3 ms·

The stoichiometric mixture of air and fuel burns exactly all the fuel. You can run richer than stoich (lower air-fuel ratio), which will produce more power and

The stoichiometric mixture of air and fuel burns exactly all the fuel. You can run richer than stoich (lower air-fuel ratio), which will produce more power and run cooler, at the expense of fuel economy. You can also run leaner than stoich (higher AFR), which will be more economical but combustion temperatures will be higher and more NOx will be produced. Throw ignition timing in the mix and you have a complicated balancing act that engine designers perform in order to produce the ideal combination of power, fuel efficiency, and emissions.

friendzis 11y ago
Throw variable air intake, variable timings and injection patterns into the mix and you get powerful, clean and efficient engine. Which leads to a question whether this cheat by VW is intentional or a (possibly intelligently executed) side effect? Given the number of moving parts and man-hours of research required to get everything right I am leaning in favour of the latter.
- tadfisher 11y ago
  Diesels are complicated to sell in the US because of their emissions profile. Part of this is because they must run lean in order to completely burn fuel and prevent soot formation. This results in very high NOx emissions compared to contemporary gasoline engines. Typically, passenger diesels employ a urea injection system to meet EPA standards, which reduces NOx as a catalyst. This system adds expense and maintenance duties. VW claimed their small diesels could meet emissions standards without such a system. What they have actually admitted to doing is writing a complicated algorithm that detects the conditions of a CARB emissions test and modifies running parameters in order to reduce NOx to acceptable levels during the test. This isn't in dispute by any party. As emissions are not tested during typical driving scenarios, and given that CARB and the EPA has measured 40x the acceptable level of NOx, I speculate that VW is enriching the fuel-air mixture when their algorithm detects the conditions of an emissions test; this would dramatically reduce NOx emissions at a measurable cost to fuel economy. As other automakers—such as Mazda with their perpetually delayed Mazda 6 diesel—are having trouble developing urea-free diesel engines for the US market, it is no wonder that someone noticed the discrepancy between USDM and EUDM emissions numbers for these vehicles. Note that the EPA does not perform emissions testing of its own; automakers self-certify. CARB is the de-facto enforcer of EPA standards.
- scott_karana 11y ago
  > Which leads to a question whether this cheat by VW is intentional or a (possibly intelligently executed) side effect? Volkswagen directly admitted it was intentional. Moot point, at least this time. :-) http://jalopnik.com/volkswagen-u-s-ceo-we-screwed-up-and-will-pay-what-we-1732165830 http://jalopnik.com/volkswagen-u-s-ceo-we-screwed-up-and-wil...
  - friendzis 11y ago
    Well, I wanted to question whether they have found out their engines are rather polluting and have invested extensively into developing such a cheat or just found out (on two separate instances) that a) it is possible to detect emissions testing and b) some engine modes have drastically lower emissions; and simply put the dots on i and went along.
sliverstorm 11y ago
The complicated balancing act gets even cooler. Modern engines can play tricks like injecting fuel into the very center of the chamber, so that the flame front does not touch the chamber walls, insulated by air. Supposedly this reduces the development of hot spots on the chamber walls (reducing NOx formation)
rcthompson 11y ago
Ah, I see, so if the fuel mix is lean, then there's some oxygen left over after all the fuel has burnt, and it goes looking for something else to oxidize, and it produces NOx.

friendzis 11y ago

Throw variable air intake, variable timings and injection patterns into the mix and you get powerful, clean and efficient engine. Which leads to a question whether this cheat by VW is intentional or a (possibly intelligently executed) side effect? Given the number of moving parts and man-hours of research required to get everything right I am leaning in favour of the latter.

tadfisher 11y ago
Diesels are complicated to sell in the US because of their emissions profile. Part of this is because they must run lean in order to completely burn fuel and prevent soot formation. This results in very high NOx emissions compared to contemporary gasoline engines. Typically, passenger diesels employ a urea injection system to meet EPA standards, which reduces NOx as a catalyst. This system adds expense and maintenance duties. VW claimed their small diesels could meet emissions standards without such a system. What they have actually admitted to doing is writing a complicated algorithm that detects the conditions of a CARB emissions test and modifies running parameters in order to reduce NOx to acceptable levels during the test. This isn't in dispute by any party. As emissions are not tested during typical driving scenarios, and given that CARB and the EPA has measured 40x the acceptable level of NOx, I speculate that VW is enriching the fuel-air mixture when their algorithm detects the conditions of an emissions test; this would dramatically reduce NOx emissions at a measurable cost to fuel economy. As other automakers—such as Mazda with their perpetually delayed Mazda 6 diesel—are having trouble developing urea-free diesel engines for the US market, it is no wonder that someone noticed the discrepancy between USDM and EUDM emissions numbers for these vehicles. Note that the EPA does not perform emissions testing of its own; automakers self-certify. CARB is the de-facto enforcer of EPA standards.
scott_karana 11y ago
> Which leads to a question whether this cheat by VW is intentional or a (possibly intelligently executed) side effect? Volkswagen directly admitted it was intentional. Moot point, at least this time. :-) http://jalopnik.com/volkswagen-u-s-ceo-we-screwed-up-and-will-pay-what-we-1732165830 http://jalopnik.com/volkswagen-u-s-ceo-we-screwed-up-and-wil...
- friendzis 11y ago
  Well, I wanted to question whether they have found out their engines are rather polluting and have invested extensively into developing such a cheat or just found out (on two separate instances) that a) it is possible to detect emissions testing and b) some engine modes have drastically lower emissions; and simply put the dots on i and went along.

sliverstorm 11y ago

The complicated balancing act gets even cooler. Modern engines can play tricks like injecting fuel into the very center of the chamber, so that the flame front does not touch the chamber walls, insulated by air. Supposedly this reduces the development of hot spots on the chamber walls (reducing NOx formation)

rcthompson 11y ago

Ah, I see, so if the fuel mix is lean, then there's some oxygen left over after all the fuel has burnt, and it goes looking for something else to oxidize, and it produces NOx.