Two-stroke cycle engine 1

[lucianfg]

Why a two-stroke cycle engine must have an intake pressure boost??

 

[dgallup]

No suction stroke.

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[Tmoose]

I wouldn't say a typical 2-stroke motorcycle, snowmobile, personal watercraft, or weed wacker has an "intake pressure boost."

 

[dgallup]

They use the crankcase to pump through the transfer ports so there is some boost going into the cylinder.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[BrianPetersen]

If the intake weren't pressurized, what do you think would happen with the exhaust and intake ports open simultaneously?

Nothing?

That would be a problem, wouldn't it?

Therefore, it needs something to push the scavenging charge through the cylinder with positive pressure.

Most questions like this can be solved by thinking.

 

[VEBill]

Somewhere I've seen some example of a two-stroke engine where it was all about the tuning of the exhaust system to scavenge out the cylinder (by means of a tuned exhaust) leaving a negative pressure that assisted with pulling in the next charge. My vague recollection is that it was an odd application, like a model airplane engine.

This might be a counter-example to the general rule.

 

[berkshire]

Ok, standard two stroke engine , induction is caused by the rising piston creating a partial vacuum in the crank case. As the piston goes over top dead center,it then starts to compress the mixture in the crank case with the port open causing a small backrush into the induction tube, the piston descends further closing the induction port, compressing the gas mixture. As the piston descends the exhaust port is opened allowing the exhaust gases to escape, The piston descends further opening the transfer ports, the compressed gas blows out the remaining exhaust from the cylinder. Now this is where the tuned exhaust comes in. The velocity of the departing gas also pulls some of the fresh charge into the exhaust and wastes power. However with an exhaust tuned to a resonant frequency at a useful RPM number, a pressure wave is reflected back to the exhaust port forcing most of the unburned fuel in the exhaust pipe back into the cylinder, just before the transfer port closes, where the charge is now compressed and burned, leading to the next cycle.
Some diehard engine tuners even tune the intake runner.
B.E.

You are judged not by what you know, but by what you can do.

 

[EHudson]

VE1BLL is perhaps referring to engines that ran without boost by using the "Kadenacy Effect". The velocity of the exhaust did create a vacuum that served to generate intake flow.

(A few old, single speed, stationary diesels I think.)

As the load and corresponding exhaust pressures increased, the vacuum pulse of the exiting exhaust tended to decrease. You can estimate the maximum intake charge pressure and figure out why the desire for higher BMEPs has left this idea in the weeds.

 

[Panther140]

Any increase in pressure of the fuel&air charge would come from the piston blocking the intake port from the cylinder while the piston is also moving downward and decreasing the volume in the crankcase.

The slightly pressurized fuel&air charge can have higher velocity entering the cylinder, which means that more time can be spent evacuating exhaust


"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[Surestick]

VE1BBL: Tuned pipes are common on RC 2-stroke engines as well as snowmobile & dirt-bike engines.

They use the reflected high pressure pulse from the convergent end of the pipe to push fuel/air mixture back into the cylinder that has escaped out the exhaust port. They only work over a narrow RPM range though (which isn't a problem in a snowmobile with a CVT).

 

[BigClive]

I have seen an experimental two-stroke engine with poppet valves running quite well without any forced induction. Having the exhaust port open before the intake creates a vacuum to draw the charge in. Starting requires some sort of assistance to give the engine the general idea of which way the flow is going. The engine I saw running had a vacuum cleaner briefly applied to the exhaust when starting.
Some years ago Toyota had a very powerful 2-stroke engine with poppet valves of about 3 litres. The engine had forced induction but I seem to recall that Toyota reported that it could be made to run quite well without the blower.
The above is presumably the general idea of what EHudson was referring to.

 

[Lou Scannon]

Starting requires some sort of assistance to give the engine the general idea of which way the flow is going.

The bi-directional nature of some 2-stroke engine configurations can be a bane or a blessing. If I'm not mistake, the large direct-drive marine 2-stroke engines use this feature as a way of achieving reverse thrust, without the cost, space and weight penalty of a gearbox. Simpler engines, such as the British Seagull, in the hands of a skilled user, can be manipulated to deliver the same benefit.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Surestick]

@Hemi,

Ski Doo & Polaris use a similar system for reverse on some of their snowmobiles. Selecting reverse cause the engine to shut down and start again in reverse. There's some additional changes to the driven CVT clutch so it will work in reverse and I'd guess the oil pump has to work bi-directionally as well.

I believe the system accomplishes this solely by manipulating spark timing to get the running engine to slow down below idle then essentially kick itself into reverse by igniting the cylinders far enough before TDC.

 

[BigClive]

I think this idea for reverse only works with piston port two-strokes which by their nature have symmetrical port timing. Quite a few of the two-stroke micro-cars of the 1950s used this system. With a poppet valve two-stroke engine the valve timing is not symmetrical - so it won't run in reverse. The whole process is powered by the exhaust blowdown and to get the first firing stroke the engine needs a bit of help from forced induction or a vacuum cleaner.

 

[BrianPetersen]

Reed-valve two-strokes will run in reverse, too. Rotary-valve, not so much.

Either way, if the ignition timing is done by simple means, it will probably not be happy about it.

The Bombardier "SDI" (semi-direct-injection) engines drive the snowmobile backwards by reversing the engine, but these engines are electronically controlled for both fuel injection timing and ignition timing, and the electronic controls manage the process of reversing the engine (very quickly, I may add), so those electronic controls are smart enough to know what to do with the injection and ignition timing when it's running backwards.

 

[Lou Scannon]

I guess on a large marine 2-stroke the savings of eliminating a gearbox can justify a little complexity in the fuel injection. Whether or not the poppet exhaust valve cam profile is symmetric is not critical, as long as the profile is "close enough" to function adequately with the engine reversed.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Overrun]

EHudson, you can add the German V-1 "jet" engine.

 

[EHudson]

Thanks, Overrun. Yes, a reed valve ram jet in the buzz bomb.

And, for the record, there were some old Mercury engines that reversed rotation for reverse.

 

[TheBlacksmith]

A couple of stories. Some of the early Ski-Doos had very little timing advance and were pull start, with the rope breaking about once a season. Some owners wound the rope backwards and the engines would readily and unknown to the owner start in reverse and back into any inconvenient object. The old direct drive Fairbanks Morse diesels were reversed by stopping, switching the crank lead with a lever and restarting. Several quick reversals (inept ship handling) would prime the exhaust stacks with raw fuel. The inevitable fireball generally did not improve the ship handling.

 

[berkshire]

The op has not visited the site since Feb,7th. Maybe we answered his question.
B.E.

You are judged not by what you know, but by what you can do.