Ford researchers present physics-based model of mass-induced fuel consumption for better insight into lightweighting benefits
A pair of researchers from Ford Motor Company’s Systems Analytics and Environmental Sciences Department in Dearborn have developed a novel physics-based model of mass-induced fuel (MIF) consumption which can be used in vehicle life cycle assessments to provide better insight—i.e., from a more firm scientific foundation—on the potential benefits of lightweighting.
To illustrate the method, they used their model to estimate the MIF values for 2013 model year internal combustion engined using the US Environmental Protection Agency’s (EPA’s) fuel economy certification data. They found MIF values in the range of 0.2−0.5 L/(100 km 100 kg). As described in a paper on their work in the ACS journal Environmental Science & Technology, the results showed that lightweighting has the most benefit when applied to vehicles with high fuel consumption and high power.
Life cycle assessment (LCA) is used to measure the environmental impacts associated with the complete life cycle of a vehicle. The use-phase, that is, vehicle operation, is the most energy consuming and accounts for approximately 60− 90% of the total life cycle energy use of conventional vehicles. Lightweighting is an effective measure to reduce the use-phase environmental impacts. However, the production and processing of lightweight materials tends to require more energy and thus generates more greenhouse gases (GHGs) than for conventional steel and iron based alloys. An accurate estimation of the impact of mass reduction on the use-phase fuel consumption is required to assess the life cycle benefit of vehicle lightweighting.
The fuel consumption reduction resulting from lightweighting is typically estimated based on a fuel-mass correlation (e.g., fuel consumption reduction value (FRV)). Accurately describing the mass-induced fuel consumption is particularly challenging in vehicle component LCAs. Current methods to estimate mass-induced fuel consumption give a wide range of results depending on assumptions regarding driving cycle, vehicle design, powertrain type, and whether the powertrain is rematched for performance equivalence.
… Fuel-mass correlation parameters are usually not available to LCA practitioners for specific vehicle models. Often, simple generic values are used which are based on literature heuristics (e.g., 6% fuel consumption reduction per 10% mass reduction). The currently available data and associated models generally do not provide guidance on estimating mass-induced fuel consumption for specific vehicle models currently in use.—Kim and Wallington
There are three fundamental issues with the existing methods of assessing mass-induced fuel consumption, say Hyung Chul Kim and Timothy J. Wallington:
The mass-induced fuel consumption of the baseline scenario is not clearly defined.
For LCA practitioners, it is very difficult to obtain the FRV or fuel-mass coefficient for a specific vehicle model under study.
The absence of a uniform approach has led to a wide range of values of FRV (Rw) used in the literature studies.
In addition to the mass-induced fuel consumption from acceleration and rolling resistance (as used in other studies), they also consider the mass-induced fuel consumption associated with the energy lost without doing useful work.
They separate the wasted fuel consumption into the mass-induced part and the balance based on the energy breakdown for the vehicle load. Then, they allocate the mass-induced wasted fuel consumption to the mass.
The rationale behind this is that the wasted fuel consumption stems from inefficiencies in delivering the engine power to overcome mass- related vehicle loads. As a result, the mechanical energy losses … decrease with lightweighting along with the fuel consumption from mass induced-loads… This is a clear distinction from the principle of existing models … where mechanical energy losses are either unaccounted for or deemed unrelated to vehicle mass.—Kim and Wallington
The model uses two steps to find the MIFs: first calculating the fuel consumption reduction value (FRV) and then estimating the gross mechanical efficiency from fuel consumption and energy loss profiles. They showed that the MIF estimated using this method can be used for both baseline and lightweight scenarios with or without powertrain adjustment for a practical range of lightweighting in which mechanical efficiency is not substantially changed.
The key policy implication of our model is that the benefit of lightweighting depends on vehicle fuel economy and power. Applying our model to the EPA fuel economy test results shows that a unit mass reduction saves more fuel from less fuel efficient vehicle models than from more fuel efficient models and from higher power rather than lower power vehicles. The existing models based on FRVs do not provide such insight because FRVs are not related significantly to vehicle fuel economy or maximum engine power …
Vehicles with an advanced powertrain such as hybrid electric vehicle (HEV), battery electric vehicle (BEV), and fuel cell vehicle (FCV) would have a different range of efficiency parameters such as thermodynamic and mechanical efficiency. Moreover, the distinct powertrain configuration of these vehicles would pose a challenge of modeling additional factors such as efficiencies of motor and regenerative brake. However, a top-down approach … can be applied to these vehicles. … Incorporating additional parameters such as motor and battery efficiencies, the present model can readily be adapted for the advanced powertrain vehicles. This would be particularly pertinent for HEVs and PHEVs whose test records are available in the EPA database. Detailed analysis on estimating MIFs for these vehicles is left for future research.—Kim and Wallington
Hyung Chul Kim and Timothy J. Wallington (2013) “Life Cycle Assessment of Vehicle Lightweighting: A Physics-Based Model of Mass-Induced Fuel Consumption,” Environmental Science & Technology doi: 10.1021/es402954w