How the Two-Stroke Thing Got Going

Admin · March 30, 2023

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Right after World War II the quintessential two-stroke was DKW’s three-speed commuter bike, the RT-125. Its feeble 4.5 hp provided the most basic transportation. Not only did the war sprinkle RTs all over Europe, but after the war, RT-inspired basic bikes—like BSA’s Bantam and Harley’s Hummer—were produced by makers all over the world. That made the RT-125 the most-copied of all motorcycles.

I have found that many people with workable four-stroke understanding are unsure just what a two-stroke is. A two-stroke is just what its name says: an engine that requires two piston strokes rather than four to accomplish the four necessary engine functions—intake, compression, power, and exhaust.

What a Two-Stroke Is

Not all two-strokes have ports through their cylinder walls (anyone remember the boardtrack four-strokes that had a ring of exhaust ports through the cylinder, exposed at BDC). Not all two-strokes use their crankcase as a charging pump, to deliver air or mixture into the working cylinder. The two-stroke “Detroit Diesel” that once powered many heavy trucks used a Roots blower as a charging pump, while the immense marine diesels that power international shipping charge their cylinders with exhaust turbo-blowers. Both types employ conventional poppet exhaust valves located in their cylinder heads. What identifies a two-stroke is only this: that each cylinder fires every time it passes through top dead center.

Before WWII supercharged two-stroke racing motorcycles had been fast, loud, and thirsty. In the 1949 meetings that set the basis for postwar racing, supercharging was banned as too expensive for war-torn Europe (DKW had in the later 1930s used large piston charging pumps, then switched to rotary blowers). The simpler system of crankcase scavenging (in which the underside of the piston pumps the air supplied to the cylinder) was clearly not supercharging because, top or bottom, the piston’s motion swept out equal volumes. This system was “charging” but definitely not supercharging.

The state of such simple crankcase-scavenged two-strokes at that time was pitiable—DKW’s RT-125 was not even remotely seen as a potential threat to “real engines”—four-strokes. Early efforts at tuning such simple engines for racing soon hit a barrier at about 8 hp and 7,500 rpm.

How Simple Two-Strokes Work

First let’s consider the workings of such simple engines, leaving aside for the moment the role of the crankcase as a charging pump.

Imagine that the engine has just fired and the fuel-air charge has burned to peak combustion pressure at around 10 degrees after top dead center (ATDC). Now the piston accelerates downward, the pressure of the combustion gas on it being transmitted to the rotating crankshaft by the connecting rod.

A simple yet effective drawing of a two-stroke. (Cycle World Archives/)

When the piston has traveled a bit over half-stroke, the pressure of the combustion gas has fallen low enough that little further energy can be recovered from it. Now the top edge of the piston begins to uncover an oval exhaust port in the cylinder wall. Residual exhaust pressure (of maybe 50–75 psi) now rushes out the port (German engineers called this “auspuff”).

Once cylinder pressure has fallen in this way low enough that fresh charge can be pumped in, a pair of transfer ports begin to be exposed by the still-descending piston. In the simplest engines there are two such transfer ports, one on either side of the single exhaust port, and aimed diagonally away from it, across the piston crown, their flows of fresh mixture converging just before hitting the opposite (non-exhaust) cylinder wall. The mixture stream is deflected upward as it hits the cylinder wall, and is then deflected twice more when it rises to hit the head, flows across its underside and, hitting the cylinder wall above the exhaust port, is deflected downward.

I have described this “scavenging loop” in some detail because every aspect has a function. Aiming the transfer ports at the opposite, non-exhaust cylinder wall makes it harder for fresh charge to reverse direction and leave the cylinder through the exhaust port. As the fresh charge travels this loop, it is to a degree protected from entraining and mixing with residual exhaust gas by being always attached to a surface. At first, that surface is the crown of the flat-topped or slightly domed piston. Then it is the non-exhaust cylinder wall, then the underside of the head, and finally, as the stream turns downward toward the exhaust port, it is the exhaust cylinder wall.

Ideally, this stream of fresh charge would act like a “piston,” pushing exhaust gas ahead of it without mixing with it. This ideal cannot be achieved; some fresh charge is always lost out the exhaust, and this is the reason for the two-stroke engine’s high emissions of unburned hydrocarbons (UHC).

How much is lost? A good four-stroke engine burns 0.5 pound of fuel per horsepower per hour, while a crankcase-scavenged two-stroke burns 0.65 lb./hp-hr, representing a waste of about 30 percent of the fuel. Today this is considered scandalous, and is the reason two-stroke streetbikes ceased production after the mid-1980s. But in the beginning, the 40 percent lower weight and smaller bulk of the two-stroke engine recommended it for applications like chain saws, portable pumps, outboard motors, and lightweight motorbikes. Although today there are technologies capable of making two-strokes remarkably clean-burning, it is seen as economically safer to build four-stroke motorcycles equipped with exhaust emissions technologies already developed by the auto industry.

The end of mass-produced two-stroke streetbikes came quickly in the mid-1980s (Mark Romanoff at English Wikipedia, via Wikimedia Commons/)

As the piston reaches bottom center (BDC) two things continue to happen in the cylinder. Exhaust gas continues to rush out through the exhaust port, and fresh charge continues to enter through the transfer ports. As the piston rises, it first closes the two transfer ports, then a few degrees later, the exhaust port.

Now compression can begin. At about 20 degrees BTDC, the timed ignition spark occurs, the mixture ignites, and the above process repeats.

As you can see, everything in two-strokes depends upon timing: that the exhaust port opens soon enough to reduce cylinder pressure in time for transfer of fresh charge to begin. That the fresh charge traveling around its guided loop doesn’t arrive at the still-open exhaust port so soon that much of it is lost to the exhaust. Workable designs were arrived at by experiment.

The Crankcase as a Scavenging Pump

The word “scavenging” deserves a few words. This is the process by which the entering fresh charge chases the exhaust out of the cylinder while it is refilling it. Clearing the cylinder of exhaust obviously depends upon how much mixture you can blow through it. If there is a bad smell in the kitchen, the more windows you open, the faster the smell dissipates.

Now consider the crankcase. A carburetor feeds fuel-air mixture through a short duct to an inlet port low in the cylinder, which is opened and closed by the lower edge of the piston skirt. Typical timings for this piston-ported valving system are inlet opens 60–75 degrees BTDC, inlet closes 60–75 degrees ATDC. Port timings controlled by the piston’s movement are symmetrical.

As the piston rises, it creates a partial vacuum in the crankcase (which is sealed, both from atmosphere and from any other cylinder’s crankcase adjacent). When the piston skirt’s edge rises enough to open the inlet port, air rushes in through the carburetor, picking up fuel on its way, and enters the crankcase. Because this inrush results in high inlet velocity, little fresh charge will be lost back out the inlet port when the piston passes TDC and begins to descend.

After the closing of the inlet port by the descending piston, crankcase pressure rises as the mixture in it is slightly compressed. When the transfer ports described above begin to open, the compressed mixture in the crankcase flows up through transfer ducts to enter and scavenge/refill the cylinder. Transfer ports are so named because they transfer fresh charge from the crankcase to the working cylinder.

At idle or low rpm, the timing of all these processes is all wrong, so it may take two, three, or even more revolutions of the crank before enough mixture accumulates in the cylinder to be spark-ignited. This is the cause of the two-stroke’s irregular firing at such times, earning the name “ring-ding.”

The above explains why crankcase-scavenged two-strokes at first had to burn fuel into which a percentage of oil (2–5 percent) had been mixed. With fuel and air rushing through the crankcase, the usual pumped recirculating oil system of a four-stroke could not function. Also, all engine bearings were of the rolling element type, which survive with very little lubrication. Later, with the coming of automatic two-stroke lubrication pumps injecting oil into the crankcase, mixing of oil and gasoline was no longer necessary.

What Made Early Two-Strokes Uncompetitive?

Late 1940s two-stroke tuners were trapped between two conflicting goals. To reduce cylinder pressure enough that cylinder filling could begin through the opening transfer ports, they needed to open the exhaust port earlier. But opening the exhaust earlier also meant that it closed later, allowing more time during which fresh charge that had traveled around its loop in the cylinder to escape through the exhaust port.

As a result, 125cc two-strokes were limited to about 8 hp at around 7,500 rpm.

They needed a way to close the exhaust port before the leading edge of the fresh-charge loop could reach it. Mechanical valves weren’t practicable—that required operating any such valve twice as often as in four-stroke engines.

Using Sound Waves as an Exhaust Valve

Fortunately, engineers developing two-stroke diesels had already tackled this problem and had concluded that a strong sound wave, created in the exhaust pipe by the sudden opening of the exhaust port, could be reflected back by a correctly shaped exhaust pipe to arrive just as the fresh charge loop was arriving at the exhaust port and starting to escape from the cylinder. Instead of a fast-acting mechanical valve, they proposed using precisely timed sound waves.

Just at the end of the 1951 season, Erich Wolf at DKW in West Germany succeeded in applying this idea. In addition to the usual exhaust of that time—a plain pipe from the cylinder’s exhaust port, ending in a megaphone—he added a convergent counter-cone (gegenkonus in German) just downstream of the megaphone. Experimentation was necessary to get the lengths right, but when the new pipe worked, the pressure wave from exhaust opening traveled down the head pipe, created a negative reflected wave in the megaphone (helping to pull exhaust from the cylinder), and next hit the convergent reflector cone behind it. That sent a positive wave reflection back up the pipe, arriving at the exhaust port just as fresh charge was reaching it and starting to escape.

The positive wave not only stopped fresh charge from escaping, it could also stuff any already-lost charge back into the cylinder just as the exhaust port was closing.

It would take time, work, and thought to get the best from this concept, which in effect reflected back exhaust pulse energy in the pipe to act as an exhaust valve, preventing the loss of fresh charge that a simple pipe-and-megaphone permitted.

This change put to work formerly wasted pressure energy in the exhaust, making it act as an exhaust valve, and potentially also as a sort of low-grade acoustic supercharger—pushing charge just lost out the exhaust port back into the cylinder, just as the piston was closing the exhaust port.

This was a powerful concept, joining a resonating duct (like that of a WWII German V1 “buzz bomb”) to a piston engine. The rapidly varying pressures in the duct were fast enough to help with the two-stroke problem of not having enough time to carry out all four necessary processes. Exhaust sound waves, moving at up to 2,500 feet per second in hot gas, could get things done quickly. And the resonance in the duct was kept going by the engine’s succession of powerful (around 100 psi) exhaust pulses.

The first part of the exhaust pipe is called the “header.” On some quite early engines the outward-tapering megaphone began right at the port, but this was found ineffective. The reason is that the initial release of exhaust from the port is sonic, forming a stationary shock through which no signal from downstream can propagate into the cylinder. That means there is no point in starting the megaphone taper at the port. Modern headers are therefore cylindrical or slightly tapered (zero to about 2-1/2 degrees) acting as a storage line for the exhaust pulse until the port outflow becomes subsonic. Then a return negative wave from a megaphone can reach the cylinder to assist in clearing exhaust gas and reducing pressure there to assist the inflow of fresh charge from the transfer ports.

We know from history that it was the East Germans who first challenged four-strokes—not DKW in the west. Why not?

I suspect that Wolf at DKW did not immediately see that the use of a reflected positive wave from the new counter-cone would allow him to open the exhaust port radically earlier.

The Rotary Disc Intake Valve

The East German challenge began with Daniel Zimmermann, who was working with highly modified DKW RT-125 engines. Using the piston skirt to open and close an intake port was a compromise because such a port opens and closes at the same timings. The earlier you open such a port, the later it must close—allowing charge loss at lower rpm. He therefore incorporated an idea previously used on the 1920s Sun-Vitesse motorcycle—a crankcase intake port controlled by a rotating disc on the crankshaft, cut away to open and close that port. With it, opening and closing timings could be set at any desired values. Typical timings were intake opens 125 BTDC, intake closes 65 ATDC.

The DKW RT-125 used a rotating disc on the crankshaft with a cutaway to open and close the intake port. (Huhu Uet, CC BY-SA 3.0 via Wikimedia Commons/)

No Good Deed Goes Unpunished

Zimmermann’s success embarrassed less imaginative state-owned manufacturers, so his reward for achievement was to be ordered by East German State Security to hand over his work and prototypes to IFA (later renamed MZ) and to keep his hands off of motorcycle two-strokes in future. He turned his attention to outboard racing. Appointed to manage IFA’s racing was Walter Kaaden.

Integrating the Two-Stroke’s Systems

Although Kaaden is widely credited with creating the modern two-stroke motorcycle engine, I believe his role was not so much that of inventor as of making several already-existing elements into an integrated system. By 1958 MZ’s 125 was making 20 hp—fully competitive with Italy’s refined Mondial and MV four-stroke singles. A 250 twin gave 48 hp. The remarkable performance of these bikes was prevented from winning world championships by unreliability.

Japan Applies Industrial-Strength R&D

Japanese two-stroke manufacturers, seeing this success, applied modern scientific methods to the problems of the Zimmermann/Kaaden synthesis and rapidly advanced the state of the two-stroke arts.

A two-stroke’s hardest-worked part is its piston—because it is heated twice as often as is a four-stroke’s piston. The big problem of the early 1960s was to combine in a single aluminum alloy the low thermal expansion that could prevent piston seizure while displaying enough hot strength to avoid either fatigue cracking or heat deformation (skirt bending or forging-down of piston ring lands).

Two-Strokes Begin to Win Championships

Suzuki was first to win a world championship with the new technology: the 50cc in 1962. Why was this first achieved in the smallest class? Because the smaller the piston, the more favorable the relationship between its heat-gathering area (the crown of the 40mm piston) and the heat outflow area of the piston’s contact with the cooler cylinder wall.

1965 Yamaha RD-56. (By Rikita (Own work) [CC BY-SA 3.0 or GFDL, via Wikimedia Commons/)

The next year, Suzuki added the 125 championship, and in 1964–65, Yamaha’s air-cooled RD-56 250 twin, its two carburetors projecting to right and left in MZ fashion, took 250 championships.

By 1975 every FIM GP roadracing class including sidecar was won by two-strokes. Traditionalists were puzzled and offended that what they considered cheap-and-nasty “ring-dings” could leave behind the sophisticated four-strokes they loved.

Nostalgia for Those Unrideable 500s

Yet many older fans today pine for the heroic “cowboy riding” of the two-stroke era, when the explosive hit of two-stroke power made every corner exit a potential highside.

Yes, we still lament the end of the two-stroke 500 beasts of grand prix competition. (Sport Rider Archives/)