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What is the result of reducing a single kilogram in weight on vehicle CO2 emissions and on electric vehicle battery range?
Although there is a correlation between vehicle weight and efficiency, which relates to CO2 emissions in traditional internal combustion engine vehicles and to driving range on a single battery charge in electric vehicles, it is not a simple direct correlation. It is not possible to provide a CO2 saving or driving range extension in exact, absolute values in relation to a 1 kg reduction in vehicle weight. In traditional fuel vehicles, empirical studies have found a range of results for change in fuel consumption and CO2 emission with change in vehicle weight. Based on this we can expect an estimated 0.75-1.25 gCO2/km decrease in CO2 emissions per kg decrease in vehicle weight for a medium-sized passenger car. In electric vehicles, the calculation is complex and there are many variables to determine the change in driving range in relation to change in vehicle weight, however based on the results of one empirical study, we can expect an estimated 0.05-0.13km increase in driving range per kg decrease in electric vehicle weight.
INTERNAL COMBUSTION ENGINE VEHICLES
When traveling at low speeds, the weight of a vehicle is one of the main factors that determine a vehicle’s fuel efficiency, and related CO2 emissions. A heavier vehicle will consume more fuel in order to accelerate, as rolling resistance is dependent on weight, or more correctly, mass. However, because of a number of other factors, to calculate exactly how a change in vehicle weight impacts fuel efficiency and corresponding CO2 emissions is complicated. A report entitled "Light weighting as a means of improving Heavy Duty Vehicles’ energy efficiency and overall CO2 emissions" found that “[t]he CO2 emissions (per km) can be related to the vehicle’s weight by the equation: CO2 (g/km) = Gradient x vehicle weight + constant”. If considering only flat terrain, the gradient can be removed from the equation however the constant coefficient needs to be determined. The constant coefficient depends on the vehicle, and the report provides values for gradients and constants based on simulations of various types of heavy-duty vehicles in Table 3.17 on page 59. Accordingly, the CO2 emission savings will vary depending on the vehicle and its related constant coefficient.
A review from the US Department of Energy reports that a linear regression analysis found that a 10% reduction in vehicle weight results in an 8% reduction in CO2 emissions, for the “model year 2008 vehicle fleet”. Other empirical studies mentioned in their review found that a 10% reduction in vehicle weight resulted in 6.3% decrease in fuel consumption for light trucks, whereas another model found it to result in a 7.6% reduction. Although exact figures vary, all studies included in the review found a 10% decrease in vehicle weight corresponded to 6-8% reduction in fuel consumption.
Another study “Fuel consumption and CO2 emissions from passenger cars in Europe — Laboratory versus real-world emissions”, notes that “[t]here are no common metrics or approaches for the measurement and quantification of the impact” of vehicle weight on the CO2 emissions. In a review of academic research on this, the author has concluded that adding weight to a vehicle can increase fuel consumption by 5-9% and CO2 emissions by 6.5-12 gCO2/km. This range is based on additions of 50-200kg under a variety of conditions, and based on passenger cars — unfortunately I was unable to locate any similar studies focused on trucks. In absolute terms, adding 100 kg in vehicle weight is reported to increase CO2 emissions by 7.5–12.5 gCO2/km for a medium-sized passenger car. As the relationship is linear, we can assume that 1 kg change in weight will result in a 0.75-1.25 gCO2/km change in emissions.
ELECTRIC VEHICLES
The batteries used in electric vehicles tend to be the heaviest component. The type of battery will therefore impact the driving range, both in terms of rolling resistance which is impacted by weight, but also in terms of power as lighter batteries will have a smaller capacity. Simulations carried out in “Analysis of Parameters Influencing Electric Vehicle Range” of a 600 kg electric vehicle carrying 100 kg of passengers and baggage, with different battery power and corresponding battery weight. Data has not been provided, but the graphs produced of vehicle weight versus driving range shows a linear relationship. The graphs show a variation of approximately 300 kg results in a difference of around 15 km for a vehicle with 100Ah battery capacity of 8kWh, or a difference in around 40 km for the same change in weight for a 300Ah capacity of 24kWh. Therefore, based on these results and a linear relationship, for 1 kg change in vehicle weight one can expect a 0.05-0.13km increase in driving range.
Extended searches of academic papers and industry reports did not return any further useful data. A “Literature Review of Electric Vehicles and Driving Range Extension” touches upon vehicle weight as one of the factors that affects driving range extension. It identifies the power to drive forward the vehicle needs to overcome four components: (1) base electric load, such as from the car heater and music system, (2) aerodynamic drag, (3) rolling resistance and (4) inertia. If the vehicle is on a gradient, then it will also need power to work against gravity. These last two components will vary with weight (or more correctly, mass) however the series of equations to determine the driving range is complex. The equation to calculate driving range is given as Eb/(Ew/D) where Eb is battery energy, D is driving distance and Ew is the average energy over one driving cycle. Ew can only be calculated by integrating the power at the wheels with respect to time. The power at the wheels (Pw) must be equivalent to the four components already mentioned, of which two are affected by vehicle mass. Rolling resistance varies with vehicle mass but has a constant coefficient, and inertia also depends on vehicle velocity. A similar approach is adopted in the “Optimal light weighting in battery electric vehicles” study. Without models or simulations to determine constant coefficients and other variables aside from weight, the equations for calculating driving range extension cannot be solved.
CONCLUSION
To conclude, the equations to determine the change in CO2 emissions or driving range in relation to vehicle weight are too complex to solve and include too many unknown factors, however from empirical studies we have an approximate range for each. These allow us to estimate a range of expected decrease in CO2 emission and increase in driving range per kilogram decrease in vehicle weight, of 0.75-1.25 gCO2/km and 0.05-0.13km respectively.