TECH5 is a regular feature where EngineLabs asks industry leaders five technical questions. This week’s guest is Aaron Neyman, product engineer at Fluidampr and torsional vibration analyst with Vibratech TVD.
EngineLabs: Basically, what is torsional vibration and how do you measure and analyze it?
Neyman: Torsional vibration is the speed fluctuation of a rotating shaft. Think of it as the vibration you feel in your steering wheel when you apply the brakes and have a warped rotor, but the steering wheel is continuously rotating at the same time. Torsional vibration can have two components, twist and rigid body motion.
In an engine lab torsional vibration is determined by RPM fluctuation in picoseconds using a high resolution laser tachometer and then translated with an FFT analyzer. On the test stand is a modified Honda F22C engine found in a race driven S2000 being tested with a Fluidampr installed.
Twist is the kind of vibration that is expected when you think of crankshaft torsional vibration. From one end of the shaft to another the vibration amplitudes change direction. When this happens there is a point where there is no vibration amplitude. This is called a nodal point. The material stresses are the highest at a nodal point. Rigid body motion is when the rotating shaft has torsional vibration but there is no nodal point along the shaft. The vibration does not change direction. When testing Fluidampr applications, torsional vibration is measured with a high-resolution laser tachometer, a high-resolution gear-tooth sensor, or an incremental encoder, on either end of the crankshaft. The data is recorded on a multiple picosecond FFT analyzer that converts the rpm fluctuation data into the angular domain. This provides us data in terms of degrees of twist or degrees of roll per rpm and can be broken down by vibration orders. Orders come into play because you have multiple cylinders firing causing multiple initial and harmonic torsional vibration events during each revolution of the crankshaft. Resonant frequencies can also be determined from the collected data and are used to properly size a damper for the application.
EngineLabs: What is the basic design difference between a viscous damper and one that utilizes elastomer construction?
Neyman: The basic difference is the way a viscous damper reduces torsional vibration amplitudes. Stock elastomer dampers function by going into resonance at a specific frequency and remove the vibration energy from the system by the opposing motion of the damping mass. The more accurate name is a tuned vibration absorber.
Viscous dampers remove vibration energy from the system by shearing a free rotating inertia ring back and forth through a highly viscous silicone fluid. This converts the vibration energy into heat, which is easily dissipated through the sealed inertia ring housing. This method provides highly durable broad band damping and the necessary protection as engine modifications change torsional vibration characteristics.
A tuned absorber only removes vibration energy at the specific resonance frequency band. It is designed with a specific amount of mass and a specific elastomer durometer to deal with the worst engine resonance and to withstand the heat that is generated from the opposing motion. Over time the heat cycles degrade the elastomer and reduce the efficiency of the tuned absorber. Viscous dampers remove vibration energy from the system by shearing a free rotating inertia ring back and forth through a highly viscous silicone fluid. This converts the vibration energy into heat, which is easily dissipated through the sealed inertia ring housing. The viscous damper housing is designed with enough heat transfer area to reach a stabilization temperature that the damper does not exceed. Because of this, the damper does not lose efficiency over time in automotive applications. Another advantage of the viscous damper design is that it allows the free rotating inertia ring to convert vibration energy to heat across a broad frequency range. Superior engine protection, broad range performance and durability are why you will find a viscous damper as original equipment in cars and trucks like the Dodge Ram with the 6.7L Cummins and Audi R8 V10. Plus, nearly all high power engines that we depend on to move our economy are protected with a viscous damper. In those industries and applications it is less about cost and more about quality and dependability.
EngineLabs: What do you mean by “broad band” versus “narrow band” protection?
Neyman: Just like it sounds. The frequency range that a tuned absorber is effective is simply a narrow band, usually 50Hz in range. The absorber is tuned to the harshest frequency range of the engine as it leaves the factory. Once anything is changed on the engine, especially the rotating assembly, the harsh frequency band will shift. A viscous damper operates across a broad frequency range because the inertia ring is not bonded to the damper housing. This allows the effective frequency range to be more of a broad bell curve and provide the necessary protection as engine modifications are made.
High power engines with long strokes typically require a larger damper. A Fluidampr performance damper for a Honda K series engine sits on top of a Vibratech TVD (parent company) damper. The Vibratech TVD heavy-duty viscous damper measures 4.7 feet in diameter and weighs 6,700 pounds. It is used on a continuous running 12,000-horsepower compressor station found in the natural gas industry.
EngineLabs: What are the tradeoffs in choosing between a lighter or heavier damper?
Neyman: Dampers are not the same as pulleys and hubs. Pulleys and hubs are generally kept as light as possible to reduce the weight of the rotating assembly. Dampers should not be treated the same way. Dampers are heavier because they contain a critical amount of weight that is needed to perform the work required to remove vibration energy from the engine. The more inertia weight present, the more amplitude reduction potential is available. High cubic inch engines with long strokes should have larger dampers than short stroke, low displacement engines. There is always a balancing point with weight. It is critical to not overload the crankshaft nose with so much weight that it causes runout, but it is also critical to use enough weight to effectively control torsional vibrations. When selecting a damper it is always best to contact Fluidampr if you have questions about what size and weight to run.
The result (of the Honda tests shown above) is a highly detailed 3D torsional vibration map across the rpm range by vibration order. Large industrial and over-the-road diesel engine manufacturers require that any individual torsional vibration order be less than 0.25 degrees peak, in order for their engines to reach rebuild intervals of 500,000 to 1,000,000 miles. Automotive OEM applications tend to be less stringent about torsional vibration control, with OEM applications sometimes in excess of 0.75 degrees peak. Even stock applications can benefit from a Fluidampr performance damper. A viscous damper can be found as original equipment in high quality automobiles and diesel trucks such as the Audi R8 V10 and Ram 6.7L Cummins. Comparative testing can also be performed on a dyno. The broad band damping abilities of a Fluidampr performance damper often shows as an increased, smoother torque curve through the rpm range.
EngineLabs: Are there special challenges in developing dampers for diesel applications?
Neyman: Diesel applications tend to have more vibration due to their nature. Over the past 15 or so years we’ve seen remarkable advancements in clean diesel technology. Techniques that we’re just now starting to see cross over to stock automotive gas engines. To achieve significant reductions in nitrogen oxide (NOx) and particulate matter (PM) in diesel, first cleaner in-cylinder combustion was addressed before selective catalyst reduction (SCR) systems were introduced. Today, cylinder mean effective pressure is raised significantly to achieve a more efficient cleaner combustion. Sky-high direct fuel injection pressure with multiple computer controlled fuel pulses per combustion cycle, plus higher turbo induction boost and optimized compression ratios above 16:1 are all utilized. When you factor in larger rotating and reciprocating components and a longer crankshaft in the case of popular inline-6 engines, it all contributes to higher vibration amplitudes. As diesel motorsports has proven, besides a cleaner burn, these advancements also allow you to get crazy power out of a smaller displacement engine. Fluidampr Performance Diesel dampers are the official damper of DIESEL Motorsports and the choice of many top teams in diesel drag racing and sled pulling because of the broad band vibration control and superior durability.