CAFE Requirements
Corporate Average Fuel Economy (CAFE) standards require improved fuel economy to 54.5 mpg for cars and light-duty trucks in US by 2025 model year. That drives automakers to seek various ways to improve fuel consumption. One main approach is to cut the vehicle weight.
Reducing a vehicle’s mass by 10% can improve fuel economy by 6-8% (DOE, 2015). In addition, for new electric cars, their weight is extremely important to the battery power demand. A 10% reduction in vehicle weight permits nearly a 10% smaller in battery size (Smock, 2010).
A midsize passenger car weights about 3400 lb (1542 kg). Many industrial experts consider a 450 lb (204 kg), or 13%, reduction of vehicle weight is necessary by 2025. The other segments of passage vehicles should have similar targets. To reduce the weight of a vehicle, its body structure is often considered a main target, for a 30% reduction. Powertrain units can be expected be reduced by 25%. Other areas, such as chassis, interior, closures, and glazing, may be targeted for 20, 14, 8, and 3%, respectively.
Environmental Concern
CO2, sometimes called Greenhouse gas (GHG), emission as a main environmental concern is also a direct benefit of weight reduction and fuel economy improvement. Figure 1 shows a clear correlation between the fuel economy and CO2 emission in the last four decades (EPA, 2016).
Figure 1. EPA report on vehicle fuel economy and CO2 emission
The CO2 emission from vehicle fuel combustion can be calculated by:
where, CO
2 emission and gas combusted are in weight, constant 0.87 is due to 87% carbon and 13% hydrogen in gasoline by weight, 44 is molecular weight of CO
2, and 12 is molecular weight of carbon. One gallon of gasoline weights about 6.3 lb. Therefore, one-gallon gasoline can produce approximately 20 lb (9.07 kg) CO
2 when burned (DOE, 2016).
CO2 generated from the material production should be considered as well. Lifecycle assessment (LCA) is a good tool for material selection. If an LCA includes the material production, the analysis results can be different to minimize the entire CO2 emission. According to the World Steel Association, steel production generates much less GHG, refer to Table 1 (ArcelorMittal, 2014). Using light materials can save vehicle weight and improve fuel economy. However, taking the vehicle mass reduction rate (say up to 50% if using aluminum) into count and the CO2 from material production steel are competitive for CO2 emission reduction.
Table 1. Greenhouse gas generated from material production
| Material | GHG (kg CO2e/kg) |
| Steels | 2.0–2.5 |
| Aluminum Alloys | 11.2–12.6 |
| Magnesium Alloys | 18–45 |
| Carbon Fiber | 21–23 |