Is it time to consider an alternative in Europe?
Reducing vehicle mass is a clear route to reducing fuel consumption and therefore CO2 emissions – the benefits depend on the road load experienced by the vehicle during the drive cycle in question.
From 2012 vehicle manufacturers will face fiscal penalties if the average CO2 emissions on the new European drive cycle (NEDC) of the new vehicles which they sell exceed target levels set by the EU (see inset 1). Therefore cost-effective vehicle mass reduction might be expected to give unequivocal benefits to manufacturers.EU emissions targets (inset 1) In 2012 65% of each manufacturer’s new cars must comply on average with the limit value curve, rising to 75% in 2013, 80% in 2014, and 100% from 2015 onwards. From 2012 excess emissions premiums for each car registered are €5 for first g/km, €15 for second g/km, €25 for third g/km, and €95 for each subsequent g/km. From 2019 the premium will be €95 for every g/km above the limit value curve. The CO2 target specific to individual manufacturers is dependent on the average mass of the new vehicles it sells. Figure 2 shows that at the reference vehicle mass, which is taken as the industry-average mass of 1372 kg, the fleet-averaged CO2 target to avoid fiscal penalties is 130 g CO2/km. The limit value curve, shown by the blue line in the graph, varies so that the target is higher for manufacturers of heavier vehicles and lower for manufacturers of lighter vehicles (although the required percentage reduction to meet the new target for vehicles well above the reference mass is larger than those required by lower mass vehicles). The target varies with vehicle mass according to equation 1.
Vehicle mass influences directly almost two thirds of the 19% of the fuel energy which makes it into supplying propulsive effort to the vehicle.
About a third of this propulsive energy is imparted to the vehicle in the form of kinetic energy or energy used in overcoming the vehicle inertia during the acceleration events which is, in this case, not recovered. Close to another third is dissipated by the vehicle rolling resistance which is related to its mass via its influence on the friction between the moving interface between the tyre and the road. Only the energy dissipated in overcoming the aerodynamic drag of the vehicle is not directly related to the vehicle mass.
The NEDC does not include any gradients but energy required to overcome these in real world driving cycles is also directly proportional to the vehicle mass.
Clearly if manufacturer A’s average vehicle mass is 1372 kg in 2012 its CO2 target for new car sales would be 130 g/km. If the manufacturer reduces its average vehicle mass by 100 kg its CO2 target becomes 125.4 g/km – a more difficult target by 4.6 g CO2/km as a reward for its mass reduction efforts. From 2015, the reference mass used in equation1 (p50) will be based on the industry average mass. Thus if, after this point, other manufacturers of less svelte vehicles cause the industry-average mass to increase by 100 kg, manufacturer A’s new target would be 120.9 g CO2/km, i.e. manufacturer A’s target would have become more severe because other manufacturers had caused the industry-average mass to increase.
There is a further disincentive for a manufacturer inclined to produce lightweight vehicles by reducing only structural mass since this course of action does not give CO2 benefits which keep pace with the reduction of the legislative limit value curve. Thus, in the case of a manufacturer reducing its fleet-averaged mass, the fleet-average CO2 target is reduced by a larger amount than the reduction in CO2 found in the performance of the vehicle on the NEDC resulting from this reduction in structural mass. Therefore simple mass reduction strategies could result in a situation where fiscal penalties for manufacturers are increased rather than reduced.
To avoid this scenario a holistic approach, where more efficient engines are used together with reduced mass structures, is required in order to give benefits. The value of parameter a in equation 1, found in practice when a
non-holistic approach to mass reduction is adopted, i.e. only the structural mass of the vehicle is reduced in isolation to the use of other technologies, is about 3.5 g CO2/km per 100 kg mass change for a C-class vehicle on the NEDC (represented by the pink line in Figure 1).
It is clear that, not only are lightweight vehicles effectively penalised by having to meet a more stringent CO2 target, the reductions in tailpipe CO2 from the simple non-holistic approach to mass reduction may be insufficient to produce a net CO2 benefit large enough to reduce the fiscal penalties to which manufacturers are liable – rather it would increase them.
If manufacturer A makes a single model and reduces the mass of this vehicle by 100 kg it will reduce its fleet-averaged CO2 emissions by about 3.5 g/km whilst the target will be reduced by almost 4.6 g/CO2. Thus there will be a fiscal penalty on 1.1 g CO2/km which at an eventual penalty of €95 per gram of CO2, amounts to €104 per vehicle sold by the manufacturer.
Significant reductions in CO2 levels below the limit value curve are only possible by adopting a holistic approach to new vehicle design and specification, where the reduction in mass is leveraged to enable similar vehicle acceleration performance to be achieved with a smaller, less powerful, but more economical engine and/or the use of modified gear ratios, enabled by the vehicle mass reduction.
This holistic approach is represented by the green curve in the graph (the value of 8.5 g CO2/km per 100 kg was used by Goede et al. in this scenario ). Clearly, further reductions in CO2 are possible via additional engine downsizing, to retain performance for example.
Of course the benefits of structural mass reduction to the fuel economy and, importantly, performance of individual vehicle models, in terms of their market appeal, and strategic positioning against competitors may be the over-riding motivation.
An alternative is to use vehicle ‘footprint’ as the parameter on which to base a variable CO2 target . Of the parameters defining the vehicle footprint, only the width, which is capped by road lane width and other practicalities, has a significant impact on vehicle energy consumption over a drive cycle via its influence on aerodynamic drag.
The possibilities for improving the efficiency of a wide car are greater than those for improving the energy efficiency of a heavy car . High mass is perceived negatively by the consumer whereas interior space, which is directly proportional to footprint, is a positive consumer attribute  and is also directly related to the ability to carry more passengers giving greater potential to reduce per capita CO2 emissions.
Whereas average vehicle mass in the EU increased by 21.7% between 1995 and 2006, average surface area (proportional to footprint) increased by only 7.7% over the same period, thus a standard based on vehicle footprint would be a more stable controlling parameter. Vehicle footprint is also positively correlated with transport safety, contrary to vehicle mass  and this has heavily influenced the US to switch to a vehicle footprint-based CO2 and fuel consumption standard . Is it time for us to consider an alternative approach in Europe?
Author: Dr Richard Pearson
Where M0 is the industry-average mass, Mv is the manufacturers fleet-averaged vehicle mass in kg, and a is the mass sensitivity parameter
[g CO2/kg]. The value of a is 0.0457 g CO2 /kg or a change in the target of 4.57 g CO2/km for every 100 kg by which the manufacturers fleet averaged-mass changes, as shown by the blue line in the graph.
- Goede, M., Droder, K., and Laue, T., ‘Recent and future lightweight design concepts – The key to sustainable vehicle developments’. International Conference ‘Leichtbau im Automobil’, 9 th-10 th November 2010.
- ‘Weight-based standards make CO2 targets harder to reach’. Background briefing: Weight vs Footprint, European Federation for Transport and Environment, April 2008.
- German, J., Bandivadekar, A., ‘U.S. EPA Light-Duty Vehicle GHG and CAFE St