Looking Back, Projecting Forward: Light Duty Vehicle Efficiency in the US
20 April 2007
|The PSFI of An and DeCicco found a linear progression in efficiency improvement over the past three decades. Click to enlarge. Data: An and DeCicco|
In the increasing debate over technology strategies and public policies to mitigate greenhouse gas emissions and begin reducing petroleum dependency, it's useful to analyze past trends in vehicle efficiency as well as to try to assess the potential impact of powertrain choices for the future.
Two papers presented at the SAE World Congress addressed those topics. Feng An from Energy and Transportation Technologies, LLC and John DeCicco from Environmental Defense presented an analysis of past trends in technical efficiency for the US light-duty vehicle fleet, including a few recommendations for the future. Emmanuel Kasseris and John Heywood of MIT’s Sloan Auto Lab assessed the potential improvement of automotive powertrain technologies 25 years into the future.
Using fuel economy alone as a measure misses trade-offs between fuel economy and other attributes of light vehicles that represent technical efficiency gains. An and DeCicco combined fuel economy with performance and size to create a “Performance-Size-Fuel economy Index” (PSFI) for quantifying technical efficiency.
Applying the PSFI to vehicle data since 1977, they found generally stronger efficiency improvement rates over the past 25 years than prior analyses had determined. Their results also suggest a diminishing growth rate for fuel economy at higher absolute levels of size and performance.
They found that relative to a 1977 baseline, technical efficiency as measured by PSFI increased an average 5.3% per year for cars and 3.1% per year for trucks.
For cars, they defined PSFI as the product of EPA’s sales-weighted horsepower-to-weight ration and sales-weighted “cubic feet mpg” statistics. For trucks, they defined PSFI as the product of EPA’s sales-weighted horsepower-to-weight ratio, wheelbase in inches, and mpg statistics.
In analyzing the results, they determined that distinct phases of technical efficiency gains have occurred over the past 30 years. These phases are:
Accelerated fuel economy growth (1977-82 for cars, 1979-81 for trucks). With the implementation of CAFE, fuel economy trends outpaced the PSFI trend, and were accompanied by reduction in performance and size.
This phenomenon was most obvious for light trucks, where the power-to-weight ratio was reduced by as much as 13% by the early 1980s to allow rapid fuel economy gains.
Limited fuel economy growth (1982-88 for cars, 1981-87 for light trucks). Fuel economy increased during this phase, but at a slower rate.
Exclusive performance and size growth. (1988-2005 for cars, 1995-2003 for light trucks.) Fuel economy remained flat, with all technology advances going into performance and/or size increases.
Accelerated performance growth (1987-95 for light trucks). Fuel economy decreased while performance and size increased.
Exclusive fuel economy growth. This situation, in which all technology advancement is used to implement fuel economy, without sacrifice of enhancement of power and size, “does not appear to have occurred for any appreciable time to date.”
Although PSFI has increased steadily over the past two decades (about 50% for cars and 30% for trucks), it is almost completely due to performance gain. Thus, the way technology advances largely were applied over the past twenty years may have hurt the potential for future fuel economy improvement, assuming that feasible rates are linked to an underlying technical efficiency trend...and that market conditions do not shift towards smaller vehicle size or lower performance.
The implication is that either an accelerated rate of technological progress, or a turning back of the clock on performance and size levels, might be needed to improve fuel economy at rates comparable to what was achieved in earlier years.
Kasseris and Heywood modeled out the potential improvements in fuel consumption and performance of a variety of powertrain technologies with a focus on the fuel consumption and GHG emissions of the US light duty fleet in 2030.
They conducted their analysis from an overall vehicle system point of view (including such aspects as improvements in aerodynamics, transmissions and tire rolling friction), and with an eye on the total fleet impact, not just on new vehicles.
For a significantly different powertrain technology to make a difference in fleet petroleum demand and GHG emissions it must first penetrate the fleet in significant numbers. Before penetrating the fleet in significant numbers, the new technology must become a significant fraction of all vehicles manufactured. For significant production to start, the new technology has to at least be close in market competitiveness with existing technologies.
Most of the new propulsion technologies now under development (such as fuel cell vehicles, electric vehicles, or plug-in hybrids), still have technical challenges to solve before they can be considered market competitive. Although new technologies could have an impact at the fleet level in the mid- to longer term future, this is less likely in the short term.
|Projected possible improvements in fuel consumption for a lower-performance sedan with a variety of future powertrains. Click to enlarge. Data: Kasseris and Heywood.|
For those reasons, the researchers limited the study to analyzing internal combustion engines using petroleum-based fuels or blended biofuels. They assess the powertrain technologies on the basis of three vehicle platforms: a lower-performance mid-size sedan, a higher-performance version of the mid-size sedan, and a representative pickup truck. They chose the Camry as the model for the sedan, and the F-150 as the model for the pickup.
They held the size and the performance of the future vehicles constant at the level of 2005 models to compare fuel consumption of the different technologies on an equivalent basis. Vehicle simulations were performed using the ADVISOR software.
Among their conclusions were the following:
The current relative advantage of diesel over gasoline engines is likely to be reduced in the future.
Turbocharged gasoline engines have the potential to become almost equivalent with low emissions diesel engines. To realize that potential, however, the limitation of knock must be dealt with.
Hybrids maintain a comparative advantage in terms of combined fuel consumption in the future, which is likely to increase. “The main reason is that hybrids, being a relatively new technology, can benefit from several component (engines, motors, battery etc.) as well as integration/architecture improvements.”
More efficient continuously variable transmissions could be a key enabling technology in improving fuel economy for hybrids and non-hybrids.
The relative fuel consumption benefits of diesel, turbocharged and hybrid powertrains are more pronounced for higher performing vehicles. “This is because sizing the engine for higher performance results in lower average engine loads.”
Future hybrids maintain a comparative advantage even for highway and aggressive driving patterns. The relative benefit remains the highest for urban driving, however.
Future gasoline engines will close the benefit gap with diesels in both urban and highway driving.
Fuel consumption results obviously varied with technology choices and drive cycles, but the study found decreases in fuel consumption ranging from 35% to more than 50% even on the more aggressive drive cycles.
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