Why buying a diesel car might not be wise

Nowadays roughly half of all new cars registered in the UK run on diesel. Fleet and private buyers alike have been enticed by the prospect of better fuel economy, lower carbon dioxide emissions and healthier residual values. However, that popularity could be about to wane.

Ironically, a widespread return to petrol engines or the adoption of electric-based technologies will likely be caused by politicians. That’s the exact same group of people who have supported diesel sales for so long by being obsessed solely with CO2 figures.

Take London, for instance. Until recently, diesel cars that produce 100 g/km or less of carbon dioxide were eligible for a 100% discount on the city’s Congestion Charge. Now those vehicles are billed the full amount and - in a move that would copy proposals already suggested for Paris - a total ban might be on the horizon for them. A total U-turn, in other words.

So what’s the problem? Well, policy-makers have seemingly just realised that diesels also belch out other nasty things, including harmful soot and oxides of nitrogen (NOx).

A diesel particulate filter (DPF) should take care of the former, but these devices have their own foibles. One major issue concerns the automatic self-emptying process known as regeneration, which is meant to burn off any accumulated particles at very high temperatures.

In a car that’s used predominantly for short journeys or stop-start urban driving the regeneration cycle may never complete successfully, leading eventually to a blocked filter. The upshot is lots of glowing dashboard warning lights and a trip to the dealer for DPF cleaning or replacement. Neither option is going to be cheap.

Some misguided people advocate permanent removal as the cure for a troublesome DPF, even though following this advice is potentially illegal. Furthermore, such a meddled-with car probably won’t pass its MOT test.

NOx is a less visible tailpipe pollutant generated by diesels. Scientific research has identified that these unpleasant gases can cause or aggravate numerous respiratory conditions like asthma, bronchitis and emphysema.

One answer to that particular concern is diesel exhaust fluid (DEF), commonly known as AdBlue in Europe. Simply put, this is a very pure aqueous solution of urea which is injected into the exhaust system. This then reacts with the NOx via a process called selective catalytic reduction (SCR), thereby converting most of it into benign nitrogen and water vapour.

Many new trucks and buses are already equipped with AdBlue apparatus as standard, and their fitment to passenger cars is becoming increasingly prevalent as manufacturers strive to meet emissions limits.

DEF is stored in a separate tank and, for cars at least, it should only need topping up at each scheduled service. Nevertheless it’s an extra expense and if the AdBlue runs out then obviously SCR is impossible. Allow that to happen and the engine is apt to either go into a reduced-power mode or refuse to start altogether.

Particulates and NOx are what economists refer to as externalities, since their effect on public health and air quality is a wider cost for society created by diesel engine users.

Direct legislative measures are often necessary to force a change in the behaviour of those responsible for externalities. And that’s precisely what’s happening in the case of diesel-powered cars.

Currently, DPF and SCR systems represent the main financial penalties (and hassles) imposed on buyers so that government pollution targets can be achieved. In the future, threats of usage restrictions and additional fiscal burdens could well be added to the list.

For some drivers, especially those doing high mileages, having a diesel might still make perfect sense. But anybody who travels a more modest distance should certainly be considering something else.

Luckily, car makers already realise that demand could shift away from diesel. The switch isn’t automatically going to mean choosing a hybrid powertrain either, because development of ultra-frugal smaller-capacity petrol motors has been rife across the industry.

A good example of a model that has benefited from this work is the Volkswagen Polo supermini. Until recently, its eco-friendly BlueMotion variant was only offered with a TDI diesel engine. Now though there’s a TSI petrol alternative, which is due to arrive at UK dealers this coming March.

Volkswagen Polo BlueMotion TSI 5-Door (2015) Front Side

Looking at the stats below, the first thing to note is that a TSI version is cheaper by £1,065. It’ll be considerably quicker, too.

The TDI boasts a clear advantage as far as fuel consumption is concerned, but to what extent does that overcome the much higher initial outlay? Time to do some sums…

At the moment, petrol can be bought for around £1.03 per litre (£4.68 per gallon), while diesel is more expensive at £1.09 per litre (£4.96 per gallon). So, taking the official combined cycle economy figures as a guide, travelling 100 miles would incur a fuel cost of £6.80 in a BlueMotion TSI or £5.44 in a BlueMotion TDI. A moderate saving of £1.36 for the TDI owner, in other words.

Apply that £1.36 to the difference in invoice prices, and a TDI would have to be driven about 78,500 miles (126,330 kilometres) more than the TSI before its premium was recouped. Okay, the calculation is somewhat simplistic, as other running costs such as depreciation and insurance have been ignored.

Even so, it does further reinforce the argument that for a lot of average-mileage new-car customers, opting for a diesel engine is clearly going to be a bad decision.

Model Polo
BlueMotion
TSI
Polo
BlueMotion
TDI
UK OTR price 3-door £14,730 £15,795
UK OTR price 5-door £15,360 £16,425
Fuel Petrol Diesel
Engine 1.0 litre,
three-cylinder,
turbocharged
1.4 litre,
three-cylinder,
turbocharged
Power 93 bhp
(70 kW / 95 PS)
73 bhp
(55 kW / 75 PS)
Torque 160 Nm
(118 lb/ft)
210 Nm
(154 lb/ft)
0-62 mph (100 km/h) 10.5 secs 12.9 secs
Top speed 119 mph
(191 km/h)
110 mph
(178 km/h)
Combined cycle
fuel economy
68.9 mpg
(4.1 l/100km)
91.1 mpg
(3.1 l/100km)
CO2 emissions 94 g/km 82 g/km

Related posts:
London Congestion Charge increasing
London Congestion Charge changes

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