Does the piston reach the max height the piston can go without fuel or oxygen I would think it can't because the oxygen molecule being squeezed in the middle. I'd say it almost fully go up but not completely is this correct?

2 Answers 2


TL DR: You are absolutely correct: the piston does not completely close the gap.

There are two points in the rotation of the crankshaft where the piston stops momentarily as it changes direction:

  • TDC - Top dead center - as the piston is traveling upwards in the cylinder
  • BDC - Bottom dead center - as the piston is traveling downwards in the cylinder

After the intake cycle, the piston starts traveling upwards on the compression stroke. As the piston travels upwards, it compresses the air/fuel mixture (only air at this point if the engine is direct injected - for simplicity's sake, I'll leave that for another time), which creates heat and helps homogenize the mixture. Just before the piston gets to TDC, the spark occurs and combustion starts occurring. In order for the combustion to continue and power to be extruded from the explosion, there has to be somewhere for the combustion to go. Without the space, something would give. That something would be a head gasket or weaker points in the metal.

Two reasons why there has to be a space left:

  1. The air/fuel can be compressed quite a bit, but it cannot be compressed to nothing. There has to be a space left.
  2. If the area were able to be compressed to nothing, some sort of metal-to-metal contact would occur as the piston reached TDC. This would cause a lot of wear to occur in the engine, which would destroy it in quick order.
  • Isn't it true that the intake valve is still open for several degrees after BDC?
    – Solar Mike
    Oct 14, 2017 at 13:58
  • @SolarMike yes but, more like in the dozens of degrees.
    – Ben
    Oct 14, 2017 at 14:27
  • @ben I do understand valve timing and overlap ....
    – Solar Mike
    Oct 14, 2017 at 14:30
  • @SolarMike so, why was is phrased as a question? you may know the intent behind the post, but it isn't clear that it was a rhetorical question.
    – Ben
    Oct 14, 2017 at 14:37
  • @SolarMike - What does that have to do with the OP's question or with what I've stated? In most cases the intake valve is open past BDC on the intake stroke to allow for the cylinder to fill better (intake charge inertia). If you have a separate question or if you don't understand the concept, please ask your own question. Oct 14, 2017 at 14:53

All engines are designed with a swept volume (the volume the piston moves through) and a clearance volume (the space between the head and the piston once it is at the top of its stroke).

The ratio between the total volume (swept + clearance) and the clearance volume is the compression ratio - around 7 to 12 : 1 for gas / petrol engines and 18 to 24 : 1 for diesel engines.

  • Thanks for the ratios as well will help in future research and studies knowing more about this.
    – DeusIIXII
    Oct 14, 2017 at 18:49
  • A good read, and a book I still have in my library is by Ricardo : estore.ricardo.com/shop/… . covers much of the basics and testing. You may find a copy in second-hand bookshops...
    – Solar Mike
    Oct 14, 2017 at 19:02
  • Thank you bud. A good informative book is what i needed just didn't know the right one
    – DeusIIXII
    Oct 14, 2017 at 19:15
  • It is informative , but old - very interesting , you can find others newer and more up to date once you start looking. The story of Cummins for example not recent though...
    – Solar Mike
    Oct 14, 2017 at 19:58

You must log in to answer this question.

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