Generally, in a IC engine, Overhead Cam setup (OHC) or Double Overhead setup (DOHC) is used for operating the inlet and exhaust valves. Those setup normally consists of valve, valve spring, cam lobe, rocker arm and several other components. But, Ducati implemented the Desmodromic valve operating system on their bikes in the middle 50's and using it still today. To understand the working of desmo system, we should be able to understand the working of conventional valve operating mechanism.
Conventional Overhead Cam valve operation
These are the type of mechanism that we see in all motorbikes today. In this setup the major components such as cam lobe, cam shaft, valve, valve lifter, return spring are present. The camshaft is connected to the crankshaft via chain or belt which is timed perfectly. As we know, during suction stroke, the lobe will push the valve down to let the fresh air fuel mixture in. After suction stroke, due to the design of the cam lobe, the valve will come to it's original position with the help of return valve spring. Then the compression and power stroke will takes place. During the exhaust stroke, the exhaust cam will push the valve down and the piston pushes the burnt air fuel mixture outside the combustion chamber and the valve is closed by the help of return spring. This is the general mechanism of valves used today and its simplified view is represented above.
Issues with conventional valve mechanism at 1950s
These mechanism is okay for low revving engines and generally does not require a high maintenance. But high revving engines faced a major problem of breaking of valve spring due to metal fatigue and valve float. Metallurgy of that time limited the resonance of valve springs to operate at high rpm. Thus the valve spring's ability to return valves to the closed position before piston reaches TDC was failed and leaded to valve floating. This caused severe mechanical damage to the engine. For overcoming this problem, Desmo mechanism was implemented.
Introduction to Desmodromic
During 50's, the above mentioned problem caused a severe problems in racing motorbikes. In 1896, the first patent is submitted by Gustav Mees. But it was rarely used due to its complications. In middle 50's, this was used in some Mercedes for racing. At the same period, Ducati has recruited a young talented engineer named Fabio Taglioni who has implemented this mechanism in their racing bikes. They were in a job of developing a powerful engines. Taglioni designed and implemented these Desmo valve mechanism in their motorbike, neglecting the limitations of valve springs there by achieving higher rpms. He said that, "The specific purpose of the desmodromic system is to force the valves to comply with the timing diagram as consistently as possible. In this way, any lost energy is negligible, the performance curves are more uniform and dependability is better". Their first bike was called as 125GP and won its first Grand Prix and got recognized as fastest 125cc racer in the world.
Working of Desmo
Desmodromic translates to controlled movement. Here, the valve spring is absent. Instead of valve springs, they used a additional cam lobe for mechanically pulling the valve to the closed position and holding it. Generally it consists of opening lobe, closing lobe, camshaft, valve, lower rocker arm and several other parts. The simplified view of the Desmo mechanism is represented in the below diagram. For simplicity, I've neglected the upper rocker arm.
The intake lobe and closing lobe are placed side by side. During Intake stroke, the opening lobe pushes the valve down therefore letting the air-fuel mixture inside the combustion chamber. After suction stroke, the closing lobe will push the the lower rocker arm. The lower part of the rocker arm is connected to the valve head. Due to this action, the rocker arm mechanically pulls the valve up and holds it till the next intake stroke. By perfectly designing the cam lobes, proper valve timing is achieved. At those days, they used bevel gears for operating the camshafts. The same operation is also carried out in the exhaust side where the valve is mechanically pulled up.
They implemented this mechanism mainly for achieving higher rpms. But in addition to that, higher torque is achieved at lower engine revs. This mechanism totally neglected the problem of valve float. It neglected a risk of causing a collision between valve and piston.
Disadvantages
• Additional weight in valve train due to extra rocker arms.
• Adjusting the valves became difficult compared to conventional setup.
• High mechanical complexity.
• Noisy
Conventional Overhead Cam valve operation
Issues with conventional valve mechanism at 1950s
These mechanism is okay for low revving engines and generally does not require a high maintenance. But high revving engines faced a major problem of breaking of valve spring due to metal fatigue and valve float. Metallurgy of that time limited the resonance of valve springs to operate at high rpm. Thus the valve spring's ability to return valves to the closed position before piston reaches TDC was failed and leaded to valve floating. This caused severe mechanical damage to the engine. For overcoming this problem, Desmo mechanism was implemented.
Introduction to Desmodromic
During 50's, the above mentioned problem caused a severe problems in racing motorbikes. In 1896, the first patent is submitted by Gustav Mees. But it was rarely used due to its complications. In middle 50's, this was used in some Mercedes for racing. At the same period, Ducati has recruited a young talented engineer named Fabio Taglioni who has implemented this mechanism in their racing bikes. They were in a job of developing a powerful engines. Taglioni designed and implemented these Desmo valve mechanism in their motorbike, neglecting the limitations of valve springs there by achieving higher rpms. He said that, "The specific purpose of the desmodromic system is to force the valves to comply with the timing diagram as consistently as possible. In this way, any lost energy is negligible, the performance curves are more uniform and dependability is better". Their first bike was called as 125GP and won its first Grand Prix and got recognized as fastest 125cc racer in the world.
Working of Desmo
Desmodromic translates to controlled movement. Here, the valve spring is absent. Instead of valve springs, they used a additional cam lobe for mechanically pulling the valve to the closed position and holding it. Generally it consists of opening lobe, closing lobe, camshaft, valve, lower rocker arm and several other parts. The simplified view of the Desmo mechanism is represented in the below diagram. For simplicity, I've neglected the upper rocker arm.
The intake lobe and closing lobe are placed side by side. During Intake stroke, the opening lobe pushes the valve down therefore letting the air-fuel mixture inside the combustion chamber. After suction stroke, the closing lobe will push the the lower rocker arm. The lower part of the rocker arm is connected to the valve head. Due to this action, the rocker arm mechanically pulls the valve up and holds it till the next intake stroke. By perfectly designing the cam lobes, proper valve timing is achieved. At those days, they used bevel gears for operating the camshafts. The same operation is also carried out in the exhaust side where the valve is mechanically pulled up.
They implemented this mechanism mainly for achieving higher rpms. But in addition to that, higher torque is achieved at lower engine revs. This mechanism totally neglected the problem of valve float. It neglected a risk of causing a collision between valve and piston.
Disadvantages
• Additional weight in valve train due to extra rocker arms.
• Adjusting the valves became difficult compared to conventional setup.
• High mechanical complexity.
• Noisy
Tags
Random talks