Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
Still, is that valid for manifold pressure? Or is that valid for the average combustion pressure in the engine after pressurized air/fuel has been added, then ignited?A simple formula for calculating BMEP (in psi) that I found some time ago was to multiply torque in lb/ft by 2473 and divide it by the cubic capacity.
Okay, I was just curious because, what he said contradicted what you said. You're generally pretty knowledgeable about this kind of stuff, but we all have gaps in our knowledge base.There is no relationship between BMEP and MAP. I already explained this.
MAP is the pressure of the air in the intake manifold.
Okay, I was just curious because, what he said contradicted what you said. You're generally pretty knowledgeable about this kind of stuff, but we all have gaps in our knowledge base.
Later, a water pump or electric generator instead of a mechanical brake could resist crankshaft rotation, but the term "brake horsepower" endures. And that brings us to BMEP (brake mean effective pressure). If brake horsepower is known, you can rearrange the classic horsepower formula to solve for pressure P. Since it's calculated from brake horsepower, this pressure is called brake mean effective pressure. It's the pressure which, if exerted for the full length of every power stoke, would cause the engine to develop the same horsepower, assuming zero friction, and atmospheric pressure in the cylinders for the full length of the non-power strokes. Of course this assumption is not attained in the real world, so BMEP is a purely theoretical value. It does give a general sense of the intensity of combustion in the cylinders.
MAP (manifold absolute pressure) is the pressure in the intake manifold. In American practice it's traditionally inches of mercury, so at sea level with the engine not running it's about 30 inches. In the old days the pilot or flight engineer would note this pressure before start. Later, during run-up, the throttle would be adjusted to the same pressure in order to accomplish certain checks.
During the war I think the Germans measured manifold pressure in atmospheres, while the British used psi. Whether these were absolute pressure or gauge pressure I don't know.
Frankly I'm confused. I'm not sure what means what. BHP was what they'd use for the engine power output?BMEP does not assume zero friction. BMAP is calculated on the brake horsepower.
Doesn't manifold pressure gauge the pressure of the engine while at a given altitude?I'm not sure if it would be classed as gauge pressure, which I would expect would relate to the air pressure at the altitude measured, rather than compared with sea level pressure.
Frankly I'm confused. I'm not sure what means what. BHP was what they'd use for the engine power output?
What did they use for steam power?
Doesn't manifold pressure gauge the pressure of the engine while at a given altitude?
OkayBrake horsepower (bhp) is for the nett power out as measure by a brake dynamometer. That means the power includes the loss due to friction.
That can be translated into horsepower, so that's fine.I'm not sure there is an equivalent in the metric system. I think it is just Watts (W) or kilowatts (kW).
Okay, that's usefulSteam engine output was brake horsepower, as explained above.
Okay, so you simply take the gauge pressure and add atmospheric pressure and you get the absolute pressure?Gauge pressure is the pressure above ambient air pressure.
Is that why the British used +PSI as a measurement? I thought it was just to be simple and easy to understand for aircrew who could vary in educational background...In the case of British boost specifications it is pressure above standard sea level pressure.
Still, is that valid for manifold pressure? Or is that valid for the average combustion pressure in the engine after pressurized air/fuel has been added, then ignited?
The manifold pressure, as I grasp it, is the air at some point in the engine's intake manifold?
People use what they are used to using, there are all sorts of ways to express pressure, it would be best to use the units of the engineers who designed the engine, or supercharger or turbo. On a normal conventionally aspirated engine the manifold pressure will be less than atmospheric pressure, people used to fit "vacuum gauges" to help them drive economically, the closer you have the engine running to atmospheric pressure the more economical it is. With a supercharger or turbo the pressure in the manifold is what that pump can supply.Is that why the British used +PSI as a measurement? I thought it was just to be simple and easy to understand for aircrew who could vary in educational background...
Okay, I was just curious because, what he said contradicted what you said.
Don't feel like the Lone Ranger, Zipper! In my years of flight instructing this was one of the most confusing topics for students to wrap their heads around. They often left the classroom not truly understanding it and did their early complex airplane work (under the eagle eye of an instructor) by rule of thumb until understanding sank in.Frankly I'm confused. I'm not sure what means what.