| Topic: | Re:Putney High Street - disgraceful example for air quality inaction | |
| Posted by: | Richard Hodges | |
| Date/Time: | 14/02/13 15:47:00 |
| No. The emissions levels for diesel-engined cars are pretty much where an automotive engineer would predict them to be. - It's the core chemistry of the diesel engine that allows the higher compression ratios that lead to higher thermodynamic efficiency. - Higher compression means higher peak pressures, means more conversion to NOx compounds, and a higher proportion of NO2 in the cylinder. - The 3-way catalytic converter system that performs on petrol engines is not practical on diesel engines, due to the different chemistry of the exhaust gasses. - The only practical catalytic system, the SCR unit, is more prone to failure and requires a much more maintenance. Failure mode can result in emissions of more harmful chemicals. The very thing that makes the diesel engine so appealing causes it to to create more NOx. Options for completing the reduction reaction are more limited. This isn't new knowledge, the science goes back to the birth of emissions research fifty-odd years ago. It's the fundamental science, and no-one's come up with a practical workaround. So someone who doesn't appear to be an engineer thought the lemon could be squeezed for more juice, when in reality it was already dry. They also didn't correctly predict the wholesale adoption of diesel in private vehicles, even though it has been tax-incentivised for thirty years. The model was wrong. It failed to follow the science. It still has to be noted how far NOx levels in either cycle have fallen - more than 90% on modern cars. We are now in the realms of scraping out the dregs of improvement by identifying the situations that give abnormal figures (which is not disappointing, it's how it has to be). The bigger opportunity, particularly it would seem for urban situations, lies in improving fuel efficiency (which should also reduce emissions per mile). The last twenty years have seen other priorities - safety, customers demanding all the trimmings - which have increased vehicle weight and limited the visible advances. That trend has started to turn again. New, very high-tech small motors are appearing, and new ideas are ready to roll. The petrol and diesel engines have a lot of life still in them. |