I am studying valve timing diagrams. Many of them show something like (for instance) the exhaust valve stays opens till ~20 degrees after TDC and the intake valve opens ~12 degrees before TDC. Wouldn't this mean that the valves are open and pushed into the combustion chamber during TDC?

If so I have two questions,

  1. Wouldn't the piston hit the valve and damage it as it reaches TDC?
  2. The explosion wouldn't be so good since there would be ports still open when the spark fires, making poor compression?

What am I missing? Am I reading the timing diagrams wrongly?

EDIT: I would like to add tags like camshaft, timing diagram, valve timing, piston, TDC etc but they are not in the predefined list.

2 Answers 2


First, remember that we are talking about four different engine cycles here: Intake; Compression; Ignition/Combustion; Exhaust. Top dead center (TDC) happens between two of these cycles: between Compression and Combustion, then again between Exhaust and Intake. The reason the piston and valve do not come into contact is because either the piston is following the exhaust valve, or the intake valve is following the piston. When I say the "follows" the piston, or visa-versa, I mean to say as the piston comes up, the exhaust valve closes which does not allow for each of them to come into contact. When I say the intake valve follows the piston, it opens slowly enough for the piston to be out of the way before any contact happens. (NOTE: Slow is a relative term here ... think piston speed v. valve open/close speed.) The valve doesn't just open completely all at once, it gradually opens as it follows the cam lobe profile. Also, this doesn't mean that valves and pistons don't share the same space, they just do it during different times of the cycles. These types of motors are referred to interference motors. If a timing belt were to slip or break while the engine is running, catastrophic effects will occur, causing major engine damage (usually requiring complete engine replacement, but at a minimum can "just" require head work or replacement).

The explosion wouldn't be so good since there would still be ports open when the spark plug fires making poor compression?

For almost the entire cycles of compression and combustion, intake and exhaust valves are all closed, allowing for a complete sealing of the combustion area and helping to absorb all of the power which is available. Hopefully not confusing the issue, I will tell you that the intake valve remains open for a short time during the beginning of the compression cycle, as well as the exhaust valve opening before the piston gets to the bottom of the combustion cycle, but specifics of why this happens is probably best left for another writ.

Needless to say, valve timing is a lot more complex than it looks on the outside. Engine designers take a lot into account when making all of these things happen so failure does not occur.

  • 2
    I still am not sure I fully understand. Even if the valve takes time to open and close, say in my example the intake valve that begins to opens at 12 degrees before TDC, the intake valve would be somewhat open when it reaches TDC no? Or is it perfectly calibrated that when it is actually open fully, the piston is beyond the TDC point so as not to hit the valve? Jul 6, 2014 at 20:55
  • @midnightBlue ... Please see edits. Believe me when I say, it's a complicated mess, it is. Also, make sure you read the last sentence, it is very true. Jul 7, 2014 at 1:37
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    @midnightBlue, you are right: the engine has to be perfectly calibrated in an interference engine. Otherwise, there could be contact between piston and valve (which is, as you might imagine, bad).
    – Bob Cross
    Jul 7, 2014 at 2:03
  • So if both valves are closed in piston is all the way to the top that is top dead centerCorrect
    – Danny
    Jul 26, 2017 at 11:50
  • @Danny - This image on wikipedia shows an animation of how valves open and close in relation to a pistons movement
    – Mauro
    Jul 26, 2017 at 13:13

Question 1: This depends on engine design. In an interference engine, the valves can collide with the pistons, so you need to design the timing diagram to prevent that. In non-interference engines, there is sufficient room in the combustion chamber that the valves will never collide with the piston.

Question 2: As @Paulster2 says, we're talking about a 4-stroke engine, so you have 2 TDC positions, only one of which has a combustion event.

The exhaust valve opens at BDC and stays open during the exhaust stroke, and potentially stays open until the start of the intake stroke. So no problems there. The intake valve opens during the intake stroke and closes at the start of the compression stroke. Both valves are closed during the compression stroke and the combustion stroke. So there's no loss of combustion pressure through the valves.

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