Much of my early career was consulting to the auto industry (in particular, Ford and GM) during the early periods of electronic engine controls and their interaction with the emissions test regime in question. I did some work with engine controls, but most of it was emissions testing automation and data reduction.
We all (executives, engine designers, test equipment designers, and regulators) knew:
- The test conditions were arbitrary but standard.
- Detecting them and switching modes would be trivial to implement and look good at first, but also illegal, immoral, and financially disastrous for the company when they were eventually detected.
- Because engineering was done to meet the regulations - which met score well on the tests - even with honest efforts and no cheating it would eventually evolve the vehicles to do well on the tests but probably not so well on other operational cycles. (You see this with "your mileage may vary".)
- Tests and design processes were VERY expensive and the companies highly competitive. They couldn't afford to engineer for BOTH the regulations and to be good all the time out of niceness: The "nice guys" would "finish last", be driven out of the market, and you'd STILL only get cars that only met the regulations. A level playing field was needed.
- So it was the responsibility of the regulators to write test specifications that modelled the driving cycle well enough that engines tuned to them would also perform adequately in general, despite the "design to the test" evolutionary pressure, and the engineers to meet the law on the tests that were imposed, not do so by explicit detect-the-tests cheats.
The executives and early-stage engineering departments were aware of the temptation for engineers to write cheats, and (at least at one I worked for) put some draconian controls in on software changes to the engine control, to prevent them. (The official explanation given to the inconvenienced engineers was "insuring regulatory compliance".)
I was told that the regulators came up with the standard test by
- instrumenting a car (with a bicycle wheel speed recorder on the bumper and some event-recording switches),
- parking behind various cars (in Denver?) and, when their owners started up, surreptitiously tailing them to their destination and recording their warm-up idle time, speeds, acceleration, braking, standing waiting for lights, etc. (but not the upslope/downslope and wind).
- picking one of these trips, which contained both city and highway driving and looked pretty typical, and adding a "cold soak" to the start (engine is not run for several hours) to standardize the starting conditions and model an initial start, and a guesstimate of a final idling period before shutdown. (To meet the cold-soak requirement, cars were pushed into the test cell by hand or things like electric pallet jacks.)
The test measures exhaust airflow volume and concentration of CO2, CO, and unburned hydrocarbons. So gasoline consumption can be easily computed by "carbon balance" - you know how much carbon is in a gallon, you measure all of it as it comes out, none is lost and only a tiny bit of burned lube oil adds any. So you get mileage for free by postprocessing the data. The regulators got the bright idea of putting this computed mileage on the stickers for customers to make objective comparisons when shopping.
It's easy to measure the average mileage of cars in the field: Just divide the odometer mileage by the gallons pumped to refill the tank, and average over several fillups to smooth out variation in how the tank was topped off. It quickly became apparent that:
- Mileage in normal service varied substantially.
- The trip defined as the standard one got substantially better mileage than was typical.
Thus was born the caveat "your mileage may vary" and a regulation change to partition the sticker mileage into separate pieces for the stop-and-go city portion and mostly-cruising highway portion. For gasoline engines, using those two, and a small nudge downward for the standard trip's deviation from the typical, gives customers a good guide.
Also because it's easy to measure, mileage numbers from the field provided feedback to limit the tendency for "design to the test" to make gas consumption evolve into complete optimization for the test. Any model that got horrible mileage in the field would soon get bad reviews, and the engineers would be on its case (if this hadn't happened before it was released.)
But emissions are NOT easily measured in the field. About the only tests there are periodic checks in some states - and they tend to use a very abbreviated cycle. They're just intended to check that the stock emissions control equipment hasn't broken or been disconnected.
So with field feedback on mileage but not emissions, the secondary selection pressure (after "do well on the standard test) is for the engine to get good mileage on other cycles without regard to whether this affects emission. Engineers, with the best intentions, would tend to design engines that pollute a bit more when off the test.
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I agree with most of what you say. But this is incomplete:
The higher temperatures and pressures (of diesels) help with CO and unburned hydrocarbons (they favor more complete combustion), but the scale of the added NOx and PM problems are much greater.
Which is true upstream of the catalytic converter. But the whole POINT of a (three-way) cat is to move oxygen from NOx to CO and unburned hydrocarbons. Get the right fuel-air mixture and any leftover oxygen, NOx, CO, and HC are all burned exactly. Getting this right with early engines - using fluid and mechanical computation - was a real pain. With software and exhaust oxygen sensors it's a much easier job.
As for particulate matter, the original emission control regulations were designed around what was current when they were imposed: gasoline engines, running the Otto cycle, which doesn't emit much PM unless horribly detuned, worn into burning lots of lube oil, or fed the wrong fuel (like accidentally topping off the tank from the green diesel-fuel pump hose). Diesels tend to put out a lot of PM, and (as big lumps of mostly carbon and unburned hydrocarbons) a surface catalyst can't do much with it. So getting that right pretty much needs to be dealt with separately.