Study: Bigger engines can boost fuel economy

An intensive 18-month telematics-based study of grocery, manufacturing, fuel, and retail truck fleet operations indicates that slightly “over-spec’ing” engines and selecting a low gear ratio improves fuel economy by 0.3 MPG

An intensive 18-month telematics-based study of grocery, manufacturing, fuel, and retail truck fleet operations indicates that slightly “over-spec’ing” engines and selecting a low gear ratio improves fuel economy by 0.3 MPG or around 5%. That equates to an approximate savings of $2,000 per tractor per year, assuming 100,000 annual miles..

Conducted by Ft. Lauderdale-based asset management firm PHH FirstFleet, the study determined that the major killer of fuel economy is driving for long stretches at high output torque ratings. According to PHH First Fleet, that leads to “over-revving” the engine or operating it at higher RPMs than engine parameters specify.

“The evidence really pointed to over-revving as the major culprit for fuel economy losses,” Mike Lewis, PHH FirstFleet president & gm, told FleetOwner. “Even the type of transmission didn’t matter as much, because we found many trucks achieved high fuel economy with manuals, while others had low few economy with automated units. It really came down to over-revving the engine.”

Increasing engine torque ratings, however, provided a simple and effective method to decrease time spent in the high output torque ranges and thus improve fuel economy, added Ezel “C.J.” Baltali, PHH FirstFleet applications engineer in charge of the study.

“That extra power allowed drivers to maintain time spent in the engine’s ‘sweet spot’ in both highway and stop-and-go applications,” Baltali told FleetOwner. “Increasing the torque decreased over-revving in both operating environments.”

He noted that over-revving shortens the life of the engine, which sustains higher costs and lower fuel economy. Yet on the same note, driving in a lower RPM range than the engine’s specified parameters may result in driver dissatisfaction and could require increased downshifting, which indirectly causes over-revving.

Baltali stressed the study found additional spec’ing and operational changes are needed to help maintain and even boost the fuel economy attained from slightly bigger engines:

  • Decrease gear ratio. Choosing a lower gear ratio improves fuel economy, reduces engine wear, increases speed at lower RPMs and may help reduce over-revving. In general, choose the correct rear axle gear ratio for the fleet’s application and needs, but consider that while higher gear ratios may improve overall performance and provide added power in lower gears, additional power causes equipment to lose fuel economy.
  • Avoid speeding. Above 70 MPH, a tractor’s engine works to overcome aerodynamic drag. If speed is decreased to 55 MPH in the appropriate gear, the horsepower demand is reduced to only 40% and less fuel is used.
  • Progressive shifting. Drivers shifting quickly in lower gears prevents over-revving, which decreases fuel consumption and increases fuel economy.
  • Aerodynamic additions. New innovations in trailer aerodynamics, as well as roof fairings, side fairings, and bumper and side skirts, provide decreased wind resistance, which reduces required horsepower and thus fuel consumption.
  • Low-profile or wide-base tires. Research indicates fuel economy can improve as much as 2% to 5% by using low-profile and/or wide-base tires and wheels. Decreased weight and lower rolling resistance are key contributors to fuel economy improvements.
  • Reduce idling. Idling achieves zero MPG and uses 1/2 to 1 gallon of fuel per hour. Shut down to save fuel while at stops.
  • APUs. Auxiliary Power Units (APUs) allow drivers to run air conditioning and other equipment without running the engine. This decreases idling, reducing fuel consumption and improving fuel economy. But because APUs add weight, payload must decrease to meet weight limits.
  • Lower weight. New materials allow for less weight and more potential fuel economy gains, although with decreased chassis and trailer weight, overall payloads may increase and offset the fuel economy improvements

“When combined with other variables including progressive shifting, APUs and chassis improvements, staying in the ‘sweet spot’ is maximized,” Baltali said. He added that PHH FirstFleet expects future fuel studies to explore the relationship between maintenance, cost-per-mile and fuel efficiency.

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