Math question about engine cylinder volume/pressure

[scanman]

Re: Math question about engine cylinder volume/pressure

It's a 4 stroke! You guys are great, BUT... You've officially CONFUSED THE HECK OUTTA ME, LOL!!! I don't really care about the thermo-crap or a difference of 2.6475 psi per cylinder. It only has one. It feels really weak when I kick it, but it's made for a 12yo. kid. I'm kinda wonderin if it should feel that way or not, & where to buy an adapter for my automotive compression tester to fit that TINY 'lil spark plug hole. Still, it WAS A LOTTA FUN readin all the argumentative math. Sumthin tells me I'm glad I don't know all that thermo-atmospheric-temperature related-pressure dependant CRAP! BTW, did anyone figure in the salinity? I am at sea level+13 feet & 40yds from the Gulf of Mexico! Glad you guys are havin fun with this!!!!!!

OH SHOOT!!! I JUST BLEW BOTH TRAILER TIRES!!! THANX GUYS!!!

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

So...you are asking what the compression numbers will be and you can't/haven't measured it?

 

[sschefer]

Re: Math question about engine cylinder volume/pressure

Enteraining thread if I do say so... LOL... The first response you got was the correct answer, I suppose. Max compression you could possibly develop would be about 130. Throw all the other stuff in the mix and you've pretty well figured it'll probably run decent anywhere between 90-125. It'll run on 60lbs probably but you won't be developing much hp. It will probably be hard to start hard and may foul the plug easily when it's cold. It'll probably start pretty easy when it's warm. A single cylinder 50cc motor should turn over pretty easy an definitely won't feel anything like kickin a 500cc motor. It's hard to tell from kickin it if the compression is low. The hard cold start symptoms and a compression guage will tell you something about the condition. You could probably tear it down and rebuild it in a couple of hours.

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

The first response you got was the correct answer, I suppose. Max compression you could possibly develop would be about 130.

Ok...found something that supports what I was saying.

Go down to "Compression ratio versus overall pressure ratio"

http://en.wikipedia.org/wiki/Compression_ratio

Talks about specific heat of air.

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

bruceb58 Heat is not a factor when doing a compression test and when your only turning the engine over a couple of revs. The thermal mass of the cylinder wall.piston and head will quickly absorb the heat.

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

bruceb58 Heat is not a factor when doing a compression test and when your only turning the engine over a couple of revs. The thermal mass of the cylinder wall.piston and head will quickly absorb the heat.

And you know this how? My engineering thermo professor would disagree. You take the measurement instantly. If oyu let it sit for 20 minutes I would agree with you.

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

And you know this how? My engineering thermo professor would disagree. You take the measurement instantly. If oyu let it sit for 20 minutes I would agree with you.

Cuzz, I ain't ever felt no hot air coming outa the spark plug hole. Your professor might say hot but my finger says cold! for the sake of sanity enough said.

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

Cuzz, I ain't ever felt no hot air coming outa the spark plug hole.

Brilliant!

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Ok...found something that supports what I was saying.

Go down to "Compression ratio versus overall pressure ratio"

http://en.wikipedia.org/wiki/Compression_ratio

Talks about specific heat of air.

Bruce,

Ok....I've done a little reading about pressure ratio, volume ratio, and specific heat ratios.....assuming an adiabatic process, here's what I get based on the equations in your wiki link.

PV^1.4 = Constant = 100,000 pa x (49 cc^1.4) = 2.32 x 10^7

P2 = 2.32 x 10^7 / ((49/8.8)^1.4) = 2,096,472 pa = 304 psi (pressure increase)

So...whats the Temp increase

PV/T = constant = 100,000 pa * 49cc/300K = 16,333

T= 2,096,472 pa * (49/8.8)/16333 = 715.9 K = 442.9 C = 829 F

Clearly these numbers....a pressure increase of over 300 psi and a temp of 829 F.....are not realistic either. "Note that in reality the ratio of specific heats changes with temperature and that significant deviations from adiabatic behavior will occur."

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Cuzz, I ain't ever felt no hot air coming outa the spark plug hole. Your professor might say hot but my finger says cold! for the sake of sanity enough said.

:facepalm:

 

[sschefer]

Re: Math question about engine cylinder volume/pressure

Cylinder temps and pressures can be precisely monitored via ionization. This is how it's done on DFI engines. You can do it too if you have the right equipment. You just need to monitor the resistance at the spark plug.

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

The engine cylinder is rated at 49ccs.
The bore is 39mm.
The stroke is 41.4mm.
The compression ratio is 8.8:1



Thanks in advance! I'm thinkin this might be an "entertaining" thread.

This has been entertaining........Just screw in a compression gauge and tell us the answer! Watch out for the hot air coming out of the spark-plug hole and this thread!!

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

This has been entertaining........Just screw in a compression gauge and tell us the answer! Watch out for the hot air coming out of the spark-plug hole and this thread!!

I'm thinking the "Thermal Mass" will absorb it quickly. :facepalm:

I have a question bigdee....if a tire is flat does it have any pressure in it?

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Ok...I think I have an estimate calculated that accounts for the specific heat of air.

Take the answer previously posted of 14.7 psi * 8.8 and again the answer is 129.4 psia...to compensate for the specific heat of air just multiply by the specific heat ratio of 1.4 and you get 181.2 psia.

Bigdee...to your post about the heat generated by compression....How does a diesel engine work?

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

You will never feel any hot air...ever! The reason is that if you ever try to feel it, it is not compressed. Any air that you feel out of a spark plug hole never got compressed in the first place. Even if it did, when a gas goes from a pressurized state to uncompressed(like the gas going through an expansion valve in an AC) it cools tremendously. If you compress it and then immediately decompress it, just like it is doing in the cylinder, you the air will end up being at the same temp as it started.

For all of you who thinks a 9:1 compression engine is going to give you 9 x 14.7, why is 180 PSI or higher a common compression measurement on a fresh engine?

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

Bigdee...to your post about the heat generated by compression....How does a diesel engine work?

I would love to hear his answer to that one!

http://www.kent4x4offroading.co.uk/how_does_a_diesel_engine_work.htm
In a diesel engine, the temperature of the compressed air reached 600?C(from the link above). A gasoline engine will be less than this because it has a smaller ratio but you can at least get the point of how much temp increase you can get.

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

Cylinder temps and pressures can be precisely monitored via ionization.

As far as I know, there are no autos in production that use ionization pressure sensors and they would never work for what is being measured here.

So what vehicles have them?

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

Ok...I think I have an estimate calculated that accounts for the specific heat of air.

Take the answer previously posted of 14.7 psi * 8.8 and again the answer is 129.4 psia...to compensate for the specific heat of air just multiply by the specific heat ratio of 1.4 and you get 181.2 psia.

Bigdee...to your post about the heat generated by compression....How does a diesel engine work?

Yes, of course compressing a volume of air generates heat and how much heat is created depends on the density and the amount of gasoline or diesel that has been mixed in as well as the compression ratio. I was just stating that for all PRACTICAL purposes heat is not a concern when checking compression. Maybe on a diesel with it's much greater compression ratio but on a cold engine with 9:1 and by just turning the engine over 2 or 3 times....no. However, IF you crank the engine LONG enough you will raise the temp and consequently the pressure on the guage.

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

Oh, I forgot to mention that... compressing air heats it and expanding it cools it. So any heat created on the compression stroke will be lost on the decompression stroke.

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Yes, of course compressing a volume of air generates heat and how much heat is created depends on the density and the amount of gasoline or diesel that has been mixed in as well as the compression ratio. I was just stating that for all PRACTICAL purposes heat is not a concern when checking compression. Maybe on a diesel with it's much greater compression ratio but on a cold engine with 9:1 and by just turning the engine over 2 or 3 times....no. However, IF you crank the engine LONG enough you will raise the temp and consequently the pressure on the guage.

The temperature increase/decrease of the air is instantaneous when it is compressed/released....the build up over time, is heat being transferred from the system to it's surroundings.