Calculating compression ratio
 

Hi,

I'm wondering. Isnt it so that the compression ratio of a engine is :
the volume of gas in cilinder at DBP(lowest, all ports are "just"closed)), compared to volume of cilinder at TBP (top) ?
Im trying to calculate the compression ratio of an OS 46FX (Glow, not diesel yet)) , but I'm already lost. Bore*stroke should be 7.5 cc according specs, but thats not the case.

Hope someone can help me out.

Best regards,

Frank

RE: Calculating compression ratio
 

Hi, you can calculate totalvolum of stroke (not all ports are "just"closed, allways from BDC to TDC) and bore and measure volume of combustion volume in cylinderhead (use syringe with sewingmachine oil to measure volume with piston at TDC and fill oil until plughole).

The formula are:

Totalvolume of cylinder + combustion volume : combustion volume = Compresion ratio

Jens Eirik

RE: Calculating compression ratio
 

Engines have a mechanical compression ratio measured using the total bore and stroke and an effective or "corrected" compression calculated from when both valves are closed on a fourstroke and from where the exhaust port is closed on a twostroke. You would amazed at how far up on the compression stroke the piston is on a Saito before the intake valve closes.

RE: Calculating compression ratio
 

Thanks,

I was wondering about the possible corrected compression ratio too, those gasses run into the cilinder pre-compressed so to speak.

Calculated and technically the stoke*bore on this .46 is 7.5 cc. If I move up the piston untill the last port closes (outlet) I only have a 5.7 cc left in the cilindre . At TDC there's only 0.4 cc left so that means I have a compression ratio of 1:14.

What volume to take, the 5.7 or 7.5 CC ? Id want to calculate the volume I need to make my conpression "room" adjustable from 1:16 to 1:20

Regards,

Frank

RE: Calculating compression ratio
 

"Compression ratio" has nothing with valves/ports when they are closed, it is wrong to calculate compresion ratio.

See at the picture from "Model Aero Engine Encyclopaedia" 1969..;)

Jens Eirik

RE: Calculating compression ratio
 

Jens,

Thanks for taking the effort to convince me :)

I'll stick to Bore*Stroke.

Best regards,

Frank

RE: Calculating compression ratio
 

The effective compression is the one that dermines the way your engine runs, the mechanical compression ratio is what it would be if it were a closed cylinder without ports or valves that are open for part of the stroke. So the effective or corrected compression ratio is the one that matters in the real world.

RE: Calculating compression ratio
 

knarf, here is an alternative to your question about the .46 sized conversions, an OS LA .46 with a Davis Conversion Head. I'll copy and paste what I had to say about it when I first got it.


I converted the LA .46 with a Davis Diesel head today and the results are great,
Davis, Oily Mix/ Plane fuel
MasterAirscrew 12x6 three blade 8,800 rock steady, does not wander, still working on the idle 2,500 so far. I'm flying it tomorrow. Throttle up is clean and crisp. I could squeeze about 9,000 rpm out of it but it wanted to hunt between about 8,900 and about 9,050, I reduced the compression slightly and it settled into 8,800 as mentioned above. This was on Davis Oily mix or Plane fuel as he calls it.

RE: Calculating compression ratio
 

hobbsy,

Your numbers are quite impressive. Have fun tomorrow.

I already ordered one head too for my magnum XL .46A. Still like to
do some diy conversions I have a lathe, i guess i should use it ;)

RE: Calculating compression ratio
 

I'm confused. I own a book called 'Modellmotoren" by Peter Demuth (1989) in which is stated that you have to calculate the compression from the point where the exhaust port is closed:

http://www.modelbouwforum.net/~foto/...s/afb12459.jpg
http://www.modelbouwforum.net/~foto/...s/afb12460.jpg

Another question: when designing a diesel-head, do you take into consideration the clearance between the piston at TDC and the head? (squish band) In my case this is 0.5mm (0.02 inch), which in my opinion, should have a considerable effect on compression ratio.

RE: Calculating compression ratio
 

Micro, he is correct, no compression happens until all the ports are closed. Check my post #7.

RE: Calculating compression ratio
 

Here is look at the difference between the Davis Diesel head on the left and the stock head on the right for a SuperTigre 4500, the piston comes even with the top of the cylinder with out the gasket. As you can see in picture #2, counting from left to right, the squish band clearance is about 4 times as great in the Davis head as in the stock head. I suspect this was done to give the contra-piston a greater effective range by softening its effect near the bottom of its travel, I don't think its effect on the compression pressure is linear.

RE: Calculating compression ratio
 

Read about compression ratio..[8D]

http://archive.dstc.edu.au/BDU/staff...ndex.html#qa13

In my early post #5 are geometrical compression ratio...

Jens Eirik

RE: Calculating compression ratio
 

I have the strong suspicion that indeed compressions ratio is calculated from the point of all ports closed.
My self converted OS10FP (with 2 piece contra :) ) and .46 OS run fine at approx 1:19 / 1:20 CR. (calculated from all ports closed, it must be the fuel i guess ) If I would calculate the CR from pure bore*stroke for both engine i would end up with a CR of approx 1:14/ 1:15.

My mix is 54/33/15/2 or so (I have hard time remembering numbers lately :) )

Best regards,

Knarf.

RE: Calculating compression ratio
 

I think you're correct on that. In my calculations, the compression increases in bigger steps as the contra is screwed in. For example, if the contra is screwed in 0.1mm from it's "TDP", compression increases with 1,9%. But if the contra is screwed out 0.1mm from "BDC", compression decreases with 6,96% If I theoretically increase the squish band from 0.5mm to 1.00mm, these numbers change to 1,53% and 3,74% successfully.

So it looks as if increasing the squish band decreases the amount the compression can be varied, but on the other side makes it possible to set the compression more precise.

Concerning the right way to calculate compression, I made sure that my contra has enough travel to set the compression between 1 : 14 and 1 : 20, regardless of what way it is measured. That way, if the head itself works, it should be possible to set the compression correctly.

RE: Calculating compression ratio
 

I don't think the actual compression ratio of our Diesels is anywhere near the 16 - 20 to 1 that we calculate using the bore and stroke. I have tried to run Davis Diesel fuel in my Saito .80 which has a 14.4 to 1 compression ration and I can't open the throttle past halfway because it has too much compression. It starts on one one flip using a glow plug with no element in it.

RE: Calculating compression ratio
 

It seems like if it will run with the davis fuel being over compressed past half throttle, it should run with lower to no ether in the fuel. If you did try low to no ether maybe using the glow plug to get it warm for start ups and then it might run with out the plug being heated.

Anyone tried that?

RE: Calculating compression ratio
 

I have a PAW19 that I start on 33/33/33 prime and the fuel tank is 70/30 mix of B & Q barbeque lighter fluid and pharmacuetical castor oil, motor starts ok, then runs undercompressed until it warms up, throttle is a little tricky if you let the motor cool down too much. full throttle I cant tell the difference!

RE: Calculating compression ratio
 

This is an interesting fuel mix option, how long does it take to warm up? When you are starting are do you only need to prime the engine with the standard diesel mix? If so how much prime is needed? Are you using a starter to get it kicked off and do you need to change the compression to start?

One other thought is you might consider adding say 5 - 10% cheap starter fluid (i.e Thrush $1.99 at WalMat) and see if you get any change easyer starting or more (or less) power, would be great if you could give prop & RPM info.

RE: Calculating compression ratio
 

Charcoal lighter fliud is usually petroleum naptha and some additives. Coleman fuel would be similar. Naptha refers to a wide range of fuel petroluem distillates. Coleman fuel is mostly naptha. It's not all that different than kerosene. I might have to give this a try.

Greg