- Engine Vibration Characteristics -
The Source of Abundant Torsional Excitation
In order to design machinery which will be driven by piston engines, it is necessary to understand the nature of the output these engines produce. Unlike a turbine or an electric motor, a piston engine does not produce a smooth output, but a very "lumpy" one. The degree of "lumpiness" depends mainly on the number of cylinders and the firing distribution. (Click Here for a complete presentation on engine torsional excitation.)
For the purposes of this analysis, the main subjects of interest are the moment of inertia and torsional signature of the engine.
GENERAL ENGINE OBSERVATIONS
The engine in the RotorWay 162F (and I presume its predecessors are similar) is a liquid-cooled horizontally-opposed 4-cylinder engine having 162 cubic inches of displacement. It is an interesting amalgam of VW design and small-block Chevy (SBC) implementations. One of the major issues with the engine system is the designed-in extremely-low mass moment of inertia of the flywheel. It is a thin aluminum production whose only apparent purpose is to carry the starter ring gear. If someone who understood dynamics had been involved in the design, a proper flywheel could have been implemented with a tiny (7 pounds) influence on overall aircraft weight, but a significant reduction in the amplitude of the torsional excitation applied to the rest of the drive system.
RotorWay claims this engine produces 150 HP at 4250 RPM, presumably corrected to sea-level, standard day conditions If that figure is correct, then the engine produces 185.4 lb-ft of torque at that RPM. (If the relationship between torque and power is unclear, we suggest you check out the quick explanation in Engine Technology.)
BMEP (Brake Mean Effective Pressure) is a very effective yardstick for comparing one engine to another in terms of how hard it is working to produce rated power, and in terms of just how effective the engine is. For practical purposes, the BMEP limit for extremely well-developed, normally-aspirated 4-stroke piston engines is about 230 psi.. (If BMEP is an unfamiliar concept, we suggest you check out the quick explanation in Engine Technology.)
If the RotorWay engine produces the advertised power, then the BMEP is 175 psi. For comparison purposes, Lycoming IO-320 engines (160 HP at 2700 RPM) and IO-360 engines (180 HP at 2700 RPM) run at a BMEP of 147 psi.
The magnitude of the mean torque which the RotorWay engine produces (185 lb-ft) is quite modest when compared to a 160-HP Lycoming IO-320 (311 lb-ft at 2700 RPM) and a 180-HP Lycoming IO-360 (350 lb-ft at 2700 RPM).
To analyze the vibration characteristics of the system, we need the effective mass moment of inertia of the complete engine at the flywheel (J1 in the model diagram on the previous page).
EPI needs that value in every system we design which has a piston engine as the power source, so several years ago we wrote a program which calculates the engine Jm from various known measurements (engine stroke, number of cylinders, bobweight, dimensions of rotating components, etc.). The calculations have proven to be within about 6% of actual measurements.
From that calculation, the RotorWay engine Jm (including the flywheel) is 0.189 in-lb-sec².
In an even-fire inline or horizontally-opposed four-cylinder engine, one cylinder fires every 180° of crankshaft rotation. This graph shows a plot of the instantaneous value of torque which an even-fire 4-cylinder engine produces at the crankshaft output flange during full throttle operation. The engine applies that torque waveform to whatever is connected to the crankshaft output flange. Notice that the torque values are displayed as a percentage of mean torque.
Torque Signature of a 4-Cylinder Engine
On any given engine, the shape and amplitude of the signal can vary somewhat from those shown, depending on the specific details of engine. However, the fact remains that the output of a 4-cylinder piston engine consists of peaks and reversals, and the peaks greatly exceed the measured mean torque of the engine.
This particular curve bears a remarkable similarity to actual data we have taken from instrumentation installed across the load cell on an engine dynamometer (as well as to data taken by engine researchers many years prior to our work). We have observed this characteristic curve on various Lycoming 4-cylinder engines as well as other high performance 4-cylinder engines we have built for customers (both inline and horizontally-opposed).
Note that the waveform contains two torque peaks per revolution, each of which is roughly 270% above mean torque, and two torque valleys which are about 200% below mean torque (and 100% below zero torque, meaning the instantaneous torque reverses and opposes the engine rotation). This waveform is an example of "second order" excitation, because there are two complete up-and-down torque pulses (cycles) per rotation of the crankshaft.
The specifics as applied to the RotorWay engine at max torque are: The value of the maximum peak engine torque is about 695 lb-ft (188 x 3.7) and occurs twice per revolution. The minimum peak engine torque is a negative value (acting against the direction of engine rotation) of about 190 lb-ft. The difference between the max and min values is an astonishing 885 lb-ft of torque. (Is it any wonder that the main V-belts flap wildly?)
Note also that this waveform is not sinusoidal, but rather approximates a sawtooth, and there is a small negative "blip" at the bottom of the valley. These characteristics mean that the engine output contains a complex mixture of higher harmonic orders. The shape of the waveform and the torque reversals make it quite apparent that designers of equipment driven by such an engine face a significant challenge, especially if it needs to be light enough to fly.
(In early May, 2003, while researching material posted on the RotorWay Owners Group forum, it became evident that more than one person had partially identified the source of the overstressing loads some time ago. Both Matthew Dock and Peter Kooiman have written articles noting that the torque signature of the engine was very peaky.)
As a separate subject, EPI has major concerns about what we perceive to be some grave problems with the valve train. Those issues are addressed on a separate page.