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Is it possible to find the combustion pressure on any ICE?

  1. Feb 9, 2007 #1
    What I am getting at is I would like to find out the psi of some engines.

    Is there a formula(s) to get good estimates or exactness of the pressure made in a cylinder.

    For example, you would need the cubic inch, static compression, cam specs, etc...and at a given position of the piston get a pressure number?

    I hope that made sense, thanks for any help or suggestions.
  2. jcsd
  3. Feb 10, 2007 #2


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    Good question.

    I keep meaning to learn Latex for moments like this, but haven't got round to it so please bear with me.

    You can express the mean effective pressure (in kPa) as:

    mep = (P*n)/(V*N)

    P is the power in kW
    n is the number of crank revolutions per power stroke (ie n=1 for a 2 stroke, n=2 for a 4 stroke)
    V is the displaced volume per cycle in cubic metres
    N is the engine speed in revs per second

    Now, this is the mean effective pressure. This is a very useful parameter when dealing with engines, because comparisons of mep are not engine-specific, such that comparisons can be made between different sizes and types of engines.

    It follows that the mep can be expressed in terms of torque, as follows:

    mep = (6.28*n*T)/V

    where T is the torque in Nm.

    Now then, peak pressures are much more difficult to calculate, because of the sheer number of different factors which can affect it: Engine speed, load, throttle position, fuel energy content, knock, volumetric efficiency, atmospheric pressure, residual cylinder temperatures, valve timing and overlap, compression ratio, spark timing, swirl, squish, tumble....

    If you want values for peak cylinder pressures, you really need to be measuring them.

    Typical values of
  4. Feb 12, 2007 #3
    Brewnog, I remember those formulas from my ICE class long ago. Is there an equation to calculate peak pressure if we make certain assumptions: full throttle/load, known boost, torque, cyl temp, valve timing. I made the assumption that peak cyl pressure occurs 15 deg ATDC. In my case, I have a solid model of an engine and I know the maximum hp and torque it will have to make. I want to do an FEA analysis on the model and I need peak cyl pressure(or a reasonable estimation) to go any further. I put together a small kinematic spreadsheet so I know piston position, velocity, acceleration, and rod angularity at any crank angle. Right now, I'm disregarding forces due to momentum. Any help would be appreciated.

  5. Feb 22, 2007 #4
    well to calculate peak pressure i would normally use chemistry and assume full combustion. but if the torque is known, then u can calculate the force translated to the piston, having the area of the piston we know that P=F/A.
    to find the force reaching the piston you would have to go through gear force translations...but i don't think its very difficult using this method
  6. Feb 24, 2007 #5
    I have the torque and I have used that to work backward to get the force on the piston and then the pressure. Only problem is in the case of detonation- cylinder pressures can increase while torque decreases. The end user is responsible for the fuel calibration and I expect the engine to see quite a bit of detonation. Using chemistry sounds like the way to go. Only issue is that this particular engine will use nitrous oxide. Typically these systems are rated in amount of horsepower added, but if I have the cross section of the nozzles and the bottle pressure, I should be able to calculate volumetric flow then mass flow. Could you show me how to set up a chemistry equation to calculate cylinder pressure? Fuel used is 110 octane racing fuel. Thanks.

  7. Feb 24, 2007 #6
    i'll work on the chemistry equation and see how to set it up. meanwhile i don't know if a formula like that could work, mayb u should check it out:
    Power= Force*distance/time distance is the distance travelled by the piston in an amount of time (which u can get from RPM)..and u have the power, then mayb u can calculate the force than the pressure. i'm not sure about the validity of this method, but mayb its worthwile that u check it. i'll tell u about the chemistry part later
  8. Feb 24, 2007 #7
    btw i would need the empirical chemical formula of the 110 octane racing fuel and NO2 amount (percentages, or moles...)
  9. Feb 13, 2009 #8
    I am new to this forum and don't really know how this works very well so I ask you to forgive me if I'm posting things in the wrong way. I have been in the process of developing an ICE which has a combustion chamber very similar to our current gas ICE engines. As we move toward building a non working prototype we are trying to estimate the torque and hp generated from this new engine. This new design, as I've stated, should be fairly easy to calculte if I can generate a certain value to the (pressure, or force from the combustion). What I have attempted to do is compare my engine, roughly a 2.9 liter, 4 cylinder engine with a 10:1 compression ratio, normally aspirated engine buring 87 octane fuel with a similar engine currently in developement. This engine is a Chevy 2.9 liter engine used for the Colorado edition truck. Now engine has a peak hp of 185 @ 5600 rpm's and 180 ft lbs of torque at 2800 rpm's. There are a few ways I can look into this. One is to calculate the average lever distance of the Chevy engine. With the stroke being approx. 4" this would be 1". If I am using the formula (force x distance = torque) would I be wrong to try to figure this equation and come to a number of 190 as a maximum force? If so, would I use this maximum force number when calculating my comparison with my engine?

    This new engine defenitely has a much better leverage advantage than the current ICE's. I need to know the estimate for its power and epa before we finalize the engineering for the prototype. Anyone have any ideas? I know this is a broad stroke and the question I'm asking may not be very simply answered. An anticipated thanks if anyone can help. Craig
  10. Feb 13, 2009 #9
    Correction, the Chevy engine has 190 ft lbs at 2800 rpm's. Thanks
  11. Feb 14, 2009 #10


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    Do you know exactly what the differences between the two engines are?
  12. Feb 14, 2009 #11
    Thanks for the response Brewnog,

    Both engines have similar displacements, compression ratio, spark plugs, fuel injected burning 87 octane fuel. The Chevy and Jones should have similar airflow through the intake and exhaust valves. Having said this the Jones Engine has a curved cylinder and operated perpendicular to the crankshaft. Its cosistantly equally distant from the center of the crankshaft throughout the cycle so its leverage is approximately 6.86x longer than the Chevy comparison.

    What I wish to accomplish is to have a reasonable value which I can then apply to the formulas above and achieve a credable estimate of its torque and hp. With this I can estimate its epa as well. I have performed all of the calculations and feel pretty good about them but I can't objectively show interested engineers the value of "force" used to determine the power output. Any help would be greately appreciated as this design has some major advantages over our current designs and has a real chance at changing our current ICE designs for a variety of applications.
  13. Feb 14, 2009 #12
    I guess another way to put it would be to determine a reasonable and credable average MEP from the Chevy. I could then use this value as a foundational starting point for the Jones calculations. I know there are many other variable which can change this but were talking about a good starting point.
  14. Feb 14, 2009 #13


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    I don't quite follow. With your talk of curved cylinders, perpendicularity, and something being constistently equidistant from the crankshaft, I don't understand how the engine works. Is this an engine of conventional design or not? If not (i.e. if it doesn't have reciprocating pistons in a cylinder), the 'usual' equations we use for performance predictions may not hold true.
  15. Feb 14, 2009 #14
    If you give me your email address I will send you a picture of an old design so you have a better understanding.

    I understand and agree with you that the normal equations won't apply to the engine. What we can somehow understand is the simple pressure from the piston (upon the connecting rod) on the Chevy engine.

    If I know the torque of this Chevy engine at a particular rpm range, I also know the average lever distance from the connecting rod to the crankshaft, I should be able to use the formula above to determine the pressure (force x distance = torque). If this pressure is a certain value from the combustion at this rpm couldn't we produce a comparative value that could be used from a similar model combustion chamber? I'm looking at a value of force that could be used to compare engines. If the Chevy is considered a certain maximum torque at 2800 rpm's, this would be its maximum pressure and torque in the power curve (right)? If I took that value (of force) and applied it to a similar combustion chamber would this be an accurate discription of the maximum force at that rpm?

    I may have to let you see the prints before you can give me your help. I may be looking at this in the wrong way. If this is the case I appologise in advance. Craig
  16. Feb 15, 2009 #15


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    Click my user name to send the email.

    Read the posts above, you can't use the average torque at a set rpm to infer instantaneous cylinder pressures. You can compare MEPs, and (given a lot of assumptions), you may be able to equate the MEPs of your two engines. But don't bank on it, it's a very sketchy assumption to say the two engines have the same MEP.
  17. Feb 15, 2009 #16
    I appreciate your response.

    Mayabe you will have an idea of how I may achieve what I need. Is it possible to compare two engines similar to the Jones and Chevy 2.9 by saying the Chevy produces this value of force to achieve this torque and this horsepower. Using this value of force one could assume reasonably that the Jones would then produce this torque and this horsepower? If this is not reasonable do you have any idea how you would compare these engines (without actually building the Jones and testing it)? I was hoping to use the idea of pressure from similar combustion chambers to compare the two. Am I wrong to move in this direction in comparing them?
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