Engine Load is a crucial parameter in modern vehicles, reflecting how hard the engine is working. OBD2 (On-Board Diagnostics II) systems provide access to various engine parameters, including two key load values: Calculated LOAD Value and Absolute Load Value. Understanding these values, often referred to in the context of “Load Abs Obd2”, is vital for diagnosing engine performance and efficiency. This article delves into these two distinct yet related parameters, helping you interpret what your OBD2 scanner reveals about your engine’s load.
Calculated LOAD Value (PID 04)
The Calculated LOAD Value, identified by PID 04 in OBD2 protocols, provides a percentage representing the current engine load relative to its peak capacity under ideal conditions. The formula for calculating LOAD_PCT is designed to standardize this value across different engine types, considering factors like airflow, temperature, and atmospheric pressure.
The standardized formula for Calculated LOAD Value is:
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 inches of Mercury Barometric pressure)
- BARO: Barometric Pressure
- SQRT: Square root
- WOT: Wide Open Throttle
- AAT: Ambient Air Temperature (in Celsius)
Key characteristics of Calculated LOAD Value (LOAD_PCT) include:
- Normalization: It reaches 1.0 (or 100%) at Wide Open Throttle (WOT) regardless of altitude, temperature, or engine RPM, for both naturally aspirated and turbocharged engines.
- Torque Indication: It directly correlates with the percentage of peak torque available from the engine.
- Vacuum Relationship: It exhibits a linear correlation with engine vacuum.
- Power Enrichment: Engine control units (ECUs) frequently use LOAD_PCT to manage power enrichment strategies, adjusting fuel delivery for optimal performance under load.
- Diesel Adaptation: Importantly, for diesel engines, the calculation adapts by using fuel flow instead of airflow in the formula, ensuring compatibility and relevance for both gasoline and diesel powertrains.
Both gasoline (spark ignition) and diesel (compression ignition) engines are mandated to support PID 04, making Calculated LOAD Value a universally accessible parameter for engine diagnostics.
Absolute Load Value (PID 43)
The Absolute Load Value, or LOAD_ABS (PID 43), offers a different perspective on engine load. It represents the normalized air mass per intake stroke, expressed as a percentage. This parameter focuses on the actual amount of air entering the cylinders relative to the engine’s displacement and maximum potential air intake.
The formula for Absolute Load Value is:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in liters]This is derived from:
air mass (g / intake stroke) = [total engine air mass (g/sec)] / [rpm (revs/min) * (1 min / 60 sec) * (1/2 # of cylinders (strokes / rev))]And fundamentally:
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 Barometric pressure)
- WOT: Wide Open Throttle
Characteristics of Absolute Load Value (LOAD_ABS) include:
- Range Variation: For naturally aspirated engines, LOAD_ABS typically ranges from 0 to approximately 0.95. For turbocharged engines, it can extend significantly, ranging from 0 to 4 or even higher, reflecting the forced induction.
- Torque Correlation: It exhibits a linear correlation with both indicated and brake torque, providing a reliable indicator of engine output.
- Control Strategy: ECUs often utilize LOAD_ABS to manage spark timing and Exhaust Gas Recirculation (EGR) rates, optimizing combustion and emissions.
- Volumetric Efficiency: The peak value of LOAD_ABS is directly linked to the engine’s volumetric efficiency at WOT, revealing how effectively the engine fills its cylinders with air.
- Pumping Efficiency: LOAD_ABS serves as a diagnostic indicator of the engine’s pumping efficiency, highlighting potential issues with intake or exhaust restrictions.
While spark ignition (gasoline) engines are required to support PID 43, compression ignition (diesel) engines are not mandated to provide Absolute Load Value.
Conclusion
Understanding both Calculated LOAD Value (PID 04) and Absolute Load Value (PID 43) is essential for comprehensive engine diagnostics using OBD2. While Calculated LOAD Value provides a normalized percentage of peak load relative to ideal conditions, Absolute Load Value reflects the actual air mass intake per stroke, offering insights into volumetric and pumping efficiency. By monitoring these “load abs obd2” parameters, mechanics and car enthusiasts can gain valuable insights into engine performance, diagnose potential issues, and ensure optimal vehicle operation.
