Engine Compression at Altitude 3

[Nobrac]

Greeting to you all. This is my first post on Eng-Tips.
Can anyone tell me how to calculate decreased cylinder compression of a naturally aspirated internal combustion engine at various altitudes? Actually, I will work the formula backwords:i.e. I have measured compression numbers and want to determine how much the decrease from OEM specs (assuming sea level) is related to altitude and how much from engine wear.

I read the thread on measuring the engine output decrease in horsepower but could find nothing in the archives on compression.

Thanks in advance for any guidance.

 

[Fabrico]

You will likely not be able to tell the difference. Another factor of a compression test is variance between the cylinders. If it is really that important, you should be doing a cylinder leak down test instead of a compression test.

 

[crysta1c1ear]

Altitude doesn't affect the compression ratio. If 50 cm3 is compressed down to 5 cm3 then the compression ratio is 10:1 regardless of how much air is actually in the initial 50 cm3.

 

[Nobrac]

Thanks guys. Perhaps I was a little inspecific. I realise the cylinder leak down test is more predictive than than the compression test. It is just that I have compression test data at sea level and at 7,800 feet. I agree that 50ccs compressed to 5 ccs is the same compression ratio. I am asking that if cylinders are reading 155 to 160 at sea level and 110 to 120 at 7,800 feet is altitude a factor or is this 100% engine wear?

Yes, perhaps I ought to start over with a leak test and lab oil sample analysis but I have this compression test data and need to make conclusions about the unit's future. Time for additional tests I do not have.

Clearly, the per cent variance between the cylinder compression numbers is probably most important of all.

 

[al1]

I believe what you are looking for is how to calculate the higher altitude compression PSI from sea level. Because of the huge variables of things like heat lost and starting ambient air density etc. I recommend that you simply use a perfect gas and adiabatic approach, as they should apply to both for comparisons. First you need a cubic inch displacement to crank angle chart. Then take the closing time in degrees of the exhaust valve to calculate your dynamic compression. Use Boyl’s law formula and simply calculate the actual compression by starting with sea level PSI 14.7 and the 7,800 starting PSI of around 11. There is too much work for me to do it for you but that is how you can actually compare the difference.
al1

 

[crysta1c1ear]

I wrote a few lines of BASIC on my PC

      REM Written in BBC BASIC using free demo version from
      REM [URL unfurl="true"]http://www.cix.co.uk/~rrussell/products/bbcwin/bbcwdemo.exe[/URL]
      
      REM Formula from [URL unfurl="true"]http://en.wikipedia.org/wiki/Atmospheric_pressure[/URL]
[COLOR=red]      
      DEF FN_pressure(altitude)=10^(5-altitude/15500)
      ratio = FN_pressure(7800 * foot) / FN_pressure(0 * foot)
      
      PRINT "Pressure at 7800 feet is ", ratio / percent, " percent"
      PRINT "155 -> ", 155 * ratio
      PRINT "160 -> ", 160 * ratio
[/color]

Prints

Pressure at 7800 feet is 70.2452195 percent
155 -> 108.88009
160 -> 112.392351

 

[Nobrac]

THank you, thank you. This gives me what I needed to make my case. I am going to try an write up a calculation work sheet on Excel using his info a a model.