Walter Kaaden's disc valve racers and the Australian Engineer

Motorcycle Engineering

Phil Irving

Each chapter of this book is based on an article written for the English magazine Motor Cycling and originally published in the 1950s or early 1960s under the pen-name of Slide Rule. Phil Irving worked in England for Velocette and Vincent. After returning to his native Australia he designed the engine of the successful Repco-Brabham G.P. racing cars.

I have extracted the material on MZ two-stroke racing engines designed by Walter Kaaden. Taken from the Clymer Publications/Speedsport edition; 5^th reprint; 1973.

I have added my comments in italics. Louis Mair  
 

Fin Design p. 181

Fin Effectiveness   p.194

… Nevertheless, there is obviously good reason for locating the exhaust at the front, or if this is not practicable, at the rear rather than at the sides, as is done indeed on the M.Z. two-stroke. Considerable lengths have been gone to in this engine to reduce the pick-up of heat in the exhaust-ports; although there are two cut in each cylinder, they  merge quickly into a single outlet, the length from bore to flange-face being less than two inches (Fig. 10.19).

Central-drive Twins   p.203

Recently, it has become a trend in racing design to construct tiny twins as two individual single-cylinder engines attached to a central component containing the primary drive and (in four-strokes) the drive to the valve-gear. This is of particular advantage for cooling in such designs as the 250 c.c. M.Z., and is still useful in the case of a four-stroke, though to a lesser extent ...

 

Inlet Port Control   p.247  

 

   This objection is overcome by the disc-valve which is used in the MZ and, no doubt, is part of the reason why the 125 c.c. model can turn out power equivalent to 170 b.h.p. per litre (Fig. 13.5).
    The valve itself is of sheet steel, 0.020" in. thick, and rotates in a space only 0.040 in. wide. It is not mounted firmly on the mainshaft, but is driven by splines so that it can centralize itself in the gap or move to one side or the other, so acting like a reed-valve during the closed periods. The periphery is cut away to open the port, which then has a

… Units have been made with just a pair of gears … if all the ratios in the box are indirect-as in the M.Z., for instance- … the engine rotates “forward”, so that a single pair of primary gears is suitable.

Basic Unit Layouts   p.315

… With a transverse four-cylinder engine, excessive width would be created if the drive were taken off one end of the shaft. … On the Rondine … taking the drive from the centre of the shaft …

GEARBOXES AND TRANSMISSION                   319

There has been a tendency in recent years for English designers to adopt what might be termed "semi-unit" construction. In this system, a flat face is machined on the back of an otherwise conventional crankcase and bolted thereto is a gearbox which has a mating surface machined on its front face. The rear half of the chaincase may be cast integrally with the drive-side half of the crankcase, as in the B.S.A. twins,* or it may be a separate bolted-on component.

In this construction, there are no shear loads...

Although at first sight it does not appear to do so, the 125 c.c. M.Z. utilizes this built-up principle, but in conjunction with an all-indirect gearbox and cross-over drive, which results in a neater and stiffer unit than would be obtained with both drives on one side.

*Early model BSA A7/A10/Kawasaki Commander with bolt-up gearbox and primary chain tensioner.

 

TWO-STROKE POWER                                  250

THE KADENACY EFFECT

It was at one time held that high power could not be obtained at high speed because of the short port opening time available. But actually the reverse is the case--it is precisely because the times are so short that full advantage can be taken of pressure-waves in the exhaust system and what is known as the "Kadenacy effect" to get the high outputs previously thought to be impossible.

٭ Kadenacy found, by ingenious experiments with a special engine, that if the exhaust ports were large and opened with sufficient rapidity the imprisoned gas would rush out at such high velocity that it continued rushing out until the port was closed again, by which time the pressure in the cylinder had become sub-atmospheric. It follows that if another valve, which may be a port or may be a poppet-valve, is opened at the right moment, enough fresh mixture may be drawn in to enable the engine to keep running without the aid of crankcase compression. But if crankcase compression is present, this effect obviously assists the entry of mixture from the transfer ports, though at the same time it has a tendency to suck a portion of this mixture out through the exhaust, as there may be a depression of 5 or 6 lb./sq. in. present in this port, immediately after the discharge of the main body of gas.

٭ Kadenacy is an adjective derived from the name of Walter Kaaden  [see Mona's comment below]

Exhaust Waves

By using a short, open pipe this negative pressure wave can reflect back as a positive wave that promptly rams some of the lost charge back into the cylinder, with a marked increase in power at the appropriate speeds. It has since been discovered that so far from needing an open pipe, it is better to use an expansion chamber with a restricted outlet, when the desired wave-effects will be intensified. A curious fact is that the outlet area may be less than that which would normally be provided on a touring machine developing about one-quarter of the power; that on the 250 c.c. twin Adler racer, for instance, is only about 5/8 in. diameter.
Representative port timings for an engine designed on these lines are: exhaust opens (and closes) 80 degrees from b.d.c.; inlet opens (and closes) 68 degrees from b.d.c. The top edges of the exhaust ports should be square to the axis in order to achieve a strong Kadenacy effect in the blow-down period of 12 degrees thus provided.
The trouble with these engines is the old one of lack of flexibility. Maximum power may be in the region of 9,000-10,000 r.p.m., but when there is little useful power available at a mere 2,000 r.p.m. less, when the resonant conditions upon which the high power depends cease to exist. Nevertheless, a lot may still be learnt by a modified application of this principle to sports engines.
An interesting recent development aimed at spreading the power over agreater range is the "boost-port" system devised by J. Ehrlich* (Fig. 13.6). In one form, two small booster

 chambers, lying between the transfer and exhaust ports, are first filled with fresh mixture through "window" ports cut in the piston below the bottom ring. Further movement of the piston seals off the chambers which then discharge their contents into the cylinder when their openings are uncovered by the piston a few degrees before b.d.c. This is a variant of a system used on the M.Z., in which a third port is located between the two transfers and opposite to the exhaust. This port is the opening of a booster-chamber which is filled via a window-port in the piston, the resulting flow of cool mixture helping considerably to ventilate the interior of the piston in which little gas movement normally occurs.

٭  Dr Joe Ehrlich lived in England where the press frequently wrote about him as a two-stroke engine designer. A German-speaker, originally from Austria, Ehrlich organised with Kaaden a form of technology transfer. Kaaden secretly gave Ehrlich old design work in exchange for racing parts he could not afford to buy.