fleshbox

Ok, having still not gotten my lude I am in a state of "what do I want to do once I got my lude?"

I was thinking at work Thursday or Friday (well, I gotta do SOMETHING with my brain whilst repairing leaking cylinders) and I had some pretty deep stuff begin to take shape. I talked to my old boss (the dept I transfered from a few weeks ago) and we couldn't completely agree about this so I thought I would post here and see if there are any engineers or the likes who have some serious input for this little discussion. So without further aideu...

These are the facts we agreed upon:
1 : Free air at sea level is approximately 14.7 PSI.
2 : PSI ratings are relative to the container in question. ie free air is 0 PSI relative to the atmosphere or the air in an air tank is 120 PSI relative to free air.
3 : Pressure ratings of compressors are values over free air (14.7 PSI atmospheric) thusly relative to the planet as a whole 15 PSI is in actuality 29.7 PSI.
4 : If something is TRUE 0 PSI you have a vaccuum (space would be 0 PSI as there is no air there)

These are the points we dissagree on to some degree or another:
1 : The air drawn into a cylinder (in a compustion engine) is approximately 14.7 PSI (atmospheric free air.)
2 : The combustion ratio of an engine is simply putting PSI (or Bar) into a ration ie a 5 : 1 compression ratio would be 14.7 * 5 or 73.5 PSI.
3 : If a combustion engine has a CR of 10 : 1 (I think thats the stock CR of an H22) the effective pressure inside the cylinder is approximately 147 PSI.
4 : If you have a FI setup putting out 15 PSI you are starting out with 29.7 PSI in the cylinder before compression. This is double volume of air. theoretically doubling the displacement of your engine.
5 : If you take that 29.7 PSI and apply a 10:1 CR to it the resulting pressure inside the cylinder is 29.7 * 10 or 297 PSI.

Ok, that is the jist of my thoughts and the conversation with my old boss. If my thoughts are correct then I can easily understand why it is so easy to blow a motor with turbo... 300 PSI is a WHOLE LOT of pressure.

Anyways, I look forward to hearing your thoughts on this. Believe it or not all of this does have some basis in practical aplication of engine mechanics for me. I tend to be a thinker, and I like to fully understand the science behind things before I do them.

Peace

__________________
-Jesse-

 

DrSeus

This is pretty interesting however I wanted to point out a couple of things.

The absolute pressure in the cylinder maybe 300 psi but there's also 1 atmosphere in the block on the other side of the rings so the pressure exterted on the rings is only 286 psi. This is a minor point but just thought I'd mention it.

Also 300 psi is really not that much. It may be more than what our stock engines can take, but in the grand scheme of things, it's low. My scuba tank regularly gets filled to 3200 psi. I'm not even supposed to let the pressure drop below 500 psi! Of course, my scuba regulator is designed to operate at these pressures.

Another example of perceived high pressure are bicycle tires. I pump up my bicycle tires up to 120 psi. Racers run 150 psi and track riders run 180 psi. While these numbers are way below 300 psi, I'm just throwing them so people can get a psi reference.

As for the H22 engine, the cause of failure to not seem to be ring failure due to compression but ring failure due to detonation. I don't know the pressures that result from a detonation but I'm sure it's much higher than 300 psi. Undoubtedly the increased pressure will add to premature ring failure with respect to an N/A engine but it may be negligable. The square area of a piston ring can't be that large.

Just thought I'd toss in my 2 cents.

kennethmina

PSI can be expressed as psia or psig.

psia = absolute value that is relative to the TRUE vacuum (assume 0 psia which is just a theoretical value that still can not be obtained by scientist);

psig = gauge value that is relative to atmospheric pressure local sea level. (local sea level can be different from standard sea level at different places)

commercial pressure gauges usually express their readings in psig which is relatively to atmospheric pressure.

The air pressure at the intake stroke into a cylinder is actually less than the atmospheric pressure due to the flow restriction such as air filter, manifold and etc. therefore when the piston pulls down (creates a small vacuum condition, just like the pipet) during the intake stroke, the pressure difference between cylinder and surrounding therefore let the air be drawn into cylinder but because of those flow restriction the flow is not fast enough to equalize the pressure between. that's why it is less than 14.7psi.

combustion ratio of an engine depends on what kind of fuel you use and at what physical condition the combustion occurred (temperature and pressure).

Compression ratio is actually indirectly related to pressure. Compression ratio is actually the ratio of total volume @ BDC (displament + clearance) to the volume @ TDC (clearance volume). However, if you assume during the compression process the air inside the cylinder is IDEAL GAS then only you are right. Otherwise, it is not true.


Wrong. Doubled the pressure means halved the volume (ideal gas case) but since you have the same displacement. so that means you doubled the amount of air. And, I personally don't think you should take account of the atmospheric pressure (14.7psi) into your calculation. your boost pressure (15psi) is the only pressure you use for your calculation coz you are interested in the pressure difference (or gauge pressure). As the surroudings and your engine have reached an pressure equalization. that's why you don't feel pressure of 14.7psi on your body. lol.....

and like I said, you can't just use the compression ratio for your calculation coz it is not right. compression ratio has nothing directly to do with pressure. it is just the ratio of volume and based on the volume changes the pressure also changes.. but they are not the same ratio unless you assume the air in that process is ideal gas.

__________________
Ken

fleshbox

Well when you calculate air demands in a pneumatic system (I know it's not really a pneumatic system) you calculate every 15 PSI as doubling the free air volume that is compressed inside the tank. ie a 1 cubic foot tank pressurized to 15 PSI contains 2 cubic feet of free air. This is at least how we figure total air volume in a system.

I hadn't thought about the addition of the fuel to the equation, however considering the addition of a liquid into the cylinder causes me to believe that the amount of pressure inside the cylinder would be higher due to liquids being less compressable than air.


From what I understand of scuba gear the air tanks do not contain pressurized air, rather they are filled with the major gasses that make up the air we breath. therefore your scuba tank that is pressurized to 3200 PSI is actually filled with a liquid. As you use gas the temperature inside the tank raises causing the liquid to "boil" and expand, pressurizing the tank. This is how CO2 works and I believe Oxygen and Nitrogen are the same.
Free air is much less compressable than gasses such as nitrogen, oxygen, etc... free air compressed to 300 PSI is pretty scary.

well thats my viewpoint. Iwould be interested to see what the pressure inside the cylinder is... I think I am going to put together a makeshift pressure guage that I can check it with and see actual PSI numbers...

Thanks mucho for your input... I love having conversations about this kind of stuff

__________________
-Jesse-

kennethmina

This is true only if you depressurize the total air back to atmospheric pressure. You can't just express the amount of air in volume coz in our case the volume of cylinder is fixed. And also with different pressure the volume can be different. So, i am not saying that you are wrong.. it is just the concept of your calculation is incorrect by saying that doubling the pressure will double the volume. It actually doubles the amount (mass) of air within the same volume instead of doubling the volume. Like i said, just the concept.

addition of the fuel will not affect too much on your calculation because it contains only 7% (AF of 15) of fuel vapor in the cylinder. and remember fuel vapor pressure is way too low than air or other gas vapor pressure. You can't compare the fuel vapor pressure with those scuba highly pressurized tank. that's why you are still be able to see liquidic fuel at the room temperature and normal atmospheric pressure condition. But you can never see any liquidic air at normal room condition coz the fuel vapor pressure is much smaller than atmospheric pressure that is not enough to boil the fuel at normal room condition.

__________________
Ken

kennethmina

prove me wrong if you can.

__________________
Ken

fleshbox

I see what your saying... And I agree totally, I wasn't trying to say it LITERALLY doubles the volume... but effectively... or as you stated, mass... I stand... er... I guess corrected



I'm not exactly sure what you are trying to say there... I think you may have gotten two separate responses mixed up... When I was talking about the scuba tank being pressurized to 3200 pounds I was talking about the difference between compressed air and compressed gases in response to Dr Seus... When I said that liquid is les compressable than air (speaking about fuel/gasoline) I was basically agreeing with the last statement in your post about assuming compression ratios as direct pressure ratings.

As far as the CR corresponding to PSI I would argue that the compression ratio is very much related to pressure rating (assuming that you are using the same pressure source for both guages) to explain... The basics of pneumatics tells us that when you pressurize a tank to 14.7 psi you have doubles the amount of air inside the tank (mass not volume as volume is a physical measurement of space and not density as you have pointed out ) This corresponds to a 2:1 compression ratio, again assuming that the CR guage was set up to read the tank. an additional 14.7 PSI would read as a 3:1 CR because you have added another atmosphere to the tank, You now have 3 atmospheres where there is only room for 1 this is a ratio of volume to volume as air does have volume and by doubling or tripling the amount of air in the tank you are comparing air volume to tank volume (tank is 1 each atmosphere of air inside the tank is one) This is how we get compression. Because The air inside the tank usually takes up more room it has to be compressed in order to fit in the tank. The 14.7 PSI atmosphere is an off handed measurement of how much air there is. Now as I think you stated The cylinder is a dynamic vessel in opposition to an air tank. The air tank will ALWAYS have the same volume (volume as in space, not volume as in air) whereas the cylinder starts out say, 2 inches in diameter by 10 inches long. This theoretical cylinder has a volume of pi(r*r)10 or 3.14159(1*1)10 that works out to 31.4159 cubic inches. Now the cylinder if filled with air (I will assume 14.7 PSIA for simplicity, knowing that the restriction of the air filter could theoretically lower this) and the piston begins its compression stroke thus altering the volume of the cylinder. Now in my little world here this cylinder has a stroke or 8.5 inches. At the end of the stroke the cylinder is now 2 inches in diameter and 1.5 inches long. To work out the volume of the cylinder... 3.14159(1*1)1.5 This works out to 4.712385 cubic inches. Now this would be represented as a CR of 6.67:1 as the final volume would fit inside the starting volume 6.67 times. However there has been a compression (obviously since it's called a compression ratio) To find the amount of compression You simply take the physical volume of air (volume being thephysical size) that was present inside the cylinder to start with (31.1459 cubic inches) and the physical volume after compression (6.67 cubic inches) and find the ratio of compression as the air is now taking up this much less space it has been compressed. With the ratio of air to compressed air you will multiply the pressure inside the cylinder before compression (The relative free air of that volume) by the compression multiplier, this being the compressed side of the ratio, or in this case 6.67. Ultimately you have this 14.7 * 6.67 = 98.049 PSI. Now the pressure inside the cylinder is 98.049 PSI. So CR is directly related to the pressure inside the cylinder, as the air inside the cylinder is being compressed pressure is raising. Which brings me back full circle to where I began...

If you start with air that is already compressed to double what is usually available and then compress it again you get hella pressure inside your motor. While I will agree that the approximately 286 pounds of pressure on the ring lands at 15 Pounds of boost probably wouldn't cause the rings to fail, Think about everything else that pressure is pushing on, valves, gasket seals, the cylinder wall (this is why sleeving is crucial in high boost I would think...) Now add to that the fact that you also have more fuel inside the cylinder... wow that leaves a ton of room for mistakes... Of course this also shows why FI builds so much power... it's like having a larger motor without having the extra weight...

Building a "bulletproof" FI motor requires a lot of knowledge, and knowing how the physics of your motor work would seemingly make building a more reliable FI motor at least a little more attainable. It is my goal to do just that... buiild the best FI motor I can possibly build. Knowing the sciencs and technology can also help fight higher costs, as the knowledge can allow me to do a little more myself...


blah blah blah... ok I'm rambling now (it's almost 3 am now and my brain is fried... I think about too much deep stuff during the day )

__________________
-Jesse-