Parasitic Oscillations

TIOC May 2005
Eaten Alive by Parasitic Oscillations
 by Kevin Cameron

As a new hi-fi amplifier is designed by an electronic engineer, particular care must be taken to be sure that unforeseen combinations of resistance, inductance, and capacitance do not permit so-called parasitic oscillations; to build up, destroying the music.

Each property of electrical circuits has a mechanical analog, and one mechanical system that is often rife with its own parasitic oscillations is the valve train. Rocker arms bend, pushrods compress and expand, camshafts deflect between their bearings, and valve stems flop from side to side while valve heads deflect like trampolines or floppy disks. All of this is invisible to us. Its symptoms are valve seat recession and valve spring and valve breakage. In many cases, after tests with a new cam that gives really good power, we have to reluctantly back up to a previous, less powerful set-up because we can't afford the DNFs [Did Not Finish] and breakages that the hot set-up produces. Valve train failures are hit-and-miss, trial-and-error, a mystery. Lighter valve train parts and stronger springs sometimes just seem to make everything worse. Is there any truth?

Percy Goodman with Alec Bennett aboard after winning the Junior TT in 1928

Back in the 1920s Percy Goodman decided it was time to put aside clattering pushrods and adopt trendy OHC [Overhead Cam] valve drive on the TT  [Tourist Trophy] Velocettes he manufactured. Soon he was driven crazy by erratic valve motion and sensibly resorted to the use of a strobe light to reveal what was happening. But naming the illness is not the same as a cure.

2019 Harley-Davidson / Vance & Hines Pro Stock drag bike with 160 cubic inch engine

When I was recently at the NHRA [National Hot Rod Association] drag nationals in Houston, I had the opportunity of conversation with Byron Hines, whose purpose-designed giant 160-cubic-inch V-twin engine has recently begun to win Pro Stock races. I asked what had made the difference after some uncompetitive seasons. The Spintron, was his answer. It showed us that our valve motion was nothing like what was in the cam profiles.


COMP® Technology Explained: Spintron
Spintron use a big electric motor to spin your engine while a variety of instrumentation is used to measure the actual trajectories of the parts you are interested in. This is different from using a strobe light in that the information you get is detailed enough to allow the flexing of each part to be isolated and understood.


🇺🇸 Working Model of a Harley V-Twin Air Cooled Motorcycle Engine at SEMA Show [4K]
Byron went on to say that valve train dynamics are particularly difficult to control in big twins because of the large variations in crank speed as each cylinder fires. The cam profiles were originally developed to work at a particular maximum rpm, provided that the camshafts turn smoothly. They do not turn smoothly because the crank does not turn smoothly; it advances in a series of fairly violent jerks. 80 percent of the recoverable energy in the hot combustion gas does its work between 10-deg ATDC and 80 ATDC. Out of the 720 crank degrees in the engine cycle, power is given during only 10%. This means that the instantaneous speed of the cam can often be much higher than its average speed. This also means that as a cylinder fires and the crank accelerates suddenly, an open valve in the other cylinder may be tossed right off its cam profile, or dropped prematurely onto its seat.

Todd Henning & ohiocaferacer

Rob Muzzy

This reveals why tuners of singles and twins found that their bikes top-ended better and faster the heavier their cranks were made. A younger generation of tuners has rejected this idea as turn-of-the-century dirt-track nonsense, reasoning that physics requires lighter flywheels to result in faster acceleration. Vintage racer Todd Henning learned the heavy crank truth in back-to-back testing of his highly tuned Honda twins, as did Rob Muzzy in 1981-83 with 1025-cc Kawasaki in-line fours. The heavier the crank, the smoother its rotation becomes, and the less power stroke disturbance, or is transmitted to the cams. Where does the lost power go when a light crank is used? Erratic valve motion is one answer, and big valve bounce after closing is another.

Power pulsing is not the only disturbance to the valve train. Consider a parallel twin, a flat twin, or an in-line four. In all of these, all the pistons stop simultaneously. This means that as pistons decelerate to a stop, the crank must accelerate rapidly because there is nowhere else for the pistons' energy to go; it's conservation of energy. Peak piston speed is about 1.5 times mean piston speed. This means that all the kinetic energy in the pistons, moving at near 100 feet per second, is suddenly dumped into the crank. This is especially bad for in-line fours, which have small-diameter cranks and little in the way of flywheel mass. This sudden crank deceleration/acceleration cycle is performed twice per revolution. No wonder new engine development usually involves coping with cam drive breakage. No wonder there are mystery failures of valve train parts.

Allan Lockheed Jr.

Allan Lockheed [Jr.] is the man behind the engine design software Engine Expert, and he talks to engine people all over the world. He has tales of engines whose valves were stable with a chain or belt cam drive, but which mysteriously began to break valve springs as soon as a much more rigid gear drive was put in its place. The slight flexibility of chain or belt took the sharp edge off the sudden crank speed variations, preventing high frequency motions from reaching the valve train. This may be why Yamaha retain a chain cam drive in their M1 in-line four MotoGP race engine. The oil film between each of the cam chain's pins, bushings, and rollers can be thought of as a kind of viscous damper. Such fluid film damping is one of the motivations inclining the designers of rocket engine turbopumps to give up rolling element shaft bearings in favor of plain journal bearings. A gear cam drive has fewer than 10 oil films between the crank and cam, but a chain has a great many more.

Phil Irving on the original Vindian

Phil Irving, designer of Vincent motorcycles, suggested construction of parallel twins with crankpins not at the traditional 360 degrees, but separated by 76 degrees. With usual rod ratios, the piston reaches maximum velocity at about 76-deg ATDC. At this point, the crank arm is at right angles to the con-rod; the condition for maximum piston velocity. This crankpin angle would therefore cause one piston to be stopped when the other was at maximum speed. The result would be that the two pistons would exchange their kinetic energy only with each other, and not with the crankshaft, eliminating one important source of cam drive disturbance.

Ford Cosworth DFV engine in a 1974 Mclaren Formula 1 race car

Wide-angle Vee engines are better in this respect than parallel twins but even they have their problems. The classic Cosworth DFV V8 GP car engine of 1967 was estimated during design to have no more than a 35 pounds-foot torque peak in its cam drive, but actual testing revealed peaks ten times greater; leading to drive failure. As the engine was already near production when this was discovered, designer Keith Duckworth had to scurry around and design a compact spring drive (analogous to what is found in clutch cush hubs) that could be incorporated into one of the gears in the drive.

Another approach is that seen on certain WW II German V-12 aircraft engines, and on late race versions of Honda's RC30. In these and other cases, small flywheels have been attached to the cams themselves.

Over Head Cam Shaft Drive Ducati 250

It is interesting to note that both Velocette and Ducati have found that changing the stiffness of cam drive towershafts can be used as a tuning measure to adapt an engine to a given race track a stiff towershaft on short courses, and a more limber one for longer tracks. The parts seem stiff and strong only in our imaginations and in our not-very-stiff protein hands. In fact, at speed, everything in engines is flexible because the amounts of energy moving from part to part can be so large.

Byron Hines noted that as useful as Spintron's instrumentation is, even more so is the experience and advice of Spintron personnel. One of the first things they suggested was that he replace his aluminum rocker arms with steel, for accurate motion depends more on stiffness than on weight. He also said that full benefit from Spintron requires making several laps through their process. A first lap involves re-configuring the cam profiles and valve train to settle the motion and eliminate float and excessive bounce. A second lap becomes necessary when it is realized that after lap one, the engine's power curve has sagged in some places. This is because before, cam timing and lift had been unwittingly chosen to at least partly compensate for the uncontrolled valve motion. Once the valve motion is settled, timing and lift are wrong. Now lap two consists of finding new optimum cam timings and profiles to again maximize power. That, in turn, brings to view new dynamic problems to be solved in lap three, and so on. The people with the greatest sophistication in all this are, naturally, NASCAR engine builders.

Jerry Branch 1950

As an extreme example of what can happen, airflow pioneer Jerry Branch was once called upon to dyno a special V-twin that was conceptually a slice off a small-block Chevy, crank and all. Being intended as a motorcycle engine, it had no flywheel other than its little piece of V8 crankshaft. Branch said that although the engine had plenty of displacement and hot tuning parts that suggested an easy 100-hp, it never made over 35 horsepower. Each time it fired, with almost no flywheel mass to smooth the pulse, it launched its valves into orbit.


[Excerpt from] Valve Float
Another aspect of unplanned valve motion is the vibratory modes of valves themselves. Particularly with rocker-arms (which exert some side-thrust), a valve can be excited laterally as it lifts, the head of the valve whipping from side-to-side on the stem; the stem possibly made more flexible by undercutting to squeeze out that last CFM [Cubic Feet per Minute] of airflow. When this flopping valve approaches the seat, one edge can hit first, causing a motion not unlike the final stages of a spun coin's motion. Or, approaching its seat squarely, the rim of the valve stops but the stem and center of the valve head keep right on going; the trampoline mode of valve flex. When the motion at the center finally stops, the valve head is quite deformed and it now snaps back, tossing itself back up off the seat in a cycle that can make several hops; with considerable lift being reached in the process. This is a cautionary tale for those who wish to carve away valve head mass in search of ultimate lightness. Or for those who wish to replace existing valve shapes with something quite different. Think about it if things don't go right. You may have made transformed those elegant chunks of metal from valves into springs.

Spintron is not cheap but all of us can afford imagination. If you have valve or spring problems, think about what has worked in your experience with your particular engine, and what has not. Careful mental sorting here can reveal a lot. Besides, what else is a body to think about while waiting for a dental appointment? Sit with the engine and rotate it through its cycle, thinking about what is happening at each point. Over time you can develop a mental picture of what may be happening and what might correct the problems. There is more to valve trains than light parts and heavy springs."


Text via Uniberp. I added the links, images & videos.