NRC report assessing fuel economy technologies for light-duty vehicles

9 June 2010

The US National Research Council published a report, titled “Assessment of Fuel Economy Technologies for Light-Duty Vehicles”, which examines fuel saving technologies for light-duty vehicles and their impact on vehicle cost. While commercially available technologies could greatly reduce fuel consumption in passenger cars, SUVs, minivans, and other light-duty vehicles without compromising vehicle performance or safety, the technologies would also increase vehicle purchase costs for consumers, sometimes by as much as several thousand dollars, concluded the committee that authored the report.

Using a 2007 base vehicle, the committee estimated the potential fuel savings and costs to consumers of available technology combinations for three types of engines: spark-ignition gasoline, compression-ignition (CI) diesel, and hybrid. The findings were:

• Adopting the full combination of improved technologies in medium and large cars and pickup trucks with spark-ignition engines could reduce fuel consumption by 29% at an additional cost of $2,200 to the consumer.
• Replacing spark-ignition engines with diesel engines would yield fuel savings of 37% at an added cost of $5,900 per vehicle.
• Replacing spark-ignition engines with hybrid powertrains would reduce fuel consumption by 43% at a cost increase of $6,000 per vehicle.

The report focuses on fuel consumption—the amount of fuel consumed in a given driving distance—because energy savings are directly related to the amount of fuel used. In contrast, fuel economy measures how far a vehicle will travel with a gallon of fuel. The relationship between these two metrics is non-linear, causing the percentage fuel economy improvement to be different from the above fuel consumption improvement figures. As fuel consumption data indicate money saved on fuel purchases and reductions in carbon dioxide emissions, vehicle stickers should provide consumers with fuel consumption data in addition to fuel economy information, the report says.

The study recommended that the National Highway Traffic and Safety Administration and the US Environmental Protection Agency review and revise fuel economy test procedures so they better reflect vehicle operating conditions and provide incentives to manufacturers to reduce fuel consumption. The report identifies an improved method that NHTSA and EPA should use to more accurately estimate a technology’s ability to reduce fuel consumption.

Engines and Technologies. Spark-ignition engines—believed to continue to be the dominant type of engine in the US for the next 10 to 15 years—have seen many technology improvements that are producing significant fuel savings. For example, the report notes the promise of cylinder deactivation—which enables a six- or eight- cylinder engine to run on fewer cylinders when full-engine power is not needed, such as on flat roads. Deactivation can reduce fuel consumption by as much as 10% at a retail vehicle price increase of $350 to $500.

Engines that run on diesel fuel may become more popular in the United States now that new emission control technologies are allowing for a wide range of diesel engines to meet federal and state air quality standards, the report says. Replacing a 2007 model spark-ignition engine with a base-level CI diesel engine—including a six-speed dual clutch transmission and other efficiencies—could reduce fuel consumption by about 33%, and with an incremental retail price cost of about $4,800 for a six-cylinder engine. Advanced CI diesel engines, expected to reach the market in the next five years, could reduce fuel consumption an additional 7% to 13%, with estimated vehicle cost increase of about $4,600 for small passenger cars to $5,900 for intermediate or larger vehicles.

In hybrid vehicle technologies, the degree of hybridization can vary from minor vehicle stop-start systems to complete vehicle redesign. A fully hybrid vehicle could reduce fuel consumption by about 50% at an estimated price increase of up to $9,000 a vehicle depending on vehicle size. The report notes that a significant part of reducing fuel consumption in full hybrids results from complete vehicle redesign that incorporates low-rolling-resistance tires, improved aerodynamics, and smaller, more efficient spark-ignition engines.

Hybrid vehicle improvements in the next 10 to 15 years will occur primarily in reducing costs of hybrid powertrain components and improving battery performance. While many challenges remain in developing lithium-ion battery technology, small, limited-range battery electric vehicles will be on the market in the next decade, the committee predicted. However, fuel-cell vehicles will not represent a significant fraction of light-duty vehicles on the road in the next 15 years.

The report also examines a range of non-engine vehicle technologies. Relatively minor changes that do not involve re-engineering the vehicle or require re-certification for fuel economy, emissions, or safety can be implemented within the next several years. They include reducing vehicle mass by using lighter materials, improving aerodynamics, or switching to low-rolling-resistance tires. Two important areas of research for long-range improvements include transmission systems and light-weighting—making vehicles very light with new materials under development.

The study was sponsored by the National Highway Traffic Safety Administration.

Source: National Academies