Two Stroke Induction

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dbabcock

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I came upon this writeup about two stroke induction methodologies that some might find interesting. It's a snowmobile thing, but the same principles apply to any two stroke engine. Older saws and some of the new race saws have used reed valves, but none that I have seen use rotary valves. Probably because of weight issues. It used to be that rotary valved engines had the highest specific outputs, next came case reed designs, then the newer piston ported units that mount the carbs higher on the cylinders to get the engine lower in the chassis. Who knows what's next.

No, it wasn't written by Ken Dunn, but it could have been.

________________________________

Many people confuse a reed valve with a rotary valve when you discuss crankcase versus cylinder mounted reed valves. The opening of a reed valve is controlled by the pressure differences on each side of the reeds while a rotary valve is opened and closed mechanically by a driving shaft.

There are three different intake control systems used on two-stroke cycle engines as used in snowmobiles. Piston port designs are still around and have been in use from the beginning. With this design, you simply have a number of holes or ports in the cylinder, the opening and closing of which is controlled by the piston. The highest port in the cylinder is the exhaust port, controlled by the top edge of the piston. As the piston travels down the bore from Top Dead Center (TDC), the next ports the piston dome allows to open are the transfer ports which are located to the sides and rear of the cylinder depending upon the engine's design. These ports transfer the fresh charge from the crankcase to the top side of the piston and help scavenge the cylinder of exhaust gasses. When the piston is at Bottom Dead Center (BDC), the exhaust port and transfer ports are wide open.

Controlled by the bottom or skirt of the piston is the intake port, located low on the back side of the cylinder. The intake port is wide open when the piston is at TDC. The intake port first opens when the piston is located about 80º Before TDC (BTDC). The piston must close the intake port at the same number of degrees After TDC (ATDC). The opening and closing of the intake port on a piston port engine is always symmetric around TDC.

Theoretically, the crankcase begins to decompress as soon as the piston begins traveling toward TDC, which is 180º BTDC. As soon as the crankcase pressure is lower than atmospheric pressure, air flow can begin from the atmosphere through the carburetor and into the crankcase. It appears that in waiting to open the intake port till 80º BTDC, we are wasting time that could be used to fill the crankcase. If, however, one opened the port at 100º BTDC, the port would have to close at 100º ATDC also. Remember the factor of symmetry of port opening and closing in the case of a piston port engine. If the intake port was open at 100º ATDC, you'd have a real cloud of fuel-air mixture coming right back out the carburetor. There is a point of no return that cannot be exceeded in intake port timing and it is controlled by the losses on the other side of TDC. The closer you get to that point, the "peakier" the engine becomes.

Reed valves are sometimes called "atmospheric valves". They can open only in one direction and will open only when the pressure in the crankcase is below the pressure in the air box which will be very close to atmospheric pressure. When the piston starts rising toward TDC, the reed valve will open shortly after BDC. When the piston crosses TDC and pressure in the crankcase begins to rise, the reed valve closes, sealing the intake port. This design allows the engine to take greater advantage of the crankcase's early decompression without losing the charge out the carburetor as the piston begins traveling back to BDC. The intake timing of reed engines is "asymmetric" about TDC, the port is open more degrees before TDC than after.

Reed valves can be mounted in the cylinder, the crankcase or in combination, cylinder/case reed arrangements. Cylinder mounted reed valves usually have openings through the skirt of the piston to allow intake through them directly into the crankcase. Passages are often used linking a cylinder mounted reed valve directly to transfer passages, particularly at the rear of the cylinder.

Today's reed cages that mount the actual reed petals flow much more efficiently than the early, flat reed designs. Most of today's engines with reed valves use a triangular style cage that mounts reed petals on both sides of the triangular casting, offering more reed surface area and smoother flow of the intake charge through them.

While the reed valve design produces a wider usable power band than a piston port engine and is economical to build, there are disadvantages to its use. The reed petal acts like a spring, never completely clearing the intake tract and, in itself, offers resistance to the flow of fresh charge into the engine. The reed petal is in constant resonation when the engine is running and this resonation or flexing causes it to fatigue and eventually fail. Those involved in competition and using high speed reed inducted engines will use phenolic or graphite composite rather than metal reeds to avoid damage to the engine in the event of a reed fracture.

The third intake system being used on snowmobile engines is found only on Bombardier-Rotax's twin cylinder, central rotary valve engines, first seen on the 1972 Blizzard 400 and 440. In this design, a single, rotating disk valve controls the opening and closing of both intake ports. The disk valve is driven at crankshaft speed by a shaft that is gear driven off the crankshaft. The design is very compact and light and allows the engine designer total control over the intake timing. The intake port can, for instance, be timed to open 140º BTDC and close 70º ATDC giving a total intake duration of 210º without a "peaky" power curve. When the disk valve clears the intake port, it exposes a wide open passage to the crankcase, there are no restrictions to the flow as is the case with a reed valve.

The peak horsepower that can be achieved from the three different intake designs on engines of equal displacement is nearly equal. The big difference between their outputs is the width of the power band and the response of the engine throughout their operating range. In the case of reed inducted two-strokes today, the cylinder mounted reeds allow lower placement of the engine in the chassis and the passages connecting directly to the transfer ports allow them to make big time power!
 
The article posted (thanks, by the way) seems to be borne out by my own experience with sno-mo eng's in the past, as well as bits and pieces dredged up talking to other motorheads involved in sno-mo racing back in the day (early 70's--early '80's)

My early experience with sno-mobile engines was limited to trying to get 10,000rpm and 50 hp out of 300cc singles, unfailingly with explosive results. One of the things we tried to do was contain the intake cloud produced by over-long intake duration with an intake bell, or veelocity stack.

In the small town I grew up in, there was a Bridgestone motorcycle dealer. These were available in sizes from 50cc up to 350cc, all rotary valve engines with oil injection. Extremely good performers, as well. I had a 100cc TMX model, as well as a 175 twin. That 100 would drub a Yamaha 125 without breaking a sweat, and the 175 would make the Yam 250 twin struggle...assuming both were stone stock.

The 350 was supposedly becoming a contender in it's roadracing class, until Yamaha and Honda informed Bridgestone that unless they ceased production of their bikes, they could expect no more tire sales.
 
Wow, I miss my Kawasaki S3, that was one hell of a bike. Rotary valve 2 cylinder 2 stroke. I always had to explain to people where the carbs were.
 
There are lots of early chain saws that used a rotary valve intake system, Homelite and Mcculloch to name a few. Then there was the IEL twin that had a rotary inake built into the lobes of the crankshaft. The fourth intake sytem that only appeared in two engines was the McCulloch BP-1 chain saw with a balanced piston that served as the intake valve. McCulloch also built a 350cc snowmobile engine in the balanced piston design but it never got past the testing stage before the snowmobile industry went into the tank and the engine went on hold. In terms of design ideas everything has been tried at least once in the chain saw industry.
 
I've heard rumours of a low production McC chain saw, and maybe the BP1 was the one...something about almost unrestrained revving ability, and the McC company pulled them out of fear of liability in case of accident. Even offered to buy back the few that had been sold, supposedly had agents track down individual saws. Extremely rare to find one now.

Mike, you're the historian/information storehouse...recognize any of the story?
 
That sounds a lot like the story of the BP-1 ! I've seen one in a private collection, owner said the guy he got it from hid it from McCulloch to keep it from being recalled.
 
The BP-1 was recalled not long after the first batch were spread around the country. It would turn up to 14,000 as soon as you pulled the trigger. It used a 2-1 reduction in the transmission so at those revs the chain speed was still pretty good. It is shown in the McCulloch section of my web page http://www.acresinternet.com/sites/cscc.nsf
Most collectors have one or two in their McCulloch section. It was another instance of McCulloch introducing an engineering innovation that blew everyone else out of the water. The BP-1 appeared in 1963.
 
Are reed valves used today?

Do any saws use reeds or has everyone gone other ways to make more power?

Was reading a post about putting reeds in a 088 and a tunes 125cc pipe.

Would the reed valve prevent the back pressure to build properly and open the reed at the wrong time?
 
I've heard stories of dealers back then burying the BP-1 in the backyard to hide them form McCulloch Salesmen hunting them down.:D
 

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