A bright light, with a heavy weight
When an exploding star led to the observation of supernova SN 2003fg in 2003, it was nicknamed the ‘Champagne Supernova’ due to its unusual brightness, and its inexplicably great mass. Many supernovae eventually succumb to their own weight, leaving behind a black hole. Are we at this stage with battery electric vehicles (BEVs)? Their prospects are currently shining brightly despite their literal weight as well as their likely wider toll on the environment, from watercourses to the seabed, due to their production. Hybrids, by contrast, tread relatively lightly on the planet, yet give off a more muted glow of past glory – perhaps more like a red dwarf. In this newsletter we want to consider a further way in which vehicle size and weight matter, and why the BEV industry must address this rapidly if it is successfully to deliver pollution reduction.
Astronomical parallels aside, we can simply say that BEVs are too big and heavy right now. Yes, there are heavy internal combustion engine (ICE) vehicles, but on average BEVs are around 40% heavier and 40% bigger like-for-like, as set out in a previous newsletter. This trend may well continue, and the weight premium increase, as BEVs come increasingly equipped with lithium ion phosphate (LFP) batteries as they are cheaper and require fewer scarce materials. It may even be the case that this weight leads to structural risks for transportation infrastructure, such as roads and car parks, although this has yet to be proven.
While this is all true, it is easy to get stuck in a pattern of ‘trading averages.’ As Senecal et al meticulously pointed out, you can only assess the decarbonisation potential of BEVs in the US by looking at local-scale grid electricity, and marginal rather than average carbon intensities. Similarly, with vehicle selection, you can only judge the benefit by understanding the marginal changes. If someone replaces a frugal gasoline car with a larger BEV, that is likely to be worse for the environment in the round. Equally, switching from a large gasoline V8 to a small city BEV is very likely to be better. It is easy to make simplistic ‘stop burning stuff’ slogans stick when you conjure up the image of a pre-particle-filter diesel being replaced by a gleaming Tesla. However, for the same investment, you are likely to get more pollutant and carbon dioxide (CO2) emissions reduction from trading those old diesels up for the latest full hybrid electric vehicles (FHEVs). So, what we mean by an ICE vehicle, and the variation in performance within that group, matter. Put another way, it is not the optimal approach to dispense with all ICE technology just because many are high-emitting, just like it would be wrong to reject all BEVs just because many are currently very heavy.
Our previous newsletter suggested that tailpipe emissions from FHEVs had reached a ‘do no harm’ status, by showing levels more than 90% below a range of air pollution legal limits. What this did not show was whether those apparently low levels were in fact sufficiently de minimis to be of little concern, or whether we were still burning stuff in a detrimental way for air quality. An immediate caveat to make is that there is a relevant difference between this European test and apparently similar US vehicles. In Europe, unlike the US, a large and increasing proportion of gasoline ICE vehicles are equipped with particle filters, which significantly reduce the particle mass and number emissions from the tailpipe. Therefore, US particle emissions remain concerning and, as a result, there is greater benefit in switching to BEVs in that market, in the absence of widespread adoption of these filters.
That said, we can look more closely at the volatile organic compound (VOC) emissions from the same test to put the results in context. We showed that there were 4.38 mg of tailpipe emissions over our EQUA test, or 0.03 mg/km. This compares to the most relevant official limit of 100 mg/km for total hydrocarbons, which puts the car more than 99% below the limit. These low emissions were compared to the 330 mg emitted from the tyres on the same test. But, still, how bad is 0.03 mg/km? A recent paper, from March 2022, in Environmental Science & Technology by Wang et al, measured the VOC emissions from four humans seated in a controlled climatic chamber, using proton transfer reaction time-of-flight mass spectrometry and gas chromatography. Without the presence of ozone, the emissions averaged 2.2 mg per human per hour, rising to 4.6 mg in the presence of ozone. Averaging these, and applying the result to the length of the EQUA test, it would imply that a human driver would emit 12 mg of VOCs in total. Therefore, during the EQUA test the driver may have emitted three times more pollution from his body than came out of the tailpipe of the car being driven. Stop metabolising stuff!
So, we can see that the levels of regulated exhaust pollutant emissions from the FHEV are now trivially low. Of those not regulated, the sub-23 nanometre, ultrafine particles are probably the greatest omission, which is being addressed by regulators soon, as set out below. What is left is a more serious concern about VOC emissions, especially from tyres. There are three main reasons we should be concerned about such emissions. First, VOCs can have a direct health effect through inhalation or contact with the skin – many are harmless, but the worst organic compounds can be carcinogenic. Second, VOCs can react in the air to create ‘secondary organic aerosols’ (SOA), i.e. new particles, for which the health and environmental effects are well described elsewhere. Third, these organic compounds have an ‘ozone formation potential’ (OFP), ground-level ozone being one of the main constituents of the smog hanging over city skylines.
From the testing on the Tesla and Kia described above, we see tyre VOC emissions of 2.2 mg/km from the Kia and 6.1 mg/km, from the larger-wheeled Tesla. Taking the ‘secondary organic aerosol formation potential’ (SOAFP) for five target compounds and the average value for the remainder from Wang et al (2017), this implies a maximum possible particle formation of between 0.03 mg/km and 0.1 mg/km. The latter is shown in the table below, which is from the Hankook tyre on the Tesla. This may sound low, but tailpipe particle emissions are now as low as 0.02 mg/km, so the tyre VOC emissions could more than five times the tailpipe mass emissions.
Using a similar approach for ozone formation, the test could have yielded up to 13.2 mg/km. There are no direct regulatory benchmarks to compare this to, however.
This shows not only that tyre size matters, but that chemical composition does as well. From over three hundred tyres tested by Emissions Analytics, the surface area of light duty vehicle tyres – from which the VOCs evaporate – can vary by around 100%, depending on whether you have a 155/60 R15 skinny summer tyre, or a 235/65 R17 specialist SUV tyre, for example. Across 73 tyre manufacturers tested, the proportion of aromatics – some of the more potentially toxic compounds, and highlighted compounds in the table above – vary in concentration from 78 to 582 micrograms per milligram of sample. In other words, the concentration of certain chemicals, and the surface area from which they can evaporate, varies significantly between tyres. As a consequence, vehicle size and weight, with the tyres that accompany that, evidently affect emissions in use, in addition to the materials required for their construction.
This shows not only that tyre size matters, but that chemical composition does as well. From over three hundred tyres tested by Emissions Analytics, the surface area of light duty vehicle tyres – from which the VOCs evaporate – can vary by around 100%, depending on whether you have a 155/60 R15 skinny summer tyre, or a 235/65 R17 specialist SUV tyre, for example. Across 73 tyre manufacturers tested, the proportion of aromatics – some of the more potentially toxic compounds, and highlighted compounds in the table above – vary in concentration from 78 to 582 micrograms per milligram of sample. In other words, the concentration of certain chemicals, and the surface area from which they can evaporate, varies significantly between tyres. As a consequence, vehicle size and weight, with the tyres that accompany that, evidently affect emissions in use, in addition to the materials required for their construction.
What does this mean for regulation? The current Euro 7 proposals are generally a sensible step, as set out in a previous newsletter, not least to make the regulations more technology neutral and tighten ultrafine emissions limits. A less prominent part of the proposal is to tighten the ‘evaporative emissions’ test. This is designed to limit the VOCs off-gassed during refuelling and the use of vehicles, for example vapour escaping from the fuel tank. The test is conducted in a controlled chamber and the escaping VOCs collected over a one-hour period when the vehicle is hot followed by 48 hours when it is cool, all with the ambient temperature varying across a ‘normal’ range. Currently, the total emissions must be less than 2 grams, although this may be significantly reduced with Euro 7. This is relevant because the test, although not specifically designed to do so, will pick up off-gassing from the vehicle’s tyres. However, this is one respect in which Euro 7 is not technology neutral: the evaporative test only applies to gasoline vehicles. So, BEVs with large tyres have no limits applied.
In conclusion, we have shown the risks of myopically looking at tailpipe emissions, and the dangers of asserting the environmental impact of vehicles simply from the type of powertrain. In short, environmental logic points towards a mixed car parc made up mainly of smaller BEVs to cover town driving and larger FHEVs for more general purposes, brought about as quickly as possible in order to get the black hole of older, dirty cars off the road as soon as possible. Vehicle weight, and hence emissions, should be minimised to allow the smallest tyres that are safe and effective. This is a lower risk approach, and from an economic point of view a ‘lower regret’ option as we would not be gambling resources of potentially up to $1 trillion globally, according to some reports, on a maxi-BEV technology that may not deliver its promises. If we pivot to this lighter approach, in ten years’ time, as with 1990s Britpop Oasis’ champagne supernova, you might not have to wipe that tear away now from your eye.