BMEP gauges in prop-liners.

[PRB]

There are a number of piston ships out there with Brake Mean Effective Pressure (BMEP) gauges. They don't all behave the same. Some of them simply indicate higher values as manifold pressure increases, while others, in addition to that, also indicate higher BMEP as RPM is reduced. This I don't understand. I've read the definition of BMEP, but it doesn't help. For the piston engine experts out there, two questions: 1) why does BMEP go up as RPM comes down? 2) Why do I care what the BMEP is anyway? Somebody went through the trouble of placing BMEP gauges in the instrument pane, so it must be important. The Cal Classics CV-240 has such gauges, and also has a long detailed document on realistic engine management, in which BMEP is never mentioned, just MP and RPM. Am I supposed to use this gauge to keep the RMP/MP combination within the desired range? Because if you just reduce the RPM at high MP levels, the BMEP needle goes way past the red mark on the gauge...

 

[Bjoern]

If the RPM comes down, you're either aborting a takeoff or you're landing, so having as much hydraulic pressure as possible for the brakes *might* just be a good thing.

Also, the gauge is probably just of cosmetical nature. FS9/FSX don't model brake systems that extensively anyways.

 

[PRB]

Actually, it's related to piston engine performance. Here's a couple sites that attempt to explain it:

http://www.epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm

http://bmepfuelandtuning.com/html/what_is_bmep_.html

I understand the definition (sort of), but still have a hard time understanding how they would be used in the cockpit. Seems no flight manual ever refers to them.

 

[Milton Shupe]

Actually, it's related to piston engine performance. Here's a couple sites that attempt to explain it:

http://www.epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm

http://bmepfuelandtuning.com/html/what_is_bmep_.html

I understand the definition (sort of), but still have a hard time understanding how they would be used in the cockpit. Seems no flight manual ever refers to them.

Seems you never got a good answer to your question Paul.

The BMEP gauge measures torque output at the crankshaft and was used to achieve maximum engine output at peak efficiency of fuel and engine heat.

Setting up for cruise after setting pitch and power, you would lean to peak BMEP then add add a bit or power. This gave best power and fuel efficiency while allowing your engine heads to run 30-40 degrees F cooler, thus increasing range and engine life.

BMEP gauges are not used in more modern engines but you can achieve teh very same results using the EGT gauge(s). Research the WAD procedure for proper use. WAD worked with BMEP and it works with EGT to extend engine life, reduce maintenance costs and fuel costs while delivering optimum cruise performance.

 

[researchpilot]

The reason your BMEP or even Manifold Pressure (whats measured in modern aircraft) increases when you decrease prop RPM is that you are increasing what is called Volumetric Efficiency. This will cause the BMEP or MP to indicate a rise. There is only so much pressure that a cylinder can take hence the red line indication on the gauge. Think of it along the lines of by slowing the crankshaft down you are giving the valves more time to stay open to shove more air into the engine, hence the rise in pressure. The opposite will be true, by increasing RPM pressure will fall.

From memory BMEP isn't a direct reading of Horse Power as such but rather has to be multiplied by RPM to give shaft/brake horsepower. You multiply that by the efficiency of the propellor to give you thrust horsepower.

 

[PRB]

Seems you never got a good answer to your question Paul...

Thanks Milton. By “WAD” do you mean Wright Aeronautical Division? I found an interesting web page on the turbocompound engines HERE.

The first web site I linked to says BMEP is: “the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output”. And goes on to add that it’s “purely theoretical and has nothing to do with actual cylinder pressures…”

So, if you have a BMEP gauge in the plane, is it measuring anything directly, or just making a calculation based on the above definition?

@researchpilot: Your post relating the rise of BMEP as RPM is reduced to volumetric efficiency maked sense. The page I linked to above discusses this as well.

 

[srgalahad]

So you really want to be a Flight Engineer?

Here are two references I found. The first tries to relate the old to the new so it explains BMEP and shows a gauge which may have been intended for a single-pilot aircraft (or no FE). It the goes on to discuss use of EGT to achieve the same results. (Are EGT parameters accurately modeled in MSFS?)
http://www.gami.com/articles/bttfpart1.php

The second comes from a far more complex bit of equipment - the FE in a KC-97
If you work through the whole article, it leads me to the conclusion that BMEP may be monitored or 'flagged' for takeoff and landing, but really only becomes a focus in cruise and is intimately bound to cruise performance/leaning, etc.
http://www.enginehistory.org/r-4360ops1.shtml

"By the time that I was involved in the intimate operations of the R-4360 engine a lot of experience had been accumulated that pretty much dictated the procedures best suited to both efficiently operate the engine and extend the TBO. In a nutshell this amounted to not using anymore than 150 psi BMEP for cruise power settings. The unfortunate in-flight engine rpm restricted range of 2,100 to 2,350 had to be considered also. The compromise that resulted from these limitations dictated that if manual leaning with spark advance were to be used, 1,735 bhp (158 psi indicated torque pressure, 2,100 rpm, which produced 150 psi BMEP) was the maximum cruise power setting to be used. So that’s how we operated the engine. We used the appropriate (and very accurate) cruise altitude charts in the performance flight manual to chose a power setting for a given aircraft weight, corrected for density altitude and always used 158 psi indicated torque pressure and the necessary rpm to provide a specific bhp that resulted in the appropriate indicated airspeed to develop the range for the mission."


Note the use of cruise altitude charts. Sounds like read the chart, set RPM, adjust MP to the table value then lean to get target BMEP which then produced the target BHP to produce the desired speed @ desired Fuel Flow.

I doubt that many FS aircraft models have those in the DOCS and I'd be surprised if most FDE's were built using them (if MSFS even has the capability - not counting the 'external' modules like Accu-Sim). . However, it matters not if automixture is used as you can't then set power/RPM and lean to get a resulting BMEP.

Having said that, the Connie Team L1049 does seem to require, or at least allow engine management using some degree of BMEP settings or limits so maybe it can be done, and generally just gets ignored (who allows engine damage anyway?? ). Now if you are trying to get an absolute max. range flight for something like the MacRobertson Race, it might be handy...

Oh, I also found one of the Dauntless Software flash cards that's part of their FE study guide and it appears the guide can be downloaded, but it might be a paid product. http://www.dauntless-soft.com/products/GroundSchool/TestBanks/Flight_Engineer/Powerplants/Q1162.asp
The download link is in the middle of the page.

Here's another bit of background that doesn't explain BMEP but discusses "cruise control' operation and this might help understand the process.
http://digicoll.library.wisc.edu/cg...UW.WIEv54no6.p0017&id=UW.WIEv54no6&isize=text

 

[robert41]

If I understand it correctly, the BMEP is measured off the engine front (prop) gears. Then converted (electrically) to drive the gauge. Meaning, it measures the engine torque? Then is converted to read a power level (BMEP) for the FE.

 

[JohnC]

This info I pulled after my undergrad education almost ten years ago, but the info came from an appropriately titled book, 'Engineering fundamentals of the internal combustion engine'. A really outstanding read for those interested.

BMEP is a way of normalizing engine performance, sort of like lift coefficient's on a wing. By normalizing output, it's a way of comparing the performance of a four cylinder engine to a six, eight, or twelve cylinder engine. Larger engines tend to produce more torque and peak power is very dependent on engine speed; and BMEP is a way of not letting those bias's creep into a comparison.

How does that apply? One of the primary calculations for BMEP is:

BMEP = BrakeTorque*Displacement_Volume/4*pi (4-Stroke engine)

This means that peak BMEP occurs at Maximum Brake Torque or MBT. While not a hard and fast rule, most torque curves peak around 2/3 of Peak Power RPM. This is related to MBT being near an engine's peak output efficiency (power out/fuel consumed), which is also known as the minimum brake specific fuel consumption or min-BSFC.

How does this apply to aircraft?

The available speed range of an aircraft's engine is usually centered between two points. The fastest available RPM is typically the peak power output. So when you throw the prop lever forward, you're kicking out the maximum possible power. The low end of the usable RPM range is typically near or at the min-BSFC. In an ideal (constant-drag) world, all cruise flight would be done here since it offers the peak in fuel economy. Once you consider lift (i.e. load) based drag, min fuel consumption during cruise is usually at a power output a little higher than what can be produced somewhere at min-BSFC*.

*Comes from The Mechanics of Flight

In either case, as you decrease RPM from max available you're moving away from peak power and towards min-BSFC and MBT; and the latter results in a higher BMEP.

Final note: FSX does contain tables in the air file that set engine torque, but they are a little wonky. You can only define the maximum Power output explicitly, while peak Torque is an auto-calc. In the air file, you apply a scalar this auto-calc'ed torque rather than explicitly define the curve. So not sure exactly how the above concepts are modeled since they vary by engine.


Hope that's helpful!