There are two types of LOAD specified in SAE J1979
One is CALCULATED LOAD VALUE and the other is ABSOLUTE LOAD VALUE.
Calculated LOAD Value (PID 04)
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 standardise and improve the accuracy the calculation
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 and is 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.
NOTE Both spark ignition and compression ignition engines shall support PID $04. See PID $43 for an additional definition
of engine LOAD.
05 Engine Coolant Temperature A –40 °C +215 °C 1 °C with
–40 °C offset
ECT: xxx °C (xxx °F)
ECT shall display engine coolant temperature derived from an engine coolant temperature sensor or a cylinder head temperature
sensor. Many diesels do not use either sensor and may substitute Engine Oil Temperature instead.
Absolute Load Value (PID 43)
The absolute load value has some different characteristics than the LOAD_PCT defined in PID 04 This definition, although restrictive, will standardise the calculation. LOAD_ABS is the normalised value of air mass per intake stroke displayed as a percent.
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]
- 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/litre 3) * cylinder displacement (litre 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.
Spark ignition engine are required to support PID $43. Compression ignition (diesel) engines are not required to support this PID.