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Nashville Scene Master Stroke

Diesel power clears the air over efficiency and mileage

By Marc Stengel

DECEMBER 7, 1999:  Since I'm going to mount a soapbox anyway, I might as well select one that catches the eye as playfully as the New Beetle from Volkswagen. Regular readers of this space will recall how easily--and willingly--I was seduced by this retro-cartoon of a car when I reviewed it for the first time in March '98. For model-year 2000, there are few reasons to alter my generally glowing opinion of this safe, spacious subcompact with an uncommon sense of humor.

Perhaps the most important change is the car's increased availability, now that production is up and "me-first" consumer frenzy is down. I'm still a little grumpy about the contortionist requirements for reaching the backseats. And (this may be a first), I think headroom up front is excessive--for this 5-ft.-sixer, at least. Sometimes I feel like I'm riding along at the bottom of a pudding bowl, and the external wing mirrors floating along at forehead level both reinforce the effect and obscure my clear view of the apex of an approaching corner.

But I remain a fan of the New Beetle; would recommend it to anyone considering an affordable subcompact; and applaud VW's deft touch in transforming a counterculture icon of the hippie-dippie '60s into a granola-chic symbol of the millennial '90s. And anyway, I don't have time to wax rhapsodic any further about the New Beetle. I've got a soapbox to climb, and I need this car to give me a leg up.

I'm going to tell you a secret, and I know you're not going to believe me, so I'm going to carry on about the matter at some length: The solution to our present concerns about tailpipe emissions and fuel economy is already with us. That's right; we can drastically cut both pollutants and fuel consumption right now without incurring any new price but a reduced one at the pump. Diesel is the secret, even though too many folks reflexively and unwisely react as if it were a dirty, little one. A diesel motor like the 1.9-liter, direct-injection, turbo-charged gem available in the New Beetle (and in VW Golf and Jetta models as well) is 40 percent more efficient at making power than a comparably sized gas model. Moreover, it not only meets the most stringent emissions standards in the U.S. and Europe, it also releases 20 percent less greenhouse-warming carbon dioxide.

Oh...it's fun to drive too. The syncopated rhythm of driving a five-speed manual mated to VW's TDI will amuse and perhaps startle anyone accustomed exclusively to the more feverish temperament of our ubiquitous gas motors. All-out sporty types, of course, are already howling in protest, I know. I concede tire-scorching burnouts to them--without regret. For those of us just looking to get around inexpensively, dependably, cleanly, I can only say, "Shame on you if you don't give diesel a try."

For me, the intellectual argument is the most compelling. It derives from the unique physics of diesel combustion. Unlike gasoline, a spark plug is not required to ignite diesel fuel. Instead, it is the massive and abrupt compression of pure, clean air that superheats the combustion chamber to about 1,000 degrees Fahrenheit. At this point, a precise quantity of diesel fuel is injected as exceedingly fine mist directly into that hot air, where the mixture ignites spontaneously, thrusting the piston down its power stroke.

Here is where the inherent genius of Rudolf Diesel's 1892 invention lies: Because of the need to compress air so dramatically, there is a significant difference between the capacity of an engine cylinder when its piston is at the bottom of the stroke compared to the capacity at the top of the stroke. This is the all-important "compression ratio"; and in VW's TDI diesel, for example, the ratio is 19.5-to-1. By comparison, a New Beetle with VW's 2.0-liter gas engine employs a 10-to-1 compression ratio. Even upon cursory examination, it's obvious that a diesel motor performs more work with each "explosion" of fuel in its combustion chamber--in other words, the higher the compression ratio, the longer the power stroke of the piston. And it is this power stroke, after all, that turns the crankshaft that drives the wheels that shove the car along.

Collateral benefits accrue to Herr Diesel's master stroke of engineering: To withstand the additional stress of higher compression ratios, diesel motors are marvelously stout, durable...dependable. Because of the precise metering of fuel to be injected, diesel combustion is complete, leaving virtually no unburnt hydrocarbons to make their way out the tailpipe, as still happens even with very clean gas motors. Modern "scrubbing" technology in a diesel exhaust system now removes the carbon-black soot that appears so noxious to most people. In actual fact, this soot is amazingly benign--and a far cry less loathsome than the carbon monoxide, nitrogen compounds, and ozone that spew invisibly out the back of gasoline-powered vehicles. It is even feasible now to refine diesel fuel from natural gas, producing a nearly odorless fuel, as clear as water, that burns even more cleanly than its petroleum-derived counterpart.

Our army of commuters isn't generally moved by the intellect, of course. It's the wallet that kicks most of us where we live. So kick this idea around: Take the 14.5-gallon capacity of the New Beetle TDI's fuel tank, and multiply it by the city/highway mileage figures of 42 and 49, respectively. Looks like a mileage range of from 609 to 710.5 to me. That's roughly the distance from Chicago to Mobile--on one tank of diesel that costs less than 20 bucks at current prices. Suddenly, an issue that was once so clouded with sooty misinformation is now crystal clear: If you're not driving a diesel, you're wasting your money, mechanical efficiency, and my clean air every time you get behind the wheel.


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