This invention allows dynamic modification of the length of the exhaust pipe in an internal combustion engine so the pipe length is optimal for scavenging efficiency depending on engine speed and other characteristics. The scroll style pipe of this invention allows a more compact package than other tunable exhaust pipes.

Name of the inventor or inventors:
Iain James Lacey Read.

Date of invention:
16 July 2009

Background
A 2 stoke (2 cycle) internal combustion engine has a necessity to control the pulses emitted from the exhaust port to scavenge and trap gasses in the system to provide optimum combustion. Optimizing, or tuning, the exhaust pipe length and geometry is considered the standard for controlling the exhaust gases.
In a two stroke internal combustion engine there is a pressure pulse which emanates from the combustion cylinder the moment the exhaust valve or port is opened. The pulse moves at the speed of sound along the pipe and will reflect back along the pipe when it encounters various geometrical phenomena. For optimum characteristics the pulse should return as the exhaust port or valve is closing. In the case of a multi cylinder 2 cycle engine, the pulse can return when one or more of the ports are closing.

The time that the port remains open is dependant on various criteria, including but not limited to, engine speed. For example: The higher the engine speed, the shorter the time the port(s) are open, therefore the shorter the exhaust pipe should be.

It is considered normal practice to size the exhaust pipe length to meet optimum performance at a single preferred point. This invention will allow two discreet lengths to be used and a single toggle between the lengths can be controlled.

Novelty:
Other solutions offer pipe lengths which are changed using valves, flappers and other geometrical features. These arrangements require packaging space to contain the redundant pipe length when not being used. This invention packages a scroll style pipe in a sealed housing and the length can be toggled between two fixed lengths.

The invention arranges the pipe length to be contained as two scrolls which can move within a fixed package space and actuated by a rotational driver such as, but not limited to, a stepper motor or linear actuator and linkage.

Invention:
The invention utilizes a closed volume with an interior scroll portion which is free to turn on a central axis. The internal scroll switches between two positions and can be used to vary the effective length of an exhaust pipe. Control of the position of the scroll can be obtained using a stepper motor or linear actuator and a linkage. The length of the effective exhaust pipe leading up to and leaving from the scroll can be maintained and the scroll length can be changed between two set lengths. A cooling jacket can be used to cool the gasses along the joint wall. The diagram shows a single helix, but multiple helixes can be put into the scroll portion to allow for greater length in a small packaging area.

The direction of the gasses is not a determining factor of the design and the flow can easily be reversed to allow for packaging constraints. Any gasses which blow by the scroll are contained with the housing assembly, this allows for clearances around the scroll to help prevent sticking and coking problems.

An example of a tuned exhaust scroll is provided below:


Tuned exhaust scroll in the shortest exhaust configuration


Tuned exhaust scroll in the longest exhaust configuration

An additional feature of the tunable exhaust scroll is that more than one can be stacked together to add additional length, tuning options, and flexibility with entry and exit points.


Stacked tunable exhaust scrolls