For car enthusiasts and professionals alike, understanding On-Board Diagnostics II (OBD2) parameters is crucial for vehicle maintenance and performance tuning. Among the plethora of data points available, engine load is a key metric. Often intertwined with discussions of engine load is the acronym AAT, which stands for Ambient Air Temperature. But how does AAT fit into the OBD2 framework, especially when we talk about engine load? Let’s delve into two important OBD2 Parameter IDs (PIDs) – PID 04 and PID 43 – to clarify the role of AAT and engine load calculations.
Calculated LOAD Value (PID 04) and Ambient Air Temperature
OBD2 PID 04, known as Calculated LOAD Value (LOAD_PCT), provides a percentage representing the current engine load relative to its maximum potential. The formula for calculating LOAD_PCT explicitly includes Ambient Air Temperature (AAT):
LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]Let’s break down this formula and highlight the significance of AAT:
- AAT (Ambient Air Temperature): Measured in Celsius, AAT is the temperature of the air surrounding the vehicle. It’s a critical factor because air density changes with temperature. Colder air is denser and contains more oxygen, which affects engine performance and load. The formula uses AAT in Kelvin (AAT+273) to account for temperature’s impact on air density.
- STP (Standard Temperature and Pressure): The formula references Standard Temperature and Pressure (25°C and 29.92 inches of Mercury), used as a baseline for calculations.
- BARO (Barometric Pressure): This accounts for altitude, as air pressure decreases at higher altitudes, affecting air density and engine load.
- WOT (Wide Open Throttle): Represents the maximum airflow achievable when the throttle is fully open under Standard Temperature and Pressure conditions.
Why is AAT Important in LOAD_PCT?
AAT is incorporated into the LOAD_PCT calculation to normalize engine load readings across varying ambient conditions. As ambient temperature rises, air density decreases. Without considering AAT, the calculated load could be artificially low simply due to warmer, less dense intake air, even if the engine is working hard. By including AAT, PID 04 provides a more accurate representation of the engine’s actual load, irrespective of the surrounding air temperature.
For diesel engines, the calculation adapts by using fuel flow instead of airflow, but the principle of normalizing for ambient conditions remains relevant.
Absolute Load Value (PID 43) and the Absence of AAT
In contrast to PID 04, Absolute Load Value (LOAD_ABS), or OBD2 PID 43, offers a different perspective on engine load and notably does not include Ambient Air Temperature in its calculation. LOAD_ABS is defined as:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]This formula focuses on:
- Air mass per intake stroke: The actual amount of air entering the cylinders during each intake stroke.
- Cylinder displacement: The engine’s swept volume, a constant value.
Why is AAT Not in LOAD_ABS?
LOAD_ABS aims to provide a more direct measure of the air mass entering the engine cylinders relative to its displacement. It represents the “normalized value of air mass per intake stroke displayed as a percent.” Unlike LOAD_PCT, which is designed to reflect load relative to peak potential under STP conditions and is influenced by temperature and pressure corrections, LOAD_ABS provides a more absolute measure of how much air the engine is actually taking in.
Because LOAD_ABS is normalized against cylinder displacement, a fixed engine characteristic, and directly reflects air mass, it inherently accounts for air density changes due to temperature and pressure without explicitly needing AAT in the formula. Warmer, less dense air will result in a lower air mass per intake stroke, and LOAD_ABS will reflect this directly.
Key Differences and Implications
- AAT Sensitivity: LOAD_PCT (PID 04) is explicitly adjusted for Ambient Air Temperature, making it sensitive to changes in AAT. LOAD_ABS (PID 43) is not directly adjusted for AAT but implicitly reflects air density changes through air mass measurement.
- Load Interpretation: LOAD_PCT indicates the percentage of peak available torque, normalized for ambient conditions. LOAD_ABS reflects the engine’s pumping efficiency and is linearly correlated with engine torque.
- Engine Types: Both spark ignition and compression ignition engines support PID 04. However, PID 43 is mandatory for spark ignition engines but optional for compression ignition (diesel) engines.
Conclusion
Understanding the AAT acronym in the context of OBD2 and engine load is crucial for accurate diagnostics. While Ambient Air Temperature is a key component in the Calculated LOAD Value (PID 04) to normalize readings across varying environmental conditions, it’s absent in the Absolute Load Value (PID 43), which provides a more direct measure of air mass intake. By grasping the nuances of these PIDs and the role of AAT, technicians and enthusiasts can gain deeper insights into engine performance and operating conditions using OBD2 data.
