Diesel and NOx

Why do diesel cars emit so much NOx ?  Here’s my answer in 6 tweets … expanded and annotated.  It turns out to be fairly complex.

  • About 80% of air is nitrogen (N₂), which is not quite an inert gas.  Nitrogen has a very slight tendency to oxidise under extreme conditions¹, producing nitric oxide (NO) and nitrogen dioxide (NO₂), which comprise the pollutant class² called NOx .
  • Reason 1: Diesel car engines compress the intake gasses about twice as much as petrol engines (the compression ratio is around 18:1 instead of 9:1). That results in hotter combustion and higher efficiency, but also generates more NOx . ³
  • Reason 2: Uneven flame.  Unlike in a petrol engine, fuel injected into a diesel engine burns immediately from the edge of the expanding fuel vapor jet. ⁴  There’s lots of oxygen available there, so the local flame temperature is high, generating more NOx .
  • Reason 3: Exhaust gas treatment is difficult.  Petrol engine exhaust is mostly low in oxygen because of close fuel/air mixture control. Low oxygen permits relatively simple catalytic reduction of NOx in a standard 3-way catalytic converter. ⁵
  • Diesel engine exhaust is often high in oxygen, because the fuel/air mixture is largely uncontrolled. There are work-arounds, notably exhaust gas recirculation into the engine intake (to lower oxygen availability), and devices that store NOx for treatment when operating conditions are favorable (lean NOx traps).
  • The best treatment option is selective catalytic reduction with ammonia generated by dosing with a urea solution.  It’s effective, but cumbersome⁶ and expensive.  Sulfur compounds once present in diesel fuel may cause it to fail, but the introduction of ultra-low sulfur fuel (2009 in Australia) has addressed that.

The expense of selective catalytic reduction appears to be what Volkswagon (including Audi and Skoda) were trying to avoid with their diesel car software cheat. If so, the real issue now for VW may be, if you remove the cheat but don’t retrofit selective catalytic reduction, how can you meet the NOx standard without destroying drivability?


  1. NOx production by oxidation of nitrogen gas typically becomes significant at local (not average) flame temperatures above about 1600 °C. There are other indirect ways that NOx can be produced during combustion.
  2. NOx is a significant pollutant. It reacts with volatile hydrocarbons (such as unburnt petrol and diesel) to form photochemical smog. It produces nitric acid droplets, a severe irritant and contributor to acid rain. Most importantly, it produces tropospheric ozone, a major environmental pollutant with adverse human health effects.
    The net greenhouse effect of NOx is actually slightly negative, mainly through its mediation of atmospheric methane oxidation. (Nitrous oxide — N₂O, “laughing gas” — is an important greenhouse gas, but is not included with NOx … even though its average composition is NO½ , which seems to belong.)
  3. Petrol and diesel engines are both thermodynamic heat engines, whose maximum theoretical efficiency is related to the ratio of the absolute temperature of the cool side (the waste heat radiator) to that of the hot side (fuel combustion).
  4. Despite the marketing hype, all diesel engines are directly fuel-injected, as are nearly all modern petrol car engines. Modern diesel car engines have “common rail injection”, meaning they use computer controlled piezo-electric valves to regulate injection from a single, high pressure fuel feed. In a diesel engine, the fuel is injected into the hot compressed air near the top of the piston stroke, where it immediately ignites by spontaneous combustion. In a petrol engine, fuel is injected earlier in the stroke (or indirectly via the inlet manifold), mixes thoroughly with the air being compressed, and is then ignited by a computer-controlled spark near the top of the stroke.
    (As a result, the pressure and temperature achievable in petrol engine compression are limited by the tendency of the fuel to spontaneously combust — called pre-ignition, “pinging” or “knocking” — which severely degrades performance and eventually causes mechanical damage. The resistance of fuel to pre-ignition is indicated by its octane rating. High octane fuel has a lower tendency to self-ignite, permitting engine designs with a higher compression ratio, higher combustion temperature and hence higher thermal efficiency. But not as high as diesel.)
  5. That is reduction in the chemical sense, meaning the energetic removal of oxygen from an oxide. Unlike more familiar oxides, decomposition of NOx is slightly exothermic, so well-suited to catalytic mediation … but not in the presence of excess oxygen, which pushes the chemical equilibrium in the opposite direction. Petrol engines “throttle” the air flow to the engine under part load to keep oxygen availability close to that required to just consume all of the fuel flow (stoichiometric balance). That makes it feasible to modestly cycle the engine operating conditions to favor alternately catalytic oxidation (of carbon monoxide and unburnt hydrocarbons) and catalytic reduction (of NOx). Diesel engines do not generally use all of the supplied oxygen, making catalytic reduction of NOx more difficult.
  6. Urea dosing needs to be precisely metered, or excess ammonia will be emitted — a pollutant in its own right. The dosing fluid of course needs replenishment, ideally coinciding with the vehicle servicing interval. Unserviced vehicles will be non-compliant.


The best simple-language source I found is here: https://www.quora.com/Why-does-diesel-exhaust-contain-more-nitrogen-dioxide-than-other-combustion-engines