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There has been discussions recently about whether electric vehicles will create more particulate pollution due to the extra weight of the large batteries these vehicles carry. Vehicles driving on roads shed particulates as friction sheds the material of brakes, tyres, road surfaces and clutches.

The basic problem with this argument and where people are going a little bit wrong is that people are comparing Tesla Model Xs with VW Ups. The argument should be about not making heavy vehicles. Electric vehicles are usually heavier but only really need to be 10-20% heavier. Please read on 🙂

Electric Vehicle weight vs Non EV weight

I compared vehicles that had an electric version and a non electric version of the same model. This is so that we can compare the differences in weight in cars that are the same in terms of size and componentry except for the difference of being electric and being non electric.

The Kia e-Niro 64kWh version (range 235) shows it is 22% heavier than the the petrol version of this car. The following table shows the results of some other cars and a popular bus. The lightest / heaviest is due to the non-ev versions varying in weight quite differently due to different models.

		Non EV Kerb weight		Percentage of extra weight of EV model
Electric Car	EV Kerb weight	Lightest model	Heaviest model	Lightest	Heaviest
Kia e-Niro 64 kWh	1,812	1,490	1,594	22%	14%
Ford Focus E	1,651	1,235	1,518	34%	9%
Kona Electric	1,685	1,233	1,496	37%	13%
Williams Dennis Enviro400EV bus	13,300	11000	11,500	21%	16%

A 64kWh battery weighs around 450kg. The difference between a Kia e-Niro and its 1.6 petrol version is 322kg (it is not 450kg because electric car componentry is lighter than combustion engine componentry). A Tesla 85kWh battery weighs 540kg.

Teslas are heavy, between 1.7t (Model3) and 2.5t (ModelX) but so are petrol Mitsubishi Shoguns (2.1t), Range Rovers (2.2t), Mercedes (2.3t) , Grand Cherokees (2.3t), Rolls-Royce (2.6t), Bentleys (2.7t). A Suzuki Celerio weighs just 0.8t. There is a great difference here in the weights of vehicles and the greater difference is not because of electric.

The Chinese 130 mile range, 24kWh, two seater Baojun E200 weighs just 0.84t, not far of the Smart Fortwo (0.73t). Just a 13% difference. The lightest four seater electric car I found was the Citroen C-Elysee which weighs 1.16t. The lightest four seater petrol cars are around 0.8t but as the electric range increases then so will the options for buying light.

I think this shows that electric cars only need to be between 10 and 20% heavier than their petrol versions. Reduce the size of the battery and this will be less, increase it and it will be more. Unnecessarily heavy cars are the biggest problem, not electric cars.

Regenerative Braking

‘Regenerative braking does not rely on frictional wear of brake materials therefore there should be no particulate emissions from cars that have regenerative brakes’.

Which components are responsible is not certain and the evidence is vague and the research is poor, however Grigoratos & Martini suggest something like the following:

35% brake wear / 17% tyre wear / 43.5% road dust resuspension / other – clutch, road wear.

Vehicles with regenerative braking will shed far fewer brake related particulates than the 35% specified above. In fact to reduce the amount of particulates from brakes all cars could just revert from disc brakes to drum brakes. We don’t need electric vehicles for that to happen although regenerative braking is an efficient technology. So, it is just the tyre wear and the road dust (and road wear) that remains and we will see a 20% increase here if the wear is proportional to the weight.

Brake and tyre composition

‘At present there is no type approval legislation covering non-exhaust emissions, nor product standards governing the composition of brake systems and tyres.’ So, tyres and brakes are made out of different materials and wear down at quite different rates and pollute differently just due to their composition. My view is that we need to legislate on the composition of brake and tyres so that fewer particulates are shed.

Road type and driving style

The type of surface vehicles drive on, the number of turns a vehicle will have to make, how quickly a vehicle accelerates and the force in which a vehicle stops will greatly affect how much wear is put on the brakes, tyres and road and will influence particulates. Town centres need to promote longer red lights and keep cars going for longer effectively reducing braking and acceleration which is also inefficient in terms of energy use.

Battey swap stations

Road users sometimes need long range and sometimes they do not. The real weight of an electric car is due to batteries that allow for very long journeys such as the Teslas. However we don’t always need that extra range and, if we swapped the battery at a battery swap station midway through our journey, then we could reach our destinations whilst using smaller lighter batteries. This technique is being developed by BJEV in China.

Conclusion

People are saying electric vehicles are no better. Well, things are usually more complicated than that type of simplification!

I think the main problem here is we are making unnecessarily large vehicles and that includes both electric and non-electric.

You would imagine a 20% difference in particulate emissions due to the 20% difference in weight between electric and non-electric. Cars will generally pollute more if they are heavier and they will pollute less if they are lighter. The size of battery (customers will see this as a car’s range), will make a big difference here. So the weight difference is significant but, it should only increase by just 20% if that weight is directly proportional to the wear to the tyres. Maybe it is not directly proportional though…I need an expert to give me the details here but I would imagine that speed has a much greater impact.

The reduction in particulate emissions using regenerative braking may offset that 20% but, you don’t need a fully electric car to do this. However, the mitigating strategies below, for reducing particle emissions from traffic will, I expect, do more to reduce an electric car’s particulate emissions. Couple this with EVs potential for having far far less of a CO2 footprint (especially if we use more renewables, maybe 50% less, maybe more), zero exhaust emissions and I see electric vehicles as a better form of transport compared with petrol or diesel cars.

There is a great difference in weight (2.5t for Tesla Model X vs Citroen C-Elysee 1.16t) and that is not necessary. We need to encourage people to buy lightweight electric vehicles and encourage business to support battery swap stations and battery swap enabled cars to reduce CO2 and improve air quality.

However trams and electric bicycles are surely the best form of transport in a town or city and, especially in the winter months in colder cities, it is solid fuel burning in homes that creates large amounts of particulate pollution (more than traffic related particulate pollution).

Mitigation strategies for reducing particle emissions from traffic – Defra

• reduce the overall volume of traffic
• lower the speed where traffic is free-flowing (such as trunk roads and motorways); and promote driving behaviour that reduces braking and high-speed cornering
• Implement regenerative braking, where that does not lead to net disbenefit on road and tyre wear NEE because of increased vehicle mass
• Establish particle mass (and/or number) and particle-associated metal emissions limits for brake pad and tyre technologies (including chemical formulation)
• Trap brake wear particles in the braking system before release into ambient air, although this technology is currently unproven
• Reduce the material that is tracked onto public road surfaces as a result of vehicle movements in and out of construction sites, waste-management sites, quarries, farms, and similar
• Wash and sweep streets and/or treat street surfaces for dust suppression; it is noted, however, that impacts on airborne PM from trials of these approaches have so far proven inconsistent and anybenefits have been short-lived in nature.

Other mitigating effects (me)

• Improve road surfaces (smoother)
• Introduce trams
• Use buses instead of driving
• Promote car sharing
• Promote bicycle and electric bicycle use
• Promote train use
• Promote battery swap stations so there is less need for large heavy batteries.
• Allow smaller and larger batteries to be swapped in at battery swap stations to increase efficiency when the range is not needed

References

Click to access jrc89231-online%20final%20version%202.pdf

Click to access 1907101151_20190709_Non_Exhaust_Emissions_typeset_Final.pdf

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