Check those initial assumptions!

I bought a science magazine (Science et Vie No. 102) last month with an article about a radical new ecofuel which would apparently incorporate the benefits of a Diesel engine but without running on Diesel fuel. It was being developed by engineers at Mercedes-Benz: the DiesOtto and Opel: the CAI engine. The idea? A fuel that can be compressed to ignite and so approach the efficiency of the Diesel engine, which - the journalist reported - is caused by better mixing, but without the attendant high-temperature and high-pressure which cause high NOx emissions. The new fuel could stand the higher pressures without pre-ignition but would not be as inert as the traditional Diesel fuel which needed much higher pressures to ignite.

At this point I hope I'm not alone in spotting the obvious error in thinking. The Diesel cycle is efficient because of those high pressures and temperatures, not because of better mixing. But remember this is Mercedes talking here - they made the first Diesel engine - so what was the result? A table was produced and this is the really good bit. They showed 15% reduction in fuel consumption, 15% reduction in hydrocarbon emissions, 80% reduction in CO2 emissions, and an almost complete elimination of NOx emissions. But wait, let's revise those numbers from an engineering perspective rather than a marketing one and point out the bits that weren't mentioned. They reduced fuel use by 15% compared to SI engines but their new engine is still less efficient than CI engines. This much was mentioned in the text but not in the headline graphic. So the laws of thermodynamics are safe after all. The hydrocarbon reduction would also obviously be less than those of the Diesel engine. With the emissions figures the marketing men were even trickier. The NOx reductions were impressive except that this time they were comparing emission levels against Diesel engines, not petrol, which would likely have the same level of NOx emissions. Worse still, the stated reduction in CO2 seems to be utterly incredible unless the laws of chemistry are to be broken too: The amount of CO2 out is totally related to the amount of fuel in, so they can't be comparing that result with either engine. The only way to reduce carbon dioxide for a given amount of fuel is to increase carbon monoxide - it's poisonous cousin - and soot. I doubt that is what they did though because that would be the very definition of an inefficient engine. So the number must be a convenient error in transcription or they are capturing carbon in some other unmentioned way.

So what we are left with after a spend of several million euros is a 15% reduction in fuel use but producing a new fuel which may very well cost more than 15% more to make and buy. Actually if they limited their top speed they could have achieved the same result for zero cost. Or they could have reduced their total weight like the Loremo car designers did to achieve 150 mpg. I find it difficult to know what to make of this saga. Did these auto engineers really know nothing about the meaning of the Diesel and Otto cycles? Are they just paying lip service to fuel efficiency, trying to falsely show us that it's not really that easy. Or is it just another demonstration of the many easy ways that dumb ideas can take root. Was it poor journalism and the actual aims of these companies were not really as high as reported?

Update: On second thoughts, despite falling well short on being as efficient as a Diesel engine I have to admit that getting rid of spark plugs etc. and removing throttle losses is after all a good idea. Since ignition problems are pretty much 80% of all IC engine problems they have made the engine more reliable while retaining the acceleration of petrol/gasoline and without the NOx emissions of Diesel. Nevertheless the PR men who advised the journalist concerned crossed well over the line of truthfulness - especially about the falseness of the extent of the CO2 savings.

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