Re: Well, let's summarise this.
@theOtherJT:
We all know the "quoted" figures are nonsense. No one has ever achieved even close to the MPG or CO2 figures that are in the brochure.
These blokes would disagree with you.
@Nigel 11:
BTW my observation of diesels is that <10% of them emit >90% of the soot, and presumably fail their next MOT. Even as a diesel driver I'd accept a 4-monthly 10-minute mini-MOT to check emissions, which might accomplish more than all the new-car regulations combined. If you failed you'd have two weeks' grace to get the car fixed or to find a new car.
I have a 2005 Citroën C5 1.6 HDi and it only meets Euro IV regulations. Had to replace the EGR valve and several parts in the air intake since it was intermittently getting into service mode and exiting it with a large cloud of soot behind me. After the replacements, it's back to being completely smokeless. Definitely better than a 2 year old car that had its DPF removed.
You know the worst part? Those cars will pass the MOT emissions tests. It's an open secret that they do test for smoke, but particulate emissions are not smoke -- those clouds of soot you sometimes see are a particularly bad case, where the emissions are several thousands times worse than the allowed limit, and worse in fact than some pre-Euro I diesel engines. Other than being clearly visible to the eye, they don't register on the MOT smoke test. First, because the type of these emissions is different. Second, because the test is being done with clutch engaged and/or in neutral. This completely invalidates the test because all manufacturers include an engine protection mode that prevents burnout due to revving it too much. The ECU will only supply as much fuel as needed to keep the engine at the high RPM, but other than running a stationary DPF burn in service mode, the engine will never actually get hot from that or emit anything comparable to actual exhaust.
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That's where a lot of misunderstanding is. Certification is performed in wind tunnels on rollers. The car being tested actually has to overcome friction, rolling resistance and headwind and is actually subject to simulated road conditions. The important difference being that the test allows very long acceleration and deceleration. Much longer than is usually the case (who accelerates to 30 mph in over a minute?). Air conditioning and heating are of course off. Radio, wipers, headlights and everything else is off, too (but those don't make as much of a difference as some people think they do). Manufacturers don't need to cheat by taking off the side mirrors, roof bars, blocking all air inlets, or anything. They are mostly inconsequential for the test, and the test tends to reflect actual, if very relaxed, driving.
It only becomes a problem when the car is on the road and the engine behaves completely differently from how it behaved in the lab.
Road testing is not going to be a catch-all, and some manufacturers will claim they were cheated if wind was very much against them or if the road conditions are worse on a given day. One thing it will definitely cut down on is unrealistic fine-tuning of on-board software to the narrow testing bands.
Biting the hand that feeds IT
