2 stroke thread (with occasional F1 relevance!)

[johnny comelately]

2023 iteration of Rotax/BRP/Ski-Doo 850 snowmobile, 180hp with (altitude compensation) turbo.

https://sledmagazine.com/ski-doo-pushes ... c-turbo-r/

This is a good expose of that little gem too:

[vorticism]

2023 iteration of Rotax/BRP/Ski-Doo 850 snowmobile, 180hp with (altitude compensation) turbo.

https://sledmagazine.com/ski-doo-pushes ... c-turbo-r/

The expansion chamber ahead of the turbine reminds me of Merc's exhaust headers.

[johnny comelately]

This may interest you then. As an example of some current 4 stroke MX ideas.

[gruntguru]

Stepped header primaries are becoming very common but the use on a turbo application is curious.

[vorticism]

Helps evacuate the cylinder just as in a two stroke. Helps reduce power required for exhaust stroke. Also advantageous during wastegate open regime.

[J.A.W.]

Stepped header primaries are becoming very common but the use on a turbo application is curious.

The Rotax/BRP/Ski-Doo 850 turbo has a 'Y' junction 2-into-1 header, & the attached expansion
chamber/diffuser cone pipe - has to be gas-pressure sealed - yet not fracture prone.

[gruntguru]

Ahh interesting, thanks J.A.W. Does that layout (with or without turbo) function the same as one chamber per cylinder in terms of expansion chamber "supercharging".

[J.A.W.]

Ahh interesting, thanks J.A.W. Does that layout (with or without turbo) function the same as one chamber per cylinder in terms of expansion chamber "supercharging".

With a 180 degree crankshaft 2-stroke, firing intervals allow for this expedient, but packaging/costs
trump max output/efficiency gains ultimately shown by running individual systems per cylinder.

A larger number of cylinders will allow closer combustion events & synergistic flows accordingly as
gas flow can be timed as continuous, or over-lapping (3 cyl 2-strokes = 6 cyl 4-strokes, pulse-wise).

[gruntguru]

. . and of course wave supercharging becomes less important when you have a hair-dryer hanging off the side.

[johnny comelately]

I always understood the step is to prevent the pressure wave going back into the engine.
We have used reverse cones with some success to achieve same .

[gruntguru]

The two stroke "pipe" is designed for the pressure pulse to go back into the engine (the pulse contains fresh charge not exhaust gas).

[johnny comelately]

The two stroke "pipe" is designed for the pressure pulse to go back into the engine (the pulse contains fresh charge not exhaust gas).

I meant the step on the 4 stroke pipes, a la F1 (at least the older NA ones), NASCAR etc

[J.A.W.]

. . and of course wave supercharging becomes less important when you have a hair-dryer hanging off the side.

That's a bit of a 'curly one' gg, if its natural vs perm, (& don't get us started on H-peroxide additives)..

[vorticism]

. . and of course wave supercharging becomes less important when you have a hair-dryer hanging off the side.

This would depend on the differential pressure between cylinder and turbine inlet. I assume never as extreme as venting to atmosphere directly, perhaps unless during WGO. Regardless, wave tuning should still be possible, pipe enlargement will lead to a pressure and velocity drop locally. Same for the 'supercharging' wave, maybe there is a small gain to minimizing crossflow beyond what is needed. Less compressor work.

[Pinger]

From this:



To this:



No mention of why the turbo was relocated to the end of the pipe.