Exhaust System for Multiple Cylinder Two-Stroke Engines

This invention relates to the exhaust system for a two stroke internal combustion engine and particularly concerns a pulse tuned exhaust system.

In prior art two stroke engines the exhaust port remains open during the charging of the air-fuel mixture. Accordingly some of the new charge of air-fuel mixture escapes into the exhaust system from where it is discharged from there. The escaped fuel is lost without producing any power but the total fuel intake to the engine is used to compute the fuel economy and the horsepower of the engine.

The present invention is directed to a two stroke internal combustion engine having at least three cylinders including

(a) an exhaust means having an exhaust cavity therein,

(b) individual exhaust port means connected to each cylinder and to the exhaust means including

(i) a port at the wall of a cylinder and

(ii) an outwardly projecting wall means defining a chamber as an extension of the port, the wall means terminating in the exhaust cavity, and

(c) exhaust pressure pulse transmission means connecting the port means of an exhausting cylinder and the port means of a charging cylinder with a pulse transmission path of a proper length to cause the exhaust pulse from an exhausting cylinder to arrive at the exhaust port means of a charging cylinder prior to the complete closing of the exhaust port whereby the air-fuel mixture which escaped from the exhaust port of the charging cylinder which is within the exhaust port means is forced back into the charging cylinder prior to the closing of the exhaust port.

FIG. 1 is an elevational view partly in section of an outboard motor employing a six-cylinder engine utilizing an exhaust system constructed in accordance with the present invention;

FIG. 2 is a horizontal section more clearly showing the exhaust port construction of the exhaust system in accordance with this invention; and

FIG. 3 is an enlarged fragmentary view of the exhaust system shown in FIGS. 1 and 2 with a conventional port unit shown in full and the exhaust port of this invention shown in phantom.

FIG. 1 illustrates an outboard motor powerhead having a two stroke, six cylinder engine 10 supported upon the upper end of a drive shaft housing 11. The outboard motor lower structure is well known. It may include an exhaust extension plate 12 positioned between the lower end of the engine and an upper drive shaft housing plate 13 which is in turn positioned on the upper end of the drive shaft housing 11.

FIG. 1 illustrates the V-6 engine 10 employing two banks of three horizontally disposed cylinders in a vertical in-line arrangement. Each of the cylinders 15 include air-fuel intake means 16 and exhaust port means 17. The exhaust port means 17 exhaust into a common exhaust means 18 having an exhaust cavity 19 therein. In the engine 10 the exhaust cavity is separated into a first and second exhaust cavity 19a and 19b. The exhaust port means 17 of the first, third and fifth fired cylinder exhaust into the first exhaust cavity 19a and the exhaust port means 17 of the second, fourth and sixth fired cylinder exhaust into the second exhaust cavity 19b.

The exhaust port means 17 include the exhaust opening or port in the cylinder wall 20 and the outwardly projecting wall means or exhaust port extension 21 defining a chamber 22. In the embodiment of FIG. 1 the exhaust port 20 is the opening in the cylinder wall and the exhaust port extension 21 is the continuation of this opening to form the chamber 22 between the exhaust port 20 and the exhaust cavity 19.

Referring to FIG. 2 the exhaust port extension 21 also includes a protruding wall 23 extending into the exhaust cavity 19. The chamber 22 and protruding wall 23 are cast integral with the engine block. The protruding walls of the chambers in the first bank are separated from the protruding walls of the chambers in the second bank by an exhaust cavity separator 24 which is formed integral with the exhaust cover assembly 25.

In the embodiment illustrated in FIG. 2 the chamber 22 is positioned at an angle from the exhaust port 20. For example the center line or plane of the chamber 22 includes an angle of 40.degree. with the center plane of the cylinders.

In two-stroke engines it has been learned that because the exhaust port remains open during the introduction of the new charge of fuel-air mixture that some of the new charge can escape through the exhaust port. It is believed that this charge is entirely lost with the exhaust gases and therefore does not provide input energy to the engine. The inventors have discovered that the escaped charge can be recovered and utilized to provide increased fuel economy as well as an increase in engine brake horsepower. First they discovered that the chamber 22 formed by the extended ports will provide containment of the escaped charge. Second as will be more fully described, they discovered that the exhaust pulse from an adjacent fired cylinder for engines having at least three cylinders can force most if not all of the escaped charge which is contained in the chamber back into the charging cylinder before the exhaust port closes. This is accomplished by an exhaust pulse transmission means 26 connecting a charging cylinder and an exhausting cylinder as will be described.

In order to use the exhaust pulse from an adjacent firing cylinder to force the escaped charge into the exhaust port 20 the exhaust pressure pulse transmission means 26 connecting the exhaust port means of an exhausting cylinder and the exhaust port means of a charging cylinder must have an exhaust pulse transmission path of a proper length to allow the exhaust pulse from the exhausting cylinder to arrive at the exhaust port means of a charging cylinder prior to the complete closing of the exhaust port so that air-fuel mixture which escaped from the exhaust port 20 of the charging cylinder which is within the extended exhaust port 21 is forced back into the charging cylinder prior to the closing of the exhaust port 20 of the charging cylinder.

The exhaust pulse transmission path length is defined as the combined length of (A) the length of the chamber 22 of the exhausting cylinder, (B) the length of the chamber 22 of the charging cylinder, and (C) the shortest distance between facing sides of two adjacent extended exhaust ports 21.

When the combined length of (A), (B) and (C) above is equal to the proper length of the exhaust pulse transmission path between the chamber 22 of an exhausting cylinder and the chamber 22 of a charging cylinder the exhaust pulse arrives at the chamber 22 of a charging cylinder before substantially any of the air-fuel mixture escapes the chamber 22 into the exhaust cavity 19 and at the exhaust port 20 prior to the closing of the exhaust port 20.

In the embodiment of FIG. 2 the exhaust pulse transmission path is shown to be between openings in the side walls of the extended port 21 of an exhausting cylinder and the extended port 21 of a charging cylinder. In this embodiment it was empirically determined that the extended port 21 must be of a specific length to provide containment of the escaped charge. But at this length one cylinder 15 can not be positioned sufficiently close to another cylinder 15 to permit the exhaust pulse to reach the charging cylinder at the proper time. It was determined that if the chamber 22 was maintained at the desired length and if the exhaust pulse transmission means included (D) a first opening 27a in the sidewall 28 of the chamber of an exhausting cylinder, (E) a second opening 27b in the sidewall of the chamber of a charging cylinder and, (F) an exhaust pulse transmission means 26 between the first and second openings 27a and 27b, the exhaust pulse from the exhausting cylinder reached the charging cylinder before its exhaust port closed. Surprisingly, even though the first and second openings 27 a and 27b in the sidewall of the chamber were below the top of the chamber it was found that the exhaust pulse continued to force most if not all of the escaped charge back into the charging cylinder. It is believed this is because the escaped charge extending above the bottom of the second opening 27b is forced back into the chamber 22 by other rebounding exhaust pulses from the walls of the exhaust cavity 24 or because the escaped charge tends to follow a path through the second opening 27b and into pulse transmission channel means 26 and therefore does not extend above the bottom of the second opening 27b.

The pulse transmission channel means 26 is shown in FIG. 2 as an open channel 26a in the outer surface of the cast engine block. The channel 26a extends in a straight path between adjacent exhaust port means. The channel 26a of the embodiment of FIG. 2 extends between the protruding walls 23 of adjacent chambers and has a cross-sectional area equal to about one-half the cross-sectional area of the chamber.

The channel 26a serves to form the pulse transmission path. Therefore the depth of the channel from the top edge of the chamber 22 affects or controls the length of the pulse transmission path and accordingly the time it takes for the exhaust pulse to travel from adjacent cylinders. Therefore increasing the depth or the position at which the channel opens from the chamber 22 shortens the distance the exhaust pulse must travel thereby permitting adjustment of the transmission path length. Also, it must be taken into consideration that the speed at which the engine horsepower peaks is inversely related to the length of the transmission path length. Therefore, the larger the distance the exhaust pulse must travel, the lower the RPM at which the engine horsepower will peak and vice versa.

The cross-sectional area of the channel 26a is also significant. If the channel 26a is too small, the exhaust pulse which may occur in the form of a pressure wave having a wave front is not effectively transmitted and if it is to large the forcing back of the escaped charge into the charging cylinder is not maximized and some horsepower and fuel economy is lost.

In the preferred embodiment of FIG. 2 in a V-6 engine rated at about 175 horsepower the chamber 22 has a cross-sectional area of about 11/2 to 13/4 square inches and a length of about 2 inches from the end of the exhaust port 20 to the exhaust cavity 19. The cross-sectional area of the chamber 22 and the exhaust port 20 are substantially equal to prevent expansion of the air-fuel mixture which escapes into the chamber 22.

The channel is about 1/2 inches wide, about 3/8 inches deep and about 21/4 inches in length between the inside edge of adjacent chambers.

FIG. 3 illustrates the prior art engine with the extended port and pulse transmission channel shown in phantom. The extended port with pulse transmission channel produced an increased efficiency and horsepower as given in the following table.

Although it has been described that the extended ports and the exhaust pulse transmission means as well as other parts are cast integral with the engine block they may be formed as separate attachable pieces. For example the extended exhaust ports 21 could be cast as a tubular member and attached between the exhaust port 20 and the exhaust means 18 and the exhaust pulse transmission means 26 can be formed of a separate part mounted between the openings in the adjacent extended exhaust ports. Also, although the exhaust pulse transmission means 26 is described as an open channel 26a it can be a tubular member or even a hole in the cast engine block or part thereof.