Like kudzu, zebra mussels, and purple loose-strife, diesel engines are poised to invade the American landscape. The environment is certainly conducive. Federal CAFE regulations and California's threatened CO2 restrictions demand a quantum leap in fuel economy of the sort that diesel promises. Rising oil prices are driving consumer demand for improved thrift, and the Green groundswell is encouraging renewable fuels like biodiesel. Over a dozen brands have announced plans to sell diesels here, but astronomical diesel-fuel prices and pending SULEV emissions standards are formidable threats. On these pages, we'll share what we know about the individual species populating this diesel invasion and examine the technology's fitness for survival.

Modern diesel engines have a lot going for them. The fuel packs about 11 percent more energy per gallon than gasoline. Compression-ignition combustion has inherent thermal-efficiency advantages due to its higher-expansion ratio, lean and unthrottled operation, and natural affinity for turbocharging. Diesels can't rev as fast as gas engines so their peak-power figures are usually lower, but their torque curves resemble Ayers Rock, so they pull like Clydesdales. To withstand higher combustion pressures, diesels are built stronger to last longer than gasoline engines.

On the downside, the heavy-duty engine construction, ultra-high-pressure direct-fuel-injection systems, and the emissions gear that's now required to scrub exhaust clean of the NOx and soot that diesels inherently produce can add nearly as much cost to a vehicle as an electric hybrid system does. (For now, ammonia carried on board in urea tanks or generated by a two-level catalyst from excess diesel fuel neutralizes NOx, and a particulate trap grabs the soot and burns it off.)

Maintaining the fuel and emissions-controls systems erodes the operating-cost advantage diesels once boasted, and the fuel has a serious image problem to overcome. Only about half of all filling stations sell it, and suburban commuters don't like tanking up at truck stops. It's slimy and smelly if you get any on you, and the American public still remembers the bad old pokey diesels that smoked and stank and sounded like a blender full of ball bearings. Test drives of modern diesels will cure most of the reputation issues one customer at a time.

Perhaps the toughest hurdle diesel faces at the moment is the alarming fuel-price discrepancy relative to gasoline. For years, diesel cost less than gasoline except during cold winters when demand for the closely related home-heating oil caused price spikes. But diesel has usually cost more since September 2004, and, in the first quarter of 2008, the discrepancy has shot up to historic highs, averaging as much as 20 percent (70 per gallon) above gasoline. That erodes the economic argument for diesel, pushing the "payback" mileage well into six figures for many vehicles, at least at current diesel prices.


There are several reasons for high diesel costs. Crude oil accounts for about half the per-gallon cost, so the recent global price spike contributes heavily. The Feds tax diesel $0.06/gallon higher than gasoline, which is only fair considering the greater wear and tear heavy diesel trucks exact on our roads. The long-overdue transition to low-sulfur diesel fuel in this country has increased production and distribution costs. But the longer-term problem is increasing worldwide demand. China and Europe are straining already tight global refining capacity, and the infrastructure has limited flexibility to shift between gasoline and diesel production. That's why Europe exports surplus gasoline to the U.S. and we export diesel to Europe. Changing the infrastructure to grossly increase diesel production per barrel would require switching from the well-established fluidized-cracking and coking processes that yield today's mix of hydrocarbon products to more gasification and steam reformation of hydrogen for use in diesel production. Why don't the fat-cat oil barons simply spend some of their obscene profits on new refineries? Because nobody wants one in his own backyard, so obtaining permission to build is difficult, even with Big Oil's lobbying budget. This means radically increasing the diesel fleet will drive near-term diesel costs even higher.

Ah, but can't we just make up the difference with biodiesel? According to the National Biodiesel Board's Jennifer Weaver, the U.S. currently has capacity to produce two billion gallons annually-enough to satisfy the renewable fuels requirement in the recent Energy Bill. Last year's production was 500 million gallons (nearly twice that of 2006), 80 percent of which came from soybeans. She also asserts that soy fuels don't stress the food supply the way corn ethanol does because the meal extracted from milling the beans still enters the food chain, and that if not for biodiesel production, much of the oil would go to waste. But biodiesel can account for only 20 percent of diesel use because it needs to be blended with petro-diesel in that concentration or less, due to its different lubricity, cetane rating, and oxidation properties. Above that, driveability problems, especially in cold weather, and fuel-aging issues crop up (it breaks down faster than petro-diesel with age or heating).

Next-generation renewable diesel fuels could change that (see "The Biodiesel Horizon" sidebar). Widely varying fuel quality has also stunted biodiesel's sales growth. General Motors markets a biodiesel fleet-use package that hardens the entire fuel system to withstand 20-percent biodiesel (B20 otherwise causes seals to swell and leak), and according to Bob Straub, GM's diesel fuel-systems specialist, many engine problems in the field have been attributable to poor-quality biodiesel sold through mainstream retailers (not fry-oil home-brewers). As you read this, standards defining the physical characteristics of B20 are (it's hoped) being agreed upon, and producers and marketers are earning BQ9000 certification for adhering to fuel-quality standards. This should enable manufacturers to design and certify vehicles to run on B20, earning themselves CAFE credits and (presumably) increasing biodiesel demand. But beware, strong demand in some regions has led to rainforest clear-cutting to grow oil-rich palm crops-an unintended consequence of the best Green intentions.

So what's our fearless prediction for diesel's prospects in this country? Technology will keep pace with emissions regulation. Big pickups and SUVs with serious work to do will be powered by diesels. When they're loaded and straining, diesels slaughter gasoline in terms of efficiency, so users in that category will certainly enjoy lower operating costs. But light-duty spark-ignition combustion efficiency will improve faster than diesel-fuel prices will fall, so economics will make gasoline/E85 and hybrid drivetrains more attractive in the mainstream passenger-car realm. But we're rooting for algae biofuels and next-gen renewable diesel to prove us wrong.

The Biodiesel Horizon
Oil derived from soy, rapeseed, palm, etc., must undergo "transesterification" (a chemical reaction with an alcohol and an acid or base catalyst) to make it more stable, freer flowing, and more like petro-diesel. The next generation of renewable diesel fuels may more closely resemble petro-diesel without further processing. Some even claim to lower engine emissions.

Pyrolysis or thermal depolymerization.
Take any long-chain hydrocarbon material -- poultry and swine offal, old tires, plastic scrap, etc. -- pressurize and superheat it with no oxygen, flash it to atmospheric pressure to evaporate water, filter out solids, and reheat the resulting liquid to get a light crude oil that can be further refined. Changing World Technologies has a plant up and running, and current oil pricing is making this approach feasible.

Catalytic Hydrotreating.
Various different catalysts react with hydrogen to remove sulfur, nitrogen compounds, and gum from hydrocarbon fuels today, and similar reactions can be used to convert vegetable oil or animal lipids into fuels that gel as low as -22 degrees F. Finnish Neste Oil has a 53-million-gallon/year facility up and running, and similar plants from various companies are expected to open in Italy, Singapore, and the U.S.

Biomass to liquid.
This concept superheats carbon-rich bio-waste and cellulosic materials in an oxygen-deprived environment to form syngas (hydrogen and carbon-monoxide), then employs a Fischer-Tropsch catalyst (usually iron or cobalt) to hydrogenate the CO yielding a diesel-like liquid hydrocarbon in the same way coal or natural gas can be converted to liquid fuels. A pilot plant producing Shell SunDiesel from wood chips is in operation.

Some of the above processes are attracting oil-company investment as a means of getting a piece of the renewable-fuels tax-credit pie using equipment and processes similar to those involved in oil refining. With oil money opening doors, expect these concepts to flourish in the long run. But don't count out 100-percent solar diesel: Solazyme claims to have bred a strain of algae that can convert CO2, water, and sunshine into a hydrocarbon that meets the proposed biodiesel standard without significant processing at a promised yield of thousands of gallons per acre of clear vertical tubes. That could radically alter the diesel landscape.

Veggie Might
The purest biofuel is straight vegetable oil. Rudolph Diesel ran his first engine on peanut oil, and if biodiesel isn't green enough to suit you (the methanol used to produce it is usually fossil-fuel-derived), you can easily find someone to convert your old diesel vehicle -- in most cases, an older Mercedes-Benz -- to run on this thicker, more viscous fuel. But there are issues to consider. This oil has to be warmed to flow properly, so you have to keep a petro-diesel tank for the engine to warm up on before switching to the heated French-fry-smelling stuff. You also have to be sure to switch back to diesel before shutting down to clear the lines of veggie oil. The oil must be exceptionally clean and free of water or you'll damage even the oldest fuel-injection systems (don't even think of running veggie oil through a modern diesel). Yes, some people get the stuff free of charge, but casting around McDonald's parking lots after hours and then filtering the oil is a greasy job. And keep a low profile or you might get hassled to pay the road taxes you're skipping out on as veggie oil isn't officially approved as a motor fuel by the EPA.

Evolving Emissions Controls
The first 50-state-compliant light-duty clean diesels employ a catalytic reaction with ammonia (NH3) to convert nitrous-oxides into harmless nitrogen. Smaller, light-duty diesels like the forthcoming Honda 2.2-liter will convert diesel fuel into ammonia in a two-stage catalyst, at a slight fuel-economy penalty of perhaps five percent. Most others generate ammonia from an on-board urea/water solution that must be refilled at oil-change intervals. The EPA will require vehicles using these selective catalytic reduction (SCR) systems to become inoperable if their urea tanks run dry.

The next generation of diesels may reduce NOx production enough inside the engine to avoid the need for ammonia reduction. Toyota is developing a smokeless rich diesel combustion (SRDC) system that drastically lowers the combustion temperature by running a high percentage of exhaust-gas recirculation (over 50 percent) and a bit of excess fuel. Nissan is at work on a SULEV-compliant (70-percent-less NOx than today) Modulated-Kinetic (MK) system that combines roughly 30-percent-cooled EGR with greatly increased swirl and late fuel injection (perhaps three degrees after top-dead-center, instead of seven degrees before). Both of these combustion methods work only during light-load operation, but excess fuel will be collected by a hydrocarbon/NOx trap catalyst, then converted to hydrogen and carbon- monoxide to reduce the NOx produced under high-load operating conditions.

Diesel's fuel-economy payoff
These two surfaces indicate the payoff mileage when opting for a diesel engine over a gasoline engine that achieves 16 mpg (average for the vehicles on sale now offering both options). Assuming a gasoline price of $3.49 (the national average at press time), the orange surface represents the worst-case diesel price 70 higher, the green surface indicates the best case at 40 under gasoline (reflecting the range experienced over the last year). Our "Miles to Payoff" engine stats on the next three pages presume these same pricing extremes.


Diesel Buyer's Guide

In dealerships now:


Dodge/Cummins 6.7L
Application: Ram Heavy Duty pickups
Stats: 6.7L/350-hp/650-lb-ft single-turbo OHV 24-valve I-6
Tech trivia: A diesel particulate filter and a NOx absorber catalyst make this the first heavy-duty diesel to meet 2010 emissions.
Option price: $7,095
Fuel-economy benefit: 50% (est*)
Miles to payoff: 50-102K (est*)
*Not rated by EPA, estimate assumes heavy duty towing/hauling operation 178.7 x 69.3 x 57.7 in


Ford PowerStroke
Application: Heavy-Duty F-Series pickups & E-Series vans
Stats: 6.4L/350-hp/650-lb-ft twin-turbo OHV, 32-valve V-8
Tech trivia: To combat turbo lag, a small variable-geometry turbo provides launch torque, then a larger fixed-geometry turbo takes over, providing 42 psi of peak boost.
Option price: $7245-$7670
Fuel-economy benefit: 50% (est*)
Miles to payoff: 51-110K (est*)
*Not rated by EPA, estimate assumes heavy duty towing/hauling operation


GM/Isuzu Duramax
Application: Chevy Silverado/Express, GMC Sierra/Savanna heavy-duty pickups/vans
Stats: 6.6L/365-hp/660-lb-ft single-turbo OHV 32-valve V-8
Tech trivia: Single variable-geometry turbo huffs up best-in-class power and torque.
Option price: $7195-$7795
Fuel-economy benefit: 50% (est*)
Miles to payoff: 50-111K (est*)
*Not rated by EPA, estimate assumes heavy duty towing/hauling operation


Mercedes-Benz 3.0L
Application: Dodge Sprinter; Jeep Grand Cherokee 3.0 CDI, Mercedes-Benz GL320 CDI, ML320 CDI, R320 CDI, E320 BlueTec;
Stats: 3.0L/154-215-hp/280-398-lb-ft single-turbo DOHC 24-valve V-6
Tech trivia: A 72-degree-bank angle and aluminum-block trim weight and optimize packaging.
Option price: $1000-$2010
Fuel-economy benefit: 23-26%
Miles to payoff: 15-188K


Volkswagen V10 TDI
Application: Touareg
Stats: 5.0L/310-hp/553-lb-ft SOHC 20-valve twin-turbo V-10
Tech trivia: Pumpe Duse cam-driven unit injectors squirt at 30,000 psi; aluminum block features plasma-deposited steel-molybdenum wear coating and cast-iron lower bedplate into which long bolts fasten the cylinder heads
Option price: $13,800
Fuel-economy benefit: 21%
Miles to payoff: 210K-never


Expected in 2009


Mercedes-Benz 3.0L BlueTEC
Application: Mercedes-Benz GL320, ML320, & R320 BlueTEC
Stats: 3.0L/208-210-hp/398-400-lb-ft single-turbo DOHC 24-valve V-6
Tech trivia: Enough AdBlue (aqueous urea) is carried onboard to last 15,000 miles-that's seven gallons in all but GL320 BlueTEC, which carries 8.5 gallons.
Option price: $1000 (est)
Fuel-economy benefit: 23-34% (est)
Miles to payoff: 17-188K


VW/Audi 3.0 TDI
Application: Audi Q7/Volkswagen Touareg
Stats: 3.0L/221-hp/406-lb-ft single-turbo DOHC 24-valve V-6
Tech trivia: Cylinder-pressure sensors and pricey NOx sensors help Audi claim this is the world's cleanest diesel.
Option price: $5000 (est)
Fuel-economy benefit: 56% (est)*
Miles to payoff: 55-105K (est)*
*Relative to base V-6 gasoline models

Expected in 2010


GM Duramax 4.5L
Application: Chevy Silverado/GMC Sierra pickups, Hummer H2 (later)
Stats: 4.5L/310-hp (est)/520-lb-ft single-turbo DOHC 32-valve V-8
Tech trivia: 72-degree block, exhaust/turbos package in the valley, intake runners cast in valve covers, water-jacketed heads, cam driven off balance shaft.


Mahindra mHawk
Application: Scorpio SUV, Appalachian pickup
Stats: 2.2L/145-150-hp (est)/300-lb-ft single-turbo
Tech trivia: Bosch and Austrian powertrain specialists AVL developed state-of-the-art piezo-injection and urea exhaust aftertreatment systems. A diesel-electric hybrid is expected to follow.


In the Pipeline:
Dodge Ram 4.5L V-8 developed with Cummins
Ford F-150 4.4L V-8 developed from Rover 3.6L
Honda Pilot/Ridgeline V-6
Hyundai Veracruz and/or Santa Fe, 3.0L V-6 developed with SeohanWarner
Nissan Maxima 3.0-liter V-6 developed with Renault
Nissan Titan V-8 developed with Cummins
Subaru Forester/Impreza 2.0L flat-4
Toyota Tundra/Sequoia V-8

Deja Dieselmania

We've been here, done this. In August 1979, we devoted 25 pages to chronicling a diesel revolution that was going to ease the acute pain we were all feeling at the pump. VW, Mercedes-Benz, and Peugeot cashed in with their Euro diesels, and GM hastily dieselized a couple V-8s with disastrous results. Tightening emissions regs, falling gas prices, and the horrendous sound, smell, and sloth of those "thundering turtles" brought a speedy end to that revolution. Here are some highlights of our eight-car comparison roundup:

Most power/torque: Cadillac Eldorado (Olds-built) 350-cu-in V-8, 125 hp/225 lb-ft
Least power/torque: VW Rabbit/Dasher 1.5L I-4, 48 hp/57 lb-ft
Quickest: Mercedes-Benz 300SD 3.0L I-5 turbo, 0-60: 15.1 sec; quarter mile: 20.0 sec @ 70.0 mph
Slowest: Mercedes-Benz 240D 2.4L I-4, 0-60: 27.4 sec; quarter mile: 25.6 sec @ 56.2 mph
Best economy, EPA city/MT observed: VW Rabbit, 41/41.8 mpg
Worst economy, EPA city/MT observed: Cadillac Eldorado, 21/21.6 mpg





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