What is "Calculated Load"?

I was going through my notes when I came across this again. I do not remember where I got it but it is still useful.

Calculated Load is required in all vehicles that use CAN communication starting in the 2005 Model Year.

The new PID is PID $04, LOAD_PCT. Here is the definition in ISO15031-5

The OBD regulations previously defined CLV as: (current airflow / peak airflow @ sea level) * (BARO @ sea level / BARO) * 100%

Various manufacturers have implemented this calculation in a variety of ways. The following definition, although a little more restrictive, will standardize and improve the accuracy of the calculation.

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 and is in °C

Characteristics of LOAD_PCT are:  Reaches 100% at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines.

 Indicates the percent of peak available torque during normal, fault-free conditions.

 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 new PID is PID $43, LOAD_ABS. It is defined as follows:

The absolute load value has some different characteristics than the LOAD_PCT defined in PID 04 This definition, although restrictive, will standardize the calculation.

LOAD_ABS is the normalized value of air mass per intake stroke displayed as a percent.

LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / litre) * cylinder displacement (litres / intake stroke)]

Derivation:

 air mass (g / intake stroke) = [total engine air mass (g/sec)] / [rpm (revs/min)* (1 min / 60 sec) * (1/2 # of cylinders (intake 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) * cylinder displacement (liters/intake stroke) based on air density at STP.

Characteristics of LOAD_ABS are:

 Ranges from 0% to approximately 95% for naturally aspirated engines, 0% – 400% 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.

These will be required in all vehicles that use CAN communication starting in the 2005 MY.