Motorcycle Exhausts - 2-Stroke Expansion Chambers

What are They and How do They Work?

Every racer of 2-strokes will tell you how important the pipe (or expansion chamber, to be more precise) is on their bike. There is no other item on a 2-stroke that will affect the performance so much. So, what is an expansion chamber, and how do they work?

The problem with such a simple design as the 2-stroke is that it is relatively hard to improve. In attempts to improve performance, engineers have changed the port timing, carburetor size, compression ratio, and ignition timing many times, but eventually they realized that there was little else they could do to get better, more usable, power.

Exhaust Port Timing

As the engineers gained more knowledge of the 2-stroke and its working principles, however, it became obvious that to increase power they needed to have a method of varying the exhaust port timing.

With a piston ported engine the exhaust port is opened and closed symmetrically about TDC (top-dead-center), so if you lowered the port to start the compression phase sooner, you automatically kept the burnt gasses in longer, which would then mix with the new charge, for instance.

A system for opening and closing the exhaust port at different points about TDC was clearly needed. After much research and development Russian engineer, Michel Kadenacy, discovered how to use the pulses (pressure waves) from the exhaust to achieve this.

Kadenacy discovered that careful design of the exhaust system could effectively use the pressure pulses to close the exhaust port without needing any additional moving mechanical parts. Taking this knowledge further, he found that the pulses were directly related to the shape, size, length, and diameter of the pipe and muffler.

Further experimentation resulted in an understanding of how and when to change the pulse direction.

So, what does this all mean in real terms?

Following the 2-stroke cycle through (on a piston ported engine), we have:

• Inlet
• Primary compression
• Transfer
• Compression
• Power
• Exhaust

Although the 2-stroke is very simple in its operation, the interaction between the phases is more complex. For instance, as the piston moves up on the inlet stroke, it is also compressing the previous charge ready to be fired. Therefore, looking at the cycles again, we have the following happening at the same time:

• Inlet - compression at the same time
• Primary compression - previous fresh charge is being transferred from the crank-cases to the top of the cylinder during the power stroke
• Compression - end of inlet phase
• Power - as the piston goes down, the fresh charge in the crank-cases gets compressed and the exhaust port opens
• Exhaust - as the burnt charge exits the exhaust port, the fresh charge is being transferred to the top of the engine.

The critical phase in relation to the exhaust occurs as the piston starts to come back up, just before the exhaust port closes, and some fresh charge starts to follow the old/burnt gases out into the pipe. If a returning pulse could push that new charge back into the cylinder just at the right time (before the piston seals it off), more power would be produced and less fuel would be wasted.

Although the effect (often referred to as the Kadenacy effect) will only work over a limited rev range, the useful power gained can be tailored to the application. For instance, a road race bike would need that power in the middle to higher rev range, a MX bike would need it in the low to middle rev range, and a trials bike at the low to middle end of the rev range.

Expansion Chamber

Having discovered the positive benefits of using the pulses, further research concluded that these pulses changed direction when the exhaust pipe (or muffler) changed size or shape. These discoveries lead to the expansion chamber system.

As the name implies, an expansion chamber exhaust consists of a chamber where gases from the exhaust phase expand into. However, the change of shape of the chamber, as it reduces in size, sets up a pulse that returns towards the exhaust port. If the returning pulse arrives at just the right time, it will push the unburnt gases back into the cylinder.

Although there have been many advances with 2-stroke technology in general, and expansion chambers in particular, the same operating principles remain. The pioneering work undertaken by engineers such as Kadenacy pushed the performance of 2-strokes to levels that are hard to beat even today.