On a slightly related note, here's a story from back when the EEPROMs for cars were on DIP chips and slightly more hackable:
The old OBD I GM cars had a feature called "highway mode" which would lean out your fuel mixture when cruising to give you better gas mileage [1]. The downside was that your car had worse emissions. This feature was known to (or discovered by) the EPA, and they forced GM to turn off the feature. GM did so not by re-writing the fuel maps and cutting out all the highway mode code, they just created a disable bit that kept the ECU from ever entering highway mode-- the EPA was satisfied with that fix. People today with an EEPROM writer (can get a cheap one for under $100), a bit of soldering skill and access to the internet can remove the disable bit and give their car highway mode.
How can you get better mileage but produce more emissions? I would assume that emissions are proportional to the amount of gas used. If you're using less gas, wouldn't you produce less emissions?
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.
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.
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.
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.
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)
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.
[I am not an expert, but] first there are controllable emission control devices (e.g. EGR) and secondly burning conditions (thermodynamic cycle) highly influence power and emissions (emission mixture), i.e. how much power and emissions you get from burning 1 g of fuel.
An intuitive analogy: when you are camping (or just use firewood in a grill) and are just starting a fire there is a lot of smoke, although the fire is relatively small. When you get it hot and burning nicely there is barely any smoke left, although you burn much more fuel per time.
For an internal combustion engine the factors determining power/emissions are mainly fuel/air mix, ignition angle (gasoline), fuel injection pattern (for direct injection engines - all "modern" diesels, VW [T]FSI gasoline) and or air/mix injection angle (variable camshaft timing).
Fun fact: modern diesel cars have a DPF designed to limit emissions (fumes mainly). Engines have special DPF cleaning mode designed to kick in in highway modes by slightly increasing exhaust gas temperature. Driving only in urban cycles causes a DPF to get clogged, engine enters deep cleaning mode from time to time (power loss, dark fumes out of "chimney") which has a negative net effect on particle emissions, which are arguably most detrimental to respiratory tract.
It seems like Nitrogen Oxide output increases as the fuel combustion temperature increases. So when you're at a cruising speed that burns fuel more efficiently, it alters the balance of waste products. Somehow.
I'll defer to anyone who claims any real knowledge about this at all, I'm just doing some motivated googling.
Roughly, a higher combustion temperature increases combustion efficiency, but also increases NOx produced. Running lean increases the air:fuel ratio, which combusts more of the fuel at higher temperatures. On the flip side, EGR [1] is one of the primary techniques to reduce NOx at the expense of efficiency. Incidentally, it's actually somewhat common for diesel truck owners to do an EGR delete to increase fuel economy.
The "somehow" is pretty straightforward chemistry. As temperature increases, oxygen in the chamber begins to react with nitrogen, producing oxides of nitrogen (NOx). Both are present in the atmosphere, but do not react at normal atmospheric temperatures.
Total emissions aren't the issue here - it's the emissions of specific regulated compounds like NOx.
Most emissions from engines are in the form of CO2, which is indeed directly proportional to how much fuel is getting burnt. However, other compounds like NOx and unburnt hydrocarbons (HC) are a product of how combustion is happening, not how much is happening.
Basically, the car can choose to trade decreased efficiency (additional CO2 emissions) for reduced NOx and HC emissions.
Note to readers: 14.7 is the stoichiometric mixture for pure octane (C8H18). The stoichiometric AFR for pump gasoline is a bit lower, but the same principles apply.
The old OBD I GM cars had a feature called "highway mode" which would lean out your fuel mixture when cruising to give you better gas mileage [1]. The downside was that your car had worse emissions. This feature was known to (or discovered by) the EPA, and they forced GM to turn off the feature. GM did so not by re-writing the fuel maps and cutting out all the highway mode code, they just created a disable bit that kept the ECU from ever entering highway mode-- the EPA was satisfied with that fix. People today with an EEPROM writer (can get a cheap one for under $100), a bit of soldering skill and access to the internet can remove the disable bit and give their car highway mode.
[1] first thing resembling a source: http://www.thirdgen.org/forums/diy-prom/408964-highway-mode....