What is it? I roughly understand what it is conceptually, but how is it measured?

 

Well, this was the wrong forum. I just thought engine load, engine forum, seems right when it's the ECU that provides the value. As near as I can tell it's the intake manifold pressure over the atmospheric pressure. I guess it uses the MAP sensor to get an atmospheric pressure reading prior to starting the bike.

 

load is resistance to engine rotation. It is measured in horse power and torque. A engine reving up in neutral has "no load" on it.

 

That's what I was struggling with, i.e. how would the ECU know? It seems it doesn't, but rather it knows how much air and fuel is being sucked into the cylinder. Rather than how much work the engine is doing it's relatively how much power it's producing. At least that's my understanding from what I've been able to find.

 

The ECU knows how much load the engine hasby monitoring how much air is being consumed during the combustion process at a given RPM. Two different methods are used on a HD engine to allow the ECM to compute this mass of air, the TPS (throttle position sensor) monitors how far the throttle blades are open and/or, depending on year, the MAP (manifold absolute pressure) sensor monitors the vacuum in the intake manifold. With this and some other info, such as air temp, cyl head temp etc, the ECM computes how much fuel to inject and spark timing for the different engine loads. Today's car and truck ECUs do this with a MAF (mass airflow) sensor which is more accurate, but it not used on Harleys due to space limitations.