Turbo/super charging a 2 stroke.

[Stihl 041 super]

Yep

Average joe the reason why turbos work on sled engines is that they have tuned pipes that help hold in the boost. Most 2 stroke saws donot have that kind of exhaust so when you put the boost to them the boost just goes right out the exhaust I know I tried it with my leaf blower.

 

[average joe]

a turbo would work fine without a tuned pipe. In most sled turbos I have seen the pipe is rube goldberged to death to the point where it doesnt function fully as a tuned pipe any way.
Likely the reason your blower didnt work had to do with fuel delivery and more importantly the porting.

 

[ErrolC]

Dear Joe..lets think about it for a minute..pertainant to piston ported petrol engines..with the piston on the power stroke (on the way down) whats going to happen if you open the transfers before you open the exh..because to close the transfers after the exh (on the compression stroke, on the way up) which is what you need for the intake ports (transfers) to continue to pressurise the cylinder before compression begins, the top of the transfer ports HAVE TO BE ABOVE the exh port..the engine won't run..

Sure you can supercharge a piston ported 2stroke but you only end up running 1-2 lbs of boost at the most..the end result is that a piped engine can give the same performance..you can only get a level of boost equivalent to the reversion pressure wave a pipe reflects back..any more pressure from within the engine just continues to flow out the pipe..no advantage....unless you run direct cylinder injection..then the excess intake air (which is air only, no fuel mix) can be used to more completely scavenge exh gases from the cylinder

 

[average joe]

Here you go Erollc:http://www.mullerperformance.com/snowmobile.html#snow
Notice the boost figures of 8 and 12lbs? Turbos do work on two stroke.
FWIW I was speculating on the port timing differances of a turbo two stroke vs a traditional two stroke. I know what your saying about the engine not operating if the transfer ports opened before the exhaust port on the exhaust stroke, but these turbo kits do exist and do work.

 

[average joe]

Here are a few more:
http://www.lako.com/turbo.html

 

[klickitatsacket]

I love this thread. For all those who hate these types of discussions becuase all people do is argue... I love them because the ideas that are floated back and forth and the differant perspectives that people think from plant seeds of thought into my brain. Here is a what if question. What would happen then if you turbo or super charged a piston port engine knowing that you are pushing the fuel through and across the piston crown,(cooling it) and then used a pipe to stuff it back into the cylinder? Just a thought using Both perspectives. Also if the duration is too long and you are wasting too much fuel, just close up the bottom of the port and widen them or add more transfer ports to optimized the effect right before exhaust port closure. Heck you could do that with out a pipe. The pipe would though help with the scavenging if you were that tight on timing.

 

[Lawn Masters]

While some of this is WAY over my head, its interesting. MAybe someone will try some of this stuff and see what happens.

 

[average joe]

Its already been tried, along with almost everything else.

 

[Al Smith]

You could do it with a direct injector. Long intake port with a tough reed on the intake. Stuff air with the blower after the piston clears the exhaust on the up stroke,inject before top dead center,hope for the best.It's all theory,who knows.I haven't quite figured out how to oil the lower end. You would have to have a real high volume blower with a fast acting waste gate.Any thing is possible,however it may not be practical.

 

[ErrolC]

Woo Hoo..I take it back..looks very much like they've sorted things..my experience and knowledge has been based on developments in the 80s/early 90s..things have moved along a bit since then obviously..thanks for the info and enlightenment.

The last of the Yamaha TZ fornula750 bikes for Daytona were pumping 180hp in the late 80s..race shops had tried turbos and found little performance advantage for the complexities added...a problem being the level of backpressure needed to hold the flow at the exh port as I explained.

Those turbo units must be getting reeaal small

Jeese thats a lot of hp "245" from something you'd be able to pick up..push that Canadian V8 saw...single handed if you could hold it!!!!!

 

[ErrolC]

reading on..

Quo:System back-pressure must be kept within narrow limits if reliability and power are to be achieved..

must be a fine balancing act

 

[shtoink]

You could do it with a direct injector. Long intake port with a tough reed on the intake. Stuff air with the blower after the piston clears the exhaust on the up stroke,inject before top dead center,hope for the best.It's all theory,who knows.I haven't quite figured out how to oil the lower end. You would have to have a real high volume blower with a fast acting waste gate.Any thing is possible,however it may not be practical.

You could also have a specially shaped piston top. I remember reading some info a while back that involved fuel being pumped in just before TDC and the special hollow shape on the top of the piston was designed to maximize the process and use less fuel. The piston actually had an inverted "V" shape and the hollow was on one side. The top of the piston nearly kissed the top of the cylinder. It was on a 4 stroke motor, but the principal is the same. Puff in vaporized fuel, and cram is as much air as you can after the exhaust port closed for a bunch of compression and there is a nice big bang.

 

[Kneejerk Bombas]

This should take to another thread containing the pic's requires on the BP-1.

Mc Bob.

http://www.arboristsite.com/showthread.php?t=19072

Awesome pictures and really cool saw!

Are there many of those around?

 

[Kneejerk Bombas]

I love this thread. For all those who hate these types of discussions becuase all people do is argue... I love them because the ideas that are floated back and forth and the differant perspectives that people think from plant seeds of thought into my brain.

Me too. Fun read.

 

[Al Smith]

More food for thought

What would happen if a person were to install,somehow a turbo charger with a spring loaded pressure activated discharge gate or perhaps differential pressure activated?Put the discharge directly over the carb[a big one].At start up or low rpm the saw would be natural aspirated.When the pressure came up from rpm's the waste gate would close pressurizing the crankcase etc.On the power side of the turbine,perhaps as the pressure wave decreased it may in fact reverse the exhaust enough to act like a tuned pipe.If this would work,you would still have the impulse cycle to pump the fuel,oil the bar etc,but a lot stronger pulse.In my madness I am thinking a reed valve engine would work best[Mac man ya know ]Alas if good old J P McCulloch wouldn't have went off the deep end,he would have figured it out 30 yrs ago.For that matter,the ole boy may have and kept it secret.Oh yes,how to oil the turbine? Impulse from the bar oiler,maybe.

 

[jimbo1490]

Crankcase scavenged two-strokes, like the ones in chain saws, hate exhaust back pressure. That's why turbocharging is not feasible for them. Actually, all piston engines hate exhaust pack pressure, but with an Otto cycle (four-stroke with valves over the piston), the higher exhaust back pressure from the turbine side is compensated almost completely by the increased intake pressure from the compressor side, so that the engine still runs like it should, mostly anyway. There are still some changes, like the need for shortened overlap of intake and exhaust events and shortened exhaust duration, generally. But these changes are easily made and can mitigate most of the problems that exhaust back pressure would normally cause.

The Otto-cycle engine depends somewhat less on the kinetic energy of its operating gas to charge and scavenge the cylinder than the crankcase-scavenged two-stroke. A crankcase-scavenged two-stroke uses a gas at a low pressure and depends mostly on velocity to get the charging/scavenging jobs done. An Otto cycle can vary the proportions of pressure and velocity to suit diffierent conditions, by changine valve timing. This can be changed somewhat with a two-stroke with port timing, but not nearly to the same extent, and the adiabatic losses will always be greater since the two-stroke engine favors a low-pressure/high-velocity type of flow.

On top of all this, the crankcase-scavenged two-stroke engine has lower peak exhaust temperatures. The turbine is essentially a heat recovery device, so with less heat available in the exhaust, the turbine has less waste heat to work with and consequently spins slower. In selecting a turbo, when it is found that a particular turbo will spin too slow, the general solution is to install a smaller unit, or one with a more restricted turbine inlet (lower A/R ratio). But this produces more back pressure, possibly compounding the problem.

Both port uniflow (Cooper-Bessemer, Fairbanks-Morse, etc.) and valve uniflow (Detroit Diesel, EMD) use turbocharging successfully. Note that these engines ALWAYS use pressure aspiration with a positive displacement pump as part of their design, even when a turbocharger is not used. So when a turbocharger is used with a uniflow two-stroke engine, it still has a seperate positive displacement scavenging pump, so it has both a turbocaharger and a supercharger. These engines will not run without pressure aspiration as there is no scavenging/charging force otherwise.

Crankcase-scavenged two-stroke engines have been successfully turbocharged, but success depends on the ability to carefully match the turbo unit with the correct event timing. In an Otto cycle engine, you can easily make even very radical changes to the event timing by simply changing camshafts. It's not nearly that easy with a crankcase-scavenged two-stroke; you wind up havig to make new cylinders.

Crankshaft-driven supercharging can be more easily adapted to small two-stroke engines, but lacks the advantage of consuming some engine output to drive the pump, whereas the turbo uses waste heat from the exhaust. This is not a trivial amount of power; in a truck diesel engine for instance, more than 50HP is dissipated across the shaft of the turbocharger, so a shaft-driven supercharger of similar capability would have to subtract that kind of power from the crankshaft to get the job done.

Jimbo

 

[jimbo1490]

shtoink,
Aren't those pics of the assembly of that GIANT Sulzer marine Diesel awesome?! I've seen them before and I'm still amazed by that degree of engineering and power.

If you liked those, the check this monster out!

http://www.lexcie.zetnet.co.uk/delticengine.htm

This is port uniflow since there are not poppet valves, only piston ports with one intake piston and one exhaust piston running in each cylinder.

Jimbo

 

[jimbo1490]

What would happen if a person were to install,somehow a turbo charger with a spring loaded pressure activated discharge gate or perhaps differential pressure activated?

Or how about a device that timed the exhaust pulses to compress intake air and then supply it to the intake side at precisley the right moment:


http://www.kfz-tech.de/ComprLader.htm

I could not find one English language page with a picture or diagram this good; I hope it will be worth a thousand words

Jimbo




Jimbo

 

[CNYCountry]

Nitrous is a way of adding more oxygen to the air fuel mix rather than a true form of forced air induction. Turbocharging or supercharging aim to flow more air through the motor by packing more air/fuel mix into the chamber and burning it.

Right, that's more correct than the generalization than I posted, I thought nitrous did something to the density of the air as a function of cooling it or something also?

 

[t_andersen]

I used to work with design of large MAN/B&W ship diesel engines. The principle of turbocharging is to make the engine "believe" that it lives in denser air with a pressure of, say, a couple of atmospheres. The engine sees a thicker air on the inlet side and at the exhaust ports. Because of the thicker atmosphere, you can burn, say, twice as much fuel in the same cylinder. As also noted by "Stihl 041" above, this calls for better cooling plus a lot of other stuff. Seems hard to do on a chain saw...