Which car to buy: Diesel or petrol?

The latest news on car sales in our country is showing a clear trend: more petrol cars are being sold than diesel (something that was unthinkable just 10 years ago). On the other hand, large cities such as Madrid and Barcelona are putting a stop to vehicles, and diesels seem to be public enemy number one because of their emissions (although they have always been there, it’s not that it has just been discovered that they emit NO_x).

Given this background, buying a new car right now is a complicated task. It seems that if we opt for a diesel, the investment is not very favourable, and the solution being promoted as an universal truth is to buy petrol vehicles. But this is not entirely true: diesel is not going to disappear in the short term, nor is petrol the solution to the problem. Everything will depend on how we use the car, and that is where the key lies. To help clarify some of these points, today I want to offer another point of view on certain claims that are repeated in an attempt to turn them into universal truths.

Diesel is going to disappear; some brands have already announced that they are stopping the production of diesel engines

This is one of those claims that contains a part that is true and a part that is inaccurate. It is true that some brands have announced they are ceasing diesel engine production. In the short term, they are changing their range and progressively removing diesels until they disappear completely. They are doing this because developing a diesel engine today is more costly than it was years ago, with the constant changes in emissions regulations. Many engines that were on the market need costly tuning processes, something that for certain brands does not make economic sense, since they would have to develop these new powertrains only for the European market (the favourite market for diesels), while they do not need them to keep selling in other markets. This means that some brands have opted for hybrid technology (the fruit of their experience in this field) or for petrol engines to replace diesels, offering an alternative without the need to develop diesel engines; Volvo and the FCA Group are some of those brands.

Meanwhile, other manufacturers continue to bet on new diesel engines, more efficient and with new technologies that allow them to meet the demanding emissions regulations. Proof of this is that over the last year new diesel powertrains have been launched, and some brands have even presented diesel-hybrid powertrains with the aim of meeting the most demanding pollution scenarios. In other words, the disappearance of diesels is a decision for each brand: some prefer to focus on other types of powertrains, while others continue to offer newly developed diesel engines.

Which engine is more reliable: diesel or petrol?

What makes an engine reliable is the robustness of its design and the fewer components it has that can fail; the more reliable it will be. But nowadays brands do not create simple engines. Quite the opposite: in the midst of the downsizing trend, there are hardly any naturally aspirated engines left on the market, and the aim is to squeeze the maximum out of small engines, obtaining great performance from them, whether the powertrain is diesel or petrol. Reliability will depend on the construction of the specific engine, because there are examples of petrol engines (even small ones) with proven reliability, and diesels that are robust well beyond half a million kilometres.

Another detail to consider is that some diesels are fitted with exhaust gas treatment systems that in many cases are a source of problems for owners, although everything depends on how the car is used. The key to the matter lies in the use the engine was designed for and its optimal operating conditions; when these are not met, this is when such problems can appear.

What should I take into account when deciding whether to buy a diesel, petrol or another type of vehicle?

The first thing to consider is how the new car will be used: will it be driven mostly in the city, on country roads, or on motorways? and then analyse the number of kilometres we cover each year. If we combine the type of use with the number of annual kilometres, it will be easier to get the right fuel. For example, for short or urban journeys the best option is a petrol or a hybrid, since the simpler and more robust the powertrain, the better, as it will cope better with day-to-day use. If we do not reach 20,000 km but still drive a mix of roads and, to a lesser extent, the city, the best option would be a hybrid, because it gives us economical running and savings even on occasional city trips.

But if we drive more than 20,000 km a year or our usual route is on motorways or main roads, that is where a diesel makes the most sense. A diesel should also be considered depending on the kind of needs we have, for example, an off-roader or for towing. A diesel engine suffers when it is driven in the wrong places, without the engine reaching its ideal temperature, or with frequent starts and stops that increase the risk of breakdowns.

In short, if you have to buy a car, it is not only a matter of deciding whether it will be diesel or petrol. The first step is to analyse how the vehicle will be used, and then weigh up the different alternatives, whether petrol, diesel or hybrid. And although the focus is currently on diesels, that does not mean they are going to disappear or that the technology is falling out of use with manufacturers.