How does a two-stroke engine work?

How the parts work together

1. Spark and power

   The spark fired at 340°, 20° before top dead centre, so at 15° the charge is burning (yellow) and its pressure, about 34 bar in the idealised chart, drives the piston down. The crown covers the exhaust and transfer windows, so the cylinder is sealed. Below the piston the skirt still leaves the intake open until 80°. At speed the moving mixture keeps flowing in for a while, but the falling piston also pushes some back, so the model draws the intake flow thinning toward 80°. Then the crankcase is sealed.

2. The exhaust opens first

   At 94° the crown passes the roof of the exhaust window and the burnt gas, still at about 3.5 bar in the chart, rushes out (orange). The transfer windows sit lower and stay covered for another 29°. In that time the cylinder pressure falls toward the crankcase pressure: in the chart the dark cylinder line drops to meet the teal crankcase line near the dashed line where the transfers open. If the transfers opened first, burnt gas would blow down into the crankcase. Meanwhile the falling piston squeezes the mixture in the sealed crankcase, to about 1.4 bar.

3. Fresh mixture loops in

   From 123° the transfer windows are uncovered (tinted blue) and the squeezed crankcase charge flows up the passages (blue lines; the cut opens the near passage from the crankcase up). In the idealised chart the cylinder is then at about 1.6 bar, just above the crankcase, and drops below it 5° later. Jennings says the cylinder must be near atmospheric pressure by then; real engines get there with help from pressure waves in the exhaust pipe, which this calculation leaves out. Real transfer ports are shaped to aim the mixture at the rear wall, as the lines show; the model's windows are plain cuts. The mixture rises up that wall, turns under the head and comes down toward the exhaust, pushing the burnt gas ahead of it. This is loop scavenging. The colours are drawn, not calculated.

4. The exhaust closes last

   Piston ports open and close at the same distance either side of bottom dead centre, so the transfer windows close at 237° but the exhaust not until 266°. Once past bottom dead centre the rising piston works against the transfer flow, which fades, and it pushes some of the new charge out of the exhaust, one reason two-strokes use more fuel. The crankcase is sealed again and growing, so its pressure falls below the outside air's.

5. Compression and intake

   With every cylinder window covered, the trapped charge is squeezed. Below, the skirt rose past the intake floor at 280°, and the crankcase pressure, down to about 0.8 bar in the chart, draws fuel and air in through the carburettor. At 340° the spark starts the next power stroke: one revolution, one power stroke.

The main parts

Cylinder barrel

A finned barrel with a 70 mm bore. The piston slides in the bore, and the ports are real windows cut through its wall. The fins pass heat to the air, and a thin gasket seals the barrel to the crankcase.

Cylinder head

Closes the top of the bore. A flat squish band, 1.2 mm above the crown at top dead centre, rings a part-spherical bowl. The 26 cc left above the crown at the top of its stroke is the combustion chamber.

Spark plug

In this model it fires 20° before top dead centre, so the charge is already burning as the piston reaches the top. The yellow flame is drawn; the pressure chart's burn is an idealised calculation, not a combustion model.

Piston and rings

The crown takes the gas pressure and also works as the valve for the exhaust and transfer ports. The lower edge of the skirt opens and closes the intake port. Two rings seal it in the bore, and a hollow gudgeon pin joins it to the rod.

Connecting rod

A rigid link, 120 mm between centres, from the gudgeon pin to the crank pin. Needle rollers sit in both eyes.

Crankshaft

Two full-circle flywheels joined by the crank pin, 32 mm off the axis, so the piston travels 64 mm. They fill most of the crankcase, which keeps its compression ratio near 1.5 : 1. It turns clockwise in the default view, the way a motorcycle's wheels turn. The holes near the pin balance the flywheels. The cut-away leaves out the flywheel nearest you so the crank pin and big end show.

Crankcase

Ball bearings and oil seals close it around the shaft, so the space under the piston works as a pump. It fills with mixture while the piston rises and the intake is open, then is squeezed when the piston comes down. In the cut-away the half nearest you is removed, except the corner of its top deck under the barrel's kept quarter, where the near transfer passage opens into it.

Exhaust port and pipe

The largest window, in the front wall. It opens first on the way down and closes last on the way up. Behind the window the duct turns from a rectangle into the round bore of the pipe. The Exhaust port height control sets its roof, measured from the piston crown at bottom dead centre: 25.5 mm by default.

Transfer ports and passages

Two windows in the side walls, each fed by a passage from the crankcase. They open after the exhaust. Real ones are shaped to aim the fresh mixture at the rear wall, as the flow lines show; here they are plain cuts. The cut opens the near passage from the crankcase up.

Intake port and carburettor

A window low in the rear wall. The skirt uncovers it near top dead centre, and the carburettor feeds fuel and air through it into the crankcase. Behind the window the duct rounds into the bore of the rubber boot.

Gas colours and flow lines

Drawn, not calculated. Blue is fresh fuel-air mixture, orange is burnt gas, yellow is burning gas, teal is mixture drawn into the crankcase. The dashed lines show which way gas moves through each open port and thin out late in a port's opening. Tinted windows show how much of each port the piston has uncovered. The crankcase charge looks denser as the computed crankcase pressure rises.

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