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How the two-stroke cycle works

A two-stroke fires once every time the crankshaft turns. In the design this page describes, the piston opens and closes the ports itself, and the crankcase, the closed space under the piston, does half the work.

What this rests on

Basis
Documents held in this archive, each listed under Where this comes from
Difficulty
Intermediate

Once every turn

A two-stroke completes its cycle in two strokes of the piston, one up and one down, which is one turn of the crankshaft, and it fires once in that turn. So it has to empty out the burned charge and fill up again in the short time the piston spends near the bottom. Two words carry the rest of this page. Mixture is what the engine takes in: on most engines fuel, air and oil together, while a direct-injection engine takes in air and puts its fuel straight into the cylinder. The charge is whatever fills the cylinder to be fired.

The piston is the valve

In the design this page describes, there are no valves in the cylinder head. Instead there are ports, openings in the cylinder wall, and the piston covers and uncovers them as it slides up and down. Some two-strokes use valves in the head instead, and they are not covered here. The exhaust port leads out to the exhaust. The transfer ports are passages that run from the crankcase up to the cylinder. With the piston high, they are covered. With the piston near the bottom, they are open.

Above the piston, squeeze and fire

As the piston rises it covers the ports and squeezes the fresh charge into the top of the cylinder. Just before the top, the spark plug fires it. The burning charge drives the piston back down, and that is the power. On the way down the piston uncovers the exhaust port first, so the burned charge has started to rush out before the transfer ports open.

Below the piston, the crankcase draws in and pushes

Underneath the piston the crankcase is closed off, except where mixture comes in and where the transfers lead up. As the piston rises, fresh mixture is drawn into the crankcase, through a reed valve on engines that use one. As the piston falls it squeezes that mixture. When the transfer ports open, the squeezed mixture flows up them into the cylinder, filling it and driving out most of what is left of the burned charge. On most of these engines the oil travels through the crankcase with the fuel and burns with it.

Piston near the top: fire above, draw in below A cylinder in section. On its flat head sits the spark plug, with a small star under it, and a tall piston rides high in the bore with a rod down to a crank in a box-shaped crankcase. Low on the right wall an opening leads out along a straight duct; a little lower on the left wall a smaller opening leads into a passage that runs down into the crankcase. The side of the piston lies across both openings. On the left of the crankcase an inlet duct carries an arrow pointing in, past a flap swung open into the crankcase. Piston high plug fires near the top charge squeezed exhaust port transfer port the piston covers both ports mixture in crankshaft crankcase draws in one-way valve open Piston near the bottom: out, then in The same section with the piston low, below both wall openings, and the crank pin under the crankshaft, and a flap lies shut across the inlet on the left. An arrow climbs the left passage and turns through the lower opening into the cylinder. A solid arrow runs out along the right duct, a dashed curve runs from the incoming flow across to the right opening, and a thin arrow inside the duct points back towards the cylinder. Piston low exhaust opens first transfer port opens next some escapes a tuned pipe's pulse pushes some back crankcase squeezes one-way valve shut

Both frames come from the same turn of the crankshaft, drawn for an engine whose fuel, air and oil pass through the crankcase together. In the direct-injection design US 5,730,099 describes, the crankcase pumps air and the fuel is put into the cylinder; under many conditions that can wait until the exhaust port has closed, but as speed and load rise the injection must start with the port still open, and a portion of the fuel escapes.

How we know: the cycle in one turn, the exhaust port opening before the transfers, some fresh charge escaping and the returning pulse are US patent 6,026,641 (line 56 of the held copy); the crankcase drawing in as the piston rises, squeezing as it falls and sending the charge up the transfer passages for the plug to fire, US patent 5,370,088 (lines 55 to 60); fuel, air and oil through the crankcase past a one-way valve that keeps the squeezed mixture in, US patent 6,379,411 (lines 138 and 143); direct injection, US patent 5,730,099 (lines 41, 42 and 100) and US patent 6,379,411 (line 110).

Not to scale.

Where some fresh mixture gets away

Near the bottom both sets of ports are open at once, and some of the fresh mixture rising from the transfers goes straight out of the exhaust unburned. That loss is typical of two-strokes. A direct-injection engine can wait to inject its fuel until the exhaust port has closed, which keeps the fuel in, but at higher speed and load it has to start earlier, while the port is still open, and some of that fuel escapes. A tuned pipe, an exhaust shaped to send a pressure wave back to the port, can push part of the escaping mixture back in. The exhaust pipe article under Exhaust in Tuning explains how.

What this page leaves out

It gives the order of events and nothing more. There is no port height, port timing or engine speed here, no engine's cylinder, and nothing about porting, oil ratios or how a pipe is tuned. It does not cover the four-stroke. Not every engine uses a reed valve to let mixture into the crankcase, and the reed valve article in this topic says so.

Where this comes from

  1. Describes the whole cycle: two strokes or one revolution per cycle, ports in the wall uncovered by the piston or valves in the head, exhaust first, the crankcase charge flowing in and expelling the exhaust with some escaping, firing just before top dead centre, and a tuned exhaust returning a pulse that pushes escaping charge back. US patent 6,026,641, on exhaust temperature control in a two-stroke engine, line 56 of the copy this archive holds.
  2. The crankcase round the crankshaft draws the charge in on the upstroke, compresses it on the downstroke and sends it up scavenge passages, and the plug fires the compressed charge above the piston. US patent 5,370,088, Two cycle engine, lines 55 to 60 of the copy this archive holds.
  3. Fuel, air and oil are drawn into the crankcase and burn together; the piston uncovers the exhaust and then the inlet near the bottom; unburned fuel leaving with the exhaust is a shortcoming of typical two-strokes; a direct-injection engine puts its fuel into the cylinder. US patent 6,379,411, Two stroke engine exhaust emissions separator, lines 103, 110 and 138 to 143 of the copy this archive holds.
  4. Under many conditions direct injection can wait for the exhaust port to close, but at higher speed and load injection starts with it open and a portion of the fuel escapes; the crankcase pumps air. US patent 5,730,099, Reduced emission two-stroke engine, lines 41 to 42 and 100 of the copy this archive holds.
  5. A tuned exhaust chamber reflects pulses that can stop the intake charge leaving through the exhaust port. US patent 5,983,633, Electronically controlled water injection system, line 61 of the copy this archive holds.
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“How the two-stroke cycle works”, BaddJet archive, https://baddjetarchive.com/tech/engine/how-the-two-stroke-cycle-works/

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