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July 5th, 2009

I noticed in the mid-1980s, about the time I became of legal driving age, that cars with manual transmissions get slightly better fuel economy than their automatic transmission equivalents.  I was surprised recently to find out that this wasn’t common knowledge.  And I’ve long wondered, what if we all drove stick-shift?  How much fuel would that save, given that nearly 3/4 of the vehicles sold in North America come equipped with automatic transmissions and the associated 1-2mpg penalty?

A friend recently asked me, why are automatics less efficient?  Now, automatic transmissions are one of the most mysterious components on a vehicle to me.  Inside the transmission are a collection of planetary gearsets, clutches, bands, hydraulic pumps, plates, valves, modulators, and pixie dust that make the car go.  All of this is typically heavier than the components of a manual transmission, and uses a portion of the engine output in its operation.  But the thing which enables the automatic transmission to work (besides the pixie dust) is the torque remover converter.  This is the device that provides a fluid coupling between the engine and transmission, and allows your engine to idle while you’re sitting still with your foot on the brake pedal … and which also accounts for some of the efficiency loss in automatics.

A proper manual gearbox is much more straightforward:  You have a clutch which engages/disengages the connection between the engine and the transmission via your left leg, and gears on the input shaft (from the engine) which engage with gears on the output shaft (to your wheels, through a differential).  No pumping losses, inefficent fluid couplings, pixie dust, or other such nonsense.

Green Technology?

Green Technology?

Blurring the line between manuals and automatics, dual clutch transmissions (DCTs) have been introduced to the mass market in recent years.  VW‘s is called DSG (direct shift gearbox); Audi calls it S-tronic; BMW calls theirs DKG, abbreviating the German “doppelkupplungsgetriebe” (literally, double clutch transmission); Porsche‘s is called PDK (Porsche DoppelKupplungsgetriebe).  These are effectively manual transmissions in which a computer does the shifting for you.  (Of course, they also have a manual mode, allowing the driver to be more involved in the process.)  Note, these boxes are vastly different than the “manumatics” of the past, such as Dodge’s “autostick” and Porsche’s “tiptronic,” which are actually planetary gearset, torque converter-based automatics that pretend to let the driver be in charge.  The new gearboxes are both more efficient than traditional automatics, and often even higher performance – faster shifts – than even the best driver-actuated manuals.  This is achieved by essentially encasing two manual transmissions (one for even gears, one for odd) in a single case – hence, the “dual” nature.  By the computer anticipating and preselecting the next gear to be chosen, shift-time is dramatically reduced.

Could these be a replacement for traditional automatics?  In my opinion, YES, and I back that up with an anecdote: A friend of mine recently bought a DSG-equipped VW Jetta.  He told me it was an automatic, for which I belittled him.  Upon seeing it, I realized it was a DSG and informed him of that fact.  He had no idea, even after driving it a few thousand miles.

A recent article in Automative Engineering International (a publication of the Society of Automotive Engineers) mentions that such gearboxes have been around since the early 1980s, but have only recently become commercially viable because of advances in electronics, sensors, and computing power on-board the vehicle.  New technology has enabled better fuel efficiency AND better performance.  …Ain’t technology great?!

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