Answers to Two-Stroke Questions

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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/)

In response to my column “A Point About Two-Stroke Scavenging,” which appeared on the CW site about a month ago, a number of folk have commented on various past levels of two-stroke technology.

Nealmeal 2000 mentions that the crankcases of modern two-strokes aren’t as “tight” (as small in volume) as they could be, speculating that this might make them less efficient as scavenging pumps.

It’s true that in the early 1960s people were in love with tight crankcases. They made con-rods almost as thin as knife blades—and then machined and attached displacer plates to the inner faces of crankcase flywheels to fill up some of that volume. The late Ron Grant, who had some good rides on Suzuki, marketed displacer rings that partly surrounded cut-down flywheels, taking up more volume in crankcases. I corresponded for a time with English racer Dave Simmonds, who in 1966 kindly sent me some tuning information for a Tohatsu 125 twin LD3 I then owned. Following his directions, I dutifully made up flywheel displacer plates and screwed them in place.

Some engines, in addition, had “deck plates” that covered the top of the crankshaft under the piston, leaving only a narrow rod slot. They even had displacer fingers that poked up inside the piston at BDC.

But as tuners learned how to use the negative and positive waves propagating in exhaust pipes, they found that supertight crankcases (some with compression ratios as high as 1.75) were reducing power, not boosting it. After further research, they realized that the long low-pressure wave generated by the megaphone part of the pipe (its divergent horn) was furiously sucking on those tiny, low-volume crankcases and couldn’t get much out of them.

This resulted in a long process of increasing crankcase volume and making more power because exhaust pipes could pull more mixture out of a larger-volume case. What had happened was that the pipe had become the real force behind scavenge flow. Yamaha, in a 1968 paper, said it had aimed at the time for a 1.5-to-1 crankcase compression ratio (meaning that case volume with piston at TDC was 1.5 times bigger than case volume with piston at BDC). When in 1972 that company built both the RD250 and RD350 twins on the same crankcase, the 350 had the high 1.5 ratio, but the 250 worked quite well at its lower 1.35 ratio. After that demonstration that you can suck more air out of a big case than you can out of a small one, here came the reed valve revolution of 1984, when the crankcase compression ratios of reed engines fell as low as 1.15. And that, ladies and gentlemen, is the saga of the large-volume two-stroke crankcase.

Even beyond that was the discovery by the late Professor Gordon Blair that exhaust pipe suction could cause reed intake valves to reopen after bottom center, allowing the pipe to pull mixture directly from the carburetor through the crankcase.

Transfer Port Fuel Injection

Reader Motorsyklist mentions the transfer port fuel injection used on KTM two-strokes.

Making two-stroke direct fuel injection work (DFI, right into the combustion chamber) wasn’t easy because there is so little time for fuel droplets to evaporate between the closure of the exhaust port and point of ignition at about 20 degrees BTDC (about 1/6 of a crank revolution). Such injectors were more expensive than the four-stroke car injectors you can buy at the NAPA store. The Orbital DFI system achieved droplet sizes in the 10 micron range, and the later Bombardier Evinrude Ficht system is said to make 40 micron droplets.

Someone then said, “Why not get more evaporation time by spraying the fuel into the transfer ports (where air velocity is very high, favoring quick droplet evaporation), timing the injection to be carried by a part of the air charge that won’t reach the exhaust port before it closes?”

This too has been successful in dramatically reducing unburned hydrocarbons in two-strokes.

Megaphone-Style Two-Stroke Exhaust Pipes

Reader cc roselle mentions the Horn of Plenty short-megaphone exhaust pipes that were used on some early Montesas, and I recall similar “blooey pipes”: short chrome megaphones run on the Italian 125 twin from Moto Rumi. Talk about loud!

Alas, the bikes were slow. Why? Megaphones reflect a negative sound wave back to the exhaust port, so if the exhaust port was opened early enough to make the engine work at higher revs, the pipe would pull much of the fresh charge into the pipe, reducing engine torque.

It takes time for cylinder pressure to drop far enough to let fresh charge enter through the transfer ports, but the more you advance exhaust opening ahead of transfer opening to achieve this, the longer the negative sound wave acts to pull much of that charge into the pipe. So in the period of the late 1940s to mid-1950s, two-strokes were trapped between these two effects. That held the peak revs of a 125 cylinder to 7,500–8,000 and peak power to the range of 8–12 hp.

The solution came from DKW engineer Erich Wolf’s 1951 invention of the baffled or “counter-cone” two-stroke pipe, which Gordon Jennings and others thereafter called an “expansion chamber.” By adding a convergent horn after the megaphone, the pipe sent back to the exhaust port a final positive wave that, when timed right, stopped or even reversed the outflow of fresh charge into the exhaust pipe. Think of this as a sonic supercharger.

It took time to work out just how tall an exhaust port was practical to use with such a pipe. But by 1962, with a lot of people working on the problem, a Suzuki with a counter-cone pipe was able to win the 50cc world championship, the first ever by a two-stroke. The work continued, with power rising every year, until by 1975 every FIM roadracing class, including sidecar, was won by two-strokes. By the time two-strokes were replaced by four-strokes to begin the MotoGP era, a 125cc two-stroke cylinder could deliver 55 hp at around 13,000 rpm.

Opposed-Piston Two-Strokes Rise Again?

Reader Celeste calls our attention to the current efforts to revive a very old idea: That of placing two pistons in a single cylinder, forming a combustion chamber between the crowns of the two pistons when at top center. One obvious advantage of this arrangement is that it eliminates the heat loss through cylinder heads, because there are none.

The big noise in this field at the moment is San Diego-based Achates Power, who in partnership with Cummins are developing an opposed-piston diesel powerplant for the US Army.

Many such engines have been successful in the past, but a cost of doing things this way is the need for either two crankshafts connected by shafts or gears, or for rocking levers or long operating rods extending from a single crankshaft, to move the opposing pistons.

Another difference is that while conventional diesels employ centrally located fuel injectors, opposed-piston engines must spray their fuel in from the sides. Can the latter be developed to work as well as the former? A three-cylinder, six-piston Achates opposed-piston diesel with geared-together twin crankshafts has given promising results in a Ford pickup truck.

For further reading, search the internet for the following: Fairbanks-Morse. It has long manufactured dual-crank opposed-piston two-stroke diesels for rail locomotives, submarines, and power generation. In Britain, the unique Napier Deltic rail and marine engine operated on similar principles. In Germany, Junkers in the 1930s and ’40s produced the Jumo 205 opposed-piston six-cylinder aircraft diesel for the Ju 52 transport plane. Truck and bus engines made by Commer in Britain and Sulzer in Europe have employed one crankshaft moving both sets of pistons via rods and rocking levers. Doxford in England built large marine diesels with a single crankshaft operating the near set of pistons through conventional con-rods, and the far set through long operating rods driven by the same crank.

These engines are all two-stroke because the compactness of the combustion space between opposing pistons leaves no room for the poppet valves used in four-strokes.

In these engines, one piston of each pair controls a set of cylinder wall exhaust ports while the other controls later-opening fresh charge ports served by a blower.

MZ engineer Walter Kaaden built and tested an opposed piston engine in his drive to make a success of two-strokes in motorcycle racing. It seems to have proven too complicated for that East German (as it then was) company’s resources.

Variable Exhaust Valves

Jayfrankbird71 mentioned the variable exhaust valves that were introduced by Yamaha in 1978 as Powervalve. Their purpose was to vary the opening point of the exhaust port, “foxing” the exhaust pipe to result in a wider rideable powerband. My experience with them began with the 1981 Yamaha TZ250 production racer, which used a Watt governor (centrifugal weights acting against a spring, producing a control movement proportional to rpm) to raise and lower “eyelids” in the exhaust ports, not quite touching the pistons.

The need for such range-broadening devices existed because without them, the useful powerband of a racing two-stroke 250 twin with an expansion chamber exhaust pipe was limited to roughly 9,600 to 11,300 rpm. It would run below and above that, but only weakly, as the exhaust pipe waves were arriving either too soon or too late to assist fresh charge pumping. With the Powervalve system working as designed, useful power did exist as low as 8,500.

Later, improved over-rev power was achieved by retarding the ignition timing, thereby dumping extra heat into the pipes. The speed of sound in a gas is determined by gas temperature; the hotter the gas, the shorter the pipe acts. This enabled pipes on 125 cylinders to stay “in step” with the pistons all the way to 12,000 rpm or a bit beyond. This was useful in first-gear corners where the shift pedal could be inaccessible.

Another system, deployed occasionally by Honda in the ’90s, was to spray water into the exhaust pipes to reduce the gas temperature in them, thereby extending their pumping action to as low as 6,500 rpm. This system is familiar to serious outboard racers, who have for decades used it to achieve stronger starts.

Any time the world of ideas becomes just too confusing, we can always retreat into the virtual reality of the smartphone, where opinion rules.

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