Tech Tidbits Regarding KTM’s 2020 1290 Super Duke R

I just reviewed the development history of KTM’s LC8 V-twin engines over the past 20 years. It was interesting to me to trace the rise of compression ratio (which is a major determinant of engine torque) with the growing sophistication of their engine control. Back in the days of carburetors compression didn’t rise above 11.7-to-one, even though the engines in question were liquid-cooled. That is surely because control of fuel mixture and ignition timing were less accurate, requiring that engines be given more protection against detonation by limiting their compression ratio.

Now that KTM engines have state-of-the-art fuel injection and ignition mapping, even engines with very large bores (108mm in this case, or about 4.25 inches—that of a 427 big-block Chevy) are able to operate safely on compression as high as 13.5. The bottom line is, the more accurately basic variables such as spark timing and mixture can be controlled, the closer the engine’s operation can be moved toward the limit (which is the appearance of knock, or detonation).

The great thing about high compression ratio is that it boosts torque everywhere not just on top, as long cam timings do, or over a limited range of rpm, as tuned-length exhaust pipes do.

Cycle World’s First Ride of this bike by much respected tester Don Canet says, in part: “Performance gains are attributed to the addition of showerhead injectors located atop each of the two intake funnels for improved air-fuel atomization at upper rpm. Airbox volume has also been increased by 25 percent and is fed via a central air inlet now located between the headlights employing revised ducting to provide a more effective ram-air effect at speed than the previous model’s dual inlets.”

Let’s take these point by point.

The normal main fuel injector, located under the throttle plate, can do a fair job of mixture formation as long as there is adequate time for fuel droplets to evaporate. Only evaporated fuel vapor and droplets smaller than roughly 10 microns can ignite and burn completely. Droplets any larger than that when the flame front reaches them cannot entirely burn, allowing some unburned fuel to pass through the engine. This is complicated by the fact that the faster the engine turns, the less time there is for evaporation. This means that the higher the engine revs, the greater the fraction of fuel that passes through it unburned, in droplet form. An obvious result is that the faster the engine turns, the leaner the effective evaporated mixture becomes.

Because a lean mixture does not provide enough fuel to fully react to all the oxygen in its intake air, a power loss is produced.

As long as the emissions test driving cycle doesn’t include high-load, high-rpm operation, we can compensate for this leanness by injecting enough extra fuel to make up for those droplets too big to fully evaporate and take part in combustion. But if we do this, we are going to lose some power because the presence of extra fuel that doesn’t burn reduces flame temperature and peak combustion pressure, and therefore power.

Just as an example, here are the times taken for evaporation of various-sized fuel droplets, in crankshaft degrees at 3,000 rpm, in a particular set of conditions described in a reference on mixture formation:

The reason large droplets take longer to evaporate is that they contain more fuel volume in relation to their surface area than do smaller droplets.

A better way to deal with this “big droplet flow-through” problem at higher revs is to add a second fuel injector to each intake stack, located in the “showerhead” position, hovering just above the throttle body’s intake bell. This position, being farther from the engine’s intake valves, provides a longer “time of flight” and increased exposure to mixing turbulence, during which fuel droplets can evaporate. To benefit from this good effect, we write our software such that at a certain rpm we start to move the fueling from the main injector under the throttle butterfly to the showerhead, so that at higher revs all the fuel is being injected by the showerhead. Because of the added evaporation time this provides, the population of fuel droplets large enough to survive combustion unburned is reduced. The emissions authorities are happy (especially if their name is Euro 5) and riders perceive a bit more power.

This can be a tough one because the engine’s intake airbox volume has competition for space—it needs to be located where conventional fuel tanks were once located—directly above the engine. As one cylinder of this large-displacement twin begins its intake stroke, the big gulp of air it takes pulls the pressure in the airbox down steeply. How far down it pulls it depends on how big the box is in relation to cylinder volume. The smaller the airbox volume, the less full the cylinders may be, reducing torque. The bigger the box, the less far down the cylinder pulls that pressure, the more fully the cylinders are filled, and the more power the engine makes.

KTM is by no means the first to have to increase airbox volume: In their time, both Ducati and Aprilia have extensively redesigned entire models to make room for bigger airboxes.

A resonant airbox can easily boost engine torque by 10 percent in a desired rpm range. This works by tuning the airbox and its intake pipe to resonate just as an empty bottle does when you blow across its open mouth. In the case of an engine airbox, the engine’s suction pulses take on the job of driving the resonance. If the engine takes air from the box at the top of its pressure cycle, and the box refills from its intake pipe after intake ends, the result is a useful net increase in engine airflow—and more power.

The major gain from the use of an airbox comes from the resonance effect, but at high vehicle speed there can be a second source of gain: the pressure of free-stream outside air upon being brought to rest in the box. At 160 mph, complete conversion of that air velocity into pressure gets us a 3-percent increase in airbox air density, and therefore a 3-percent power gain. On a 180-hp engine, 3 percent is 5.4 hp. Yes, please.

But how much gain we actually get depends on where we take the air in, and on how smoothly we decelerate the air in the process of converting its kinetic energy into pressure energy.

The whole enchilada in ram air is called “Q,” which is the pressure resulting when moving air is brought to a stop. But on a curved surface, such as the front of a fairing, that only occurs where the curved surface is perpendicular to the airflow—roughly at its center. That’s why the airbox intakes of MotoGP bikes are central, and to achieve that the air is often routed through the steering head of the chassis.

The farther from center we locate the intakes (as with the 1290’s previous dual intakes), the less ram pressure we can recover, because the external airflow at points away from the center of pressure is already accelerating around the fairing—and is losing pressure.

The shape of the intake duct is also important. To smoothly decelerate the entering air, the duct must gradually widen, but in airbox installations of the early 1990s, the intake ducts were just crude pieces of corrugated plastic hose from the pool supply. Bad! So, to recover more of the pressure potential in ram air, we must shape the duct as carefully as those leading from atmosphere to jet engine intakes.