This article presents a discussion on a special engine valve design, associated particularly with the Italian motorcycle manufacturer, Ducati.
Most conventional Internal Combustion (IC) engine valves consist of a spring. The valve is actuated by a rotating cam lobe via a rocker arm. The rocker arm pushes against the spring, which in turn lifts the valve off its seat. As the cam lobe moves away, the spring relaxes back to its original configuration, thereby returning the valve up to its seat. However, using valve springs is not without its disadvantages, as discussed later.
A desmodromic valve system addresses these disadvantages. It consists of two cams and two rockers, one each for positive opening and closing of the valve. It completely does away with the conventional springs. This design is being used in high performance Ducati bikes since 1968.
WHY- DISADVANTAGES OF SPRING VALVE DESIGN
- It was observed that at high engine RPM, continuous and quick contraction of the metal spring caused its failure due to fatigue. However, this was later addressed to a large extent by using alloyed spring material with better endurance and fatigue strength.
- The major problem with spring design is that of valve float at high revs. There is a limit to the speed at which the spring can retract to its original shape and return the valve back to its seat once the rotating cam lobe has moved away. With increasing speed, the cam lobe leaves and returns so quickly that the spring fails to catch up. Valve momentum drastically exceeds the spring’s ability to close it completely before the piston reaches the top dead center (TDC) during the compression stroke. This may result in the piston hitting the valve, thereby causing serious damage to both. Also, as the valve is slightly off its seat when combustion begins, combustion gases would escape, deteriorating engine efficiency.
The easiest way to overcome the second problem is to use proper valve spring pressure, i.e., by stiffening the spring. This eliminates valve float to an extent at high speeds, but poses another problem. The engine has to work harder to open the valve at all speeds. This causes greater frictional losses, wear and tear and high heat generation.
This is where desmodromic valve design comes in handy. It is sans any spring and so need not overcome static energy of the spring. It is entirely cam-rocker operated.
HOW- DESIGN & WORKING
Fig. 1 DESIGN
Look at Figure 1. Cam 1 and Rocker 1 are responsible for opening the valve. Cam 2 and Rocker 2 are for closing it. The cams rotate on a common shaft whereas the rockers rotate on separate shafts. The two ends of rocker 1 have been labelled as A1 and B1 as shown. A2 and B2 are the two ends of rocker 2. A1 of rocker 1 is actuated by Cam 1 and B1 sits over the valve head. A2 of rocker 2 is actuated by Cam 2 and B2 is connected to the valve.
Fig. 2 (a) VALVE OPENING (b) VALVE CLOSING
- Valve opening (Fig. 2 a)
As Cam 1 rotates, its nose pushes A1 of rocker 1 up. Thus, B1 moves down, pushing the valve down and off its seat. This is valve opening.
It is to be noted that B2 of rocker 2 moves down along with the valve. So, A2 moves up. The rotation of the two cams are timed and adjusted in such a way that just when Cam 1 starts pushing A1 up, the flatter portion of Cam 2 is in position and allows space for A2 to move up.
- Valve closing (Fig. 2 b)
As Cam 2 rotates further, its rounded portion pushes A2 of rocker 2 down. Thus, B2 moves up, pulling the valve up and back to its seat. This is valve closing.
It is to be noted that B1 of rocker 1 moves up along with the valve. So, A1 moves down, ready once again to be pushed up by cam 1 for the next cycle. The rotation of the two cams are timed in such a way that just when Cam 2 starts pushing A2 down, Cam 1 nose has moved away and allows space for A1 to move down.
This design is genius as it eliminates valve float idea completely since there is no spring to keep up with. It is just metal on metal. However, desmo system has to deal with sliding friction between cam and rocker arm. Contact points on rocker arm are hard chromed to reduce wear. Also, the design involves precise calculations and hence is quite expensive.