Is there a theoretical equation that is used to define engine load?
It's not theoretical, but real. According to SAE International SAE J1979 / ISO 15031-5 (dated: 2014-08-11), calculated engine load is calculated by the following equation:
LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) *
(BARO/29.92) * SQRT(298/(AAT+273))]
- STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg BARO,
- SQRT = square root
- WOT = wide open throttle
- AAT = Ambient Air Temperature (in °C)
Characteristics of LOAD_PCT are:
- Reaches 1.0 at WOT at any altitude, temperature or rpm for both naturally
aspirated and boosted engines.
- Indicates percent of peak available torque.
- Linearly correlated with engine vacuum
- Often used to schedule power enrichment.
- Compression ignition engines (diesels) shall support this PID using fuel
flow in place of airflow for the above calculations.
The second part of this portion (Characteristics) gives you a lot of the information you are seeking. The percentage given by the equation indicates percent of peak available torque.
What parameters does an ECU look at to sense and determine the engine load? I don't think there is a single tell-all sensor which returns the engine load; there are likely several signals being combined here.
There isn't a single sensor it uses to figure this out. For a gasoline (or spark ignition) engine, it utilizes the Air Intake Sensor (IAT), Manifold Absolute Pressure (MAP) sensor, Throttle Position Sensor (TPS), and Engine Coolant Temperature (ECT) sensor to do calculations and to discover if the engine is at the ready point to make the calculations. The percentage variable can be read from the ECU using PID $04. According the the standard, both compression ignition (diesel) and spark ignition (gasoline) systems are required to maintain this.
Some ECU's appear to make a distinction between relative and absolute engine load. What is the difference between the two?
You've seen the above equation for calculated engine load. Below is the calculation for absolute engine load:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) *
cylinder displacement in liters]
- air mass (g / intake stroke) = [total engine air mass (g/sec)] /
[rpm (revs/min)* (1 min / 60 sec) * (1/2 # of cylinders (strokes / rev)]
- LOAD_ABS = [air mass (g)/intake stroke] / [maximum air mass (g)/intake
stroke at WOT@STP at 100% volumetric efficiency] * 100%.
- STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg (101.3 kPa)
- WOT = wide open throttle
The quantity (maximum air mass (g)/intake stroke at WOT@STP at 100%
volumetric efficiency) is a constant for a given cylinder swept volume.
The constant is 1.184 (g/liter 3) * cylinder displacement (liter 3/intake
stroke) based on air density at STP.
Characteristics of LOAD_ABS are:
- Ranges from 0 to approximately 0.95 for naturally aspirated engines,
0 – 4 for boosted engines
- Linearly correlated with engine indicated and brake torque,
- Often used to schedule spark and EGR rates,
- Peak value of LOAD_ABS correlates with volumetric efficiency at WOT.
- Indicates the pumping efficiency of the engine for diagnostic purposes.
As you can probably see, this equation relies on the flow of air and basically engine displacement. As it states in the body, this correlates with volumetric efficiency (how completely a cylinder fills with air on the intake stroke) at WOT. This variable can be read from the ECU on PID $43. It is only required by the standard on spark ignition systems.