October 21, 2015 Vol. 12 No. 20

Almost all truck operators watch fuel consumption to one extent or another, some rather casually, some very, very carefully. And a few of the latter even take action based on what the numbers tell them. In every case the data being used comes directly from the engine’s electronic control module (ECM).

But hold on…

Relying on the electronic data from a truck engine’s computer may not be the most accurate gauge of fuel economy, according to a new report from the respected Performance Innovation Transport Group (PIT). The differences seem to be small but I’ll let you judge for yourselves.

In fact, one of PIT’s key conclusions is that “ECM data should be used with caution.”

The Quebec-based not-for profit engineering and research outfit has produced a study that details electronic control module (ECM) data precision and accuracy from four engine manufacturers — Cummins, Detroit, Mercedes, and Volvo.

The study compares that data with actual test track fuel consumption, in trials conducted in the fall of 2014 on 14 different vehicles. The evaluations followed the Joint TMC/SAE Fuel Consumption Test Procedure Type II. The fuel measurement gravimetric method allows test accuracy within 1%. 

The results of the gravimetric measurement, converted to fuel volume, were compared with the fuel consumption data provided by the ECM, which was retrieved using onboard computers or engine scan tools. Fuel specific gravity was measured with a hydrometer.

Fuel consumption data provided by engine ECMs is generally regarded as an industry standard for testing.

“The ability to measure fuel consumption accurately and precisely and defend the results is critical for writing vehicle specifications, for establishing effective maintenance practices, and for training drivers to operate vehicles as fuel-efficiently as possible,” said PIT director Yves Provencher. “Our tests show that engine ECMs present different levels of precision and accuracy, and that ECM data should be used with caution for evaluating the impact of a fuel saving technology or operational practice.”

PIT’s other conclusions were:

ECM data precision and accuracy varies between engine manufacturers and among engine models from a single manufacturer.

For a given vehicle, ECM data precision and accuracy will even vary from test to test.