Cetane is to diesel what octane is to petrol – a measure of how easily the fuel ignites under compression. Cetane booster additives are designed to raise the cetane number slightly so that combustion starts more cleanly. In reality, that can mean easier cold starts, quieter running and marginally better fuel efficiency. However, many “premium” diesels already contain extra detergents and cetane improvers straight from the refinery.

Diesel can become waxy or gel-like in very cold temperatures, which can block filters and fuel lines. Anti-gel additives lower the temperature at which crystals form, so the fuel continues to flow.

However, in the UK, road diesel sold between roughly mid-November and mid-March is legally required to meet winter-grade EN 590 specifications, with a cold filter plugging point (CFPP) down to –15°C. That means daily-driven cars and vans that refuel regularly don’t usually need extra anti-gel.

Where such additives are useful is when a car or van has been sitting for months with summer diesel still inside when winter arrives, or you’re driving in exceptionally cold conditions and want extra protection.

Lubricity additives help replace the lubricating properties lost when sulphur was removed from modern diesel, protecting the high-pressure fuel pump and injectors from wear by forming a protective film over internal components.

Some additives are marketed as emissions reducers or diesel particulate filter (DPF) cleaners, and it’s important to understand how these actually work. Part of the emissions benefit can come from cleaner injectors and higher cetane, which encourage better combustion and slightly less soot. Many DPF-focused additives contain metal catalysts (often iron or cerium compounds) known as fuel-borne catalysts or FBCs.

These don’t magically stop soot forming, but they lower the temperature at which soot burns inside the DPF and help it clean itself at lower exhaust temperatures – a process known as regeneration. DPF additives make that process more likely to be successful on vehicles that do a lot of short, low-speed journeys.

Modern UK diesel typically contains up to seven percent biodiesel, known as B7. Biodiesel is hygroscopic, which means it absorbs water more readily and that can encourage microbial growth, commonly called diesel bug.

This particularly affects vans, campers, and motorhomes, or any diesel vehicle that sits idle for weeks or months. Diesel bug can form a sludge that clogs filters and fuel lines. In such cases, biocide additives can be useful to prevent or treat contamination in stored fuel.