Enlightened Confusion

Wired had an article today entitled “VW’s Prius – Killing Diesel gets 62 mpg“.  Kudos to VW for more incremental improvements to not only the fuel efficiency, but also the emissions performance of their engines.

Like most Autopia articles, the story is light on facts and mostly designed to produce a storm of comments.  This time using the over the top headline to draw the partisians; one of their trademarks.  Naturally, it had the desired effect, and a mild hybrid vs. diesel flamewar was the result…not to mention lots of pageviews for Wired.

In any article like this that stresses the fuel efficiency of diesels, some commenter always insists that it’s the ultimate proof that we’re all wasteful idiots in the US because we don’t all drive diesels.  If we were smart, like the Europeons, we’ld all drive diesels.  First off, it isn’t true; they do have both diesel and petrol vehicles in Europe, although diesels are more popular than they are here.  So, it’s a fair question:  Given the higher energy potential of diesel and the the better fuel efficiency of the engines, why don’t we all drive diesels?

Well, the answer lies with crude oil refinement.  Refining the crude is essentially a distillation process where the component hydrocarbon marterials separate at different temperatures.  The shorter hydrocarbon chains like LPG and gasoline separate at lowest temperatures, the kerosine and diesel separate next, and, finally, the heavier fuel oil and tar products separate at the highest temeratures.  In the end, the average refinery gets about 20 gallons of gasoline and 9  gallons of diesel from every 42 gallon barrel of crude.

Now through processes known as cracking, some of the long hydrocarbon chain distillates can be broken into shorter chains to maximize gasoline production.  Other processes can combine shorter chains to maximize diesel production, but there are limits to what can be done with these processes.  Currently, the most diesel optimized refinery on the planet manages to turn about 42% of its crude to diesel.

So, considering that base percentages of distillates, the limits of the optimization processes, the fact that the marterial transports side of our transportation system is based on diesel, and, worldwide, the increase in demand for diesel is outstripping the demand for gasoline, I don’t think we’ll all be driving diesels any time soon.  Then again, there’s always biodiesel.