Compression Ratio

  • henrymski
  • henrymski's Avatar Topic Author
  • Offline
  • User
  • Posts: 83
  • Thanks: 2

Compression Ratio

29 Nov 2005 06:45
#10871
If you don't know the compression ratio of an engine, how do you determine it? Assume there is no published info on the engine. TIA.

Please Log in or Create an account to join the conversation.

  • steell
  • steell's Avatar
  • Offline
  • User
  • Posts: 6850
  • Thanks: 208

Re: Compression Ratio

29 Nov 2005 06:58
#10873
Put the timing mark on TDC for #1, then measure the amount of gas/oil/kerosene is takes to fill the cylinder. That will give you the compression volume, divide the displacement by the compression volume to obtain the compression ratio.

If it's a stock motor, the specs are probably listed on the web somewhere, so you could use Google and save a lot of mess.

Post edited by: steell, at: 2005/11/29 09:59
KD9JUR

Please Log in or Create an account to join the conversation.

  • Ron G
  • Ron G's Avatar
  • Offline
  • User
  • Posts: 57
  • Thanks: 2

Re: Compression Ratio

29 Nov 2005 13:50
#10929
steell wrote:
divide the displacement by the compression volume to obtain the compression ratio.

Divide the displacement + the compression volume by the compession volume to obtain the compression ratio.

CR=(piston displacement+combustion chamber volume)/combustion chamber volume

Ron

Please Log in or Create an account to join the conversation.

  • steell
  • steell's Avatar
  • Offline
  • User
  • Posts: 6850
  • Thanks: 208

Re: Compression Ratio

30 Nov 2005 06:35
#11035
One of the nice things about this place, if I screw up there is always someone that will correct my mistake :D

Thanks!
KD9JUR

Please Log in or Create an account to join the conversation.

  • APE Jay
  • APE Jay's Avatar
  • User
  • Thanks: 0

Re: Compression Ratio

30 Nov 2005 09:08
#11063
I was just about to correct that when I scrolled down and saw Rons post.

Jay

Please Log in or Create an account to join the conversation.

  • JR
  • JR's Avatar
  • Offline
  • Sustaining Member
  • Posts: 2943
  • Thanks: 458

Re: Compression Ratio

30 Nov 2005 19:05
#11162
If stock you might find the info listed here www.bikez.com/brands/index.php
1980 kz750E1, Delkevic exhaust

Please Log in or Create an account to join the conversation.

  • TwoSeven
  • TwoSeven's Avatar
  • Offline
  • User
  • Posts: 34
  • Thanks: 0

Re: Compression Ratio

10 Dec 2005 19:29
#12564
It will still only give an approximate as you also need to know the VE (volumetric efficiency) of the engine.

For example, a 600cc 4-stroke with a 14cc head area is going to have rough compression ratio of about 10.7:1 ratio. However, if the VE is only 90% then it means that the compression ratio will drop to about 9.7:1 or so.

It means less mix being sucked into engine, so less to be compressed.

Please Log in or Create an account to join the conversation.

  • steell
  • steell's Avatar
  • Offline
  • User
  • Posts: 6850
  • Thanks: 208

Re: Compression Ratio

10 Dec 2005 19:46
#12567
It will give you an exact (static) compression ratio, not an approximate one. Calculating the Volumetric Efficientcy in order to determine the dynamic compression ratio is a whole nother ball game, and I doubt that was the question.
KD9JUR

Please Log in or Create an account to join the conversation.

  • nads.com
  • nads.com's Avatar
  • Offline
  • User
  • Posts: 1000
  • Thanks: 20

Re: Compression Ratio

10 Dec 2005 20:41
#12574
volumetricly speaking, the dynamics of the efficiency of the compression are not subjects of displacement?

Please Log in or Create an account to join the conversation.

  • steell
  • steell's Avatar
  • Offline
  • User
  • Posts: 6850
  • Thanks: 208

Re: Compression Ratio

10 Dec 2005 22:12
#12584
Volumetric efficiency is stated as a percentage of the engines total airflow potential. (Total airflow potential, or pumping potential = displacement X rpm.) Volumetric efficiency is determind by measuring airflow into the engine while it is running.

For more info:

www.yawpower.com/martech.html
KD9JUR

Please Log in or Create an account to join the conversation.

Powered by Kunena Forum