Due Tempi & Le Espansioni Theory

How Two Strokes and Expansion Chambers Work

two-stroke_c (530K)

(I think this fabulous animation was created by Joseph Schuster. It doesn't show a Rumi engine, but this is a similar design.)

A two stroke ("due tempi" in Italian) is an engine in which each piston moves 2 stokes for each firing of the spark plug. Two stoke engines do not have valves or the associated cams. The intake of air and fuel and exhaust are controlled by ports in the cylinder walls that are covered and uncovered by the movement of the piston. Compared to 4 stroke engines, 2 stroke engines are light (no valve train) and they fire twice as often creating more power than a similar 4 stroke. Additionally, oil is mixed with the fuel (called premix) and is used to lubricate the crankcase. The animation above illustrates the engines function.

When the piston moves toward the spark plug (up), the following things happen: Below the piston, air and fuel (along with the premixed oil) are sucked into the crankcase through the intake port uncovered by the piston. The oil in the fuel lubricates the big and small ends of the crank, cylinder walls and other bearings in the crankcase. Above the piston the waste gasses are forced out the exhaust port, and after that port is closed off by the rising piston the charge is compressed for ignition.

After ignition the piston moves away from the spark plug (down) and the following things happen: The power generated by the ignition of the charge turns the crank. Above the piston the spent charge starts to escape out the exhaust port as it is uncovered by the piston. Below the piston the fuel and air mixture in the crankcase is being compressed. The intake port opens next as the piston uncovers it and the air and fuel is forced through the intake port into the combustion chamber. This charge being forced into the combustion chamber also pushes the spent charge out the exhaust port.

Now if that all makes sense, it may be clear that there is inefficiency with the intake and exhausts ports: both happen to be open at the same time during some of the process! Why doesn't the intake charge just slide across the piston and out the exhaust port? Well, to some degree it does. Note that Rumi's have a deflector on the piston crown; a ridge that is designed to divert the charge some and help prevent the intake from just sliding through the combustion chamber. But this is where the expansion chamber exhaust comes into effect. The expansion chamber is designed so that at certain RPMs the exhaust gasses resonate just right and help suck out the spent gasses and then reflect back and compress the charge back into the combustion chamber! This is what two stroke riders mean when they say the engine "comes onto the pipe" at the right RPM the engine becomes much more powerful as the expansion chamber starts to help the engine run much more efficiently. Obviously, this effect can greatly improve a two stroke engine's performance!

Note that the engine in the illustration above has a reed valve between the carburators and the crankcase. This valve prevents the intake charge from escaping out the carburators. Rumi's do not have this valve and this explains why Rumi's never rust behind the engine: they are always covered by a thin film of oil that escapes the carburators!

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