Following on from previous articles about the differences between petrol and diesel engines, and explaining why your fuel economy doesn’t usually match the official figures for your car, The Car Expert looks more closely at the suitability of diesel cars for use in city driving.
One of the most frequent questions submitted to The Car Expert is “Are diesel cars suitable for city driving?” The answer is… maybe!
A diesel engine generates its power slightly differently from a petrol engine, and usually yields better economy and load-carrying ability, albeit with less power and a slower response time. This is why buses and trucks usually run on diesel – they are big, heavy vehicles carrying large loads over long distances.
But what about city driving? Does the extra cost of a diesel engine yield savings in running costs for urban drivers or those who only take short journeys?
Diesel in the city – fuel economy
Firstly, short journeys. Any engine is very inefficient when it is cold, so the first 15 minutes or so of any journey will not yield very good fuel consumption regardless of the advertised fuel economy of the vehicle. So if your journey involves a short commute to work or trip to the shops, you will get quite poor fuel economy in either a diesel or petrol car.
Secondly, stop-start driving. Even once your engine has warmed up thoroughly, it is still not operating efficiently if you are constantly accelerating, braking and sitting at traffic lights.
And because a diesel engine tends to lag and be a bit more sluggish off the mark than an equivalent petrol engine, but stronger once it gets up and running, there is a tendency to put your foot down harder to get it to respond and then have to lift off or even brake again to slow it back down as it starts to take off over about 20mph.
This sort of driving uses more fuel than smoothly applying accelerator and brake. It is certainly possible to drive a diesel smoothly, but it takes a bit of practice and constant anticipation. Meanwhile, sitting idling at traffic lights uses fuel regardless of whether its petrol or diesel, so you are simply wasting fuel with both.
Mechanical issues – the Diesel Particulate Filter (DPF)
Diesel cars built since about 2008 (sometimes earlier) come fitted with a diesel particulate filter (DPF). This device collects the black soot particles you see belching from older diesel engines, especially when the engine is cold. The DPF stores this soot until the filter unit reaches a certain temperature and then proceeds to burn off the soot. It still ends up in the atmosphere, but as much finer, invisible particles rather than ugly black smoke, and is less of a health hazard (allegedly, but this is disputed).
This is good, right? Well, yes, but…
To get the filter trap hot enough to burn off the soot, the engine needs to have been running for at least 15-20 minutes and it then takes another 10-15 minutes to burn the soot and clean the filter.
When DPFs were first launched into the car world, most people didn’t realise this – including the dealers who sold the cars – and so customers were unaware they even had such a device. If the driver does not regularly (ie – about once a week to once a fortnight) go for a drive of at least half an hour, the filter trap clogs up. In addition to not filtering the diesel soot properly, continued or repeated clogging will eventually destroy the filter, requiring a very expensive replacement.
So, if your regular use of your car does not involve a regular drive of 20-30 minutes, you need to make a special trip just to clear your DPF every week or two, which is hardly helping your fuel bills.
Cars built since about 2012-ish usually feature better-designed DPFs and improved engine computer controls which allow faster and more efficient burning off of the soot, so it is now less of a problem than before, but it is still a problematic issue for dealers who have to deal with irate customers objecting to paying hundreds of pounds to replace their DPF when they weren’t made aware of it.
This is even more of an issue for used car buyers who have almost certainly not been given a proper explanation by the dealer and consequently are more likely to run into trouble.
Misfuelling – filling up with the wrong fuel
One other hazard is misfuelling, and it happens a lot more often than you’d think – according to the AA, 150,000 people do it each year, or once every three-and-a-half minutes!
Putting diesel fuel in a petrol engine is very bad news, and putting petrol in a diesel engine is even worse. When switching from one sort of engine to another, such as when you buy a new car or in multi-car households, it is an easy mistake to fill up the tank with the wrong fuel.
Damage caused by mis-fuelling is not normally covered by warranty, so you could be up for thousands of pounds if the engine has been damaged. Even if you don’t destroy your engine, it is still an expensive and time-consuming exercise to have the car towed away, drained and cleaned out before you can drive it again.
This isn’t a problem of diesel engines per se, but if you are considering switching from a petrol car or already have another petrol car in the household, it’s worth keeping in mind.
All of the above suggests that buying a diesel car for mainly city driving is not a great idea. However, it also depends on your overall mileage and how much load (passengers, luggage, trailers, etc) you are putting on the car.
The very broad advice usually given around the industry is that an annual mileage of over 10,000 miles/year over three years is about the threshold for choosing a diesel car over a similar petrol one. However, it also depends on the type of car you are looking at, how long you’re planning to keep it, what sort of deals you can get on one or the other, and so on.
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