This invention allows dynamic and continuous 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.

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

Date of invention:
16 July 2009

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.

A mechanism to control the length(s) of the exhaust pipe as the engine speed changes allows for increased control of exhaust pulse timing.

This invention allows for continuous variable exhaust pipe length between two limits without the use of valves.

Other solutions offer pipe lengths which are changed using valves, flappers and other geometrical features. These arrangements do not allow for continuous variability and in most cases the pipe length is toggled between two or more fixed lengths. This invention allows for continuous control of exhaust pipe length between two distinct values which can be set to match engine speed limits.

The same way a trumpet uses valves to change the length and therefore pitch of the instrument, a trombone will allow for control of the pitch between the notes and offers continuous scale. It is obvious that the packaging space required for a solution similar to a trombone over that offered by a trumpet is a limiting factor when considered for internal combustion applications.

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.

The invention utilizes a closed volume with an interior scroll portion which is free to turn on a central axis. The internal scroll can turn through 270 degrees 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. 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 tunable exhaust scroll is below.

Multi-position exhaust scroll tuned to the shortest exhaust length

Multi-position exhaust scroll tuned to an intermediate exhaust length

Multi-position exhaust scroll tuned to the longest exhaust length

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