the injectors - which introduce the fuel into the combustion chamber - are to be considered so called "rapid" actuators, i.e., they can operate with performances which may even vary from cycle to cycle, the combustion air - and generally speaking the management thereof - is associated with so-called "slow" actuators, i.e., actuators operating with time constants which are longer than in the rapid actuators, and which generally do not allow for an immediate response from a cycle to the following one. Examples are the fluid-dynamic transients of the intake and exhaust system, the response delay of the turbocharger, etc.

The result of the difference between the response time scale of the injectors and of the combustion air temporarily leads the engine to operate with a relative air-to-fuel ratio which is significantly lower than in stationary conditions (the values of the relative air- to-fuel ratio X may fall to about 1.5) , which causes a much higher tendency to the generation of nitrogen oxides NOx. As a comparison, the emissions of nitrogen oxides during a fast transient, with an increase of the fuel mass which is markedly higher than the increase of the mass of combustion air, may amount to about twice as high as the emissions in stationary or nearly stationary conditions.

Object of the Invention

The invention aims at solving the technical problems outlined in the foregoing. Specifically, the invention aims at limiting the generation of nitrogen oxides during the transient operating mode of a hydrogen spark-ignition internal combustion engine, specifically in a transient operating mode with a transition from lower to higher loads .

Summary of the Invention

The obj ect of the invention is achieved by means of a method having the features set forth in the claims that follow, which are an integral part of the technical disclosure provided herein in relation to the invention .

Brief description of the Figures

The invention will now be described with reference to the annexed Figures , which are provided by way of non-limiting example only, wherein :

Figure 1 shows a li ft diagram of intake valves (valve li ft as a function of a crank angle ) as a result of the implementation of a method according to the invention ( curves EVmin, EVmax ) , which is superimposed to a li ft diagram of intake valves without the implementation of the method according to the invention ( curve no_WT ) ,

Figure 2 shows a time diagram of the evolution of an opening instant of the intake valves as a result of the implementation of a method according to the invention ( solid line curve ) , which is superimposed to a diagram representing the evolution without the implementation of the method according to the invention (no_VTT ) , and to a diagram representing an evolution in time of a torque request by means of the accelerator pedal ,

Figure 3 shows a time diagram of the evolution of the volumetric ef ficiency as a result of the implementation of a method according to the invention ( solid line curve ) , which i s superimposed to a diagram representing the evolution without the implementation of the method according to the invention (no_WT ) , and to a diagram representing an evolution in time of a torque request by means of the accelerator pedal ,

Figure 4 shows a time diagram of the evolution of the firing timing as a result of the implementation of a method according to the invention ( solid line curve ) , which is superimposed to a diagram representing the evolution without the implementation of the method according to the invention (no_WT ) , and to a diagram representing an evolution in time of a torque request by means of the accelerator pedal ,

Figure 5 shows a time diagram of the evolution of the pressure in the intake mani fold as a result of the implementation of a method according to the invention ( solid line curve ) , which is superimposed to a diagram representing the evolution without the implementation of the method according to the invention (Pman_noWT ) , and to a diagram representing an evolution in time of a torque request by means of an accelerator pedal ,

Figure 6 shows a time diagram of the evolution of the driving torque as a result of the implementation of a method according to the invention ( solid line curve ) , which is superimposed to a diagram representing the evolution without the implementation of the method according to the invention ( Torque_noWT ) , and to a diagram representing an evolution in time of a torque request by means of an accelerator pedal ,

Figure 7 shows a time diagram of the evolution of the relative air-to- fuel ratio X (AFR/AFR_st , i . e . , the ratio between the actual air-to fuel ratio and the stoichiometric air-to- fuel ratio ) as a result of the implementation of a method according to the invention ( solid line curve ) , which is superimposed to a diagram representing the evolution without the implementation of the method according to the invention ( lambda_no_WT ) , and to a diagram representing an evolution in time of a torque request by means of an accelerator pedal , and

Figure 8 shows a time diagram of the evolution of the emissions of nitrogen oxides as a result of the implementation of a method according to the invention ( solid line curve ) , which is superimposed to a diagram representing the evolution without the implementation of the method according to the invention (NOx_noWT ) and to a diagram representing an evolution in time of a torque request by means of an accelerator pedal .

Figures 1 to 8 all refer to the same sparkignition internal combustion engine , which is supercharged by means of a turbocharger and is hydrogen- fuelled, and they also describe the same operation during a transient with an increase of the load . The diagrams refer to an internal combustion engine having two intake valves , but the method described may be used, with the unavoidable adj ustments in order to compensate for a di f ferent internal fluid dynamics , irrespective of the number of intake valves .

Detailed Description

The method according to the invention may be implemented in hydrogen spark-ignition internal combustion engines , especially those having an architecture derived from compression-ignition engines . As generally known, such internal combustion engines comprise at least one cylinder having a longitudinal axis , along which a piston is reciprocally movable , and a head which defines , for each cylinder and each respective piston, a combustion chamber . Moreover the engine comprises , for each cylinder, at least one intake valve and at least one exhaust valve , each of them having a respective li ft diagram comprising an opening instant, a closing instant and a motion according to a lift law between the opening instant and the closing instant. The description in the following is developed with reference to an engine comprising two intake valves and two exhaust valves, but it is understood that the method may be applied irrespective of the number of such valves (i.e., at least one intake valve and at least one exhaust valve) .

In some applications, the opening instant of the lift diagram of the at least one intake valve occurs earlier than a closing instant of the at least one exhaust valve, so that the anticipation defines an overlap interval of the intake and exhaust valves (in the following, for brevity, "valve overlap") .

In other applications, however, it is possible to operate with a timing of the intake valves which does not lead to any overlap interval, i.e., with a timing wherein the opening instant of the intake valves occurs after the closing instant of the exhaust valves.

The invention envisages to use the regulation of the intake timing as a means to act on the filling coefficient of the engine (and therefore on the volumetric efficiency) with a response speed higher than the speed which normally characterizes the supply system of combustion air. The action of modulating the engine filling, specifically of increasing the filling during a transient (e.g., when a torque increase is requested by the user) contrasts the reduction of the relative air-to-fuel ratio X and keeps the air-to-fuel ratio at values (of about 2) whereat no significant production of nitrogen oxides takes place.

In various embodiments, the method according to the invention comprises giving the engine a torque request increase control (therefore going from a lower to a higher load) and controlling, in response to the torque request increase control, a delay of the opening instant of the intake valves, in order to obtain an increase of the volumetric efficiency.

In preferred embodiments of the invention, which are shown in Figures 1 to 8, varying the timing of the intake valves with a delay of the opening instant of the intake valves is controlled by means of a variable valve timing for the intake valves. In other words, the lift diagram remains constant, but it may be translated to an earlier or a later time (by varying the timing of the closing instant and, of course, of the opening instant) thanks to the variable valve timing. This means that a timing variation concerning the opening instant is transferred "rigidly" to the closing instant, and vice versa. By way of reference only, without limiting any of the embodiments of the invention, the lift diagram of the intake valves of the engine in Figures 1 to 8 covers a width of crank angles between 140 crank degrees and 190 crank degrees, measured at a lift of 0.6 mm (i.e. the angular ends of the lift diagram - the opening instant and the closing instant - are conventionally chosen as points whereat the valve lift amounts to 0.6 mm) .

In some embodiments, especially in the embodiments which feature an overall timing diagram with a valve overlap at positive values (i.e., with a closing instant of the intake valves occurring after the opening instant) , the delay of the opening instant of the intake valves corresponds to a reduction of the width of the valve overlap.

By way of example, Figure 1 shows the timing variation concerning the intake valves, both as regards the excursion of the valve overlap from a position of minimum/low volumetric efficiency (curve EVmin) to a position of maximum/high volumetric efficiency (curve EVmax ) , and as regards a condition of constant timing (noWY) without the variable valve timing, i . e . , without the implementation of the method according to the invention .

The diagram of Figure 1 shows what has been described in the foregoing : thanks to the variable valve timing, it is possible to regulate the timing of the intake valves at low loads , by envisaging an earlier closing of the intake valves in comparison with the case of a constant timing, due to the absence of variable valve timing ( in the case of embodiments having a valve overlap at positive values , this corresponds to an overlap having a width higher than in the case of constant timing, due to the absence of a variable valve timing) .

Speci fically, it is necessary to consider the angular range Al between the opening instant of the intake valves at low loads , with the implementation of the method according to the invention, and the opening instant of the intake valves in the case of constant timing . It wi ll be observed that , with the implementation of the method according to the invention, the opening instant occurs earlier with respect to the case of constant timing, therefore leading to an earlier closing of the intake valves .

At the beginning of the transient , the variable valve timing is controlled to vary the timing of the intake valves by delaying the opening instant thereof ( and consequently by delaying the closing instant thereof , as envisaged by the invention ) . In this regard it is necessary to observe the angular range A2 between the opening instant o f the intake valves , after the regulation performed by the variable valve timing ( curve EVmax ) , in the case of implementation of the method according to the invention ( corresponding to a condition of maximum/high volumetric ef ficiency) and the opening instant of the intake valves in the case of constant timing . The total angular excursion between the curve EVmin and the curve EVmax equals the sum of the ranges Al + A2 , the latter representing the total reduction of the valve overlap ( as well as , according to the invention, the total delay of the closing instant of the intake valves ) between the timing with high volumetric ef ficiency following the start of the transient and the timing with low ef ficiency before the start of the transient .

The following Figure 2 shows the evolution in time of the opening instant of the intake valves ( IVO in the diagram) as a result of the intervention of the variable valve timing . The dashed line curve is obviously stationary because , without the intervention of the variable valve timing ( if the variable valve timing is not present ) , the opening instant o f the intake valves is constant in time .

The variation of the opening instant IVO occurs immediately after the reception of an accelerator pedal control , which derives from an increased torque request by the vehicle driver . The operation of the pedal illustrated herein is stepwise , and has a slightly more rapid dynamics than the intervention of the variable valve timing . Moreover, the diagram shows the ranges Al and A2 described in the foregoing .

It should be noted, moreover, that when higher load stationary operation conditions are established - end of the "pedal" curve after the step - the timing of the intake valves is returned to the timing taking place without the variable valve timing . This is not mandatory, and speci fically the diagram of Figure 2 must be construed as showing a timing correction in an opposite direction, when new stationary conditions are achieved again (which may or may not lead to a timing equal to the case without variable valve timing) .

Figure 3 shows a filling diagram (volumetric ef ficiency) of the intake mani fold during the transient : the diagram graphically shows the ef fect of the timing variation of the intake valves on the filling of the manifold during the transient , and speci fically :

- the partiali zation ef fect caused by the timing at low loads (EVmin) , which leads to a lesser filling than in the case of constant timing (noWT ) , passing through a condition of low filling, and

- the filling of the mani fold in the transition towards the timing of the transient end (EVmax ) which is more rapid than in the case of constant timing (no WT ) .

As will become evident from the analysis of the following diagrams , this balances the ef fect o f the increase of the fuel mass entering the combustion chamber, and improves the engine response during the transient , while at the same time reducing the peaks of nitrogen oxide generation .

The diagram in Figure 4 shows the evolution in time of the firing timing . Speci fically, the diagram shows the firing angle with reference to the top dead centre firing, and speci fically the range of crank degrees after top dead centre firing (ATDCF) . The delay increase of the firing shi fts the combustion towards the exhaust step, therefore bringing about a slight anti-lag ef fect on the turbocharger, which in this way may increase the number of revolutions while equally increasing the amount of air supplied to the internal combustion engine .

The diagram of Figure 5 shows an advantageous aspect of the implementation of the method according to the invention . As may be observed, the timing at low loads in the method according to the invention (Pman_WT ) leads to a pressure in the intake mani fold which is higher than the pressure present in the case of constant timing ( curve Pman_no_WT ) . This means that , at the beginning of the transient , there is present an amount of pressure which improves the response during the transient , thereby contributing to increase the amount of air input into the engine and to mitigate the relative air-to- fuel ratio reduction and which, in some operating conditions , may be taken advantage of in order to obtain a moderate anti-lag ef fect on the turbocharger . In short , the amount of pressuri zed air in the mani fold may be vented towards the inlet of the turbocharger unit , speci fically towards the inlet of the turbine , at the beginning of the transients , favouring the vent by means of the variable valve timing and the fill ing modi fication consequent thereto . In the operating points whereat such ef fect is stronger, this results in an increase of the number of revolutions of the turbocharger, which in turn leads to an even better transient response and to a further reduction of the nitrogen oxide emissions .

Figure 6 shows the evolution in time of the torque delivered by the engine during the transient . It will be observed that the increase rate of the torque in the case of the method according to the invention ( Torque_WT ) is generally higher than in the case of constant timing ( Torque_noWT ) , the improvement resulting from the combination between a better volumetric ef ficiency of the intake mani fold, the reserve of pressure in the intake mani fold, and the anti-lag action on the turbocharger : of course , each of such phenomena will have a di f ferent importance according to the operating point wherefrom the transient starts , and according to the target operating point of the transient .

Figures 7 and 8 show the ef fect of the method according to the invention on the variation of the relative air-to- fuel ratio ( Figure 7 ) and on the emissions of nitrogen oxides NOx . It is easily observed that , in the case of the method according to the invention ( lambda_WT ) , the oscillation of the relative air-to- fuel ratio has a definitely smaller width than in the case of constant timing ( lambda_noWT ) , and as a consequence the peak of nitrogen oxide emission ( Figure 8 ) , which appears without the implementation of the method according to the invention, is completely eliminated .

It will be appreciated, therefore , that the delay of the closing instant of the intake valves during the transient , according to the invention, whether or not it results in a width variation of the valve overlap ( in engines which, in stationary condition, operate with zero or negative valve overlap, i . e . in the absence of a valve overlap, obviously the very concept of a valve overlap "width" loses its meaning) , neutrali zes - by means of a synergistic combination of partial ef fects - the ef fects of an increase o f the fuel mass entering the combustion chamber on the relative air-to- fuel ratio , while keeping the latter at values which prevent the generation of nitrogen oxides .

Of course , the implementation details and the embodiments may amply vary with respect to what has been described and illustrated herein, without departing from the scope of the present invention, as defined by the annexed claims .