18 replies to this topic

[Bluehair]

With the recent domination of 4-strokes in GP Moto, I had this thought...Has anyone ever successfully supercharged or turbocharged a two-stroke? With the proper combination of valves and/or ports, this seems like a possibility. Imagine the performance of an open-class kart, motocrosser, or personal watercraft.

[LB_13]

YES! Supercharged twostroke engines has some very intresting history.

I can post some pictures if I can find some of those old books.

[Wolf]

Bluehair- all two-strokes are supercharged, one way or the other. It is neccessary for their proper operation. The most usual way is using downward movement of the piston to compress the air in the sump which then goes into the intake.

[LB_13]

Originally posted by Wolf
Bluehair- all two-strokes are supercharged, one way or the other. It is neccessary for their proper operation. The most usual way is using downward movement of the piston to compress the air in the sump which then goes into the intake.

There is a big differens. A normal twostroke engine is not defined as a supercharged engine!

Still loocking for my old books...

[Chickenman]

Supercharged andor Turbocharged two stroke Diesels are very common. In fact the Roots Superchager used on all Top Fuel dragsters was originally derived from the Scavenger Blower off of GMC " Jimmy " Diesels.

Two Stroke Diesels require a blower ( Scavenger ) to function properly. Two stroke gasoline engines do not.

[rdrcr]

Wolf, technically your mistaken but I get your theory...

The two-stroke engine draws air/fuel in at the same time as it pushes out exhaust Expansion / Power. In the two-stroke engine, as in the four-stroke engine, the expanding combustion products force the piston down to provide the power. Pressure and temperature start to decrease from their maximum values. As the piston moves downwards - but before BDC - exhaust blowdown is begun. This is done by opening an exhaust valve at the cylinder head, or by having the piston uncover ports at the sides of the cylinder.

Intake & Scavenging.

As the pressure drops and atmospheric pressure is nearly reached inside the cylinder, intake slots at the side of the cylinder are uncovered and pressurized air/fuel forces its way in. This mix pushes out remaining exhaust gases a nd fills the cylinder - a process known as “scavenging.” The piston passes BDC and on its way up quickly covers the intake port and exhaust ports (or, the exhaust valves are closed). Finally, all valves/ports closed, and the piston finishes compressing the air/fuel as it moves back up to TDC. A spark plug fires as the piston reaches TDC, ignition occurs, and the cycle repeats

Compression.

Note that in the two-stroke cycle, the air/fuel mixture must enter under pressure, in order to force out the exhaust gases. This is done in one of two ways: with a supercharger (compressor) that compresses the air/fuel before it enters the cylinder, or mo re commonly, by redesigning the crankcase so that it acts as a compressor during the power stroke (in other words, as the piston descends, it turns the crankshaft and compresses the air in the crankcase) This crankcase compression replaced the earlier tec hnology which used blowers to push in the air/fuel mixture.

In the case of crankcase compression, the crankcase is no longer filled with lubricating oil. In practice, what this means is that lubricating oil must be mixed into the air/fuel in a carburetor in order to coat the moving parts in the crankcase. This oil is composed of heavier hydrocrabons than fuel and less chemically reactive; when burned, it produces severe emissions problems. The oil also reduces the efficiency of the fuel combustion, because it does not completely burn.

Superchargers allow precompression of the air/fuel without requiring crankcase compression, and avoid mixing lubricating oil with the fuel, but add expense. Essentially, a supercharger is a blower that increases the pressure of the intaken air. It may be powered off the crankshaft (thus parasitically consuming some of the developed power), or powered by a turbine driven by the exhaust gas flow, in which case it is called a turbocharger.

Examples of supercharging two-strokes date back to the 30's at least... with the use of them by DKW in their GP bikes. Current applications are found in light aircraft, ultralights, etc. I haven't seen them recently in the unlimited classes of Karting. There isn't enough benefit to off set the weight vs. a strong 250 twin.

[LB_13]

rdrcr

[Wolf]

Richard- please enlighten me on this one... The way I see it, supercharging is incresing intake pressure (relative to the standard atm. pressure)... And that is done with crankcase compression and other (more common with 4-T technology) means. Or am I still missing something?

BTW, I think Puch (in '30ies) used additional piston on the same conrod (Y shaped) for forcing the mixture into the cylinder.

[Bluehair]

Great stuff rdrcr!! I've always been a little mislead about the crankcase compression technique. How many pounds of "boost" can be generated in this manner? How much of this pressure is lost during scavenging (prior to compressing the remaining intake charge)? It seems to me that the VOLUME of pressurized mixture will be no more than the swept volume in the cylinder, and will therefore, not be as effective as applying larger amounts of boosted intake by some other means. Obviously, the ports or valves would require reconfiguring.

[Wolf]

Bluehair- the volume that can enter the cylinder is swept volume... The pressure is what makes the difference, because the bigger pressure is bigger amount (mass) of the mixture will enter the cylinder. But I guess the 'boost' is limited because unlike with TC engines, it affects efficiency (in this case thermal efficiency, and with SC mechanical efficiency because SC is considered to be accessory).

[AS110]

ALL 2 stroke diesels use a supersharger,this is part of the scavange system,if a turbo is fitted,it is used to boost above atmospheric for more power.

99% of petrol 2strokes built in the last 40years use the piston as a double pump to fill the cyl.Other methods can and have been used,they are just not very cost/power efective.Boosting above atmospheric comes under the same problem.A 2stroke offers simlicity for lots of power,although if you look at modern 2stokes it doesn't look like that.They are doing just fine making as much horsepower as a 4stoke of twice the size and much more complex.

If they need to do it they will.

[Bluehair]

Wolf: Let's assume that we have a 125cc 2-stroke. The volume under the piston is likely to be even less than 125cc prior to being compressed. So then, after compression, the intake charge is is "blown" into the cylinder where it can expand (until the piston blocks the intake port and begins to compress the gas again). My point, is that once the intake port is blocked, the cylinder only contains about 125cc of gas (if it were under normal atmospheric pressure). On the other hand, a supercharged 125cc 4-stroke has the ability to pack in the equivalent of 250cc or more gas into the cylinder (again, measuring this volume as if it were under normal atmospheric pressure).

[AS110]

Have you ever seen a reciprocating compressor (Atlas Copco) layed out as a Vtwin or V4? These use a very large low pressure cyl to pump into a smaller high pressure cyl.

In the 50s there was a funny little British commercial vehicle called the Trojan with a 2stroke engine of similar design.A large piston pumped into the smaller working cyl,giving one would believe something for nothing,more charge pumped in than capacity.The Trojan was no horse but a mole and dug itself into history.

It worked on compressors,but not a petrol engine.I don't know the mathmatics of why this stuff doesn't work,but it sure doesn't.Could someone blind us with science?

[desmo]

Anyone else remember the Norton Wulf progam. It was a 500 twin 2-stroke in which each cylinder was used to charge its opposite? The motorcycle never made it to production, but Bernard Hooper is apparently still developing the concept for unmanned aircraft applications at the Wolverhampton Business Airport.

See Here for More.

[Melbourne Park]

The Australian Orbital Engine Company's fuel injection system for 2 strokes uses a sideways blast of air to divert a continous stream of fuel into the cyclinder. The longer the duration of air, the greater the amount of fuel; no air, no fuel is diverted into the cylinder. I would imagine that this blast could possably increase air pressure. Mercury I think have used the system for some years and there are a number of car companies who intend to produce 2 stroke cars suing the technology in the next few years. Although it hasn't yet come to pass ...

[rdrcr]

Originally posted by Wolf
Richard- please enlighten me on this one... The way I see it, supercharging is increasing intake pressure (relative to the standard atm. pressure)... And that is done with crankcase compression and other (more common with 4-T technology) means. Or am I still missing something?

Wolf, I always thought that the term "Supercharging" was given to an added on mechanically driven device. Where as "Turbocharging" was a device driven by exhaust gases. If an engine doesn't have either device then I'd consider it a normally aspirated engine regardless of how the combustion chamber gets filled.

Originally posted by Bluehair
...On the other hand, a supercharged 125cc 4-stroke has the ability to pack in the equivalent of 250cc or more gas into the cylinder (again, measuring this volume as if it were under normal atmospheric pressure).

Bluehair, I think you mean to say 125cc 2-Stroke right? Keeping apples with apples... I think that the case area is a bit larger than 125cc's but only 125cc worth of mixture gets in the combustion chamber due to the swept area that Wolf commented on. There is only so much time for the area to fill with mixture, porting helps improve flow but without the additional supercharging or turbocharging of the mixture, the engine will only be able to ignite what's available in the cylinder area.

[Christiaan]

I read an old issue of Britsh Car (the one when they first previewed the Ferrari F355, yes that old) and Toyota had a concept two stoke with a roots type blower. The blower blew are from the bottom of the cylnder at TDC so wash out the exhaust gases. I thought that was intersesting.

[Ray Bell]

Originally posted by Bluehair
Wolf: Let's assume that we have a 125cc 2-stroke. The volume under the piston is likely to be even less than 125cc prior to being compressed. So then, after compression, the intake charge is is "blown" into the cylinder where it can expand ....

It's quite a job to design the crankcase so that this actually happens. So much space has to be allowed for rotating and reciprocating parts... have a look at a modern 2-stroke engine crankcase some time.

Here's a couple of diagrams of a supercharged 2-stroke diesel that hasn't been mentioned so far in this thread... and it surprises me that nobody has mentioned the pre-war Auto-Union GP engines, too...





And another point to consider with conventional port-induction 2-strokes is that the ports prevent the full compression ratio being realised.

[desmo]

The return of the Commer double-knocker! Calculating actual CRs for conventional 2 strokes isn't straightforward, as one runs into the problem of ascertaining their 'trapping efficiency.'