What is the Four-Stroke Cycle?

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

Every time someone in this household finishes a carton of milk and leaves it on the counter with its cap on, it reminds me of how heat engines work. In the fridge, the carton is at 42 degrees Fahrenheit, but on the counter, after it has warmed up, it's at 68 degrees, a rise of 26 degrees. And here's the point: If the cap is on, the sides of the carton are bulging from the heat expansion of the air inside it. I unscrew the cap and hear the psssh of that air rushing out. If that pressure pushed a piston or spun a turbine wheel, power could be produced.

The milk-carton effect intensifies with higher temperature. If we mix the air inside a sealed volume with the right amount of gasoline and then ignite it, two big things happen:

  • The temperature of the air is driven upward roughly 4,500 degrees Fahrenheit by combustion, which converts chemical energy into heat energy.
  • This temperature rise multiplies the pressure of the air by roughly seven times.

Why did it take so long to demonstrate the workability of Otto’s four piston strokes—intake, compression, power (expan­sion), and exhaust?

I think the major reason was that, in 1860, every engineer's thinking was conditioned by the example of the then-common double-acting piston steam engine, which produced power on every piston stroke. It did so by means of intake valves, which admitted high-pressure steam alternately both above and below its piston, and exhaust valves, which released the expanded steam after it had done its work. With steam engines thus producing two power strokes per crankshaft revolution, who would even consider a concept that produced only one power stroke for every two crankshaft revolutions?

Beginning in about 1860, thousands of primitive internal combustion engines fueled by city-illuminating gas were built to operate on a crude two-stroke principle devised by Étienne Lenoir. An intake valve opened at TDC, and as the piston descended it drew in a mixture of air and gas for roughly one-third of its stroke. The valve closed, the piston passed a flame port, and the partial vacuum in the cylinder drew in the flame, igniting the gas and air. The result was a feeble rise in pressure (because only one-third of a stroke’s worth of mixture had been drawn in) and so very little power. But many such engines were sold because of their convenience; they eliminated the steam engine’s firebox and a man to stoke it, and they eliminated the boiler, the danger of its explosion, and the man whose job it was to add feedwater as needed. The only thing Lenoir’s engine needed was a connection to a city gas main.

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Otto built at least one such engine but saw it was incapable of much improvement—1) because it drew in only one-third of a stroke’s worth of mixture, and 2) because igniting its mixture at low pressure limited the peak combustion pressure (that is, when not-very-much is multiplied times seven, the result is still not very much).

Somehow Otto was able to make the leap to understand that the advantages of drawing in a full stroke's worth of mixture greatly outweighed the apparent advantage of Lenoir's crude two-stroke—that it produced more frequent power strokes.

Otto also came to understand one other super-important fact. If the combustion of a correct fuel-air mixture inside a sealed volume raises its pressure seven times, doesn't it make sense to compress that mixture before igniting it? That way, the factor of seven would be applied to an already compressed gas, rather than to gas at atmospheric pressure. The result would be a tremendous increase in peak combustion pressure—and much more power. To compare: Igniting the charge without pre-compression means peak pressure is about atmospheric pressure, 14.7 psi, times seven, or 103 psi. But by first compressing it at, say, five-to-one, combustion would raise peak combustion pressure to more like 500 psi. Big difference.

…The advantages of drawing in a full stroke’s worth of mixture greatly outweighed the apparent advantage of Lenoir’s crude two-stroke—that it produced more frequent power strokes.

This sounds like something for nothing, but it is not. In the no-compression engine, the extra energy goes out the exhaust valve as hotter exhaust gas. But in the engine with five-to-one pre-compression, the exhaust gas is cooler because the energy difference is now acting as extra pressure on the piston crown. More power!

  • The first down-stroke of the piston draws in a full-stroke charge of fuel and air through an intake valve, which closes near bottom center.
  • The following up-stroke with valves closed pre-compresses that charge.
  • Ignition occurs with the piston close to its top center position, and the resulting high combustion pressure expands against the piston, driving it and its load through the next down-stroke.
  • An exhaust valve opens near bottom center and the next piston up-stroke pushes out all exhaust gas except that remaining above the piston at TDC. Near TDC the exhaust valve closes and the intake valve opens, and then the engine cycle repeats.

The great strength of the four-stroke cycle is that there is one dedi­cated separate piston stroke for each of the four necessary functions. Its weakness is that the rugged, heavy power piston spends half its time performing the light-duty “housekeeping” functions of intake and exhaust. However, in our emissions-controlled era, the four-stroke cycle’s separation of functions has made it uniquely adaptable to emissions-control technologies. Its separate intake stroke also allows easy power-boosting by super­charging or turbo­charging.

In surviving applications of the two-stroke cycle (large marine engines, some truck, locomotive, and tank engines), the housekeeping functions are performed by an external blower during the time the piston spends near bottom center. This allows each power piston to give power once per revolution. In the simple motorcycle two-strokes of the past, the place of the blower was taken by the crankcase and underside of the piston, acting as a pump.