Two-stroke 2.0

Two-stroke 2.0

And then we talk about the first half of the eighties when the simplest two-stroke principle with the piston-driven crankcase rinse (think of the once famous Jawa's) was no longer sufficient because there was not enough power to get out of it.

Getting power from two branches is not such a problem. The largest sea-going vessels also have two-stroke (diesel) engines.

A two-stroke is very sensitive in terms of power to changes to its in and exhaust system. And the plan of approach was therefore put in that corner. We are going to talk about membranes, boxes and sliders.

All membranes

A membrane is a membrane or plate that can move. Such a membrane ensures a smooth, almost imperceptible separation between two spaces or two substances. In the inlet path, such a membrane ensures that the mixture can flow into the cylinder but cannot flow back. Because that backflow is a thing of the old-fashioned two-stroke. When a two-stroke 1.0 is running, the piston opens and closes the outlet, inlet, and flushing ports. That opening and closing causes problems for the gas / mixture stream from a little more revs. Because that mixture flow has a certain mass of inertia and it must be stopped and started again and again. And that actually only works 'well' at certain speeds. Yamaha devised a tool with another membrane to make the gas flow start a little easier. The inlet channel was connected with a 'potty'. If the membrane closes, the mixture can flow to the jar. When the membrane opens, the buffered contents can go back into the inlet channel.

Controlling gas flows

Controlling gas flows also applies to the outlet side where the gases are considerably hotter. Already in 1978, Jawa - and that company has made an impressive contribution to two-stroke technology - patented an outlet port slider that controlled the opening and closing of the outlet port at the desired speeds. The thereby improved flushing resulted in fewer leakage losses and more power. Yamaha bought the patent and brought it to production maturity under the name 'power valve'. It consisted of a cylinder with 'bites out' that could rotate about its length. The exhaust port was not opened until late and at low revs. At high revs the bites cleared the way for optimum exhalation.

Honda was inspired by Yamaha's intake box system and devised the 'ATAC' (Automatic Torque Amplification Chamber) box for the exhalation side of the blocks. It worked a bit differently: At Honda, the box closed at high speeds. This resulted in the block having a more voluminous outlet at low speeds than at high speeds. The system made such a Honda block less sensitive to the correct speed.

Bombardier Rotax did it too

In Europe, Rotax made a contribution in the bag with a system with a flat slider in the shape of the main outlet channel. That valve is opened by the pressure in the exhaust channel itself. Rotax had attached the stem of the barge to a diaphragm and the slide is pushed down by a spring in the outlet channel. At the start and at low speeds the exhaust port opens so that the engine pulls well. The diaphragm is connected to the outlet port and if the pressure exceeds a certain value, the barge is pushed up. That keeps the exhalation perfect over the range of 9000-12000 rpm where the power increases to 30%.