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