Which is cleaner: diesel or petrol?

by Alex

Expert answer:

Quick answer

Comparing diesel to petrol depends on what you measure. Diesel often delivers fewer carbon dioxide emissions, but it can produce more harmful nitrogen oxides and particulates. Modern filters improve diesel’s cleanliness, yet driving habits and UK emissions rules also impact real-world results.

Detailed answer

When deciding which is cleaner—diesel or petrol—it’s helpful to understand the strengths and weaknesses of each. Diesel engines typically produce lower carbon dioxide emissions, which can look impressive if you’re worried about overall greenhouse gases. Yet, there’s a flip side: diesel exhaust may contain more nitrogen oxides (NOx) and particulates that harm local air quality. Petrol engines, by contrast, usually emit fewer of these irritants but create more CO2, which can add to global warming. This trade-off makes it tough to give a simple, one-size-fits-all answer.

In the UK, diesel and petrol technologies have both evolved dramatically. Newer diesel vehicles feature advanced particulate filters, selective catalytic reduction (SCR), and enhanced engine controls. These innovations target NOx and soot, aiming to keep the exhaust cleaner. Meanwhile, modern petrol engines use direct injection or hybrid setups to minimise emissions and boost efficiency. The real winner in a cleanliness contest often comes down to which model year you’re looking at and how well the vehicle is maintained.

Driving habits count as well. For longer motorway trips, diesel engines shine. Their higher compression ratios and efficient combustion save fuel, which lowers CO2 output and reduces visits to the pump. On the other hand, frequent city trips with constant stopping and starting can gum up a diesel particulate filter. If that filter doesn’t regenerate, the engine might run less cleanly. Petrol cars can handle short hops more gracefully, although they could emit more CO2 in the grand scheme of things.

Then there’s the regulatory environment. Low-emission zones in places like London penalise older, dirtier diesel cars with higher charges or restrictions. If you own a newer diesel that meets strict standards, you’ll likely fare better. However, if your diesel is from an older generation, you might face extra costs. Petrol cars aren’t immune to these restrictions, but the clampdown often targets diesel vehicles first, given their more notable NOx emissions.

When it comes to maintenance, both diesel and petrol engines need care, but diesel cars may require more specific attention. Diesel injectors, turbochargers, and particulate filters are sensitive to soot buildup and clogged parts. Missing service intervals or skimping on recommended maintenance can cause a diesel to pump out clouds of smoke and lose its efficiency advantage. Regular servicing and occasional longer runs help keep the filter in tiptop shape, ensuring a cleaner drive.

So which is cleaner? For some drivers, especially those who rack up motorway miles and maintain their vehicles well, diesel can prove cleaner overall by reducing CO2 and running efficiently. Others, particularly city dwellers or short-distance commuters, might find a petrol option creates less trouble with NOx or clogged filters. Ultimately, it’s all about balancing the different emissions, understanding the vehicle’s design, and being aware of local rules. Consider what kind of driving you do most often, how you maintain your car, and how stringent your local emissions laws are. That combination of factors helps you find the right answer for your situation, rather than relying on a blanket statement.

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