Multi-cylinder diesel engine with variable valve actuation

Details Multi-cylinder diesel engine with variable valve actuation Multi-cylinder diesel engine with variable valve actuation

1999-10-08

2001-05-29

Walberg, Teresa (Department: 3742)

Internal-combustion engines

Poppet valve operating mechanism

With means for varying timing

C123S090120, C123S090160

Reexamination Certificate

active

06237551

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to multi-cylinder Diesel engines for motor-vehicles, in particular for trucks. The invention relates particularly, even if not exclusively, to engines of this type which are supercharged by means of a turbo-supercharger actuated by the exhaust gases.
In previous Italian patent application No. TO96A000327, which was still secret at the priority date of the present application, the Applicant has proposed a gasoline engine comprising:
at least one intake valve and at least one exhaust valve for each cylinder, each provided with respective spring means biassing the valve to a closed position, for controlling respective intake and exhaust conduits, a cam shaft, for actuating the intake and exhaust
valves of the engine cylinders by means of respective tappets, each intake valve and each exhaust valve being driven by a cam of said cam shaft,
each of said tappets being adapted to cause the respective intake or exhaust valve to open, against the action of said return spring means, by means of the interposition of hydraulic means including a chamber of fluid under pressure,
the chamber of fluid under pressure associated with each intake or exhaust valve being adapted to be connected, by means of a solenoid valve, to an outlet channel, in order to uncouple the valve from the respective tappet and cause the valve to close rapidly under the action of the respective return spring means,
electronic control means for controlling each solenoid valve, in order to vary the opening time and stroke of the respective intake or exhaust valve, as a function of one or more parameters of operation of the engine.
The above described system provides a variable control on the opening of the intake and exhaust valves, without modifying the mechanical elements which control the valve movement. Indeed, whereas in a conventional timing system the movement of each intake or exhaust valve is univocally determined by the geometry of the mechanical elements which control the valve (cam, rocker arm, tappet), in the above described system the solenoid valve controlling the chamber under pressure associated with a given valve can be caused to open at any time which is desired so as to empty said chamber from the oil under pressure and cause the intake or exhaust valve to close rapidly, under the action of the respective return spring means, even during a stage in which the respective cam would tend to keep said valve opened.
SUMMARY OF THE INVENTION
The object of the present invention is that of applying the above-mentioned variable valve actuation system to a Diesel engine, so as to obtain the possibility to provide different selectable modes of operation of the engine.
In view of achieving this object, the invention provides a multi-cylinder Diesel engine, comprising all the above indicated features and further characterized in that:
each cam of the cam shaft has a profile such as to tend to cause the respective intake or exhaust valve controlled thereby to open not only during a conventional opening stage within a regular cycle of operation of the engine, but also at some additional stages of the cycle,
and in that said electronic control means are adapted to cause each solenoid valve to open, so as to keep the respective intake or exhaust valve closed, during said conventional stage and/or during one or more of said additional stages during which the respective cam would tend to cause the valve to open, so that the engine can be selectively operated according to different modes of operation by controlling said solenoid valves.
According to a first important preferred feature of the invention, said electronic control means are adapted to provide a mode of operation of the engine as a brake, wherein no fuel is injected into the cylinders, and wherein each solenoid valve is controlled so as to keep the intake and exhaust valves of each cylinder closed substantially during each stage when the respective piston raises from the bottom dead centre (BDC) to the top dead centre (TDC), so as to exploit the power absorbed at each piston upstroke to obtain a braking effect, said electronic means being also adapted to allow each exhaust valve, by closing the respective solenoid valve, immediately before the piston reaches the TDC, under the action of the respective cam.
As it is clearly apparent, the above-mentioned mode of operation of the engine as a brake represents a first example of application of the above described principle, according to which the cams of the cam shaft have a profile such as to cause the respective valves to open not only during the conventional opening stage within a regular cycle of operation of the engine, but also at some additional stages, whereas the respective solenoid valve is opened at determined stages in order to cause the valve to remain closed at these stages, or to cause the valve to be driven differently, even if the cam would tend per se to cause the valve to open. In the above-mentioned case, the cams have profile portions adapted to drive the valves in the mode of operation as a brake, which profile portions are “enabled” by the solenoid valves only when the operation as a brake is desired. As it will be shown more in detail in the following with reference to the annexed drawings, in the case instead of a regular operation of the engine, the solenoid valves are controlled by the above-mentioned electronic control means so as to “enable” the cam profile portions which control the regular opening cycle of the valves and “to disable” the profile portions which are exploited during the mode of operation as a brake.
Reverting in particular to the mode of operation as a brake, it is to be noted that in this mode each upstroke of the piston from the BDC to the TDC is exploited in order to obtain the braking effect, so that the piston has an active stroke each time it raises, according to a two-stroke cycle of operation. The power absorbed in this manner is very high and amongst to 100-120% of the rated output with regular combustion operation. For instance, if at a speed of 2400 rpm the engine output in combustion operation is 257 Kw, the power absorbed as a brake at the same speed would be 305 Kw. Since the exhaust valve is opened at the end of each piston upstroke, during the downstroke the compressed air is not able to return the whole power previously absorbed during the piston upstroke, which would happen instead if the air volume trapped within the cylinder would be always the same, thus behaving as a king of spring.
According to a further preferred feature of the invention, which relates to the case of a Diesel engine supercharged by a turbo-supercharger actuated by the exhaust gases, in the above-mentioned mode of operation of the engine as a brake, the exhaust valve is held opened also during a first part of the piston downstroke from the TDC, so as to draw air from the exhaust conduit into the cylinder, in order to decrease the air flow fed to the turbo-supercharger. This is done because, since in the mode of operation as a brake the engine operates according to a two-stroke cycle, the air flow which is fed to the turbo-supercharger in this mode would tend to be double than that normally fed to the turbo-supercharger during regular combustion operation of the engine. This would cause an excessive lowering of the turbo-supercharger efficiency and a resulting very poor compression of the air fed to the cylinder. By leaving the exhaust valve opened during the first part of the piston downstroke, a portion of the air previously fed to the exhaust conduit is taken into the cylinder, which decreases the flow through the turbine of the turbo-supercharger thus keeping the turbo-supercharger efficiency at acceptable levels.
Finally, yet in the said mode of operation of the engine as a brake, during the final part of the piston downstroke the intake valve is held opened, so as to allow the cylinder to be filled fully in view of the following upstroke, in which the intake and exhaust valves are substantially closed, so as to commenc

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