When delving into the world of OBD2 diagnostics, understanding engine load is paramount. Among the myriad of Parameter IDs (PIDs) available, two stand out when discussing engine load: Calculated LOAD Value (PID 04) and Absolute Load Value (PID 43). Often abbreviated as Abs Load Obd2, PID 43 provides a distinct perspective compared to PID 04. This article aims to clarify the differences between Calculated Load and ABS Load OBD2, helping you interpret these values accurately for better vehicle diagnostics and performance understanding.
Calculated LOAD Value (PID 04)
The Calculated LOAD Value, identified as PID 04 in OBD2 standards, is essentially an estimation of the current engine load as a percentage of the maximum possible load under specific atmospheric conditions. The OBD regulations define this parameter using a somewhat complex formula, which can be represented as:
LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]Where:
- WOT: Wide Open Throttle
- STP: Standard Temperature and Pressure (25°C, 29.92 inches of Mercury)
- BARO: Barometric Pressure
- AAT: Ambient Air Temperature (in Celsius)
- SQRT: Square Root
Key characteristics of the Calculated LOAD Value (PID 04) include:
- Normalization: It is designed to reach 100% (or 1.0 in decimal form) at Wide Open Throttle (WOT) regardless of altitude, ambient temperature, or engine RPM.
- Torque Indication: It provides a relative indication of the percentage of peak torque currently being produced by the engine.
- Vacuum Correlation: It exhibits a linear correlation with engine vacuum, which is a useful diagnostic indicator.
- Power Enrichment Scheduling: Engine control units (ECUs) often utilize this value to schedule power enrichment strategies, adjusting fuel delivery for optimal performance.
- Universal Application: Crucially, PID 04 is mandated for both spark ignition (gasoline) and compression ignition (diesel) engines. For diesel engines, the calculation substitutes airflow with fuel flow to achieve a comparable load estimation.
Absolute Load Value (PID 43) – ABS Load OBD2
In contrast to PID 04, the Absolute Load Value (PID 43), often referred to as ABS Load OBD2, offers a more direct and normalized measure of air mass intake relative to the engine’s displacement. It represents the air mass per intake stroke as a percentage of the theoretical maximum air mass the engine could ingest per stroke at standard conditions and 100% volumetric efficiency. The formula for ABS Load OBD2 is:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]This can be further broken down as:
LOAD_ABS = [total engine air mass (g/sec)] / [rpm (revs/min) * (1 min / 60 sec) * (1/2 * # of cylinders (strokes / rev))] / [1.184 (g / intake stroke) * cylinder displacement in litres] * 100%Key characteristics of ABS Load OBD2 (PID 43) are:
- Range Variation: For naturally aspirated engines, ABS Load OBD2 typically ranges from 0 to approximately 0.95 (95%). However, for boosted engines (turbocharged or supercharged), it can exceed 1.0, often reaching values up to 4.0 or even higher under high boost conditions.
- Torque Linearity: Similar to PID 04, ABS Load OBD2 exhibits a linear correlation with both indicated torque and brake torque, making it a reliable indicator of engine output.
- Spark and EGR Scheduling: ECUs frequently use ABS Load OBD2 to schedule spark timing and Exhaust Gas Recirculation (EGR) rates, optimizing engine performance and emissions.
- Volumetric Efficiency Correlation: The peak value of ABS Load OBD2 directly correlates with the engine’s volumetric efficiency at Wide Open Throttle (WOT), providing insights into how effectively the engine is breathing.
- Pumping Efficiency Indication: ABS Load OBD2 serves as a diagnostic parameter to assess the engine’s pumping efficiency, revealing potential issues with intake or exhaust restrictions.
- Spark Ignition Specific: It’s important to note that PID 43 is mandatory for spark ignition engines but is NOT required for compression ignition (diesel) engines under OBD2 regulations. Diesel vehicles may not report this PID.
Key Differences and Applications: ABS Load OBD2 vs. Calculated Load
While both Calculated LOAD Value (PID 04) and Absolute Load Value (PID 43) / ABS Load OBD2 provide valuable information about engine load, their calculation methods and applications differ:
- Normalization and Range: PID 04 is normalized to 100% at WOT under any conditions, making it a relative measure within the current atmospheric context. ABS Load OBD2 (PID 43), however, is normalized against the engine’s displacement and theoretical maximum air intake, allowing it to exceed 100% in boosted applications and providing a more absolute measure of load relative to engine potential.
- Diesel Engine Support: PID 04 is universally supported across both gasoline and diesel engines, albeit with different calculation parameters for diesels. ABS Load OBD2 (PID 43) is primarily focused on spark ignition engines and is not mandated for diesel engines.
- Diagnostic Focus: Calculated LOAD (PID 04) is useful for general engine condition monitoring and understanding load relative to current operating conditions. ABS Load OBD2 (PID 43) is particularly valuable for performance diagnostics, tuning, and understanding volumetric and pumping efficiency in gasoline engines, especially those that are boosted.
In conclusion, understanding both Calculated LOAD Value (PID 04) and Absolute Load Value (PID 43) / ABS Load OBD2 enhances your ability to interpret OBD2 data comprehensively. While PID 04 offers a broader, universally applicable view of engine load, ABS Load OBD2 provides a more detailed and performance-oriented perspective, especially for gasoline and boosted engines. By correctly interpreting these PIDs, you can gain deeper insights into your vehicle’s engine performance and diagnose potential issues more effectively.
