The Relationship Between Mass Reduction and Fuel Economy
Steel has been the material of choice for automotive applications for generations, and there’s a reason for that. It is a flexible material that has been able to continuously evolve to keep up with ever changing vehicle design challenges. And the challenges of better fuel economy are no different. Evolving types of Advanced High-Strength Steels (AHSS) are becoming increasingly stronger at thinner grades and formable for complex shapes. Consequently, automakers are boasting mass reductions that are approaching those achieved by the ULSAB, ULSAB-AVC and FSV projects, which demonstrated the extensive use of AHSS in automotive light-weighting applications.
An fka study, Determination of Weight Elasticity of Fuel Economy for Conventional ICE Vehicles, Hybrid Vehicles and Fuel Cell Vehicles, researched mass savings versus fuel consumption and considered the influence for different vehicle classes, driving cycles and powertrains.
A statistic often seen in the media is that a 10% reduction in mass can result in a 6 to 8% reduction in fuel consumption. The study concludes that weight elasticity values can vary from 1.9 to 8.2% (Figures 3a and 3b) depending on driving cycle, vehicle size, powertrain selection and whether or not the powertrain is adjusted for equivalent acceleration for the reduced weight vehicle.
Weight elasticity values of 6 to 8% are possible with powertrain resizing for equivalent acceleration using conventional gasoline powertrains. The effect of powertrain resizing has more influence on fuel savings than does mass reduction, especially for urban driving cycles. Therefore, these impressive fuel economy gains of 6 to 8% usually are not realized in real vehicle designs for several reasons:
- Vehicle manufacturers do not have enough engine and powertrain system options to apply to every incremental step in vehicle weight.
- Market forces have caused significant increases in acceleration performance rather than the resizing of powertrains to equivalent performance.
- Data collected by the U.S. National Highway and Traffic Safety Administration in its “2004 Automotive Fuel Economy Update” and by the European Automobile Manufacturers Association indicate significant gains in engine technology over the past two decades, but this technology is applied to vehicle acceleration performance rather than to fuel economy.
The fka study concludes that when engine and powertrain system resizing is not achieved, weight/fuel consumption elasticity values of only 2–4% are applicable.
The fka study also considers advanced powertrains, such as hybrids and fuel cell vehicles. The study concludes that these advanced powertrains, which take advantage of regenerative braking, do not see the same large variation in weight/fuel consumption elasticity with powertrain resizing as conventional internal combustion engines do. Historically, the often-stated weight elasticity figure of 8% has not been achieved. Such a high reduction in fuel consumption will be almost totally out of reach as hybrid and fuel cell powertrains become more widely used.
Update: We’ve seen the results of this fka study played out in the Life Cycle Assessment studies we have conducted. Have a look at our Light Duty Truck and Sport Utility Vehicle case studies, conducted with the UCSB Automotive Greenhouse Gas and Energy Model. The fuel economy predicted by the model, resulting from the mass reduction, lines up with fuel economy reported by OEMs on similar vehicles.