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Two stroke gasoline engine exhaust pipes are shaped strange. They expand into a wide diameter and then shrink down to a very small diameter where the exhaust gasses emerge into the environment.

  • Why are they shaped in such a strange way?

  • What is the large expansion in the exhaust pipe for?

  • Why does the diameter reduce so dramatically at the end?

enter image description here

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That's an expansion chamber exhaust (invented by Limbach). It's worthwhile to note that not all two-stroke engine exhausts feature an expansion chamber - some omit it. – Mikaveli Jan 15 at 11:46
up vote 11 down vote accepted

There is this nice animation of a 2-stroke engine with an expansion chamber

enter image description here Source

It works like this:

While moving downwards after ignition, the piston exposes the exhaust opening, and the burned gas streams into the exhaust pipe like a (high pressure) shock wave.

Due to the inertia, this gas will create a slight vacuum wave behind it, which helps to suck out more burned gas, but also fresh gas as soon as the intake opening is uncovered.

The first cone helps to increase the vacuum: When the gas travels through the pipe with a certain speed, it travels through a certain volume per time. If the cross section is increased and the speed remains the same, the wave travels through a larger volume. This creates more ... uhm... vacuum volume behind the wave. It's a bit hard to explain.

OK, now we have fresh gas in the cylinder, but also in the manifold. The shock wave now hits the right cone and gets reflected. I.e. you now have a shock wave running towards the cylinder. It hits the fresh gas right in the moment when the intake opening is covered by the piston, and presses this gas into the cylinder. When the piston covers the exhaust opening, too, the fresh gas already is under some pressure.

This way, the exhaust forms some kind of compressor, increasing the volumetric efficiency / power of the motor.

The shape of the exhaust is very critical with respect to timing: The length of the pipe between exhaust opening and first cone defines when the amplified vacuum wave reaches the cylinder - it should be there when the intake openings are exposed and fresh gas can be sucked out of them. And the distance to the second cone defines when the reflected shockwave reaches the cylinder. Again: This should happen when the intake is already covered, and the exhaust is not yet.

This means the exhaust pipe is designed for a specific RPM, where you get the maximum gain in power.

However, the angles of the cones allow to broaden the RPM range where the motor develops its power, at the cost of maximum power.

For example, here are power / torque curves for three exhaust systems on the same scooter:

enter image description here

First, it's notable that the motor starts to develop some power right above 5000RPM because there the exhaust starts to work.

  • The blue curve is a quite narrow peak with a maximum at 7700 RPM. it looses power quite rapidly for higher/lower RPMs.
  • The red curve is shifted to higher RPM, where it should develop more power - but instead, it's designed for a wider RPM range. So, the max. power is similar to that of the blue curve, but the RPM range is about twice as wide.

Finally, the design depends on the needs of the vehicle, too. A motor cross bike usually has a transmission with several fixed gears and so runs over a wide RPM range. In contrast, scooters usually have a variomatic, which allows the motor to run on a certain, constant RPM.

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Great answer +1 – Zaid Jan 17 at 19:31
    
Aye. +1 with a desire to +5 it (i guess assigning bounties might be the closest we get to that, but my rep on this site is still rather limited). – Hennes Jan 20 at 16:20
    
Is that horizontal scale Revolutions Per Minute (RPM)? It's labeled as U/Minute These numbers seem way high. Are these exhaust curves for racing machines? I can't imagine a scooter engine would last very long at 9000 RPM's. – zipzit Jan 30 at 11:22
    
@zipzit: Yes. For comparison: My 50ccm scooter has 3.1kW at 6000RPM (legal street version). Those exhausts have about 8kW at 7700-8300RPM, maybe on a 70ccm motor. Scooter Attack is a shop for street and racing scooters, so it's definitely possible these are racing machines. – sweber Jan 30 at 14:28
1  
U/Minute is probably "Umdrehungen pro Minute" (rotations per minute). – Hennes Mar 15 at 8:12

Briefly:

The exhaust pipes are shaped to suck gasses out of the combustion chamber over a range of RPMs.

Somewhat longer:

Imagine a 2 stroke engine without an exhaust*1. When the exhaust valve opens, burned gasses leave the cylinder. Leave the valve open for a long enough time and the cylinder will be at ambient air pressure.

Now run the engine at a fixed speed and add a straight pipe as exhaust. When the exhaust valve opens a wave of compressed gasses will flow into the pipe, progress to the end of the pipe, and expand into the surrounding atmosphere. At this time a reverse (low pressure) wave will enter the pipe and travel back to the cylinder. Time this right and you can suck some of the remaining burned gasses out of the cylinder.

Now a straight pipe only works for one small range of RPMs. But if you change the pipe to a V-shape it will work more gradually.

Now the big questions:

  1. Is this the only reason? (No idea, it is one of the reasons to shape an exhaust pipe).
  2. How does this interact with sound damping? (Which I suspect is the second part of the exhaust which you have shown).

Sadly I will have to leave those two for others to answer.


Straight exhaust pipe


Additional information from – Perkins who has more detailed knowledge. Put into the answer since I understand that comments will eventually disappear from answers.

It's a little more complicated. Two-stroke engines have new fuel-air mixture coming into the cylinder at the same time as the exhaust is leaving. Sucking sufficient quantity of exhaust out of the cylinder is trivial. The wierdly-shaped exhaust pipes like the one pictured are actually tuned for a specific RPM range, and they first suck a larger quantity of fuel-air through the engine, and then the pressure wave bounces off the second narrow part and pushes the extra fuel-air back into the cylinder at a higher pressure, reducing fuel waste and increasing performance.


*1: Ignore the risk of dirt getting into the engine.

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You're almost right. It's a little more complicated. Two-stroke engines have new fuel-air mixture coming into the cylinder at the same time as the exhaust is leaving. Sucking sufficient quantity of exhaust out of the cylinder is trivial. The wierdly-shaped exhaust pipes like the one pictured are actually tuned for a specific RPM range, and they first suck a larger quantity of fuel-air through the engine, and then the pressure wave bounces off the second narrow part and pushes the extra fuel-air back into the cylinder at a higher pressure, reducing fuel waste and increasing performance. – Perkins Jan 15 at 18:44
    
This low pressure wave doesn't come from the end of the exhaust - it will automatically follow the high pressure wave, and the shape of the exhaust helps to tune the low pressure. – sweber Jan 17 at 13:02

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