OBDII (On-Board Diagnostics II) is a standardized system that allows external electronics to interface with a vehicle’s computer and access diagnostic data. Among the numerous parameters available through OBDII, Calculated Load Value (PID 04) and Absolute Load Value (PID 43) are crucial for understanding engine performance. This article delves into the definitions and calculations of these essential OBDII parameters.
Calculated Load Value (PID 04)
Calculated Load Value (CLV) represents the engine’s current airflow compared to the maximum airflow possible at sea level, adjusted for atmospheric conditions. While older definitions existed, a standardized formula now ensures accuracy:
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: Barometric pressure
- SQRT: Square root
- WOT: Wide Open Throttle
- AAT: Ambient Air Temperature (Celsius)
Key characteristics of LOAD_PCT:
- Reaches 1.0 (100%) at WOT regardless of altitude, temperature, or RPM.
- Indicates the percentage of peak available torque.
- Correlates linearly with engine vacuum.
- Used for power enrichment scheduling.
- For diesel engines, fuel flow replaces airflow in the calculation.
Both gasoline and diesel engines must support PID 04. PID 43 offers an alternative definition of engine load.
Absolute Load Value (PID 43)
Absolute Load Value (LOAD_ABS) provides a different perspective on engine load compared to PID 04. It represents the normalized air mass per intake stroke as a percentage:
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 or 101.3 kPa)
- WOT: Wide Open Throttle
Characteristics of LOAD_ABS:
- Ranges from 0 to ~0.95 for naturally aspirated engines and 0 to 4 for boosted engines.
- Correlates linearly with engine indicated and brake torque.
- Used for spark and EGR rate scheduling.
- Peak LOAD_ABS correlates with volumetric efficiency at WOT.
- Indicates engine pumping efficiency for diagnostics.
Gasoline engines are required to support PID 43, while diesel engines are not.
Conclusion
Understanding both Calculated Load Value (PID 04) and Absolute Load Value (PID 43) is crucial for comprehensive engine diagnostics and performance analysis. While PID 04 provides a percentage of peak available torque, PID 43 offers insights into air mass utilization and volumetric efficiency. These parameters, along with other OBDII data, empower technicians and enthusiasts to assess engine health, optimize performance, and troubleshoot issues effectively.
