MAP direct correlation to FE?

[sword_guy8]

I'll turn on the data logging in ALDLdroid and post the results. Yeah, 2/3 throttle is damn near atmospheric, especially at low RPM. Obviously, your MAP will change with RPM regardless of where you put the throttle. At a very high RPM, you're MAP will definitely decrease, even with 2/3 throttle.

[user removed]

Map is load. Best bfsc is about 80% of atmospheric pressure (generally 27-30 inches of mercury) in a range of RPM generally between 1500 and 2700 rpm. That can vary greatly from huge displacement low rpm diesels to ultra high output racing gasoline engines.

Hold the throttle perfectly steady and you could calculate the grade your are on by the change in map due to the load increasing and decreasing as your load changes because of the grade changes, even without moving the throttle. Also by the change in speed of the vehicle itself.

edit; of course disregarding many other potential variables in aero drag,rolling resistance, and gear selection, plus many others.

regards
mech

[sword_guy8]

Atmospheric pressure is 29.92 inHG. So you mean 80% of that? That would be just about 24.

[user removed]

An engine is a type of lever, using fuel to rapidly heat compressed atmosphere, then extracting useful work out of the rapid expansion of that atmosphere when the fuel is ignited.

That lever is it's longest when 100% of the available atmospheric pressure enters the cylinder during the intake stroke.

regards
mech

[user removed]

Originally Posted by sword_guy8

Atmospheric pressure is 29.92 inHG. So you mean 80% of that? That would be just about 24.

Above 80% the ecu enrichens the mixture for more power and mpg suffers due to enrichment.

regards
mech

[betasniper]

Engines have very different throttle position torque demand responses. So it would be impossible to use TPS as an indicator of acceleration demand among different cars or even possibly the same car.

Note: these are programmed throttle mappings of formula 1 cars so they may not necessarily be the same as road cars, but the Idea stands I think.

[oldtamiyaphile]

Originally Posted by sword_guy8

What I discovered was NOT peak FE based on engine RPM. In fact, engine RPM has nothing to do with it. The only thing I've found which has a direct correlation to FE is manifold pressure.

This is really simple if you think about what you're actually measuring.

At a constant load/MAP, as speed rises, so does RPM, and this means your fuel usage also goes up (if you watch your Gallon/Hr you'll see this).

However when measuring MPG, as your speed/rpm increases, you're also increasing miles traveled. So although you're using more fuel, you're traveling more miles and if load is constant, the two cancel each other out.

[sword_guy8]

What you say is true, but I'm not increasing my speed. Only my RPM. I have cruise control on. Here's what's really happening: as I get to a hill in 4th gear, there is a much higher load on the engine while cruise tries to keep up (throttle position increases quite a bit). When I downshift into 3rd, engine RPM increases and MAP decreases (if I go from 4th to 3rd when the MAP is initially 24, then it will drop to 20). This results in the engine being in a more powerful RPM range, and the cruise will dial back the throttle. So, what's really occurring is that the throttle needed to maintain speed up a hill is lower when the engine is at a slightly higher RPM. In doing more testing, I've found that MAP is a good way to keep track of what gear to be in, but at the end of the day, less throttle is more MPG (regardless of engine RPM). In other words, if I maintain throttle position at any given speed and shift gears, MPG barely changes.

I still assert, though, that 20inHg is a sweet spot (for my engine) as far as MPG is concerned. My next testing, which will be conducted someday in the future, is to use a CVT and constantly hold the MAP at certain values and take measurements at each.

Basically, what I've been doing is finding the balance between available engine power and demand. Having the engine at a place where power is not available (low RPM) when there is a high demand for power will result in overloading of the engine and decreased MPG. Inversely, with the engine at a place where available power is much higher than demand will decrease MPG considerably as well.

Last thing I'm going to touch on here is that my MPG readings are instantaneous, so miles traveled doesn't make a difference. It's MPH at that instant, which I am keeping at a constant value.

[sword_guy8]

Originally Posted by Old Mechanic

Map is load. Best bfsc is about 80% of atmospheric pressure (generally 27-30 inches of mercury) in a range of RPM generally between 1500 and 2700 rpm.

I just checked a bunch of BSFC charts. I can't find any where MAP is part of the chart. They're all measured in torque and RPM. And yes, best BSFC is 80%.....of available TORQUE.

[user removed]

The chart is the result of load and MAP. Load and MAP create the chart. Can you tell which engine is in a car by looking at the exhaust?

regards
mech