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Given the high level of concern about climate change, what can or should the worried motorist do?

If one accepts that global warming due to man-made emissions is a growing issue that society has to face, but also that mobility is a vital part of commerce, industry and living life in general, what is the best type of car to own?

We all know that tailpipe CO2 emissions are the chosen instrument of compliance for manufacturers and users alike; company car tax bills are aligned with them.  However, the tailpipe is only one part of a complicated picture. Most people will be aware that the CO2 emissions associated with electricity generation is ignored when electromobility is being championed, but what about the embedded CO2 in making the vehicle in the first place?

Green Company Car - Professor James Turner, dept of Mechanical Engineering

Professor James Turner, dept of Mechanical Engineering, University of Bath

Making automotive traction batteries is enormously energy intensive, and there are significant carbon emissions associated with supplying the necessary energy.

Consultant Ricardo recently published a study in which it stated that 1 kWh of automotive battery has a carbon intensity of 150-200kg, using electricity at current CO2 levels. A big battery, say 100 kWh, would therefore have 15-20 tons of fossil CO2 associated with its manufacture. At normal fuel consumption rates, this is equivalent to well over 50,000 miles of motoring in a combustion-engined car. Throw in the emissions generated in the production of electricity used to charge the battery during vehicle use and it is easy to get to over 100,000 miles of travel before an electric vehicle sees any benefit over a combustion-engined one.

So, the first thing for the concerned citizen to do is not to buy a vehicle with a big battery until we have significantly decarbonised the grid. Keep it small – about 10-20 kWh should do most people.

This, however, means frequent charging, and the infrastructure most definitely is not there to support this yet. Also, most people travel because they have to, so expecting the public to waste time on long trips in frequent charging stops is, I would suggest, naive. How do you address this?

Essentially, we need to go the PHEV route. There are significant benefits in this – more of the vehicle’s battery capacity is used more often (and any unused kWh in a battery is a complete waste of the CO2 invested in making it), and a smaller battery also uses lower amounts of rare raw materials; the uncomfortable one here is cobalt, because of the way it is mined and the associated moral implications.

“Modern combustion-engined vehicles actually clean the air in polluted cities”

The other thing is to plug in whenever you can, which cannot be stressed enough. More miles are then driven on electricity – which will mean less fossil CO2 emitted, especially if you can charge during daylight hours. While many see plugging in as a chore, at least there’ll be some financial benefit. Really, anyone who buys a PHEV and claims any kind of benefit associated with it ought to be monitored for this – but that’s another story. What we could really do with is more widespread adoption of inductive charging, so that when the car is parked it charges without intervention. This overcomes the “chore” bit.

But all of this means that the vehicle still has to have a combustion engine. Fortunately, this is not bad news (unless you are a grandstanding politician). The highest emission level that we have is EU6d with Real Driving Emissions testing. The upshot is that modern combustion-engined vehicles actually clean the air in polluted cities. They may be emitting fossil CO2 but at least the air from the tailpipe is cleaner than what went into the air filter – thanks to the high-temperature combustion process and the use of a chemical plant in the exhaust, all controlled by some of the most advanced computer systems in mass production. And even more remarkably, since they both meet the same emissions regulations, it now doesn’t matter whether you have a petrol or diesel engine.

Of course, when we have “greened” the grid more, and we no longer have to worry so much about embedded carbon, there is an argument that we should move increasingly towards full electric vehicles. But by then we may have carbon-neutral “e-fuels” with no fossil carbon in them – but again, that’s another story.

What we can say is that there is more life in the combustion engine than most people realise, which is just as well since it is the thing that keeps transportation widely affordable.


Dr James Turner

Professor of Engines and Energy Systems, Department of Mechanical Engineering, University of Bath