Updated: Jun 18, 2020
Engine Control Units need to be able to inject fuel proportionally to the amount of air an engine is ingesting at any moment. Due to the vast amounts of driving conditions a vehicle operates through, the ECU needs to have accurate mathematical models to turn various sensor inputs into a calculated amount of air. These "Air Models" are used in conjunction with fueling calibrations to deliver the right ratio of air and fuel needed for an efficient combustion cycle.
There are many different mathematical Air Models used in the industry, and this article will focus on the two you will most commonly see in Motorsports applications.
Speed Density tuning relies on a calibrated "Volumetric Efficiency" (VE) percent for each engine speed (RPM) and air density (typically Manifold Air Pressure). For each engine speed, and manifold pressure, there are values in a VE table to specify how efficiently the motor is running in those conditions (Below).
A VE of 100% at any RPM and Pressure would imply that the cylinders are able to completely fill with air at Standard Temperature and Pressure (STP)... A VE of 50% would mean that the motor is being throttled to the point that it's only filling half of each cylinder with air each cycle. Temperature is a factor when calculating air density. Cold air is denser than hot air. Because of this, a Speed Density airflow model requires a temperature sensor to truly get an accurate calculation.
In a forced induction application, or an NA application with proper resonance tuning, it's possible to see tables where VE exceeds 100%.
Minimum Required Inputs:
- Volume of one Cylinder
- Table or equations to calculate VE Pros:
- Fast response times
- MAP readings are accurate and consistent
- Can be easily used to calculate transient fueling
- Can accurately account for intake air lost out the exhaust during overlap Cons:
- IAT readings can be thrown off by heat soak or hot radiator airflow
- 3-dimensional nature of VE makes it processor intensive
- Difficult to model on a motor with variable cams, as VE% changes with cam phase
The simplest way to model airflow is to measure it. Mass Airflow sensors of different types gather data about the air passing through them, and convert that data into an electric signal that an ECU can interpret. Most air flow meters that you will encounter operate on the principle of a "hot-wire" sensor. On these sensors, there is an electrical element exposed to the oncoming airflow, that is heated by a fixed voltage. As air flows across the element, cooling it, the electrical resistance of the hot wire changes, causing more current to flow through the circuit. These changes of current are converted by the circuitry in the MAF to either a 0-5v analog signal, or a frequency based (Hz) output for the car's computer to read.
The output from a MAF is converted to a mass of airflow by a 2-dimensional table in the ECU. In the example below, a MAF Frequency of 9,000Hz would result in an airflow quantity of 1,800 lb/hr.
While the principles of a MAF's operation will rule out factors like temperature (Cold dense air will cool the element more, calculating to more airflow), the MAF is sensitive to placement. If the same MAF element is placed in a smaller tube, the velocity of air crossing the sensor will increase, causing it to read higher. Conversely, if you put the same MAF in a larger intake tube, air velocity drops, causing the MAF to read lower. Minimum Required Inputs:
- Table or equations to convert MAF signal to Mass Flow Pros:
- Fewer environmental variables to skew readings
- Easy to tune for different configurations
- Can easily measure air charge in a variable cam motor, as cams change phase Cons:
- Calculating Transient Fueling is complicated, and delayed
- Air that is blown out the exhaust during overlap needs to be calculated for
- Any intake air that does not pass through the MAF will not be measured (Vacuum Leak)
- Sensor element can get dirty, throwing off calculations
Advantages are apparent for both MAF and MAP based Air Models. Blending them with a capable PCM will give you the best possible calculated fueling. The instant response time of the MAP sensor coupled with the consistent steady state accuracy of a MAF will provide for any situation you can throw your vehicle into. When we tune vehicles, we aim to accurately calibrate all of the available Air Models in the ECU. After we have correctly and thoroughly calibrated the Air Models, we can simply command a specific Air Fuel Ratio, and hit the target every time. For example; If we want to change a tune from running at a WOT lambda of .85 (12.5:1 on a Gasoline scale) to .78 (11.5:1), it only takes a few keystrokes. This flexibility and accuracy allows us to target different Air Fuel Ratios at different RPMs and loads, to give the motor the fueling it needs in every condition.