Energy Exchange

Energy exchange

TDC

Kevin Cameron

WE ALL KNOW THAT THE FEWER CYLINders an engine has, the larger the flywheel mass it needs. There is a simple reason for this, namely that the flywheel must store enough energy to perform the next compression stroke, even at idle speed. The bigger the cylinder, the more energy it takes to compress the air in it, and the more flywheel is needed to store that energy.

There is also another reason. In a single-cylinder engine, at top and bottom center, the piston is stopped. But 78 or 79 degrees after top center, or 101 degrees after bottom center, the piston will be at its maximum speed.

At redline in a modern engine, this peak instantaneous piston speed is close to 60 miles per hour, so there is a fair amount of energy involved. But a moment later, the piston will be stopped again. Where does the energy come from, and where does it go?

It comes from the crankshaft, naturally. As the piston accelerates, the crank slightly decelerates. Then as the piston decelerates, it gives its energy back to the crankshaft again-minus, of course, any friction loss involved in the transfer.

You will still find people with a superstitious belief that any form of reciprocating motion-like that of valves or pistons—is a complete loss. This is not true at all. As we all learned in high school, energy is neither created nor destroyed, although it can be transformed from one form into another. Therefore the piston and crankshaft simply exchange energy, minus the friction involved in defining their motions.

The result of this energy exchange between piston and crank is that the crankshaft’s instantaneous rpm is continually varying. The greater the flywheel mass, the smaller this rpm variation becomes, and the smoother the rotation.

In similar fashion, the energy required to lift a valve against the pressure of its valve spring comes from the crankshaft, through the cam drive. But this energy is not lost, either, for as the valve begins to close, the energy stored in the valve spring is given back to the cam, exerting a positive torque on it, restoring energy to the cam and the crankshaft. This energy exchange is not perfectly efficient, of course, because there is some friction loss.

Crankshaft rpm variation caused by the piston starting and stopping is superimposed on another one caused by the engine’s power strokes, so you can see that the delivery of power is anything but smooth. With combustion kicking the crankshaft back up to speed every four strokes, and with the back-and-forth exchange of energy between piston and crank, crank rotation is similarly unsmooth. Some form of torsional shock absorber may be needed between the engine and the gearbox, to smooth out all this rattling and banging. In the old single-cylinder engines, the torsional absorber was a rotary cam-and-ramp affair, and in more recent engines it takes the form of springs or rubber elements built into the clutch drive gear. Often there is a rubber-element cush drive built into the rear wheel, as well.

People from the manufacturers’ testing departments know that there is a minor but measurable power loss in these torsional absorbers, so for racing applications, engines may be designed without them. The idea is that the frequent parts replacement that’s normal in racing will prevent failure. When Harley’s VR Superbikes first appeared at Daytona, they had solid clutch hubs, but several of them broke. Yoshimura Suzuki has also run such solid hubs.

Let’s consider other engine layouts in the light of this piston-inertia question. When you make a 180-degree or 360-degree Twin, you again have all pistons starting and stopping together, just as with a Single, so these engines have a lot of crank-speed variation. But look at a 90-degree V-Twin. When one piston is stopped, the other is near its point of maximum velocity, so instead of exchange of energy between piston and crank, this is direct exchange of energy from one piston to another on the same crankpin. This engine will be torsionally pretty smooth-as will be any engine with a fairly large vee-angle.

These days, though, most motorcycles have inline four-cylinder engines, with so-called flat cranks that put two pistons at TDC while the other two are at BDC. This arrangement is necessary to give a regular firing interval, but because all pistons start and stop together, it is just as torsionally rough as a Single or a 180/360 Twin.

On the other hand, a V-Four of the kind that Honda and Yamaha have made for so many years is torsionally smooth. Another clever idea is the “quartered” crank of Yamaha’s TRX850, whose crankpins are at 90 degrees to each other. This, in a parallel-Twin, gives a smoother piston-topiston energy exchange that leaves the crankshaft rotation largely unaffected.

There is also a second-order exchange of energy between pistons and crankshaft, caused by the twice-per-revolution forward-and-back swing of the connecting rod. Just as a person striding rises and falls, so the swing of the rod causes a double-time variation in piston height, called secondary vibration. We are bothered by the buzzy high frequency of secondary vibration, as the whole engine shakes with it, and it also imposes a rapid flutter in crankshaft rpm.

What appears to be a smoothly operating mechanical system is in fact reverberating richly with vibrations at various frequencies that can both add and subtract in their effects. The parts that must transmit these vibrationscrank, cams and gears-are themselves torsionally springy, sometimes encouraging build-up of torsional resonances. Small wonder, therefore, that it’s still not easy, even in this age of computer simulation, to make engines 100 percent reliable under all conditions. Like it or not, the testing department is still necessary, running engines to destruction to discover where, if at all, these vibrations are adding up to partskilling amplitudes.