Medium- and heavy-duty vehicles use 26% of all U.S. transportation liquid fuels. That’s expected to increase at least until 2035, according to the Department of Energy (DOE) and the Energy Information Administration (EIA). Also of concern are the greenhouse gases emitted by these trucks, begging the question: what can we do about it?
In 2008, I was asked, along with 18 other committee members from academia and industry, to participate in a study on how to improve fuel economy in medium- and heavy-duty vehicles. The recommendations that resulted were published in the 2010 National Research Council (NRC) report: Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles. And, it’s from these recommendations that the government drafted the most recent mandates on fuel efficiency and CO2 emissions for medium- and heavy-duty trucks and buses.
As part of the NRC committee, I chaired the group tasked with writing Chapter 4, “Powertrain Technologies for Reducing Load-Specific Fuel Consumption”. This included diesel engines, gasoline engines, transmission and driveline technologies, and hybrid powertrains. I previously participated in the NRC review of the 21st Century Truck Partnership projects, which has DOE goals for brake thermal efficiency of 50% and 55%.
The diesel engine technologies identified for potential fuel consumption reduction were:
- Turbocharging, including turbocompounding
- Variable valve actuation
- Advanced exhaust gas recirculation
- Electric-driven accessories
- Engine friction reduction
- Alternative combustion cycles
- Improved DPF and SCR systems
- Thermal insulation of ports and manifolds
- Improved work extraction from the combustion process
- Electronic controller calibration management
- Bottoming cycles
A roadmap was included in the report for improving diesel engine thermal efficiency from 42% in 2008 to 49.1% by 2016 and 52.9% by 2019 using some of the above technologies. A similar approach was taken for gasoline engines and it was concluded that gasoline engines could reduce their fuel consumption by up to 24%. Even with this improvement, diesel engines will continue to have lower fuel consumption than gasoline engines by 6% to 24%. In addition, it was observed that, due to the cost of diesel engine aftertreatment systems, diesel engine penetration in medium-duty trucks decreased from 76% in 2004 to 58% in 2008.
As part of this research, we also found that automatic transmissions can reduce fuel consumption by decreasing driver variability and hybrids, both electric and hydraulic, can reduce fuel consumption by 5% to 50% depending on the application and duty cycle.
Overall, the report concluded that three technologies could best contribute to fuel consumption reduction from medium- and heavy-duty vehicles. These technologies are advanced diesel engines, hybridization and aerodynamics.
While reducing fuel consumption in medium- and heavy-duty vehicles is a complex problem with no one “silver bullet” solution, development of new technologies—like the Achates Power opposed-piston, two-stroke diesel engine—will go a long way in helping manufacturers and fleet owners meet the new fuel efficiency and emissions standards for 2014 and beyond (especially when those technologies can be produced for less!).