In general mechanical terms, the word desmodromic is used to refer to mechanisms
that have different controls for their actuation in different directions.
A desmodromic valve is a reciprocating engine valve that is
positively closed by a cam and leverage system, rather than by a more
conventional spring. The term desmodromic derives from two Greek roots, desmos (controlled, linked) and dromos (course, track).
The valves in question are those in a typical four-stroke engine that allow the air/fuel mixture
into the cylinder at the beginning of the cycle and allow exhaust
gases to be expelled at the end of the cycle. In the conventional four-stroke engine, a spring is used to apply
pressure to the valve and return it to the valve seat or closed position. The
valve is either directly or indirectly opened by the camshaft.
The word itself comes from the Greek words desmos (δεσμός, translated as
"bond" or "knot") and dromos (δρόμος, "road" or
"way"). Denoting this way the major characteristic of the valves
being continuously bound to the camshaft, a tied way.
The common valve spring system is satisfactory for
traditional mass-produced engines that do not rev highly and are of a design
that requires low maintenance.At the period of initial desmo development, valve
springs were a major limitation on engine performance because they would break
from metal fatigue. Vacuum melt processes developed in the
1950s helped remove impurities in the steel used to make valve springs,
although after sustained operation above 8000 RPM often springs would still
fail. The desmodromic system was devised to remedy this problem.Furthermore, as maximum RPM increases, higher
spring pressure is required to return the valve, leading to increased cam drag
and higher wear on the parts at all speeds, problems addressed by the
desmodromic mechanism.
Fully controlled valve movement was thought of in the
earliest days of engine development, but devising a system that worked reliably
and was not overly complex took a long time. Desmodromic valve systems are
first mentioned in patents in 1896 by Gustav Mees, and in 1907 the Ariès is
described as having a V4 engine with "desmodromique" valve actuation,
but details are scarce. The 1914 Grand Prix Delage used a desmodromic valve system
(quite unlike the present day Ducati system).
Azzariti, a short lived Italian manufacturer from 1933 to
1934, produced 173 cc (173 ml) and 348 cc twin cylinder engines,
some of which had desmodromic valve gear, with the valve being closed by a
separate camshaft.
The Mercedes-Benz W196 Formula One racing car of 1954-55, and the Mercedes-Benz 300SLR sports racing car of 1955 both
had desmodromic valve actuation.
In 1956, Fabio Taglioni, a Ducati engineer, developed
a desmodromic valve system for the Ducati 125 Grand Prix, creating the Ducati
125 Desmo.
He was quoted to say…
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.
The engineers that came after him continued that development,
and Ducati holds a number of patents relating to desmodromics. Desmodromic
valve actuation has been applied to top-of-the-range production Ducati motorcycles since 1968, with
the introduction of the "widecase" Mark 3 single cylinders.
In 1959, the Maserati brothers introduced one of their final designs: a
desmodromic four cylinder, 2000cc engine for their last O.S.C.A. Barchetta.
In modern engines, valve spring failure at high RPM has been
mostly remedied. The main benefit of the desmodromic system is the prevention
of valve float at high rpm. It has the primary disadvantages of
complexity, since there are more components, and lack of understanding, which
prevents people from straying from the well-known conventional valvetrain with
its valve springs.
In traditional sprung-valve actuation, as engine speed
increases, the inertia of the valve will eventually overcome the spring's
ability to close it completely before the piston reaches TDC(Top Dead Centre). This can lead to several problems. First,
and most damaging, the piston collides with the valve and
both are destroyed. Second, the valve does not completely return to its seat
before combustion begins. This allows combustion gases to escape prematurely,
leading to a reduction in cylinder pressure which causes a major decrease in
engine performance. This can also overheat the valve, possibly warping it and
leading to catastrophic failure. In sprung-valve engines the traditional remedy
for valve float is to stiffen the springs. This increases the seat pressure of
the valve (the static pressure that holds the valve closed). This is beneficial
at higher engine speeds because of a reduction in the aforementioned valve
float. The drawback is that the engine has to work harder to open the valve at
all engine speeds. The higher spring pressure causes greater friction (hence
temperature and wear) in the valvetrain.
The desmodromic system avoids this problem, because it does
not have to overcome the static energy of the spring. It still needs to work
against the inertia of the valve opening and closing, and that force still
depends on the effective mass of the moving parts. The effective mass of a
traditional valve with spring includes one-half of the valve spring mass and
all of the valve spring retainer mass. However, a desmodromic system must deal
with the moment-of-inertia of the two rocker arms per valve, so this advantage
depends greatly on the skill of the designer. Another disadvantage is the
contact point between the cams and rocker arms. It is relatively easy to use
roller tappets in conventional valvetrains, although it does add considerable
moving mass. In a desmodromic system the roller would be needed at one end of
the rocker arm, which would greatly increase its moment-of-inertia and negate
its "effective mass" advantage. So desmo systems have generally
needed to deal with sliding friction between the cam and rocker arm and
therefore may have greater wear. The contact points on most Ducati rocker arms
are hard-chromed to lessen this wear issue. Another possible disadvantage is
that it would be very difficult to incorporate hydraulic valve lash adjusters
in a desmodromic system, so the valves must be periodically adjusted.
Before the days when valve drive dynamics could be analyzed
by computer, desmodromic drive seemed to offer solutions
for problems that were worsening with increasing engine speed. Famous examples
of successful desmodromic engines were Mercedes-Benz W196 and Mercedes-Benz 300 SLR racing cars. Since those days,
lift, velocity, acceleration, and jerk curves for cams have been modeled by
computer to reveal that cam dynamics are not what they
seemed. With proper analysis, valve adjustment, hydraulic tappets, push rods, rocker arms, and
above all, valve float, became things of the
past...without desmodromic drive.
Today most automotive engines use overhead cams, driving a flat tappet to achieve the shortest,
lightest weight, and most inelastic path from cam to valve, thereby avoiding
elastic elements such as pushrod and rocker arm. Computers have allowed for fairly accurate
acceleration modelling of valvetrain systems.
Before numerical computing methods were readily available,
acceleration was only attainable by differentiating cam lift profiles twice,
once for velocity and again for acceleration. This generates so much hash
(noise) that the second derivative (acceleration) was uselessly inaccurate.
Computers permitted integration from the jerk curve, the third derivative of
lift, that is conveniently a series of contiguous straight lines whose vertices
can be adjusted to give any desired lift profile.
Integration of the jerk curve produces a smooth acceleration
curve while the third integral gives an essentially ideal lift curve (cam
profile). With such cams, that mostly do not look like the ones
"artists" formerly designed, valve noise (lift-off) went away and
valve train elasticity came under scrutiny.
Today's cams have mirror image (symmetric) profiles with identical positive and
negative acceleration while opening and closing valves. An asymmetric cam
either opens or closes valves more slowly than it could, speed being limited by Hertzian contact stress between curved cam and flat
tappet from accelerating the mass of valve, tappet and spring.
In contrast, desmodromic drive uses two cams per valve, each
with separate rocker arm (lever tappets). Maximum valve acceleration being
limited by cam-to-tappet galling stress, is governed by moving
mass and cam contact area. Rigidity and contact stress are best achieved with
conventional flat tappets and springs whose lift and closure stress is
unaffected by spring force, both occurring at the base circle where spring load is minimum and contact radius
is largest. Curved (lever) tappets of desmodromic cams cause higher contact stress
than flat tappets for the same lift profile, thereby limiting rate of lift and
closure.
With conventional cams, stress is highest at full lift, when
turning at zero speed (engine cranking), and diminishes with increasing speed
as inertial force of the valve counters spring pressure, while a desmodromic
cam has essentially no load at zero speed (in the absence of springs), its load
being entirely inertial, and therefore increasing with speed. However, its
greatest inertial stress bears on its smallest radius. Acceleration forces for
either method increase with the square of velocity resulting from kinetic energy.
Desmodromic valve drive was often justifiedby claims that springs could
not close valves reliably at high speed and that the forces caused by suitably
strong springs exceeded what cams could withstand. Since then, valve float was analyzed and found to be caused
largely by resonance in valve springs that generated oscillating compression
waves among coils, much like a Slinky. High speed photography showed
that at specific resonant speeds, valve springs were no longer making contact
at one or both ends, leaving the valve floating before crashing into the cam on closure.
For this reason, today as many as three concentric valve
springs are sometimes nested inside one other; not for more force (the inner
ones having no significant spring constant), but to act as snubbers to reduce
oscillations in the outer spring.
An early solution to oscillating spring mass was the mousetrap or
hairpin spring used on Norton Man engines. These avoided resonance but were
ungainly to locate inside cylinder heads. Today, Formula One racing engines use
gas springs that have no resonant parts,
their working parts having an insignificant mass compared to the force of their
compressed gas.
Valve springs that do not resonate are progressive, wound with varying pitch or varying diameter
called beehive springs from their shape. The number of active coils in
these springs varies during the stroke, the more closely wound coils being on
the static end, becoming inactive as the spring compresses or as in the beehive
spring, where the small diameter coils at the top are stiffer. Both mechanisms
reduce resonance because spring force and its moving mass vary with stroke. This
advance in spring design removed valve float, the initial impetus for
desmodromic valve drive.
While the desmodromic system is not ideal in a practical
world of mechanics, it still survives and performs without problem. While it
can be more expensive to maintain than traditional spring actuated valve
systems, many aftermarket precision machined components can extend the
maintenance interval to that of spring actuated systems (in comparable
motorcycles).
While newer, high performance, pneumatic systems may follow
more specific design and engineering specifications (computer aided) they are
typically limited to race only applications (Formula 1, Moto GP, etc.).
Currently, there is no method of determining longevity or extended maintenance
intervals of such systems in practical, everyday, systems such as the
automobile.
While the design can be noisy, it is typically overridden by
road noise from tires and other engine components such as intake and exhaust
noise. Though stated above the noise is "uncomfortably loud in engines
with four or more cylinders", if true, this is limited (in terms of
Ducati) to the MotoGP and MotoGP Race Replica bikes, which are the only current
production desmodromic motors that feature four cylinders and are intended for
racing. (Note that exhaust noise levels can exceed 110 dB on full race
systems.)
Famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars, and most commonly,
modern Ducati motorcycles.
Ducati motorcycles with desmodromic valves have won numerous
races and championships, including Superbike
World Championships from
1990–92, 1994–96, 1998–99, 2001, 2003–04, 2006 and 2008. Ducati's return to Grand Prix motorcycle
racing was
powered by a desmodromic V4 990 cc engine in the GP3 (Desmosedici) bike, which went on to claim several victories,
including a one-two finish at the final 990 cc MotoGP race at Valencia, Spain in
2006. With the onset of the 800 cc era in 2007, they are generally still
considered to be the most powerful engines in the sport, and have powered Casey Stoner to the 2007 MotoGP Championship and Ducati to the
constructors championship with the GP7 (Desmosedici) bike.
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