Article by Doug Switzer, Moto/ology Publisher.
OK, here at Moto/ology, we bring you another installment of Manic Motors where you get to contemplate some more weird and wonderful diversities of engine design.
This time around, we’re not going to jump into an entirely new engine configuration, but we’re simply going to look at an interesting alternative method of valve actuation.
In the so-called “normal” set-up in a typical 4-stroke, internal combustion engine, we have two or more valves feeding and exhausting the goods from the cylinders. These valves are usually of the “poppet” valve design that pops open and closed with the aid of a camshaft and any associated actuation gear. The effectiveness of these valves depends on a precise design incorporating split-nanosecond timing and positive effectiveness in opening and sealing the valve ports repeatedly and at the exact, perfect moment. The traditional method of operation has the camshaft’s eccentric lobe forcing the valve open when required and then utilising a very strong valve spring to force the valve closed at the exact, precise moment compression is initiated and hold it there until the exhaust phase is required. OK, fair enough. We should also point out this arrangement has served the industry very well for a very long time. There are, however, some bugaboos that can come about with this system.
The first, most obvious issue happens when the engine is revved to very high speeds and is under load. A very undesirable phenomena known as “valve float” or “bounce” can occur, and it really screws things up. The situation happens when the valves start moving so quickly and under so much load that they overcome the closing force of the valve springs and literally “bounce” or “float” on their seats, destroying the controlled sealing of the cylinder. As can be imagined, this leads to all kinds of problems including reduced efficiency, backfiring, component interference and damage and even catastrophic destruction of the engine itself.
Early in the development of the four-stroke engine when the metallurgy didn’t exist to make super-duty valve springs, an alternative was developed. It’s been said there is evidence that Karl Benz even postulated this technique as far back as the 1880s. Slightly later, in 1896, a certain Gustav Mees supposedly mentioned in some patent applications the idea of using the camshaft to control both the opening and closing of the poppet valves. This idea was later picked up by some bright bulb who proposed using the valve actuating gear and camshaft itself to achieve this action. This system was used in a very advanced marine engine built by Austin of England in 1910. This unit boasted 12 cylinders, an overhead cam design with desmodromic valve actuation and produced a then-astounding 300hp. Other examples of engines with desmodromic valve systems include a 1914 Delage Grand Prix car, some experimental Azzariti motorcycles of the mid-1930s, the legendary Mercedes-Benz W196 and 300SLR racing cars of the mid-1950s, the Maserati-designed 2-litre O.S.C.A. engine of 1959 and of course, the most modern iteration, the Ducati “Desmo” motorcycles developed from the 1950s to the present day.
If you noticed, most of the aforementioned engines, motorcycles and cars were for racing applications. As I said before, in motorsports, the positive action of desmodromic systems would effectively eliminate the bouncing valve spring problem as it eliminated the springs themselves. Now, this would be all well and fine, but there were certain obstacles to overcome. The first and most obvious was: how on earth do you positively close the valve on its seat without smashing the valve into the seat and destroying them both? The answer was, once again, timing… and a very precise manufacturing and tuning set-up.
It is this precise set-up that creates the greatest drawbacks of the desmodromic system… complexity and cost. The expertise and intricate timing required to get the closing and opening of the valves just right, requires specialized equipment and know-how. The additional hardware, (another camshaft, actuators and/or followers) required for the secondary action of closing the valves adds extra precision components and weight. A secondary issue arises when any of these precise components go slightly out of whack and cause a failure. The results can be catastrophic and expensive. While Ducati has made the system quite reliable, failure is still a real issue and as we all know, if anything can go wrong, it usually will.
That Austin V12 Marine engine of 1910 was a fantastic piece of technology for its time and when installed in a speedboat named “Irene 1”, it set quite a few records and raised quite a few eyebrows.
Later, in 1914 when the French Delage company fielded their advanced grand prix car, once again folks noticed. Unfortunately, the onset of WWI, the “Great War”, stifled further development and with the wartime rise of advanced metallurgy in the fabrication of spring steels, the development of desmodromics fell by the wayside. There were some attempts at using the technology in the 1930s, but nothing of real noteworthiness came about until Mercedes-Benz built their ground-breaking grand prix racecars and endurance-racing sportscars of the mid-1950s.
The Mercedes Connection
Mercedes-Benz, along with the Auto Union company had enjoyed a phenomenally successful prewar period with their exotic and sophisticated racing machines. After the dust had settled from the devastation that was WWII, Mercedes once again flexed their technical capabilities and started working on a new generation of racing cars to demonstrate their manufacturing quality and technical superiority. The Mercedes-Benz W196 grand prix racer featured a supercharged, fuel-injected straight eight-cylinder overhead cam engine with… you guessed it, a desmodromic valve system. It revved like a demon and with the supercharger whine, it sounded like one as well. In the hands if drivers like Juan Manual Fangio and Stirling Moss it was very successful. Not happy with just the open-wheel GP wins, Mercedes decided to develop a super-aerodynamic closed, enveloped-body version of the W196 and with this machine they captured several speed records as well as a few road course wins. The closed bodywork was at one point deemed to be in conflict with the Formula One rules of the day and eventually banned.
Wanting to further their reach and knowing their aero bodywork would eventually be disallowed in formula car racing, Mercedes had decided to launch a parallel program in sportscar racing and developed the wonderful 300SLR sportscars for endurance and closed-circuit racing. These cars shared much of the technology of the grand prix cars including their desmodromic valve operating system.
They went on to success in the Targa Florio, Mille Miglia and other international venues and were leading Le Mans at the time of the devastating 1955 disaster.
Around this same time, a talented Italian engineer named Fabio Taglioni was working for the Ducati Motorcycle Company and had become enamoured with the prospect of a high-revving, hi-output motorcycle racing engine. To achieve his goal, he stipulated a desmodromic valve system and the legendary Ducati “Desmo” series of motorcycles was born. The first Desmo was a 250cc GP racing machine built and campaigned in 1956. Ducati went on to build bikes of other displacement configurations and classes and achieved victories and much notoriety.
The Ducati company has embraced and continued the development of the desmodromic valve actuating system to this day. Although some of their street machines use conventional valve spring systems for ease of maintenance and general ease of manufacture, many of their advanced racing machines still use the now well-developed desmodromic system. We believe Ducati is at present, the only major motoring manufacturer that still uses the system and uses it successfully.