The weight of passenger cars has risen on average in Europe by nearly 30 % over the last thirty years. This weight increase reflects the demand for greater comfort such as air conditioning, power steering or increasing the size of the passenger compartment, but also by implementing regulations concerning safety and the environment. There are also technical reasons such as the considerable increase in the internal pressure of diesel engines which require a substantial resizing.
However it has become urgent to drastically reduce the fuel consumption of vehicles which has led today to turning the trend around and considerably reduce the weight of vehicles while maintaining the progress of recent years.
Weight and energy
Weight has a direct effect on the energy consumed by a vehicle, irrespective of the type, when accelerating and going uphill, and also affects tire rolling resistance.
Weight also plays a part in the energy required for braking. This explains the advantage of recuperative (or regenerative) braking, when traveling downhill and in urban areas as it allows the battery to be recharged rather than losing it as heat or wear on brake pads. Thermal vehicles fitted with a stop & start system and electric vehicles also enjoy this advantage.
Weight and comfort
Improving comfort is generally accompanied by a weight increase. This is the case for example of electric window controls, air conditioning, power steering, sound insulation, hydraulic suspension and other electric and electronic equipment.
Dynamic comfort, on bumpy roads for example, is closely dependent on the vehicle’s weight and relationship between the weight of the body and of the non-suspended elements (hubs, wheels, tires, shock absorbers, etc.).
Reducing the weight of vehicles without limiting comfort is a difficult challenge for automakers.
Weight and vehicle horsepower
From the very beginning the quest for horsepower has dictated the changes to thermal motors, bringing an increase in the number of cylinders and causing an increase in weight. Since the 1990s efforts have been focused on improving engine efficiency which is essentially obtained by lowering the maximal power, which in turn means the motor is operating closer to its normal charge in the most frequent conditions. The extra power needed during acceleration phases is supplied by a turbocharger or an electric motor (hybridization). The outcome is a substantial reduction in the number of cylinders and the size (downsizing), which can also result in reducing the weight of part of the chassis.
Weight and road safety
The laws of physics relative to impacts are well known: in the event of a collision between two vehicles of different weight, it is better to be in the heavier vehicle. But in the case of an accident involving only one vehicle the lighter it is then the crash generates less energy. Current regulations are in favor of designing vehicles able to absorb part of the energy of the impact while transmitting as little as possible of the energy to all those involved in the collision. Vehicle safety design is currently guided more by an endeavor to transmit the minimum amount of energy to the users concerned by a collision rather than transforming the vehicle itself into an assault tank. Secondary safety.
An increase in weight may, however, be associated with the protection of the occupants and other users in the event of an impact. This involves lateral reinforcements, a deformable front structure, airbags and pop-up hoods, etc.
Weight and the environment
In an increasing number of countries environmental regulations mean that some vehicles have to be fitted with catalytic converters in some countries and particle filters or NOx traps in others. All this equipment adds to the general trend of increased vehicle weight.
Weight and electric vehicles
Comparing the weight of an electric vehicle to that of a conventional vehicle is not as easy as it first appears. In fact, you need to be able to compare two vehicles in the same range, with the same horsepower and autonomy, which is often only a textbook case since electric vehicles are intended for different market segments than conventional vehicles, in terms of autonomy in particular.
An example is the Renault Fluence Z.E. which exists in a conventional diesel version and an electric Li-ion battery version:
0 – 100 km/h
66 kW/90 hp
70 kW/95 hp
85 – 135 km
The weight difference of 250 kg is due to the batteries. The specific energy of the lithium battery pack is currently in the region of 100 Wh/kg. However, it is anticipated that this value will be doubled by 2015.
Note: 100 Wh/kg from a battery pack may seem ridiculous compared with the 10 kWh/l from fuel but we should not forget that the battery supplies all the energy whereas every liter of fuel needs 10 m3 of air, i.e. 12 kg for its combustion process! The actual specific energy of fuel + air is therefore 770 Wh/kg but one of the major advantages of a conventional engine is the fact that the air does not have to be transported.
Why and how to reduce weight?
Weight consumes energy during every acceleration phase or when driving uphill. Less energy is recovered when slowing down, braking or going downhill, hence the need to reduce the weight of vehicles.
A few key figures:
- A 100 kg reduction in weight gives fuel savings of 0.4l/100 km and -10 gCO2/km
- On average a local gain of 1 kg means an overall gain of 1.5 kg. (For example reducing the weight of the motor brings a reduction in weight of the cradle…)
Reducing the weight of a vehicle involves three types of action:
- Reducing the weight: of the motor by reducing its size (downsizing), of the platform and the bodywork. The resulting loss of horsepower can be compensated for by adding a turbo charger or an electric motor.
- Optimizing the design of the main structures and accessories to an ISO impact rating (for safety).
- Using different materials: high resistance steels, aluminum, magnesium, composites, reinforced plastics…
Significant weight gains can also be obtained by redesigning the architecture of vehicles to match needs. The issue is nonetheless not that simple as market segmentation cannot be absolute: could we seriously prohibit the owner of a basic entry level vehicle who mainly uses his vehicle in the city from occasionally taking it on the freeway? This would appear to be difficult. Automakers are well aware of this sensitive issue and have been working on it for several years now. Today’s small urban cars can use the freeway in satisfactory conditions
Reduction of weight in figures
If important weight gains are possible they all have a cost. Simulations done by automakers give the following estimates:
For an urban vehicle: the average weight could be lowered from 1,300 kg to 900 kg for a cost in the order of 2,500 US dollars. What would the return on investment be for the user?
A weight gain of 400 kg would result in fuel savings of 1.6l/100 km or 320 liters annually for traveling 20,000 km/per year. At 1.5 dollars the liter the savings would be in the region of 450 US dollars per year, hence an amortization period of roughly 5 years.
Note: Annual combustion of 320 fuel liters roughly produces 960 kg of CO2.