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I'm struggling to find information on how engine load is defined. Is it really just a fancy word for torque demand?

There are three parts to this question:

  1. Is there a theoretical equation that is used to define engine load? If so, what is it? I couldn't find anything definitive in the Bosch Automotive Handbook.

  2. 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.

  3. Some ECU's appear to make a distinction between relative and absolute engine load. What is the difference between the two?

  • what is that book (Bosch Automotive Handbook) like ? is it good as a reference manual for a backyard mechanic ? – amphibient Jun 10 '15 at 15:49
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    @amphibient : It's a handy reference for automotive design/state-of-the-art. It's catered more towards engineers than a DIY-er but I wouldn't recommend against having a copy on hand :) – Zaid Jun 10 '15 at 16:34
  • It's worth noting that there is a layman's usage for "Engine Load" for vehicles without an MAF sensor. That is simply the ratio of throttle position sensor to RPM. This is regularly referenced on motorcycles as they rarely have an MAF sensor. – justinm410 Sep 7 '16 at 14:37
  • Am interested in this topic as well, am figuring out how auto trans ecu (EAT) determins shift/lockup by means of torque% input from ECU/ECM Points to ponder : actual MAF vs projected AF value (mitsubishitechinfo.com/data/GS41/2009/13/HTML/…) – user34833 Jan 10 '18 at 0:13
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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))]
Where:

- 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]

Derivation:

- 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%. 

Where:

- STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg (101.3 kPa)
  BARO
- 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.

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    great writeup. what i would like to know what sensing mechanism(s) the engine (or ECU) uses to determine input factors based on which the torque demand is determined. IOW, if the vehicle is going uphill or is towing a heavy weight, the torque demand is bigger. how are those demands gauged ? – amphibient Jun 10 '15 at 15:58
  • @amphibient - I'm sure throttle position, mass air flow, and rpms are the big ones. As you are going up hill you have a certain amount of rpm to maintain a speed. In order to maintain that speed, you'll need more throttle, which will allow more air to flow through. More air flow at a lower rpm will indicate a higher torque demand. This is a SWAG on my part, but believe it to be an educated one. – Pᴀᴜʟsᴛᴇʀ2 Nov 4 '15 at 23:04
  • Hi, this is a great answer. I am trying to determine if the engine is reaching stall. Can the engine load be used to determine this? – Mohammed Lokhandwala Dec 20 '15 at 13:00
  • @MohammedLokhandwala - Please ask this as its own question and refer back to this answer if it helps explain what you are trying to ask. – Pᴀᴜʟsᴛᴇʀ2 Dec 20 '15 at 13:04
  • Why would LOAD_PCT use the MAP sensor and not the MAF? My 98 626 doesn't even have a MAP sensor, yet the ECU supplies a "Calculated Load" PID. – Robert S. Barnes Jun 6 '16 at 8:55
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Engine load is measured by the MAF sensor. In other words, it measures how much air (and fuel) you're sucking into the engine and then compares that value to the theoretical maximum. When I modified my Subaru's fueling, boost and ignition maps, all the tables plotting engine load to RPM referenced engine load in CFM (Cubic feet per minute). It basically means that because my turbo flows e.g. 350cfm max, the closer the actual airflow is to that, the higher the load percentage.

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