The Ongoing Golf TDi Diesel debate

9 05 2005

This past week I drove my nifty Golf TDI down to NJ for a quick visit to a client. It was a great drive, but it gave me time to think about the moral arguments in my head about my car.
On the one hand, it can’t be denied that getting 42-43mpg nowadays is a good thing. Diesel prices in Boston run about $2.49 a gallon (though down in NJ it was down to $2.15. What’s up with that?). I can run almost 600 miles on a single tank of gas, and the car is _FUN_ to drive. This was one of rare long distance drives where I could really wind it out and have fun with it (go go gadget Merritt Parkway, a road that is high on my list of East Coast Speedways.
On the other hand, there are questions about the cleanliness of the Turbo Diesel Injected, or ‘TDI’ engine.
It goes without saying that the TDI engine in the current model Golf is vastly superior to the old Diesel engines that went into the VW Rabbits in the 70’s. They’re far cleaner and more efficient. You don’t see current generation TDI’s with black soot all over the back of the car. But California, New York, and Massachusetts have forced VW to withdraw the TDI because of questions about the emissions.
I did a little digging trying to find out the reason behind this. If I’m driving a car that is dirtier than a standard gasoline engine, I’m really not gaining much…
The folks over a TDI Club have one of the most detailed and complete FAQ’s about these engines (heck, about ANY topic) that I’ve ever seen.
The relevant section is in the part about emissions…

TDI vs. gasoline – The TDi emissions levels are among the lowest ever for Diesel powered engines. All TDi powered Volkswagens sold in the US meet so-called “Tier 1” emission limits. The TDi is often “cleaner” overall than gasoline powered cars. CO2 emissions are 25% less than a conventional gasoline powered engine. CO, HC and NOx emissions are less than previous Volkswagen Diesels. Diesel fuel has lower evaporative emissions than gasoline. Diesel fuel also requires less energy intensive refining than gasoline.

Diesel engines generally emit higher amounts of NOx and particles than equivalent gasoline powered cars, even though CO and HC emissions may be lower, and total emissions are lower due to much better fuel consumption. The current TDI Volkswagens typically emit slightly somewhat lower than the Tier 1 limits for NOx and particles (around 0.052 g/mi of particulate matter [PM] and 0.82 g/mi of NOx per EPA data), but the CO and HC emissions are far below the Tier 1 limits and well below the emissions of the equivalent gasoline engine.

Further down, there’s the section specifically about why California and NY (and now Massachusetts) have withdrawn these engines:

California and New York issues – The state of California places limits on the “fleet average” emissions of auto manufacturers. Currently, a manufacturer can only sell a certain proportion of “Tier 1” vehicles in relation to the number of “LEV” or low-emission vehicles. Certain other states have copied the California legislation. Volkswagen has stated that the withdrawal from sale of 2000-model TDI vehicles from certain states is due to these reasons. Furthermore, California has declared diesel exhaust to be a toxic air contaminant, although other studies dispute this conclusion. The situation in New York is the same.

My interpretation of this document (and it’s long, and quite detailed), is that the TDI engine is in fact more efficient on all fronts than a gasoline engine. MPG for MPG, it is dirtier in particulates, but because of it’s uncommonly high mileage, it is still cleaner than a standard-mileage car.
Compared to ultra-efficient gas-hybrid engines (such as those in the Toyota Prius) it appears that the TDI still puts out less of the more gruesome chemicals (Formaldehyde and Benzine – in Benzine’s case, by an order of magnitude), but can be considered ‘dirtier’ due to particulate emissions.
I’m also intrigued by this comment:

The emission levels from diesel engines tend to remain more-or-less constant throughout the useful life of the engine, whereas gasoline engines have many more emission-related components which deteriorate and lead to higher and higher emissions as the engine gets older.

So for me, I think I can keep driving my TDI with a clear conscience. It is a high performing, very clean engine that gets great mileage. It is not the ultra-clean engine that a pure electric or Hydrogen based vehicle may be, but it is available now, the price is good on it, it has 4 doors and is very zippy to drive. I think I’ll keep it.




3 responses

9 05 2005

At a gues, the cost is cheaper in New Jersey because there are refineries there.

9 05 2005

Dunno if it applies equally for diesel, but state taxes would account for about 0.07/gallon difference ‘tween MA and NJ.

9 05 2005

MPG per MPG doesn’t seem like an effective standard for comparison – as I understood it, the accepted “objective standard” is “emissions per mile travelled” regardless of fuel economy. Or, in some cases, per passenger-mile, which lets busses get away with those nasty old-school diesels.
Emissions is (at least theoretically, I imagine there’s a correlation, but I bet it’s far from perfect) not on the same axis as fuel economy. Heck, doesn’t fuel economy improve slightly when you take the catalytic converter out of a car?

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