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Don't try this at home!

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An emissions thought experiment

Increasingly simplistic calls to #Stopburningstuff and #Stickyourselftothings have recently been accompanied by another call: that anyone who challenges the virtues of battery electric vehicles (BEVs) should shut themselves in their garage alongside their idling internal combustion engine (ICE) vehicle for an hour, to see whether they emerge to tell the tale.  This is a rather unethical, and arguably shocking, call that can only cheapen the decarbonisation debate.  It is one thing coming from fringe on-line influencers, but quite another when it comes from well-known, vocal academics.  

The tastelessness of the proposal aside, as Emissions Analytics is committed to using independent testing to understand real-world emissions questions, we have taken the challenge in the form of a thought experiment.  The conclusions from previous newsletters are that modern ICE vehicles are extremely clean relative to older ones, while BEVs have low but non-zero pollutant and carbon dioxide emissions, leaving the optimal policy more finely balanced than typically thought.  Contradicting this, does the challenge, while flippant, hold an essential truth?

Traditional thinking is that toxic fumes would quickly overwhelm you, while a BEV would sit there passively emitting nothing.  Such an experiment tests in microcosm the effects of vehicles on the wider environment.  To see what the likely effects would be, we can forecast pollutant concentrations in our test laboratory using Emissions Analytics’ real-world test data from tailpipes and non-exhaust sources, together with the latest academic research.  

Without wanting to spoil the result, as with so much in the decarbonisation debate, the answer is nuanced and highly sensitive to the selection of the vehicles.  So, it should be emphasised in the strongest possible terms: don’t actually try this at home!  

A typical single European garage is about three metres wide, six metres long, and three metres high – so, about 54 m3 in volume, which is similar to one of our laboratories.  Let’s assume a constant ambient temperature of around 20 degrees Celsius, which is in line with the temperature used for a vehicle certification test.  Vehicle emissions should be estimated from cold start, with the engine having been soaked at the same temperature overnight.  For the thought experiment, it is assumed that no pollutants escape the laboratory, although some gas egress would be necessary to avoid gradual pressurisation of the space.  To avoid accusations of sophistry, we will assume that the ICE vehicle does not have stop/start or other cut-out system engaged, so it is idling throughout.

The laboratory is assumed to contain air with standard background gas and particle concentrations, as shown in the table below.  Also indicated in the table are guides as to when concentrations of each pollutant start having negative human health or cognitive effects (the ‘threshold of harm’), and the immediate danger levels.  These danger levels have been compiled from multiple sources, many from the world of occupational safety.  For some, there are some wide variations in views, but we have tried to pick a fair midpoint for the purposes of illustration.  The references are listed at the end of the newsletter.

First, let’s first take the controversial one: a diesel.  Diesel is a fuel that burns around a quarter more efficiently than gasoline and was for a generation pushed as a route to decarbonisation, only then to be undermined by excessive real-world NOx emissions.  Many air quality problems we suffer today arise from these excesses.  In this case, we have taken a 2021 Volkswagen Passat 2.0 litre 148 bhp front-wheel drive automatic vehicle certified to the latest, strict Euro 6d-ISC-FCM emissions standard.  The results are modelled by using the second-by-second data from an actual real-world test by Emissions Analytics on its EQUA test route, using a Portable Emissions Measurement System (PEMS) augmented by sampling of VOCs onto thermal desorption tubes for later GCxGC-TOF-MS analysis.

During this hypothetical experiment, the vehicle would suck in and then emit about 30 m3 of gas – mainly nitrogen – which is equivalent to about 55% of the total laboratory air volume.  The colour coding indicates whether the concentration after one hour is below the no-harm level (green), or between the no-harm and immediate danger level (amber).  We have also considered whether the process of combustion would use sufficient oxygen from the air to create an asphyxiation risk.  Clear to see is that there is no red, which would indicate immediate danger. 

The most dangerous pollutant, therefore, is nitrogen dioxide, but the forecast levels are still half of the recommended immediate danger levels, despite the amount of air in the laboratory being relatively small.  Even with this worst-case pollutant, if the volume of the laboratory were just 1,438 m3 (about 27 single garages, or the interior volume of an Airbus A380 aircraft), the amount of air would be sufficient to dilute the NO2 to the point of no harm.  Put this car in the open air, and you can see why this powertrain is no longer a problem from an urban air quality point of view.

The experiment was then extended to a modern gasoline vehicle: a Renault Clio 1.0 litre 88 bhp front-wheel drive manual vehicle certified to the Euro 6d-TEMP-EVAP emissions standard.  The main difference in the outcome is for NO2, which is now at a negligible level, and CO, which is about double the diesel vehicle.  Carbon monoxide is rightly feared as highly poisonous gas to humans, and even modern gasoline vehicles emit a significant amount when the engine is cold, but after about two minutes the catalytic converter brings it down to low levels, even in dynamic driving.  As this engine is smaller than the diesel one, the total amount of gas ingested and then exhaled in one hour is only around 12 m3, or 22% of the total volume of the laboratory.

So, for both ICE vehicles, idling in a single garage for an hour, is likely to be negative for your comfort, health and enjoyment, but not fatal.  But are we covering everything?  If we sit alongside a BEV, are there any effects at all?

As we showed in a previous newsletter, VOCs don’t just come out of the tailpipe, but also ‘off-gas’, or evaporate, from the surface of car tyres, as they are substantially made from components of crude oil.  For the Tesla Model Y tested, we found that total VOCs from the tyres was 0.26 grams over one hour.  If these tyres were the only source of VOCs, they would lead to 1.2 ppm in the laboratory at 20 degrees Celsius.  The distinctive smell experienced when entering a tyre warehouse is caused by these VOCs.

There are two further non-exhaust sources of VOCs.  First, fuel evaporates from the fuel tank of an ICE vehicle, even though most gasoline vehicles have ‘canister’ systems to capture as much as possible.  The US Environmental Protection Agency Tier 2 regulations limited these emissions to 0.05 g/mile (0.03 g/km), and they have been tightened significantly since.  If 60 km were travelled in one hour, this would mean evaporative emissions of 1.8 grams.  

Second, a recent academic paper on unreported VOC emissions from road transport highlighted the issue of VOCs from the evaporation (not usage) of screenwash, which contain a mix of mainly alcohols, and which the authors termed “non-fuel, non-exhaust” emissions.  The paper proposes a distance-specific emissions factor of 58 mg/km.  To convert this to grams per second for the purposes of our experiment, we assume the same average speed of 60 km/hour as above for the test cycle the emissions factor was derived for.  That implies total emissions over one hour of 3.5 grams.

In total, these non-exhaust VOCs add up to between 3.8 and 5.6 grams over the hour.  The lower end of the range is for the BEV, as there would be no fuel evaporative emissions, although this may be offset by larger tyres, which is a trend with battery vehicles due to their weight.  The totals for the exhaust alkanes and aromatics were 0.07 grams for the diesel and 0.11 grams for the gasoline.  Therefore, the non-exhaust sources are around 50 times higher than the exhaust VOCs.  In summary, it is best to not to dwell in a small, sealed space, whether it contains an idling ICE vehicle or a BEV that is switched off.

To this approach there is one important caveat.  Change the car to an older one, and the outcome may not be as favourable.  Older gasoline vehicles can have much higher CO emissions, a gas that can have rapid and terminal effects, while older diesel vehicles are famous for their elevated NOx emissions.  The Volkswagen Passat examined here had NOx emissions of 19 mg/km when tested on Emissions Analytics’ combined EQUA route.  This is 76% below the regulatory limit, which is typical of the current generation of diesels.  Wind back only five years, and the emissions would have been more like 400 mg/km.  Being locked in the garage with that car would lead to a poor health outcome.  This is why, fundamentally, the Ultra Low Emission Zone in London, and similar schemes in other countries, are beneficial to air quality, as it is the older vehicles that are a disproportionate source of pollution.

The conclusion from this analysis, apart from avoiding academics with unethical experiments, is that how we think about vehicle emissions is ripe for a complete overhaul.  Most of the impacts come from the tailpipe of older vehicles, and from non-exhaust sources on new vehicles.  Any properly functioning modern vehicle, operating in the open air, will contribute a negligible amount to air quality problems from the tailpipe.  The carbon dioxide problem remains, however, which is the subject of extensive discussion elsewhere.

It remains true that ICE vehicles produce a range of potentially highly toxic compounds from combustion, but at current concentrations when rapidly diluted in the open air, they cease to be a major problem.  But this insight points to the next major are of concern: inside the vehicle cabin.  Pollutants from older vehicles enter through the ventilation system, and VOCs evaporate from the interior materials, to which the driver and passengers are exposed over extended periods within a sealed cabin.  Without the benefits of dilution and filtration in a poor ventilation system, the health exposures can be significant.  We will look at this in our next newsletter…

 

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