compression

[SteveS]

I am looking at a TO 30 with purchasing in mind. The compression tests at 130# after 3 puffs on each cylinder. Is good? Also this tractor has 16.9 x24 turf tires. In general does this detract from the value? Would I be able to trade for regular tires and wheels easily?

 

[Jerry/MT]

To do a proper compresssion test, the engine must be at operating temperature, the throttle plate must be wide open, and the air cleaner should be disconnected unless you know that air cleaner has minimum restriction. (It"s easy to loosen the clamps on the rubber connecter between the air tube and the carb inlet and slide it out of the way.) It"s easier on the starter if you remove all the plugs. The minimum acceptable pressure is 90 psi( somepeople will say 80-85 is an acceptable minimum) and there should be less than 10 psi difference between cylinders.

 

[justin14054]

Steve,

Turfs=Beauty is in the eyes of the beholder (what someone plans to use the tractor for).Mowing lawn=people want turfs so they do not dig there lawn up.Me=Ag only (yes turfs are a price reduction=for me).just as an FYI I have been trying to trade off a good set of turfs mounted on 8n rims for months now.For me I figure $500 reduction for turfs(This is me only) other people may be looking for that
Justin

 

[phil(va)]

Compression sounds great if done correctly. Trading for wheels and tires is problematic. They are too heavy and bulky to ship anywhere. You have to find a local trade to make it work. Check with your local ag tire store(s). If they have no interest, I would say your chances are not good. If you want the tractor for mowing, you are ok as is. If you want to plow, maybe you should look elsewhere, unless buying and having two sets of wheels and tires appeals to you. My two cents.

 

[John H TO20]

I've been thinking about switching my TO20 to turf tires but haven't because of the expense. Are the turf tires you'd like to get rid of anywhere near Missouri?

 

[Don Hooks]

The theoretical maximum compression on the original engine is somewhere between 95 and 105 depending on altitude, temperature, etc. A compression of 130 (presuming the guage is accurate) calculates to a compression ratio of 8.8 to one which suggests the head may have been machined - which may be a good thing if it was part of a complete rebuild. On the other hand, it will require precise ignition timing or premium gasoline to prevent preignition pinging. It will probably put out an additional 5 or 6 HP over stock.

 

[Bob (Aust)]

Don

Can you give me the forumla to convert compression ratio to theoretical maximum cylinder compression?

I assume it is: cylinder compression / atmospheric pressure = compression ratio?

Or compression ratio x atmospheric pressure = max cylinder compression?

Obviously best calculated in ISA conditions (International Standard Atmosphere 1.00 Bar at +15 deg C/59 deg F)?

Thanks

Bob in Oz

 

[Jerry/MT]

Respectfully,compression ratio is a volume ratio; specifically, the total volume of the cylinder at BDC(Vbdc) divided by the volume at TDC (Vtdc). So dividing the absolute level of pressure (compression gauge pressure + atmospheric pressure) by the atmospheric pressure gives you a pressure ratio not "the compression ratio."

The "theoretical" pressure ratio as a function of the compression ratio for a constant mass of air in the cylinder is as follows:

p2= patm x (Vbdc/Vtdc)^g

where g depends on the ratio of the specific heats for the gas that you are compressing. If it"s mainly air at normal pressures and temperatures you can approximate g as g=1.4 so for a 6.5 compression ratio engine like the Z129,Continental,

p2 = p1 x (6.5)^1.4 = 13.74 x p1

So p2 with a p1 of 14.7 psia ( pi=patm impies that the throttle plate is wide open as is the choke plate the the air cleaner restiction is non-existant or vanishingly small)
p2= 14.7 x 13.74= 202 psia or, 187.3 psig.

This equation assumes that the "mass of air in the cylinder is constant and that g is constant".
However, our real engine has valves that are not closed at BDC so the mass in the cylinder isn"t constant.There"s is generaly some gas vapor mixed with the air and as the temperature goes up due to compression, so the value of g chnages. So you see the theoretical equation doesn"t fit the real situation.

A newer TO-30 will have a compression pressure gauge reading of about 120 - 125 psig or 134.7-139.7 psia so AS AN APPROXIMATION, if you multiply the ideal absolute pressure by 0.68, subtract the atmospheric pressure from it, you"ll have an idea what the max compression gauge pressure should be.


I might add that carbon in the combustion chambers can have the effect of raising the compression ratio and the compression pressures because of the reduction of VTDC as can "shaving the head". These can lead to increases in the compression pressure and higher gauge readings.

hope this helps.

 

[Jim in OH]

(quoted from post at 21:17:51 11/30/07) ....
...However, our real engine has valves that are not closed at BDC .......

Nice post... and don't forget about the cooling of the charge... you equations assume adiabatic compression which is also not quite the real situation... (but of course lumped into the 0.68 fudge factor)

 

[Jerry/MT]

Yeah I didn"t want to get into the whole litany of caveats about thermally and calorically perfect gas, adiabatic cylinder walls, heat transfer from the manifold etc, etc. The biggy I think is the valves being open and the gas being a mixture of air and fuel and the temperature effect on the specific heat ratio. As you point out, the factor of 0.68 covers all the non-ideal stuff.

Since I know what the answer ought to be I can then develop the factor. That"s kinda" cheatin".(The car guys in Detroit can do this analytically I"m sure.) Wonder how well this factor works on other engines? Have any idea, Jim? My WAG would be the factor would vary from low of 0.60 to a high of 0.70 for engines in the 6-6.5 compression ratio range.

 

[Don Hooks]

Thanks! John (UK) has cautioned us about jumping to conclusions and I took enough thermo classes to know better. You have piqued my interest and I think I will get out those 50 year-old engineering texts and try to improve my understanding. I WAS just looking at a closed system and I even forgot to add in atmospheric. The model was basically an isothermal piston compressor and that is just not close enough to real life.

 

[Jerry/MT]

Don,
It"s been 0ver 40 years since I took Thermo in engineering school but I worked in the field and even though I retired in "99 some of that knowledge stayed with me.

If you"re interested in a practical reference on IC engines, "Internal Combustion Engines" by Obert, (2nd Edition) is a good reference and It can be had in used form for around $10-$20. It has some real practical info though some of the data presented as emperical can easily can be analytically derived. The book was written when test stand data was king and engine analysis was in it"s infancy.

 

[Jim in OH]

I put together an ideal calculation for the Ferguson TO30 a while back... and since we know what the answer is (I used 29.2 Hp at the PTO), I seem to recall that the factor was right around 0.7... probably very close to your 0.68... I think the calc is on a memory stick that I left at work... so I can"t check... I also tried to see where it all went... that wasn"t very successful without getting some pressure and temperature measurements that I can"t make.. the point of finding it was to find ways to improve it... flow (pressure drop) and compression ratio were two of the most likely major culprits...

 

[Don Hooks]

Thanks Jerry! I DIDN'T work in the field - I crosstrained into environmental and retired in '03. Some of the core curricula stayed with me but I seldom used any thermo that was not open cycle. I will look for the book you recommended.

 

[SteveS]

The turfs are in southern Michigan. I think I will swap them onto an 8N I will be selling in the spring.