Uneven firing plays winning tune for Yamaha

  • 31-Dec-2008 10:29 EST
2. Rossi aboard YZR-M1.JPG
­Valentino Rossi astride the Yamaha YZR-M1 MotoGP racing machine. The YZR-MI won three ­MotoGPs in a row in April, May, and June.

“I might have gone absent without leave today had we lost Le Mans,” said Masao Furusawa, Executive Officer of Yamaha Motor Co. and Senior General Manager of Engineering Operations, Motorcycle Headquarters, when telling the secret of Yamaha’s winning formula in the MotoGP racing series. In April and May, Yamaha won three races in a row, the Grand Prix of China, Portugal, and France. The French Grand Prix at Le Mans saw Yamaha YZR-M1s finish in first, second, and third positions, justifying Furusawa’s technical discussion.

Furusawa’s success with Yamaha is in part due to his long service to the company. Furusawa was born in February 1951 on the western island of Kyushu, Japan. He graduated from the mechanical engineering department of the Kyushu Institute of Technology in 1973 and joined Yamaha the same year.

Furusawa climbed up in Yamaha’s engineering echelon; he was appointed General Manager of the off-highway vehicle Engineering Department Division, working with all-terrain vehicle snowmobiles, in 1998; General Manager of the Engineering Department, Global Engineering and Manufacturing, Motorcycle Operations in 2001; and Senior General Manager of Engineering Operations, Motorcycle Headquarters in 2005, before coming to his current management position.

MotoGP is the pinnacle of the world motorcycle road racing series, a Formula One on two wheels, fiercely contested by the leading OEM marques including Honda, Yamaha, Suzuki, and Kawasaki of Japan and the 2007 World Champion, Ducati, of Italy.

MotoGP’s governing regulations stipulate that engines must be four-stroke, have reciprocating pistons (no rotary or oval pistons), and be naturally aspirated of less than 800-cm3 displacement, reduced from the previous 990 cm3 in 2007 to curtail race speeds, as with the case of Formula One. Maximum fuel tank capacity has also been decreased from 22 to 21 L (5.8 to 5.6 gal). Minimum weight of the MotoGP motorcycle with four cylinders is 148 kg (326 lb), whereupon 7.5 kg (16.5 lb) must be added for each additional cylinder. Furusawa cited that a typical MotoGP machine should be capable of exceeding 320 km/h (199 mph) in a straight line, with its engine producing over 200 hp (149 kW).

As an engineer, theoretician, and avid racing enthusiast, Furusawa analyzed Yamaha’s race-winning YZR-M1 transverse inline four-cylinder engine and employed a 90° crankshaft and adopted irregular—or odd—interval firing.

“Uneven- or irregular-interval firing has been employed in racing engines—the so-called ‘Big Bang,’ with more than one cylinder firing simultaneously,” Furusawa explained. “Then there is the ‘Long Bang,’ with crank phases out of sync. Uneven-interval firing race engines have been known to improve lap times versus even-interval firing ones. How and why they work has not been clearly defined,” he said.

“Some maintain that closely spaced, multicylinder combustion pulses press the driving tire’s contact patch against the road surface harder, to a degree that the tire slips or spins, then enables the tire’s ‘recovery’ during the following non-combustion interval, so that pent up energy generates a stronger grip on the next power pulse—a rather dubious supposition that I do not subscribe to at all,” he continued.

Furusawa observed that, by simultaneous two-cylinder combustions, peak torque would double, producing a momentary burst of power, but conversely the total number of combustions decreases, thus obtaining the same total.

His ideal would have been a multicylinder, even-interval-firing engine with minimum fluctuations in revolutions, thereby getting the maximum amount of propulsion without inducing tire slippage. Ultimately, he added, something like an electric motor.

He said that uneven-interval firing itself was not the primary purpose of his design and development team, but that the 90° crankshaft was a fruit of strenuous work in minimizing fluctuations in engine revolutions. The natural result was that combustion intervals had become uneven because of the crankshaft design.

Furusawa’s team then pushed the uneven-interval-firing envelope to an extreme in one variation of the M1 engine that had all combustion occurring in a single revolution, the “Ultimate Long Bang,” Furusawa described. The exercise produced a change in the bike’s total traction, but no improvement in lap times.

Yamaha put the 90° crank YZR-M1 through its paces at the Mugello circuit, the MotoGP Italian Grand Prix track. The post-2007 machine shed 20% of its displacement in the regulation change, lost about 10 km/h (6 m­ph) in top speed—now about 330 km/h (205 mph)—­yet its cornering speeds improved by 2 to 8 km/h (1 to 5 mph), thus producing better lap times.

The throttle-opening ratio spread of the MotoGP machine is opposite to that of a Formula One car, noted Furusawa. A Honda F1 engineer confirmed that at the fast Monza circuit, the F1 Italian GP track, full throttle would account for massive 74% of all running, whereas a MotoGP rider at Mugello, another fast track, would crack the throttle wide open for only 25% of the race. More notable, Furusawa elaborated, is that less than 10% open throttle accounts for roughly 30% of the race.

“Whichever the engine configuration may be, inline or V-formation, targeted weight distribution should be no different, about 50/50. It could be a fraction of that at either end, but I am not going into that,” Furusawa said. “I believe the inline engine is more advantageous when fitted within a shorter wheelbase, which is more agile. In the V-configured engine, the rear bank tends to shift the mass rearward, which must be offset by lengthening the wheelbase.”

Engine power is obviously important; more so is its good traction and driveability on partial throttle. Ubiquitous inline four-cylinder engines in road cars and motorcycles employ 180° crankshafts and even-interval firing. Such engines are relatively simple in design and construction (by racing standards) with reciprocating first-order inertia forces balanced internally, while even-interval firing reduces vibration in the low- and mid-speed zones. However, the design is entirely unsuitable for MotoGP, where inertia forces and inertia torque would become huge in the usable revolution range of 15,000 rpm.

“What the rider wants is combustion torque proportionate to the throttle work, not inertia torque,” said Furusawa, who drew an analogy to signal-to-noise ratio (SNR), an electrical engineering term. “Combustion torque is a signal, and inertia torque is noise. Unfortunately, noise increases proportionately to the square of revolutions, greatly deteriorating the SNR.”

A typical road vehicle inline four may go up to about 7000 rpm on the high end, which is well within the engine’s effective combustion torque zone. In the MotoGP application, winding to 15,000 rpm, inertial torque would be significant. The rider must make best use of the signal buried deep within a large noise, and that would do no good to the essential connectivity between the throttle and rear tire.

Furusawa used the basics to illustrate his point; a single-cylinder engine does not turn as smoothly as might be imagined. It runs faster at top dead center (TDC) and bottom dead center (BDC), because the piston and crank pin are perfectly aligned, thus the crank exerts no effect. With the crank at 90 and 270°, the crank pulls, fluctuating revolution.

His theoretical SNR graph shows a 180° engine model at wide-open 15,000 rpm, assuming no fluctuations in rpm, in which noise/inertia torque is greater than signal/combustion torque. The compounded torque value that drives the motorcycle is, therefore, reduced. In the 90° crank engine, noise/inertia torque is almost negligible; what little is there is generated by the leaning connecting rods. Net driving torque nearly matches the value of combustion torque and is efficiently transmitted to the rear tire at the rider’s command.

Verification of the Yamaha SNR theory was performed by directly measuring fluctuations in rear tire revolutions during cornering using frequency analysis. In the 180° crank YZR-M1, speed fluctuations by second order in revolutions occurred regardless of throttle opening. The 90° crank version, on the other hand, displayed speed fluctuations of the second order in only 1.5 revolutions when the throttle was opened. Furusawa concluded that the 90° crank engine transmitted the signal/combustion torque singularly and effectively to the driving wheel.

Furusawa recounted racer Valentino Rossi’s first reaction to the YZR-M1 in January 2004. Rossi was the World MotoGP champion in 2002 and 2003, riding for Honda. The young Italian had just switched camps to Yamaha, widely rumored to prove that it was not the song but the singer.

Furusawa put Rossi on both the 180° and 90° crank YZR-M1. On the 180° machine, he commented that it was as power-oriented as his previous Honda mounts. On the 90° crank version, he responded, “Very sweet!” The bike responded precisely to his throttle command, producing optimal rear-wheel traction during cornering, and demanded less physical strain on a long run.

He observed that it somehow felt a little short of acceleration but, in fact, had better lap times. His riding style on the successive Hondas had been very aggressive, often wildly sliding the rear tires. His instinct told him that the old technique would not make the Yamaha go faster, but that he should adjust to a smoother style.

Rossi won the MotoGP World Championship astride the Yamaha YZR-M1 in 2004 and 2005. The 2008 season was still young, with only eight races run out of 18, when Furusawa revealed his unique SNR theory and the smallest hint of the YZR-M1 technology.

Furusawa imparted his wisdom, “Yes, the rider may adapt himself to a specific machine. On the other hand, we will respond to his demand if he wants more power. It works two ways. Winning is not by a single force, but the total efforts of the rider, machine, organization, motivation, and human and material resources. I want them all.”

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