Cylinder head pressure at expansion phase

  1. Does anyone know what the pressure is inside the cylinder of an internal cumbustion gasoline engine at the expansion phase? ive been doing some research and ive seen answers ranging from 200psi to 600psi. seems like a significant range, does anyone know where i can get somewhat more accurate information? i dont have a specific engine in mind, But i have to assume there is an industry standard for this pressure.
     
  2. jcsd
  3. Mech_Engineer

    Mech_Engineer 2,299
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    I'm assuming you mean the power stroke in a 4-stroke engine...

    Unless you have a specific engine in mind, you won't be able to get a good number. There is no "industry stardard" on pressures, it's completely engine dependent.
     
  4. thanks for the reply. is there a way to look it up based on the specific engine - is there a table or website that quotes these measurments?
     
  5. Mech_Engineer

    Mech_Engineer 2,299
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    This is a pretty rare measurement because it is difficult to get. Overall, the pressure is not constant during the power stroke (obviously) so what you might find is a peak pressure or average pressure. It's not really a factory-supplied metric.

    I would guess the peak pressure in an engine is an order of magnitude greater than what you heard before which could be an average of some kind (200-600psi). High-compression engines could have just a test air compression of 150psi or more, so during a power stroke I would not be suprised if during combustion the pressure spiked to thousands of psi, considering the amount of torque engines can put out.
     
  6. Thanks for the info. i didnt think it would be thousands of psi, and you are correct i am curious of the peak pressure, not the average. i was thinking 600psi might be closer to correct, because ive seen references that the compression phase (which seems to be documented better) is 150-200psi, so i assumed with ignition, the explosion would create a significant increase over the compression pressure. I guess i just didnt account for the explosion creating that significant of an increase.
    I appreciate your help in trying to figure this out.
    Thanks!
     
  7. Mech_Engineer

    Mech_Engineer 2,299
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    Well, I did a little searching of my own and it seems you are correct in assuming peak pressures will be around 200 psi. High performance engines may be higher, but probably not more than 400 psi or so...
     
  8. WOW....thanks for looking into it, that is really nice of you. do you have a reference i can take a look at?
     
  9. Mech_Engineer

    Mech_Engineer 2,299
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    I looked around at a couple of different sites and discussions. One interesting paper I found here discusses measuring the pressure in a Skoda engine with about 67 hp:

    http://www3.fs.cvut.cz/web/fileadmin/documents/12241-BOZEK/publikace/2004/Kragujevac-Bl.pdf

    I also ran accross a paper from Harvard that discusses a computational model used to try and estimate the pressure in a spark ignition ICE. The calculations are then compared to actual measurments on an engine.

    http://www.hcs.harvard.edu/~jus/0303/kuo.pdf

    Both papers end up measuring and calculating a peak pressure in the cylinder of about 200 psi.
     
  10. brewnog

    brewnog 2,791
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    Your figures are of the right kind of order. I'd work on 30bar (435psi) being a general maximum peak cylinder pressure for a typical automotive SI engine.
     
  11. From doing some digging around the web, I found that most engines are designed for PCP to occur around 10° - 15° ATDC. If we know the torque and the stroke we can work backwards to determine the amount of pressure the piston sees.


    Example L91 (GM 1.6L) engine:


    Bore & Stroke: 3.11" & 3.21"


    CR: 9.5:1


    Torque Peak: 107 Ft-Lbs @ 3600 RPM


    Take the peak torque of 107 Ft-Lbs, divide that by half the stroke in feet, 0.13375' we get 800 POF (pounds of force). If PCP occured at 90° ATDC than we would only need 105.3 PSI to generate full torque. However as stated above engines are designed for PCP at 10° - 15° ATDC at which point we take the sin(10°) and get: 606.5 PSI or sin(15°) and get: 406.9 PSI.

    So with this particular engine that is our range between 406.9 and 606.5 PSI.
     
  12. Mech_Engineer

    Mech_Engineer 2,299
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    Hmm, after re-reading the Harvard paper dealing with cylinder pressure, it would appear the maximum pressure in the cylinder at full load at 4000 rpm was about 5320 kpa, which equates to 771 psi, and the peak pressure occured at 20 degrees after TDC.
     
  13. NateTG

    NateTG 2,537
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    Um...
    You're ignoring the mechanical effects of the pushrod linkage.
    You're assuming that the torque rating (which should be a time-average) is the same as the instantaneous torque at ignition.
    You're also ignoring that the torque rating is the net torque and that while one cylinder is firing, other cylinders are getting compressed, going through power strokes, or venting exhaust. (The L91 is a 4-cylinder engine.)
     
  14. hello ....can the people who reply later give me the following information or any help regarding this ? .....i want to know the pressure at which the exhaust waste gases leave the IC Engine chamber en route to the Three way catalytic converter ......
     
  15. can someone here tell me the pressure at which the exhaust waste gases leave the IC Engine chamber en route to the three way catalytic converter
     
  16. NateTG has a very lucid point. The losses are enormous and I think unaccountable. I think also that there are throttling losses accosiated with actual running conditions. However, minimum pressure should be able be to calculated if losses are put aside for the moment. Then accountable losses just add up to more pressure.
     
  17. any idea , what is the material inside a internal combustion engine need to be change when i change the gasoline fuel into hydrogen fuel.? Also what is the lubricant that will suitable for this hydrogen fuel. thx
     
  18. brewnog

    brewnog 2,791
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    Please don't submit multiple posts on the same topic. Your question has been answered elsewhere.
     
  19. I have tabulated cylinder pressure data for an diesel engine of relatively old design (pre-1956) that shows a maximum cylinder pressure of 785 psig at 15 deg past TDC.
     
  20. ok, sry for that, i am a newbie in this forum,
     
  21. NateTG makes a very good point when he says, "You're assuming that the torque rating (which should be a time-average) is the same as the instantaneous torque at ignition.
    You're also ignoring that the torque rating is the net torque and that while one cylinder is firing, other cylinders are getting compressed, going through power strokes, or venting exhaust. (The L91 is a 4-cylinder engine.)"

    If you could see the instantaneous torque output from an IC engine, you would observe that there is considerable pulsation in it. This is the reason that IC engines are notorious for exciting torsional vibration in the machinery trains that they drive. The rated torque is most definitely an average torque, averaged over all cylinders and over a full thermodynamic cycle which is two crank revolutions for a 4-stroke engine.

    In a multicylinder engine, when one cylinder is on the power stroke, another cylinder is on the compression stroke, so the net output torque of the engine is reduced from that of the firing cylinder by the amount required to compress the second cylinder (assuming just two cylinders for simplicity). Furthermore, there are many losses in an IC engine that also reduce the output torque including bearing friction (the main bearings are heavily loaded during part of the cycle!) and power required to drive the cam shaft and distributor, water pump, oil pump, fan, generator, etc. All of these so called "parasitic loads" reduce the available output shaft torque because they come straight off the crank shaft before any torque is delivered to the outside world.
     
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