Power plants – Internal combustion engine with treatment or handling of... – Pulsed – timed – tuned or resonating exhaust
Reexamination Certificate
2001-04-27
2004-02-03
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Pulsed, timed, tuned or resonating exhaust
Reexamination Certificate
active
06684633
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for receiving the exhaust gas output of a reciprocating piston internal combustion engine, and more particularly discloses a muffler assembly that is particularly advantageous for use with a two-stroke gasoline engine, which assembly provides both an exhaust scavenging function and a supercharging function, and in addition, comprises a low profile and compact design.
BACKGROUND OF THE INVENTION
This invention discloses an exhaust gas handling assembly for an internal combustion engine, which is especially useful in small two-stroke gasoline engines such as in radio-controlled airplanes and wheeled vehicles for ground travel, such as motorcycles and all terrain vehicles. The device is commonly referred to as a muffler, but this is a term that is too restrictive for all the functions performed by the device. While the device does serve to muffle or dampen the noise of combustion in such internal combustion engines, it also serves at least two other critical functions, exhaust scavenging and fuel-charge densification.
The present invention has been found to be particularly advantageous when used on a two-stroke, internal combustion, piston and crankshaft type engine which burns a volatile fuel such as gasoline and/or alcohol, and which utilizes valving consisting of ports formed through the wall of the piston cylinder, controlled by movement of the piston within the cylinder to alternately expose and cover up said ports.
A typical two-stroke engine has one or more intake ports formed through each cylinder wall and one or more exhaust ports formed through the cylinder wall, usually located on the opposite side of the cylinder from the intake ports. These ports are positioned such that the piston opens and closes them in a carefully controlled sequential manner to allow intake and exhaust of the fuel/air mixture and the products of combustion, respectively. Many such engines pump the fuel/air mixture through the crankcase of the engine into the intake port in the cylinder wall.
During a normal intake/compression/combustion/exhaust cycle of the two-stroke piston-cylinder combination, when the exhaust port is opened by movement of the piston away from its blocking position over the port, a high-pressure exhaust gas pulse starts down the exhaust tube. The piston continues down and the exhaust pressure bleeds off into the tube. This occurs at around 90-110 degrees from piston Top Dead Center (TDC). At about 15-25 degrees later, the intake ports on the other side of the cylinder are exposed by the piston, and, because of crankcase compression, a fuel/air mixture begins to flow through the intake ports and into the cylinder while exhaust gas is still moving out the exhaust ports. After a small fraction of a second, the pressure pulse moving down the exhaust tube reaches an open area, or expansion chamber, and this starts an expansion wave back toward the exhaust ports. This expansion wave creates an action at the exhaust ports, which serves to draw additional flow of exhaust from the cylinder, including a portion of the new fuel/air charge entering through the intake ports.
As the expanding exhaust pulse reaches the end of the expansion chamber, it impinges the narrowed end of the tube at the downstream end of the chamber and is compressed, thereby creating a strong compression wave that moves back up the tube to the exhaust port. This results in some of the escaped fuel/air charge being pushed back into the cylinder before the piston closes the exhaust ports, thus achieving the desired charge-densification effect in the cylinder.
The “tuning” of the muffler is dependent upon the length and volume of the expansion chamber and its distance down the tube from the exhaust ports. This chamber effectively locates the positions of the expansion part of the tube, and the compression portion. The remaining portion of the exhaust tube downstream from the expansion chamber has little effect on the “tuning” of the exhaust.
Some exhaust mufflers, which are also commonly called “tuned pipes” or “tuned exhaust extractors”, which are currently available commercially for small two-stroke engines are sufficiently “tuned” to allow optimum scavenging of exhaust from the cylinder of the engine and a charge-densification of the incoming fuel/air mixture. This occurs by the advantageous utilization of the above-described impulse/compression wave nature of the exhaust muffler. There are also mass effects involved in exhaust processes, i.e., the volume of exhaust gas in a system does not move through the pipe with a smooth, linear velocity. The velocity rises and falls along with the pressure waves, so that being “in tune” with these differences amplifies the pressure differences. The expansion portion of the exhaust gas wave moving out of the cylinder, through the exhaust valve, and down the muffler tube serves to establish a subnormal pressure condition just outside the exhaust valve, which aids in removing additional combustion products from the cylinder while the cylinder interior is exposed to the open exhaust port. Shortly thereafter, the compression wave passing back up the muffler to the cylinder serves to “supercharge” the incoming fuel/air charge that has begun to exit the open exhaust port by forcing the charge back through the exhaust port and into the cylinder, thereby increasing the density of the fuel/air mixture in the cylinder before the compression and combustion cycles are achieved.
Unfortunately, prior art muffler devices for small two stroke gasoline engines offer chamber designs that are many times longer than the diameter of the cylinder in which the fuel/air mixtures are combusted. The most prevalent of such muffler devices commercially available for two-stroke gasoline engines suffers from having a length as much as 6-30 times the diameter of the cylinder it is attached to. The specific length of the tuned pipe is primarily a function of the RPM at which the engine designer wishes to “tune” the system. Often a particular torque curve is desired for an optimum match-up with the particular airframe chosen, and this can be achieved by designing the system to be longer or shorter. A short length tube will be utilized for a high-RPM, low torque engine, and a long length tube will be used for a low-RPM, high torque engine. This length is used to create the compression/expansion wave actions referred to above which establish the scavenging and densification functions previously described. If such muffler chamber is not properly sized, the two-stroke engine exhaust will not be “tuned” and performance of the engine will suffer drastically.
However, when the muffler chamber is properly sized for optimum performance, it results in a muffler having a physical presence that is many times larger than the entire engine to which it is attached. In the world of small engines, this is very undesirable for several reasons. One reason that such bulky and cumbersome exhaust device is undesirable is the ugly aesthetics that it presents. The present commercially available muffler is a long, cigar-shaped tube that must extend down the side of the vehicle to which it is attached. For those who desire authenticity in the appearance of their small gasoline-powered vehicles, the presence of such a bulky and obvious attachment, often extending down the full length of the airplane or land vehicle on which it is used, greatly mars the owner's enjoyment of the vehicle. This is particularly true in the field of radio-controlled (RC) airplanes and cars.
In addition to the aesthetically unpleasant feature of current muffling devices, they also are very aerodynamically inefficient, causing unbalanced weight and drag on the vehicles, especially on the RC airplane.
Commercially available “tuned” mufflers for small two-stroke engines generally comprise a long, cigar-shaped tube/chamber combination that begins with a small diameter next to the exhaust port of the engine cylinder. At this point the cross-sectional area of the muffler may be approximately th
Caddell Michael J.
Denion Thomas
Tran Diem
LandOfFree
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