Patent Publication Number: US-10774774-B2

Title: Internal combustion engine having two fuel injectors per cylinder and control method therefor

Description:
CROSS-REFERENCE 
     The present application is a division of U.S. patent application Ser. No. 16/222,968, filed Dec. 17, 2018, which is a continuation of U.S. patent application Ser. No. 15/439,258, filed Feb. 22, 2017, which is a continuation-in-part of International Patent Application No. PCT/IB2016/053184, filed May 30, 2016, which claims priority to U.S. Provisional Application No. 62/298,053, filed Feb. 22, 2016, and U.S. Provisional Application No. 62/167,959, filed May 29, 2015, the entirety of all of which is incorporated herein by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The present technology relates to internal combustion engines having two fuel injectors per cylinder and methods for controlling such engines. 
     BACKGROUND 
     Two-stroke internal combustion engine burn a mixture of fuel and oil. Many two-stroke internal combustion engines use a carburetor to supply the mixture of fuel, oil and air to the combustion chambers of the engine. The mixture of fuel, oil and air flows from the carburetor, to the crankcase, then through scavenge ports to the combustion chambers. 
     Such carbureted engines have some drawbacks. They do not provide for a precise control of the fuel being supplied. They also produce a substantial amount of polluting emissions. 
     One of the advancements made to two-stroke internal combustion engines consists in replacing the carburetor with a throttle body and a port fuel injector that injects fuel upstream of the combustion chamber. The use of the port fuel injector allows for a more precise control of the fuel quantity being delivered and has helped reduce the amount of polluting emissions being produced. 
     In recent years, the port fuel injectors have been replaced in some two-stroke engines by direct fuel injectors such as the E-TEC™ fuel injector from BRP™. The direct fuel injectors inject fuel directly into the combustion chambers of the engine. As a result of the use of the direct fuel injectors, the engine performance has improved, even more precise control of the fuel quantity being injected is possible, less oil is used, and the amount of polluting emissions, such as carbon monoxide, has been reduced even more. 
     Although direct fuel injection has helped improved two-stroke engines, as the demand for even higher performance engines continues, the use direct fuel injectors has some drawbacks. For example, at very high engine speed (RPM), there is a very short period of time during which fuel can be injected and then mix with the air in the combustion chamber. As such, the fuel may not have the time to properly atomized, resulting in larger droplets of fuel being combusted, which emits more soot than when the fuel has properly atomized. 
     Although some of the above drawbacks could be resolved by switching to a four-stroke engine, doing so would result in losing the advantages typically associated with two-stroke engine, namely a simplified construction, more power (two-stroke engines have an explosion in each combustion chamber at every revolution, four-stoke engines at every two revolutions), and a lighter weight. These advantages of two-stroke engines are important features for vehicles such as motorcycles, snowmobiles and other recreational vehicles. 
     There is therefore a desire for a two-stroke engine having the advantages associated with direct fuel injection while addressing at least some of its drawbacks. 
     SUMMARY 
     It is an object of the present technology to ameliorate at least some of the inconveniences present in the prior art. 
     According to one aspect of the present technology, there is provided a method for controlling a two-stroke internal combustion engine. The engine has at least one combustion chamber, at least one direct fuel injector for injecting fuel directly in the at least one combustion chamber, and at least one port fuel injector for injecting fuel upstream of the at least one combustion chamber. The method comprises determining a first fuel quantity to be supplied to the at least one combustion chamber; determining a ratio of the first fuel quantity to be injected by the at least one direct fuel injector; determining a ratio of the first fuel quantity to be injected by the at least one port fuel injector; injecting a second fuel quantity in the at least one combustion chamber using the at least one direct fuel injector; injecting a third fuel quantity upstream of the at least one combustion chamber using the at least one port fuel injector, a sum of the second and third fuel quantities being initially greater than the first fuel quantity; and decreasing at least one of the second and third fuel quantities over time such that the sum of the second and third fuel quantities equals at least the first fuel quantity. 
     In some implementations of the present technology, the sum of the second and third fuel quantities is at least initially greater than the first fuel quantity when the ratio of the first fuel quantity to be injected by the at least one port fuel injector has changed from 0 percent to more than 0 percent. 
     In some implementations of the present technology, the first fuel quantity corresponds to a combination of a base fuel quantity and a correction factor. The base fuel quantity is determined based at least in part on engine speed and throttle position. The correction factor is determined based at least in part on at least one of atmospheric pressure, air temperature, engine temperature and exhaust temperature. 
     In some implementations of the present technology, the second fuel quantity is initially greater than the ratio of the first fuel quantity to be injected by the at least one direct fuel injector. The third fuel quantity is initially greater than the ratio of the first fuel quantity to be injected by the at least one port fuel injector. 
     In some implementations of the present technology, the second fuel quantity is initially the first fuel quantity, is held constant for a first period of time and is then decreased until the second fuel quantity equals the ratio of the first fuel quantity to be injected by the at least one direct fuel injector. 
     In some implementations of the present technology, following the first period of time, the second fuel quantity is held constant for a second period of time at a value between the first fuel quantity and the ratio of the first fuel quantity to be injected by the at least one direct fuel injector. 
     In some implementations of the present technology, following the second period of time, the second fuel quantity is reduced linearly until the second fuel quantity equals the ratio of the first fuel quantity to be injected by the at least one direct fuel injector. 
     In some implementations of the present technology, the third fuel quantity is initially a value between the first fuel quantity and the ratio of the first fuel quantity to be injected by the at least one port fuel injector and is then decreased until the third fuel quantity equals at least the ratio of the first fuel quantity to be injected by the at least one port fuel injector. 
     In some implementations of the present technology, the third fuel quantity is decreased linearly. 
     In some implementations of the present technology, the third fuel quantity corresponds to a combination of the ratio of the first fuel quantity to be injected by the at least one port fuel injector and at least one correction factor. 
     In some implementations of the present technology, the at least one correction factor includes a phase-in correction factor, the phase-in correction factor being greater than or equal to 100 percent. The third fuel quantity corresponds to the ratio of the first fuel quantity to be injected by the at least one port fuel injector multiplied by the phase-in correction factor. 
     In some implementations of the present technology, the at least one correction factor further includes a fuel trapping efficiency correction factor based at least in part on engine speed and throttle position, the fuel trapping efficiency correction factor being greater than 100 percent. The third fuel quantity corresponds to the ratio of the first fuel quantity to be injected by the at least one port fuel injector multiplied by the phase-in correction factor, multiplied by the fuel trapping efficiency correction factor. 
     In some implementations of the present technology, decreasing at least one of the second and third fuel quantities over time comprises decreasing both the second and third fuel quantities over time. 
     In some implementations of the present technology, the second fuel quantity is decreased faster than the third fuel quantity. 
     In some implementations of the present technology, the second fuel quantity is greater than the ratio of the first fuel quantity to be injected by the at least one direct fuel injector; and the third fuel quantity is equal to the ratio of the first fuel quantity to be injected by the at least one port fuel injector. 
     In some implementations of the present technology, the second fuel quantity is initially greater than the first fuel quantity. 
     In some implementations of the present technology, the second fuel quantity is then decreased until the second fuel quantity equals at least the ratio of the first fuel quantity to be injected by the at least one direct fuel injector. 
     In some implementations of the present technology, the engine has at least one exhaust valve movable between at least a fully lowered position and a fully opened position. When the at least one exhaust valve is in the fully opened position, the second fuel quantity to be injected by the at least one direct fuel injector is greater than 0 and the third fuel quantity to be injected by the at least one port fuel injector is greater than 0. 
     In some implementations of the present technology, the at least one direct fuel injector supplies fuel to produce one of a stratified charge and a homogeneous charge. When the at least one direct fuel injector supplies fuel to produce the stratified charge, the third fuel quantity to be injected by the at least one port fuel injector is 0. 
     According to another aspect of the present technology, there is provided a method for controlling a two-stroke internal combustion engine. The engine has at least one combustion chamber, at least one direct fuel injector for injecting fuel directly in the at least one combustion chamber, and at least one port fuel injector for injecting fuel upstream of the at least one combustion chamber. The method comprises: determining a first fuel quantity to be supplied to the at least one combustion chamber; determining a ratio of the first fuel quantity to be injected by the at least one direct fuel injector; and determining a ratio of the first fuel quantity to be injected by the at least one port fuel injector. If the ratio of the first fuel quantity to be injected by the at least one port fuel injector has changed from more than 0 percent to 0 percent: injecting a second fuel quantity in the at least one combustion chamber using the at least one direct fuel injector, the second fuel quantity being initially less than the first fuel quantity; stopping to inject fuel using the at least one port fuel injector; and increasing the second fuel quantity over time such that the second fuel quantity equals the first fuel quantity. 
     In some implementations of the present technology, the first fuel quantity corresponds to a combination of a base fuel quantity and a correction factor. The base fuel quantity is determined based at least in part on engine speed and throttle position. The correction factor is determined based at least in part on at least one of atmospheric pressure, air temperature, engine temperature and exhaust temperature. 
     In some implementations of the present technology, the second fuel quantity is initially held constant for a first period of time and is then increased until the second fuel quantity equals the first fuel quantity. 
     In some implementations of the present technology, following the first period of time, the second fuel quantity is increased linearly until the second fuel quantity equals the first fuel quantity. 
     According to another aspect of the present technology, there is provided a method for controlling a two-stroke internal combustion engine. The engine has at least one combustion chamber, at least one direct fuel injector for injecting fuel directly in the at least one combustion chamber, and at least one port fuel injector for injecting fuel upstream of the at least one combustion chamber. The method comprises: determining a first fuel quantity to be supplied to the at least one combustion chamber; determining a ratio of the first fuel quantity to be injected by the at least one direct fuel injector; determining a ratio of the first fuel quantity to be injected by the at least one port fuel injector; determining which of a phase-in control, a phase-out control, and a regular control is to be used, the phase-in control being used when the ratio of the first fuel quantity to be injected by the at least one port fuel injector has changed from 0 percent to more than 0 percent, the phase-out control being used when the ratio of the first fuel quantity to be injected by the at least one port fuel injector has changed from more than 0 percent to 0 percent, the regular control being used when the ratio of the first fuel quantity to be injected by the at least one port fuel injector has remained at 0 percent or has remained above 0 percent; injecting fuel using at least one of the at least one direct fuel injector and the at least one port fuel injector according to the one of the phase-in control, the phase-out control and the regular control that has been determined; when fuel is injected according to the phase-in control, determining if a condition that would result into too much fuel being supplied to the at least one combustion chamber is present; and if the condition is present, reducing a quantity of fuel being injected during phase-in control. 
     In some implementations of the present technology, the condition corresponds to the phase-in control having been used more than or equal to a predetermined number of times within one of a predetermined amount of time and a predetermined amount of engine cycles. The quantity of fuel being injected during the phase-in control is reduced if the phase-in control has been used more than or equal to a predetermined number of times. 
     In some implementations of the present technology, during phase-in control, injecting fuel comprises: injecting a second fuel quantity in the at least one combustion chamber using the at least one direct fuel injector; injecting a third fuel quantity upstream of the at least one combustion chamber using the at least one port fuel injector, a sum of the second and third fuel quantities being initially greater than the first fuel quantity; and decreasing at least one of the second and third fuel quantities over time such that the sum of the second and third fuel quantities equals a quantity of fuel to be injected during regular control for a corresponding engine speed and throttle position. 
     In some implementations of the present technology, decreasing at least one of the second and third fuel quantities over time comprises decreasing both the second and third fuel quantities over time. 
     In some implementations of the present technology, during phase-out control, injecting fuel comprises: injecting a second fuel quantity in the at least one combustion chamber using the at least one direct fuel injector, the second fuel quantity being initially less than the first fuel quantity; stopping to inject fuel using the at least one port fuel injector; and increasing the second fuel quantity over time such that the second fuel quantity equals a quantity of fuel to be injected during regular control for a corresponding engine speed and throttle position. 
     According to another aspect of the present technology, there is provided a two-stroke internal combustion engine having a crankcase, a cylinder block connected to the crankcase, the cylinder block defining at least one cylinder, each of the at least one cylinder defining at least one exhaust port, a cylinder head connected to the cylinder block, the cylinder block being disposed between the crankcase and the cylinder head, a crankshaft disposed at least in part in the crankcase, at least one piston disposed in the at least one cylinder, the cylinder head, the at least one cylinder and the at least one piston defining at least one combustion chamber, a least one connecting rod connecting the at least one piston to the crankshaft, at least one scavenge port fluidly communicating an interior of the crankcase with the at least one combustion chamber, at least one throttle body connected to at least one of the crankcase and the cylinder block for supplying air to the interior of the crankcase via at least one air intake port, air flowing from the at least one throttle body to the interior of the crankcase, from the interior of the crankcase to the at least one scavenge port, and from the at least one scavenge port to the at least one combustion chamber, each of the at least one throttle body having a throttle plate, at least one direct fuel injector fluidly communicating with the at least one combustion chamber for injecting fuel directly in the at least one combustion chamber, and at least one port fuel injector fluidly communicating with the interior of the crankcase for injecting fuel upstream of the at least one combustion chamber. 
     In some implementations of the present technology, the at least one port fuel injector injects fuel directly in one of the at least one throttle body, the interior of the crankcase and the at least one scavenge port. 
     In some implementations of the present technology, the at least one port fuel injector is connected to the at least one throttle body and injects fuel directly in the at least one throttle body. 
     In some implementations of the present technology, the at least one direct fuel injector is connected to the cylinder head. 
     In some implementations of the present technology, each of the at least one port fuel injector injects fuel upstream of a corresponding one of the at least one air intake port and downstream of the throttle plate of a corresponding one of the at least one throttle body. 
     In some implementations of the present technology, the at least one air intake port is formed in the cylinder block. 
     In some implementations of the present technology, the at least one cylinder is two cylinders, the at least one exhaust port is at least two exhaust ports, the at least one piston is two pistons, the at least one combustion chamber is two combustion chambers, the at least one connecting rod is two connecting rods, the at least one scavenge port is at least two scavenge ports, the at least one throttle body is two throttle bodies, the at least one air intake port is at least two air intake ports, the at least one direct fuel injector is two direct fuel injectors, and the at least one port fuel injector is two port fuel injectors. 
     Implementations of the present technology each have at least one of the above-mentioned object and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present technology that have resulted from attempting to attain the above-mentioned object may not satisfy this object and/or may satisfy other objects not specifically recited herein. 
     Additional and/or alternative features, aspects and advantages of implementations of the present technology will become apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present technology, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
         FIG. 1  is a right side perspective view of a snowmobile; 
         FIG. 2  is a perspective view taken from a front, left side of the internal combustion engine of the snowmobile of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the engine of  FIG. 2  taken through line  3 - 3  of  FIG. 4 ; 
         FIG. 4  is a cross-sectional view of the engine of  FIG. 2  taken through line  4 - 4  of  FIG. 3  with a drive pulley of a CVT mounted on a crankshaft of the engine; 
         FIG. 5  is a schematic diagram of an electronic control unit (ECU) of the snowmobile of  FIG. 1  and various components connected to the ECU; 
         FIG. 6  is a logic diagram of a method for controlling the engine of  FIG. 2 ; 
         FIG. 7  is a graph illustrating a phase-in control of the method of  FIG. 6 ; 
         FIG. 8A  illustrates a fuel ratio map; 
         FIG. 8B  illustrates an exhaust valves position map; 
         FIG. 9  illustrates a fuel trapping correction map; 
         FIG. 10  is a graph illustrating a phase-out control of the method of  FIG. 6 ; 
         FIG. 11  is a logic diagram of an alternative method for controlling the engine of  FIG. 2 ; 
         FIG. 12  is a graph illustrating a phase-in control according to the method of  FIG. 11 ; and 
         FIG. 13  is a graph illustrating a phase-out control according to the method of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     The method for controlling an internal combustion engine will be described with respect to a snowmobile  10 . However, it is contemplated that the method and system could be used in other vehicles, such as, but not limited to, on-road vehicles, off-road vehicles, a motorcycle, a scooter, a three-wheel road vehicle, a boat powered by an outboard engine or an inboard engine, and an all-terrain vehicle (ATV). It is also contemplated that the method and system could be used in devices other than vehicles that have an internal combustion engine such as a generator. The method and system will also be described with respect to a two-stroke, inline, two-cylinder internal combustion engine  24 . However, it is contemplated that the method and system could be used with an internal combustion engine having more than two cylinders or having a configuration other than inline, such as a V-type engine. 
     Turning now to  FIG. 1 , a snowmobile  10  includes a forward end  12  and a rearward end  14  that are defined consistently with a forward travel direction of the snowmobile  10 . The snowmobile  10  includes a frame  16  that has a tunnel  18 , an engine cradle portion  20  and a front suspension assembly portion  22 . The tunnel  18  consists of one or more pieces of sheet metal arranged to form an inverted U-shape that is connected at the front to the engine cradle portion  20  and extends rearward therefrom along the longitudinal axis  23 . An internal combustion engine  24  (schematically illustrated in  FIG. 1 ) is carried by the engine cradle portion  20  of the frame  16 . The internal combustion engine  24  is described in greater detail below. Two skis  26  are positioned at the forward end  12  of the snowmobile  10  and are attached to the front suspension assembly portion  22  of the frame  16  through a front suspension assembly  28 . The front suspension assembly  28  includes shock absorber assemblies  29 , ski legs  30 , and supporting arms  32 . Ball joints and steering rods (not shown) operatively connect the skis  26  to a steering column  34 . A steering device in the form of handlebar  36  is attached to the upper end of the steering column  34  to allow a driver to rotate the ski legs  30  and thus the skis  26 , in order to steer the snowmobile  10 . 
     An endless drive track  38  is disposed generally under the tunnel  18  and is operatively connected to the engine  24  through a CVT  40  (schematically illustrated by broken lines in  FIG. 1 ) which will be described in greater detail below. The endless drive track  38  is driven to run about a rear suspension assembly  42  for propulsion of the snowmobile  10 . The rear suspension assembly  42  includes a pair of slide rails  44  in sliding contact with the endless drive track  38 . The rear suspension assembly  42  also includes a plurality of shock absorbers  46  which may further include coil springs (not shown) surrounding one or more of the shock absorbers  46 . Suspension arms  48  and  50  are provided to attach the slide rails  44  to the frame  16 . A plurality of idler wheels  52  are also provided in the rear suspension assembly  42 . Other types and geometries of rear suspension assemblies are also contemplated. 
     At the forward end  12  of the snowmobile  10 , fairings  54  enclose the engine  24  and the CVT  40 , thereby providing an external shell that protects the engine  24  and the CVT  40 . The fairings  54  include a hood and one or more side panels that can be opened to allow access to the engine  24  and the CVT  40  when this is required, for example, for inspection or maintenance of the engine  24  and/or the CVT  40 . A windshield  56  is connected to the fairings  54  near the forward end  12  of the snowmobile  10 . Alternatively the windshield  56  could be connected directly to the handlebar  36 . The windshield  56  acts as a wind screen to lessen the force of the air on the driver while the snowmobile  10  is moving forward. 
     A straddle-type seat  58  is positioned over the tunnel  18 . Two footrests  60  are positioned on opposite sides of the snowmobile  10  below the seat  58  to accommodate the driver&#39;s feet. 
     Turning now to  FIGS. 2 to 4 , the internal combustion engine  24  and the CVT  40  will be described. The internal combustion engine  24  operates on the two-stroke principle. The engine  24  has a crankshaft  100  that rotates about a horizontally disposed axis that extends generally transversely to the longitudinal axis  23  of the snowmobile  10 . The crankshaft  100  drives the CVT  40  for transmitting torque to the endless drive track  38  for propulsion of the snowmobile  10 . 
     The CVT  40  includes a drive pulley  62  ( FIG. 4 ) coupled to the crankshaft  100  to rotate with the crankshaft  100  and a driven pulley (not shown) coupled to one end of a transversely mounted jackshaft (not shown) that is supported on the frame  16  through bearings. The opposite end of the transversely mounted jackshaft is connected to the input member of a reduction drive (not shown) and the output member of the reduction drive is connected to a drive axle (not shown) carrying sprocket wheels (not shown) that form a driving connection with the drive track  38 . 
     As can be seen in  FIG. 4 , the drive pulley  62  of the CVT  40  includes a pair of opposed frustoconical belt drive sheaves  64  and  66  between which a drive belt (not shown) is located. The drive belt is made of rubber, but it is contemplated that it could be made of metal linkages or of a polymer. The drive pulley  62  will be described in greater detail below. The driven pulley includes a pair of frustoconical belt drive sheaves between which the drive belt is located. The drive belt is looped around both the drive pulley  62  and the driven pulley. The torque being transmitted to the driven pulley provides the necessary clamping force on the drive belt through its torque sensitive mechanical device in order to efficiently transfer torque to the other powertrain components. 
     As discussed above, the drive pulley  62  includes a pair of opposed frustoconical belt drive sheaves  64  and  66  as can be seen in  FIG. 4 . Both sheaves  64  and  66  rotate together with the crankshaft  100 . The sheave  64  is fixed in an axial direction relative to the crankshaft  100 , and is therefore referred to as the fixed sheave  64 . The fixed sheave  64  is also rotationally fixed relative to the crankshaft  100 . The sheave  66  can move toward or away from the fixed sheave  64  in the axial direction of the crankshaft  100  in order to change the drive ratio of the CVT  40 , and is therefore referred to as the movable sheave  66 . As can be seen in  FIG. 4 , the fixed sheave  64  is disposed between the movable sheave  66  and the engine  24 . 
     The fixed sheave  64  is mounted on a fixed sheave shaft  68 . The fixed sheave  64  is press-fitted on the fixed sheave shaft  68  such that the fixed sheave  64  rotates with the fixed sheave shaft  68 . It is contemplated that the fixed sheave  64  could be connected to the fixed sheave shaft  68  in other known manners to make the fixed sheave  64  rotationally and axially fixed relative to the fixed sheave shaft  68 . As can be seen in  FIG. 5 , the fixed sheave shaft  68  is hollow and has a tapered hollow portion. The tapered hollow portion receives the end of the crankshaft  100  therein to transmit torque from the engine  24  to the drive pulley  62 . A fastener  70  is inserted in the outer end (i.e. the left side with respect to  FIG. 4 ) of the drive pulley  62 , inside the fixed sheave shaft  68 , and screwed into the end of the crankshaft  100  to prevent axial displacement of the fixed sheave shaft  68  relative to the crankshaft  100 . It is contemplated that the fixed sheave shaft  68  could be connected to the crankshaft  100  in other known manners to make the fixed sheave shaft  68  rotationally and axially fixed relative to the crankshaft  100 . It is also contemplated that the crankshaft  100  could be the fixed sheave shaft  68 . 
     A cap  72  is taper-fitted in the outer end of the fixed sheave shaft  68 . The fastener  70  is also inserted through the cap  72  to connect the cap  72  to the fixed sheave shaft  68 . It is contemplated that the cap  72  could be connected to the fixed sheave shaft  68  by other means. The radially outer portion of the cap  72  forms a ring  74 . An annular rubber damper  76  is connected to the ring  74 . Another ring  78  is connected to the rubber damper  76  such that the rubber damper  76  is disposed between the rings  74 ,  78 . In the present implementation, the rubber damper  76  is vulcanized to the rings  74 ,  78 , but it is contemplated that they could be connected to each other by other means such as by using an adhesive for example. It is also contemplated that the damper  76  could be made of a material other than rubber. 
     A spider  80  is disposed around the fixed sheave shaft  68  and axially between the ring  78  and the movable sheave  66 . The spider  80  is axially fixed relative to the fixed sheave  64 . Apertures (not shown) are formed in the ring  74 , the damper  76 , and the ring  78 . Fasteners (not shown) are inserted through the apertures in the ring  74 , the damper  76 , the ring  78  and the spider  80  to fasten the ring  78  to the spider  80 . As a result, torque is transferred between the fixed sheave shaft  68  and the spider  80  via the cap  72 , the rubber damper  76  and the ring  78 . The damper  76  dampens the torque variations from the fixed sheave shaft  68  resulting from the combustion events in the engine  24 . The spider  80  therefore rotates with the fixed sheave shaft  68 . 
     A movable sheave shaft  82  is disposed around the fixed sheave shaft  68 . The movable sheave  66  is press-fitted on the movable sheave shaft  82  such that the movable sheave  66  rotates and moves axially with the movable sheave shaft  82 . It is contemplated that the movable sheave  66  could be connected to the movable sheave shaft  82  in other known manners to make the movable sheave  66  rotationally and axially fixed relative to the shaft  82 . It is also contemplated that the movable sheave  66  and the movable sheave shaft  82  could be integrally formed. 
     To transmit torque from the spider  80  to the movable sheave  104 , a torque transfer assembly consisting of three roller assemblies  84  connected to the movable sheave  66  is provided. The roller assemblies  84  engage the spider  80  so as to permit low friction axial displacement of the movable sheave  66  relative to the spider  80  and to eliminate, or at least minimize, rotation of the movable sheave  66  relative to the spider  80 . As described above, torque is transferred from the fixed sheave  64  to the spider  80  via the damper  76 . The spider  80  engages the roller assemblies  84  which transfer the torque to the movable sheave  66  with no, or very little, backlash. As such, the spider  80  is considered to be rotationally fixed relative to the movable sheave  66 . It is contemplated that in some implementations, the torque transfer assembly could have more or less than three roller assemblies  84 . 
     As can be seen in  FIG. 4 , a biasing member in the form of a coil spring  86  is disposed inside a cavity  88  defined radially between the movable sheave shaft  82  and the spider  80 . As the movable sheave  66  and the movable sheave shaft  82  move axially toward the fixed sheave  64 , the spring  86  gets compressed. The spring  86  biases the movable sheave  66  and the movable sheave shaft  82  away from the fixed sheave  64  toward their position shown in  FIG. 5 . It is contemplated that, in some implementations, the movable sheave  66  could be biased away from the fixed sheave  64  by mechanisms other than the spring  86 . 
     The spider  80  has three arms  90  disposed at 120 degrees from each other. Three rollers  92  are rotatably connected to the three arms  90  of the spider  80 . Three centrifugal actuators  94  are pivotally connected to three brackets (not shown) formed by the movable sheave  66 . Each roller  92  is aligned with a corresponding one of the centrifugal actuators  94 . Since the spider  80  and the movable sheave  66  are rotationally fixed relative to each other, the rollers  92  remain aligned with their corresponding centrifugal actuators  94  when the shafts  68 ,  82  rotate. The centrifugal actuators  94  are disposed at 120 degrees from each other. The centrifugal actuators  94  and the roller assemblies  84  are arranged in an alternating arrangement and are disposed at 60 degrees from each other. It is contemplated that the rollers  92  could be pivotally connected to the brackets of the movable sheave  66  and that the centrifugal actuators  94  could be connected to the arms  90  of the spider  80 . It is also contemplated that there could be more or less than three centrifugal actuators  94 , in which case there would be a corresponding number of arms  90 , rollers  92  and brackets of the movable sheave. It is also contemplated that the rollers  92  could be omitted and replaced with surfaces against which the centrifugal actuators  94  can slide as they pivot. 
     In the present implementation, each centrifugal actuator  94  includes an arm  96  that pivots about an axle  98  connected to its respective bracket of the movable sheave  66 . The position of the arms  96  relative to their axles  98  can be adjusted. It is contemplated that the position of the arms  96  relative to their axles  98  could not be adjustable. Additional detail regarding centrifugal actuators of the type of the centrifugal actuator  94  can be found in International Application Publication No. WO 2013/032463 A2, published Mar. 7, 2013, the entirety of which is incorporated herein by reference. 
     The above description of the drive pulley  62  corresponds to one contemplated implementation of a drive pulley that can be used with the engine  24 . It is contemplated that other types of drive pulleys could be used. 
     The engine  24  has a crankcase  102  housing a portion of the crankshaft  100 . As can be seen in  FIGS. 2 and 4 , the crankshaft  100  protrudes from the crankcase  102 . It is contemplated that the crankshaft  100  could drive an output shaft connected directly to the end of the crankshaft  100  or offset from the crankshaft  100  and driven by driving means such as gears in order to drive the drive pulley  62 . It is also contemplated that the crankshaft  100  could drive, using gears for example, a counterbalance shaft housed in part in the crankcase  102  and that the drive pulley  62  could be connected to the counterbalance shaft, in which case, the crankshaft  100  does not have to protrude from the crankcase  102  for this purpose. A cylinder block  104  is disposed on top of and connected to the crankcase  102 . The cylinder block  104  defines two cylinders  106 A,  106 B ( FIG. 5 ). A cylinder head  108  is disposed on top of and is connected to the cylinder block  104 . 
     As best seen in  FIG. 4 , the crankshaft  100  is supported in the crankcase  102  by bearings  110 . The crankshaft  100  has two crank pins  112 A,  112 B. In the illustrated implementation where the two cylinders  106 A,  106 B are disposed in line, the crank pins  112 A,  112 B are provided at 180 degrees from each other. It is contemplated that the crank pins  112 A,  112 B could be provided at other angles from each other to account for other cylinder arrangements, such as in a V-type engine. A connecting rod  114 A is connected to the crank pin  112 A at one end and to a piston  116 A via a piston pin  118 A at the other end. As can be seen, the piston  116 A is disposed in the cylinder  106 A. Similarly, a connecting rod  114 B is connected to the crank pin  112 B at one end and to a piston  116 B via a piston pin  118 B at the other end. As can be seen, the piston  116 B is disposed in the cylinder  106 B. Rotation of the crankshaft  100  causes the pistons  116 A,  116 B to reciprocate inside their respective cylinders  106 A,  106 B. The cylinder head  108 , the cylinder  106 A and the piston  116 A define a variable volume combustion chamber  120 A therebetween. Similarly, the cylinder head  108 , the cylinder  106 B and the piston  116 B define a variable volume combustion chamber  120 B therebetween. It is contemplated that the cylinder block  104  could define more than two cylinders  106 , in which case the engine  24  would be provided with a corresponding number of pistons  116  and connecting rods  114 . 
     Air is supplied to the crankcase  102  via a pair of air intake ports  122  (only one of which is shown in  FIG. 3 ) formed in the back of the cylinder block  104 . It is contemplated that the air intake ports  122  could be formed in the crankcase  102 . It is also contemplated that there could be more than one air intake port  122  per cylinder  106 . A pair of throttle bodies  124  is connected to the pair of air intake ports  122 . Each throttle body  124  has a throttle plate  126  that can be rotated to control the air flow to the engine  24 . One or more throttle cables connected to a throttle lever are used to change to position of the throttle plates  126 . In an alternative implementation, a throttle motor  127  (schematically shown in dotted lines in  FIG. 5 ) could be used to change the position of the throttle plates  126 . It is also contemplated that each throttle plate  126  could be actuated by its own throttle motor  127 . A pair of port fuel injectors  125 A,  125 B is connected to the pair of throttle bodies  124  (i.e. one fuel injector  125  per throttle body  124 ) to inject fuel directly in the throttle bodies  124  as will be described in greater detail below. It is contemplated that the fuel injectors  125 A,  125  could alternatively be connected to the crankcase  102  and/or the cylinder block  104  to inject fuel directly in the crankcase  102  or in the scavenge ports  130 . A pair of reed valves  128  ( FIG. 4 ) are provided in each intake port  122 . The reed valves  128  allow air and fuel to enter the crankcase  102 , but prevent air from flowing out of the crankcase  102  via the air intake ports  122 . 
     As the pistons  116 A,  116 B reciprocate, air from the crankcase  102  flows into the combustion chambers  120 A,  120 B via scavenge ports  130 . It is contemplated that each combustion chamber  120 A,  120 B could communicate with multiple scavenge ports  130 . Fuel is injected directly in the combustion chambers  120 A,  120 B by direct fuel injectors  132 A,  132 B respectively as will be described in greater detail below. The direct fuel injectors  132 A,  132 B are mounted to the cylinder head  108 . In the illustrated implementation, the direct fuel injectors  132 A,  132 B are E-TEC TM  fuel injectors, however other types of direct fuel injectors are contemplated. The direct fuel injectors  132 A,  132 B can supply a fuel to produce a stratified charge or a homogeneous charge depending on the operating conditions of the engine  24 . The fuel-air mixture in the combustion chamber  120 A,  120 B is ignited by spark plugs  134 A,  134 B respectively (not shown in  FIGS. 2 to 4 , but schematically illustrated in  FIG. 5 ). The spark plugs  134 A,  134 B are mounted to the cylinder head  108 . 
     To evacuate the exhaust gases resulting from the combustion of the fuel-air mixture in the combustion chambers  120 A,  120 B, each cylinder  116 A,  116 B defines one main exhaust port  136 A,  136 B respectively and two auxiliary exhaust ports  138 A,  138 B respectively. It is contemplated that each cylinder  116 A,  116 B could have only one, two or more than three exhaust ports. The exhaust ports  136 A,  136 B,  138 A,  138 B are connected to an exhaust manifold  140 . The exhaust manifold  140  is connected to the front of the cylinder block  104 . Exhaust valves  142 A,  142 B mounted to the cylinder block  104 , control a degree of opening of the exhaust ports  136 A,  136 B,  138 A,  138 B. In the present implementation, the exhaust valves  142 A,  142 B are R.A.V.E.™ exhaust valves, but other types of valves are contemplated. It is also contemplated that the exhaust valves  142 A,  142 B could be omitted. It is also contemplated that the auxiliary exhaust ports  138 A,  138 B could be omitted. 
     The position of the exhaust valves  142 A,  142 B is determined by an electronic control unit (ECU)  164 , described in more detail below, at least in part based on the throttle position and the engine speed. The ECU  164  makes this determination using an exhaust valves position map  167  shown in  FIG. 8B . It should be understood that the exhaust valves position map  167  shown in  FIG. 8B  is an exemplary map. Different engines and/or desired performance characteristics could require different exhaust valves position maps. In the exhaust valves position map  167 , the throttle position is given as a percentage of opening of the throttle plate  126 , with 0% being a minimum position of the throttle plate  126  and 100% being a wide-open throttle plate position. It is also contemplated that the position of the exhaust valves  142 A,  142 B could be determined using one or more algorithms. It is also contemplated that the ECU  164  could have multiple exhaust valves position maps  167  corresponding to different modes of operation of the engine  24  that can be selected by a user of the snowmobile  10 . 
     As can be seen in the exhaust positions map  167  shown in  FIG. 8B , in the present implementations, the exhaust valves  142 A,  142 B can have one of four positions: a fully lowered (FL) position, a first intermediate (I 1 ) position, a second intermediate (I 2 ) position, and a fully opened (FO) position. These positions have also been labeled in  FIG. 3 . It is contemplated that the exhaust valves  142 A,  142 B could have less or more than four positions. The FL position is the position in which the exhaust valves  142 A,  142 B restrict fluid flow through the main exhaust ports  138 A,  138 B the most. The FO position is the position in which the exhaust valves  142 A,  142 B restrict fluid flow through the main exhaust ports  138 A,  138 B the least or, in some implementations, not at all. The I 1  position is a position of the exhaust valves  142 A,  142 B that is intermediate the FL and FO positions. The I 2  position is a position of the exhaust valves  142 A,  142 B that is intermediate the I 1  and FO positions. Once the ECU  164  has determined the position of the exhaust valves  142 A,  142 B, the ECU  164  sends a signal to an exhaust valves actuator  186  ( FIG. 5 ) to move the exhaust valves  142 A,  142 B to this position. In the present implementation, the exhaust valves actuator  186  is an electric motor that pushes or pulls on a push-pull cable  187  ( FIG. 4 ) that moves both exhaust valves  142 A,  142 B together. 
     An alternator  144  ( FIG. 4 ) is connected to the end of the crankshaft  100  opposite the end of the crankshaft  100  that is connected to the drive pulley  62 . It is contemplated that the alternator  144  could be connected to another shaft operatively connected to the crankshaft  100 , by gears for example. The alternator  144  is turned by the crankshaft  100  and generates electricity that is supplied to a battery (not shown) and to other electrical components of the engine  24  and the snowmobile  10 . 
     As can be seen in  FIG. 4 , the alternator  144  has a stator  148  and a rotor  150 . The stator  148  is disposed around the crankshaft  100  outside of the crankcase  102  and is fastened to the crankcase  102 . The rotor  150  is connected by splines to the end of the crankshaft  100  and partially houses the stator  148 . A housing  152  is disposed over the alternator  144  and is connected to the crankcase  102 . A cover  154  is connected to the end of the housing  152 . 
     As can also be seen in  FIG. 4 , a recoil starter  156  is disposed inside the space defined by the housing  152  and the cover  154 , between the cover  154  and the alternator  144 . The recoil starter  156  has a rope  158  wound around a reel  160 . A ratcheting mechanism  162  selectively connects the reel  160  to the rotor  150 . To start the engine  24  using the recoil starter  156 , a user pulls on a handle  163  ( FIG. 3 ) connected to the end of the rope  158 . This turns the reel  160  in a direction that causes the ratcheting mechanism  162  to lock, thereby turning the rotor  150  and the crankshaft  100 . The rotation of the crankshaft  100  causes the pistons  116 A,  116 B to reciprocate which permits fuel injection and ignition to occur, thereby starting the engine  24 . When the engine  24  starts, the rotation of the crankshaft  100  relative to the reel  160  disengages the ratcheting mechanism  162 , and as such the crankshaft  100  does not turn the reel  160 . When the user releases the handle  163 , a spring (not shown) turns the reel  160  thereby winding the rope  158  around the reel  160 . It is contemplated that the recoil starter  156  could be omitted. 
     In the present implementation, the drive pulley  62  and the alternator  144  are both mounted to the crankshaft  100 . It is contemplated that the drive pulley  62  and the alternator  144  could both be mounted to a shaft other than the crankshaft  100 , such as a counterbalance shaft for example. In the present implementation, the drive pulley  62 , the alternator  144  and the recoil starter  56  are all coaxial with and rotate about the axis of rotation of the crankshaft  100 . It is contemplated that the drive pulley  62 , the alternator  144  and the recoil starter  56  could all be coaxial with and rotate about the axis of rotation of a shaft other than the crankshaft  100 , such as a counterbalance shaft for example. It is also contemplated that at least one of the drive pulley  62 , the alternator  144  and the recoil starter  56  could rotate about a different axis. In the present implementation, the drive pulley  62  is disposed on one side of the engine  24  and the alternator  144  and the recoil starter  56  are both disposed on the other side of the engine  24 . It is contemplated the alternator  144  and/or the recoil starter  56  could be disposed on the same side of the engine  24  as the drive pulley  62 . 
     The fuel injectors  125 A and  132 A will now be described in more detail with respect to  FIG. 3 . The fuel injectors  125 B and  132 B are similarly arranged with respect to the components associated with the combustion chamber  120 B and as such will not be described in greater detail herein. 
     The port fuel injector  125 A injects fuel directly in the throttle body  124  that fluidly communicates with the combustion chamber  120 A at a location between the throttle plate  126  and the air intake port  122 . It is contemplated that the port fuel injector  125 A could inject fuel at other positions upstream of the combustion chamber  120 A, such as in the interior of the crankcase  102  or the scavenge port  130 . It is also contemplated that the port fuel injector  125 A could inject fuel at positions upstream of the throttle plate  126 . The fuel injected by the port fuel injector  125 A flows with the air flowing through the throttle body  124  into the crankcase  102 , then through the scavenge port  130  and into the combustion chamber  120 A to be combusted. The port fuel injector  125 A is connected to the top of the throttle body  124 . The port fuel injector  125 A is angled relative to a central axis of the throttle body  124  such that fuel is injected by the port fuel injector  125 A flows generally toward the bottom of the throttle body  124  and toward the air intake port  122 . When the port fuel injector  125 A is initially actuated following a period where it has not been used to inject fuel while the engine  24  is in operation, a portion of the fuel injected by the port fuel injector  125 A that flows into the crankcase  102  sticks to surfaces of the engine  24  that are downstream of the port fuel injector  125 A, such as the surfaces of the interior of the crankcase  102 , of components of the engine  24  that are in the crankcase  102 , and of the scavenge port  130 . The method described in detail below compensates for this portion of fuel that does not reach the combustion chamber  120 A. 
     The direct fuel injector  132 A injects fuel directly into the combustion chamber  120 A toward the piston  116 A. More specifically, the direct fuel injector  132 A injects fuel into a domed portion of the combustion chamber  120 A defined by the cylinder head  108 . The direct fuel injector  132 A injects fuel at an angle to a reciprocation axis of the piston  116 A such that fuel injected by the direct fuel injector  132 A flows generally toward the piston  116 A and away from the exhaust ports  136 A,  138 A. 
     The fuel injectors  125 A,  125 B,  132 A,  132 B are connected by fuel lines and/or rails (not shown) to one or more fuel pumps that pump fuel from a fuel tank  161  ( FIG. 1 ) of the snowmobile  10 . 
     Turning now to  FIG. 5 , the ECU  164  of the snowmobile  10  and various components connected to the ECU  164  will be described. The ECU  164  is used to control the operation of the engine  24  by control the actuation of its components such as the fuel injectors  125 A,  125 B,  132 A,  132 B, the spark plugs  134 A,  134 B and the throttle motor  127  (should one be provided). Although a single ECU  164  is illustrated, it is contemplated that the various tasks of the ECU  164  could be split between various electronic modules. To control the operation of the engine  24 , the ECU  164  receives multiple inputs from sensors which will be described below. Using these inputs, the ECU  164  obtains information from control maps, such as the control maps  166 ,  168  which are described in greater detail below, and control map  167  described above, and uses information from these maps to control the engine  24 . The control maps are stored in an electronic data storage device, such as a hard disk drive or a flash drive. It is contemplated that instead of or in addition to the control maps, the ECU  164  could use control algorithms to control the engine  24 . In the present implementation, the ECU  164  is connected with the various components illustrated in  FIG. 5  via wired connections; however it is contemplated that it could be connected to one or more of these components wirelessly. 
     An engine speed sensor  170  is disposed in the vicinity of the crankshaft  100  in order to sense the speed of rotation of the crankshaft  100 , commonly referred to as the engine speed. The engine speed sensor  170  sends a signal representative of the speed of rotation of the crankshaft  100  to the ECU  164 . It is contemplated that the engine speed sensor  170  could alternatively sense the position of an element other than the crankshaft  100  that turns with the crankshaft  100 , such as the rotor  150  of the alternator  144  for example, and be able to determine the engine speed from the speed of rotation of this element. 
     An engine temperature sensor  172  is mounted to the engine  24  to sense the temperature of one or more of the engine coolant, the crankcase  102 , the cylinder block  104  and the cylinder head  108 . The engine temperature sensor  172  sends a signal representative of the sensed temperature to the ECU  164 . 
     An exhaust temperature sensor  174  is mounted to the exhaust manifold  140  or another portion of an exhaust system of the snowmobile  10  to sense the temperature of the exhaust gases. The exhaust temperature sensor  174  sends a signal representative of the temperature of the exhaust gases to the ECU  164 . 
     A throttle position sensor  176  is mounted to one of the throttle bodies  124  to sense an angular position of its throttle plate  126 , commonly referred to as the throttle position. The throttle position sensor  176  sends a signal representative of the throttle position to the ECU  164 . It is contemplated that the throttle position sensor  176  could sense the position of both throttle plates  126 . It is also contemplated that two throttle position sensors  176  (one per throttle body  124 ) could be provided. It is also contemplated that the throttle position sensor  176  could alternatively sense the position of a component used to actuate the throttle plate  126 , such as the position of a shaft of the throttle motor  127 , should one be provided, and that the ECU  164  could determine the throttle position from the position of this component. 
     A throttle lever position sensor  178  is mounted to the right handle of the handlebar  36  of the snowmobile  10  to sense an angular position of a throttle lever (not shown). The throttle lever is actuated by the driver of the snowmobile  10  such that the driver can control the desired speed and acceleration of the snowmobile  10 . The throttle lever position sensor  178  sends a signal representative of the throttle lever position to the ECU  164 . 
     An atmospheric air pressure sensor  180  is mounted to the snowmobile  10 , in the air intake system for example, to sense the atmospheric air pressure. The atmospheric air pressure sensor  180  sends a signal representative of the atmospheric air pressure to the ECU  164 . 
     An air temperature sensor  182  is mounted to the snowmobile  10 , in the air intake system for example, to sense the temperature of the air to be supplied to the engine  24 . The air temperature sensor  182  sends a signal representative of the air temperature to the ECU  164 . 
     An exhaust valve position sensor  184  ( FIG. 4 ) senses the position of the exhaust valves  142 A,  142 B and sends a signal representative of the this position to the ECU  164 . The ECU  164  uses this signal to determine if the exhaust valves  142 A,  142 B are in the position determined as described above. 
     It is contemplated that one or more of the sensors  170 ,  172 ,  174 ,  176 ,  178 ,  180 ,  182 ,  184  could be omitted. It is also contemplated that one or more of the sensors  170 ,  172 ,  174 ,  176 ,  178 ,  180 ,  182 ,  184  could be used only under certain conditions. 
     The ECU  164  uses the inputs received from at least some of the sensors  170 ,  172 ,  174 ,  176 ,  178 ,  180 ,  182 ,  184  to retrieve one or more corresponding control maps  166 ,  167 ,  168  and to control the port fuel injectors  125 A,  125 B, the direct fuel injectors  132 A,  132 B, the spark plugs  134 A,  134 B, the throttle motor  127  (should one be provided), and the exhaust valves actuator  186  using these inputs and/or the control maps. 
     The ECU  164  is also connected to a display (not shown) provided on the snowmobile  10  near the handlebar  36  to provide information to the user of the snowmobile  10 , such as engine speed, vehicle speed, oil temperature, and fuel level, for example. 
     Turning now to  FIGS. 6 to 10 , a method for controlling the engine  24  will be described. For simplicity, the method will be described with respect to only the cylinder  106 A and its associated components. It should be understood that the method is also being carried out in the same manner with respect to the cylinder  106 B and its associated components. It should also be noted that the time values t 1  to t 5  in  FIGS. 7 and 10  are intended to merely indicate the sequence of events, that the spacing between subsequent time values is not necessarily representative of a relative amount of time between these events, and that the values of t 1  to t 3  in  FIG. 7  do not correspond to the values of t 1  to t 3  in  FIG. 10 . The method begins at step  200 . 
     Following step  200 , at step  202  the ECU  164  determines the primary fuel quantity to be supplied to the combustion chamber  120 A. The ECU  164  makes this determination in two parts. The first part consists in determining the base fuel quantity to be supplied and the second part consists in determining a correction factor. 
     To determine the base fuel quantity, the ECU  164  first determines the engine speed and throttle position from the signals received from the engine speed sensor  170  and the throttle position sensor  176  respectively. Then, using the engine speed and throttle position, the ECU  164  retrieves from a base fuel quantity map (not shown) a corresponding base fuel quantity. It is contemplated that other operating conditions of the engine  24  and/or snowmobile  10  could be taken into consideration to determine the base fuel quantity. It is also contemplated that the base fuel quantity could be calculated by the ECU  164  using an algorithm. 
     To determine the correction factor, the ECU  164  first determines one or more of the atmospheric pressure, the air temperature, the engine temperature and the exhaust temperature from the signals received from the atmospheric pressure sensor  180 , the air temperature sensor  182 , the engine temperature sensor  172  and the exhaust temperature sensor  174  respectively. The ECU  164  then determines one or more secondary correction factors corresponding to the one or more of the atmospheric pressure, the air temperature, the engine temperature and the exhaust temperature from one or more maps or using an algorithm. The ECU  164  then combines the secondary correction factors to obtain the correction factor. It is contemplated that the correction factor could be a combination of correction factors of other operating conditions of the engine  24  and/or snowmobile  10 . 
     Once the ECU  164  has determined the base fuel quantity and the correction factor, the ECU  164  combines the two to obtain the primary fuel quantity. For example, if the ECU  164  determines that the base fuel quantity to be injected is Y mm 3  and that the correction factor is 104%, then the primary fuel quantity to be injected determined at step  202  is 1.04Y mm 3 . A correction factor is applied in the present implementation since the base fuel quantity map has been calibrated for specific operating conditions (temperature, pressure, etc.). The correction factor accounts for the difference(s) between the operating conditions at which the base fuel quantity maps has been calibrated and the actual operating conditions of the engine  24 /snowmobile  10 . Alternatively, it is contemplated that the primary fuel quantity could be obtained from multiple fuel quantity maps, each one of which would be calibrated for different operating conditions, and as such no correction factor would be required. It is also contemplated that multiple fuel quantity maps could be used in combination with a correction factor. For example, multiple maps for different air temperatures could be provided with a correction factor being used for variations in atmospheric air pressure. 
     Once the primary quantity of fuel to be injected in the combustion chamber  120 A has been determined at step  202 , then at step  204  the ECU  164  determines the ratio of the primary fuel quantity that is to be injected by the direct fuel injector  132 A (hereinafter the % DI) and the ratio of the primary fuel quantity that is to be injected by the port fuel injector  125 A (hereinafter the % PFI). In the present implementation, the ECU  164  makes this determination using the fuel ratio map  166  shown in  FIG. 8A . It should be understood that the fuel ratio map  166  shown in  FIG. 8A  is an exemplary map. Different engines and/or desired performance characteristics could require different fuel ratio maps. The fuel ratio map  166  provides the % PFI as a percentage for a given throttle position and engine speed. Generally, the % PFI values will be higher for an engine  24  calibrated to offer high performance and/or high acceleration compared to an engine  24  calibrated to offer fuel economy and/or low emissions. In the fuel ratio map  166 , the throttle position is given as a percentage of opening of the throttle plate  126 , with 0% being a minimum position of the throttle plate  126  and 100% being a wide-open throttle plate position. The ECU  164  determines the % PFI by retrieving from the fuel ratio map  166  the % PFI corresponding to the engine speed and throttle position used above at step  202 . The ECU  164  determines the % DI by subtracting the % PFI from 100%. It is contemplated that the fuel ratio map could provide the % DI instead of the % PFI. It is also contemplated that the % DI and % PFI could be determined using one or more algorithms. It is also contemplated that the ECU  164  could have multiple fuel ratio maps  166  corresponding to different modes of operation of the engine  24  that can be selected by a user of the snowmobile  10 . 
     As can be seen in  FIG. 8A , at low engine speeds and/or low throttle positions, the % PFI is 0%. This means that at low engine speeds and/or low throttle positions fuel is to be supplied to the combustion chamber  120 A only by the direct fuel injector  132 A (i.e. the port fuel injector  125 A supplies 0% of the fuel). In some implementations, over a range of engine speeds and a small range of low throttle positions, the direct fuel injector  132 A supplies fuel to produce a stratified charge. In one implementation, the direct fuel injector  132 A supplies fuel to produce a stratified charge only at less than 4500 RPM and a throttle position of less than 5%. Outside of this range, the direct fuel injector  132 A supplies fuel to produce a homogeneous charge. As can be seen in  FIG. 8A , in the range of engine speeds and throttle positions where the direct fuel injector  132 A supplies fuel to produce a stratified charge, fuel is to be supplied to the combustion chamber  120 A only by the direct fuel injector  132 A. As can be seen by comparing the fuel ratio map  166  of  FIG. 8A  to the exhaust valves position map  167  of  FIG. 8B , whenever the exhaust valve  142 A is in the FO position, the % PFI is greater than 0%. In other words, whenever the exhaust valve  142 A is in the FO position, fuel is injected by both the direct fuel injector  132 A and the port fuel injector  125 A. As can also be seen by comparing the fuel ratio map  166  of  FIG. 8A  to the exhaust valves position map  167  of  FIG. 8B , whenever the % PFI is 0%, the exhaust valve  142 A is either in the FL position or the Il position. In other words, whenever fuel is injected by the direct fuel injector  132 A only, the exhaust valve  142 A is never in the FO position. 
     Then, at step  206  the ECU  164  determines if the % PFI determined at step  204  is greater than 0. It should be understood that determining if the % DI is less than 100% would be equivalent. 
     If at step  206  the ECU  164  determines that the % PFI is not greater than 0% (i.e. the % PFI is 0%), then at step  208  the ECU  164  determines if the port fuel injector  132 A is being phased-out, meaning that it was previously being used and should no longer be used. For example, if in the previous cycle the engine speed was 6000 RPM and the throttle position was 30%, the % PFI was 45%, and now the engine speed is 6000 RPM and the throttle position is 25%, the % PFI is 0%. Since the % PFI has changed from 45% to 0%, the port fuel injector  132 A is being phased out. If in two consecutive cycles the % PFI changes from a non-zero value to 0%, the ECU  164  determines that phase-out occurs. If in two consecutive cycles the % PFI remains 0%, the ECU  164  determines that no phase-out occurs. For ease of visualization, the border between zero and non-zero values of % PFI in the fuel ratio map  166  has been drawn using a dash-dot line in  FIG. 8A . For ease of comparison between the maps  166 ,  167  of  FIGS. 8A, 8B  a corresponding dash-dot line has been drawn in  FIG. 8B . 
     If at step  208  the ECU  164  determines that phase-out is not occurring, then the ECU  164  continues at step  210  and controls the engine  24  (i.e. the fuel injectors  125 A,  132 A, the spark plug  134 A . . . ) according to the regular control and then returns to step  202 . It should be understood that for purposes of the present application, the regular control does not refer to any particular control, but is rather intended to mean any control to be used for the current operating conditions of the engine  24 /snowmobile  10  other than the phase-in and phase-out controls described below. 
     If at step  208  the ECU  164  determines that phase-out is occurring, then the ECU  164  continues at step  212  and controls the engine  24  (i.e. the fuel injectors  125 A,  132 A, the spark plug  134 A . . . ) according to the phase-out control and then returns to step  202 . In the phase-out control, the ECU  164  stops injecting fuel using the port fuel injector  125 A and increases the quantity of fuel injected by the direct fuel injector  132 A. The quantity of fuel injected by the direct fuel injector  132 A is increased from the % DI of the primary fuel quantity determined at step  204  to 100% of the primary fuel quantity. The phase-out control will be described in greater detail below with respect to  FIG. 10 . 
     Returning to step  206 , if the ECU  164  determines that the % PFI is greater than 0%, then at step  214  the ECU  164  determines if the port fuel injector  132 A is being phased-in, meaning that it was previously not being used and should now be used. For example, if in the previous cycle the engine speed was 6000 RPM and the throttle position was 25%, the % PFI was 0%, and now the engine speed is 6000 RPM and the throttle position is 30%, the % PFI is 45%. Since the % PFI has changed from 0% to 45%, the port fuel injector  132 A is being phased-in. If in two consecutive cycles the % PFI changes from 0% to a non-zero value, the ECU  164  determines that phase-in occurs. If in two consecutive cycles the % PFI does not change or changes from one non-zero value to another non-zero value, the ECU  164  determines that no phase-in occurs. 
     If at step  214  the ECU  164  determines that phase-in is not occurring, then the ECU  164  continues at step  210  and controls the engine  24  (i.e. the fuel injectors  125 A,  132 A, the spark plug  134 A . . . ) according to the regular control and then returns to step  202 . 
     If at step  214  the ECU  164  determines that phase-in is occurring, then the ECU  164  continues at step  216 . At step  216 , the ECU  164  increases a counter N by  1 . Then at step  218 , the ECU  164  determines if the value of the counter N has changed by more than or equal to a predetermined number of times X during a predetermined period of time “t”. Alternatively, it is contemplated that at step  218 , the ECU  164  could determine if the value of the counter N has changed by more than or equal to a predetermined number of times X during a predetermined period of cycles, one cycle corresponding to one full rotation of the crankshaft  100 . The purpose of steps  216 ,  218  will be explained in more detail below. 
     If at step  218  the ECU  164  determines that the counter N has not changed by more than or equal to the predetermined number of times X during the predetermined period of time “t”, then the ECU  164  continues at step  220  and controls the engine  24  (i.e. the fuel injectors  125 A,  132 A, the spark plug  134 A . . . ) according to the phase-in control and then returns to step  202 . In the phase-in control, the ECU  164  injects fuel using the port fuel injector  125 A and the direct fuel injector  132 A. The quantity of fuel injected by the direct fuel injector  132 A is initially more than the % DI of the primary fuel quantity and is decreased to the % DI of the primary fuel quantity. The quantity of fuel injected by the port fuel injector  125 A is initially increased to more than the % PFI of the primary fuel quantity. As such, during the phase-in control, the sum of the fuel quantities injected by the direct fuel injector  132 A and the port fuel injector  125 A is initially greater that the primary fuel quantity. The reason for this is that it has been found that when fuel is being injected by the port fuel injector  125 A after a period of time where the port fuel injector  125 A has not been used, a portion of the fuel injected by the port fuel injector  132 A sticks to and coats the surfaces of the interior of the crankcase  102  and the surfaces of components housed therein. As such, not all of the fuel initially injected by the port fuel injector  125 A makes it to the combustion chamber  120 A. If only the % DI of the primary fuel quantity was initially injected by the direct fuel injector  132 A and only the % PFI of the primary fuel quantity was initially injected by the port fuel injector  125 A, then less than the primary fuel quantity determined at step  202  would be supplied in the combustion chamber  120 A. Therefore, more than the % DI of the primary fuel quantity is initially injected by the direct fuel injector  132 A and more than the % PFI of the primary fuel quantity is initially injected by the port fuel injector  125 A to compensate for the fuel lost to the above-mentioned sticking and coating to ensure that a sufficient quantity of fuel is supplied to the combustion chamber  120 A. Once the various surfaces are coated with fuel, it has been found that very little to no fuel is lost to the above-mentioned coating and sticking, as such the amount of fuel injected by the direct fuel injector  132  and port fuel injector  125 A can be reduced. It is contemplated that the direct fuel injector  132 A could initially inject only the % DI of the primary fuel quantity and that the port fuel injector  125 A could inject more than the % PFI of the primary fuel quantity. It is also contemplated that the port fuel injector  125 A could initially inject only the % PFI of the primary fuel quantity and that the direct fuel injector  132 A could inject more than the % DI of the primary fuel quantity. The phase-in control will be described in greater detail below with respect to  FIG. 7 . 
     If at step  218  the ECU  164  determines that the counter N has changed by more than or equal to the predetermined number of times X during the predetermined period of time “t” (or number of cycles), then the ECU  164  goes to step  222 . At step  222 , the ECU  164  makes a record that the fuel quantities to be injected during the subsequent step  220  by one or both of the direct fuel injector  132 A and the port fuel injector  125 A during phase in control should be less than they would be had the method proceeded directly from step  218  to step  220 . If at step  218  the ECU  164  determines that the counter N has changed by more than or equal to the predetermined number of times X during the predetermined period of time “t” (or number of cycles), this is an indication that the amount of time (or number of cycles) between subsequent uses of the phase-in control is small. As such, the above-mentioned fuel sticks to and coats the surfaces of the interior of the crankcase  102  and the surfaces of components housed therein did not have the time to evaporate and/or to be entrained by air to the combustion chamber  120 A since the port fuel injector  125 A has stopped injecting fuel. As such, injecting the same amount of fuel injected by the direct fuel injector  132 A and the port fuel injector  125 A during phase-in control as when no fuel coats these surfaces would result in too much fuel being supplied toward the combustion chamber  120 A. Hence, step  222  reduces the quantity of fuel that will be injected at step  220  by one or both fuel injectors  125 A,  132 A. It is contemplated that steps  216 ,  218 ,  222  could be omitted. It is also contemplated that instead of or in addition to steps  216 ,  218 , the ECU  164  could determine following step  214  and prior to step  220  if any other conditions is present that would result in too much fuel being supplied to the combustion chamber  120 A during phase-in control. If such a condition is present, the ECU  164  would proceed to step  222  prior to proceeding to step  220 . 
     Turning now to  FIGS. 7 and 10 , phase-in and phase-out controls will be described in more detail with respect to the exemplary graphs shown in  FIGS. 7 and 10 . For simplicity, the phase-in and phase-out controls will be described with respect to only the cylinder the  106 A and its associated components. It should be understood that the phase-in and phase-out controls are also being carried out in the same manner with respect to the cylinder  106 B and its associated components. The graph of  FIG. 7  represents phase-in control for a % PFI that changes from 0% to 50%, for example when the engine speed is 5000 RPM and the throttle position changes from 25% to 30% (see  FIG. 8A ). The graph of  FIG. 10  represents a phase-out control for a % PFI that changes from 50% to 0%, for example when the engine speed is 5000 RPM and the throttle position changes from 30% to 25% (see  FIG. 8A ). In both graphs, the quantity of fuel injected by the direct fuel injector  132 A is represented by line  300  and the quantity of fuel injected by the port fuel injector  125 A is represented by line  304 . 
     With respect to  FIG. 7 , phase-in control begins at time t 1 . Before time t 1 , the port fuel injector  125 A injects no fuel (i.e. % PFI equals 0%) and the direct fuel injector  132 A injects the primary fuel quantity (i.e. % DI equals 100%). The phase-in control begins at t 1  when the % DI and % PFI determined at step  204  described above are both 50% in the present example. However as can be seen, the quantities of fuel injected by the direct fuel injector  132 A and the port fuel injector  125 A are greater than 50%. 
     From time t 1  to time t 2 , the fuel quantity injected by the direct fuel injector  132 A is held constant at 100% of the primary fuel quantity. Then at t 2 , the fuel quantity injected by the direct fuel injector  132 A is reduced to a value between 100% and the % DI determined at step  204  (i.e. 50% in the present example). In the present example, this value is 75%, but other values are contemplated. The fuel quantity injected by the direct fuel injector  132 A is then held constant at 75% up to time t 3 . Starting at time t 3 , the fuel quantity injected by the direct fuel injector  132 A is reduced linearly until it reaches 50% (i.e. the % DI of step  204 ), and is then held at this value until the % DI changes. The above manner in which the fuel quantity injected by the direct fuel injector  132 A is reduced is only one of the various manners contemplated. For example, it is contemplated that the fuel quantity injected by the direct fuel injector  132 A could decrease linearly or non-linearly from time t 1  to time t 4 . 
     At time t 1 , the fuel quantity injected by the port fuel injector  125 A is increased to inject a quantity of fuel corresponding to the % PFI of the primary fuel quantity combined with a pair of correction factors. It is contemplated that in other implementations, there could be more or less than two correction factors and that the correction factors could differ from the ones described below. 
     The first correction factor is a phase-in correction factor. The phase-in correction factor is a factor that is applied to initially increase the quantity of fuel injected by the port fuel injector  125 A above the % PFI of the primary fuel quantity for the reasons discussed above and then decreases over time (or cycles). In the present implementation, the phase-in correction factor is initially the same regardless of operating conditions, but it is contemplated that it could vary based on operation conditions such as engine speed or throttle position. In the example shown in  FIG. 7 , the phase-in correction factor is initially 110%, is then reduced linearly to reach 100% at time t 5  and is then constant. In the example shown, for a % PFI of 50%, it means that the quantity of fuel to be injected by the port fuel injector  125 A resulting from the combination with the phase-in correction factor is initially 55% (i.e. 50%×110%) at time t 1  and is reduced linearly to 50% (i.e. 50%×100%) at time t 5 . This is illustrated by line  302  in  FIG. 7 . It is contemplated that the phase-in correction could be reduced non-linearly or in steps. 
     The second correction factor is a fuel trapping efficiency correction factor. Fuel trapping efficiency is the ability to keep fuel freshly supplied in the combustion chamber  120 A and preventing it from escaping to the exhaust system without being combusted. The direct fuel injector  132 A has a higher fuel trapping efficiency than the port fuel injector  125 A because the direct fuel injector  132  can inject fuel in the combustion chamber  120 A later in the cycle when the exhaust ports  136 A,  138 A are closed or almost closed, so less fuel can escape. To compensate for the fuel supplied from the port fuel injector  125 A that escapes through the exhaust ports  136 A,  138 A, the fuel trapping efficiency correction factor is applied to the quantity of fuel calculated above with the phase-in correction factor. In the present implementation, the ECU  164  obtains the fuel trapping efficiency correction factor using the PFI fuel trapping efficiency correction map  168  shown in  FIG. 9 . It should be understood that the map  168  shown in  FIG. 9  is an exemplary map. Different engines and/or desired performance characteristics could require different maps. The map  168  provides the fuel trapping efficiency correction factor as a percentage for a given throttle position and engine speed. In the map  168 , the throttle position is given as a percentage of opening of the throttle plate  126 , with 0% being a minimum position of the throttle plate  126  and 100% being a wide-open throttle plate position. The ECU  164  determines the fuel trapping efficiency correction factor by retrieving from the map  168  the fuel trapping efficiency correction factor corresponding to the engine speed and throttle position used above at step  202 . It is also contemplated that the fuel trapping efficiency correction factor could be determined using one or more algorithms. In the example illustrated, for an engine speed of 5000 RPM and a throttle position of 30%, the fuel trapping efficiency correction factor is 155% as can be seen from  FIG. 9 . This factor is then applied to the fuel quantity corresponding to line  302 . 
     The ECU  164  determines the fuel quantity to be injected by the port fuel injector  125 A by multiplying the % PFI of the primary fuel quantity by the phases in correction factor and by the fuel trapping efficiency correction factor. Therefore, in the present example, at time t 1 , the port fuel injector  125 A initially injects a fuel quantity corresponding to 85,25% of the primary fuel quantity (i.e. 50%×110%×155%) and then reduces this quantity linearly to reach 77,50% (i.e. 50%×100%×155%) at time t 5 . This is illustrated by line  304  in  FIG. 7 . As can be seen by comparing lines  300  and  304  in  FIG. 7 , the quantity of fuel injected by the direct fuel injector  132 A decreases faster than the quantity of fuel injected by the port fuel injector  125 A. 
     With respect to  FIG. 10 , phase-out control begins at time t 1 . Before time t 1 , the port fuel injector  125 A injects the fuel quantity corresponding to line  304  (i.e. % PFI of the primary fuel quantity with fuel trapping efficiency factor applied) and the direct fuel injector  132 A injects the % DI of the primary fuel quantity, which in the present example is 50%. The phase-out control begins at t 1  when the % DI and % PFI determined at step  204  described above are 0% and 100% respectively. Therefore, as can be seen, the ECU  164  stops injecting fuel with the port fuel injector  125 A (line  304  is at 0%). However as can be seen, the quantity of fuel injected by the direct fuel injector  132 A is not immediately raised to 100%. This is because even if the fuel injector  125 A is stopped, some of the fuel that is present in the crankcase  102  and coats the various surfaces therein will continue to be supplied nonetheless. Therefore, the fuel quantity injected by the direct fuel injector  132 A remains constant at the % DI (i.e. 50%) until time t 2 . At time t 2 , the fuel quantity injected by the direct fuel injector  132 A is increased by a predetermined amount. From time t 2 , the fuel quantity injected by the direct fuel injector  132 A is increased linearly until it reaches 100% of the primary fuel quantity at time t 3 . The fuel quantities injected by the direct fuel injector  132 A and the port fuel injector  125 A will then remain the same until there is a change in the operating conditions. It is contemplated that the fuel quantity injected by the direct fuel injector  132 A could be increased differently than described above. 
     Turning now to  FIGS. 11 to 13 , an alternative method for controlling the engine  24  will be described. For simplicity, the method will be described with respect to only the cylinder  106 A and its associated components. It should be understood that the method is also being carried out in the same manner with respect to the cylinder  106 B and its associated components. It should also be noted that the time values t 1  to t 3  in  FIGS. 12 and 13  are intended to merely indicate the sequence of events, that the spacing between subsequent time values is not necessarily representative of a relative amount of time between these events, that the values of t 1  and t 2  in  FIG. 12  do not correspond to the values of t 1  and t 2  in  FIGS. 7, 10 and 13 , and that the values of t 1  and t 2  in  FIG. 13  do not correspond to the values of t 1  and t 2  in  FIGS. 7 and 10 . The method begins at step  400 . 
     Following step  400 , the ECU  164  proceeds to step  402  where it determines the primary fuel quantity to be supplied to the combustion chamber  120 A. From step  402 , the ECU proceeds to step  404  where it determines the ratio of the primary fuel quantity that is to be injected by the direct fuel injector  132 A (hereinafter the % DI) and the ratio of the primary fuel quantity that is to be injected by the port fuel injector  125 A (hereinafter the % PFI). Steps  402  and  404  correspond to steps  202  and  204  described above respectively. As such, steps  402  and  404  will not be described herein in detail. It is contemplated that the manner in which the ECU  164  determines the primary fuel quantity at step  402  and the ratios at step  404  could differ from the manner described above with respect to step  202  and  404 . 
     From step  404 , the ECU  164  proceeds to perform steps  406  and  410  in parallel, and from step  406  and  410  the ECU  164  proceeds to perform steps  408  and  412  respectively in parallel. It is contemplated that steps  406 ,  408 ,  410  and  412  could be all be performed in series or that only some of steps  406 ,  408 ,  410  and  412  could be performed in series as long as step  406  is performed prior to step  408  and as long as step  410  is performed prior to step  412 . 
     At step  406 , the ECU  164  determines for the quantity of fuel to be injected by the direct fuel injector  132 A (i.e. % DI) determined at step  404 , how much of this fuel would stick to the surfaces in the engine  24  (i.e. accumulate) and how much of the already accumulated fuel would evaporate back in the air to be combusted. There are many known ways to make this determination. Some of these are based on the X-Tau model developed by Charles Aquino and described in the  1981  SAE paper entitled “Transient A/F Control Characteristics of the 5 Liter Central Fuel Injection Engine” (SAE 910494) and in U.S. Pat. No. 5,474,052, issued Dec. 12, 1995, the entirety of both of which is incorporated herein by reference. In the present implementation, the base quantity of fuel to be injected by the direct fuel injector  132 A that accumulates on the surfaces is obtained from a map as a function of engine speed. A correction factor based on temperature is then applied to this base quantity. It is contemplated that the correction factor could be based on a factor other than temperature or on a combination of factors. In the present implementation, the base quantity of fuel that evaporates from the surfaces is obtained from a map as a function of engine speed. A correction factor based on temperature is then applied to this base quantity. It is contemplated that the correction factor could be based on a factor other than temperature or on a combination of factors. Then at step  408 , the ECU  164  combines numbers obtained at step  406  to determine the net quantity of fuel that will accumulate by injecting the quantity % DI with the direct fuel injector  132 A (i.e. the quantity of fuel that accumulates minus the quantity of fuel that evaporates). 
     Steps  410  and  412  correspond to step  406  and  408  respectively, but in steps  410 ,  412  the determination is made based on the quantity of fuel % PFI to be injected by the port fuel injector  125 A. As such, steps  410  and  412  will not be described in detail herein. It should be noted that the amount of fuel accumulating from the direct injector  132 A is typically less than the amount of fuel accumulating from the port fuel injector  125 A. 
     Following steps  406  and  412 , at step  414  the ECU  164  combines the quantities of accumulated fuel determined at steps  408  and  412 , thereby determining the total quantity of fuel that are expected to accumulate on the surfaces of the engine  24  should the quantities of fuel determined at step  404  be injected by the direct fuel injector  132 A and the port fuel injector  125 A. This total quantity of fuel represents the quantity of fuel which needs to be compensated during the actual fuel injections in order to obtain the desired air/fuel ratio based on the quantity of fuel determined at step  402 . If at step  414 , there is a net accumulation of fuel on the surfaces of the engine  24  (i.e. more fuel accumulates than evaporates), then the total quantity of fuel to be injected has to be more than the quantity of fuel determined at step  402  in order to maintain the air/fuel ratio. If at step  414 , there is a net evaporation of fuel from the surfaces of the engine  24  (i.e. more fuel evaporates than accumulates), the quantity determined at step  414  has a negative valued, and the total quantity of fuel to be injected has to be less than the quantity of fuel determined at step  402  in order to maintain the air/fuel ratio. 
     As will be understood from the following steps, in the present method, the compensation for the quantity of fuel determined at step  414  is entirely handled by adjusting the quantity of fuel to be injected by the direct fuel injector  132 A. The port fuel injector  125 A is not used to compensate for the quantity of fuel determined at step  414 . It is contemplated that in an alternative implementation the compensation for the quantity of fuel determined at step  414  could be entirely handled by adjusting the quantity of fuel to be injected by the port fuel injector  125 A and that the direct fuel injector  132 A could not be used for this compensation. It is also contemplated that in another alternative implementation the compensation for the quantity of fuel determined at step  414  could be handled by adjusting the quantities of fuel to be injected by both the direct fuel injector  132 A and the port fuel injector  125 A. 
     From step  414 , the ECU  164  proceeds to step  416  where it determines what percentage X of the quantity of fuel to be injected by the direct fuel injector  132 A determined at step  404  (i.e. % DI) the quantity of accumulated fuel determined at step  414  represents. For example, if at step  402  the total fuel quantity is 10 mg and at step  404  the ratio of fuel to be injected by the direct fuel injector  132 A is 50%, then the quantity of fuel to be injected by the direct fuel injector  132 A determined at step  404  is 5 mg (i.e. 50% of 10 mg). Then, if at step  414  the total quantity of accumulated fuel is 6 mg, then the percentage X at step  416  is 120% (i.e. 6÷5×100=120%). 
     Then from step  416 , the ECU  164  proceeds to steps  418  and  420  which it performs in parallel. At step  418 , the ECU  164  first calculates the actual quantity of fuel to be injected by the direct fuel injector  132 A and then causes the direct fuel injector  132 A to inject this quantity of fuel. The actual quantity of fuel to be injected by the direct fuel injector  132 A is the quantity of fuel determined for the direct fuel injector  132 A at step  404  (% DI) plus the amount of fuel to be compensated because of fuel accumulation (Acc. Fuel, step  414 , which has a negatively value if there is a net evaporation). As such for the example provided above where the quantity of fuel to be injected by the direct fuel injector  132 A determined at step  404  is 5 mg, the total quantity of accumulated fuel is 6 mg at step  414 , and the percentage X is 120% at step  416 , then the direct fuel injector  132 A needs to inject 11 mg of fuel (i.e. (1+1.2)×5 mg=11 mg). As such, the direct fuel injector  132 A needs to inject 110% of the quantity of fuel calculated at step  402 . At step  420 , the ECU  164  causes the port fuel injector  125 A to inject the quantity of fuel determined for the port fuel injector  125 A at step  404 . So for the above example, this is 5 mg of fuel to be injected by the port fuel injector  125 A (i.e. 50% of 10 mg). Therefore, for the above example, the actual total quantity of fuel to be injected is 16 mg (i.e. 11 mg from DI+5 mg from PFI, or 10 mg initially determined at step  402 +6 mg to compensate for the accumulated fuel from step  414 ), or 160% of the quantity initially determined at step  402  (i.e. 16÷10×100=160%). As a result, of the 16 mg of fuel injected by the two fuel injectors  125 A,  132 A, 6 mg accumulate on the surfaces of the engine  24 , and 10 mg get mixed with the air in the combustion chamber  120 A, which corresponds to the quantity of fuel calculated at step  402  and therefore the desired air/fuel ratio is achieved. It should be understood that the above calculations are for a given time and that the quantities will vary over time. It is contemplated that step  418  and  420  could be performed in series. In such a case, step  420  would be performed first in most cases as the fuel injected by the port fuel injector  125 A takes some time to reach the combustion chamber  120 A, whereas the fuel injected by the direct fuel injector  132 A is injected directly in the combustion chamber  120 A. 
     From steps  418  and  420 , the ECU  164  returns to step  402  and the method is repeated. 
     Turning now to  FIGS. 12 and 13 , phase-in and phase-out controls with respect to the method of  FIG. 11  described above will be described in more detail with respect to the exemplary graphs shown in  FIGS. 12 and 13 . For simplicity, the phase-in and phase-out controls will be described with respect to only the cylinder  106 A and its associated components. It should be understood that the phase-in and phase-out controls are also being carried out in the same manner with respect to the cylinder  106 B and its associated components. The graph of  FIG. 12  represents phase-in control for a % PFI that changes from 0% to 50%. The graph of  FIG. 13  represents a phase-out control for a % PFI that changes from 50% to 0%. In both graphs, the quantity of fuel injected by the direct fuel injector  132 A is represented by the dotted line  450  and the quantity of fuel injected by the port fuel injector  125 A is represented by the solid line  452 . 
     With respect to  FIG. 12 , phase-in control begins at time t 1 . Before time t 1 , the port fuel injector  125 A injects no fuel (i.e. % PFI equals 0%) and the direct fuel injector  132 A injects the primary fuel quantity (i.e. % DI equals 100%). The phase-in control begins at t 1  when the % DI and % PFI determined at step  404  described above are both 50% in the present example. However as can be seen, the quantity of fuel injected by the direct fuel injector  132 A is greater than the 50% of step  404 . Using the same example that was used above with respect to the description of the method of  FIG. 10 , where the quantity of fuel to be injected by the direct fuel injector  132 A determined at step  404  is 5 mg, the total quantity of accumulated fuel is 6 mg at step  414 , and the percentage X is 120% at step  416 , then the direct fuel injector  132 A needs to inject 11 mg of fuel (i.e. (1+1.2)×5 mg=11 mg). As such, the direct fuel injector  132 A needs to initially inject 110% of the value determined at step  402  as shown at t 1  (i.e. value determined at step  418 ). Over time, this quantity gradually goes down as can be seen until time t 2  where the direct fuel injector  132 A injects 50% of the value determined at step  402  (i.e. the value of step  404 ). This is because as time goes by, the surfaces of the engine  24  get saturated with fuel and a balance between accumulation and evaporation is reached (i.e. the quantity at step  414  is zero). At time t 1 , the port fuel injector  125 A injects 50% of the value determined at step  402  (i.e. the quantity determined at step  404 ) and this quantity remains constant. Following time t 2 , the ratios of fuel injected by the direct and port fuel injectors  132 A,  125 A remain the same until the ECU  164  determines at step  404  that they should change. 
     With respect to  FIG. 13 , phase-out control begins at time t 1 . In the present example, before time t 1 , the port fuel injector  125 A and the direct fuel injector  132 A each injects 50% of the quantity of fuel determined at step  402 . The phase-out control begins at t 1  when the % DI and % PFI determined at step  404  described above change to 100% and 0% respectively. Therefore, as can be seen, the ECU  164  stops injecting fuel with the port fuel injector  125 A (line  452  is at 0%). However as can be seen, the quantity of fuel injected by the direct fuel injector  132 A is not immediately raised to 100%. This is because even if the port fuel injector  125 A is stopped, some of the fuel that is present in the crankcase  102  and coats the various surfaces therein continues to evaporate and therefore continues to be supplied to the combustion chamber  120 A nonetheless. Therefore, the quantity of accumulated fuel at step  414  is negative, which means that at step  418 , less than 100% of the quantity determined as step  404  is to be injected. For example, for the example where the quantity of fuel at step  402  is 10 mg and the quantity determined at step  414  is −2 mg (i.e. net evaporation of 2 mg), then at step  418 , the direct fuel injector injects 8 mg or 80% of the quantity of step  402 , not 100%. From time t 1 , as the fuel that coats the various surfaces dry up, less fuel evaporates, and the quantity of fuel injected by the direct fuel injector  132 A is increased gradually until it reaches 100% of the primary fuel quantity at time t 2 . The fuel quantities injected by the direct fuel injector  132 A and the port fuel injector  125 A will then remain the same until there is a change in the operating conditions. 
     In the above-described methods there may be operating conditions where the determined quantity of fuel to be injected by the direct fuel injectors  132 A,  132 B exceeds the maximum quantity of fuel that the direct fuel injectors  132 A,  132 B are capable of injecting. It is contemplated that under such conditions, the direct fuel injectors  132 A,  132 B inject the maximum quantity of fuel that they are capable of injecting. The difference between the determined quantity of fuel to be injected by the direct fuel injectors  132 A,  132 B and the maximum quantity of fuel that the direct fuel injectors  132 A,  132 B are capable of injecting is added to the quantity of fuel to be injected by the corresponding port fuel injectors  125 A,  125 B such that the total quantity of fuel injected still corresponds to the total quantity of fuel to be injected that has been determined by the methods. 
     Similarly, in the above-described methods there may be operating conditions where the determined quantity of fuel to be injected by the port fuel injectors  125 A,  125 B exceeds the maximum quantity of fuel that the port fuel injectors  125 A,  125 B are capable of injecting. It is contemplated that under such conditions, the port fuel injectors  125 A,  125 B inject the maximum quantity of fuel that they are capable of injecting. The difference between the determined quantity of fuel to be injected by the port fuel injectors  125 A,  125 B and the maximum quantity of fuel that the port fuel injectors  125 A,  125 B are capable of injecting is added to the quantity of fuel to be injected by the corresponding direct fuel injectors  132 A,  132 B such that the total quantity of fuel injected still corresponds to the total quantity of fuel to be injected that has been determined by the methods. 
     Also, in some implementations, the direct fuel injectors  132 A,  132 B and the port fuel injectors  125 A,  125 B have a minimum quantity of fuel that they can inject with precision. Below this minimum quantity, it cannot be precisely determined the quantity of fuel actually being injected. As such, in some implementations, should the above described methods determine that the quantity of fuel to be injected by the direct fuel injectors  132 A,  132 B or the port fuel injectors  125 A,  125 B is less than their corresponding minimum quantity, the minimum quantity will be injected. As a result, the total quantity of fuel injected will be slightly higher than the total quantity of fuel to be injected that has been determined by the methods. Alternatively, the quantity of fuel injected by the fuel injectors  132 A,  132 B or  125 A,  125 B that are not injecting their minimum quantity of fuel can be reduced slightly to compensate for the extra quantity of fuel being injected by the fuel injectors  132 A,  132 B or  125 A,  125 B that are injecting their minimum quantity of fuel. 
     In the above-described methods, in some implementations, should one of the direct fuel injector  132 A and the port fuel injector  125 A become defective, the operating conditions of the engine  24  will be limited to operating conditions where the total quantity of fuel to be injected can be completely supplied by the non-defective injector  132 A,  125 A. The same thing occurs should one of the direct fuel injector  132 B and the port fuel injector  125 B become defective. In one implementation, should one of the injectors  125 A,  125 B,  132 A,  132 B become defective, the operating conditions of the engine  24  are limited to forty percent of the maximum engine power, the exhaust valves  142 A,  142 B are limited to an intermediate position (i.e. they will not fully open) and a “check engine” light (not shown) or some other visual indicator is turned on to indicate to the user that something is wrong with the engine  24 . 
     Modifications and improvements to the above-described implementations of the present technology may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present technology is therefore intended to be limited solely by the scope of the appended claims.