I'm learning about engines, and I understand the basics of how a 4-stroke engine works, but I've seen lots of true or false questions such as "The power stroke is shorter than the intake stroke" or "The power stroke is longer than the power event". What's the difference between strokes and events, and where can I see more about how long each one takes?

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    $\begingroup$ You can also check out some of the questions over on Mechanics.SE since your question isn't unique to airplanes. $\endgroup$ – JPhi1618 Sep 11 at 19:26
  • $\begingroup$ This is confusing due to terminology (mis)use. Three things that get confused are the four processes in an Otto cycle engine, the mechanical motion of the piston, and the opening/closing of valves. Stroke can mean any of the four processes or the mechanical motion of the piston. Event has been used to describe the processes and an opening/closing of a valve. You may need to clarify which thing you mean if a conversation gets confusing. $\endgroup$ – Pilothead Sep 13 at 17:02
  • $\begingroup$ Wikipedia might help: en.wikipedia.org/wiki/Four-stroke_engine But this is not really an aviation question. $\endgroup$ – jamesqf Sep 13 at 17:38
  • $\begingroup$ If you want to understand engine timing, study timing diagrams and their explanations from camshaft companies, not wikipedia. $\endgroup$ – Pilothead Sep 13 at 19:27

The duration of the stroke is simply the time the piston takes to run from the bottom to the top of the cylinder or vice versa, half a rotation. So if the engine is running 2000 rpm, or 33.33 revolutions per second, or 16.66 strokes per second, the stroke duration is .06 seconds, half of one full revolution.

That's the physical mechanical stroke. However the "events" overlap the strokes. Ignition on an aircraft piston engine starts normally around 24 degrees before Top Dead Center, so the power "event" actually starts before the compression stroke is completed. Same with exhaust; the exhaust valve may open slightly before the power stroke is complete (piston not quite at bottom dead center) and the intake valve may open just before exhaust stroke is complete.

This is all done to take advantage of inertial effects of the mass of the air fuel charge. There is a lot of overlap of all these events with the mechanical stroke and is a function of valve and ignition timing, so for a given engine you need to look at a diagram of the timing points for ignition and valve opening/closing for that particular engine.

  • $\begingroup$ It seems your answer is math challenged. If a stroke is half a revolution you need twice as many strokes as revolutions, not half. $\endgroup$ – Pilothead Sep 13 at 6:43
  • $\begingroup$ The stroke duration, time from TDC to BDC, is half a revolution. Don't follow you. $\endgroup$ – John K Sep 13 at 11:34
  • $\begingroup$ If duration drops, frequency goes up. F=1/D. They move in opposite directions. By definition it is mathematically impossible to halve both frequency and duration at the same time. $\endgroup$ – Pilothead Sep 13 at 15:32
  • $\begingroup$ It is not useful to describe duration with time units, as that is engine speed dependent. The appropriate units for engine timing is degrees of crankshaft rotation, as this is consistent no matter what speed. $\endgroup$ – Pilothead Sep 13 at 16:33

Stroke in this context means the duration of one of the four processes (intake, compression, power, exhaust) in a four stroke engine. An illustration shows the mechanics:

enter image description here

Duration of each process is measured in crankshaft degrees and is shown with a timing diagram:

enter image description here

Events are points in time which can be used to identify the beginning or end of a process.

In this example, the intake stroke starts with the intake valve open event (15 degrees before top dead center) and ends with the intake valve close event (30 degrees after bottom dead center). The intake close event also starts the compression stroke, which ends with the ignition event (35 degrees before TDC). The ignition event begins the power stroke, which ends with the exhaust valve open event (50 degrees before BDC). This starts the exhaust stroke which continues to the exhaust valve close event (20 degrees after TDC).

The strokes do not align with TDC and BDC because it takes time for a valve to open or close, for a flame to combust the entire fuel/air charge, or for air/fuel/burnt gasses to start or stop moving. Some may argue that the power stroke does not begin until TDC, and you may notice that the intake and exhaust strokes overlap. The timing diagram makes everything easy to see.


Valve timing events are controlled by the camshaft(s). For a more detailed discussion of timing events, see this article:

The Timing Events - The valve timing events occur in this order of importance (well, this is debatable, somewhat). 1. Intake valve closing (IVC) 2. Intake valve opening (IVO) 3. Exhaust valve closing (EVC) 4. Exhaust valve opening (EVO)

enter image description here

  • $\begingroup$ No. The very definition of an engine's "bore and stroke" is the cyl diameter and the length of piston travel from TDC to BDC. The intake/compression/power/exhaust "events" overlap with the strokes due to ign and valve timing, as my answer said. $\endgroup$ – John K Sep 13 at 11:59
  • $\begingroup$ @JohnK OP is asking about "intake stroke", "power stroke" and which is longer. This obviously refers to the processes which differ in duration, not the mechanical piston motion which is always 180 degrees. $\endgroup$ – Pilothead Sep 13 at 15:37
  • $\begingroup$ so.... when the intake and exhaust valves are open at the same time, is that the haust-in stroke? OP is confused about strokes and events. "Power event" starts at ignition, before completion of the compression stroke. The flame has started, but the piston is still compressing the mixture and no power is being generated until the piston passes TDC. Stroke is a geometric term related to piston travel, not timings of the valve and ignition. $\endgroup$ – John K Sep 13 at 18:49

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