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Inside KTM’s 890 Duke R

Cycle World logo Cycle World 7/29/2020 Kevin Cameron
a person riding on the back of a motorcycle: KTM went through great effort in the redesign of its first parallel twin used in the 890 Duke R. Was this for more power or to battle regulatory constraints? © Provided by Cycle World KTM went through great effort in the redesign of its first parallel twin used in the 890 Duke R. Was this for more power or to battle regulatory constraints?

Okay, folks, here are the figures from the CW dyno, comparing the previous 790 with this year’s complete redesign with increased bore and stroke. Peak power is up 11.7 percent, and torque is boosted 3.7 percent. To get these increases, the Austrian company had to give this engine a new crankcase, and new crankshaft, con-rods, pistons and valves, a new cylinder head, new balance shaft, and new cam profiles with greater lift.

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Displacement is up 12.6 percent, and compression jumps from 12.7 to 13.5—two changes that normally have a big effect in boosting peak torque. But when I got out my colored pencils and graphed CW’s 790 and 890 power and torque curves on the same sheet of graph paper, the torque curves were so close that they crossed each other in three places. Close.

I thought the above was an expensive list of engine changes to gain just 3.7 percent in peak torque, so I asked if we could run a retest. I didn’t want to write something that made KTM look like it had paid big bucks for new die-casting molds and production programming to get so little return without a confirmation of results. 

A few weeks later I heard from our test rider and dyno guy Michael Gilbert that he’d rounded up another 890, and shortly he sent me two more runs. But instead of the hoped-for (or expected) big step-up in torque, the new curves were pretty close to what we’d measured before. Mind you, the torque curves are lovely and flat from 5,500 to 9,000 rpm, as well as impressively high from 3,000 rpm, where torque is close to 80 percent of peak. That’s a great gas-it-and-go torque curve of the modern type. But I had expected a torque increase that just wasn’t there.

That jolted me out of an obsolete perspective: A hot-rodder’s reasonable expectation that major engine changes will bring a big jump in performance. Sorry, we now live in the Euro 5 world, so what KTM has cleverly done is make all those expensive changes to keep and even somewhat increase the performance of this motorcycle while making it pass Euro 5.

a close up of a map: 2020 KTM 890 Duke R Dyno Chart. © Provided by Cycle World 2020 KTM 890 Duke R Dyno Chart. a close up of a map: 2019 KTM 790 Duke Dyno Chart. © Provided by Cycle World 2019 KTM 790 Duke Dyno Chart.

Euro 5, scheduled for introduction in January 2020, tightens carbon monoxide (CO) by 12 percent, unburned hydrocarbons (UHC) by 41 percent, and oxides of nitrogen (NOx) by 33 percent. It also sets standards for particulate and evaporative emissions, requires OBD-II self-diagnostic capability, and raises the system durability requirement from 20,000 kilometers to vehicle lifetime.

a person riding on the back of a motorcycle: KTM went through great effort in the redesign of its first parallel twin used in the 890 Duke R. Was this for more power or to battle regulatory constraints? © Jeff Allen KTM went through great effort in the redesign of its first parallel twin used in the 890 Duke R. Was this for more power or to battle regulatory constraints?

We would normally expect this 890′s boost in displacement and compression ratio to give it not 60 pound-feet of peak torque but more like 65 to 67 pound-feet. Where is that missing torque? I believe we could find it in the mapping of the ECU. To pass Euro 5, engineers have to avoid the highest-temperature combustion that generates NOx, and they have to lean out fuel-air mixture where possible to meet UHC. This is now the cost of making the motorcycle compatible with wider social goals such as cleaner air in urban areas.

And there’s more. Engineers are saying that we can expect to see fewer supersport four-cylinder engines in future because Euro 5 is so difficult for them to meet.

Here is a major reason why—as the mixture is ignited, turbulent flame roils radially outward from the central spark plug. Burning releases heat, causing a steep rise in combustion chamber pressure. That pressure, which may peak as high as 1,200 psi at peak torque, occurring at about 11 degrees ATDC (after top dead center), forces unburned mixture into the crevice volume defined by the top piston ring land and the ring’s vertical and back clearances in its groove. As much as 2 percent of the charge can be compressed into those tiny volumes. When combustion ends and piston motion expands and cools the combustion gas, that unburned mixture comes streaming up out of the crevice volume. Dramatic Schlieren photography made with MIT’s square quartz cylinder test engine shows this gas—a major source of UHC, continuing to emerge through the whole exhaust stroke. That unburned mixture stays unburned. 

a close up of a map: 2020 KTM 890 Duke R Dyno Chart. © Robert Martin 2020 KTM 890 Duke R Dyno Chart.

One way to minimize ring crevice volume is to move the rings up as high as possible on the piston, and to minimize the clearance above and behind the rings. That has already been done. To get further reductions we have to reduce the total length of piston ring sealing, which is the circumference (distance around) of one piston, times the number of cylinders.

Why not just eliminate this crevice volume by tightening clearances? Rings must have enough vertical and back clearance for combustion pressure to press them out to seal against the cylinder wall, and downward to keep them on the bottoms of their grooves. Make clearance too tight and ring sticking may occur, destroying ring seal.

I did the arithmetic to compare the seal length of a Honda CBR600 with that of this new 890 KTM. Result? The KTM twin, despite having 48 percent more displacement, has almost 50 percent less total seal length. That has to translate to a useful reduction in HC—just by building a twin instead of a four.

And there’s more. I see more and more outstanding torque curves like these in recent engines, and I suspect they are not just the result of simple-minded “more torque is more better” thinking. They are also what happens when Euro 5 or similar limits apply pressure to reduce valve overlap. Overlap is the period around TDC at the end of the exhaust stroke, during which the intakes have begun to open but the exhaust valves have not yet completely closed. Overlap is important to emissions because it is the major “window” through which some fresh charge, entering the cylinder through the intake valves, can get short-circuited straight out the exhausts to be detected as UHC in emissions test cycles.

RELATED: Will Exhaust Emissions Limits Kill Motorcycling?

It just so happens that the shorter valve timings that result from reduced overlap can also produce extremely broad and very rideable torque curves. How much overlap is a lot, and how much is a little? Back in the days of two-valve engines, greater valve weight needed more time for valve springs to control it, so engines were built with as much as 80 degrees of overlap. By contrast, some Harley engines have been given zero or negative overlap, and engineers at Ducati have called their “Diavel” engine “the eleven-degree” because of its very short overlap.

Not so long ago, the 13.5-to-1 compression of this new 890 was considered radically high, and very likely to cause engine knock on ordinary pump gasoline. How do modern engines avoid knock (a.k.a. “detonation”) with such high compression? Years ago, before we had the luxury of digital mapped ignition timing and fueling, a 13.5 engine would have knocked itself silly the first time the owner took a big handful down at 3,000 revs (lugging can lead to detonation) but today the combination of fast combustion and electronic knock detection/prevention make knock a thing of the past. Then why so much compression if it’s not to boost torque? High compression doesn’t just make torque—it also reduces fuel consumption and therefore carbon dioxide emissions (motorcycle CO2 emissions are not regulated in Euro 5 but must be reported).

Looking at the power and torque curves we see that the new 890 makes its peak power of 106 up at 9,870 rpm, up 660 revs from the 790′s peak. Why rev a bigger engine higher than a small one? If we compare the net, stroke-averaged combustion pressures of these two designs, we find they are reduced almost 10 percent in the new design. If you need more power for a new model but Euro 5 is making it harder to achieve high combustion pressure, to get it you have to boost one or both of the other two major variables in engine power: cylinder displacement and rpm. KTM used both to realize the 890′s gain of 11.7 percent in peak power.

a close up of a toy: KTM made many changes to the 790 Duke’s parallel twin to arrive at the 890 Duke R engine. © Provided by Cycle World KTM made many changes to the 790 Duke’s parallel twin to arrive at the 890 Duke R engine.

The journal insert plain bearings most used in modern automotive practice consist of a thin layer of aluminum-silicon alloy on a steel backing. In increasing cylinder bore by 3 percent and stroke by 4.7 percent, the 890′s inertia loads from bigger pistons, moving through a longer stroke at higher peak rpm told the engineers they needed to make some changes. They redesigned the pistons to be 10 grams lighter than those of 790, not 6 percent heavier as you’d expect from their larger bore. And to handle the added loads of the longer stroke and higher peak revs, they adopted con-rod bearing shells with copper-based rather than aluminum-based bearing material, in all-new con-rods.

a close up of a map: 2019 KTM 790 Duke Dyno Chart. © Robert Martin 2019 KTM 790 Duke Dyno Chart.

Probably because idle can be a problem at high compression ratio, crankshaft rotating mass has been increased 20 percent.

To further refine mixture control, each throttle body has its own manifold pressure sensor and operating variables are separately mapped for each cylinder.

Motorcycling is changing to adapt, both to Euro 5 emissions limits and to riders who want a more natural upright riding position. The sportbike era was exciting, but the sudden rush of high-rpm four-cylinder power is now being replaced with Euro 5-compatible short valve timing that gives strong rideable torque across a wide range. To control costs in our post-2008 and COVID-19 economy, engines have fewer parts. That means twins like this 890. Enjoy your ride.

a close up of a toy: KTM made many changes to the 790 Duke’s parallel twin to arrive at the 890 Duke R engine. © KTM KTM made many changes to the 790 Duke’s parallel twin to arrive at the 890 Duke R engine.
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