Going deeper8 min read5 / 9

Two-strokes: ports & scavenging

A two-stroke fires every single crank revolution — twice as often as a four-stroke — by making one radical trade: it throws away the two dedicated breathing strokes and swaps gas while the power stroke is still finishing. The result is the best power-to-weight of any simple piston engine, paid for in fuel that escapes unburned.

Ports instead of valves

The classic small two-stroke has no valvetrain at all. Holes — ports — are cut in the cylinder wall near BDC, and the piston itself covers and uncovers them as it sweeps past. In the demo's timing (crank degrees after firing TDC, one 360° cycle):

$\text{EO}$: exhaust port opens (blowdown begins) [110°]
$\text{TO}$: transfer ports open (fresh charge in) [122°]
$\text{TC}$: transfer closes [238°]
$\text{EC}$: exhaust closes — trapped compression starts [250°]

Note the symmetry about BDC (180°) — a piston edge cannot open a port at a different angle than it closes it. The exhaust port sits higher in the wall than the transfer ports, so it opens first: cylinder pressure blows down for a few degrees before the fresh charge arrives, otherwise exhaust would blast backwards into the intake system.

Fig. 1. Near BDC with both port sets uncovered. The descending piston has pressurised the crankcase, which pushes fresh charge (green) up the transfer duct; the stream is aimed up the wall away from the exhaust, loops across under the head and sweeps the burnt gas (orange) out. The dashed arrow is the price: some charge skips the loop and short-circuits straight across to the exhaust port.

Scavenging — and the short-circuit problem

For roughly 130° of crank around BDC, intake and exhaust are open simultaneously. The incoming charge must physically push the burnt gas out — a process called scavenging — without either mixing into it or following it out of the port. Loop scavenging (Fig. 1) aims the transfer streams up the back wall so the flow makes a U-turn, sweeping the chamber from the top down. It mostly works. What escapes anyway is the two-stroke’s defining loss:

Trapping efficiency — the fraction of delivered charge that is actually in the cylinder when the exhaust port shuts — runs 65–80% for a simple carburetted two-stroke. The remainder, raw fuel included, goes straight out the exhaust. This is why old two-strokes smell of petrol and why emissions law killed them in cars and most motorbikes.
The crankcase does the pumping: the descending piston pressurises the sealed crankcase, which then blows the charge up the transfer ducts. The oil that would normally sit in that crankcase must therefore be mixed into the fuel and burned — the blue smoke.

The effective compression ratio

Compression cannot start until the exhaust port closes at 250°, so the gas is only squeezed from the trapped volume, not from BDC:

r_{\text{eff}} = \frac{V(\text{EC})}{V_{\text{TDC}}} \;<\; r_{\text{geom}} = \frac{V_{\text{BDC}}}{V_{\text{TDC}}}

(1)

The demo computes its compression exactly this way — from the port-close volume — which is why its two-stroke PV loop is visibly shorter and lower than the Otto loop beside it, even at the same geometric ratio. The thermodynamics that follow are pure Otto (fast premixed spark burn), just applied to less charge over less stroke.

Twice the bangs ≠ twice the power

Naively, firing every 360° instead of every 720° should double specific power. Reality delivers perhaps +50–70%: the effective stroke is shortened by the ports, scavenging is imperfect, and some residual exhaust always stays behind. Still, per kilogram there is nothing simpler that comes close — which is why chainsaws, outboards and model aircraft, where mass matters more than fuel economy, remain two-stroke strongholds.