>From the Norbye book.

Paul Lamar
_________________________________________
NSU DEVELOPS THE WANKEL

95

In mid-1957, a new and larger Wankel engine was designed and built-the DKM-125. Its chamber volume was 125 cc. (7.6 cubic inches).
Ultimately, its power output was raised to 28.4 horsepower at 17,000 r.p.m. (outer rotor and output shaft), which equalled 11,300 r.p.m. at the inner rotor. The
inner rotor had a 65 mm. radius and a width of 38 mm. The Rje ratio was 6.85: 1, which allowed compression ratios up to 17.6:1.
Spring-loading was not required for the apex seals, because the apex seal swing angle was limited to 26 degrees. The DKM-125 was fully balanced and did not have
any mechanical vibration. Bearing loads resulted from gas pressure only, because both the inner and outer rotors revolved around their own centers of gravity.
Both rotors revolved in the same direction and were connected by phasing gears that positioned the two rotors but had nothing to do with transmission of the
engine torque. The outer rotor was made of nitrided steel but, despite the use of high-grade metal, it tended to distort from true epitrochoidal shape under
centrifugal force. The rotating inertia in the outer rotor was so high that it had a detrimental effect on the engine's ability to accelerate-it wanted to run
at a constant speed. The inner rotor, instead Q of being one complete casting, was built up from a number of components, including separate side plates and a
variety of seals and seal parts. Initially, the inner rotor had no provision for cooling (the outer rotor was water-cooled). Later, water cooling was added to
the inner rotor-with considerable complication.

Port sealing was a big problem because of the high rubbing velocity of the outer rotor. It was found to be almost impossible to make a satisfactory gas seal for
the exhaust port, where high gas temperature aggravated the already unfavorable conditions. The problem also was aggravated by the fact that friction losses in
transmissions increase with a rise in gear rotational speed, and the output shaft revolved at outer rotor speed. The stationary nature of the mainshaft added to
the difficulty of providing a suitable transmission. Efforts were made to limit the operational speed without engendering unacceptable sacrifices in power
output.
Volumetric efficiency was found to be 98% at 7,000 r.p.m. and 70% at 16,000 r.p.m. The DKM was tested at speeds up to 25,000 r.p.m. in order to prove the
mechanical safety of the engine. Peak torque was reached at 8,000 r.p.m. and peak mean effective pressure was 120 psi.
The engine ran most economically at 12,000 to 14,000 r.p.m.


The DKM-125 had been designed without regard to installation problems and transmission requirements. The dual rotation made a stationary outer casing necessary
if the engine was to be installed in a vehicle. This would have added substantially to the weight and cost, and the obvious course was to redesign the power
unit with vehicle installation and power transmission in mind. This work was carried out by Dr. Froede and the research staff at NSU, the result of which was a
concept that made the Wankel engine truly practical. Dr. Froede discarded the idea of adding an extra casing and concentrated on redesigning the DKM with a
stationary outer housing. In other words, he decided to prevent the outer rotor from rotating, and to confine rotation to the inner rotor and the shaft.
This line of thought led him to the invention of what he called "kinematic inversion" and to the introduction of eccentric rotor movement. These principles were
first combined in a new engine called the KKM. KKM stands for Kreiskolbenmotor, which means "circuitous piston engine."

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