Cracking the Throttle, Cracking the Code

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Almost 30 years ago Rob Muzzy, known as “Mr. Superbike” in his era, made the pronouncement that “the harder you work to make four-stroke horsepower, the more your engine acts like a two-stroke.”

Nowhere is this more true than during initial throttle—the sequence of events that occurs as the rider begins throttle-up to accelerate out of a turn.

This is of particular interest because Matt Oxley published an interview with KTM engine designer Kurt Trieb last month on the MotoMatters website, and their discussion touches on this problem.

The Strange Technologies of Initial Throttle

Understanding the dynamics in play here has become crucial to four-stroke performance, because throttle-up problems can destroy one of a four-stroke’s greatest potential strengths: its ability to deliver smooth torque from just off zero throttle angle.

Harking back to Muzzy’s observation, the problems leading to unsmooth throttle-up are very similar between highly tuned four-strokes and the two-stroke 500s of the 1975‒2001 era. Conditions within both engine types lead to exhaust-gas dilution of the fresh charge entering the cylinders, enough to result in tire-upsetting irregular combustion.

An example from 1998 was Anthony Gobert on the Vance & Hines Ducati, closely pursued by Mat Mladin on a Yosh Suzuki, at the old Loudon, New Hampshire, AMA National. Ducati at the time had years of World Superbike fuel-injection experience and its big V-twins had three injectors per cylinder: a main and a small vernier injector under each butterfly, plus a showerhead over the bellmouth. Suzuki had only that year adopted injection on the GSX-R, and was still on the initial slope of the learning curve. Gobert’s system was clearly very smooth in its throttle-up, for he was getting on the gas much earlier than his rival. It was easy to see why—as Mladin began to open the throttle, there was at first no response, then a couple of big da-dits, followed by the engine abruptly catching up to the throttle. No matter how brave you are there is nothing to be gained by falling, so Mladin was having to postpone his acceleration until his bike was more upright—enough that its rear tire footprint could take the hit without stepping out. Advantage Gobert.

Related: Patience At Neutral Throttle

This was exactly the problem that made the big two-strokes so very hard to ride smoothly off corners. As the rider began to turn the throttle, a small volume of fresh charge was being squirted into a cylinder full of inert, incombustible exhaust gas. That degree of dilution made it extremely unlikely that at the instant of the spark there would be fresh mixture in the plug gap. The result was nothing, followed by a series of misfires. As the rider rolled on more throttle, a point was reached at which every few crank revolutions there was enough accumulated mixture in a given cylinder to make it fire—pop, pop, pop—giving the rear tire a series of yanks. More throttle yet and firing began to occur every four revolutions, then jumping to every two (meaning a sudden doubling of torque) and finally, at maybe 30 percent throttle, combustion would smooth out.

When you are leaned over at a high angle in midcorner, your tires are giving nearly 100 percent of their grip just to stay on their line. Asking them to additionally handle all that popping and banging was just a ticket to the gravel. So a two-stroke riding style developed around postponing throttle-up until you’d lifted your bike up enough (or suddenly pushed it up, as Mick Doohan, Dani Pedrosa, and others did) that the tires had the extra footprint to take the hit.

Four-Strokes—The Ideal Case

Now consider the four-stroke case. As we learned in school, in the four-stroke cycle, as the piston rises on its exhaust stroke, the exhaust valves close at top dead center (TDC) and the intakes open to begin the following intake stroke. Pure fresh charge is admitted to the cylinder as soon as the rider cracks the throttle, and as the crank completes the circle by rising on compression (the intakes having closed at BDC), there’s nothing but lovely, 100 percent ignitable fresh charge between the piston and the head. When the spark jumps the plug gap, a flame kernel is created with near 100 percent probability, and with the engine firing smoothly, the rider begins the drive early and can easily match engine torque to what the rear tire can handle.

That’s the fairy tale; now for the reality. While opening and closing the valves at TDC and BDC actually works quite well on big Harley V-twins, such short timings also cause them to run out of breath at 5,000 rpm. For best power in high-speed four-strokes, opening valves earlier and closing them later boosts torque at higher revs; MotoGP engines are now revving to 18,500. This changes the situation inside the cylinders as a rider initiates throttle-up.

Effect of Valve Overlap

To open the intake valves farther, sooner, they begin to lift many degrees before TDC. To allow time to return the exhaust valves to their seats without bouncing, the exhausts are closed several degrees ATDC. This results in a period around TDC at the end of the exhaust stroke when both exhausts and intakes are slightly open simultaneously: the overlap period.

Added torque is possible if a negative exhaust-pipe wave is timed to arrive at the cylinder during overlap. The wave “sucks” exhaust residuals out of the combustion space, then starts the intake process even before the piston starts down on its intake stroke. But at some lower rpm (such as that at which riders start their drives off turns) it is instead a positive wave that arrives during overlap. This stuffs extra exhaust into the combustion space, then backflows it out through the intake valves to partially fill the intake ports. When the piston does move downward on its intake stroke, it is this exhaust gas that is initially drawn into the cylinder.

Now comes the similarity to a two-stroke. With all that exhaust gas in the combustion chamber and intake ports, the small volume of fresh charge that the rider sends in by cracking the throttle may be so diluted by exhaust that at small throttle firing is irregular. And that’s just what I heard one afternoon at Laguna as one of Muzzy’s long, low Kawasaki 750 Superbikes accelerated toward turn four: irregular initial response to throttle. Aha, I thought, they’ve “cammed up.”

Taking Up Driveline Backlash

As we all know from rolling the throttle on and off, there is backlash in motorcycle drivelines—most of it in the dogs’ angular clearance necessary to make transmissions shift well, plus a tiny bit in the final chain drive. Problem is, as the rider throttles up, this backlash must be taken up before the engine can drive the rear wheel. During that short time the crankshaft accelerates, so when the backlash is taken up, it hits with something of a clunk—sometimes a clunk that’s enough to mess with tire grip. We learned from Aprilia MotoGP team manager Romano Albesiano that there is a special bit of engine control software that “gentles the clunk” so that no matter how tempestuously you turn the throttle, that backlash will be clunkless so tire grip continues uninterrupted.

Activities in the Combustion Chamber

As the rider is throttling up, valve overlap plus wave action in the exhaust pipes can create a mess in the combustion chamber. Some exhaust gas may have back-flowed into the chamber, but at this late stage there is none of the “tumble” charge motion that an instant before had greatly accelerated combustion. That being so, there may be little gas movement to mix fresh charge and leftover exhaust gas. This inhomogeneity is the source of misfiring on low throttle, and it also leads to idle instability.

Related: Initial Throttle versus Accelerating

This problem becomes worse as we make the bore bigger and stroke shorter, because this configures the combustion space into quite a thin disc with a large diameter, within which charge motion is sluggish. In MotoGP the disc’s diameter is the maximum bore limit of 81mm and its thickness is determined by the engine’s compression ratio; the higher the ratio, the thinner the disc. How do we stir the contained gas so that, 300-odd degrees later, the ignition spark finds a well-mixed concoction that ignites easily and steadily, rather than alternating pockets of unmixed fresh charge and exhaust gas that pop and bang?

We’ve all seen efforts made on certain production bikes to solve this or related problems by adding one or more extra spark plugs. The idea here is that adding ignition points increases the chances of avoiding a misfire.

Then I remembered something that John Wittner, who built Moto Guzzi Battle of the Twins bikes in the late ‘80s/early ‘90s, said long ago. Wittner described a conversation with an Italian Formula 1 engineer who had said that when the value of tumble-charge motion had reached its limit, something more might be accomplished by adding an amount of axial swirl (rotation of the charge around the cylinder axis). While swirl might not be the answer to intermittent combustion at throttle-up, it does suggest that some kind of engineered charge motion other than tumble might approach a solution.

That Tiresome Word Compromise

If peak power alone could win races, cam timings would include generous valve overlap, and intake-valve closure would be considerably delayed past bottom center to allow complete cylinder filling. But “big cams” make it very hard to achieve something that definitely can win races: early, smooth throttle-up. This is part of the reason why repeated attempts by Formula 1 organizations (Ilmor, BMW, Cosworth) to design successful MotoGP engines have so far failed. They make big power, but they also upset the fragile traction a MotoGP bike needs.

Remember those 900 hp V-10 F1 engines of the early 2000s? Just saw off three pairs of cylinders, package them to fit a bike, and we’re golden! Long-serving test rider Jeremey McWilliams has referred to such attempts as “high-siders.” F1 cars have acres of soft rubber on the pavement and use 100 percent of their tread width, so fiddly problems with combustion roughness on initial throttle hardly exist. Bikes have comparatively tiny footprints and never have more than about one third of their tread width on the pavement. Big difference.

As a result, power must compromise with drivability. Why must life be so complicated?

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