A four-stroke engine fires once every two crankshaft revolutions (720 degrees), using separate strokes for intake, compression, power and exhaust, while a two-stroke fires once per revolution (360 degrees) by combining those jobs in two strokes. The two-stroke is lighter and has no valves but burns more fuel and oil and gives dirtier exhaust, so a settlement building or salvaging engines should expect the four-stroke to last longer and the two-stroke to be simpler to make.
Part of Internal Combustion
The two ways of packaging the engine cycle into strokes, what each gains and loses, and which suits a settlement’s needs.
The Strokes Are Merged In A Two-Stroke, With A Cost
The carrier describes the four-stroke cycle: intake, compression, power, exhaust, with both valves closed during compression and power, and the crankshaft turning twice (720 degrees) per cycle. A four-stroke needs a valve train to open and close its valves, as Valve Train describes.
A two-stroke squeezes the same jobs into two piston strokes, one crankshaft revolution (360 degrees). As the piston rises it compresses the charge above it and draws fresh mixture into the crankcase below; ignition drives the piston down, compressing the mixture in the crankcase; near the bottom the piston uncovers ports in the cylinder wall, the exhaust leaves and the fresh charge flows in from the crankcase. This port-and-crankcase arrangement is the common small design (recalled description, flagged). It needs no valves or camshaft, which is its great simplicity.
The cost is that the fresh charge and the exhaust are in the cylinder at the same time, so some fresh mixture leaves with the exhaust unburned, and some exhaust stays behind to dilute the next charge. The crankcase also has to be sealed and used as a pump, so it cannot hold a normal oil sump, and the engine is lubricated by oil mixed into the fuel (recalled practice, flagged). That mixed oil is partly burned in the cylinder.
Comparing The Two On A Settlement’s Terms
| Feature | Four-stroke | Two-stroke |
|---|---|---|
| Power strokes per revolution of a single cylinder | One every second revolution | One every revolution |
| Valves and camshaft | Needed | None (ports) |
| Lubrication | Oil sump | Oil mixed into the fuel |
| Fuel economy | Better | Worse; some charge escapes |
| Weight for a given power | Heavier | Lighter |
| Wear life | Longer | Shorter |
The carrier’s discussion and its fuel options assume the four-stroke throughout, and the conversion advice for wood gas and ethanol is written for it. A two-stroke running on producer gas has a particular problem: the oil must come from somewhere other than the fuel, because the gas carries none.
Worked Example: Power Strokes Per Minute
For a single cylinder at 3,000 revolutions per minute, an assumed speed:
- Four-stroke: one power stroke per two revolutions, so 3,000 ÷ 2 = 1,500 per minute.
- Two-stroke: one per revolution, so 3,000 per minute.
On paper the two-stroke makes twice the power strokes at the same speed. In practice it gives rather less than twice the power of a four-stroke of the same size, because the charge is diluted and part of it is lost, and the real ratio must be measured, not assumed. The four-stroke’s separate strokes are the reason it is the better user of its fuel.
Common Mistakes
- Running a two-stroke on straight fuel. With no oil in the fuel the piston and bearings seize quickly.
- Using the wrong oil quantity. Too little wears the engine; too much fouls the plug and smokes.
- Fitting a two-stroke where hours of reliable running are needed. The four-stroke’s sump lubrication wears longer.
- Assuming a two-stroke cannot be carburetted for ethanol. It can, but the oil mixing must be re-thought; see Fuel Systems.
Safety
Exhaust, fuel and moving parts are the same hazards in either cycle
Run engines outdoors with the exhaust away from people, because carbon monoxide is odourless. A two-stroke’s fuel is premixed in a can, so label the can and keep it away from flames. Hand-starting kicks back if the timing is wrong; see Ignition Timing.
What’s Next
- Learn next: Cylinder Block — the casting that holds either cycle
- Combine with: Engine Theory — why the cycle matters at all