I asked this on Reddit earlier this week, but nobody had much technical insight.

Can anyone shed light on why exhaust gas from a typical London bus smells entirely different from a coach or truck's exhaust? The latter smell like diesel always did, whereas London busses emit this strange, to me at least, intolerable smell of weirdness. When I walk past a bus, I take the deepest possible breath, and hold it until I'm clear of the bus. When I drive, I close the windows and set the airflow to recirculate instead of taking in outside air. I avoid ricing my bicycle behind a bus, at all cost.

  • It was suggested on reddit bio diesel can cause this type of exhaust gas. Someone else suggested very high combustion temperature cause less pollution and that adblue is a factor.
    – Jepper
    Dec 12, 2022 at 21:27
  • Haha ... sorry, I didn't read your comment prior to throwing the answer out there. Dec 12, 2022 at 22:26

2 Answers 2


While biodiesel may make up a different smell of diesel exhaust, my guess is meeting stringent exhaust pollution regulations requires selective catalytic reduction (SCR), diesel particulate filter (DPF) to remove visible diesel particulates, and Adblue/DEF to reduce oxides of nitrogen at a high temperature. Exhaust from diesel engines using emissions equipment eliminates the smoke and diesel exhaust smell. With tight diesel emissions controls, the smell is very different and less obnoxious than original non emissions equipped diesel engines. I grew up liking original diesel exhaust but have allergies. As an adult, following NYC buses in regular traffic, gone are old time diesel smells. No more black smoke and smells cleaner. The same occurred when gasoline engines underwent emissions controls over several decades. Original exhaust smelled terrible. Once EFI, O2 sensors and catalytic converters were mandated, exhaust smells aren't terrible compared to non emissions engines.

  • I'll accept this. I too like the smell of old diesel!
    – Jepper
    Dec 17, 2022 at 8:37

I'm going to go out on a limb and suggest what you are smelling is biodiesel. According to the London Government website, it says:

Around a third of London's 9,500 buses are running on 20 per cent blends of biodiesel (B20).

Biodiesel has a different smell and this could really be what you're smelling, but I wouldn't know for sure.

  • Biodiesel also smells differently based on what it's made from, which can be just about anything. At the State Fair in my state they run tractors pulling people on trams around running 100% corn biodiesel and it smells strangely like popcorn.
    – jwh20
    Dec 13, 2022 at 11:03
  • @jwh20 - I've heard the same kind of thing, such as using oil used for cooking fries and the biodiesel exhaust smelling like them. Pretty wild, but it does sort of make sense. Dec 13, 2022 at 12:45
  • There are biodiesels with less smell such as Neste NEXBTL fuel that should be fully compatible with petrodiesel even in 100% concentrations.
    – juhist
    Dec 13, 2022 at 16:58

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