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Volumetric Effeciency & Indicated Mean Effective Pressure Question
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Posted by: racer92
08/24/2003, 17:39:55

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Hello everyone,  got a question for you.  I am not very versed in this topic so please bear with my ignorance of this issue.

I own/race a sprint car.   In the years Ive been around the dirt track, my best friend and I have always built our own race engines.  We both are relatively educated on issues related to engine building and have a machine shop.  (we also have a Superflow flow bench)

My son is wanting to do a Science Fair project this year with the topic concerning the kind of work we do with cylinder heads.  Meaning, we modify the ports & runners to increase airflow and therefore increase volumetric effieciency.  We use our Superflow bench to keep tabs on how much flow we are increasing and how many CFMs we are adding to the fuel/air charge in the cylinder.  

My son wanted to take a set of cylinder heads,  and prove that modified airflow and increasing volumetric effieciency will result, usually, in increased horsepower.  Hence, a faster sprint car.   We have the flow bench to prove/document what we are doing with the heads,  however unless we build the motor and put it on a dyno, we cannot really prove the increase of indicated horsepower.

So we had the idea of using the formula of PLANK to create a theoretical engine, and let PLANK tell us how much appreciable indicated (or brake) horsepower we are adding by increasing the volumetric effieciency of the cylinder head ports.

IHP = PLANK/33,000

P - indicated mean effective pressure

L - length of stroke

A - area of piston head in square inches

N - number of power strokes per minute

K - number of cylinders

Using this simple formula, one can get a reasonable idea of horsepower being created at a given RPM.  And with this formula, one could dyno a theoretical engine, without ever having built it yet.  (granted this isnt accurate, but a good tool for estimation purposes to use in lieu of actual dynometer data)

Now, here is where I am in dire need of assistance.  

Does anyone know a formula, or know where to find data on how to determine Indicated Mean Effective Pressure without having the actual motor to get it from?   In his experiment, my son is wanting to use PLANK to get to indicated horsepower, but without the IMEP, he cant get there.  Now, you can get IMEP if you already know the indicated horsepower;  as you back the IMEP out going in reverse.  But in our instance, we do not know IMEP.

Further, is there a formula that could correlate the increase in IMEP that would be realized when volumetric effeciency (VE) is increased?  Meaning, if we can get a cylinder head to flow 45% better with alterations to port shape, size and such - - how can we predict what the increase in IMEP will be if the VE is increased 45%?

In our real world experience, when we build motors we merely get a dyno pull to tell us how we did in regards to the motors performance.  But this is just one situation where we are having to test a imaginary engine, and what it would do based upon the increase of airflow in the heads.

Thanks in advance for any help!


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Re: Volumetric Effeciency & Indicated Mean Effective Pressure
Re: Volumetric Effeciency & Indicated Mean Effective Pressure -- racer92 Post Reply Top of thread Forum
Posted by: taffycrook

11/26/2003, 18:12:37

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imep is the mean pressure available throught the stroke to do all the mechanical work before all the friction losses have been taken into account.

You can only find it from actual tests or indicator diagrams.

For your needs bmep would be more help but this too is only available from torque figures. But it relates to real bhp figure not the indicated power which is the power before frictional losses are taken into account.


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Re: Volumetric Effeciency & Indicated Mean Effective Pressure
Re: Volumetric Effeciency & Indicated Mean Effective Pressure -- racer92 Post Reply Top of thread Forum
Posted by: hnic

11/13/2003, 03:38:38

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IMEP/BMEP are really specific torques, in fact some people use that principle to record BMEP. IMEP on test engines is usually determined by indicator cards but there are formulas to determine it mathematically, one being work/(V1-V2). It would take some research to find a formula to go with what you have determined, but it should be do-able. A good book about engines is The Internal Combustion Engine in Theory and Practice by Charles Fayette Taylor. It is available at many libruaries or you could find other engine books in the 621.43 or TJ785 section.

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