Clean Diesel vs. Gasoline: Introduction
Clean Diesel vs. Gasoline: Engines
Clean Diesel vs. Gasoline: Fuels/Emissions
Clean Diesel vs. Gasoline: True Costs
Clean Diesel vs. Gasoline: Summary

Clean Diesel vs. Gasoline: Introduction

Kudos to technology and good ol’ creative thinking. Without you, we’d live in a world devoid of pairings like vanilla ice cream and chocolate chip cookie dough, or cell phones that allow us to access our email or browse the hardware store’s assortment of hammers. There are countless such examples of matches painfully obvious with the benefit of hindsight, but on the flip side are marriages that will be forever puzzling. That’s especially true with words. In fact, until recently the words “clean” and “diesel” were not normally paired together. That changed with the debut of a new generation of so-called oil burners, including Volkswagen’s Jetta TDI, Jetta SportWagen TDI, and Touareg 2 TDI. Not only do these models run on cleaner fuel, they also deliver more torque and superior fuel economy* while providing what many classify as a fun driving experience. Best of all, VW’s TDI engine-equipped vehicles meet the same strict emissions standards set for gasoline-powered vehicles.

Clean Diesel vs. Gasoline: Engines

Drive down any U.S. highway and you’re apt to encounter more diesel engines than you can shake a sustainably-harvested stick at, but chances are the vast majority will be powering big rigs delivering goods. In contrast, diesel-powered passenger cars have accounted for only 3.6 percent of all vehicles registered in the U.S.,** though that just might change with the nationwide availability of clean diesel fuel and Volkswagen’s TDI models.

Today’s diesel engine can trace its roots back to the early 1890s, at which time Rudolf Diesel engineered and patented a more efficient version of the gasoline-powered internal combustion engine (ICE). Both diesel and gasoline engines are technically ICEs, meaning their inner workings consist of a crankshaft, connecting rods, pistons, and intake and exhaust valves. And both require the introduction of a volatile fuel to operate, but that’s where the similarities end. Inside a gas engine, a precise air/fuel mixture is pumped directly into a cylinder, at which time a spark plug ignites the mixture, causing a mini explosion that creates power by pushing down on the piston and rotating the crankshaft.

A diesel engine, on the other hand, lacks spark plugs, a distributor, and an electronic ignition system. Instead, air is pumped into the cylinder, compressed to a much greater degree, and as a result, gets very hot. When the hot air is at maximum compression, fuel is sprayed into the cylinder, causing a mini explosion without the need for a spark.

That high-compression detonation is what caused the clatter sound often associated with old diesels, used in everything from Mercedes-Benz sedans to the Volkswagen Dasher and boasting not-so-memorable statistics like 0-60 mph runs of more than 15 seconds. Modern engine designs have eliminated much of the clatter, and the introduction of turbochargers offered a big boost in terms of performance. But diesels still must deal with tremendous internal pressure relative to their gas competitors. As an example, the Volkswagen Jetta uses a cast iron engine block and alloy cylinder heads in gas and TDI models.

With that in mind, diesels are typically built with stronger blocks and parts, but in the case of the Jetta TDI, engineers were able to keep the car’s overall curb weight in line with the gas-powered variant. Thanks to the turbocharger, Volkswagen’s 2.0-liter four-cylinder TDI engine generates a respectable 140 horsepower at 4,000 rpm and 236 lb.-ft. of lively torque at only 1,750 rpm. Compare that to the Toyota Corolla’s 1.8-liter four-cylinder engine with 132 horsepower and 128 lb.-ft. of torque, and the Honda Civic’s 1.8-liter four-cylinder that’s good for 140 horses and 128 lb.-ft. of twist.

One final issue focuses on difficult starts in cold weather, a result of a need for hot air in the diesel’s combustion process. This situation has been addressed by direct injection and the use of advanced engine computers.

Clean Diesel vs. Gasoline: Fuels/Emissions

In technical terms, diesel fuel has an energy density of about 147,000 BTUs; gasoline has an energy density of 125,000 BTUs. In other words, it takes less diesel fuel to do the same work as a gallon of gas. Combine that with a diesel engine’s design, and you’ve got an engine/fuel combination that is 30-35 percent more efficient than its gasoline-powered competition.

But how is diesel “clean”? We all remember the black clouds of smoke and noxious fumes. Gasoline and diesel fuel are both derived from crude oil, but diesel was subjected to less refining and as a result had a heavier odor and a dark color that resulted in black clouds of sooty exhaust.

But that was then, and the result was emission levels that were not accepted in all states.
Things started to change in 2007 when all new diesel vehicles sold in the United States were required to run on ultra-low sulfur diesel (ULSD) fuel. ULSD has a sulfur content of 15 parts per million (ppm), compared to 500 parts per million in regular diesel fuel, which will be completely erased from the U.S. marketplace by the end of 2010. Much of the dirty, black smoke that many people associate with diesel exhaust is due to rich sulfur content; with more refined ULSD, less sulfur leads to cleaner emissions.

In the case of the Jetta TDI and Jetta SportWagen TDI, ULSD works in concert with a particulate filter in the exhaust system, creating a clean diesel car. As the name implies, the filter traps harmful particulates, including nitrogen oxide and hydrocarbons. As more particulates accumulate, back pressure builds in the exhaust to the point where a sensor triggers more fuel to be injected into the system, which initiates the burning of the particulates and creates cleaner emissions.

The Touareg TDI, on the other hand, uses AdBlue® technology. In this case, urea is blended with the exhaust to neutralize much of the noxious emissions. Thanks to the cleaner fuel and advanced emissions/exhaust systems, diesel-powered vehicles like the Jetta TDI and Touareg TDI meet strict Bin 5 emissions ratings, and qualify for a lean burn technology tax credit of up to $1,300, depending on the model.*** And, as an added bonus, TDI drivers can fill their tanks with either 100 percent ULSD or B5 (a blend consisting of five percent biodiesel and 95 percent ULSD) and still be fully covered by the Volkswagen’s new car limited warranty.

Clean Diesel vs. Gasoline: True Costs

Sometimes, the cheap way isn’t always the right way, and there’s no denying that new clean diesel models can be pricier than their gas competitors. That premium covers the added cost of designing and building engines with beefier internal parts, more precise tolerances, and increasingly complex emissions systems.

But not so fast: there are a number of additional important factors to consider, many that shed positive light on Volkswagen’s clean diesel models. Consider that a diesel engine will never need a new distributor or replacement spark plugs and wires, will offer a fuel economy advantage of 30-35 percent on average, and will typically outlast its gas counterpart by a wide margin. Factor in available federal tax incentives*** plus the fact that Volkswagen’s TDI models follow the same service interval as their non-TDI siblings, and the case for diesel starts to look a little stronger. Falling diesel prices aren’t anything to complain about either.

Serving to improve the situation yet again is Volkswagen’s Carefree Maintenance Program,† under which scheduled maintenance is covered by the manufacturer for the first three years or 36,000 miles of ownership.

Of special interest to Jetta TDI owners will be the particulate filter in the exhaust system that transforms hydrocarbons into cleaner emissions (including water and nitrogen). This diesel-specific part needs to be replaced at 120,000 miles. The Touareg 2 TDI’s AdBlue urea technology requires a fill-up every 6,000-10,000 miles, which is initially covered by the no-charge scheduled maintenance program during the first three years or 36,000 miles (whichever occurs first).†

Clean Diesel vs. Gasoline: Summary

For some, the image of an old diesel firing off black smoke will be a hard one to forget. But the fact is the days of dirty diesels are gone, and considering incentives and maintenance costs, clean diesel technology is much more affordable than most people think.

*Jetta TDI EPA estimates 30 mpg city / 41 mpg highway. Your mileage will vary.

**Percent of diesel passenger vehicles registered in 2005. Source: Dieselforum.org

***Tax credit submitted under Notice 2006-9 and Internal Revenue Code, Section 30B relative to the 2009 Volkswagen Jetta 2.0L TDI Sedan and SportWagen and Touareg 2 TDI. Available to taxpayers who purchased a qualified vehicle on or before December 31, 2010. Leased vehicles are not eligible for the tax credit. Only a limited number of full tax credits are available. Please consult a tax professional for more information or go to http://www.irs.gov.

†The Volkswagen Carefree Maintenance Program covers the vehicle’s 10K, 20K and 30K scheduled maintenance intervals except for the Volkswagen Routan. The Routan Carefree Maintenance Program covers the vehicle’s 6K, 12K, 18K, 24K, 30K and 36K scheduled maintenance intervals. Coverage is during the term of the new vehicle limited warranty, at no additional charge. The program covers three years or 36,000 miles, whichever comes first. See dealer or vehicle maintenance program booklet for details. Does not include routine wear and tear on parts such as brakes, tires, wiper blades, light bulbs, etc.