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anachranerd
10-03-2008, 06:33 PM
Hey all,

Hopefully I am not asking something stupid here.

But in reading about Engine performance, and ECU maps, etc, I always see charts of Engine Load vs. RPM. What exactly is engine load? I always assumed that since an engine is maxed out at a certain RPM, that that is loading...What determines load? Is it RPM, torque,,,dont understand...

Thanks :)

MrWOT
10-03-2008, 07:07 PM
Heh, manifold vac vs. rpm is more like it.

anachranerd
10-03-2008, 07:51 PM
So it is a function of how much air is being pulled into the engine?

MrWOT
10-03-2008, 07:54 PM
Nah, you can have almost no air flowing and a lot of load, it's more like throttle position vs. torque production ability? Not really sure how to explain.

anachranerd
10-03-2008, 08:00 PM
Hummm, yeah...I am searching the web like crazy right now and get a bunch of different explanations.

For example this site (http://fordfuelinjection.com/?p=4) says


Load is the a measurement of how hard an engine is working which is measured in percent. Coasting down hill is considered very low engine load. Pulling a weighted trailer uphill is considered high engine load. Engine load is usually measured by taking a reading off the inlet air flow The main factor that chooses which EFI system to use is AIR FLOW. Air flow is the biggest unknown factor when calculating fuel ratios and timing curves. Air flow is so unpredictable that people get grant money to study wind tunnels. So to decide which fuel management system to use depends on how unpredictable the air flow into your engine is

I havent towed a heavy load in a long time...so if i remember correctly if I am towing a weighted trailer uphill, I may be flooring the gas pedal, introducing a lot of air into the system, but my engine still may only reach 2000 rpm's because it has to work very hard against gravity. That would be a high load situation right?

T-spoon
10-04-2008, 01:22 AM
Yeah, I guess a general (though not necessarily 100% accurate) way to look at it is how much power is needed to produce the same acceleration. If you're on a flat road or going downhill, you don't need a lot of torque to maintain the same speed, but going up a steep incline you do need torque. The heavier the vehicle, generally the higher load on any surface. RPM isn't really an indicator of load AFAIK, but gearing vs speed. If you go up a steep hill at 30 MPH in 4th gear, it will still be a high load even though you're probably lugging the engine with too low RPM and hardly able to move and being unkind to the tranny with the throttle wide open. Same would go for a low load situation coasting downhill but for some reason if you wanted to you could downshift and hit redline with little or no throttle because long as the tranny is engaged with the engine the car's wheel rotation from rolling downhill will spin the crank without needing much fuel or air (comparatively) to move the pistons.

Maybe it has something to do with the fact that just like a bicycle going downhill the RPMs of the sprockets and chain are just as high as if you were in a high gear on a level road cranking away at the pedals, but in the level scenerio it is requiring a lot of energy from you to keep the RPMs up, but going downhill you don't need to supply much if any. Compare that to going up a steep hill on the bike, you're in a low gear, low RPM of gears and chain but having to supply a LOT of energy to keep it moving uphill. Which of course, is all about inertia.

So load vs. rpm is something useful to know because just RPM by itself doesn't tell you about how hard the engine is working to produce power, it just tells you how fast the crank is spinning, yeah?

anachranerd
10-04-2008, 01:32 AM
So load vs. rpm is something useful to know because just RPM by itself doesn't tell you about how hard the engine is working to produce power, it just tells you how fast the crank is spinning, yeah?

Right,,,,I am going to be firing up this engine on a stand as soon as my harness is done. So in that circumstance, I have a tranny mounted, but no car on the tranny. So no load. So the engine could spin up to 6000 rpm and have no load other than friction. Hummmm....I need to think about this some more. Time to go crack open a cold one... :bigthumbu

T-spoon
10-04-2008, 01:46 AM
Right,,,,I am going to be firing up this engine on a stand as soon as my harness is done. So in that circumstance, I have a tranny mounted, but no car on the tranny. So no load. So the engine could spin up to 6000 rpm and have no load other than friction. Hummmm....I need to think about this some more. Time to go crack open a cold one... :bigthumbu

Well, I'd guess there's /some/ load, but very very little, like the weight of the internals and such.

alltracman78
10-05-2008, 11:26 PM
Engine load is airflow [or pressure in a MAP based system] x RPM.

It's an arbitrary number that's read as a percentage of the whole.
Max load [100%] would be max airflow x max RPM.
Min load [somewhere around 16%, the ECU doesn't see or accept zero in this case] would be min airflow x min RPM.

Then you have everything in between.

anachranerd
10-07-2008, 04:06 PM
So, if the engine is running without a car hooked to it, and it drawing as much air as it can(~1900 cc's I guess) than it is at 100% load?

alltracman78
10-07-2008, 05:44 PM
Technically, yes.
If you were at max RPM, and max airflow, you would be at 100% load.

However, with the engine on a stand, max RPM isn't going to pull max airflow.

Load is how much work the engine is doing.
On a stand it's not working as hard as if it were pulling your car.
That's why on a dyno it's hard[er] to get max boost, less load than when you're actually pulling the car.

MrWOT
10-07-2008, 05:48 PM
It's accelerating against the rotating mass, yes. So yeah, technically.... But uh... BOOM!

anachranerd
10-08-2008, 10:09 PM
Ok, so when an engine is pulling a load, how does that draw more air? I just dont understand why it pulls more air if it is tryign to force the crankshaft/transmission/axles/wheels to turn. Guess my physics is a little rusty...

Brian

alltracman78
10-08-2008, 10:22 PM
How much air [and fuel] the engine needs is dependant on how much load [work] is on the engine.
The engine is always "pulling" a load as long as it's running.
The more load, the more air. Higher RPMs require more air, so does adding more weight for the engine to move. You will reach max airflow at a lower RPM if you are pulling a trailer.

That's all I can add for the moment, I just got a big job dumped on me and we close in 2 hours, so I have to go.