PETER Spinney (May 19) gives the correct wording of the scientific definition of efficiency but the wrong context. Useful work is not defined in the sense of being worthwhile, so whether a car's journey is useful or not doesn't enter into it. You can in the scientific sense calculate the useful work in raising a guillotine blade to the top of its frame. Not everyone would regard that as useful in the wider sense.
The appropriate mechanical figure of merit for a car is the proportion of the fuel energy that appears at the road wheels. Tests have shown that is about 20% whether you burn petrol in a car engine or burn fuel in a power station to charge batteries for an electric car.
That is not really surprising. The French physicist Sadi Carnot laid down the principles of the limiting efficiency of engines 170 years ago. There is little prospect of any significant improvement in petrol engine efficiency. The limits are fundamental and not technological.
Indeed, one recent development, the catalytic converter, has reduced the efficiency of internal combustion engines. An engine fitted with a catalytic converter will produce more carbon dioxide than a similar engine without one. In engineer's language, the converter is an obstruction in the exhaust system and that increases the back pressure. The loss in efficiency is readily calculable.
The converter was justified on the grounds that it reduced NOX and SOX gases which were more harmful that the carbon dioxide which it produced.
I am quite puzzled by Peter Spinney's statement that cars are six to eight times less polluting than 10 years ago. It seems devoid of scientific sense. There is no global index by which all forms of pollution can be combined into a single number, but there is one way in which he might get such a figure. Most of the cars that ran on leaded petrol have been phased out in that period so lead pollution is enormously reduced, but if that is what is meant the statement is quite misleading.
The only way to compare relative pollutions at two different times is to give pollution profiles for then and now. Anyone can pull a single figure out of thin air, and, with a little ingenuity, justify it.
Chris Parton,
40 Bellshill Road, Uddingston.
May 19.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules hereComments are closed on this article