Math question about engine cylinder volume/pressure

[scanman]

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

The question is; How many kpa/psi should I see on the compression tester?

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

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

Atmospheric pressure is 14.7psi x 8.8 = 129.36 psi.......I guess?

 

[dwco5051]

Re: Math question about engine cylinder volume/pressure

In an ideal world this would hold true. When you start figuring in leakage around rings and cranking speed this will vary. Also in what position are the throttle plates. Restricted air flow can cause a vacuum in the intake and thus not a full charge of air in the cylinder. Also 14.7 (29.92" Hg) is the average at sea level so that theoretical figure will vary dependent on local weather conditions. A good answer would be less than 130 psi.

 

[JB]

Re: Math question about engine cylinder volume/pressure

You don't measure absolute pressure with a guage, you measure guage pressure, which is the difference between ambient and absolute pressure.

Example: If you try to measure ambient pressure you will get a reading of zero, not 14.7psi.

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

Atmospheric pressure is 14.7psi x 8.8 = 129.36 psi.......I guess?

"The question is; How many kpa/psi should I see on the compression tester?"

Absolutely no way of figuring this unless you have data on all the variables....I was giving you a theoretical answer on compression ratio.....but what you see is what it is and that guage is showing the amount of atmospheric pressure that is being compressed.

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Assuming air as an ideal gas.....P1*V1=P2*V2...solve for P2......P2 = (P1*V1)/V2. V1 = (49.5 cc + X) based on bore and stroke provided. X is the clearance volume between the Head and Piston at TDC. V2 = X. Compression Ratio = 8.8 = (49.5 + X)/X therefore X = 6.3 cc.

V1 = 55.8
V2 = 6.3
P1 = 14.7 psia

Therefore P2 = 130.2 psia....(130.2-14.7) = 115.5 psig

As posted earlier.... since X is constant for both the top and bottom of the stroke you can simply take the compression ratio of 8.8 * 14.7 = 129.4 psia and hence 114.7 psig.

This is in a perfectly sealed cylinder assuming air behaves an an ideal gas. The difference between 115.5 and 114.7 is rounding error. It also assumes an isothermic system (no change in temperature)

 

[scanman]

Re: Math question about engine cylinder volume/pressure

I'm really glad to have math experts at my fingertips! From the posts above I'm guessin anything above 90psi would be acceptable pressure to create combustion? This is a 50cc Baja dirt bike I picked up at a yard sale.

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

"The question is; How many kpa/psi should I see on the compression tester?"

Absolutely no way of figuring this unless you have data on all the variables....I was giving you a theoretical answer on compression ratio.....but what you see is what it is and that guage is showing the amount of atmospheric pressure that is being compressed.

You have to subtract 14.7 from 129.36 to get gauge pressure. 129.36 is absolute pressure.

 

[tswiczko]

Re: Math question about engine cylinder volume/pressure

http://www.asknumbers.com/kilopascal-to-psi.aspx

 

[bigdee]

Re: Math question about engine cylinder volume/pressure

You have to subtract 14.7 from 129.36 to get gauge pressure. 129.36 is absolute pressure.

Yeah your correct it is to easy to forget about the 14.7psi around us at sea level......I hope no one gets confused on tire pressure now. 74.7psi in those little trailer tires!

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Yeah your correct it is to easy to forget about the 14.7psi around us at sea level......I hope no one gets confused on tire pressure now. 74.7psi in those little trailer tires!

Tire pressure is labeled as gauge pressure, should be no confusion........just trust the gauge

 

[JB]

Re: Math question about engine cylinder volume/pressure

You didn't tell us it is a 2 stroke! You must recalculate the stroke from the opening of the exhaust ports, not from the bottom of piston travel.

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

Also, nobody mentioned the fact that by reducing the volume you are also increasing the temperature which causes the pressure to go up. The ideal gas law says that pressure is inversely proportional to volume but that is assuming a constant temp. By exerting work and decreasing the volume, the energy you are exerting increases the temperature...a lot!.

The absolute volume of the cylinder means absolutely nothing here. Its the compression ratio that determines the amount of pressure you are measuring. It could be 50cc or 500cc. The PSI you measure will be exactly the same.

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Also, nobody mentioned the fact that by reducing the volume you are also increasing the temperature which causes the pressure to go up. The ideal gas law says that pressure is inversely proportional to volume but that is assuming a constant temp. By exerting work and decreasing the volume, the energy you are exerting increases the temperature...a lot!.

The absolute volume of the cylinder means absolutely nothing here. Its the compression ratio that determines the amount of pressure you are measuring. It could be 50cc or 500cc. The PSI you measure will be exactly the same.

Yes...I did in my post....

It also assumes an isothermic system (no change in temperature)

Agreed that the total volume is irrelevant unless you don't know the compression ratio. I could blow the dust off my thermodynamics book if we want to torture ourselves....

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

Yes...I did in my post....

Of course we are definitley not not talking isothermic here so any calculations assuming so are not correct. Temperature is actually a huge factor.

Not uncommon for a 8.8 compression ratio to cause compression readings of 180 PSI. You need to factor in the specific heat of air which is around 1.4.

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

Of course we are definitley not not talking isothermic here so any calculations assuming so are not correct. Temperature is actually a huge factor.

Was only an assumption used in order to simplify the math, of course there will be a temp increase when compressing air. The question is how much? Does the relatively small instantaneous increase in temperature due to the compression, effect the answer enough to justify taking it into account? The answer is no....we're splitting hairs.

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

The specific heat of the air is what all the previous calculations don't take into account. And that is definitley not splitting hairs. As a mechanical engineer, I am surprised you didn't mention this. Forget your thermodynamics?

As a scuba diver you also know that when tanks are filled, they are put into a tank of water. This is removing all the heat added to the air during the compression process. If you don't do this, as the air cools the pressure would drop causing a low fill.

 

[1980Coronado]

Re: Math question about engine cylinder volume/pressure

The specific heat of the air is what all the previous calculations don't take into account. And that is definitley not splitting hairs. As a mechanical engineer, I am surprised you didn't mention this. Forget your thermodynamics?

As a scuba diver you also know that when tanks are filled, they are put into a tank of water. This is removing all the heat added to the air during the compression process. If you don't do this, as the air cools the pressure would drop causing a low fill.

No, I haven't forgotten my thermo....I have nightmares about that class ....I'm not discounting the fact that there is heat generated by the work being done. It definitely is part of the physics involved. However, I do not believe it would add enough heat to cause the calculated pressure to be off by some order of magnitude....The cylinder is a very small volume and the compression happens very quickly. I just don't think the assumptions made in the calc. would cause the results to be off by more than a few percent....a couple of psi maybe. I'm not curious enough to blow the dust off the thermo book to figure it out though ...Just saw your 180 psi...big difference from 115.....may have to rethink this....hmmm. I don't have any idea what the compression numbers should be on a dirt bike! I'd much rather just look it up in a service manual....I'm lazy ....We also assumed dry air for the record....air has water in it...just sayin. I mean, how far do we want to take this.....

There's a big difference between the scuba tank analogy and a small cylinder. A scuba tank is a much larger volume and the pressures are 30 times greater. With the cylinder, your talking a delta P of 100+ psi, with a scuba tank you're talking about a delta P of 3000 psi. Much more work being done to compress a much larger volume of air to a much higher pressure. That translates into a much larger amount of heat being generated. Wouldn't you agree?

I'd like to know if the compression was actually measured by the OP....what was it? Now I need to know...I'm being tortured!

 

[hrdwrkingacguy]

Re: Math question about engine cylinder volume/pressure

This about sums it up...

 

[bruceb58]

Re: Math question about engine cylinder volume/pressure

On a new engine with between 8 and 9 compression ratio, the compression will be at the 180 PSI level or so. Obviously if you just do the ratio with pressure vs volume, you won't get that number.

This about sums it up...

The OP wanted an entertaining thread and he is getting it!