The Ups and Downs of Diesel

by Edward Niedermeyer

Published: June 23rd, 2009

Autoweek figures that the “dramatic decline in diesel fuel price” is what’s causing VW and Mercedes diesel sales to increase as a percentage of total sales in recent months. But only VW’s Jetta, and M-B’s ML, GL and R Classes have diesel options to take advantage of the mini-boom. To be fair, though, not many would have predicted a year ago that Jetta diesel sales would approach 4k units per month (3,862 in May). But will it last?

According to etrucker.com, diesel prices have risen for seven straight weeks for a total increase of 43.1 cents per gallon (national average). Of course, the national average for gasoline is also up .67 cents in the last 50 days. On the other hand, a look at the good old government price tracker shows that California’s averages display the greatest price discrepancies with gas at about $3 and diesel at $2.79.

Any guesses as to the percentage of these pricy diesels being bought in the Golden State? Based on an uptick in street sightings here in Portland, OR, not all the new Jetta TDIs are moving to Cali. But if the economy (specifically real estate) is ever going to recover, isn’t this boomlet bound to bust?

Edward Niedermeyer

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Pch101 on Jul 02, 2009
in the real world TDIs get substantially better mileage than comparable gas engines. Again, those of you who tout the MPG just don't understand this at all. The MPG of gas and diesel can't be compared because the fuels are different! Diesel contains more oil, so a barrel refined to produce a higher ratio of diesel produces fewer gallons of fuel overall. Making more diesel for you takes away a disproportionately higher amount of gas from the rest of us. MPG is only relevant when comparing like fuel types (gas to gas or diesel to diesel). MPG cannot be used to compare different vehicle types. Study some chemistry, and this will become more obvious. Just because you purchase fuel by volume does not mean that it makes for a relevant scientific comparison when discussing "efficiency."

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U mad scientist on Jul 02, 2009
When will that be? And, just to be clear, are you arguing that (comparable generation) gasoline engines can overcome the inherent advantages of diesels? Absolutely. 5-10% is perhaps noteworthy, but not overwhelming. Hint - diesels are more efficient (even when measured by BSFC) and will continue to be. (BTW, note that the LHV (usable energy per mass) of diesel is only about 1% higher than gasoline.) You realize that list is not using any modern gas engine. I noted FSI for a reason. - Why should maximum power be the metric for comparing automobile engines? Because it measures how fast a car can accelerate, which is the point of an engine. - What’s most important for real-world driving is torque in the most commonly used RPM ranges. That's why they invented transmissions. BTW, in the future, try to use the relative curves instead of absolute value to avoid embarrassment. - In 2004 VW offered 3 engines in the US Jetta wagon with a 5 speed manual transmission. They specifically offer the only expensive gas engine as a performance model, and the diesel with high "mileage" green number for dumb people who don't understand density. - Finally, remember the Audi LeMans TDI engines. Whadayouknow - they get better mileage and still won! You realize they were only really competing against another diesel, right? (and just lost) If you want to discuss le man technicalities we can do that, but you need to realize this is a nuanced issue surrounding cell capacity (pit strategies) and minimum weight.

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Grunculus on Jul 02, 2009

agenthex writes, but provides no links:
You realize that list is not using any modern gas engine. I noted FSI for a reason. The 2008 Orbital engine (Revtech report) seems pretty modern to me. YMMV. A 1.2 FSI doesn't seem very comparable to a 1.9TDI PD. # Performance: 0-62mph in 16.5 secs, max speed 97mph, 50.1mpg # Tech: 1198cc triple, FWD, 64bhp, 83lb ft, 1143kg, 151g/km CO2 Perhaps you can post some better numbers and some links to support your case? If not, there's not much point in continuing. Cheers, Scott.

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U mad scientist on Jul 02, 2009

Sorry, I mistyped. The 1.2 tsi is a turbo direct injection. I got some detailed spec data on the powertrain itself at one time, but google is failing me at the moment. But for simplicity, just assume the euro cycle instead of analyzing BSFC plot, you can compare to same (or lower/slower in this case) power diesel and the economy is still well within range of my estimate. 4.2 l/km (65kw diesel) + 20% or 5.5 l/km (77kw gas) -20% density diff ~= 10% more fuel mass use for engine with 15% more power.

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