Ask Kevin

ASK KEVIN

WHY ALL THAT WASTED SOUND ENERGY IN MOTOGP?

Q: At Phillip Island last month I was once again reminded of the huge and glorious noise made by the MotoGP bikes. It’s loud enough to deafen the crowd and shake your insides even standing well back from the track. Why aren’t bigger efforts made to use this energy for forward motion?

RHYS DAN I ELL TULLERA, NEW SOUTH WALES, AUSTRALIA

A: When Alison Balsom plays the trumpet part in Sound the Trumpet," it is quite loud even though the pressure she can apply to generate that lovely clear sound is roughly 1 psi.

In an internal-combustion engine running on full throttle, peak combustion pressure occurs at close to 11 degrees after top dead center (ATDC), and as the piston accelerates downward on its power stroke, that pressure falls rapidly-so rapidlythat by half stroke, or about 80 degrees ATDC, 85 percent of the pressure energy in the gas has been extracted by the piston. As expansion continues, the pressure continues to fall. At the point where the exhaust valves begin to open, roughly 100 psi remains in the cylinder. This substantial pressure generates the sound you hear at MotoCP events.

Why not continue to expand this gas, extracting further energy from it? This is a question of practicality. If, for example, we postponed the opening point of the exhaust valves to bottom dead center, we would find that as the piston began to rise on its exhaust stroke, negative work was being performed to push out exhaust gas that with earlier exhaust valve opening would have rushed out from its own energy. This is a case of, “Pay me now or pay me later.” Either way, the exhaust gas must leave the cylinder before the intake stroke can begin (if there is still some exhaust pressure as

the intakes begin to open, exhaust will blow through the opening intake valves and fill the intake ducts).

Atthe end of the era of piston steam engines, the range of ships was greatly increased by use of multiple expansion engines. High-pressure steam from the boiler acted first in a small-diameter highpressure cylinder, and the exhaust from this cylinder would then drive a larger intermediate-pressure cylinder, and in the case of triple-expansion steam engines, steam exhausted from the intermediatepressure cylinder would drive an even bigger low-pressure cylinder. This is not practical as a form of internal-combustion engine because a specialized low-pressure cylinder, operating on only 100 psi, would generate considerable friction and comparatively little power (peak pressure in an unsupercharged four-stroke racing engine is approximately 1,200 psi).

Therefore, it has been more practical to usethat100 psi exhaust pressure to generate waves in the exhaust pipe whose action can assist gas exchange. When the exhaust valves begin to open, a blow-down wave of pressure rushes into the exhaust pipe. As it comes to the first pipe expansion, it expands in all directions—including back

the way it came-sending a wave of negative pressure back to the cylinder. In a certain range of rpm, this negative wave arrives during valve overlap, that period around TDC at the end of the exhaust stroke when the exhaust valves have not yet closed and the intakes are beginning to open. The lowpressure wave first extracts the exhaust gas from the clearance volume above the piston (an amount roughly equal to a tenth of the cylinder’s displacement) and then, acting on the fresh charge waiting behind the intake valves, gets it moving into the cylinder, thereby beginningthe intake process early. These two effects boost torque.

Because positive and negative waves alternate in the pipe, at some lower rpm it will be a positive exhaust wave that returns to the cylinder during overlap. It will pump more exhaust back into the cylinder, through the intake valves, and into the intake ducts. Because this dilutes the next intake charge with exhaust gas, the result is reduced torque, an rpm region that is the dreaded flat spot.

Anyway, you are quite right to ask why not convert all that noise into power.

But the practicalities of the internalcombustion engine make it difficult to do!

-Kevin Cameron