So...I was taking a Facebook quiz sponsored by "How Stuff Works" about engine technology. One question asked, during what stroke did the spark plug fire. Knowing that the spark fires a few degrees before top dead center, I answered the compression stroke. Well, the quiz considered that wrong and said the correct answer was the combustion stroke (which, frankly, I've always seen and heard referred to as the power stroke). If you consider the four strokes as occurring between top dead center and bottom dead center (or vice versa), then the answer the quiz considered as correct was actually incorrect.

But this got me thinking: do the strokes, in fact, overlap? For instance, when the piston comes up for the exhaust stroke, the exhaust valve is actually held open for a few degrees after top dead center, since the exhaust gasses have enough momentum to "pull" more exhaust out of the combustion chamber even after the intake valve has opened. And at the end of the intake stroke the intake valve is usually held open a few degrees after bottom dead center, because the momentum of the incoming air or air-fuel mixture will continue drawing in more air. And if we consider the power stroke as occurring when the spark plug fires or gasoline is injected into the combustion chamber, then the power stroke overlaps with the compression stroke.

So...between dead centers or overlap?

Edit:My vote goes for between dead centers; otherwise, it's just too dang ambiguous.

  • Weird. I always thought of it as the end of the compression stroke as well. Since I've never seen an argument about it amongst my friends I've never really viewed it any other way. Apr 10, 2016 at 22:43
  • 1
    I've taken one or two other "How Stuff Works" quizzes, and I think their name is a bit of a misnomer...just my $0.02 worth.
    – BillDOe
    Apr 10, 2016 at 23:49
  • 1
    They should rename it to "How Stuff Might Work" Apr 10, 2016 at 23:51
  • 1
    This may be where the difference between the terms "four stroke" and "four cycle" comes into play. I would truly love to answer this question, but what I'd be saying is supposition. You are correct about them being wrong. I believe the "stroke" denotes TDC to BDC with the travel of the piston. A cycle, IMHO, could start before TDC/BDC or end after, with overlap in between. Again, JMHO. I don't have anything which proves my thinking. Apr 11, 2016 at 0:35

2 Answers 2


Theoretically, they are all distinct stages. However in reality, varying engine speeds need the valve timing and the ignition timing to change. That is why variable valve timings exist ("VTEC just kicked in yo!") and variable ignition timing.

The quiz is in a way correct because, it is the most general form of the 4 stroke engine. How many degrees after the BDC the intake valve remains open or how many degrees after the TDC the exhaust valve closes, and even how many degrees before or after the TDC the spark plugs fires depend on the engine speed. This changes from manufacturer to manufacturer, engine to engine.

Long story short, it is not correct to say that the spark plug fires a certain number before or after the TDC but it is correct to say that the spark plug fires in the combustion/power stroke.

Strictly speaking, the combustion stroke, is whichever stroke the chemical energy of the fuel is converted to mechanical energy of the engine (Irrespective of when the spark plug fires. The spark may fire before or after TDC).


While the plug fires before tdc, and therefore a mixture that is starting to burn is being compressed - hence absorbing work i.e. not developing power, thus the OP's answer compression stroke would be correct. Power is developed from the burning & expanding combustion mixture only on the downward travel i.e. after tdc, so the start of the power stroke is after tdc until it gets to bdc, but effectively once the exhaust valve is opened then the energy from the expanding gases will be used to force the gases out down the exhaust port.

It is useful to think about the total energy of the system i.e. enter image description here

where Q is total energy, W is work and U is internal energy.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .