HEM Data Products

Fuel Economy Calculations from OBD Data

The simplest way to calculate a light duty (LD) car or truck’s fuel economy is to acquire fuel consumption from the Injector Flow Rate. Instantaneous Fuel Economy can be calculated from two parameters: Vehicle Speed and Injector Flow Rate. Note that delta time is included in each parameter and therefore cancels out.

Sample standard OBD-II Parameters from LD Engine Controllers are:

  • Engine speed
  • Engine load (absolute & calculated)
  • Vehicle speed
  • Temperatures: coolant, ambient, catalyst
  • Accelerator pedal and throttle position
  • Diagnostic Trouble Codes (DTCs)
  • Mass Air Flow (MAF) Rate to engine
  • Manifold Air Pressure (MAP)
  • O2 sensor data, Lambda (A/F)
  • Fuel trims (long and short term)

PID $5E defines the Injector Flow Rate in SAE standard J1979 for OBD-II, however, it is seldom used. Another option is to use Enhanced OBD (EOBD). Some OEMs provide Injector Flow Rate as part of EOBD data, but again, it is rare.

So then how do we calculate fuel consumption to obtain fuel economy?

Fortunately, most vehicles provide mass air flow (MAF) rate to calculate fuel consumption. In addition, we need to know stoichiometric A/F ratio, vehicle speed, lambda, and fuel density. Therefore, the most common approach is to use MAF. From our experience, this calculation is used to report fuel economy on the instrument cluster.

Even though this approach is most commonly used, Chrysler and some Honda vehicles don’t often provide MAF.

If no injector flow data or MAF is available, then we need to use MAP and the ideal gas law. Using MAP is the least accurate. To improve the MAP-based calculation, a computer model of the engine’s volumetric efficiency is required.

In the next eNewsletter we will present two studies that compare fuel economy calculations based on MAF and Fuel Injector Flow:

  • A single Prius HEV
  • 111 Light Duty vehicles from an OEM’s fleet

Plot comparing fuel consumption calculated from MAF and Injector. The engine reports the injector volume every 10 cycles, and this delay shows up in the plot. The sample rate is 10 samples/second.
*Click image to enlarge*

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