reed versus rotery valved

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Reed vs. Rotary Valve Engines Explained
by Phil Mickelson
Snowmobile News

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!

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Tuned Pipe Theory here I have also included expansion chamber theory