I've read on the internet, in a couple of maintenance books and asked a few buddy mechanics and they all seem to point to the fact that a healthy diesel engine should not produce black smoke and that the common reason why this happens is because of too much fuel to not enough oxygen leading to incomplete combustion.

If the fuel is not completely burnt, then all that fuel is wasted which is not what I would expect in a racing context, so what gives? Why don't they cut on the amount of fuel injected into the engine so as to maintain a proper fuel/air ratio? Or better yet, increase the amount of air being rammed into the engine (at least as long as it wouldn't hurt the power to mass ratio)?

To add more confusion, I took a look at some footage of the Audi R18 TDI and I failed to notice any black smoke, which makes perfect sense, given the nature of the type of contest it is taking part in (efficiency anyone?).

So why are diesel dragsters wasting fuel and producing soot?

  • Drag racing is world apart from production consumer cars, drag diesel engines run advanced injector timing and lots of fuel delivery, since diesels to not control power using an intake throttle plate like gasoline engines, they use the amount of fuel injected to control power delivered, lots of fuel lots of power.
    – Moab
    Commented Aug 6, 2016 at 15:44
  • See that's just it. What is the point of adding more fuel if you don't add the air to burn that fuel thus generating more power? I'm not arguing that power delivered is or isn't controlled by the amount of fuel, I agree with you on that. But, to me, the presence of smoke equals the presence of unburnt fuel. Is that not the case? If just injecting more fuel produces more power then why do we turbo charge diesels? Edit: I never mentioned consumer cars. The R18 TDI is a 3.7 litre V6 530-ish HP diesel engine used in Audi's Le Mans car that won 16 of 33 races it took part in. Commented Aug 6, 2016 at 16:15
  • I don't know for sure, but would bet one of the biggest reason for this is heat. If you saturate the cylinders with fuel, you lower the heat produced during the combustion cycle. This helps prevent problems with detonation. Detonation or pre-ignition would kill the big engines. Nitromethane can burn in the absence of oxygen, due to its chemical nature. At stoich values is 1.7:1 versus gasoline's value of 14.7:1 ... You can just cram a bunch more fuel in there and not worry so much about the oxygen levels. It's why they can get 10k hp out of the engine. Commented Aug 6, 2016 at 18:14
  • Even car and truck diesels produce lots of soot when you kick down the pedal. Most of this is filtered out today, so you don't see it very often these days. I guess there's not enough time to vaporize all the fuel / mix it with the air, so there are many droplets which don't combust completely. Yet, more of these drops give more fuel burnt in total, so more power.
    – sweber
    Commented Aug 6, 2016 at 18:32
  • 1
    @ dlu: I personally don't enjoy the smoke but I guess it's possible. I honestly don't know if over fueling would reduce the times. What I'm saying is wouldn't adding more air to match that amount of extra fuel improve the times?Or is this a "Law of diminishing returns" scenario: more air+more fuel=bigger bang needing stronger HEAVIER engine => less overall performance? Conclusion: i'll just stick to just using more fuel which also has the benefit of reducing the amount of heat in the engine? Commented Aug 8, 2016 at 18:34

3 Answers 3


The Primary Reason for Excessive Soot in Diesel Dragsters is Incomplete Combustion

Something to keep in mind regarding diesel dragsters is that they do not use any emissions equipment to more effectively burn unburned fuel and oil contained in diesel fuel which is essentially kerosene.

Emissions devices NOT on a diesel dragster

  • Exhaust Gas Re-circulation devices or EGR's are absent

  • Oxidation catalyst devices are absent

  • DEF injection systems where ammonium or urea are injected into the system as an fluid are absent

Additionally, excessive valve overlap is tuned into the system to ensure the intake charge flushes the combustion chamber of exhaust gasses. These leads to excessive unburned fuel escaping from the exhaust valve and into the atmosphere, this unburned fuel is partially burned in the exhaust and is turned into DEP (diesel exhaust particles) which are primarily just carbon.

This is what you see with the thick black smoke from diesel dragsters, carbon.

  • 2
    So the extra fuel is used to purge the combustion chamber of exhaust gases. Thorough and to the point. Commented Aug 12, 2016 at 14:04

From a practical perspective: It is most likely that they still see a power increase by progressively injecting more and more fuel - even with the extraordinary soot production, which isn't a limiting factor in these conditions.


Diesel fuel is heavy in oil residual as it is far less refined than is gasoline. However please understand though both burn in internal combustion engines diesel runs on different processed product. Gasoline is measured via Octain and this is a completely different line as Cetain, the processing method of diesel. Jet fuel, Kerosine and diesel share this proccess! Now as to Tractor pull rig's and the plume of heavy soot this took tons of compression, huge valves to flow it all and injectors with pencil size holes where an normal diesel has injector sprayer holes as small as a doctors smallest needles.

  • Diesel is an oil. Which is why it's actually called a fuel oil. You are partially wrong or ill informed as to why there is more soot. Unlike gasoline, you can run diesel much greater than stoich and it will still burn. This actually produces more power/torque. It also helps cool the engine as the residual fuel which comes out as the black ploom absorbs some of the heat, which is important when you're stuffing 100+ psi of boost into those blocks under racing conditions. Commented Jan 22, 2023 at 15:53

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