Ahh. The Diesel car. Once heralded as the next big thing in emissions control, now vilified as the cause of our emission woes. Who is to blame? It’s a compound issue. Government gave subsidies and investment, car manufacturers disproportionately pushed these vehicles on to members of the public who wouldn’t benefit. The net result is more Diesel cars on our road than ever. According to ACEA, the European Automotive Manufacturers Association, the percentage of Diesel cars making up new passenger cars in the UK in 2016 was a whopping 47.7%. It’s been steadily rising since 1999 when it was 13.8%. So, with all these new Diesel cars on Britain’s streets what’s the problem?
"There is more particulate matter, made up from compounds like Diesel soot and ash particulates"
Diesel cars have a unique emission ‘fingerprint’. It’s different from a typical spark-ignition engine (petrol and most alternative fuels) in that there is more particulate matter, made up from compounds like Diesel soot and ash particulates. This can be released as either individual particles or chains, most is in the sub-micrometre range. Hence, it is easy to get deep into the lungs and cause a reduction in lung capacity. Particulate matter may also be carcinogenic (cancer-causing) and if it is lodged in the lung tissue problems may result. Another aggravating issue is the difference in the emissions based on factors such as temperature. When the engine is cold, more particulates are produced as the cold engine block draws heat from the combustion process and prevents the complete burning of fuel. This is a major concern in Britain’s towns and cities where most drivers will be making short trips where the engine doesn’t have time to warm up fully.
“But wait!” I hear you cry, “what about Diesel Particulate Filters!?” Well, yes. They are a thing. DPFs are designed to filter out this particulate matter and modern ones are surprisingly effective, they run at nearly 90% efficiency. Still, there’s 10% of particulate emissions leaking out which adds up with the volume of Diesel cars on our roads. DPFs come with their own problems too. Users of Diesel cars in cities where they make only short trips at low speed will find the filters clog very quickly, as the process of ‘regeneration’ (i.e. burning the particulate into ash) requires high exhaust temperatures, something which is achieved only at sustained high engine speeds in most cars. Worse still, people sometimes remove them either to save money rather than replacing a clogged filter or under the belief the filters reduce efficiency. Figures given to the Guardian newspaper by the Department for Transport in April 2016 indicate that since a new rule requiring a visual inspection to be made that a DPF is in place implemented in 2014 1,188 vehicles had been failed because of it. The true figure is likely larger as some people remove only the innards leaving a convincing bit of metal in its place.
"Diesel cars emit much less CO2 than spark-ignition cars and so it's understandable that government would want to prioritise this"
This doesn’t account for the other emissions either. In the noughties, successive governments were obsessed with CO2 emissions, it was the buzzword of the time. Diesel cars emit much less CO2 than spark-ignition cars and so it’s understandable that government would want to prioritise this. Unfortunately, Diesel cars also produce much more NOX (Nitrogen Oxides), around 20% more. NOX compounds are ozone damaging and contribute to acid rain as well as lung tissue damage.
New technology goes a long way to solving these issues. Various elements can be added to Diesel engines to reduce NOX and other emissions. However, per the SMMT (the Society of Motor Manufacturers and Traders) the average age of the UK car on the road in 2015 was 7.8 years. People hold onto their cars for a long time, and the new technology just isn’t in them. Additionally, the technology doesn’t solve the issue either. It just alleviates it somewhat. It’s probably better to avoid Diesel for now until better solutions are more widely available. Of course, this doesn’t solve issues in commercial transport and shipping where Diesel will reign king for long to come. This is because Diesel engines have a higher potential torque output than others allowing them to drive heavier vehicles so investigation of alternative powertrains is also vital to help reduce emissions, making the environment better not only for the planet but also our health.