GB2043172A

GB2043172A - System for scavenging and charging the cylinders of a two- stroke internal combustion engine

Info

Publication number: GB2043172A
Application number: GB8005137A
Authority: GB
Original assignee: Sulzer AG; Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1979-02-16
Filing date: 1980-02-15
Publication date: 1980-10-01
Also published as: DK59080A; CH638592A5; JPS55117028A; DE2907533C2; DE2907533A1; DK147380B; IT1148752B; GB2043172B; JPS5920849B2; IT8019805A0

Abstract

The cylinder head 3 is formed with two circular rows of air or mixture inlet ports 7 distributed uniformly and concentrically with the axis of the cylinder over the cross- section. The inlet ports 7 are closed by an intermittently moved valve slide 11 or respective valve slides (24), Figs. 4 to 9 (not shown), which are movable over a surface of the cylinder head facing the combustion space and biased into engagement with the surface. As a result of the uniform distribution of the charge entering the swept space (1) of the cylinder, a longitudinal advancing "displacing piston" of charge is formed which expels the combustion gases through piston-controlled exhaust ports (5), to provide optimum scavenging. <IMAGE>

Description

SPECIFICATION System for scavenging and charging the cylinders of a two-stroke internal combustion engine The invention relates to a system for scavenging and charging the cylinders of a two-stroke internal combustion engine, particularly but not exclusively a diesel engine.

Optimum scavenging and charging of the cylinders of a two-stroke internal combustion engine with air, or a mixture of fuel and air, is of course theoretically achieved when a maximum charge of scavenging air enters the cylinder chamber in the form of a "piston" substantially over the cylinder cross-section and expels the combustion gas in the cylinder by displacement. If this is to occur, the size of the air ducts must provide a maximum net flow area (which is the product of the geometric flow area and the flow coefficient) and the displacing "piston" of air must remain substantially a discrete element with very little break-up into stream lines and/or eddies i.e., the mixing zone which arises when air or gas mixture enters the combustion-gas-filled cylinder chamber must be very small.

Known scavenging systems do not come anywhere near achieving displacement scavenging of this kind. Of the known scavenging systems in which the air scavenges the cylinder chamber in an axial longitudinal flow operative in the direction of the cylinder axis, the systems which have recently become popular have air inlet ports directly controlled by the engine piston disposed around the cylinder periphery and have externally controlled exhaust ports which are disposed in the cylinder head. In such systems scavenging occurs in the same direction as the compression stroke of the engine and they are little, if at all, suited to the formation of an air "piston" acting as a displacing element. In these systems, since the scavenging air flows into the cylinder substantially radially, a swirl must be imposed on the flow through the inlet ports if a stable axial flow is to be produced.Unfortunately, the swirl inhibits or makes difficult the formation of a displacing "piston" of air which can sweep the cylinder cross-section fully, since a central core which is scavenged little, it at all, remains.

The scavenging systems which were used in eariier decades and which were based on reverse longitudinal scavenging have as closing elements for the air ports either, in the case of inlet ports distributed around the wall of the cylinder, sleeve valves, or, in the case of air ducts in the cylinder head, poppet valves opening into the cylinder chamber.In the case of inlet ports distributed around the cylinder wall, a swirling component must be produced near the cylinder head, and so it is impossible to form a "displacing piston" of charge air; in the case of the conventional sleeve valves, where the charge flows radially through the sleeve, because of the nature of the construction, there is always a gap between the sleeve and the cylinder wall so that the sleeve cannot seal completely with the cylinder wall during the compression, expansion and combustion strokes. Consequently, however carefully the sleeve valve components are machined and produced, this kind of construction cannot seal the combustion chamber in a manner comparable with poppet valves.

In the case of ports which extend through the cylinder head and which are closable by poppet valves, the valve elements prevent a smooth entry of the charge air and result in a double deflection of the entering charge, with the result that the gas flow experiences disturbances causing it to break up into a large number of discrete stream lines. In this way a relatively long and extended mixing zone arises, and so there is only a limited possibility of forming a "displacing piston" of charge air.

It is an object of the invention to devise a scavenging and charging system in which a displacing "air piston" may be built up and maintained as far as possible; it is another object to ensure that the closure elements for the inlet ports to the cylinder chamber provide a sealing effect similar to that provided by poppet valves in the closed position, but do not cause the disturbances in the air flow previously referred to.

Accordingly the present invention provides an arrangement for scavenging and charging a cylinder of a two-stroke reciprocating internal combustion engine comprising a number of inlet ports disposed in the head of the cylinder and each closable by means of a valve slide and one or more exhaust ports each being closable by the piston, in which each valve slide is disposed to slide on a surface of the cylinder head facing the combustion space, being biassed into engagement with that surface, and provided with means to move it intermittently between a closed position in which it closes the associated inlet port or ports and an open position in which the port or ports are substantially unobstructed for an axial flow pf charge into the cylinder.

The ignition pressure in the cylinder in operation presses against the valve slides which are disposed on the cylinder head in the combustion space and which are thus held in sealing engagement with the head, the slides engaging with the cylinder head like a poppet valve. The load on the valve slide decreases immediately the pressure in the cylinder has decreased, e.g. as a result of the exhaust ports opening at the end of a power stroke with the piston descending. The valve slide may be then be moved on the cylinder head without clearance but also without being subjected to loads resulting from gas pressure. Consequently, the arrangement has the advantages of poppet valves so far as sealing tightness is concerned and the advantage of sleeve valves so far as movement is concerned.

The use of valve slides also makes it possible for a large number of inlet ports, preferably at least 10 inlet ports to be used, distributed over at least two concentric circles in the cylinder head.

The inlet ports may be arranged so that the flow through them is free from transverse deflection relative to their axes and not relative to the axis of the cylinder, since the streams issuing from any air port widen unavoidably, a flow completely free of transverse deflection can be achieved only in the port itself.

The lack of transverse deflections in the ports leads to a considerable increase in the flow coefficient so that in all there is a very high net flow area for the charge entering the cylinder chamber, thus ensuring that a very large quantity of air or gas mixture flow through the cylinder head axially of the cylinder-i.e. without radial components and is distributed substantially uniformly over the cylinder cross-section, into the cylinder chamber.

The required flow properties can be further improved if the entry cross-sections of the inlet ports in the cylinder head are such that the flow becomes free from detachment upon entering the ports and/or if the exit ends of the ports are such that the air flow entering the cylinder chamber experiences optimal widening so as homogeneous scavenging is concerned. These two features can be provided by a slight funnel-shaped rounding of the edges of the inlet ports at the entry to the combustion space. For optimum widening of the streams (and "optimum" is to be understood in the sense of uniform scavenging) the widening streams should cover the complete cross-section of the cylinder chamber substantially uniformly after a short initial distance of flow into the cylinder and such distance should lose so much of their individual kinetic energy as not to interact by eddying.

For the scavenging and charging of the cylinder the system increases the inlet flow time-cross-sectional area from which the resulting replacement-cross-sectional area for the scavenging flow is determined, the latter cross-section being a simple measure of the flow resistance operative during scavenging.

The inlet flow time-cross-sectional area, which may be defined as follows: L A.a= Vn ----Vn (1) 2w where A is the mean inlet port area at right angles to the flow in square inches, CL is the inlet duration in degrees of crank angle, L is the delivery ratio, V is the displacement volumn, w is the mean inlet velocity, and n is the number of revolutions per minute. (For a more detailed explanation reference may be made to Chapter 7 of P.H.Schweitzer "Scavenging of Two-Stroke Cycle Diesel Engines" New York 1973, particularly pages 58 and 59); thus reflects for given timings the resistance to be overcome by the scavenging flow, and can be depicted (for the flow entering the cylinder chamber) as the area below the curve e in Fig. 10 of the accompanying drawings which also shows the open cross-section of the exhaust ports (curve a) plotted against the crankshaft angle--i.e. against time.

The increased flow time-cross-sectional area therefore corresponds to a considerably reduced gas flow resistance, and so the crank angle necessary for scavenging can be reduced and the effective stroke of the engine therefore lengthened. Also, the exhaust gases remain hotter, since they are displaced without appreciable mixing with the scavenging air, so that the efficiency of any exhaust gas turbine utilised is improved and, with the corresponding waste heat utilization, the overall efficiency is also improved.

Also, of course, displacement scavenging is much cleaner for a given excess of scavenging air than any other form of scavenging. Consequently, more air is available for combustion at a given engine mean pressure and at a given charging pressure, resulting in a decrease in specific fuel consumption.

Another advantage of the scavenging system is that there is no catching or jamming of solid combustion residues as in the case of poppet valves, since the slide valve always scapes such residues off the orifices of the air ducts; accordingly, all those edges of the or each valve slide which are adjacent the cylinder head and any cylinder head edges which are near the slides can, with advantage, have sharp edges.

Also, after-charging of the cylinder is possible with the system by control on the inlet flow means of the known "tuned pipe" principle or by the use of mechanical control elements such as check valves. In the "tuned pipe" principle, the dimensions of the inlet air conduits to the cylinder head are so adapted to the open time of the air inlet ports to the cylinder chamber, that the oscillating air column in the inlet conduits has a half-period directing additional charge air into the cylinder chamber shortly before closure of the inlet ports after the closure of the exhaust ports by the ascending piston, i.e. in the time n shown in Fig. 10.

The considerations of a maximum overall open cross-section for the inlet ports together with an adequate bearing surface for the or each valve slide on the cylinder head surface leave few possibilities for the cross-sectional shape of the inlet ports and for accommodating the means and elements for producing the necessary motion of the or each valve slide.

Thus disregarding complicated cross-sectional shapes which are difficult to produce, the inlet ports are preferably circular in cross-section; consequently, the inlet ports are preferably arranged in at least two circular rows of longitudinal bores of the same diameter within a row but of different diameters in each row, the rows being concentric with the cylinder axis. In this event, a single valve slide can be provided which is annular and formed with orifices corresponding in number and size to the ports in the cylinder head, the diameters of the ports and of the orfices being less than half the between-centres spacing of the ports in the associated row. In this arrangement, when the orifices are in line with the ports in the open position of the valve slide, the orifices form the final port of the inlet port to the cylinder.

The elements for moving the or each valve slide are preferably arranged to product little, if any, resistance in the charge air flow. They can take the form, for instance, of pins which extend through the cylinder head and which extend through the inlet ports and which may include resilient elements to bias the valve slides into sealing engagement with the cylinder head. Additional bores can be provided for the driving pins so as to permit over-run in the open and/or closed position, thus leaving the flow area of the port free from such pins in the open position.

Conveniently a hydraulic actuator of known design per se may be used for actuating the valve slides with the position of the or each valve slide being determined by the actuators.

Also, combustion can be improved by forming (after the scavenging just before the end of the charging of the swept space, i.e. just before the closing of the inlet ports by the valve slide) a known ring or toroidal turbulence by the valve slide closing the inlet ports of turbulence by the valve slide closing the inlet ports of two rows at different instants of time; for instance, there can be slight angular offsets of the valve slide orifices of one row of inlet ports relative to the orifices associated with another row.

In order to promote a fuller understanding of the above, and other aspects of the invention, some embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a longitudinal cross-section through a cylinder of an internal combustion engine showing a first embodiment of the invention, the view being on the section line I-I of Fig. 2, Figure 2 is a section on the line ll-ll of Fig.

1, Figure 3 is a section on the line Ill-Ill of Fig. 2, Figure 4 is a partial plan view of a cylinder head as seen from the cylinder chamber showing a second embodiment of the invention which each single air duct is closed by its own slide, Figure 5 is a view similar to Fig. 4 of the second embodiment but with the air ducts open, Figure 6 is a view on the line VI-VI of Fig.

4, Figure 7 is a view on the line VII-VII of Fig.

5, Figure 8 is a view of Fig. 6 looking in the direction of an arrow B therein, Figure 9 is a view similar to Fig. 3 looking on to the cylinder head of the second embodiment from the outside, and Figure 10 shows graphically the patterns of the open cross-sections of the exhaust ports and inlet ports plotted against the crank angle a-i.e. against time.

In the arrangement of Fig. 1 the swept space 1 of a cylinder 2 of a diesel engine (not shown in greater detail) is closed at the top by a cylinder head 3 having at its centre a fuel injector 4. The wall of the cylinder 2 is formed with exhaust ports 5 which a piston 6 exposes as it descends to bottom dead centre and closes as it rises again.

Disposed in the head 3 are air inlets ports 7 for supplying air-in the case of diesel engi nesor a fuel-air mixture in the case of petrol or gas engines. The inlet ports 7 are arranged in two circular rows 9, 10 concentric with the cylinder axis (Fig. 2) and take the form of longitudinal bores in the head 3, the bores being of one diameter in row 9 and of a smaller diameter in row 1 0. The inlet ports connect the swept space 1 to an air chamber which is shown schematically at 8. The inlet ports 7 are opened and closed in timed manner by an annular valve slide 11 formed with orifices 1 2 corresponding in arrangement, shape and size to the inlet ports 7. The valve slide 11 is shown in an open position in the left-hand half of Figs. 1 and 2 and a closed position in the right-hand half.

The annular valve slide 11 engages in an annular recess 1 3 in the head 3 and is retained by actuating elements or pins 14 which extend through the ports 7. The pins 14 are carried by resilient arms 1 5 which are in the form of spring strips, and are thus biassed and cause the valve slide 11 to bear upwardly on the base of the recess 1 3 in the cylinder head 3 even when there is a negative pressure in the swept space 1.All the arms 1 5 are connected to a sleeve 1 6 which may be rotated by a drive (not shown) for moving the valve slide 11 thus opening and closing the inlet ports 7 as diagrammatically illustrated by a lever 1 7. The drive, which is preferably a hydraulic device of known design per se, moves the valve slide 11 intermittently at a cadence corresponding to the desired engine timing; in the present example the movement is oscillating but in other constructions (not shown) it could be progressive movement in one direction.

So that air may enter the inlet ports 7 in the head 3 undisturbed, the arms 1 5 are so shaped and guided as not to extend over the cross-sections of the ports 7, at least in the open position of the valve slide 11 (Fig. 3).

Another feature necessary to ensure that the air flow is not disturbed is that the pins 14 be disposed outside the effective cross-sectional area of the ports 7 when the valve slide 11 is in its end positions; consequently, the head 3 is formed with recesses or port bores 1 8 for the pins 14, the ducts extending parallel to the ports 7. Such recesses 18 may for production reasons be produced by drilling and over some of their periphery the bores 1 8 overlap with the ports 7 and are thus open to them.Thus, the ports 7, through a pin 14 moves along an arc during a movement of the valve slide 11, are associated with additional bores or chambers 1 8 which receive the pins 14 in the end position of the valve slide 11, and which are of a diameter such that the pins 14 may leave completely the cross-section of the ports 7. The arc along which the pins 1 4 move is preferably of greater diameter than the circle 9 which interconnects the centres of the ports 7 so that the pins 14 are free in all directions in the bores 18-i.e.

they do not strike the walls thereof. The number of pins 1 4 is preferably from four as in the present case up to a maximum number corresponding to the number of ports 7.

As Fig. 2 shows, the ports 7 in the two circular rows 9, 10 are distributed uniformly and symmetrically over the cross-section of the swept space 1 and the head 3, to ensure uniform charging of the space 1 over its cross-section with a large number of air streams. The size and number of the ports 7 are determined by two factors; the need for maximum possible total open cross-section, for the sake of very low flow velocities for a given volume of scavenging air and therefore a gentle entry of the air streams, and the contrary requirement that, with the valve slide 11 in a closed position, there must be an overlap sufficient to seal the space 1 from the ports 7 (including the additional bores 18). In the embodiments shown there are 8 longitudinal ports 7 in each of the two rows 9, 10 and so the valve slide 11 makes an oscillating rotating movement of 22.5 .

As mentioned the valve slide 11 is shown in an open position in the left-hand halves of Figs. 1 and 2, whereas in the right-hand sides of Figs. 1 and 2 the position of the orifices 1 2 in relation to the ports 7 and the bores 1 8 the valve slide 11 in a closed position can be seen. Fig. 2 shows the overlaps in the closed position between, on the one hand, the ports 7 and bores 1 8 and, on the other hand, the orifices 1 2 in the valve slide 11 and also shows the minimum overlap between the bores 1 8 and the orifices 1 2.

Since the valve slide 11 is not actuated directly by the piston 6 or crankshaft, the crankshaft just providing power for the hydraulic drive for rotating the valve slide 11, the opening and closing of the ports 7 can proceed substantially instantaneously-i.e., very rapidly-while the piston 6 makes only a very slight movement in the cylinder 1.

For maximum reduction of disturbances to the flow at its entry into the ports 7 and into the cylinder swept space 1 and for maximum reduction of cross deflections and detachments relatively to the axis of the ports 7, the entry edges 1 9 of the ports 7 and the exit edges 20 of the orifices 1 2 are rounded slightly in a funnel shape, whereas those edges 21 and 22 (Fig. 7) of the ports 7 and of the orifices 1 2 which are near one another are sharp, thus ensuring that any combustion residues sticking to the latter edges are scraped off in the movements of the slide.

In the second embodiment, shown in Figs.

4 to 9, the single axial valve slide 11 of Fig.

1 is replaced by a valve slide 24 for each port 7, the latter again being in the form of a longitudinal bore extending through the cylinder head 3.

Each slide 24 is pivotable by its own actuating pin 14 guided in its own separate longitudinal bore in the cylinder head 3. Each valve slide 24 is formed, in the surface facing the combustion or cylinder swept space 1, with a recess 25 whose base 26 (Fig. 8) is in the form of an inclined plane and arranged to be abutted by a stationary catch or the like 27 during the final part of the closing movement of the valve slide 24. The catch 27 is secured on a rod or the like which extends in another separate bore through the cylinder head and is secured, in that surface of the head 3 which is remote from space 1, by means of a screwthread and of a nut 29; the separate bore for the catch 27 being sealed by the catch bearing axially on the surrounding part of the head 3 against the gas pressure inside the cylinder. The function of the catch 27 is to form in addition to the eccentric pin 14 a second locating point for the closed valve slide 24; since for reasons of space the catch 27 cannot be disposed diametrically opposite the pin 14 if the opening of the valve slides 24 is not to be impeded without wasting space, the catch 27 is disposed in mirror image positions of the pin 14 as seen in a plane passing through the centre of the particular port 7 concerned and the axis of the cylinder.

Each pin 14, to which the associated valve slide 24 is rigidly secured, is biassed outwardly by a spring element 30 and is rigidly connected at its free end projecting from the cylinder head 3, to a pivot lever 31 having a guide pin 32 engaging in a slot 34 in an arm 33 (Fig. 9). Each valve slide 24 of each circle or row 9, 10 is connected to a common arm 33. All the arms 33 are rigidly secured to the central sleeve 1 6 which is oscillated about the axis of the cylinder by a drive (not shown) for opening and closing all the valve slides 24 together. As in the first embodiment (Fig. 3, spring element 15), the arms 33 are shaped so as not to impede the flow of air from the air chamber 8 into the longitudinal ports 7 at least with the valve slides 24 in their open position.

The invention is not of course limited to the embodiments shown. For instance, the longitudinal ports 7 can abe arranged in more than two rows, and more than a single annular valve slide formed with orfices can be provided. Also, the inlet ports can have their axes, which are parallel to one another, disposed at an angle to the cylinder axis and not parallel thereto as in the embodiments shown.

Another possibility is to arrange for the discrete streams to widen to different extents in various directions, for instance, by the funnellike curvatures of the exit edges 20 of the ports 7 or orifices 1 2 varying around the periphery. It should be understood that in effect the orifices 1 2 in the valve slides form the exit or outlet end of the inlet ports into the swept chamber 1.

Claims

1. An arrangement for scavenging and charging a cylinder of a two-stroke reciprocating internal combustion engine comprising a number of inlet ports disposed in the head of the cylinder and each closable by means of a valve slide and one or more exhaust ports each being closable by the piston, in which each valve slide is disposed to slide on a surface of the cylinder head facing the combustion space being biassed into engagement with that surface, and provided with means to move it intermittently between a closed position in which it closes the associated inlet port or ports and an open position in which the port or ports are substantially unobstructed for an axial flow of charge into the cylinder.

2. An arrangement as claimed in Claim 1, in which said inlet ports are disposed over the cross-section of the cylinder in a circular row which is substantially concentric with the axis of the cylinder.

3. An arrangement as claimed in Claim 1 or 2, in which said inlet ports are disposed over the cross-section of the cylinder in two or more circular rows which are substantially concentric with the axis of the cylinder.

4. A scavenging arrangement as Claimed in Claim 1, 2 or 3, in which at least 10 inlet ports are distributed over the cross-sectional area of the cylinder head.

5. A scavenging arrangement as claimed in Claim 1, 2, 3 or 4, in which the inlet ports are longitudinal bores in the cylinder head.

6. A scavenging arrangement as claimed in Claim 2 or 3, in which a single valve slide is provided formed with orifices corresponding in number and size to the inlet ports in the cylinder head, the diameters of the inlet ports and of the orifices being less than half the between-centres distance of the inlet ports in the associated row.

7. A scavenging arrangement as claimed in Claim 6, as dependent on Claim 3, comprising a plurality of such valve slides.

8. A scavenging arrangement as claimed in Claim 6 or 7, in which the valve slide orifices are so arranged as to open or close the inlet ports of two rows at different instants of time.

9. A scavenging arrangement as claimed in any preceding Claim, in which entry crosssections of the inlet ports in the cylinder head are such as to ensure a non-detaching charge flow at the entry into the inlet ports.

1 0. A scavenging arrangement as claimed in any preceding Claim, in which the exit ends of the inlet ports or of the valve slide orifices are such as to produce optimum widening of the charge flow entering the cylinder chamber to promote uniform scavenging.

11. A scavenging arrangement as claimed in any preceding Claim, in which any edge of a valve slide which moves over said head surface, and any edge on the cylinder head which moves over a valve slide is sharp.

1 2. A scavenging arrangement as claimed in any preceding Claim as dependent on Claim 6, in which the orifices in the or each valve slide are of slightly larger diameter than the associated inlet ports in the cylinder head.

1 3. A scavenging arrangement as claimed in any preceding Claim in which said means for moving the or each valve slide is such as not to detract from low charge flow resistance in said ports.

1 4. A scavenging arrangement as claimed in any preceding Claim in which said means for moving the valve slide comprises pins which extend through the cylinder head and which are associated with resilient bias means arranged to cause the associated valve slide to bear on said surface of the cylinder head.

1 5. A scavenging arrangement as claimed in Claim 14 in which the pins extend through respective inlet ports in the cylinder head.

16. A scavenging arrangement as claimed in Claim 1 5 in which additional bores to accommodate the pins are provided as lateral extensions of the inlet ports and are such as to accommodate the pins in the opened position and in the closed position of the valve slide.

1 7. A scavenging arrangement as claimed in any preceding Claim in which the or each valve slide is so moved by means of a sleeve which is disposed on the cylinder head con centric with the cylinder axis.

1 8. A scavenging arrangement as claimed in any preceding Claim as dependent on Claim 3, in which cross-sections of the inlet ports of the various rows are different in size.

19. A scavenging arrangement for a twostroke reciprocating internal combustion engine substantially as herein described with reference to Figs. 1, 2 and 3 or Figs. 4, 5, 6, 7, 8 and 9 of the accompanying drawings.