Thursday, January 7, 2010

Carnot Knowledge: Rudolf Diesel's Awesome Idea

Rudolf Diesel (1858-1913)

Ordinary gasoline engines are powered by the sparked ignition of gasoline vapor compressed with air. The heat of combustion and increased exhaust gas pressure drives pistons, doing useful work derived from the chemical energy stored in the fossil fuel. Gasoline engines behave according to the Otto Cycle and the ideas originally date from the mid-19th century.

Rudolf Diesel, a German engineer, understood the engines of his day and had the radical idea of compressing air inside the piston until it became so hot that fuel would spontaneously ignite when it contacted the hot pressurized air, thus not requiring a spark plug. In essence, Diesel reengineered the existing Otto cycle and invented engines that behaved according to the Diesel Cycle.

Diesel based his ideas on his understanding of the thermodynamics of heat engines, a young science begun by the French engineer Sadi Carnot and who later became known as the father of thermodynamics.

US Patent No. 542,846: "Method of and Apparatus for Converting Heat into Work (Link) was awarded to Diesel and has a clear and concise explanation of how and why diesel motors work. In Diesel's own words (or more likely those of his patent attorney):
The gases in the cylinder are now permitted to expand with gradual introduction of fuel and expansion is so regulated that the decrease in temperature by expansion counterbalances the heat produced by the combustion of the fresh particles of fuel. The effect of combustion will therefore not be increase in temperature or pressure, but increase in actual energy exerted.
Diesel also solved another important problem that still limits the efficiency of gasoline engines, viz., the tendency for gasoline motors to knock or ping due to "predetonation." Autoignition is precisely what diesel motors are supposed to do, albeit in a controlled way.  In a diesel motor, the air and fuel are pressurized separately and then mixed. Because diesel motors burn at hotter temperatures than gasoline engines do, they have a tendency to "burn air," forming nitrogen oxides from the normally inert N2 and O2 that make up the air we breathe. Precious metal catalysts are used to convert the nitrogen oxides back into oxygen and nitrogen.

Today, diesel motors find widespread use in nearly all commercial transportation applications: trucks, trains, ships, submarines, and, as I learned from Theo Boehm, even in aviation (BTW, did you know that aviation gasoline still has lead? Link--fine particles of lead oxide (or actually lead chloride or bromide) rain down on us everyday. Europeans use diesel motors far more commonly than we do for personal transportation.

I'm sold on diesels. I own a 2003 VW Golf Diesel (TDI) and I love it. It gets around 43 MPH on the highway and not much less in city because it's a stick. Another advantage to owning a diesel in CA is that they are exempt from smog-testing.


  1. We are on our third Jetta TDI as my wife's daily driver. This one is a 2006 model with the DSG transmission.

    The car is great, and my wife likes it enough to have bought it from the leasing company at the end of the lease.

    our highest mpg is on highway trips where we've broken the 50 mpg barrier a couple of times. The car is fun to drive, with a lot of torque and good acceleration.

    The DSG transmission is a marvel. It's actually a manual gearbox, no clutch pedal and a computer that controls the clutch. It shifts wonderfully, and increases the overall enjoyment of driving the car.

    It's too bad that other carmakers, like Honda and Toyota, that have excellent diesel engines in Europe don't sell them in cars manufactured for the US. Imagine a minivan with Honda's 40 mpg V6 diesel. Home run.

  2. Yay! another happy dieseler!
    My wife wants a new Audi diesel next year. The state government here in Sacramento hates diesels and seems to do everything in its power to prevent their greater use. I don't know who is paying them off or why, but I'd sure like to see them change their collective minds, by force of ballot box if neccesary.
    OTOH, there's so much wrong with CA state government that thinking and talking about diesels is almost a waste of time.