Internal-combustion engines – Two-cycle – Whirl through piston-controlled ports
Reexamination Certificate
1999-02-10
2001-03-06
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Two-cycle
Whirl through piston-controlled ports
C123S07900R
Reexamination Certificate
active
06196171
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns an improvement to two-stage internal combustion engines with loop scavenging of the type
having at least one variable volume working chamber delimited by a cylindrical wall in which a piston slides, the mobile top face of the piston and a fixed cylinder head,
operating in accordance with the two-stroke cycle, with a loop scavenging system via the cylinder head, controlled by at least one inlet valve cooperating with a seat, preferably of generally conical shape, to cause the working chamber to communicate cyclically with an inlet cavity communicating with means for supplying air to the engine and by at least one exhaust valve cooperating with a seat, preferably of generally conical shape, to cause the working chamber to communicate cyclically with an exhaust cavity communicating with the combustion gas exhaust system of the engine,
and in which said inlet and exhaust valves are disposed so that the air entering the working chamber through the inlet valve causes scavenging of at least a substantial part of the burned gases in the chamber and their evacuation via the exhaust valve.
BACKGROUND OF THE INVENTION
In engines of the above type the difference AP in the gas pressure between the means supplying the engine with air at pressure P and the engine combustion gas exhaust system is relatively low and in practice is imposed by the specifications of the supercharged air supply means.
Scavenging can take place only during a limited part of each cycle, another and large part of the cycle being devoted to compression and expansion of the gases renewed in the chamber.
As a result the geometry and the operation of the inlet and exhaust valves play a decisive role in the efficiency, the power and the speed of the engine.
Increasing the size of the valves rapidly runs up against a geometrical limit imposed by the dimensions of the cylinder head while increasing the valve lift, that is to say the distance the valve moves away from its seat, and the lift speed, which are determined by the profile of the cams opening and closing the valves, is rapidly limited by mechanical constraints imposed by the permissible contact pressure between the nose of the cam and the components that it actuates.
This limits performance, i.e. the permeability of the cylinder head, the efficiency of use of the air passing through the cylinder head, i.e. the ratio between the mass of air enclosed in the working chamber at the end of scavenging to the mass of air passing through the cylinder head, and the scavenging efficiency, i.e. the ratio between the mass of air and the total mass of gas enclosed in said chamber at the end of scavenging.
Patent application EP-A-0 673 470 (or U.S. Pat. No. 5,555,859 or WO 95/08052) has made it possible to improve significantly on the above limitations by providing a single inlet valve and a single exhaust valve,
said inlet and exhaust valves being coaxial circular cylinders, preferably coaxial with the cylindrical wall of the working chamber, the coaxial arrangement being such that the inlet valve is outside the exhaust valve,
the seat of the inlet valve being attached to the cylinder head and oriented so that the pressure of the drive fluid contained in the working chamber exerts a force that tends to press said valve onto its seat, said seat being in the immediate vicinity of the periphery of the upper part of said cylindrical wall inside which the piston slides, and in contact with the cylinder head,
said exhaust valve having a tubular part the inside wall of which slides on a fixed hub carried by the cylinder head, to which it is sealed by sealing means, and the end of which towards the chamber has a bearing surface coaxial with said tubular part so that it can cooperate with a seat provided inside the lower part at the same end of the chamber as said inlet valve, enabling communication of said exhaust cavity with the working chamber, by virtue of the annular space delimited radially by the inside wall of the inlet valve and by the outside wall of the exhaust valve.
This arrangement optimises and controls scavenging and doubles the actual lift of the exhaust valve because the inlet and exhaust valves lift in opposite directions.
If means are provided to cause the inlet air passing through the inlet valve to rotate, axi-symmetrical centrifugal layering can be achieved and the fuel can be injected into a hot central area that is relatively impoverished in oxygen, to obtain the advantages described in the above patent and in patent application FR-A-2 690 951.
BRIEF SUMMARY OF THE INVENTION
The present invention proposes to improve further the performance of engines having concentric inlet and exhaust valves, in particular as defined above, enabling a choice between reducing the scavenging time and consequently increasing the usable expansion stroke of the engine and therefore its efficiency, or, for a given angular duration of scavenging during the upward travel of the piston, a high permeability of the cylinder head enabling the rotation speed of the engine to be increased.
Another objective of the invention is to reduce significantly wear of the valves and their seat and in particular the inlet valve and its seat, the exhaust valve and its seat being generally better protected by the effects of lubrication by the carbon deposits caused by combustion gases moving towards the exhaust.
The invention consists in a two-stroke internal combustion engine with loop scavenging, preferably of the compression ignition type, of the kind described in the preamble, and preferably in which the inlet and exhaust valves are coaxial circular cylinders, preferably coaxial with said cylindrical wall, preferably with the inlet valve outside the exhaust valve, and preferably having the other features of the valves of concentric valve engines of the type described hereinabove, characterised in that, for at least one of said inlet and exhaust valves the surface of the valve downstream of the bearing surface of said valve, in the direction of flow through it, on the one hand, and the surface of a part extending the seat of said valve, with which said bearing surface cooperates, and also situated downstream of said seat, on the other hand, are configured to constitute a substantially isentropic diffuser discharging into the cavity downstream of the valve.
“Isentropic diffuser” means a divergent nozzle in which the flow of gas through the nozzle is slowed and compressed virtually isentropically.
The outlet into said downstream cavity has a discharge section greater than the geometrical flow section of the valve in the fully open position between the bearing surface of said valve and its seat.
“Geometrical flow section of the valve” means the minimum unrestricted flow section between the lifted valve and its seat. This section remains in the vicinity of the bearing surface, i.e. the areas of contact between the closed valve and its seat, but its axial position can vary with the lift of the valve.
Accordingly, flow downstream of the valve is effected in a diffuser whose section increases progressively, at least on approaching the cavity downstream of the valve, i.e. the working chamber in the case of an inlet valve and the exhaust cavity in the case of the exhaust valve, the discharge section of the diffuser, i.e. the section through which the diffuser opens into said cavity, being greater than the flow section of the valve in the fully open position between the bearing surface of the valve and its seat.
In a preferred embodiment, the ratio between the discharge section where the flow from the valve enters the cavity downstream of the latter, in the direction of flow, and said geometrical flow section of the valve in the fully open position is at least equal to the critical ratio calculated for the value of the ratio of the pressures of the fluid flowing in said valve on either side of the latter during normal operation of the engine.
The critical ratio is defined as that for which the speed of the flow reaches the speed of sound at the throat of th
Larson & Taylor PLC
McMahon Marguerite
S.N.C. Melchior Technologie
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