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Showdown: Diesel vs. Petrol

By Stuart Watson - Last updated: Tuesday, February 21, 2017

Petrol-PumpsAs I sit in traffic this morning on my daily commute to work the radio tells me that the UK is facing legal action from the European Commission due to unacceptably high levels of air pollution around the country. Several causes are identified, but the one singled out for specific criticism is the diesel engine.

Diesel engines have experienced press similar to a B list celebrity from a soap opera in the last few years. Not that long ago they were dull and boring, always there but only in the background working away without much praise. Sure you could buy one, but they were slow, expensive and sounded like a tractor.

Then, all of a sudden they got a leading role and were centre stage, with everyone talking about them. They even appeared in celebrity Big Brother, winning Le Mans several times on the trot. Why wouldn’t you want this wonder engine that was cheap on fuel, ‘good’ for the environment and had sporting pedigree?

As ever though, fame was too much and the fall from grace is now complete, with the VW emissions scandal and the revelation that the nitrous oxides which diesels put out are killing us as we sleep. Surely we are only one step away from a teary eyed confessional to Piers Morgan.

So after poking a bit of fun, let’s now ask the question what can we learn from this and how can we avoid it happening again. The reason, as with many technical issues, comes down to understanding the core science of the problem and drawing up your technical requirements as to how you want to address it.

Reducing CO2: The Rise of the Diesel

Car-ParkIn the late 1990s it was recognised that to combat climate change car emissions needed to improve significantly. This resulted in a voluntary agreement by car manufacturers to cut CO2 emissions by 25% from 1998 levels within 10 years. All very laudable stuff, the only problem was that by over focusing on the CO2 other pollutants cars produced were not as strictly controlled. Car manufacturers quickly worked out that to meet the new requirements, the low CO2 producing diesel engine would be key and invested heavily in a previously neglected technology, rapidly bringing benefits in all areas. The only problem: those pollutants, such as the previously mentioned nitrous oxides and particulates, suddenly became much more of an issue.

What’s most frustrating about this development is that any engineer with a basic understanding of combustion cycles could tell you these characteristics of the diesel cycle. This is why at the start of any project you must take a significant amount of time to first fully understand the problem. This means working with all stakeholders, not just those that happen to be around the table in a kick off meeting. Sometimes, as with the emissions problem, it requires technical experts who understand the science and can see consequences several steps down the path. In other situations, those stakeholders may not be clear and could be the actual users who will be experiencing the product first hand or those who have to deal with end of life issues such as recycling and disposal. This approach is not quick and to do it properly takes money, but it is valuable and the cost of skipping it can be to render project worthless or worse.

Responsive legislation has since been issued in the form of the Euro 6 diesel emissions standard, but with the average age of the UK car estimated to be over 7 years, this will take a sometime to take into account. Consequently, drastic measures, such as banning diesel cars, which only a few years ago tax schemes encouraged, are being discussed. It will be interesting to see what further lessons have been learnt when the new Real Driving Emissions tests start to takes effect.

Hindsight is a wonderful thing

It would be interesting to speculate how popular the diesel would now be if the original legislators had 20/20 foresight when drawing up the legislation the car manufacturers have worked to for the couple of decades. Diesels would still have been targeted for investment and would have improved in performance, if not as much as now. Would those development funds have been diverted into other ‘green’ technologies such as hybrids or petrol turbocharging? But it’s doubtful the established petrol engine would have seen a huge leap forward over what we have now, while electric and hybrid vehicles would still be hamstrung by their battery range. The bigger change probably would have come if the marketing budgets were spent telling us to buy small, green electric cars rather than big oil burners.

It very easy to criticise though, and I would like to say that action taken back in the late 90’s has had a real effect on reducing new car CO2 levels, so one has to be supportive of the broad goal. We should also remember the auto manufacturers only tend to sell cars that people want to buy and diesel engines do fit very nicely in heavy, inefficient SUVs. Is it just coincidence that sales of these have gone hand in hand with the diesel engine’s popularity? Driving habits also hugely influence fuel economy and hence emissions. Maybe if we want to live in a cleaner world, as with most things, we all just need to take a bit more individual responsibility and really question our decisions based on the evidence. It is after all what engineers and scientists pride themselves on professionally.

In the interest of full disclosure, the Watson home is a 2 car household with both a petrol hatchback and diesel MPV.


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AuthorStuart Watson


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