RESONANCE

No engine design should ignore the potentially destructive and energy wasting effects of resonance, but very little "anti-resonance" development has been applied since the IC engine was first introduced.

It is not easy to start an IC piston engine which is not fitted with a flywheel. However, it is possible and furthermore, it can be very illuminating. At low speeds in particular, such an engine will exhibit alarming instability, sometimes resulting in catastrophic destruction of the crankshaft. Even if no significant damage occurs, the excessive vibrations give a very clear indication that the gas expands much very much faster than is generally realised, and the engine is easily stalled. What is very clear from such experiments is that the flywheel's main function is to smooth out the cycle, not only by presenting a load to the pistons, but also providing a degree of drive. From this it can be gathered that there are certain 'shock' forces acting between the pistons and the flywheel. Get the engine speed just right, and these 'shocks' will occur at a frequency which will cause the crankshaft to resonate. Rather like the proverbial opera singer's sound waves causing a glass to shatter, or a gusting wind to destroying a major, traffic carrying, bridge, so too might the crankshaft shatter.

Most engines suffer catastrophic failure under heavy loading. As much as any other factor, this is because the engine speed changes so slowly that the resonant 'nodes' persist for longer than they otherwise would. Any person who has experience of testing any piston engine on a heavy-duty dynamometer, will know about resonance. It's a pity that engine designers tend to ignore it.

Any 'point' on a smoothly rotating flywheel, will describe a motion with respect to the centre (hub) which will follow a sinusoidal directional velocity curve. The "big-end" bearing of the crankshaft directly equates to such a spot, and so it too will strive to attain sinusoidal motion. However, a quick look at the relative motion of the piston (at the other end of the connecting rod) will show that it cannot be following such a velocity curve, and would not, even if it were possible to 'cajole' the gasses to impart such forces. That slight difference in the two motions is the first of a number of inherent instabilities which affect most engines.

But it's not just the crank assembly which makes the conventional engine 'sing' like a junior school orchestra. The camshafts twist, the valve springs chatter, and the valves dance on their seats. As to gas-flow resonance, that's both a contributer and a casualty. The flywheel wobbles, and the engine case itself acts like a concert hall, transmitting, reflecting and a modulating the whole cacophony.

As far as 'endemic' resonance is concerned, the worst design of all is the basic two stoke. With its fixed (relative to cycle) gas-flow characteristics, it is as much a resonant chamber as is a Bugle.

Resonance can rarely be designed out of a structure. For the most part, it has to be accommodated. The Millennium Bridge over the River Thames has hydraulic or pneumatic dampers: So has the VLB engine - in its excess air. But all a single damper can do is shift the resonant frequency. Even complex dampers can never completely eliminate the effects of resonance, as they, too, become a part of the structure (although, such systems can bring the resonant frequency well below dangerous levels). The easiest way to ensure that a structure does not resonate, is to see to it that it is never subjected to sympathetic motive forces - don't bang the tuning fork, and it won't sing.

The VLB chamber is more akin to a trombone than a bugle. By varying the exhaust timing, the resonant frequency of its chambers can be shifted. By ensuring that those engine components which would impart, or be subjected to, sympathetic motive power, never 'match', the design is able to avoid catastrophic situations. And by arranging for the pistons to share the same velocity curve as the flywheel, the 'shock' element is removed from the assembly.

The VLB 'linear to rotary converter' plays a very significant part in avoiding resonance. The roller bearing follows a perfectly circular path around the inside of a perfect circle, and the actual surface to surface contact point 'moves' at a constant velocity. Power is delivered in both directions, and at the direction change point (with the pistons at maximum velocity) any tendency to 'vibrate' is both dampened by the oil-slide and limited by the 'high gradient' path of the oval.

(As a matter of interest, the components of the VLB engine are designed to have differing resonant signatures for the benefit of both the engine testing system, and the ECU. Each 'motor unit' of the engine employs a pair of ceramic 'microphones' as monitors, enabling resonance to be detected at onset, and counter measures taken).

Few would be foolish enough to argue that high flywheel speeds are not a short-cut to better engine efficiency. For one thing, they get closer to the expansion characteristics of the fuel - but there is more to it than that. If the piston is firing twice as fast, it is producing nearly twice as much energy, but for no extra weight. There are greater inertial losses, of course, but the BHP 'net' profit is very high. So why don't all of our engines run faster? Because they would disintegrate.

The VLB engine is able to comfortably reach 20,000 RPM, simply because that is what it was designed to do.