Questar Auto Technologies has released a new white paper, The Hidden Fuel Cost of Mechanical Degradation, detailing how degraded diesel aftertreatment systems can significantly increase fuel consumption across commercial fleets.

According to Questar's analysis of a mixed-class commercial fleet, vehicles operating with degraded diesel particulate filter (DPF) and selective catalytic reduction (SCR) systems consumed enough excess fuel to create an additional operating cost of approximately $25 to $30 per vehicle per day.

The study found that aftertreatment system health emerged as the leading mechanical contributor to excess fuel consumption. Vehicles with aftertreatment health scores below 40 on a 100-point scale consistently used more fuel than comparable vehicles operating under similar conditions with healthy systems.

For trucks traveling approximately 186 miles (300 kilometers) per day, the fuel penalty amounts to roughly 4.5 additional gallons of diesel consumed. Across fleets with multiple affected vehicles, the resulting annual fuel waste can reach tens of thousands of dollars.

"Fleets have long understood that mechanical degradation can increase operating costs, but until now had no way to accurately identify and quantify the cause," said Dr. Yuval Shalev, vice president of data science and AI at Questar and author of the white paper. "This analysis shows how advanced technology can uncover hidden economic losses inside normal fleet operations and turn them into maintenance intelligence that operators can take action on."

Measuring the Impact of Mechanical Condition

To conduct the analysis, Questar examined thousands of vehicle operating days across a mixed-class commercial fleet. Predictive models were developed using hundreds of operating variables, including engine speed, road speed, gear position, coolant temperature, ambient temperature, vehicle type, and manufacturer.

The models estimated how much fuel each vehicle should consume under specific operating conditions and compared those predictions with actual fuel use. The difference between predicted and actual consumption, referred to as the "fuel gap," was then analyzed in relation to vehicle health scores.

Questar built separate models for idle fuel consumption and driving fuel efficiency, allowing researchers to identify the impact of mechanical condition in both operating environments.

Among the findings:

• Vehicles with degraded aftertreatment systems burned approximately 0.05 additional gallons of fuel per hour while idling.
• Fuel consumption while driving increased by approximately 1.48 gallons per 100 miles.
• Excess fuel costs averaged $25 to $30 per vehicle per day.

One fleet vehicle highlighted in the report showed more than 10% excess fuel consumption while idling due to aftertreatment degradation.

Implications for Fleet Maintenance

According to the report, the findings reinforce the value of proactive maintenance programs focused on the health of DPF and SCR systems.

The study notes that clogged DPFs can increase exhaust back pressure, forcing engines to work harder, while degraded SCR systems may trigger more frequent regeneration events that consume additional fuel.

Questar says the resulting fuel gap may also serve as an early warning indicator of emerging aftertreatment issues before diagnostic trouble codes or warning lights appear. The metric can also be used to verify whether maintenance and repair work successfully restores vehicle efficiency.

The company believes similar analysis techniques could be applied to other vehicle systems, including brakes, tires, and transmissions, where mechanical degradation may create hidden operating costs.

The full whitepaper, The Hidden Fuel Cost of Mechanical Degradation, is available from Questar Auto Technologies.