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Diesel, fuel of the future

A decade ago, we were all being encouraged to buy diesel cars to reduce CO2 emissions. More recently, reports of the health toll caused by urban air pollution – much of it down to older generations of diesel engines – together with VW’s infamous ‘dieselgate’ scandal, seemed to mean that the days of diesel were numbered. But perhaps we should think again.

A Bloomberg story this week suggests that Tesla shock means global gasoline demand has all but peaked. The basis of the headline comes from the latest report on the future demand for energy: “The International Energy Agency forecasts that global gasoline consumption has all but peaked as more efficient cars and the advent of electric vehicles from new players such as Tesla Motors Inc. halt demand growth in the next 25 years.”

There can be little doubt about the continuing increase in the efficiency of conventional engines, but I’m not convinced that public subsidies, a surge of new models and the enthusiasm of early adopters signals widespread ownership of shiny new Teslas in the near future, nor sounds the death knell for the internal combustion engine just yet. And any increase in the efficiency of cars has to be set against rising car ownership in large emerging economies. But let’s assume that the forecast is essentially correct.

However, a drop in demand for petrol does not necessarily mean less use of oil. The Bloomberg headline comes purely from projected trends in the market for passenger cars, by far the biggest user of petrol. Meanwhile, further expansion of the road freight, aviation and maritime sectors is foreseen, using heavier fractions of crude oil. For road transport, this means diesel and, as heavy bunker oil is phased out, diesel consumption will also increase for ships. As an aside, the fastest growing use of oil looks set to be as a raw material for the chemicals industry, notwithstanding the increasing use of biological processing of biomass, particularly in the fine chemicals sector.

But it’s also a bit early to write diesel cars off just yet. Volkswagen’s tinkering with the results of emissions tests was a misguided aspect of the company’s strategy to create a market for diesel cars in the American market (not only misguided, but unnecessary, since it seems that VW engines have among the best emissions figures in real use). In the meantime, we have to remember that the main attraction to drivers is the better economy (and hence lower emissions and road tax in the UK). Diesel cars simply make better use of extracted oil.

But in today’s monochrome world, diesel cars have gone from being good to bad, simply because of their contribution to urban air pollution. A tightening of emissions standards reduced the output of particulates (PM10 and particularly PM2.5), but the attention of campaigners and politicians has only recently turned to the parallel problem of nitrogen oxides, emitted to a greater extent by diesel than petrol vehicles.

In fact, this has not been ignored. EU emissions standards have progressively tightened over the years, with the latest (Euro 6) engines now on the market. The improvement has been a dramatic one, as the following table shows (grams per kilometre):
















Euro 3





Euro 4





Euro 5a





Euro 6






The comparison with petrol engines is an interesting one. Because particulate emissions were not a problem for petrol engines, there were no standards set until recently. Now, the legislated standard for both diesel and petrol engines is the same. The carbon monoxide emission limit for petrol engines is still double that of diesels, which is a function of their intrinsically less efficient combustion process (and hence also higher CO2 emissions for engines of equivalent power output).

The only factor which is still somewhat worse for diesels is NOx emissions, with a limit of 0.08g/km compared with 0.06g/km for petrol engines. However, it is clear that today’s diesel engines are far cleaner than even a decade ago. There is still a problem with air quality in urban areas, and we should remember that these figures are for a standard, idealised driving cycle rather than the stop/start of city driving. But the steady renewal of the fleet of cars will continue to improve air quality. In time, it is likely that low-emission zones will be set up, with older vehicles being charged more to enter, so accelerating the improvement.

With the Euro 6 standard now being met, both petrol and diesel engines are pretty much as clean as each other, so a more nuanced view of diesels is needed. Rather than demonising them, if we want to maximise the efficiency of our resource utilisation, we should be encouraging uptake of the latest models, which themselves will doubtlessly be improved still further in coming years.

But however clean the engines become, urban air quality is likely always to be less good than in the countryside (although, while acknowledging remaining problems, we shouldn’t forget what vast improvements have been made over the past 50 years). Even electric cars and buses will still be a source of particulates, from brakes and tyres in particular. The PM2.5 and other pollutants that are produced (not just from road traffic) tend to be trapped in narrow streets. Ours is not a perfectible world, although we can continue to improve.

For now, we need to recognise that there will be significant numbers of older, more polluting vehicles on the roads for some time to come, so we have to find ways to cope with that. But when replacements are bought, in many cases there are still good reasons to make that a diesel.