# How to calculate BMEP from P-V Graphs?

• nb89
In summary, the conversation discusses the process of calculating BMEP from a P-V trace for different engine conditions. The suggested methods include integrating PdV to determine work and using a planimeter or numerical integration in Excel. The importance of using gage pressure and average pressure is also highlighted.

#### nb89

I have a P-V trace for a wide open throttle and a part load. Is it possible to calculate the BMEP from these graphs?

BMEP = ( Hp x 13000) / ( liter displacement x RPM)

that doesn't really help considering all i am given are values of different pressures and volumes

If you have pressure and volume you may integrate PdV to determine work, then divide by time to determine power.

For example, if you have a table of values that give pressure and volume, you should be able to take average pressure (gage pressure, not absolute) and change in volume, multiply to obtain work, then sum all work done over a single cycle (360 degrees of crankshaft rotation for a 2 stroke, 720 degrees for a 4 stroke) and divide by time it takes for that cycle to get power.

Sure, integrate to find the area enclosed by the PV curve; this gives you the IMEP.

Q_Goest is bang on the money with this one, I used a similar type of thing in my masters disseration. Designing a vintage V8 racing engine using simulation techniques. huzzah.

Integrating real curves is a *****, either find a program that can make approximate curves from a set of data points or break out the graph paper.

xxChrisxx said:
Integrating real curves is a *****, either find a program that can make approximate curves from a set of data points or break out the graph paper.

If you have no better software than Excel, you can use a trapezoidal numerical integration approach reasonably well so long as the x-interval is pretty small compared to the sample length.

But when it comes to a manual method, use a planimeter! I've mentioned before with PV diagrams, they're absolutely fantastic and much better than counting squares :)

Those are two marvellous suggestions. I feel especially silly about the numerical integration in excel as that is probably the easiest way to do it, I should have known that but never even thought to use the trapezoid rule. :(

xxChrisxx said:
Those are two marvellous suggestions. I feel especially silly about the numerical integration in excel as that is probably the easiest way to do it, I should have known that but never even thought to use the trapezoid rule. :(
Actually, if you used the average pressure between two datapoints then you used the trapezoid rule! :)

## 1. How do I determine the BMEP from a P-V graph?

The BMEP, or brake mean effective pressure, can be calculated by dividing the area under the P-V curve by the displacement volume of the engine. This can be done by hand using a ruler to measure the area or by using computer software.

## 2. What units should be used when calculating BMEP?

BMEP is typically measured in units of pressure, such as pounds per square inch (psi) or kilopascals (kPa). However, it is important to use consistent units throughout the calculation to ensure accuracy.

## 3. Can BMEP be calculated for any type of engine?

Yes, BMEP can be calculated for any type of engine as long as the engine's P-V curve is available. However, it is important to note that BMEP is typically used for internal combustion engines.

## 4. How does BMEP relate to engine performance?

BMEP is a measure of the average pressure exerted on the piston during the power stroke of an engine. It is used to evaluate the efficiency and power output of an engine. Higher BMEP values indicate better engine performance.

## 5. Are there any limitations to using BMEP as a performance measurement?

While BMEP is a useful metric for evaluating engine performance, it does have some limitations. It does not take into account factors such as engine size, speed, or fuel type, and it may not accurately reflect real-world driving conditions. It is best used in conjunction with other performance measurements for a more comprehensive analysis.