Today's Improved Diesel Engine Technology
Diesel 101: Part 2
By Bill Siuru
If you haven’t driven a diesel powered car lately, you would be surprised at how much they have improved. Gone are clanking engines, smelly exhaust, and anemic performance.
Diesel rattle is eliminated by a combination of direct injection (DI), common rail (CR) fuel distribution, unit fuel injectors, and pilot injection. With DI, fuel is injected directly into the cylinder rather than into a small side chamber as with older indirect injection (IDI). This helps provide the fine, high-pressure mist of fuel needed to eliminate knocks and rattles. High pressure also means better fuel atomization, resulting in increased engine efficiency for more power and better fuel economy
The common rail system uses an engine-driven pump that produces extremely high pressure fuel delivered to the electrically-operated injector at each cylinder, via a single thick-walled tube – the “common rail.” Besides reducing characteristic diesel noise, the CR system can greatly increase injection pressure over the older distributor pump type of injection systems, thus injecting a far finer mist of fuel to enhance DI engine efficiency.
Diesel fuel has to be injected at very high pressures to counter the huge compression pressure in the diesel engine. The higher the pressure, the more power is produced and the cleaner the exhaust emissions. Another advancement is pilot injection, which injects a small amount of fuel prior to the main injection, resulting in a more gradual increase in combustion chamber temperature. This eliminates the diesel knocking and rattling caused by a sudden increase in temperature. All this is made possible by sophisticated electronic injection control systems.
Today’s diesel engines are usually turbocharged and most are intercooled. Turbochargers compress the air supplied to the engine, or in automotive terms, improve engine “breathing.” More air means more fuel can be combusted, leading to increased power output. Exhaust gases spin the turbocharger’s turbine at speeds of up to 150,000 rpm, which rotates an air pump that provides a “boost” of air higher than atmospheric pressure for more powerful combustion. Because the turbocharger is driven by engine exhaust, temperatures are very high. Countering this is an intercooler, either an air-to-air or water-to-air heat exchanger, that’s used to cool down the hot compressed air exiting the turbocharger. Cooler air takes up less volume, so more air can be delivered to the cylinders to produce more power.
A variety of emission control devices are used to reduce diesel exhaust emissions. These, plus ever more sophisticated electronic engine management control and fuel injection systems, have allowed diesels to meet increasingly stringent emissions rules. The most common devices include oxidation catalysts, particulate filters, exhaust gas recirculation (EGR), and selective catalytic reduction (SCR). Catalytic processes or filters reduce carbon monoxide, hydrocarbons, and particulate matter in diesel exhaust. Particulate filters, typically muffler-like devices installed in the exhaust system, physically trap particles in the engine exhaust before they can leave the tailpipe. Selective catalytic reduction uses a catalyst and a liquid reagent to reduce oxides of nitrogen (NOx) emissions. Exhaust gas recirculation recycles a portion of the exhaust back into the engine to reduce NOx emissions as well.
Many of these exhaust emission control technologies are very sulfur intolerant and have short lifetimes if subjected to the high sulfur content of diesel fuel previously sold in the U.S. Thus, ultra low sulfur diesel (USLD) fuel is required. EPA regulations now limit the sulfur content of on-highway diesel fuel to 15 parts-per-million (ppm) by weight, down from the previous 500 ppm. While the EPA mandates that 80 percent of diesel fuels sold for use by on-road trucks be ULSD, growing to 100 percent by June 2010, USLD is now available virtually everywhere. Only ULSD is available in California and Canada.
It is expected that ULSD will encourage the development, sale, and use of growing numbers of very clean diesel vehicles in the U.S., like it already has in Europe. While diesel-powered passenger vehicles represented just 3 percent of the U.S. market in 2004, J.D. Power and Associates predicts this will increase to about 11 percent by 2012.
Recognizing American’s demand for vehicles offering good fuel economy without compromising utility, cargo capacity, and performance, Mercedes-Benz is one of those leading the way by offering several diesel models. The E320 CDI, which has been offered for several years, has been replaced by the E320 BLUETEC luxury sedan. The E320 BLUETEC is the only diesel-powered luxury sedan available in the U.S. that can deliver an estimated 780 miles on a tank of fuel. In addition, there is the new ML320 CDI, R320 CDI, and GL320 CDI – all SUVs. As of now, these four diesel models are not available in California, Maine, Maryland, New York, and Vermont because they do not pass the more stringent emission requirements in these five states. 
What would make it possible for Mercedes-Benz, or for that matter any automaker, to offer a 50-state diesel? While other approaches are under development, an immediate one is Mercedes-Benz’ BLUETECH technology, which includes a Selective Catalytic Reduction system with a urea-based injection system, rather than the DeNOx catalytic converter used now on U.S.-shipped cars. This approach meets very strict Euro-5 requirements for cleaning up nitrogen oxides but does require an aqueous urea solution. When AdBlue – an odorless, non-toxic solution stored in an onboard tank – is injected into the pre-scrubbed exhaust, ammonia is released. This causes NOx to be reduced to harmless nitrogen and water in the downstream SCR catalytic converter. Injection is controlled by the engine management system.
If the AdBlue system was used on U.S. cars, they could probably meet emission standards in all fifty states. However, this system does require carrying another fluid aboard vehicles to work and would require EPA approval. DaimlerChrysler has lobbied the EPA to get them to accept urea injection systems in the U.S. The company says since an average of one gallon of AdBlue is needed for about 2,300 miles of driving, AdBlue tank can be refilled during scheduled maintenance, taking it out of the hands of vehicle owners, EPA’s big objection. In Europe, there are now around 1,500 public-access AdBlue refueling sites, ranging from the Arctic Circle to southern Spain and from Ireland to Moscow, with several producers supplying AdBlue.
DaimlerChrysler’s Jeep unit also offer the Jeep Grand Cherokee CRD that uses a Mercedes-Benz-built 3.0 liter diesel engine, again only a 45-state vehicle. Jeep has also shown a BLUETECH version that could be a 50-state vehicle, if it reaches production.
It already offers a diesel version of the Chrysler 300 in Europe.
Volkswagen long sold TDI versions of its Golf, Jetta and new Beetle, but ceased sales because they didn’t meet the more stringent emission requirements that started in 2006. In all, VW has sold some 800,000 diesel cars in the U.S. Now, this automaker plans a return in 2008 with 50-state diesels using BLUETECH and AdBlue to meet the required Tier 2 Bin 5 / CA LEV II emissions standards. VW already offers a Toareg SUV with a 5 liter, V-10 diesel engine. Audi, the third member of a joint BLUETECH project with DaimlerChrysler and VW, plans to sell a Q7 SUV here with a V-12 diesel engine.
With diesel sales running at the 50 percent mark in Europe, every major European automaker has several diesels in their catalog and are well-poised to sell diesel models in the U.S. For example, BMW plans on offering one or more vehicles with a 50-state compliant, 3.0 liter twin turbo diesel engine in 2008. Jaguar, Saab, and Opel all have diesel versions, and several of these engines are the basis for some Saturn models. Ford and GM say they foresee diesels in their U.S. offerings and do have European models that could be sold here, if diesel sales really take off. Likewise, Asian manufacturers like Nissan, Toyota, Honda and Hyundai, which offer diesels in other markets, are planning to offer them here, too. Aimed at the 2009-2010 timeframe, they should be 50-state compliant.
The Department of Energy estimates that a 30 percent market penetration of light-duty diesel vehicles by 2020 would reduce U.S. net crude oil imports by 350,000 barrels per day. In California alone, gradually increasing the use of currently-available clean diesel technology in cars, pickups, and SUVs to levels seen today in Europe could save the state 110 million gallons of gasoline per year by 2010, and up to 840 million gallons per year by 2030.
Diesel is on the rise in America: The forces holding it back – namely long-held stigmas and emissions concerns – are quickly being overcome by technology and now, of course, cleaner diesel fuel.
A new generation of clean diesel vehicles selling in large numbers opens the door for biodiesel to play a bigger role as well. This clean-burning fuel is derived from domestically produced agricultural products and runs in a diesel engine with little or no modifications, reducing both emissions and energy dependence.
Plus, diesels could factor favorably into the future of the popular hybrid as well. Just imagine the fuel economy that could be achieved by combining the fuel-saving benefits of hybrid-electric technology with an inherently efficient diesel engine. For example, DaimlerChrysler has developed a “mild hybrid” that combines the 3-liter BLUETEC V-6 diesel with a high-torque electric motor. With all this potential, we expect to be hearing a lot more about diesel in the years ahead.
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