US3805750A

US3805750A - Two cycle engine with auxiliary exhaust ports

Info

Publication number: US3805750A
Application number: US00233588A
Authority: US
Inventors: W Tenney
Original assignee: Individual
Current assignee: Yamaha Motor Co Ltd
Priority date: 1972-03-10
Filing date: 1972-03-10
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23

Abstract

In two cycle internal combustion engines of the type which utilize the underneath side of the power piston as a scavenge pump piston and which have a central exhaust port area in the cylinder wall flanked on both sides by scavenge ports, the addition of auxiliary exhaust port areas adjoining or abutting each side of the main exhaust port area, these additional areas extending the upper part of the exhaust port area circumferentially as far as approximately halfway around the cylinder bore. These additional auxiliary exhaust port areas are located at least in part in a portion of cylinder wall overlying and located directly above the portions of the flanking or side scavenge ports located next to the main, central exhaust port area.

Description

United States Patent [191 Tenney [451 Apr. 23, 1974 TWO CYCLE ENGINE WITH AUXILIARY [21] Appl. No.: 233,588

11,519 3/1956 Germany 123/73 R OPTIONAL -RESONANT OR PULSE TUNED EXHAUST SYSTEM Primary ExaminerWendell E. Burns Attorney, Agent, or Firm-Dugger, Johnson & Westman [5 7] ABSTRACT In two cycle internal combustion engines of the type which utilize the underneath side of the power piston as a scavenge pump piston and which have a central exhaust port area in the cylinder wall flanked on both sides by scavenge ports, the addition of auxiliary exhaust port areas adjoining or abutting each side of the main exhaust port area, these additional areas extending the upper part of the exhaust port area circumferentially as far as approximately halfway around the cylinder bore. These additional auxiliary exhaust port areas are located at least in part in a portion of cylinder wall overlying and located directly above the portions of the flanking or side scavenge ports locate next to the main, central exhaust port area. i

14 Claims, 7 Drawing Figures R EED OR ROTARY INLET VALVE Pmisminmzsw 3.805750 I SHEET 1 OF 4 OPTIONAL RESONANT PULSE TUNED EXHAUST SYSTEM REED 0R ROTARY INLET VALVE PATENTED PR 2 1914 SHEET 3 UP 4 I OPTIONAL RESONANT OR PULSE TUNED EXHAUST SYSTEM TWO CYCLE ENGINE WITH AUXILIARY EXHAUST PORTS BACKGROUND OF THE INVENTION engine or its components, bottom or lower portions of the engine or its components, directions or motions upward or downward etc., as in my copending patent application Ser. No. 224,756, filed Feb. 9, 1972, (Attorneys Docket No. T05.209).

2. Field of the Invention The field of the invention is that of the two piston internal combustion engine of the type which utilizes the underneath side of the power or work piston as a scavenge pump piston and which has a central main exhaust port area flanked on either side by scavenge portareas, these port areas being located in that portion of the cylinder bore situated next above the upper or timing edge of the power piston head when it is in the bottom dead center position. All of these port areas are covered and uncovered by the timing edge of the piston head and adjoining piston wall areas in the course of the normal up and down motion of the iston within the cylinder bore. Additionally, the flanking or side scavenge port areas extend around the cylinder bore wall toward the exhaust port area to such a degree that at least part of these side scavenge port areas occupy the same 180 sector of cylinder bore wall as the main exhaust port area.

3. Description of Prior Art In two cycle internal combustion engines of the subject type a very widely used and successful arrangement of the scavenge port and exhaust port areas has come into existence during prior years. This arrangement has the distinguishing feature of a central exhaust port area located in the cylinder wall so as to be covered and uncovered by the upper or timing edge of the power piston (together with its adjoining piston wall area) in the course of the normal up and down travel of the piston in the cylinder bore, flanked on both sides by scavenge port areas which are similarly piston timed astto being open or closed to the power cylinder contents. The exhaust port area is located in the lower portion of the cylinder bore and in direction of piston travel extends from a lower edge substantially at the timing edge of the piston head when it is in bottom dead center position, up to an upper edge which may be located some 30 percent up to 50 percent or somewhat more of the piston stroke higher up on the cylinder wall. In cylinder circumferential measurement, the central exhaust port area generally occupies an arc or sector of cylinder wall corresponding to l5-25 percent of the wall circumference.

At each side of this central exhaust port area are located scavenge port areas which will be referred to hereinafter as side scavenge port areas. These side" scavenge port areas extend circumferentially into the same 180 sector of cylinder wall in which the main exhaust port area is centered. in addition to the side scavenge port areas there may also be provided additional scavenge port areas which are located in the segment of cylinder wall which lies diametrically opposed to, or opposite the central exhaust port area. These port areas will hereinafter be referred to as opposite scavenge port areas. It should be noted at this point that the side and opposite" scavenge port areas often merge into one another without any clear cut line of demarcation. In direction of piston travel, these scavenge port areas extend from a lower edge located at a level substantially at the timing edge of the piston head with the piston at bottom dead center position,

up to a normal height of more or less 18-28% of the piston stroke higher up on the cylinder wall. The upper level of the scavenge port height is generally set as high in the cylinder as feasible without encountering cylinder gas pressures too much higher than scavenge gas pressure at the time of port opening during high speed operation, as explained further in my aforementioned copending application Ser. No. 224,756. In the terms of circumferential cylinder wall width, the scavenge port area may occupy substantially the entire cylinder circumference not occupied by the central exhaust port area, when both side and opposite scavenge port areas are utilized. If only side scavenge port areas are utilized, then a lesser circumferential width is utilized, located at each side of the main, central exhaust port area.

Scavenge ports are defined in these areas by suitable port bars and adjoining cylinder wall areas. Also, suitable transfer passageways are provided so as to connect these ports with the scavenge pump system. The transfer passageways in cooperation with the scavenge ports, piston head and adjacent cylinder wall area are so designed as to direct the scavenging medium upwardly in the area of the cylinder bore contents located oppositely in the cylinder from the central exhaust ports. The upward flowing scavenge gas column or stream extends to the cylinder head surface which is crosses and thus acts to displace the remaining combustion gases downwardly, in the area of the cylinder bore cross section on the same side as the exhaust port area. The downwardly flowing, spent combustion gases then exit through the exhaust port area. Between'the generally upward and downwardly flowing gas column or stream on opposite sides of thecylinder bore, there usually exists a central vortex or core area which consists of a mixture of fresh charge gases and spent combustion gases.

The loop type gas motion thus taking place within the cylinder bore, originating from the scavenge ports and then progressing upwardly along the portion of the bore opposite the exhaust port area, across the cylinder head, then downwardly along the exhaust port side of the cylinder bore and finally out the exhaust port, constitutes a stable, effective gas flow path for scavenging of the spent gases from the cylinder followed by charg: ing with fresh gases. A principal reason for the stability and effectiveness of this flow path appears to lie in guidance of the gases up, across and then downwardly, through impingement and consequent clinging of the upward moving portion of the gas column or stream to the cylinder wall surface on the opposite side from the exhaust port area. When the column or stream reaches.

the cylinder head it flows across the cylinder head surface and then folws downward along the cylinder wall surface located on the same side as the central exhaust port area. The tendency of an impinging jet of gases to adhere to and follow along a finite, solid surface is well known.

Engines of the subject type, having the cylinder porting and consequent scavenge gas flow pattern just described, have achieved widespread usage because of their mechanical simplicity, light weight, low cost and high power output. The research carried out in the course of this invention has shown, however, that the power output is limited by the running speed which can be attained before the power cylinder gas pressure at the time of scavenge port opening becomes enough higher than the scavenge pump gas pressure to result in loss of power caused by interference with transfer of charges out of the scavenge pump into the power cylinder, or by contamination of fresh charge gases by protrusion of combustion gases into the scavenge pump system. It is possible to achieve high running speeds by utilizing suitably large scavenge port areas and adjoining transfer passageways leading out of the scavenge pump system, in combination with suitably large exhaust port areas which equal or exceed the gas flow through capabilities of the scavenge port system, but still higher running speeds have not been possible of attainment because of lack of exhaust port blowdown area. Applicants U. S. Pat. No. 3,612,014 and copending patent application Ser. No. 224,756 are examples of methods useful in obtaining high gas flow through capabilities of the scavenge port system. Matching flow through capabilities of the exhaust port area often can be attained simply by extending its width and height. Simple width extension is limited by interference with the adjoining side scavenge ports, and height extension is limited principally by displacement and consequent loss of fresh charge out of the exhaust port caused by the upward motion of the piston in the cylinder bore, following bottom dead center position. With the aid of resonant exhaust pipes, pulse tuned exhaust systems, etc., it has been found feasible to extend exhaust port height up to a level in the cylinder bore equal to 50 percent (and in some cases even more) of the piston stroke. Within these limitations as to width and height of the exhaust port area, the designer can usually provide sufficient exhaust port system flow through capability to match the scavenge port flow through capability.

However when large enough scavenge port system flow through capabilities are used in combination with matching exhaust port system flow through capabilities, running speeds taking advantage of the total flow through capabilities of the port systems have generally not been successfully attained, even with engine designs which are not held back by mechanical limitations such as result from high piston speeds. Under these circumstances the gas dynamics of the exhaust blowdown take over and limit the engine running speed which consequently limits the engine power output. In other words, with such prior art engines the running speed and power output are not limited by the combined flow through capabilities of the scavenge port system and exhaust port system, but by the lack of blowdown area provided by the exhaust port system.

The exhaust port flow through area of such prior art engines is limited, as already described, by the circumferential width of cylinder wall available between the two flanking, side scavenge port areas, and by the height on the cylinder wall, as measured above the piston timing edge with the piston in bottom dead center position, which the designer can successfully utilize as the upper edge of the exhaust port. The exhaust port blowdown area can be considered, for the purposes of this patent, to be the area bounded at its upper edge by the upper edge of the exhaust port, at its lower edge by the position of the timing edge of the piston head when it starts to uncover the normal height scavenge ports, and at its side edges by the outer side edges of the exhaust port. Of course, the blowdown area also forms part of the flow through area. Nearly all prior art engines of the subject type have been limited as to width of the exhaust blowdown area by width available between the two adjacent side scavenge port areas. As a result, they have been limited as to running speed and power output, not by the cylinder and port system gas flow through capability, but by this capability as limited by exhaust blowdown area.

A modified cylinder port arrangement which provides for a greater exhaust blowdown area than that described above is to be seen in the Sulzer arrangement described and illustrated in Paper No. 710579 of the Society of Automotive Engineers, Two Pennsylvania Plaza, New York, New York, 10001, entitled Scavenging Model Solves Problems in Gas Burning Engine authored by Nabi Dedeoglu. In this arrangement there are provided additional exhaust port areas which are located beyond the normal outer side limits of the exhaust ports; i.e., the limits brought about by the adjacent side edges of the side scavenge ports. These additional exhaust port areas extend for an additional distance around the cylinder wall circumference, so that they overlap and overliethe nearest edges of the side scavenge ports and consequently occupy a portion of the same cylinder wall sector as the side scavenge ports, but in a location directly above these ports.

It follows that with this Sulzer arrangement of the exhaust ports the exhaust blowdown area is increased and the engine running speed and power output might also be increased. However, in this arrangement what is normally the main central exhaust port area does not extend substantially from the upper edge of the port down to the timing edge of the piston head when the piston is in bottom dead center position, but instead only approximately halfway down. As a result, the flow through area is severely limited, and the small flow through area becomes the limiting factor instead of the blowdown area. Thus little or nothing is gained as to running speed capability and consequent power capability.

A modified design which utilizes auxiliary exhaust port areas occupying the same are or sector of cylinder wall as the scavenge ports and thus increases the exhaust blowdown area, is illustrated by the Krupp design shown on Page 313 of Der Ladung swechsel der Verbrennungskraftmaschine, by Prof. Dr. Hans List (Springer-Verlag in Vienna, 1950, printed in Austria). However, this design has the auxiliary exhaust port areas located above the opposite scavenge ports, and in fact has no side scavenge ports as such and no scavenge ports which occupy the same are or sector of cylinder bore wall in which the exhaust ports are centered. Thus the design differs fundamentally from that of the subject invention. It does not utilize the areas directly adjacent the side of the main, central exhaust port area for use as additional blowdown areas. It gives a different and less efficient scavenge and exhaust gas flow pattern. It makes the exhaust manifolding unduly and unnecessarily complex. Also, the engine is of the separate scavenging pump type instead of the power piston underside scavenging pump type of the resent invention.

SUMMARY OF THE INVENTION In engines of the subject type, the addition of auxiliary exhaust port areas which extend into the same are or sector of cylinder bore wall as is occupied by normal height (or less than normal height) side scavenge ports, markedly increases the exhaust port blowdown area as well as increasing the exhaust flow through area. In prior art engines of the subject type in which the running speed is not limited by mechanical or other considerations, the running speed and hence power output has generally been limited by lack of exhaust blowdown area. In such engines the marked increase in blowdown area made possible by the present invention (without upsetting the otherwise favorable scavenge and exhaust gas flow pattern in the cylinder) results in capability for markedly increased running speeds and correspondingly increased power output. Thus it is a principal object of the invention to increase the power output of engines of the subject type through addition of the subject auxiliary exhaust areas.

Since very high exhaust ports are required in order to provide sufficient exhaust port blowdown area as required for high power output in prior art engines of the subject type, the power band (i.e. the rpm range over which high torque operation can be attained) in such engines is often narrow and limited by the necessity for using rpm tuned" exhaust systems in order to prevent undue loss of fresh charge gases out through the exhaust port during the port closing phase of the cycle. By using the much wider exhaust port blowdown area afforded by the subject invention, an equal blowdown area can be attained with a lesser height of exhaust port. The lesser height exhaust port need rely less on rpm tuned exhaust systems to prevent fresh charge loss during the port closing phase, and hence the power band is widened. Widening of the power band is of greater importance than increasing the peak horsepower, in many applications. Accordingly, it is an object of the invention to widen the power band through use of the subject auxiliary exhaust port areas.

One of the principal limitations on high engine rpm and high power outputs in prior art engines has been the mechanical type of limitation brought on by high bearing loads and friction which result from increasing piston speeds. In an engine cylinder of a given piston displacement, shortening the stroke and increasing the bore raises the rmp level at which piston speed induced mechanical limitationstake over. However, little or no power advantage results from use of short stroke type cylinder designs if the rpm is already limited by lack of exhaust blowdown area. Thus it is an object of the invention to permit advantage to be takenof the added power capabilities inherent in short stroke cylinder designs through provision of necessary additional exhaust blowdown area.

Additionally, short stroke cylinders of themselves tend to reduce the weight and bulk of engines which have their other design parameters fixed. With less bulk, less material of construction is required which in turn tends to reduce cost of the engine. Accordingly, it is an object of the invention to reduce weight, bulk and stroke cylinder designs. For purposes of this patent, it is considered that short stroke cylinders utilize a bore to stroke ratio greater than that of the 1:1 to 1.25:] ratio range which has generally resulted in best power output in prior art engines.

Another principal limitation on high running speeds and high power outputs has been the limitation on scavenge port system flow capacity described more fully in my copending patent application Ser. No. 224,756. By use of the extra height scavenge ports of that patent application, the flow through capacity of the scavenge port system can be greatly increased, resulting in increased high rpm and power output capability. Full advantage of this increased capability cannot be taken without corresponding increase in exhaust flow through area, and more importantly the more difficult to obtain exhaust blowdown area. It is an object of this invention to permit full advantage to be taken of extra height scavenge port arrangements by providing the necessary additional exhaust port blowdown area (and to a lesser degree increased exhaust port flow through capacity) required to match the additional scavenge gas flow capability provided by such arrangements.

It is also an object of the invention to overcome the inherent shortcomings of the superficially somewhat similar appearing Sulzer and Krupp exhaust port systems described above, and of other prior art systems of similar type or nature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a two cycle engine utilizing auxiliary exhaust ports constructed in accordance with the present invention; r

FIG. 2 is a sectional view taken as on line 2--2 in FIG. 1;

FIG. 3 is a sectional view taken as on line 3-3 in FIG. 1;

FIG. 4 is a flat layout of a portion of the cylinder wall of the device of FIG. 1 when viewed in direction toward the main central exhaust port area;

FIG. 5 is a vertical sectional view of a two cycle engine utilizing auxiliary exhaust ports constructed in accost of subject type engines by increasing the power advantages which can be had with utilization of short cordance with a modified form of the present invention wherein the scavenge port arrangement includes both normal height and extra height scavenge ports and taken as on line 5-5 in FIG. 6;

FIG. 6 is a sectional view taken as on line 66 in FIG. 5; and g FIG. 7 is a sectional view taken as on line 7-7 in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 14, a rear compression two cycle engine 15 is shown. It includes a crankcase 16 which can be made in two sections which are suitably fastened together. Crankcase 16 encloses and supports a rotatable crankshaft 17. A connecting rod 19 isattached to a crank pin 18 on crankshaft l7, and rod 19 is also connected at its opposite end to a piston pin 20 in a piston 21. The piston 21 is sli-dably mounted in a cylinder bore 22 of a cylinder 23 attached to the crankcase 16. The cylinder 23 is air cooled as shown and utilizes cooling fins 24. A cylinder head 23A is attached to the top of the cylinder 23 to close the open end of bore 22. As shown, the engine has spark ignition, utilizing a spark plug 233, but any desired ignition method can be used in engines having the auxiliary exhaust ports of the present invention.

The cylinder bore 22 in which the piston 21 reciprocates during a normal piston stroke is provided with a main or central exhaust port area 25. The exhaust port area 25 extends from a lower edge on a level with the piston timing edge in bottom dead center position to a top or distal edge located at a desired height in the cylinder. As shown, the central exhaust port area opens through a short exhaust passageway 27 into an exhaust pipe 28 which, as shown, leads to an exhaust system 29 that may optionally be a resonant or pulse tuned exhaust system.

A pair of auxiliary exhaust port areas 26 are provided in the cylinder on opposite lateral sides of the main central exhaust port area 25. In FIG. 2, it can be seen that the auxiliary exhaust port areas 26 open into exhaust passageways 32 defined in the cylinder, and the passageways 32 in turn open into the passageway 27 and pipe 28 for discharge of exhaust gases to the exhaust system. The main exhaust port area 25 is separated from the auxiliary exhaust port areas by

port bars

33, 33 which form retainers for the piston rings to ride upon.

The upper edges of themain exhaust port area 25, and of the auxiliary exhaust port areas 26 as shown both extend to substantially 50 percent of the piston stroke above bottom dead center position of the piston timing edge 21A, although the main exhaust port upper edge heights can be varied as may prove desirable. The height of the upper edges of the auxiliary exhaust port areas 26 may be similarly varied.

The auxiliary exhaust ports are located substantially within the same 180 are or sector of the cylinder bore wall in which the main exhaust port area 25 is centered. In FIG. 2, it can be seen that the'cylinder wall sector extending from an outer edge 26A ofa first of the auxiliary exhaust port areas 26 to the outer edge 26B of a second of the auxiliary exhaust port areas 26 as shown is somewhat less than 180. The sector of cylinder wall occupied by these exhaust port areas could be less than that shown or could be slightly more than I80".

In the embodiment of the invention shown in FIGS. l-4, the engine utilizes a conventional scavenge port arrangement for introducing scavenging and charging gases into the cylinder bore. As shown, the cylinder wall is provided with side scavenge ports which are positioned on either side of the central exhaust port 25. The side scavenge ports include a pair of scavenge ports 35 one of which is positioned along each side of the cylinder and which occupy a substantial portion of the 180 sector of the cylinder wall centered opposite from the center of the central exhaust port area. These scavenge ports have significant flow capacity in relation to the total flow capacity of the scavenge system. These scavenge ports 35 open into transfer passageways 36 which, as shown in FIG. 1 extend downwardly alongside the cylinder bore and open into the crank chamber 37, which comprises a scavenge pump system chamber.

In addition to the scavenge ports 35, there are also a pair of side scavenge ports 40 which lie along the sides of the cylinder, adjacent to the edges of the central exhaust port area 25. These ports 40 extend into the same 180 sector of the cylinder in which the central exhaust port area 25 is centered. The auxiliary exhaust port areas 26 are positioned over portionsof the scavenge ports 40, and are separated from the scavenge ports 40 by a divider 41. Each of the ports 40 opens into a suitable transfer passageway 42. The passageways 42 open to the scavenge pump chamber 37.

A reed or rotary inlet valve (shown schematically at 38) controls flow of fresh charge gases into chamber 37. A carburetor or other suitable air or air-fuel inlet device is provided at 39. When the piston is on its upstroke toward the distal end of the cylinder from position shown in FIG. 1, a fresh charge will be drawn into the chamber 37 through the inlet valve 38, and then, on the downstroke of the piston, the inlet valve 38 will close and the fresh charge will be compressed in the chamber 37. After the piston timing edge 21A uncovers the upper edges of the

scavenge ports

35 and 40 the fresh charge of gases will flow under scavenge pump pressure through transfer passagewaysout via the scavenge ports into the cylinder bore.

The entry angles of the scavenge ports in cooperation with the surfaces of the associated transfer passageways adjacent the scavenge ports are designed to direct the gases flowing into the cylinder in a direction generally toward the wall of the cylinder opposite from the central exhaust port area 25, causing the scavenge gases to flow in a column or stream upwardly along the wall of the cylinder opposite from the central exhaust port area.

Referring to FIG. 4, a flat layout of the cylinder of FIGS. 1 and 2 is illustrated. The flat layout illustrates the overlap between the auxiliary exhaust ports 26 and the underlying scavenge ports 40, which occupy a portion of the same are or sector of the cylinder wall as main exhaust port area 25. Dotted lines 45 depict an optional port divider bar that may be used, if desired, to provide additional support for the piston rings. In addition, as shown in the dotted

lines

46, 47 and 48, the width of the auxiliary exhaust ports 26 can be varied if desired. The width of the bars 33 may also be varied as desired, or these bars might even be dispensed with under some circumstances. It will be noticed that the auxiliary exhaust ports 26 overlap the scavenge ports 40 by a substantial amount, and in any case the overlap should extend for a circumferential distance at least equal to more than 5 percent of the piston stroke. The area available for exhaust blowdown can thus be greatly increased.

When the piston is descending during the power stroke under combustion gas pressure, the piston timing edge 21A will first uncover the upper edges of the main central exhaust port area 25 and the auxiliary exhaust port areas 26 to permit high pressure gases to start to blow down through the passageways provided. This blow down continues as the piston moves downwardly. It can readily be seen that the additional flow capacity provided by the auxiliary exhaust port areas 26 greatly increases the total overall exhaust blowdown area so that the pressure in the cylinder reduces much more rapidly than with the normal configuration.

After the scavenge ports are uncovered fresh charge gases will be introduced into the cylinder through the transfer passageways 36 and 42 from the scavenge pump system. The

scavenge ports

35 and 40 and their transfer passageways direct the flow toward the opposite side of the cylinder bore from the central exhaust port area 25 as previously explained. This results in a gas column or stream flowing upwardly along the cylinxder wall section opposite the main exhaust port area The upwardly flowing scavenge gas stream passes across the cylinder head surface, and thus acts to displace combustion gases in the cylinder downwardly in the area of the cylinder bore cross section on the same side as the exhaust port area. The downwardly flowing spent combustion gases then exit out through the exhaust port areas. This flow pattern constitutes a stable, effective gas flow path for scavenging of the spent gases from the cylinder followed by charging with fresh gases from the scavenge pump system.

In the form of the invention shown in FIGS. 1-4, there is shown no scavenge port directly oppositefrom the main or central exhaust port, although such an opposite scavenge port can be added to the arrangement. If such an opposite scavenge port is utilized the entry angle will be selected so as to cooperate with the flows from the other scavenge ports to aid in obtaining the desired scavenge gas flow pattern. When a piston skirt valved crankcase inlet port is utilized and thisport is located on the opposite side of the cylinder from the exhaust port area 25, there is usually not much room left over for opposite scavenge ports. The arrangement shown without opposite scavenge ports has thus been widely used in the past. However, if the piston skirt valved crankcase inlet port is located on the same side of the cylinder as the exhaust port, (see US. Pat. No. 3,612,014) or if a reed or rotary inlet valve system is utilized as shown, there is ample space left over for installation of an opposite scavenge port. Of course the auxiliary exhaust ports are also useful and applicable when opposite scavenge ports are included.

Another variation of the scavenge port arrangement which is often used is.that in which the side scavenge

ports

35 and 40 are combined into one without any intervening port bar such as that shown at 43. Of course, the use of only one large scavenge port on each side of the cylinder does not depart from the teaching of the present invention.

. By use of the auxiliary exhaust port areas 26 overlappingthe side scavenge ports 40 extending into the same 180 sector of cylinder wall in which the main exhaust port area is centered, it is possible to achieve higher useful engine runningspeeds. With the more rapid exhaust blowdown resulting from addition of the auxiliary exhaust port areas, higher engine rpm can be attained before onset of unduly high combustion gas pressures at the time of scavenge port opening causes performance deterioration through undue blow back into the transfer passageways and scavenge pump system. Also, maximum engine power will usually be increased even without increasing the engine running speed. This is due to the fact that at the running speed for maximum power output when only the usual central exhaust port area is provided, there generally occurs a considerable amount of blow back through the scavenge ports, which at the least interferes with the transfer of fresh charges from the scavenge pump system into the power cylinder. The more rapid blowdown which results from addition of the auxiliary exhaust port areas reduces or eliminates the blow back and hence also reduces or eliminates the interference with transfer of charges, in turn resulting in increased power output even without increase in engine rpm. Similarly, maximum power output can be boosted through use of higher'and consequently increased area scavenge ports in order to take advantage of themore rapid blowdown which occurs when the addedauxiliary exhaustport areas are utilized.

Maximum power output may also be maintained without reduction, when lower exhaust port heights are used in combination with the added auxiliary exhaust port areas. With lower exhaust portheights, less reliance need be placed on use of rpm tuned exhaust systems to minimize lossof fresh charge gases out through the exhaustport duringthe port closing phase of the piston stroke. Thus the power band can bewidened and widening of the ower band is in many instances of more importance than increasing the maximum power output.

It should be noted that although the auxiliary port areas serve principally "to increase the exhaust blowdown area, they also serve to raise the total of exhaust port flow through area. It is usually not difficult to provide sufficient exhaust port flow through capability to equal or exceed the scavenge port system flow through capability, so the increase of exhaust port flow through area is generally of lesser importance. When extra heightscavengeport systems are utilized, however, the scavenge port flow capability can be increased to the point that matching increase in exhaust port flow through capability becomes of greater importance. FIGS. 5, 6 and 7 illustrate the use of the additional auxiliary exhaust port areas in combination with extra heightscavenge ports having piston skirt controlled additional valving.

Reference is now made to FIGS. 5, 6 and 7 wherein a two cycle engine shown generally at 55 includes a crankcase 56 of usual design, that can be made in two sections which are suitably fastened together. The crankcase 56 encloses a crankshaft 57 rotatable on suitable bearings. A crank pin 58 on crankshaft 57 rotates in a connecting rod 59. The connecting rod 59 in turn is connected to a'piston pin 60 of a piston 61. The piston *61 is slidably mounted in a cylinder bore 62 of a cylinder 63 attached to the crankcase 56. The cylinder 63 is air cooled as shown.

The engine illustrated utilizes extra height scavenge ports for increasing the scavenge port area as fully illustrated and described in my copending patent application Ser. No. 224,756 filed Feb. 9, I972 (Attorneys Docket T05.209). The description therein is incorporated by reference.

A central exhaust port area 65 is defined in the cylinder and opens to a short exhaust passageway 66 and to an exhaust pipe 67 which is connected to the outer end of the passageway 66. The exhaust pipe 67 may lead to an optional resonant or pulse tuned exhaust system indicated schematically at 68.

Central exhaust port area 65 extends around the periphery of the cylinder the desired amount, and is restricted in its total width by other requirements for the circumferential wall space, including scavenge port area requirements. In addition to the central exhaust port area 65, in the present invention there are auxiliary exhaust port areas 70 located on either side of the central exhaust port area. The auxiliary exhaust port areas overlie the normal height scavenge ports which extend into the of arc sector of cylinder wall in which the main exhaust port area 65 is centered.

The auxiliary exhaust port areas 70 lead to exhaust passageways 71 that serve to discharge exhaust gases into the exhaust system. Cylinder wall sections forming port bars 73 are positioned between the auxiliary port areas 70 and the central exhaust port area 65.

The cylinder shown is provided with normal height side scavenge ports 110 adjacent the sides of the main exhaust port area 65. The scavenge ports 110 extend into the 180 are or sector of the cylinder wall in which the main exhaust port area 65 is centered. As shown, the auxiliary exhaust port areas 70 overlie the scavenge ports 110 and are separated from the ports 110 by dividers 115.

In addition to the scavenge ports 110 the engine has another pair of normal height side scavenge ports 106, and a pair of extra height side scavenge ports 100 that overlie the ports 106. An opposite extra height scavenge port 90 is defined in the cylinder wall opposite the main exhaust port area 65.

The extra height scavenge port 90 opens from a transfer passageway 91. As shown, passageway 91 has a steeply inclined upper wall adjacent the port 90 to direct the fresh charge gas flow thru the scavenge port 90 in a desired upward direction along the portion of the cylinder opposite from the main exhaust port area 65. The lower end of the transfer passageway 91 ends at a crankcase outlet port 93 in the lower portion of the cylinder wall. The port 93 is valved by the piston wall and a control port 95 defined in the piston wall. The valving at port 93 is such that there is no communication between the cylinder bore 62 and the scavenge pump system until the piston has descended to position where control port 95 opens the outlet port 93. In this way communication between the scavenge pump system and the cylinder is delayed until the cylinder gas pressure has blown down to a desirable level.

The extra height side scavenge ports 100 are connected to transfer passageways 101, which communicate with crankcase outlet ports 102 provided in the lower cylinder wall on a level with crankcase outlet port 93. The crankcase outlet ports 102 are valved by the piston wall and large control ports 103 defined in the piston wall. Passageways 101 are not open to the scavenge pump system until the lower edges of their respective piston wall control ports 103 uncover the ports 102.

The piston wall control ports 103 are substantially longer in longitudinal axial direction of the cylinder than the control port 95, and are also used, as shown, to uncover crankcase outlet ports 104. Each outlet port 104 opens to a transfer passageway 105 (see FIG. 6) which leads to a normal height side scavenge port 106.

The opening of the side scavenge ports 106 to the scavenge pump system is controlled by the upper portion of the piston and piston head timing edge 61A. When the piston uncovers normal height scavenge ports 106, the control ports 103 will already have uncovered the crankcase outlet ports 104.

The normal height side scavenge ports 1 l communicate through transfer passageways 111 directly to the scavenge pump system. When the piston uncovers port 110, the scavenge pump system will be open to the cylinder bore through the port 110. Again, the operation of theextra height and normal height scavenge ports, and the valving thereof is fully set forth in my aforementioned application Ser. No. 224,756.

As mentioned in the introduction to this specification, normal height ports extend to a height in the cylinder of 18 to 28 percent of the length of the piston stroke above the level of the piston head timing edge in bottom dead center position. The extra height ports, such as ports and 100, have distal edges extending to a height of more than 28 percent of the piston stroke. As shown in the drawings, the height of the distal edges, or upper edges of these extra height ports is substantially more than 28 percent of the piston stroke.

An inlet port 83 for the scavenge pump system is defined in the cylinder wall, below the main exhaust port area, as shown in FIG. 5, and a suitable connecting tube 84 is provided for connecting the inlet port to a carburetor or other inlet device for introducing a fuel-air, or air charge into the scavenge pump system.

The inlet port 83 as shown is valved by the wall of the piston 61 in the manner well known in the art. Fresh charge gases are drawn into the crank chamber through the port 83 because of the reduced pressure in the crank chamber as the piston moves on the upstroke and uncovers port 83. The operation of an inlet port, such as port 83, valved by the piston skirt is disclosed in my US. Pat. No. 3,602,014. Inlet valving by reed valves, rotary valves, or other suitable inlet valve devices may likewise be utilized.

The engine 55 thus includes the auxiliary exhaust port areas for improved blowdown. The use of both extra height scavenge ports and normal height scavenge ports as shown in FIGS. 5-7 increases the total scavenge port flow through area substantially and the auxiliary exhaust port areas 70 also increase exhaust port flow through area. The combination of extra height and normal height scavenge ports and main and auxiliary exhaust port areas thereby provides greater total flow through area and results in improved engine operation.

In the engine 55 the scavenge gas flow also is directed by the scavenge ports and associated transfer passageways so as to result in a gas column or stream flowing upwardly along the opposite wall of the cylinder from the main exhaust port area and then across the cylinder head and then downwardly along the cylinder wall area on the same side as the exhaust ports, resulting in a stable, efficient scavenge gas flow pattern which facilitates clearing exhaust gases out of the cylinder via the exhaust ports.

What is claimed is:

1. In a two cycle engine having a scavenge pump system of the rear compression type and having a cylinder, a piston mounted for reciprocation in said cylinder and having a piston head port timing edge, said engine having a bore diameter substantially equal to or greater than the length of the piston stroke, a main exhaust port area defined in said cylinder and extending from a reference line substantially along the level of the piston head port timing edge in the piston bottom dead center position, thence to a distal edge and having side edges spaced apart circumferentially in said cylinder, at least one scavenge port means defined in said cylinder, said scavenge port means being defined at least in part by lower edges that are substantially aligned with said reference line and which extend toward a distal end of the cylinder but not farther than said main exhaust port, said scavenge port means including scavenge ports arranged in a substantial portion of the sector of the cylinder centered opposite from the center of the main exhaust port area, and at least one of said scavenge port means having portions thereof circumferentially closer to a side edge of said main exhaust port area than 90 of arc from the center of said main exhaust port area, and auxiliary exhaust port means opening from said cylinder adjacent the main exhaust port area, said auxiliary exhaust port means circumferentially overlapping at least a portion of said one scavenge port means but terminating circumferentially at a region such as to leave effectively free of said auxiliary exhaust port means a significant portion of the 180 opposite cylinder sector occupied by said scavenge port means so that the scavenge port means extends circumferentially over a greater portion of said 180 opposite cylinder sector than any effective portion of said auxiliary exhaust port means.

2. The combination as specified in claim 1 wherein the circumferential length of overlap between said auxiliary exhaust port means and said one scavenge port means is greater than 5 percent of the length of piston stroke in said cylinder.

3. The combination specified in claim 1 wherein the distal edge of some of the scavenge port means other than portions of said one scavenge port means overlapping said auxiliary exhaust port means extends from said reference line more than 28 percent of the piston stroke toward the distal end of said cylinder, and means separate from the piston head port timing edge controlling communication between the said some scavenge port means and the scavenge pump system and timed to permit substantial portions of said some scavenge port means and said exhaust port means to be uncovered by the piston without permitting fluid communication between said some scavenge port means and the scavenge pump system.

4. The combination as specified in claim 2 wherein there are a pair of auxiliary exhaust port means, one of said auxiliary exhaust port means extending circumfer entially from each of the sides of said main exhaust port area, said auxiliary exhaust port means being positioned substantially within a 180 sector of cylinder centered on said main exhaust port area.

5. A two cycle engine having a cylinder defined by a wall, a piston having a piston head port timing edge, said piston reciprocating in said cylinder in a piston stroke, the diameter of the cylinder being substantially equal to or greater than the length of the piston stroke, a scavenge pump system for said engine comprising the underside of the piston as a scavenge pump piston, scavenge port means open to the interior of said cylinder, transfer passageways defined in said engine between the scavenge pump system and said scavenge port means, a main exhaust port area open to said cylinder and positioned circumferentially between said scavenge port means in one wall portion of said cylinder and extending from a lower edge substantially along a reference line defined by the piston head port timing edge in piston bottom dead center position to a distal exhaust port edge, said scavenge port means and adjacent transfer passageways being positioned to di' rect scavenge gas flow from said scavenge pump in a stream adjacent a wall portion of said cylinder opposite from said exhaust port area to form a scavenge gas stream flowing along said opposite wall portion, said scavenge port means extending from substantially adjacent said reference line in the same axial direction as said main exhaust port and including some scavenge ports of significant flow capacity located in the 180 sector of the cylinder wall centered opposite from the main exhaust port area, said scavenge port means extending into the sector of cylinder wall in which the main exhaust port area is centered at least on one side of said main exhaust port area, and at least one auxiliary exhaust port area defined. in said cylinder, said auxiliary exhaust port area being positioned adjacent said main exhaust port area and at least partially circumferentially overlapping portions of said scavenge port means which extend into the 180 sector of cylin der wall in which the main exhaust port area is centered but terminating circumferentially at a region such as to leave effectively free of said auxiliary exhaust port means a significant portion of the 180 opposite cylinder sector occupied by said scavenge port means so that the scavenge port means extends circumferentially over a greater portion of said 180 opposite cylinder sector than any effective portion of said auxiliary exhaust port means.

6. The engine of claim 5 wherein the main and auxiliary exhaust port areas have side edges, and the total circumferential distance covered by these exhaust port areas between the extreme side edges thereof is not substantially more than a 180 sector of cylinder wall.

7. The combination as specified in claim 5 and port bar means dividing said auxiliary exhaust port area from said main exhaust port area.

8. The combination as specified] in claim 5 wherein said scavenge port means include extra height scavenge port means, means separate from said piston head timing edge valving said extra height scavenge port means, said extra height scavenge port means extending to upper edges on a level substantially greater than 28 percent of the piston stroke in regions of said cylinder other than where the auxiliary exhaust ports overlie said scavenge port means.

9. The engine of claim 5 wherein the circumferential length of overlap between the auxiliary exhaust port area and said portions of said scavenge port means is greater than 5 percent of the length of piston stroke in said cylinder.

10. The engine of claim 5 wherein said main exhaust port area is symmetrical about a bisecting line, and there are two auxiliary exhaust port areas, said auxiliary exhaust port areas being positioned on opposite sides of said main exhaust port area.

11. The engine of claim 10 wherein said scavenge port means comprise scavenge ports positioned on opposite sides of said main exhaust port area and which extend on both sides of the main exhaust port area into the 180 sector of cylinder wall in which the main exhaust port area is centered. 1

l2. Thecombination as specified in claim 8 wherein the extra height scavenge port means includes an opposite scavenge port substantially diametrically opposite fromthe main exhaust port area, a transfer passageway connected to said opposite scavenge port and having surfaces adjacent said opposite scavenge port to direct flow from the scavenge pump system through said opposite scavenge port in direction toward the end of said cylinder opposite from said reference line, said means valving said extra height scavenge port means preventing communication between said extra heightscavenge port means and said scavenge pump system until after substantial portions of the main and auxiliary exhaust port areas and the extra height scavenge port means are uncovered by said piston during each power stroke of the engine.

passageway means connected between said scavenge pump system and said opposite scavenge port, said transfer passageway means being defined by surfaces adjacent the opposite scavenge port positioned to direct flow through the opposite scavenge port toward an end of said cylinder opposite from said reference line.

Claims

1. In a two cycle engine having a scavenge pump system of the rear compression type and having a cylinder, a piston mounted for reciprocation in said cylinder and having a piston head port timing edge, said engine having a bore diameter substantially equal to or greater than the length of the piston stroke, a main exhaust port area defined in said cylinder and extending from a reference line substantially along the level of the piston head port timing edge in the piston bottom dead center position, thence to a distal edge and having side edges spaced apart circumferentially in said cylinder, at least one scavenge port means defined in said cylinder, said scavenge port means being defined at least in part by lower edges that are substantially aligned with said reference line and which extend toward a distal end of the cylinder but not farther than said main exhaust port, said scavenge port means including scavenge ports arranged in a substantial portion of the 180* sector of the cylinder centered opposite from the center of the main exhaust port area, and at least one of said scavenge port means having portions thereof circumferentially closer to a side edge of said main exhaust port area than 90* of arc from the center of said main exhaust port area, and auxiliary exhaust port means opening from said cylinder adjacent the main exhaust port area, said auxiliary exhaust port means circumferentially overlapping at least a portion of said one scavenge port means but terminating circumferentially at a region such as to leave effectively free of said auxiliary exhaust port means a significant portion of the 180* opposite cylinder sector occupied by said scavenge port means so that the scavenge port means extends circumferentially over a greater portion of said 180* opposite cylinder sector than any effective portion of said auxiliary exhaust port means.

4. The combination as specified in claim 2 wherein there are a pair of auxiliary exhaust port means, one of said auxiliary exhaust port means extending circumferentially from each of the sides of said main exhaust port area, said auxiliary exhaust port means being positioned substantially within a 180* sector of cylinder centered on said main exhaust port area.

5. A two cycle engine having a cylinder defined by a wall, a piston having a piston head port timing edge, said piston reciprocating in said cylinder in a piston stroke, the diameter of the cylinder being substantially equal to or greater than the length of the piston stroke, a scavenge pump system for said engine comprising the underside of the piston as a scavenge pump piston, scavenge port means open to the interior of said cylinder, transfer passageways defined in said engine between the scavenge pump system and said scavenge port means, a main exhaust port area open to said cylinder and positioned circumferentially between said scavenge port means in one wall portion of said cylinder and extending from a lower edge substantially along a reference line defined by the piston head port timing edge in piston bottom dead center position to a distal exhaust port edge, said scavenge port means and adjacent transfer passageways being positioned to direct scavenge gas flow from said scavenge pump in a stream adjacent a wall portion of said cylinder opposite from said exhaust port area to form a scavenge gas stream flowing along said opposite wall portion, said scavenge port means extending from substantially adjacent said reference line in the same axial direction as said main exhaust port and including some scavenge ports of significant flow capacity located in the 180* sector of the cylinder wall centered opposite from the main exhaust port area, said scavenge port means extending into the 180* sector of cylinder wall in which the main exhaust port area is centered at least on one side of said main exhaust port area, and at least one auxiliary exhaust port area defined in said cylinder, said auxiliary exhaust port area being positioned adjacent said main exhaust port area and at least partially circumferentially overlapping portions of said scavenge port means which extend into the 180* sector of cylinder wall in which the main exhaust port area is centered but terminating circumferentially at a region such as to leave effectively free of said auxiliary exhaust port means a significant portion of the 180* opposite cylinder sector occupied by said scavenge port means so that the scavenge port means extends circumferentially over a greater portion of said 180* opposite cylinder sector than any effective portion of said auxiliary exhaust port means.

6. The engine of claim 5 wherein the main and auxiliary exhaust port areas have side edges, and the total circumferential distance covered by these exhaust port areas between the extreme side edges thereof is not substantially more than a 180* sector of cylinder wall.

8. The combination as specified in claim 5 wherein said scavenge port means include extra height scavenge port means, means separate from said piston head timing Edge valving said extra height scavenge port means, said extra height scavenge port means extending to upper edges on a level substantially greater than 28 percent of the piston stroke in regions of said cylinder other than where the auxiliary exhaust ports overlie said scavenge port means.

11. The engine of claim 10 wherein said scavenge port means comprise scavenge ports positioned on opposite sides of said main exhaust port area and which extend on both sides of the main exhaust port area into the 180* sector of cylinder wall in which the main exhaust port area is centered.

12. The combination as specified in claim 8 wherein the extra height scavenge port means includes an opposite scavenge port substantially diametrically opposite from the main exhaust port area, a transfer passageway connected to said opposite scavenge port and having surfaces adjacent said opposite scavenge port to direct flow from the scavenge pump system through said opposite scavenge port in direction toward the end of said cylinder opposite from said reference line, said means valving said extra height scavenge port means preventing communication between said extra height scavenge port means and said scavenge pump system until after substantial portions of the main and auxiliary exhaust port areas and the extra height scavenge port means are uncovered by said piston during each power stroke of the engine.

13. The combination of claim 12 wherein said extra height scavenge port means includes extra height scavenge ports on opposite sides of and adjacent said opposite scavenge port.

14. The combination of claim 3 wherein said some scavenge port means includes an opposite scavenge port centered substantially diametrically opposite from the center of said main exhaust port area, and transfer passageway means connected between said scavenge pump system and said opposite scavenge port, said transfer passageway means being defined by surfaces adjacent the opposite scavenge port positioned to direct flow through the opposite scavenge port toward an end of said cylinder opposite from said reference line.