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I'm curious as to what, if anything, would preclude variable displacement from being used in diesels as well as petrol engines. Diesels also have performance/economy tradeoffs like this gasoline engines, so it stands to reason that tweaking compression should give similar benefit. I've got a feeling that rumors of diesel's impending demise are greatly exaggerated.



Diesel's fatal flaw is particulate emissions.

Attempts to mitigate those, without cheating (e.g., VW), have proven difficult.


Difficult, yes, but doable: mazda's SKY-D diesel features 14:1 compression and Tier II Bin 5 / Euro VI compliant emissions without Urea. The lower emissions were achieved by lowering the compression ratio and lengthening the exhaust manifold, so that the exhaust gasses can cool. As the result, mazda is the only manufacturer selling diesel passenger cars in Japan.


Volkswagen cheated to try to get a less expensive system in place. DPFs and urea injection are pretty well proven technologies and work pretty well.


They work well enough on new vehicles coming out of the factory.

The real problem is 5-10 years later when these systems need repair or replacement. For many the economic temptation to have them "nulled" is too great (fuel efficiency is improved by removing the DPF) - and that once-clean diesel is now a toxic, polluting nightmare.

This is a huge problem in the UK - and why we need to phase out diesel in private/light motor vehicles.


How do they pass the emissions test?


In the UK, there is no emissions test for particulate or NOx emissions, other than the subjective "does it emit visible smoke?".

A visual inspection for the presence of the DPF is required, but this is easily defeated by installing a look-alike "null" filter.

The people doing the MOT testing can be pretty shady anyway. Since any mechanic can operate as a testing facility, it's often the same people who remove DPFs that will then pass them in the MOT inspection.


Seems like fixing that system would be more sensible than outlawing diesel cars, no?


I agree with you. A strict emissions test regime would be a good first step. But politicians have known about this for years and little has been done - apparently it's a too hard (or too expensive) problem to fix.

The thing is, even brand-new "clean" diesels are still worse polluters, for toxic particulates and NOx, compared to petrol and petrol-electric hybrid vehicles.

London already has a disincentive on diesels coming into effect by 2019, when pre-Euro 6 (i.e. older than September 2015) diesel vehicles will be subject to a £12.50/day charge to drive in London.

There are calls to extend this to cover all diesel vehicles, eventually leading to a total ban:

https://www.theguardian.com/environment/2016/jul/18/ban-dies...

Licensing of new diesel taxis will also soon be prohibited in London, and all single-deck busses will be zero-emission by 2020 (and double-deck busses will, at minimum, be hybrids).




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