From Ravi Nadkarni
John Griffiths explains clearly the benefits of cool-flame combustion (5 June, p 28). But low-temperature combustion must decrease the Carnot efficiency. This theoretical maximum efficiency for converting heat into mechanical energy is determined solely by the temperature difference between heat source and sink.
Typical combustion temperatures for an engine using cool-flame ignition may be around 1800 °C. This is lower than with spark ignition. What is the penalty in decreased efficiency?
John Griffiths replies:
• Typical combustion temperatures for an engine using cool flames is around 1600 °C, reducing the engine’s maximum efficiency by around 15 per cent. However what legislators are interested in are the benefits – lower emissions of pollutants such as soot and nitrogen oxide.
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Wrentham, Massachusetts, US
