Readers Pick Apart Ducati’s Superquadro Mono

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The crankcase is split vertically on Ducati’s Superquadro Mono. (Ducati/)

Reader Yash comments that singles require a “heavy flywheel to improve tractability [and] eliminate rough idle.

“…it takes a heavy toll on acceleration and top speed.”

Yash is right that these are serious concerns, but there are many thousands of successful modern singles—four-stroke MX engines are prime examples. And, yes, it’s possible to provide too little flywheel, as was the case in the first year of Supermono racing, when big singles so often stalled in slow corners—sometimes to the point of being comical. Since then the sophistication of ECU programming has corrected this situation.

As to the “heavy toll on acceleration,” Yash is right again. When the late C.R. Axtell tested the Cosworth Norton on his dyno, he said it was “the slowest-accelerating engine I’ve seen.” It was a parallel twin (both pistons moving together) with both a massive central crank flywheel and a pair of hefty primary balancer shafts carrying eccentric weights.

In the case of Ducati’s Superquadro single, you’ll notice that the amount of flywheel mass on the crankshaft itself is quite limited. The designer has taken care to make crank and balancer rotating masses add up to a workable and appropriate total.

Flywheel mass on the Superquadro’s crankshaft is fairly little. (Ducati/)

Loss of top speed is also listed by Yash as one of the ills of big singles. Hmm, well, in past conversations with former Harley-Davidson racing manager Dick O’Brien and “Mr. Superbike” himself, Rob Muzzy, I was told that neither man had ever had much luck with light cranks and top speed at Daytona. In their (extensive) experience, a certain level of crank mass was essential to achieving high top speeds.

Joe Broussard writes, “I would love to see detailed images of just how Ducati makes, [and] uses, their desmodromic system.”

On the internet are to be found many images and even engineering drawings of Ducati desmo systems, but the basics are that each valve is opened by a normal-looking cam lobe, operating through a normal-looking short pivoted finger follower. The interesting part is what closes the valve. A roughly L-shaped closing rocker is pivoted in the middle and forked on the valve end to pull up against a round collar fixed to the end of the valve stem (by press-fitted collets for which there is a special “squeezer” for installation). The other end of the closing rocker bears against a mostly circular closing cam of rather large diameter, providing motion that is quite precisely complementary to that of the normal-looking opening cam.

The opening cam (1) is conventionally shaped while the closing cam (2) has a flat spot to allow the valve to be opened.

Mr. Broussard also speculates that springless desmo operation may give Ducati a significant power advantage.

Although it seems intuitively right that the added resistance of valve springs must consume a bunch of power, giving Ducatis an edge in acceleration and top speed, I believe this to be an illusion created by having manually rotated assembled engines during timing checks. Valve spring engines are stiff! We know also that valve train friction versus rpm displays a U-shaped curve: Friction is high at idle and low revs because there is not enough sliding velocity to generate full-film lubrication. As the engine revs into its working range, valve train friction falls. Then at higher revs it rises again because of increased power being consumed in more rapidly shearing oil films—just as happens between pistons and cylinders or between crank journals and their plain bearings.

Ducati is winning races now but recall that sixth, seventh, and eighth places were regarded by many as “reserved for Ducati” in its doldrum years after Casey Stoner’s 2007 World Championship. Did desmo valve drive somehow stop being an advantage in those years?

Also, if we theorize for discussion’s sake that desmo gives Ducati a 5 percent horsepower advantage (15 hp!), we must ask how the pneumatic-valve competition handle the resulting 11,000 watts of extra valve train friction (there are 746 watts in 1 hp, so 15 x 746 = 11,190). Do all the non-Ducatis have hulking oversized oil coolers? Those 11,000 watts are equivalent to the heat given off by the red-hot elements of 15 kitchen toasters. Do they regularly suffer scoring and seizure in their valve trains?

Reader “Two Dakars” writes, “…please explain how they only need a single spark plug at 116mm bore when the Husqvarna 701/KTM 690 has a twin [spark] plug arrangement for a 105mm bore?”

KTM’s 690cc LC4 single uses twin spark plugs; why doesn’t the bigger-bore Superquadro Mono use them? (KTM/)

In answer, I must say that I was impressed by the apparent ease with which Ducati kept its 90-degree twin Superbikes competitive during the years of enlargement during which the original 851 (92mm bore) swelled to 1,199cc (112mm bore). Meanwhile, in the early years some Japanese Superbikes encountered trouble in making bigger-bore engines burn as well as previous ones of smaller bore. When I asked Ducati engineer Claudio Domenicali (he is today Ducati’s CEO) why this difference existed, he replied, “I cannot speak for the Japanese, but in our own case we vary intake downdraft angle and runner diameter to achieve the desired results.”

The desired results were sufficient mixture turbulence during combustion to rapidly shred and widely distribute the flame initiated by the ignition spark. It is not the speed of combustion itself which burns from the ignition point out to the cylinder wall, but rather this turbulent shredding and transport process, and it achieves rapid combustion by greatly increasing the area of flame surface. Typical speeds for this are in the range of 50 to 200 feet per second. For the Superquadro’s 116mm bore, flame travel distance is half of this, or about 2.3 inches. At a 150 feet/second effective flame speed and 9,750 rpm, this would require 0.00153 second, which is one quarter of a crank revolution, or 90 degrees. Because combustion is roughly evenly divided on TDC, this would permit ignition at 45 degrees BTDC. This is just an example—I don’t yet know what ignition timing the SQ single requires.

Please bear in mind that combustion in engine cylinders is not, as so often described, an “explosion.” TNT, a high explosive first used in the Russo-Japanese War of 1905, reacts at 22,000 feet/second, while dynamite is a relative slowpoke at 8,860 feet/second.

While there are cases of engine manufacturers having to add one or two extra spark plugs per cylinder to either speed up ignition or eliminate regions of high emissions generation, Ducati appears to have the experience and methodology to achieve the combustion it wants without them.

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