Of all the environmental boogeymen, who would think the diesel engine would be the new green machine? 


Diesels are clanking, smoking slugs fit only to toil in the slow lane, or so common wisdom has it. But the reality is quite different and has already arrived in desirable form in some very respectable showrooms.


This new reality is based on a triad of new engines, evolving post-combustion clean-up and the fuel itself. Pivotal of the three is the fuel. Thanks to new Ultra Low Sulfur Diesel requirements by the Environmental Protection Agency, the latest engines and clean-up technologies have begat clean diesels powerful enough to retire gasoline hybrids.


How so? For starters diesel is energy dense, packing more energy per pound than any other available fuel, including gasoline. Distilled in the same refineries as gasoline, diesel fuel traditionally has been high in sulfur. But whereas traditional domestic Low Sulfur Diesel is allowed 500 ppm (parts per million) sulfur, the new ULSD mandated in California, Maine, Massachusetts, New York and Vermont last October and nationwide by the EPA, is allowed 5 ppm sulfur. That’s a 97 percent reduction, just a teaspoonful in a tanker truck of fuel.


What does the EPA have to say?


Practically speaking, ULSD fuel is already the norm. The EPA calls for 100 percent ULSD in everything but offroad versions. The fuel is already in widespread use and its extra cost of four to five cents per gallon has been absorbed by consumers.


So how exactly is diesel fuel cleaner? Without sulfur, diesel burns cleaner to begin with and several new post-combustion devices can then be used to scrub the exhaust. Simplest of these is a particulate trap. That’s a screen in the exhaust pipe which catches what little soot the new crop of advanced engines produce. Oxidation converters are similar to catalytic converters on gasoline engines, but concentrate on oxides of nitrogen, the diesel’s emission bug-a-boo.


The chicken or egg situation between clean diesel fuel and the oxidation catalysts mirrors unleaded gasoline and catalytic converters in 1975. One must have the other to work.

Trump card of post-combustion clean-up is urea injection. Such systems mist an odorless, biodegradable, ammonia-based liquid into the exhaust stream downstream of the oxidation catalysts. The resulting ammonia and exhaust gas cocktail reacts with a Selective Reduction Catalyst to reduce oxides of nitrogen into water and nitrogen. SCR’s can also reduce carbon monoxide.


All of this wouldn’t matter if diesels were still weak, heavy, slow-revving engines as no one would buy them. But thanks to turbocharging, computer modeling of engine design and computer control of diesel fuel injection, modern diesels are speedy powerhouses. They are also gasoline quiet, don’t smell and never smoke. The most modern diesels are nothing like the still clattering pickup trucks popular right now; the experience is absolutely car-like.


To fully understand their power and economy advantages consider how diesel engines differ from gasoline powerplants. Where a gas engine controls its output by limiting air intake with a throttle plate and ignites its fuel with a spark, diesel engines have no throttle and control engine power and speed strictly via fuel injection. The “spark” in diesels comes from very high compression ratios; the air in the diesel combustion chamber is squeezed so tightly that when the fuel is injected directly into the cylinder it burns immediately.


Diesel power is thus controlled by the amount of fuel injected while engine speed is controlled by when the fuel is injected. For 100 years these tasks were controlled mechanically. Using modern common rail technology of high pressure pumps, this system is now computer controlled, allowing a host of power, emission and noise-reduction tricks.


One of these is high fuel injection pressures-up to 26,000 psi. This pulverizes the fuel into tiny droplets that burn more completely. Another is rapid-cycling piezoelectric injectors that not only allow minute control of the volume of fuel injected, they can pulse the fuel into the combustion chamber in a series of short bursts. The naturally raucous diesel combustion cycle is thus quelled using less fuel and forming fewer pollutants.


Cumulatively, modern diesel technology is thus inherently quieter, more powerful and fuel efficient. With no throttle restriction, higher energy content in the fuel and extreme combustion pressures, diesels typically garner 30 percent greater fuel economy than comparable gasoline engines. And less fuel burned means less greenhouse gas CO2. After a century with a dirty reputation, diesel is suddenly looking green.


Diesel power is also desirable. It’s true diesel combustion still calls for heavy internal engine parts to withstand the high cylinder pressures, and those heavy parts restrict engine rpm. But computer modeling of engine design has allowed safe weight reduction, and the modern crop of diesel passenger car engines are much lighter and faster revving.


The relatively narrow diesel powerband is occasionally accounted for with more transmission gears-seven are not unusual-and diesels produce such prodigious torque they often don’t need much gearing. The big torque is a boon during stop-and-go driving where it helps launch even heavy cars from low speeds.


Whose leading the pack?


Not surprisingly, Mercedes-Benz leads the passenger car diesel vanguard with its E320 BLUETEC currently available in 45 states. Mercedes has been dedicated to diesel passenger cars since the mid-1930s and their diesel-tech lead is considerable. They use Bluetec as an umbrella term for their suite of clean diesel technologies-oxidizing and SCR catalysts, urea injection (AdBlue in Mercedes-speak), particulate and NOx traps-and have joined with Audi/Volkswagen and their DaimlerChrysler sister company Jeep in licensing this technology.


The Mercedes E320 BLUTEC luxury sedan employs a 3.0-liter turbocharged V6 diesel rated at 208 hp at 3800 rpm and 388 lb-ft of torque from 1600 to 2400 rpm. With a zippy 0-60 performance in the mid 6 second range, the $52,325 E320 BLUETEC can set the pace in traffic and still returns 27 mpg city and 37 mpg highway EPA ratings. With a 21.10 gallon fuel tank a remarkable highway cruising range of 700 miles is possible.


Currently AdBlue urea injection is not used on the E320 BLUETEC, but maybe next year to meet the incredibly stringent 50-state Tier II bin 5 EPA emission standards. The EPA is not comfortable with emission controls requiring maintenance, and AdBlue needs its fluid filled during regular maintenance at the dealership. The EPA’s decision on urea injection is pending, but it’s thought the advantages are too compelling to not allow it.


Many other automakers plan to be going green soon! Working off their diesel-powered Le Mans win recently, Audi has revealed a whopping 500 horsepower and 738 ft-lbs of torque 6.0-liter V12s for its Q7 SUV, but expect to see a 3.0-liter diesel stateside in the Q7 next year. It will be 50-state legal with AdBlue injection. Audi also has 2.7- and 4.2-liter six and eight cylinder engines in Europe.


Volkswagen’s 2008 entry is a 2.0-liter 4-cylinder in Jetta TDI sedans; an estimated $25,000 typically equipped. Its smaller 140-horsepower engine does not require urea injection and should return an estimated 45 mpg highway.


Speaking of a Jetta, this Audi fan clearly isn’t impressed…


Sorry, that was too funny for me to skip sharing. Plus, this is one of the longer articles I’ve written to date, so there. Back to the topic at hand though.

Domestic builders should not be counted out. GM and Ford do purchase much of their existing light truck turbo-diesel engines, but they’ve at least been in the diesel market. Other European brands are also diesel savvy-over 50 percent of new car sales in Europe are diesel today-and should rapidly adapt clean diesel technology.


The bottom line is to expect a raft of clean, quiet diesel cars starting next year. Today only 5 percent of U.S. vehicle sales are diesel powered, but like the diesel, that is about to change. Then again, how long do we have before the whole industry goes electric on us???