Break Mean Effective Pressure

Howard Gibson · Jun 2, 2023

Shortround6 said:
I could be very wrong but I think what they had were torque meters?
Or I am confusing things?

For some reason the radials got torque meters and the V12 engines did not. Something to do with the reduction gears. With Torque and RPM you can figure out the actual power and you can work backwards from torque to figure out the BMEP.
With the appropriate conversions (or 2nd input) done inside the gauge your needle can run over the outside dial giving you HP or ft/lbs of torque or BMEP?

Engines are tested on dynamometers. Toirque can be measured with a brake. You know the engine speed. A horsepower is 550 ft.lb/sec, or 33000ft.lb/min. The term "brake horsepower" should be obvious.

Shortround6 · Jun 3, 2023

The big radials were fitted with torque meters attached to the reduction gears for measurements in flight.
Not common during WW II but very common post war on multi engine planes.
What I don't know is if the some meters were set up with different scales or units of measurement to make inflight measurements/calculations easier. Flight engineer on a 4 engine transport doesn't want to do the math calculations every time they change throttle settings.

pbehn · Jun 3, 2023

Shortround6 said:
The big radials were fitted with torque meters attached to the reduction gears for measurements in flight.
Not common during WW II but very common post war on multi engine planes.
What I don't know is if the some meters were set up with different scales or units of measurement to make inflight measurements/calculations easier. Flight engineer on a 4 engine transport doesn't want to do the math calculations every time they change throttle settings.

Maybe to tell the engineer if an engine had lost a cylinder or two?

Simon Thomas · Jun 3, 2023

The torque-meters measured the force on the fixed gear of the reduction gearset. That works well for Farman or planetary type gears which have a fixed gear, but doesn't work on spur gears reduction sets used on V's.
They were used to set the cruise BMEP drop to find the optimum mixture strength for a given power setting.

emu27 · Jun 3, 2023

I could be very wrong but I think what they had were torque meters

Torque meters are a feature of turbo props and turbine helicopters for setting power.

BHP = torque ft lbs*output shaft speed RPM/5255

Extract from the Super Constellation manual on manually leaning the R-3350 ie not using the AUTO LEAN function. It should be noted that torque meters are part and parcel of the BMEP system. The particular formula to calculate BHP in the Constellation installation was BMEP*RPM/236

AMCKen · Jun 4, 2023

The formula I use to calculate bmep is:
hp x 33000 / piston area(sq ins.) x stroke(in feet) x 1/2 of the rpm
1/2 the rpm since every other stroke is the power stroke.
A Packard Merlin 11 with 2270hp@3000 gives 363.5psi bmep

HP52 · Jun 9, 2023

Shortround6 said:
I could be very wrong but I think what they had were torque meters?
Or I am confusing things?

For some reason the radials got torque meters and the V12 engines did not. Something to do with the reduction gears. With Torque and RPM you can figure out the actual power and you can work backwards from torque to figure out the BMEP.
With the appropriate conversions (or 2nd input) done inside the gauge your needle can run over the outside dial giving you HP or ft/lbs of torque or BMEP?

I first came across BMEP in relation to a not well advertised "sort of crash". It was in a Constellation fully fuelled, with reserves to make a diversion from perth to Forrest West Australia from Mauritius. On take-off the FE saw one BMEP (torque meter) needle flicker and the pilot aborted the takeoff. The overshoot area was a sugarcane plantation and they went through the fence; unknown to everyone. All the stones in the field had been collected over the years and piled into cairns.
As the Connie tore through the boundary fence, pretty quickly the undercarriage was wiped of and she slithered on her belly.

The crew didn't waste time bundling everyone out, Within two minutes they had a good fire but everyone was out and there were no casulties.
- HP52

Break Mean Effective Pressure

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