2-stroke power valve system
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The 2-stroke power valve system is an improvement to a conventional two-stroke engine that gives a high power output over a wider RPM range.
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[edit] Operation of a 2-stroke engine
A stroke is the action of a piston travelling the full length of its cylinder. In a two-stroke power valve system, one of the two strokes combines the intake and the compression stroke, while the other stroke combines the combustion and exhaust stroke.
As the piston travels upward in the cylinder, it creates a vacuum in the crankcase; this draws fresh air and atomized fuel from the carburetor through a hole in the cylinder wall. As the piston continues travelling upward, transfer ports that are responsible for delivering the fresh air-fuel mixture to the cylinder are closed off, thus trapping the combustible mixture. As the piston reaches the top of the cylinder, the mixture in the cylinder is compressed to the point of ignition.
The second stroke begins once ignition has taken place. The combustion-or power—stroke begins as the air-fuel mixture is ignited. The burning fuel creates pressure in the cylinder above the piston and forces it downward. As the piston passes the midpoint of the downstroke, the exhaust port to the side of the cylinder starts to open and initiates the flow of burned fuel out into the expansion chamber or muffler through the manifold.
As the piston is forced downward, positive pressure builds up in the crankcase where the air-fuel mixture remains from the previous intake-compression stroke. Shortly after the exhaust port is uncovered by the downward travel of the piston, the transfer ports begin to be uncovered. The transfer ports act as a passage through which the air-fuel mixture moves from the crankcase into the cylinder above the piston. The rush of the fresh air-fuel mixture as it enters the cylinder helps to push out residual exhaust gases. Once the piston reaches the bottom of the stroke, the second cycle is completed and the process is repeated.
[edit] Engineering design improvements
The only moving parts inside a two-stroke engine are the crankshaft, the connecting rod and the piston. This means two-strokes are very simple engines. Because there is a combustion-stroke whenever the piston travels downward, they are capable of producing tremendous power. It is the same simplicity in design, however, that makes a two-stroke engine less fuel-efficient. At the bottom of the power stroke, the transfer ports, which deliver fresh fuel, are open at the same time as the exhaust port. This allows a significant amount of fresh fuel to run straight through the engine without ever being available for power production. Properly designed exhaust systems help minimize the amount of raw fuel loss in the exhaust process, but a two-stroke engine will always waste some fuel.
Many producers of two-stroke performance bikes fit them with the exhaust valve systems. A valve is normally situated alongside the exhaust port. To make a two-stroke engine high-powered, the cylinder is given large ports, particularly the exhaust port. The main problem with this is that it makes the engine produce very little power at low RPM (revolution per minute) and have a high fuel consumption.
In a race bike, this is not a problem as the engine will be operating at high RPM almost all the time. However, in a road/commuter/dirt bike, this presents a problem. To give more low RPM power, as well as enable the engine to be able to produce a lot of high RPM power, a power valve system is used. This also smoothens out the power band, apparent in a lot of high powered two-stroke bikes, especially dirt bikes.
Some power valve systems simply restrict the exhaust gases after they have exited the combustion chamber, while others (such as Suzuki AETC) have valves that actually protrude to almost as if they are in the combustion chamber. The AETC system replicates the size and shape of a cylinder with small ports when they are closed giving good low rpm power, and when open replicate an engine with large exhaust port(s) to give good high RPM power. Yet another system opens and closes a sealed cavity in the head; V-TACS uses this system. On the most basic level, the power valve system simply varies the size of the exhaust port, allowing a small port to be used at low RPM's, and a larger port at a higher RPM. On a more complex level, however, many power valve systems change the compression ratio as well, allowing the optimum compression for low and high RPM's.
With the power valve system, there is a "crossover point". This is the RPM whereby the engine produces the same amount of power, whether the power valve is open or closed. By opening the valve, it allows the engine to produce more power at higher RPM; however, if it is open at low RPM, the engine will produce less power (if it is below the crossover point) than when it is closed. To get the most power out of the engine, the power valve must be open just as the crossover point is reached. The same theory is used with adjustable-cam timing on four-stroke engines, such as the Honda's VTEC.
The advantage of these systems over an engine that has a large exhaust port is that at low RPM, the power valve engines will make more power and use less fuel. It will also produce more power at high RPM than an engine with a small exhaust port.
[edit] Suzuki AETC and Super AETC
AETC and Super AETC Suzuki engines, Automatic Exhaust Timing Control: The two-blade version was fitted to the VJ21 RGV250, and the three-blade version, to the VJ22 RGV250 and Suzuki RG150.
With the AETC system, the power-valve systems are normally partially closed at low RPM; when closed, it enables the engine to make more power. Up to a certain point, however, power drops off as the engine is unable to expel enough gases out of the exhaust. When the power-valve is opened, it allows more gases to flow out of the exhaust port. This system is recognizable by a small box above the exhaust outlet; the power-valves are situated in this box. Depending on the valve, they may be made of two (older version) or three (newer version) separate blades.
[edit] YPVS
YPVS Yamaha engines, Yamaha Power Valve System: It was fitted to later models of the RZ/RD two-stroke road bikes and dirt bikes such as the DT125/250 enduros and YZ125/250 motocrossers among others.
Yamaha was actually the first company to produce consistent results with their YPVS in their race bikes. The 1977 OW35K was the first race bike to incorporate the power valve system and it won the Finnish GP in 1977. Kadenacy effect was harnesed and controlled to a point that gave Yamaha great advantage over all the other manufacturures throughout the late 70's and into the mid 80's. The first street bikes with YPVS were the RZ 350/RD350 LC and RZ 500 GP Replica in 1983-84.
[edit] Honda V-TACS
The "V-TACS system" works differently from the "AETC system" and it will only work when it is used in conjunction with a tuned muffler. Tuned mufflers/expansion chambers increase power but only at the RPM they are designed for and can actually cause a power loss outside their tuned RPM. "V-TACS system" takes advantage of using an expansion chamber without losing power outside the expansion chamber's tuned RPM. Within the head and cylinder of the engine, there is a chamber that is sealed by a valve. This sealed chamber is vented onto the exhaust port when the valve is open. At low RPM this valve is open, this has the effect of increasing the exhaust manifold volume and negating the power loss that would normally be apparent at low RPM with an expansion chamber. At mid RPM the valve is closed, this enables the expansion chamber to work. It is identified by the head and cylinder, being much larger than normal for its displacement, the cylinder is also cast with the wording VTACS on it.
V-TACS was a foot-operated power valve system made by Honda on some of its small two-stroke bikes and scooters, like the Honda FC50.
[edit] Kawasaki KIPS
Kawasaki uses a power-valve system called KIPS (Kawasaki Integrated Powervalve System) on their two-stroke bikes.
