With Direct Injection Would a 2 Stroke Need An Expansion Chamber?

by Steve

With the basic understanding of how an expansion chamber works by forcing or rebounding unburned fuel/air mixture back in to the cylinder through the exhaust port, and the fact that Direct Injection would not allow unburned fuel mixture into the exhaust, would there be any benefits to having an expansion chamber? And here is another head scratcher, would direct injection eliminate the 'Power Band'??

Does anyone have any feedback on this? How about the PWCs or Skidoos with direct injection? Are they still using expansion chambers?

It will be a great day when the carburetor on a two stroke motorcycle is replaced by direct injection or maybe something even better.

Two Stroke Lover

Comments for With Direct Injection Would a 2 Stroke Need An Expansion Chamber?

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Sep 24, 2014
DI two-stroke
by: John

A direct injected two-stroke engine would benefit from an expansion chamber the same way that a carbureted two-stroke engine does. The only difference being the positive pressure wave generated by the expansion chamber would push fresh air only (no fuel is present yet) into the combustion chamber just as the exhaust port closes. Next the DI system would add fuel to the air. So, we'd get the supercharging effect created by the pipe without the high emissions normally associated with this type of engine.

Oct 05, 2013
Expansion Chamber
by: Anonymous

Some of you are trying to answer the wrong problem here. Of course dfi deals with fuel, but if you had an expansion chamber on an engine with dfi, it would direct pure air back into the cylinder instead of gas/air. Therefore, the mixture would be leaner. Less gas = smaller boom. You would have to tune the dfi to give it more gas if you put an expansion chamber on. This way, it will have higher compression and a BIGGER boom, while maintaining a proper fuel/air mixture. Thoughts?

Mar 19, 2013
by: Duke

The rule about expansion chambers I was told back in the 1960's, still holds true today regardless of fuel delivery -- Long and thin does not win, short and fat is where it is at.

Feb 09, 2012
Maybe Not, Or Maybe Different...
by: Motopinion

I'm no engineer but as far as I can see DI addresses part of the 2T problem - getting the perfect amount of fuel in after the exhaust port's closed.

Basic 2T design means you will still lose some of the charge pressure (air) that comes up the transfer ports out the exhaust, since it's a ruddy great hole in the cylinder wall way before TDC.

Exhaust valves (YPVS and all the rest) were invented to address this to some extent (making the hole artificially lower/the properly sealed cylinder space artificially bigger), and of course the back pressure from the expansion chamber tries to help by stuffing that lost charge back in.

I see a lot of DI projects parcel up with clever exhaust valves to keep as much incoming charge in the chamber as possible, so I guess there may be 'changes' to expansion chambers to suit.

I think the theory is shorter/smaller chambers are about top-end power, so my eternal hope is they can address bottom-end with clever valvery and make smaller pipes to limit the amount of pay I direct credit to FMF.

Shane Mac,

Mar 27, 2011
Expansion Chamber
by: Anonymous

You need expansion chamber. Without expansion chamber, there is nothing to return the vacuum pulse back and scavenge unburnt fuel/air back into the combustion chamber (supercharge effect). No expansion chamber means no "powerband"

As for direct injection.. Direct injection only controls fuel, not air. Throttle bodies on a DI engine control the air flow. Principle is still the same... just controlled differently.

Dec 04, 2009
Thanks Tim
by: Steve A

Thanks Tim.

Your comment definitely sheds some light on the the subject. So there is a benefit from the expansion chamber if DFI is used. So without the expansion chamber the amount of trapped air would be lower and we would lose the "Super Charging" effect. Hmmm sounds like there may still be a power band with DFI...

2 Stroke Lover

Nov 16, 2009
by: Tim Hickox

DFI doesn't change what the expansion chamber does. Don't confuse AIR with FUEL. DFI controls FUEL; the expansion chamber changes the amount of AIR that remains trapped in the cylinder when the exhaust port closes.

Nov 15, 2009
Good Question
by: kenny

Hi Steve. A very good question. Some petrol cars have a single point injection system that injects fuel into an inlet manifold whilst still using a carb-type system to provide air to the mix. An idea perhaps?

I heard that they used to have two-stroke diesel trucks that required a turbo to pressurize the air too make it run! A turbo two-stroke dirt bike whoohooo!

I think that the expansion chamber gives you compression too. I know that when I dinged my one thanks too a pine tree, my bikes motor performance changed for the worst.

Back in the day they used to tune two-stroke engines by starting with a long as muffler and cut sections off until the desired tuning was met. That was until a very smart man thought of the expansion chamber.

No powerband? The day that happens is the day I buy a four-stroker! lol

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