Recent downspeeding findings

Other options for further fuels savings for the future will likely include downspeeding. North American Council for Freight Efficiency released a report on the Trucking Efficiency effort, which is the result of a joint venture with the Carbon War Room, an organization of comprised of citizens, entrepreneurs, investors and industry leaders who work to accelerate the adoption of business solutions to reduce carbon emissions and advance a low-carbon economy. Recently, it distributed its eighth Confidence Report on vehicle downspeeding, which focused on transmissions, engine parameters, lower rolling resistance (LRR) tires and light-weighting.

According to the report, downspeeding in trucking specifically entails reducing the 65 MPH cruise engine RPM from 1,600 RPM or higher to less than 1,200 RPM, corresponding to a reduction in rear-axle ratios from over 4.00:1 to around 2.47:1. Recent product announcements and new product launches are even more aggressive with downspeeding, offering ratios as low as 2.28:1 and even 2.08:1. Downspeeding can be adopted in one of two configurations:

• A direct drive transmission combined with a very fast axle ratio (between 2.64:1 and 2.47:1).
• An overdrive transmission and somewhat slower rear axles (approximately 3.36:1 and faster).”

The research drew the following conclusions:

• Downspeeding is an idea that has been around a long time, but recent complementary technologies have made it more attractive and practical.
• Fleets in long haul operation should consider downspeeding in their powertrain for improved fuel efficiency and driver satisfaction.
• Some manufacturers are already at work on aggressive downspeeding with rear axle ratios at 2.28:1 or lower.

The benefits of downspeeding include improved fuel economy, better drivability and reduced noise. The consequences, however, may lead to the potential for driveline failure and upfront cost of more durable components.

The key findings noted in the report are: When optimally applied, downspeeding will improve fuel efficiency and lower the operating RPM of the engine under cruise conditions, while helping in other areas such as noise reduction and improving drivability. In addition, downspeeding alone can save 2% to 3% off the fuel bill. However, specifying a downsped engine without looking at the entire powertrain can have negative consequences such as increased risk of driveline failure or insufficient horsepower, according to the report. Optimal truck design will see downsped powertrains in either of the two configurations spec’d with other technologies, including AMTs, certain rear-axle ratios, modified engine torque levels that may be restricted to certain gears, carefully chosen electronic engine parameters and reinforced drivelines.This package of multiple fuel efficiency technologies results in about 3% to 6% fuel savings overall and reduces the negatives posed by adopting downspeeding exclusively.

Downspeeding is at a tipping point, with rear-axle ratios of 2.47:1 and engine RPMs of 1,100 to 1,300 now common offerings among powertrain manufacturers. And “aggressive downspeeding” is just around the corner, with manufacturers poised to offer rear-axle ratios of around 2.08:1, and even lower engine cruise RPMs of just 900 to 1,000.