A Point About Two-Stroke Scavenging

Discussing blower scavenging on two-stroke motorcycle engines. (Robert Martin/)

Fellow enthusiast Jim Anderton writes to speculate about whether blower scavenging, as opposed to the usual use of a two-stroke motorcycle engine’s crankcase as a scavenge pump, would have worked on the two-stroke bikes of the 1960s through the mid-’80s.

The answer is a definite yes; two-stroke cylinders don’t care where their fresh air or fuel-air mixture comes from as long as there’s enough to do a proper job of chasing the exhaust from the previous cycle out of the cylinder while refilling it.

On typical 1990s-era Yamaha, Suzuki, and Kawasaki two-stroke motorcycle engines the crankcase was used as the scavenge blower.

On typical 1990s-era Yamaha, Suzuki, and Kawasaki two-stroke motorcycle engines the crankcase was used as the scavenge blower. (The SB Image/)

In fact, prior to the writing of new FIM competition rules in 1947-49, blower scavenging was considered essential for proper two-strokes. The German firm DKW built a series of very loud and fuel-hungry two-stroke racers in the 1930s employing separate large-volume piston blowers at first, then rotary blowers.

The long-serving 71-series two-stroke Detroit truck/bus diesels employed four exhaust valves above each cylinder, with a ring of fresh charge ports exposed just above the piston at bottom dead center (BDC). Those fresh charge ports were supplied with fresh air by a rotary Roots blower. As the piston descended on its power stroke, at a point the exhaust valves would open, allowing time for the spent exhaust gas to leave the cylinder before the ring of fresh charge ports were exposed by piston motion to begin the process of refilling the cylinder with air.

On the familiar Yamaha, Suzuki, and Kawasaki two-stroke motorcycle engines of the late 20th century, the crankcase was used as the scavenge blower. As the piston rose on compression, the partial crankcase vacuum beneath it allowed fresh charge to enter the crankcase through a carburetor and some form of intake valve; reed, rotary disc, or piston-controlled. Once the crankcase was filled, the piston descended, raising crankcase pressure to drive the fresh charge up around the piston through two or more “transfer” ports and into the cylinder. This process of chasing out exhaust gas while refilling the cylinder with fresh stuff is called “scavenging.”

Because the FIM had banned supercharging for postwar motorcycle racing, reasoning the simpler the formula, the fuller the grids, separate scavenge blowers for two-strokes were no longer legal. But because the top and bottom of the piston sweep out the same displacement, the result was charging, but not supercharging. That was legal.

No one at the time had the slightest idea that by 1975, simple crankcase-charged two-strokes would dominate every FIM roadracing class, including sidecar.

Related Content: Two-Stroke Engines: Defining Their Purpose

But the answer to Mr. Anderton’s question is a definite yes. In the early 1990s, Toyota developed a six-cylinder two-stroke with blower scavenge and Detroit-like exhaust valves in the head. It was said to have been a real torque monster, far outperforming the usual four-stroke V-8s, because it fired twice as often. Since the crankcase no longer acted as a scavenge pump, it could be lubricated in the conventional four-stroke way—by a pumped recirculating oil system—and employ durable plain journal bearings.

Two-stroke streetbike engines disappeared from our showrooms in the 1980s because, being crankcase-scavenged by fuel-air mixture, the inevitable loss of some fresh charge out the exhaust port during scavenge released a lot of unburned hydrocarbons into the air. But by scavenging with pure air and then injecting the fuel later so that none could be lost out the exhaust, direct fuel injection (DFI) and transfer-port injection (TPI) two-strokes instantly became very low emitters. That’s why the automotive industry was interested in them.

Because some exhaust gas is naturally recirculated in two-strokes, their combustion is cooler than four-stroke combustion, stopping most generation of nitrogen oxides (NOx) at their source. The result was a simple, low-bulk, low-emissions engine.

Related Content: Lightness And Simplicity Are The Basic Appeals Of A Two-Stroke Motorcycle Engine

According to the late Professor Blair, the automotive world abandoned the idea of building such DFI or TPI two-strokes for two major reasons. Firstly, no one can predict what the clean-air agencies of nations will want next from the automotive industry. Therefore it is better to stay with four-strokes, the majority solution, and be able to use whatever emissions-reduction technologies the majority come up with rather than to go it alone with a different technology. And secondly, at least in Detroit, the majors insisted upon developing their two-stroke tech from zero rather than hiring experienced two-stroke engineers. As soon as the expense of doing it that way rose to a certain level, red lights flashed in the accounting departments and the projects were cut off.

At one time in the early/mid-’90s I counted 26 two-stroke engine development projects within the world automotive industry. About the only legacy of that work is the fine-droplet-size fuel injection technology now being used in gasoline direct injection four-strokes (GDI).

As you inspect your latest consumer purchases and see the familiar words “made in China,” consider that the giant container ships bringing this flood of low-priced manufactured goods to our shores are powered by enormous two-stroke marine diesels employing separate blower scavenging. Because of their high compression ratio, low pumping loss, and modest friction (80 rpm is typical) they deliver just about double the fuel efficiency of conventional four-stroke auto engines.

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