Should diesel vehicles be scrapped?

Posted on February 10th, 2016 by Rob Marshall


With diesel being 2015’s automotive ‘bad guy’, should you scrap your ‘dirty diesel’, to make away for one of the new generation of ‘clean’ petrol cars? The fuel has never enjoyed a particularly clean image and it seems as though its main advantage, championed all those years ago, of a relatively low level of greenhouse gas output, appears to have been forgotten in the haze of media reports that lambaste diesels for choking our lungs with cancer-causing NOx and particulate pollution.

However, it has not been realised fully that petrol engines are far from saintly and, if every motorist plumped for petrol power, the problem of urban pollution would be far from solved – in some cases, it could be made worse. Starting at the forecourt, the person holding the nozzle is likely to receive several hefty whiffs of Volatile Organic Compounds, as they evaporate from the fuel, unlike the oilier diesel. Petrol engines also emit not only 15% more CO2 per mile than diesels, they also produce more carbon monoxide, benzene and hydrocarbons.

Perversely, some direct injection petrol engines also have a tendency to kick-out high levels of NOx. Interestingly, while particulate levels are lower than those emitted by diesels, some petrol engines produce greater quantities of smaller particulate matter, i.e. finer than PM2.5, which is even more hazardous for human health, than bigger soot particles that are visible with the naked eye.

Hybrid and Plug-In Hybrid Vehicles seem to make a more sensible proposition, for urban use especially, even if one discounts their energy intensive production processes. However, catalytic converters need to work at approximately 300 degrees Celsius, to convert effectively the various toxins into less harmful gases. Unfortunately, the temperature falls rapidly, when the engine ceases to run, meaning that the real-world emissions are likely to be far higher, when the engine is crawling to its destination with a low battery, in a cold British city, compared to a warm laboratory. Recently, I drove the lightly-facelifted version of the UK’s most popular Plug-in Hybrid, the Mitsubishi Outlander PHEV, and I asked the manufacturer if any special means had been taken to ensure that the catalytic converter temperature was kept at its optimum, when batteries alone provide propulsion, or when the engine is running at low speeds in traffic. The fact that I have not received a satisfactory response in over a month might be an answer in itself…

There are no simple conclusions to address the current problem of real-world emissions. However, it is clear to me that scrapping perfectly serviceable diesel-powered cars and replacing them with petrol and hybrid alternatives does not make environmental sense, at this stage of my research. Plus, in today’s ‘Austerity Britain’, should the taxpayer really hand over even more cash to the motor industry, when the funds have more pertinent uses?