Great Plains Diesel Technologies
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The Age of Steam

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September 19, 2014

Shrouded in vapor, the mysteries of heat engines began to give up their secrets during the age of steam and iron.  One of the early masters was French Army engineering officer Sadi Carnot, named after a medieval Persian poet then in vogue, son of one of Napoleon’s best generals.  Even with the colorful title “Reflections on the Motive Power of Fire” and even without calculus, sales of Carnot’s short little book did not catch fire with the public of 1824.  Apparently, despite the original being in French, this subject is not very romantic.

Nevertheless, Carnot described timeless ideas in simple but exact language.  These ideas are worth repeating here because they are the pathway to maximum fuel economy and because Rudolf Diesel designed his engine to use them to advantage.  Our continuously-controllable fuel injector offers the ability to get on the straight and narrow pathway of Carnot and at last realize the full fuel economy potential of the diesel engine.

His significant idea is that any heat is wasted when not used to expand a substance such as the gas in the cylinder of an internal combustion engine.  In Carnot’s (translated) words:

“The necessary condition of the maximum is, then, that in the bodies employed to realize the motive power of heat there should not occur any change of temperature which may not be due to a change of volume.  Reciprocally, every time that this condition is fulfilled the maximum will be attained.  This principle should never be lost sight of in the construction of heat engines; it is its fundamental basis.  If it cannot be strictly observed, it should at least be departed from as little as possible." (Carnot, S., “Reflections on the Motive Power of Fire,” Dover Reprint, 2000)

Another way to think about it is that heat is both A) stored mechanical work and B) rapidly perishable.

Since internal combustion engines first appeared, lack of technological capability has limited how well the combustion of gasoline and diesel fuels have been controlled, allowing useful combustion heat to be transferred away from the gas and to the radiator or out the exhaust without doing work.  In other words, Carnot’s maxim of not changing temperature without changing volume has yet to be fulfilled in ordinary vehicles; burning the fuel in a typical engine requires getting rid of excess heat not converted into work.

The seemingly strange results of maximum fuel economy, the result of superior fuel injection control capability, are that the familiar radiator would either shrink or disappear and the exhaust would be very cool.
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