In 2007 the National Highway Traffic Safety Administration (NHTSA) requested that the US National Academies provide an objective and independent update of the technology assessments for fuel economy improvements and incremental costs contained in the 2002 National Research Council (NRC) report Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards.

NHTSA also asked that the NRC add to its assessment technologies that have emerged since that report was prepared. To address this request, the NRC formed the Committee on the Assessment of Technologies for Improving Light-Duty Vehicle Fuel Economy. The statement of task directed the committee to estimate the efficacy, cost, and applicability of technologies that might be used over the next 15 years.

The report, Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy estimates the potential fuel savings and costs to consumers of available technology combinations for three types of engines: spark-ignition gasoline, compression-ignition diesel, and hybrid.

While each technology on its own has benefits, and is often considered independently, there can be positive and negative interactions among individual technologies, and so the technologies must be integrated effectively into the full vehicle system, the report says.

Committee’s estimates of effectiveness (shown as a percentage) of near-term technologies in reducing vehicle fuel consumption. Benefits highlighted in green are incremental to those obtained with preceding technologies shown in the technology pathways. Source: NRC. Click to enlarge.

Integration requires that other components of the vehicle be added or modified to produce a competitive vehicle that can be marketed successfully. Thus, although the fuel consumption benefits and costs discussed here are compared against those of representative base vehicles, the actual costs and benefits will vary by specific model. Further, the benefits of some technologies are not completely represented in the tests used to estimate corporate average fuel economy (CAFE). The estimate of such benefits will be more realistic using the new five-cycle tests that display fuel economy data on new vehicles’ labels, but improvements to test procedures and additional analysis are warranted.

Given that the ultimate energy savings are directly related to the amount of fuel consumed, as opposed to the distance that a vehicle travels on a gallon of fuel, consumers also will be helped by addition to the label of explicit information that specifies the number of gallons typically used by the vehicle to travel 100 miles.

—“Assessment of Technologies”

Spark-ignition technologies. Spark-ignition (SI) engines will continue to be the primary source of propulsion for light-duty vehicles in the United States over the time frame of the report (next 15 years). There have been and continue to be significant improvements in reducing the fuel consumption of SI engines in the areas of friction reduction; reduced pumping losses through advanced valve-event modulation; thermal efficiency improvements; cooled exhaust gas recirculation; and improved overall engine architecture, including downsizing, the report says.

An important attribute of improvements in SI engine technologies is that they offer a means of reducing fuel consumption in relatively small, incremental steps. This approach allows automobile manufacturers to create packages of technologies that can be tailored to meet specific cost and effectiveness targets, as opposed to developing diesel or full hybrid alternatives that offer a single large benefit, but at a significant cost increase. Because of the flexibility offered by this approach, and given the size of the SI engine-powered fleet, the implementation of SI engine technologies will continue to play a large role in reducing fuel consumption.

—“Assessment of Technologies”

The committee identifies cylinder deactivation as one of the more effective in reducing fuel consumption; this feature is most cost-effective when applied to six-cylinder (V6) and eight-cylinder (V8) overhead valve engines, and typically reduces fuel consumption by 4 to 10% at an incremental retail price equivalent (RPE) increase of about $550. Others include:

• Stoichiometric direct injection typically affords a 1.5 to 3 percent reduction in fuel consumption at an incremental RPE increase of $230 to $480, depending on cylinder count and noise abatement requirements.

• Turbocharging and downsizing can also yield fuel consumption reductions.

  Downsizing is an important strategy applicable in combination with technologies that increase engine torque, such as turbocharging or supercharging.

• Downsizing simultaneously reduces throttling and friction losses because downsized engines generally have smaller bearings and either fewer cylinders or smaller cylinder bore friction surfaces. Reductions in fuel consumption can range from 2 to 6% with turbocharging and downsizing, depending on many details of implementation.

  This technology combination is assumed to be added after direct injection, and its fuel consumption benefits are incremental to those from direct injection. Based primarily on an EPA teardown study, the committee’s estimates of the costs for turbocharging and downsizing range from close to zero additional cost, when converting from a V6 to a four-cylinder (I4) engine, to almost $1,000, when converting from a V8 to a V6 engine.

• Valve-event modulation (VEM) can further reduce fuel consumption and can also cause a slight increase in engine performance, which offers a potential opportunity for engine downsizing. There are many different implementations of VEM, and the costs and benefits depend on the specific engine architecture. Fuel consumption reduction can range from 1% with only intake cam phasing, to about 7% with a continuously variable valve lift and timing setup.

  The incremental RPE increase for valve-event modulation ranges from about $50 to $550, with the amount depending on the implementation technique and the engine architecture.

The committee considered variable compression ratio, camless valve trains, and homogeneous-charge compression ignition. However, it concluded that because of questionable benefits, major implementation issues, or uncertain costs, it is uncertain whether any of these technologies will have any significant market penetration in the next 10 to 15 years.

Compression-ignition technologies. The committee found that replacing a 2007 model year SI gasoline power train with a base-level compression-ignition (CI) diesel engine with an advanced 6-speed dual-clutch automated manual transmission (DCT) and more efficient accessories packages can reduce fuel consumption by about 33% on an equivalent vehicle performance basis. It estimated the incremental RPE cost of conversion to the CI engine is about $3,600 for a four-cylinder engine and $4,800 for a six-cylinder engine.

• Advanced-level CI diesel engines, which are expected to reach market in the 2011-2014 time frame, with DCT (7/8 speed) could reduce fuel consumption by about an additional 13% for larger vehicles and by about 7% for small vehicles. Part of the gain from advanced-level CI diesel engines comes from downsizing.

• The estimated incremental RPE cost of the conversion to the package of advanced diesel technologies is about $4,600 for small passenger cars and $5,900 for intermediate and large passenger cars.

An important characteristic of CI diesel engines is that they provide reductions in fuel consumption over the entire vehicle operating range, including city driving, highway driving, hill climbing, and towing. This attribute of CI diesel engines is an advantage when compared with other technology options that in most cases provide fuel consumption benefits for only part of the vehicle operating range.

—“Assessment of Technologies”

The committee found that market penetration of CI diesel engines will be strongly influenced by both the incremental cost of CI diesel power trains above the cost of SI gasoline power trains and by diesel and gasoline fuel prices.

Further, while technology improvements to CI diesel engines are expected to reach market in the 2011-2014 time frame, technology improvements to SI gasoline and hybrid engines will also enter the market. Thus, competition between these power train systems will continue with respect to reductions in fuel consumption and to cost. For the period 2014-2020, further potential reductions in fuel consumption by CI diesel engines may be offset by increases in fuel consumption as a result of changes in engines and emissions systems required to meet potentially stricter emissions standards.

—“Assessment of Technologies”

Hybrid vehicle technologies. For the most basic hybrid systems that reduce fuel consumption by turning off the engine while the vehicle is at idle, the committee pegged the fuel consumption benefit at up to about 4% at an estimated incremental RPE increase of $670 to $1,100. The fuel consumption benefit of a full hybrid may be up to about 50% at an estimated incremental RPE cost of $3,000 to $9,000 depending on vehicle size and specific hybrid technology.

A significant part of the improved fuel consumption of full hybrid vehicles comes from the complete vehicle redesign that can incorporate modifications such as low-rolling-resistance tires, improved aerodynamics, and the use of smaller, more efficient SI engines, the committee noted.

In the next 10 to 15 years, improvements in hybrid vehicles will occur primarily as a result of reduced costs for hybrid power train components and improvements in battery performance such as higher power per mass and volume, increased number of lifetime charges, and wider allowable state-of-charge ranges.

...A number of different lithium-ion chemistries are being studied, and it is not yet clear which ones will prove most beneficial. Given the high level of activity in lithium-ion battery development, plug-in hybrid electric vehicles will be commercially viable and will soon enter at least limited production. The practicality of full-performance battery electric vehicles (i.e., with driving range, trunk space, volume, and acceleration comparable to those of vehicles powered with internal-combustion engines) depends on a battery cost breakthrough that the committee does not anticipate within the time horizon considered in this study.

However, it is clear that small, limited-range, but otherwise full-performance battery electric vehicles will be marketed within that time frame. Although there has been significant progress in fuel cell technology, it is the committee’s opinion that fuel cell vehicles will not represent a significant fraction of on-road light-duty vehicles within the next 15 years.

—“Assessment of Technologies”

Resources

• Assessment of Technologies for Improving Light Duty Vehicle Fuel Economy (National Academies Press, 2011)