Complexity, Anyone?

Complexity, anyone?

TDC

Kevin Cameron

ONE SPRING DAY A FEW YEARS BACK, I was spying my way along Daytona’s pit lane when I saw one of the Yoshimura Superbikes with its usual single oxygen sensor in the exhaust collector pipe. This sensor compares the percentage of oxygen in the exhaust gas with that in the atmosphere, allowing the ECU to constantly correct the mixture.

As I looked more closely I could also see plugged, threaded holes in each of the four header pipes. Clearly, at some previous time there had been one oxygen sensor for each cylinder-probably for dyno testing or more instrumented track testing. What might those four separate oxy sensors reveal?

All racing and sport motorcycle engines have tuned intake-tract lengths. As one cylinder’s suction stroke generates low pressure, a wave of that low pres¡ sure propagates back up the intake pipe I to its open end. There it reverses sign and is reflected back toward the engine as a wave of positive pressure. The game in intake tuning is to make that positive pressure wave arrive just as the intake valves are closing, so its pressure is added to the pressure in the cylinder, thereby boosting torque. The existence of this effect is what makes it worthwhile in some cases to employ a variable intake length, which widens the rpm range across which a torque boost (10 percent or more) is produced. Sign me up.

But even with variable intake length, some lower rpm will result in a negative pressure wave arriving as the intake valves close, with a resulting drop in torque.

Now let’s add to this the effects of the exhaust pipe. Its header length is chosen so that in a desired range of rpm, there is a negative wave that reflects back to the cylinder just as the exhaust valves are closing. This negative wave extracts any remaining exhaust gas above the piston and begins to draw in fresh air through the intake valves, which are just beginning to open. This boosts torque by 1) removing inert exhaust gas and 2) replacing it with fresh charge-all while the piston is essentially stopped near TDC.

Now the downside. At some lower rpm, the engine gets out of step with the exhaust waves, so it’s a positive wave that arrives at the cylinder just as the exhaust valves are closing. This blows more exhaust gas back into the cylinder, and may even push some all the way back into the intake airbox through the justopening intake valves. This causes the affected cylinder to lose torque, and any exhaust blown into the airbox causes even more torque loss when it is drawn in by the next cylinder’s intake stroke. What’s worse, fuel-injection may be taking place at the same time, so the exhaust wave blows not only exhaust gas but fuel back into the airbox, where it may make the next cylinder’s intake process overly rich.

We must not forget that the airbox itself has a resonance-its internal pressure falling late in each cylinder’s suction stroke and refilling between them. At its tuned rpm, the engine’s suction strokes benefit from positive pressure in the box. But at other engine rpm, the box works against the engine, which is why so many new bikes have trick devices such as butterfly valves in the airbox inlet ducts or a big “saxophone valve” in one side of the airbox, to kill its anti-resonance.

Now let’s try to sum all this action up in our minds. Concentrate! I don’t know about you, but I can’t do it. There’s too much happening, what with intake tuning, exhaust pressure waves, fuel standoff being blown back into the airbox and so on. Therefore factory engineers may just put oxygen sensors into each of the four header pipes and see which cylinders go rich at certain rpm and throttle openings. Then they can play with

adjusting the quantities of fuel injected for each cylinder, or can even delay injection so that no fuel is present when wave action is most likely to blow it back.

In fact, on my recent trip to Ducati, engineer Claudio Domenicali said that, “At peak torque, our engine might use only 190 grams of fuel per horsepower per hour. But in other conditions, the fuel consumption can be several times greater.”

For you folks who regularly work on dynos, that’s a very economical 0.42 pound per hp-hour-just what you might expect of a high-compression engine.

Thinking of the chaotic interplay of multiple colliding wave actions in the airbox, with fresh air pulsating in through one or more inlet pipes, and fuel standoff being shared, it would look like a miracle if all cylinders always received the same mixture strength. You just might want to detail í some engineers noted for their extreme patience and ingenuity to plow through all this complexity and sort it out. They would want instrumentation-at a minimum they’d want oxygen sensors on all four header pipes. And, using our imagination, we can see why teams might want one central airbox intake rather than two coming in at odd angles, causing merry little unpredictable breezes that carry fuel stand-off to whoknows-where.

Let’s consider another complicationsudden acceleration. Many old bike and car carburetors use tiny accelerator pumps to supply the extra fuel that’s needed because air accelerates faster than fuel (it’s 580 times denser than air). But the insides of the throttle bodies may run wet with fuel on part-throttle, so that when the rider snaps the grip, that fuel promptly evaporates, acting like nature’s accelerator pump. It’s tempting when doing fuel mapping to just type in a percentage of enrichment for rapid throttle movement, hoping that’s good enough. But it would be more accurate to use instrumentation to find out exactly how much extra fuel each cylinder needs, individually.

Dear me. I thought fuel-injection was so simple, so perfectly digital. The computer tells the injector how many milliseconds it should squirt, and that accurately metered fuel is supposed to be what the cylinder actually gets. Sorry, as the old song says, “It ain’t necessarily so.” That’s just the beginning of getting the fueling right. □