WO2018224979A1

WO2018224979A1 - Two-stroke internal combustion heat engine with fuel direct injection

Info

Publication number: WO2018224979A1
Application number: PCT/IB2018/054052
Authority: WO
Inventors: Vincenzo MATTIA; Davide PERINO; Attilio GIULIANI; Giuseppe EVANGELISTA; Vincenzo DOINO; Nicola TRENTANI
Original assignee: Vins Srl
Current assignee: Vins Srl
Priority date: 2017-06-06
Filing date: 2018-06-06
Publication date: 2018-12-13
Anticipated expiration: 2019-12-06
Also published as: IT201700061734A1

Abstract

A two-stroke internal combustion heat engine (1) with fuel direct injection, wherein the heat engine (1) has: at least one cylinder (2) with an internal combustion chamber (7) having a cylindrical symmetry around a first longitudinal axis (Al); a piston (8) mounted inside the cylinder (2) to slide in a reciprocating manner; a cylinder head (11) closing the combustion chamber (7) at the top; a spark plug (12) mounted through the head (11) and having a pair of electrodes (13) at the bottom; and an injector (14), which is arranged through the side wall of the cylinder (2), has a second longitudinal axis (A2) and is provided with an injection nozzle (15). The injection nozzle (15) of the injector (14) faces the head (11) to spray fuel towards the cylinder head (11), and the second longitudinal axis (A2) is inclined to form an acute angle with the first longitudinal axis (Al).

Description

"TWO-STROKE INTERNAL COMBUSTION HEAT ENGINE WITH FUEL DIRECT INJECTION"

PRIORITY CLAIM

This application claims priority from Italian Patent Application No. 102017000061734 filed on June 6, 2017, the disclosure of which is incorporated by reference.

TECHNICAL FIELD

The present invention relates to a two-stroke internal combustion heat engine with fuel direct injection.

PRIOR ART

The two-stroke internal combustion heat engine was invented by Dugald Clerk in 1879 and differs from the more widespread four-stroke internal combustion heat engine mainly due to the different alternation of the active phases (useful or power phases) in relation to the revolutions of the crankshaft. In fact, if the four-stroke engine has an active phase (namely the expansion phase transforming the chemical energy into thermal energy and then into kinetic energy) for every two revolutions of the crankshaft, the two-stroke engine has an active phase for each complete revolution of the crankshaft. Structurally, a two-stroke internal combustion heat engine normally has no classic intake and exhaust valves, which are replaced by the "openings", i.e. non-circular slits that are obtained directly on the cylinder and are opened and closed by the reciprocating motion of the piston.

The two-stroke internal combustion heat engine is an extremely simple, compact, light and inexpensive engine and for this reason it has almost always been used in small displacement mopeds (generally up to 125-150 cc) and in all small-sized applications (small generators, chainsaws, lawn mowers, small outboard motors) . However, the present two- stroke internal combustion heat engines are not able to meet the new regulations on heat engine emissions. Mainly, since there is a strong intersection between the intake phase and the exhaust phase (during the intersection both the intake openings and the exhaust openings are open at the same time), a significant part (up to 40-50%) of the fresh intake (namely, the air that is taken in the cylinder through the intake openings) exits directly into the exhaust without being affected by the combustion. In the case of fuel direct injection, the fresh intake is already a mixture of air and fuel and therefore the intersection between the intake phase and the exhaust phase causes a dispersion of unburnt hydrocarbons into the environment. To substantially reduce the amount of unburnt hydrocarbons that are dispersed in the environment, it has been proposed to use fuel direct injection in the cylinder, thus injecting the fuel only when the exhaust openings are closed (in this case, during the intersection, the fresh intake exiting the exhaust openings is composed only of fresh air) . However, a good mixing of the fuel injected directly into the cylinder with the fresh air taken in the cylinder through the intake openings normally requires very sophisticated and therefore high-cost injection systems, which nullify the economic advantages and the compactness for which the two-stroke internal combustion heat engine is chosen .

The patent application WO2004106714A1 describes a two- stroke internal combustion heat engine with fuel direct injection, in which the fuel is injected directly into the cylinder by means of an injector, which has a nozzle oriented so as to direct the fuel jet against a wall of the cylinder head arranged next to a spark plug mounted in a central position. However, the two-stroke internal combustion heat engine described in the patent application W02004106714A1 has various operating irregularities, since at many driving speeds it does not allow obtaining an optimal mixing between the fuel injected directly into the cylinder and the fresh air taken in the cylinder through the intake openings.

The patent application WO2009044225A1 describes a two- stroke internal combustion heat engine with fuel direct injection, in which the fuel is injected directly into the cylinder by means of an injector having a nozzle oriented so as to direct the fuel jet against a wall of the cylinder head arranged around a spark plug mounted in a central position. The injector generates a fuel jet having an internally hollow conical shape due to a conical hole inside the fuel jet, and the injector is sized so that the central hole of the fuel jet includes inside it the spark plug electrodes. As a consequence, the fuel injected by the injector wets the bottom wall of the cylinder head arranged around the spark plug but does not wet the spark plug electrodes. However, also the two-stroke internal combustion heat engine described in the patent application W02009044225A1 has some operating irregularities, since at some driving speeds it does not allow obtaining an optimal mixing between the fuel injected directly into the cylinder and the fresh air taken in the cylinder through the intake openings .

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a two- stroke internal combustion heat engine with fuel direct injection, said heat engine being free from the drawbacks described above and, at the same time, being easy and inexpensive to manufacture.

According to the present invention, it is provided a two- stroke internal combustion heat engine with fuel direct injection as claimed in the appended claims.

The claims describe preferred embodiments of the present invention forming an integral part of the present disclosure .

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings showing a non-limiting embodiment, in which:

• Figure 1 is a schematic and plan view of a two-stroke internal combustion heat engine with fuel direct injection manufactured in accordance with the present invention;

• Figure 2 is a schematic and sectional view of a cylinder of the heat engine of Figure 1;

• Figure 3 is an enlarged view of a detail of Figure 2; · Figure 4 is a schematic and longitudinal section view of an injector of the heat engine of Figure 1; and

• Figure 5 is a schematic view of an exhaust duct of the heat engine of Figure 1.

PREFERRED EMBODIMENTS OF THE INVENTION

In Figure 1, the reference number 1 indicates as a whole a two-stroke internal combustion heat engine with fuel direct injection .

In the (non-limiting) embodiment shown in the accompanying figures, the heat engine 1 is a V-twin engine, namely it comprises two twin cylinders 2, which are arranged at an angle between them (at an angle of 90° as shown in Figure 1) and rise from a common crankcase 3 containing a crankshaft .

An exhaust duct 4 ending with a silencer 5 originates from each cylinder 2. The two exhaust ducts 4 are completely independent, i.e. they do not have any type of exhaust gas intersection between them and, as far as possible, are straight to minimize the pressure drops.

At a central area of the crankcase 3 arranged between the two cylinders 2 there is an intake duct 6 taking in fresh air (i.e. air containing about 20% oxygen and coming from the external environment), which is necessary for combustion. In particular, in the crankcase 3 it is integrated a filter box, which is arranged in a front position between the two cylinders 2, houses an air filter, has a structural function (namely contributes to confer rigidity and solidity to the crankcase 3) and has an integrated single trumpet through which the fresh air taken in enters .

As shown in Figure 2, each cylinder 2 internally comprises a combustion chamber 7, which has a cylindrical symmetry around a longitudinal axis Al and in which a piston 8 slides in a reciprocating manner. The side wall of each combustion chamber 7 has an intake opening 9, which receives fresh air from the intake duct 6 and is cyclically opened and closed by the movement of the piston 8. The side wall of each combustion chamber 7 has at least one exhaust opening 10 (opposite the intake opening 9), which expels the exhaust gases towards the exhaust duct 4 and is cyclically opened and closed by the movement of the piston 8.

Each cylinder 2 comprises a cylinder head 11, which closes the combustion chamber 7 at the top; namely, the cylinder head 11 is a sort of cover, which delimits at the top the combustion chamber 7. A spark plug 12 is arranged (screwed in) through each cylinder head 11, said spark plug having at the bottom a pair of electrodes 13 arranged inside the combustion chamber 7. Cyclically (i.e. at the end of the compression phase), a spark is fired between the electrodes 13, thus causing the ignition of the mixture of air and fuel in the combustion chamber 7. The spark plug 12 is arranged in the centre of the cylinder head 11 and therefore in the centre of the combustion chamber 7; in other words, the spark plug 12 is coaxial with the longitudinal axis Al of the combustion chamber 7.

Each cylinder 2 comprises an injector 14, which injects directly into the combustion chamber 7 and is arranged through the side wall of the cylinder 2 (i.e. it is arranged externally with respect to the cylinder head 11) . In other words, a through opening is obtained through the side wall of the cylinder 2, opening into the combustion chamber 7 and housing the injector 14.

The injector 14 ends with an injection nozzle 15 (oriented towards the combustion chamber 7) through which fuel is injected and having a cylindrical symmetry around a longitudinal axis A2 (please note that the longitudinal axis A2 , besides forming the central cylindrical symmetry axis of the injector 14, also determines the fuel injection direction) ; the injection nozzle 15 of the injector 14 is turned towards the cylinder head 11 to spray the fuel towards the cylinder head 11. The longitudinal axis A2 of the injector 14 forms with the longitudinal axis Al of the combustion chamber an angle a, preferably comprised between 52° and 62° (in the embodiment shown in the attached figures, the angle a is 57,5°) . Furthermore, the longitudinal axis A2 of the injector 14 is oriented to intersect the electrodes 13 of the spark plug 12 so that the extension of the longitudinal axis A2 passes through the electrodes 13 of the spark plug 12. In particular, the longitudinal axis A2 of the injector 14 intersects the longitudinal axis Al of the combustion chamber 7 at the electrodes 13 of the spark plug 12 (i.e. the intersection of the two longitudinal axes Al and A2 is superimposed on the electrodes 13 of the spark plug 12) .

The injector 14 generates a fuel jet 16 having a conical shape (the vertex of the cone being arranged near the injection nozzle 15) and centrally having a hole 17 (i.e. a fuel-free zone) , which is conical too (the vertex of the cone being arranged near the injection nozzle 15) . In other words, the fuel jet 16 generated by the injector 14 has the shape of a conical casing due to the presence of the central hole 17, i.e. it has an internally hollow conical shape. The fuel jet 16 having a conical shape has a conical symmetry about the longitudinal axis A2 of the injector 14; analogously, a conical hole 17 inside the fuel jet 16 has a conical symmetry around the longitudinal axis A2 of the injector 14. The central hole 17 of the fuel jet 16 of the injector 14 is sized so that it houses the electrodes 13 of the ignition spark plug 12. As a consequence, the fuel injected by the injector 14 (namely the fuel constituting the fuel jet 16) wets the lower wall of the cylinder head 11 arranged around the spark plug 12, but does not wet the electrodes 13 of the ignition spark plug 12.

Please note that each injector 14 is arranged in an intermediate position of the side wall of the cylinder 2 so that the injector 14 is not covered by the piston 8 during the fuel injection (which however occurs when the piston 8 has already closed the intake opening 9 and above all the exhaust opening 10) and is covered by the piston 8 during the combustion (i.e. when the combustion chamber 7 shows a maximum pressure and a maximum temperature) . In this way, each injector 14 is protected by the corresponding piston 8 against the high pressures and temperatures generated by the combustion in the combustion chamber 7, since during the combustion the piston 8 is interposed between the injector 14 and the combustion chamber 7.

As shown in Figure 4, each injector 14 has a substantially cylindrical symmetry around the longitudinal axis A2 and is controlled to inject fuel through the injection nozzle 15, which enters directly into the combustion chamber 7 (impacting against the lower wall of the cylinder head 11 around the electrodes 13 of the spark plug 12) . Each injector 14 comprises a supporting body 18 having a cylindrical shape with a variable section along the longitudinal axis A2 and a feeding channel 19 extending along its length to feed the pressurized fuel to the injection nozzle 15. The supporting body 18 houses an electromagnetic actuator 20 at its own upper portion and an injection valve 21 at its own lower portion; in use, the injection valve 21 is actuated by the electromagnetic actuator 20 to regulate the fuel flow through the injection nozzle 15, which is formed at the injection valve 21.

The electromagnetic actuator 20 comprises an electromagnet, which is housed in a fixed position inside the supporting body 18 and which, when excited, moves a movable anchor made of ferromagnetic material along a longitudinal axis A2 from a closed position to an open position of the injection valve 21 against the action of a closing spring 22, which tends to keep the movable anchor in the closed position of the injection valve 21.

The movable anchor is part of a movable unit, which further comprises a shutter or pin 23 having an upper portion integral with the movable anchor and a lower portion cooperating with a valve seat 24 (having a truncated- conical shape) of the injection valve 21 to adjust the fuel flow through the injection nozzle 15. The pin 23 ends with a shutting head 25 having a truncated-conical shape, which sealingly rests against the valve seat 24, whose truncated- conical shape reproduces in negative the truncated-conical shape of the shutting head 25. Please note that the shutting head 25 is arranged on the outside with respect to the supporting body 18 and is pushed by the closing spring 10 against the supporting body 18. As a consequence, in order to move from the closed position to the open position of the injection valve 21, the shutting head 25 moves along the longitudinal axis A2 downwards, i.e. with a direction corresponding to the fuel supply direction.

In the open position of the injection valve 21, the shutting head 25 is separated from the valve seat 24, thus creating a fuel passage opening having a circular crown- shaped section and a truncated-conical shape. As a consequence, the fuel injected through the injection nozzle 15 exits with an internally hollow conical shape, whose opening angle is substantially identical to the opening angle of the shutting head 25 (exactly corresponding to the opening angle of the valve seat 24) .

According to a preferred (but non-limiting) embodiment, the heat engine 1 comprises an electronic oil intake system that supplies the lubrication oil at the base of each cylinder 2 in a calibrated way (and only when necessary) .

Please note that the heat engine 1 completely lacks an intake valve; in other words, each intake opening 9 is opened/closed only by the reciprocating movement of the piston 8 and there is no other moving mechanical element, which opens/closes the intake opening 9. According to a preferred (but non-limiting) embodiment, the intake of fresh air provides a dual front lamellar pack intake.

According to a preferred (but non-limiting) embodiment shown in Figure 3, each cylinder 2 is provided with an exhaust valve 26 (e.g. guillotine-shaped), which is driven by an electric actuator 27 and allows opening and closing the exhaust duct 4 immediately downstream of the exhaust opening 10. The electric actuator 27 driving each exhaust valve 26 is mechanically completely independent of the rotary motion of the crankshaft, and can thus drive the exhaust valve 26 to open/close the exhaust duct 4 immediately downstream of the exhaust opening 10 completely independently of the angular position of the crankshaft (i.e. of the axial position of the piston 8) . Generally, the exhaust valve 26 is used to completely close the exhaust duct 4 immediately downstream of the exhaust opening 10 during the intake phase (i.e. when fresh air enters each combustion chamber 7 through the exhaust opening 9), thus avoiding that the fresh air entering each combustion chamber 7 partially escapes through the exhaust opening 10 during the intake phase.

According to a possible embodiment, the intake duct 6 of the heat engine 1 (feeding both cylinders 2) is provided with a butterfly valve, which is servo-controlled by an electric actuator regulating the flow of fresh air taken in. It is thus possible varying the flow of fresh air respectively taken into the cylinders 2 through corresponding intake openings 9. According to an alternative embodiment, the intake duct 6 of the heat engine 1 (feeding both cylinders 2) lacks a butterfly valve (i.e. no movable mechanical element regulates/varies the flow of fresh air respectively taken in the cylinders 2) and therefore the cylinders 2 always take in the maximum possible flow of fresh air.

Please note that a combustion under stoichiometric conditions in the heat engine 1 generally generates less HC (i.e. unburnt hydrocarbons) and more NOx (i.e. nitrogen oxides and their mixtures), while an excess air combustion generally generates more HC and less NOx (thus reducing the combustion temperature peak due to the presence of an excess of fresh air) . Therefore, an excess air combustion can be always chosen (by getting rid of the intake butterfly valve) to minimize the NOx generation, trying to limit the HC generation by other means. The absence of the butterfly valve has further advantages, as it allows a reduction of weight, bulk and cost (by completely avoiding a component) and also allows increasing the performance (having no intake losses due to the passage of fresh air through the butterfly valve) . Obviously, even in the presence of the intake butterfly valve, the engine control may still, at least in some engine points, operate an excess air combustion to reduce the NOx generation.

An excess air combustion has the further advantage of constant large amounts of molecular oxygen in the exhaust gases, thus allowing an efficient operation of the oxidizing catalysts mounted in the exhaust ducts 4.

As shown in Figure 5, each exhaust duct 4 comprises a metallic tubular duct 28, which originates from the corresponding cylinder 2 and ends in the corresponding silencer 5; inside the tubular duct 28, the exhaust gases flow from the cylinder 2 to the silencer 5 and are then released into the external environment.

According to a preferred but non-limiting embodiment, each exhaust duct 4 comprises a pre-catalyst device 29, which is arranged inside the tubular duct 28 near the cylinder 2. Each exhaust duct 4 comprises a catalyst device 30, which is arranged inside the tubular duct 28 near the silencer 5 (or downstream of the pre-catalyst device 29) .

A further tubular duct 31 is arranged inside the tubular duct 28, is separated and spaced from the tubular duct 28 (i.e. the wall of the tubular duct 31 does not touch the wall of the tubular duct 28, with the exception of a limited area 32) and houses the catalyst device 30. The tubular duct 31 is cantilever mounted within the tubular duct 28 so that the tubular duct 31 is mechanically coupled to the tubular duct 28 only on one side, at an anchoring area 32 adjacent to the silencer 5. The catalyst device 30 is arranged at one end of the tubular duct 31 opposite the end where the anchoring area 32 is located. Preferably, the tubular duct 31 is larger at the catalyst device 30 (namely, where the catalyst device 30 is housed in the tubular duct 31) and is smaller downstream of the catalyst device 30.

The presence of the tubular duct 31, which is cantilever mounted within the tubular duct 28 and houses the catalyst device 30, allows a thermal insulation of the tubular duct 28 with respect to the catalyst device 30, thus considerably reducing the heat exchange between the catalyst device 30 and the external environment. In this way, the catalyst device 30 more quickly reaches its optimal working temperature when the heat engine 1 is cold started and the external temperature of the tubular duct 28 is reduced near the catalyst device 30.

According to a possible embodiment, each catalyst device 30 is provided with its own heater (e.g. an electric thermo- resistance) , which is actuated when the catalyst device 30 is cold for rapidly increasing the temperature of the catalyst device until it reaches the optimal working temperature in a few tens of seconds.

The heat engine 1 comprises an electronic control unit 33 (schematically shown in Figure 1), which regulates the operation of the heat engine 1 by driving the fuel injection through the injectors 14 (i.e. by setting both the amount of fuel to be injected and the instant of the injection), by driving the ignition of the spark plugs 12 (i.e. by setting the instant to fire the spark), by driving the lubrication oil injection, by controlling the position of the exhaust valves 26 and, if present, by controlling the position of the intake butterfly valve (i.e. by adjusting the flow of fresh air taken in the cylinders 2) . An angular position sensor (i.e. an angular encoder) is coupled to the crankshaft and provides in real time the angular position of the crankshaft; through the position sensor, the control unit 33 determines the angular position of the crankshaft and deriving this datum over time determines the angular speed (i.e. the rotation speed) of the crankshaft. Furthermore, a flow (or flow and temperature) sensor can be provided that detects the flow of fresh air taken in (if present downstream of the butterfly valve) and, possibly the temperature of the fresh air taken in. Furthermore, for each exhaust duct 4 it is provided a corresponding lambda probe, which detects the oxygen concentration inside the exhaust gas. Finally, a temperature sensor is provided, which detects the temperature of a cooling liquid of the heat engine 1 (in case of liquid cooling) and/or a temperature sensor, which detects the temperature of a component of the exhaust duct 4 (e.g. of the catalyst device 30) .

The electronic control unit 33 generally uses maps, which are pre-loaded in its own non-volatile memory, providing the values to control the injectors 14, the spark plugs 12, the lubrication oil intake, the exhaust valves 26 and the intake butterfly valve according to the accelerator control operated by the driver, depending on the rotation speed and the oxygen concentration in the exhaust gas. Such maps are generally parametrized (i.e. vary) based on the temperature of a cooling liquid or the temperature of a component of the exhaust duct 4 (e.g. of the exhaust catalysts 30) .

In use, in each cylinder 2 the fuel is injected directly into the combustion chamber 7 by the corresponding injector 14 during the intake/compression phase. In particular, the fuel is injected when the piston 8 has just closed the intake opening. In these conditions, the fuel sprayed by the injector 14 finds a relatively low pressure in the combustion chamber 7 and therefore requires a low pressure (e.g. between 3 and 8 bars) to be injected. The fuel jet ejected by the injection nozzle 14 impacts against the lower wall of the cylinder head 11 around the spark plug 12 (namely around the electrodes 13 of the spark plug 12), but without wetting the spark plug 12 (namely the electrodes 13 of the spark plug 12) . The fuel impact against the lower wall of the cylinder head 11 is followed by the so-called secondary atomization (i.e. fuel drops bounce from the bottom wall of the head 11 dispersing in the combustion chamber 7) and by a violent heating of the fuel (the cylinder head 11 is the area with the highest temperature of the whole combustion chamber 7), which favours a rapid vaporization of the fuel. In this way, a high flammability of the air-fuel mixture is ensured at all operating modes of the heat engine 1, also taking into account that the fuel remains in the combustion chamber 7 before the spark is fired for a relatively long time and that the electrodes 13 of the spark plug 12 are located right in the middle of the fuel impact area against the wall of the cylinder head 11 (i.e. the area where fuel is concentrated after injection) .

As previously stated, the combustion inside the cylinders 2 can be controlled only by adjusting the amount of injected fuel, thereby operating with (even large) excess air. The NOX generation can thus be remarkably reduced while keeping the HC generation within limits.

When the heat engine 1 is cold started (i.e. when the cylinder head 11 of each cylinder 2 is at room temperature) , it is generally necessary to increase the amount of injected fuel to increase the intake in the combustion chambers 7 and thus compensate for the failed fuel vaporization due to the fuel heating against the cylinder heads 11 with an increase of the primary atomization (generated by the fuel injectors 14) and of the secondary atomization (generated by the impact against the cylinder heads 11) . Obviously, as the heat engine 1 heats up, the increasing intake in the combustion chambers 7 is progressively reduced; in other words, during the progressive heating of the heat engine 1, the electronic control unit 33 progressively reduces the larger amount of injected fuel until reaching a steady state condition.

The embodiments described herein can be combined without departing from the scope of protection of the present invention .

The heat engine 1 described above has numerous advantages. First, the heat engine 1 described above has a particularly low emission of pollutants so as to be approved according to the most recent "Euro4" and "Euro5" regulations applying to motorcycles and mopeds .

Furthermore, the heat engine 1 described above has excellent operating regularity at all operating speeds. This result is obtained thanks to the fact that the fuel injected into each cylinder 2 does not impact against a small and limited area of the lower wall of the head 11, but impacts against almost the entire lower wall of the cylinder head 11, with the only exclusion of the central area housing the electrodes 13 of the spark plug 12. As a consequence, the injected fuel spreads in a much higher volume, thus favouring an optimal mixing with the air at all operating speeds.

Finally, the heat engine 1 described above is particularly light, compact and inexpensive if compared to a similar four-stroke internal combustion heat engine providing the same performance.

LIST OF REFERENCE NUMBERS OF THE FIGURES 1 heat engine

2 cylinders

3 crankcase

4 exhaust duct

5 silencer

6 intake duct

7 combustion chamber

8 piston

9 intake opening

10 exhaust opening

11 cylinder head

12 spark plug

13 electrodes

14 injector

15 injection nozzle

16 fuel jet

17 central hole

18 supporting body

19 feeding channel

20 electromagnetic actuator

21 injection valve

22 closing spring

23 pin

24 valve seat

25 shutting head 26 exhaust valve

27 electric actuator

28 tubular duct

29 pre-catalyst device

30 catalyst device

31 tubular duct

32 anchoring area

33 electronic control unit a angle

Al longitudinal axis A2 longitudinal axis

Claims

1. A two-stroke internal combustion heat engine (1) with fuel direct injection; wherein the heat engine (1) comprises :

at least one cylinder (2), which comprises, on the inside, a combustion chamber (7) having a cylindrical symmetry around a first longitudinal axis (Al);

a piston (8), which is mounted inside the cylinder (2) so as to slide in a reciprocating manner;

an exhaust duct (4), which originates from the cylinder

(2) ;

at least one intake opening (9), which is obtained in a side wall of the combustion chamber (7) ;

at least one exhaust opening (10), which is obtained in the side wall of the combustion chamber (7) ;

a cylinder head (11), which closes the combustion chamber (7) at the top and has a lower wall;

a spark plug (12), which is mounted through the cylinder head (11) and has, at the bottom, a pair of electrodes (13) arranged inside the combustion chamber (7); and

an injector (14) , which is arranged through the side wall of the cylinder (2), and has a cylindrical symmetry around a second longitudinal axis (A2) and is provided with an injection nozzle (15), through which the fuel is directly injected into the combustion chamber (7);

wherein the injection nozzle (15) of the injector (14) faces the cylinder head (11) so as to spray the fuel towards the cylinder head (11) ;

wherein the second longitudinal axis (A2) of the injector (14) is inclined to form an acute angle with the first longitudinal axis (Al) of the combustion chamber (7); wherein the injector (14) generates a fuel jet (16) having an internally hollow shape due to a cone-shaped hole (17) present inside the fuel jet (16) ; and

wherein the injector (14) is sized so that the central hole (17) of the fuel jet (16) comprises, on the inside, the electrodes (13) of the spark plug (12) and, as a consequence, the fuel injected by the injector (14) wets the lower wall of the cylinder head (11) arranged around the spark plug (12), while it does not wet the electrodes (13) of the spark plug (12);

the heat engine (1) being characterized in that:

the second longitudinal axis (A2) of the injector (14) is oriented so as to intersect the electrodes (13) of the spark plug (12) .

2. A heat engine (1) according to claim 1, wherein the second longitudinal axis (A2), besides constituting the central cylindrical symmetry axis of the injector (14), also determines the fuel injection direction.

3. A heat engine (1) according to claim 1 or 2, wherein the cone-shaped fuel jet (16) has a conical symmetry around the second longitudinal axis (A2) .

4. A heat engine (1) according to claim 1, 2 or 3, wherein a conical hole (17) inside the fuel jet (16) has a conical symmetry around the second longitudinal axis (A2) .

5. A heat engine (1) according to any one of claims 1 to 4, wherein the second longitudinal axis (A2) of the injector (14) intersects the first longitudinal axis (Al) of the combustion chamber (7) at the electrodes (13) of the spark plug (12) .

6. A heat engine (1) according to any one of claims 1 to 5, wherein the acute angle formed between the second longitudinal axis (A2) of the injector (14) and the first longitudinal axis (Al) of the combustion chamber (7) ranges from 52° to 62° .

7. A heat engine (1) according to any one of claims 1 to 6, wherein the injector (14) is arranged in an intermediate position of the side wall of the cylinder (2) so that the injector (14) is not covered by the piston (8) during the fuel injection and is covered by the piston (8) during the combustion.

8. A heat engine (1) according to any one of claims 1 to 7, wherein the injector comprises:

a supporting body (18) having inside a feeding channel (19) to feed the pressurized fuel to the injection nozzle (15) ;

a valve seat (24) with a truncated-conical shape; and a shutting head (25) with a truncated-conical shape, which is part of a movable equipment, is designed to sealingly rest against the valve seat (24), is arranged on the outside with respect to the supporting body (18) and is pushed against the supporting body (18) by a closing spring (10) .

9. A heat engine (1) according to any one of claims from 1 to 8, wherein the spark plug (12) is coaxial to the longitudinal axis (Al) of the combustion chamber (7) .

10. A heat engine (1) according to any one of claims from 1 to 9, wherein:

there is no intake valve coupled to the intake opening (9) ; and

there is an exhaust valve (26), which is arranged immediately downstream of the exhaust opening (10) and is operated by an electric actuator (27) .

11. A heat engine (1) according to any one of claims from 1 to 10 and comprising an intake duct (6) , which ends in the intake opening (9) and is provided with a butterfly valve, which is servo-controlled by means of an electric actuator and adjusts the flow of the fresh air taken in.

12. A heat engine (1) according to any one of claims from 1 to 10 and comprising an intake duct (6) , which ends in the intake opening (9) and is not provided with a butterfly valve.

13. A heat engine (1) according to any one of claims from 1 to 12, wherein:

the heat engine (1) is a V-twin engine and comprises two twin cylinders (2) that are arranged at an angle relative to one another and project from a common crankcase (3) containing a crankshaft;

in a central area of the crankcase (3) between the two cylinders (2) it is arranged an intake duct (6), through which the fresh air needed for the combustion is taken in; and

a filter box integrated in the crankcase (3) is arranged between the two cylinders (2), houses an air filter and has a single trumpet through which the fresh air taken in enters.

14. A heat engine (1) according to claim 13, wherein the filter box has a structural function and helps to confer rigidity and solidity to the crankcase (3) .