It is often thought that there is no valve in the 2-stroke engine which opens and closes the port by the height of the piston in the cylinder in contrast to the 4-stroke engine which opens and closes the intake and exhaust valves by the camshaft. In reality, however, there are parts such as reed valves and rotary disc valves. Let's take a look at what happens when a reed valve breaks and explain the role of valves in a 2-stroke engine through actual problems.
The 2-stroke valve prevents the intake air from blowing back.
Debuting in 1965 as a derivative of the YG-1 introduced in 1963, the Yamaha YGS-1 was one of Yamaha's earliest models to feature a rotary disc valve. The rotary disc valve cover is hidden behind the clutch, so to check it, the clutch must be removed first.
While a four-stroke engine rotates by burning a mixture of air during the intake, compression, combustion, and exhaust processes, a two-stroke engine rotates while simultaneously performing the intake, compression, combustion, and exhaust processes.
In a 4-stroke engine, the mixture is drawn into the combustion chamber where the top of the piston and the intake and exhaust valves are located, but in a 2-stroke engine, the mixture first enters the crankcase. In order for the mixture to flow into the crankcase, the pressure inside the crankcase must be lower than atmospheric pressure.
The pressure inside the crankcase varies with the stroke of the piston, with the internal pressure rising when the piston descends and falling when the piston ascends. At this time, the mixture flows into the crankcase and at the same time, compression of the mixture takes place in the combustion chamber.
When the mixture burns and the piston is pushed down, exhaust gases are released from the exhaust port, and at the same time, the pressure in the crankcase rises due to the pushed down the piston, and the scavenged air is pumped into the combustion chamber through the scavenging port. When the piston rises above the bottom dead center, the pressure in the crankcase drops, and a new mixture flows into the combustion chamber. It is called a two-stroke engine because it combines two of the four work processes in a four-stroke engine and it is called a two-stroke because it performs combustion in two jobs, or processes.
Unlike a 4-stroke engine, a 2-stroke engine does not have engine oil stored in the crankcase. Instead, the engine mixes oil with air and gasoline from the carburetor to lubricate the pistons and cylinders, as well as the crankshaft and connecting rod bearings.
In a 2-stroke engine, the airflow inside the engine is controlled by the reciprocating pistons and by the position of the pistons in relation to the ports on the cylinder wall. 2-stroke engines have no camshafts or intake and exhaust valves, so there is no friction loss to move them. There is no friction loss to move them. In addition, while the 4-stroke engine performs the intake, compression, combustion, and exhaust processes with two revolutions of the crankshaft, the 2-stroke engine performs intake, compression, combustion, and exhaust at the same time, resulting in one degree of combustion per revolution of the crankshaft and strong torque.
However, due to the simultaneous intake, compression, combustion, and exhaust processes, there are problems such as overlap between the two processes, resulting in unburned air being exhausted.
For the 2-stroke engine, although it has no valves, it actually exists in the form of reed valves and rotary disc valves. As explained earlier, the intake process of a 2-stroke engine utilizes the crankcase pressure drop when the piston rises. In the scavenging process, a part of the mixture flows back to the carburetor side.
The reed valve or rotary disc valve is there to stop that reverse flow. There are several types of reed valves, such as piston reed valves and crankcase reed valves, but in all cases, they are located at the entrance to the crankcase and allow the mixture drawn in from the carburetor to enter the crankcase, but stop it from blowing back into the carburetor. Older 2-stroke engines built before the early 1970s did not have reed valves. There is a form of piston valve, but other than that, some kind of valve is used to prevent backflow.
- Point 1: A 2-stroke engine that performs compression, combustion, and exhaust at the same time provides strong torque, but the overlapping processes make it difficult to separate the flow of the air mixture and exhaust.
- Point 2: There is a check valve mechanism such as a reed valve or rotary disc valve to prevent the mixture sucked into the crankcase from flowing back to the carburetor side.
The rotary disc valve is directly connected to the crankshaft
A bakelite disc blocks the intake to prevent the mixture from flowing back into the crankcase. The notch in the upper left corner of this image is the time the valve is open. The cover side also has an o-ring and oil seal to prevent compression leaks. If this oil seal falls off, the engine will not start because the mixture will leak.
In contrast to the reed valve, which is a thin metal or plastic valve that opens and closes autonomously depending on pressure changes in the crankcase, the rotary disc valve is actuated by the crankshaft. The rotary disc valve was first used for racers in the 1950s to prevent the backflow of primary compression in the crankcase, but its greatest feature was that it was driven directly by the crankshaft, allowing the valve to open and close at any time.
In terms of the flow of the times, it went from piston valves to rotary disc valves to reed valves (there are two types: piston reed valves and crankcase reed valves), but in production motorcycles, Yamaha's small-displacement motorcycles started using rotary disc valves in the early 1960s. The latest engine to inherit this trend was the Yamaha YB-1 (2-stroke model), which continued to be sold until 1999. In addition to Yamaha, the Suzuki RG400/The 500 Gamma, Kawasaki KR250, and AR125 also used rotary disc valves.
The rotary disc valve system, in which the intake port on the side of the crankcase is opened and closed by a disc with part of its circumference cut off, is characterized by the fact that the carburetor is mounted horizontally to the engine. The rotary disc valve system is characterized by the fact that the carburetor is mounted horizontally in relation to the engine, which is a slight disadvantage when it comes to reducing the engine width, and only a few models have adopted this system.
- Point 1: The rotary disc valve, which is turned directly by the crankshaft, allows the intake timing to be set as desired.
- Point 2: Since the carburetor is mounted on the extension of the crankshaft, it is disadvantageous for the slimness of the engine.
Valve opening/closing timing deviation causes engine malfunction
When I open the throttle to idle, the engine stops. I suspected the rotary disc valve because there was no problem when I checked the carburetor and ignition system. I removed the carburetor from the valve cover and stepped down on the kick pedal while looking into the crankcase from the intake port. The valve did not close regardless of the piston position and I could see inside the case. This means that the intake mixture is flowing backward into the case, so the engine speed cannot be increased.
In order to keep the thin disc valve airtight and rotating smoothly, the valve cover was given sufficient strength and rigidity even though it was made over 50 years ago. The structure of the carburetor attached to the end of the round tube on the upper right is similar to that of the Yamaha YB-1, which was manufactured until nearly 2000.
When the cover was removed, part of the bakelite valve had cracked and fallen off and was free to rotate off the center fitting. This would leave the valve open, and when the engine was revved, it would blowback and cause a lack of primary compression. On the other hand, if the intake was stuck in the closed position, the engine would not start at all.
In contrast to the sound valve (right), the partially cracked valve has also dropped out of its engagement with the metal part. If the valve cracked 50 years after manufacture, there would be no way to complain about this.
Before reed valves with low intake resistance and excellent durability were put to practical use in the 1970s, the rotary disc valve, which could prevent the backflow of primary compression in the crankcase, was an advantageous mechanism in terms of both practicality and performance.
To ensure the durability of the valve, Yamaha used a plastic material called Bakelite, while Suzuki used a thin metal plate. Yamaha later changed the material to an engineering plastic, which was used until the final model of the YB-1.
The feature of the rotor disc valve is that it can open and close the intake port in synchronization with the crankshaft rotation = piston position, but what happens if the disc valve is damaged? This is the actual case of the Yamaha YGS-1 introduced here.
This engine starts with one kick. It also idles but stops when I open the throttle. It was in a state. I checked the carburetor setting and ignition timing, but there was no problem, and the last thing I arrived at was the rotary disc valve.
If you remove the carburetor set in the rotary disc valve cover on the right side of the engine and slowly step down on the kick pedal while looking inside the cover, you can see the inside of the crankcase all the way through, even though the intake should be opened and closed by the disc, and it is not blocked by the disc! So I remove the cover and remove the disc. So I took off the cover to check the disc itself and found that a part of the bakelite cover was cracked and was not linked to the crankshaft and was idling.
This means that the mixture that is sucked into the crankcase will blowback freely, and even if it manages to hold its own at low engine speeds around idle, it will blow back more as the throttle is opened, and the engine will stop for lack of new mixture. The motorcycle itself is over 50 years old, so it's not surprising that the bakelite discs, which would have been durable enough when new, have deteriorated over time.
Fortunately, I had a parts engine, so I was able to divert the rotary disc from here and confirm that the engine turns well from idle to full throttle.
Since rotary disc valves were used in a limited number of models at that time, I don't think there are many cases of rotary disc-related problems, but engine malfunctions that blowback due to valve breakage can also occur with reed valves.
In particular, when the sponge type air cleaner element deteriorates and disintegrates into a sponge cake shape, part of it sticks to the valve and causes a poor seal, it can lead to symptoms such as poor start-up, so it is a good idea to check the condition of the reed valve if you have any questions.
There were no genuine parts available for repair, so I took out the disc valve from the engine that I had kept for parts and transplanted it. These parts are the lifeline for the rare old motorcycle.
Although there are no intake and exhaust valves in a 2-stroke engine, it is important to understand the importance of reed valves and rotary disc valves to prevent the backflow of primary compression.
- Point 1: If the disc valve is damaged due to aging or trouble, and the opening and closing timing of the crankcase inlet is shifted, it will directly lead to engine malfunction.
- Point 2: Poor sealing of the reed valve can also cause engine malfunction, so it is necessary to check if you notice any abnormalities.
See YAMAHA Moto Index Page
See SUZUKI Moto Index Page
See Accessories for YAMAHA YB-1
see Accessories for SUZUKI RG500 Gamma
