Vehicle diagnostics have become increasingly sophisticated, and On-Board Diagnostics II (OBD2) systems play a crucial role in modern automotive repair and maintenance. Among the wealth of data provided by OBD2, Parameter IDs (PIDs) are essential for understanding various engine metrics. This article delves into Obd2 Pid Definitions, specifically focusing on two key parameters related to engine load: Calculated LOAD Value (PID 04) and Absolute Load Value (PID 43). Understanding these PIDs is vital for diagnosing engine performance and efficiency.
Calculated LOAD Value (PID 04)
The Calculated LOAD Value (LOAD_PCT), identified by PID 04, provides a standardized way to measure engine load as a percentage of peak airflow. It’s designed to reflect the current engine airflow relative to its maximum potential at wide open throttle (WOT) under standard temperature and pressure (STP) conditions.
The formula for LOAD_PCT 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 in Hg)
- BARO = Barometric Pressure
- SQRT = Square root
- WOT = Wide Open Throttle
- AAT = Ambient Air Temperature (°C)
Key characteristics of Calculated LOAD Value:
- Reaches 1.0 (or 100%) at WOT, regardless of altitude, temperature, or RPM for both naturally aspirated and boosted engines.
- Indicates the percentage of peak available torque.
- Shows a linear correlation with engine vacuum.
- Frequently used in engine control strategies, such as power enrichment scheduling.
- For compression ignition (diesel) engines, the standard mandates the use of fuel flow instead of airflow in the calculation, ensuring PID 04 support across engine types.
Absolute Load Value (PID 43)
The Absolute Load Value (LOAD_ABS), or PID 43, offers an alternative perspective on engine load. It represents the normalized value of air mass per intake stroke, also expressed as a percentage. This PID focuses on the actual air mass entering the cylinders relative to the engine’s potential maximum air intake.
The formula for LOAD_ABS is:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]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,
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 Absolute Load Value:
- Ranges typically from 0 to approximately 0.95 for naturally aspirated engines and 0 to 4 for boosted engines.
- Demonstrates a linear correlation with both indicated and brake torque.
- Often employed to manage spark timing and EGR (Exhaust Gas Recirculation) rates.
- The peak value of LOAD_ABS is linked to volumetric efficiency at WOT.
- Serves as an indicator of the engine’s pumping efficiency, valuable for diagnostic purposes.
- Spark ignition engines are required to support PID 43, while support is optional for compression ignition (diesel) engines.
Conclusion
Understanding obd2 pid definitions like Calculated LOAD Value and Absolute Load Value is crucial for anyone involved in automotive diagnostics and repair. While both PIDs provide insights into engine load, they utilize different calculation methods and offer unique perspectives on engine operation. By monitoring and interpreting these parameters, technicians and enthusiasts can gain a deeper understanding of engine performance, identify potential issues, and ensure optimal vehicle operation.
