Abstract:
With reference to Figure, the present invention provides a fuel injection system for an internal combustion engine which delivers fuel to be mixed with charge air for subsequent combustion in a combustion chamber of the internal combustion engine. The fuel injection system comprises a fuel injector which functions as a positive displacement pump and dispenses in each operation thereof a set quantity of fuel;
       a mixing chamber into which the fuel injector dispenses fuel; and a gas supply passage for supplying gas to the mixing chamber to entrain the fuel dispensed into the mixing chamber in a flow of gas which passes through the mixing chamber into the combustion chamber. The mixing chamber is connected to the combustion chamber to deliver fuel and gas into the combustion chamber separately from the charge air and a depression in the combustion chamber is used to draw gas through the gas supply passage into the combustion chamber. An inlet valve controls flow of charge air into the combustion chamber and the inlet valve is kept closed for an initial part of an intake stroke of the engine so that the depression is created in the combustion chamber.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an internal combustion engine having a fuel injection system. 
   Most internal combustion engines in automobiles currently use fuel injection systems to supply fuel to the combustion chambers of the engine. Some fuel injection systems have fuel injectors which inject fuel directly into a combustion chamber of an engine. It is a problem to ensure that such fuel is properly atomised. 
   Most fuel injection system are designed to meter fuel accurately and are not fuel atomisation devices. It is recognised that a finely atomised fuel spray will improve air fuel mixing and will help reduce engine emissions. It is therefore advantageous to incorporate an atomisation feature into the fuel injector. This is difficult with conventional injectors since if the atomisation process has any variable effect on the pressure difference across the injector this can alter the flow rate of fuel through the injector and cause incorrect fuel quantities to be delivered to the engine. Therefore, choosing an effective atomisation process is very limited with the conventional fuel injection systems and the current “state of the art” injection systems overcome this problem by using a complex highly controlled high pressure fuel system where the high kinetic energy in the fuel can aid atomisation. 
   The sophisticated and highly developed fuel injection systems currently available are ideal for use in internal combustion engines in automobiles. However, there are many other applications for internal combustion engines where such a level of sophistication is not appropriate and too costly. For instance, small single cylinder engines as used for lawn mowers, chain saws, small generators, mopeds, scooters, etc are built to very tight cost targets and so cannot afford the cost of a sophisticated fuel injection system nor the additional power required to run a fuel pump. To date, such small engines have used traditional carburettor technology and relied on a gravity fed fuel supply. However, it is now the case that such small engines will face the same type of exhaust gas emission legislation as the engines in automobiles and so must be modified in such a way as to meet emissions targets. Therefore, a cheap and simple system of fuel injection is required for such small engines. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, there is provided an internal combustion engine having a fuel injection system which delivers fuel directly into a combustion chamber of the engine for mixing with charge air delivered separately to the combustion chamber via an inlet valve, the fuel injection system comprising 
   a fuel injector which functions as a positive displacement pump and dispenses in each operation thereof a set quantity of fuel; 
   a mixing chamber into which the fuel injector dispenses fuel; and 
   a gas supply passage for supplying gas to the mixing chamber to entrain the fuel dispensed into the mixing chamber in a flow of gas which passes through the mixing chamber into the combustion chamber; wherein: 
   the mixing chamber is connected to the combustion chamber to deliver fuel and gas into the combustion chamber separately from the charge air and a depression in the combustion chamber is used to draw gas through the gas supply passage into the combustion chamber; and 
   the inlet valve controls flow of charge air into the combustion chamber and the inlet valve is kept closed for an initial part of an intake stroke of the engine so that the depression is created in the combustion chamber. 
   According to a second aspect of the invention the present invention provides an internal combustion engine having a fuel injection system which delivers fuel directly into a combustion chamber for mixing with charge air delivered separately to the combustion chamber via an inlet valve, the fuel injection system comprising: 
   a fuel injector which functions as a positive displacement pump and dispenses in each operation thereof a set quantity of fuel; 
   a mixing chamber into which the fuel injector dispenses fuel; and 
   a gas supply passage for supplying gas to the mixing chamber to entrain the fuel dispensed into the mixing chamber in a flow of gas which passes through the mixing chamber into the charge air; wherein: 
   the mixing chamber is connected to the combustion chamber to deliver fuel and gas into the combustion chamber separately from the charge air and a depression in the combustion chamber is used to draw gas through the gas supply passage into the combustion chamber; 
   the inlet valve controls flow of charge air into the combustion chamber and the inlet valve is kept closed for an initial part of an intake stroke of the engine so that the depression is created in the combustion chamber; and 
   the fuel injector dispenses an amount of fuel which is fixed for each and every operation of the injector. 
   According to a third aspect of the invention the present invention, the present invention provides an internal combustion engine having a fuel injection system which delivers fuel directly into a combustion chamber for mixing with charge air delivered separately to the combustion chamber via an inlet valve, the fuel injection system comprising: 
   a fuel injector which functions as a positive displacement pump and dispenses in each operation thereof a set quantity of fuel; 
   a mixing chamber into which the fuel injector dispenses fuel; and 
   a gas supply passage for supplying gas to the mixing chamber to entrain the fuel dispensed into the mixing chamber in a flow of gas which passes through the mixing chamber into the charge air; wherein: 
   the mixing chamber is connected to the combustion chamber to deliver fuel and gas into the combustion chamber separately from the charge air and a depression in the combustion chamber is used to draw gas through the gas supply passage into the combustion chamber; and 
   the inlet valve controls flow of charge air into the combustion chamber and the inlet valve is kept closed for an initial part of an intake stroke of the engine so that the depression is created in the combustion chamber; and 
   fuel and gas leaving the mixing chamber pass through an atomising nozzle prior to mixing with the charge air. 
   According to a fourth aspect of the invention the present invention provides an internal combustion engine having a fuel injection system which delivers fuel directly into a combustion chamber for mixing with charge air delivered separately to the combustion chamber via an inlet valve, the fuel injection system comprising: 
   a fuel injector which functions as a positive displacement pump and dispenses in each operation thereof a set quantity of fuel; 
   a mixing chamber into which the fuel injector dispenses fuel; and 
   a gas supply passage for supplying gas to the mixing chamber to entrain the fuel dispensed into the mixing chamber in a flow of gas which passes through the mixing chamber into the charge air; wherein: 
   the mixing chamber is connected to the combustion chamber to deliver fuel and gas into the combustion chamber separately from the charge air and a depression in the combustion chamber is used to draw gas through the gas supply passage into the combustion chamber; 
   the inlet valve controls flow of charge air into the combustion chamber and the inlet valve is kept closed for an initial part of an intake stroke of the engine so that the depression is created in the combustion chamber; and 
   the fuel injector dispenses an amount of fuel which is fixed for each and every operation of the injector; 
   the fuel and gas leaving the mixing chamber pass through an atomising nozzle prior to mixing with the charge air; and 
   the atomising nozzle further includes a pintle, the pintle being operated simultaneously with the fuel injector. 
   According to a fifth aspect of present invention, there is provided a method of delivering fuel into a combustion chamber separately from charge air delivered via an inlet valve to the combustion chamber, the method comprising the steps of: 
   dispensing a set quantity of fuel from a fuel injector to a mixing chamber; and 
   entraining the fuel in the mixing chamber in a flow of gas, with the flow delivering the fuel to the combustion chamber via an atomising nozzle; wherein: 
   a depression is created in the combustion chamber in an early part of an intake stroke of the engine by keeping closed the inlet valve and the depression is used to draw through the atomising nozzle the gas used to entrain the dispensed fuel. 
   Internal combustion engines that make use of embodiments of the invention can do away with complicated, heavy and expensive fuel injection systems. Instead, they may make use of a cheaper and simpler system that does not require the pressure within the inlet passage to be monitored or the provision of a fuel pump and pressure regulator to maintain a constant pressure differential between the fuel and the charge air. Rather, the fuel injector of the current invention dispenses a known quantity of fuel at a fixed flow rate independent of the pressure of the charge air. The vacuum drawn in the combustion chamber by piston motion while the inlet valve is closed is used to draw in air through the mixing chamber to entrain injected fuel and atomise the fuel as the fuel and air mixture is drawn through the atomising nozzle. There is no need for an air pump as used in known gasoline direct injection engines. The ability to deliver fuel in this way also allows a simpler apparatus for dispersing the fuel in the charge air and the use of low cost effective atomisation processes without effecting the accurate fuel quantity being delivered, so allowing simple small engines to benefit from well atomised accurate full flow rates. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention shall now be described with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic representation of a fuel injector of the present invention arranged for direct injection into the combustion chamber; 
       FIG. 2  is a schematic representation of a fuel injector of the present invention arranged for direct injection into the combustion chamber wherein the sonic nozzle is of the pintle type; 
       FIG. 3  shows a cross-sectional view of a nozzle of the fuel injector of  FIG. 2 ; and 
       FIGS. 4   a - 4   d  show alternative nozzle orifice shapes. 
   

   DETAILED DESCRIPTION 
   A first embodiment of the invention uses a fuel injector  210  and nozzle  276 . The fuel injector  210  provides direct injection of fuel into the combustion chamber  630  of an engine.  FIG. 1  shows an internal combustion engine in which a piston  620  cooperates with a cylinder to define a combustion chamber  630 . Also shown are an inlet valve  614  controlling flow of charge air into the combustion chamber  630  and an exhaust valve  616  controlling flow of exhaust gas from the combustion chamber  630 . A sonic nozzle  276  of the fuel injector  210  is arranged to dispense fuel directly into the combustion chamber  630  of the engine. The fuel injector  210  comprises a fuel inlet  240 , a fuel outlet  214  and a fuel chamber  216 . The fuel inlet  240  of the fuel injector  210  is connected to a supply of fuel and communicates via spring-loaded one-way inlet valve  222  with the fuel chamber  216 . A second spring-loaded one-way outlet valve  224  controls the flow of fuel out of the fuel chamber  216  to the fuel outlet  214 . 
   The fuel chamber  216  itself is defined by a piston  220  which is slidably located within a body of the fuel injector  210 . The piston  220  is acted upon by a biasing spring  211  and surrounded by a solenoid  213 . An end plate  215  is connected to the piston  220  at an end remote from the fuel chamber  216  and extends radially outwardly from the piston across an end face of the solenoid  213 . The solenoid  213  is connected by a line (not shown) to an engine control unit (also, not shown). 
   Starting from a condition in which the piston  220  is biased to its uppermost point within the body of the fuel injector  210  by the biasing spring  211  (i.e. the point at which the fuel chamber  216  has its greatest volume), the fuel chamber  216  will be primed with fuel ready for injection. Energisation of the solenoid  213  then acts to pull the end plate  215  into contact or near contact with the solenoid  213 . The piston  220  moves downwards against the force of the basing spring  211  to reduce in volume the fuel chamber  216 . This causes the positive displacement of fuel from the fuel chamber  216 , the one-way outlet valve  224  opening to allow the piston  220  to expel fuel from the fuel chamber  216  to the fuel outlet  214  while the one-way inlet valve  222  remains closed. 
   Once the solenoid  213  is de-energised, the biasing spring  211  will force the piston  220  upwardly and the end plate  215  away from the solenoid  213 . The upward motion of the piston  220  will cause the fuel chamber  216  to increase in volume and this will have the effect of closing the one-way outlet valve  224  and opening the one-way inlet valve  222 . The moving piston  220  draws fuel from the fuel inlet  240  into the fuel chamber  216  to fully charge the fuel chamber  216  ready for the next dispensing of fuel. 
   The fuel injector  210  is constructed so that the piston  220  has a set distance of travel in each operation. The piston  220  moves between two end stops. Thus, in each operation of the fuel injector  210 , the piston  220  displaces a predetermined quantity of fuel and a predetermined quantity of fuel is dispensed out of the fuel outlet  214 . The amount of fuel dispensed by the fuel injector  210  is constant for each and every operation. 
   Having been dispensed from the fuel chamber  216 , the fuel is forced via the fuel outlet  214  to a mixing chamber  218  and then via an atomising nozzle  276  to the combustion chamber  630 . The atomising nozzle  226  of the present invention is a sonic nozzle (also known in the art as a critical flow venturi, or critical flow nozzle). The atomising nozzle could also be an air-blast nozzle. 
   A schematic diagram of a sonic nozzle is shown in  FIG. 3 . The nozzle comprises a venturi  350 , the internal dimensions of which narrow to provide a throat  302 . The fluid upstream  352  of the throat  302  is provided at a higher pressure than that downstream  354  of the throat. The fluid flows into the nozzle and is accelerated in the narrowed throat region. The velocity of the fluid in the narrowed region approaches the speed of sound. Once this condition has been realised the flow rate through the sonic nozzle will remain constant even if the downstream pressure varies significantly, provided, of course, that the pressure differential across the nozzle continues to exceed the threshold valve. Thus in the present case a constant fuel flow rate into the charge air is achieved. It should be noted that a sonic nozzle will provide a constant flow rate regardless of the abruptness of the change in downstream pressure provided that the downstream pressure remains at less than about 85-90% of the upstream pressure. 
   In the current invention the passage of fuel through the sonic nozzle  276  also aids in dispersing the fuel into the charge air. In fact, since the velocity of the fuel passing through the venturi  350  approaches the speed of sound, the nozzle  276  acts as a highly efficient atomizer breaking the liquid fuel up into a mist of tiny particles. Generally, the finer the spray of fuel in the charge air, the better the combustion process achieved. While the exact operation of sonic nozzles in atomizing fuel is not well understood, it is thought that the passage of the liquid fuel through the shock waves in the high velocity region of the sonic nozzle produces very high shear stresses on the liquid surface and cavitation bubbles within the liquid, both of these processes leading to very fine atomisation and dispersion of the fuel into the charge air. 
   In conventional fuel injection systems the pressure differential between the fuel and charge air must be constantly regulated to allow the amount of fuel dispensed by the injectors to be accurately determined. This prevents the use of sonic nozzles. However, in the current invention the fuel injector does not require the fuel-to-charge air pressure ratio to be precisely controlled. Hence, the use of sonic nozzles becomes possible. 
   In conventional fuel injection systems, the fuel is pressurised and the or each fuel injector simply acts as an on/off switch to control the amount of fuel dispensed. In contrast, the present fuel injector is intended to be operated using a pulse. The fuel injector  210  in each operation dispenses a fixed volume of fuel. Due to changing load conditions on the engine, the amount of fuel to be injected for combustion will have to be increased or decreased. To meet this requirement the injector  210  is operated by a pulse count injector method which uses multiple operations of the fuel injector  210  in each engine cycle. When the engine is at the part of the cycle at which fuel injection must occur, multiple operations of the fuel injector  210  take place. To increase or decrease the amount of fuel dispensed, the number of operations of the injector  210  is adjusted accordingly. For example, under normal loading conditions the number of operations may be, say, ten. For higher load conditions the number is increased to fourteen, for example, or for reduced load conditions the number of pulses may be reduced to, say, six. 
   For a conventional engine with a fuel injection system the timing of the fuel injection is critical. Both the duration for which the on/off valves are open, and the point in the engine cycle at which the fuel is dispensed must both be accurately controlled. The combination of a pulse count injection system with a sonic nozzle overcomes many of the timing problems associated with the prior art. In a pulse count injection system using a sonic nozzle the volume of fuel delivered in each engine cycle is easily determined. Successive operations of the fuel injector  210  (in a single engine cycle) in a pulse count injection system can be easily provided for. 
   In an alternative embodiment, the piston  220  may be configured to deliver a number of different volumes of fuel. This may be achieved by only partially retracting the piston  220 . There are other ways of implementing such a variable volume injection device, for example a diesel fuel injector with a variable stroke can be used to give a variable, but known quantity of fuel. 
   The mixing chamber  218  is located between the fuel chamber  216  and the nozzle  276 . The mixing chamber  218  is connected to receive air via an air bypass  270 , orifices, e.g.  252 . are shown allowing this. This is a passage which communicates with both the mixing chamber  218  and a region where air is at atmospheric pressure. 
   During operation of the fuel injector  210 , the piston  220  moves to expel the fuel from the fuel chamber  216 . The fuel then passes through the mixing chamber  218  and on through the sonic nozzle  276 . The fuel is expelled under the pressure provided by the piston. The dispensing of the fuel is timed to coincide with low pressure conditions inside the combustion chamber  630 . As the fuel is expelled, the low pressure conditions in the combustion chamber  630  draws air from the air bypass passage  240  and the air flows through the mixing chamber  218  and entrains the fuel in the mixing chamber  218 , the fuel and air passing through the atomising nozzle  276  into the combustion chamber  630 . The flow through the high velocity region in the nozzle  276  causes the stream of fuel to be broken up. This improves the break up and atomisation of the stream of fuel as it is ejected from the sonic nozzle  226 . 
   The opening of the inlet valve  614  of the engine is delayed at the start of the intake stroke of the engine and movement of the piston  620  is used to create a partial vacuum in the combustion chamber  630 . The fuel is dispensed into the mixing chamber  218  with the partial vacuum drawing air from the air bypass passage  270  to entrain the fuel. An electrically operated valve  600  (comprising a spring biased valve member  602  and an electrical coil  603 ) is used to control flow of air through the air bypass passage  270  so that air can only be drawn through the passage  270  during the intake stroke of the engine (and not the expansion stroke) and so the gas cannot flow out of the combustion chamber  630  via the bypass passage  240 . The valve member  602  seals on a seat  650  to prevent flow of air from an air inlet  601  via connecting passage  651  to the air bypass passage  240 . 
   Whilst the passage  270  has been described above as an air bypass  270 , the passage  270  is not limited to supplying air but could alternatively be connected to a gas supply to provide an alternative gas to aid in atomisation or combustion. One such example of another gas that could be used is exhaust gas from the engine (i.e. exhaust gas recirculation). 
     FIG. 2  shows a second embodiment of the invention. This is similar to the embodiment shown in  FIG. 1 . The fuel injector is located for direct injection of fuel into the combustion chamber of the engine. However, this embodiment includes a different type of sonic nozzle. In this case, the sonic nozzle consists of an outer tube  710  through which fuel entrained in air (or exhaust gases) flows. A pintle  720  is provided across the end of the tube inside the combustion chamber. The closure is connected to an actuating rod  730  located centrally of the outer tube  710 . Importantly, the pintle  720  abuts against the outer tube  710 . The abutting surfaces of both the pintle  720  and the outer tube  710  are chamfered. 
   Fuel supplied by supply line  742  is dispensed from the fuel mixing chamber  216  of the injector  210 . At the same time the pintle closure is opened allowing fuel and air to be dispensed into the combustion chamber  630 . Air (or exhaust gases) flows through passage  741  to entrain the dispersed fuel in mixing chamber  743  and deliver it to the combustion chamber. The pintle  720  is opened only when the piston in the combustion chamber is moving to draw air into the cylinder in the intake stroke. The chamfered shape of the pintle causes a spray of fuel forming a conical shape extending outwards from the pintle. Actuation of the pintle may be by means of a solenoid  740  or other means. Again, in this embodiment there is no requirement to monitor and tightly regulate the pressure in the mixing chamber  743  or the combustion chamber. A sonic velocity is achieved as the fuel is forced through the narrow gap between the closure  720  and the tube  710 . 
   An engine with a fuel injection system as described above can be used to power a device such as a gardening device, e.g. a lawn mower, a hedge trimmer, a chain saw, a lawn aerator, a scarifier and a shredder. 
   The nozzle  276  can have orifices of different shapes such as shown in  FIGS. 4   a  to  4   d  to improve the atomisation of the fuel in the inlet passage. The orifice of a standard sonic nozzle, when a cross-section is taken perpendicular to the flow direction, is circular (see  FIG. 4   a ). Alternative shapes of the nozzle orifices may be provided, for example a linearly extending orifice ( FIG. 4   b ), a cruciform shape ( FIG. 4   c ) or alternatively a plurality of smaller dispersed nozzles, each having a circular orifice ( FIG. 4   d ). All of these allow the control of the fuel mist  230 . The plurality of smaller dispersed nozzles provides improved atomisation.