Diesels have always been more fuel-efficient than gasoline engines, thanks to their high compression ratios and ability to run very lean fuel mixtures. Diesel fuel also contains more heat energy per gallon than gasoline (about 138,700 BTUs versus 125,000 BTUs for gasoline), but the heavier hydrocarbons in the fuel tend to produce more HC and soot in the exhaust. Higher combustion temperatures in a diesel also produce more oxides of nitrogen (NOx). These factors, combined with a traditional diesel engine’s noisy piston clatter, exhaust odor, hard cold starting characteristics and sluggish performance, have limited their use in passenger cars — until recently.

A number of factors have combined in recent years to create a bright future for the latest generation of Clean Diesel cars and trucks. Common rail high-pressure, direct-injection systems with sophisticated electronic controls have transformed today’s diesels into clean-running, quiet and powerful powerplants more than capable of satisfying the most demanding drivers. Today’s diesel-powered import cars and SUVs drive and feel no different than their gasoline-powered counterparts, while delivering 20% to 30% better fuel economy.

Another force that’s driving a move toward more diesels is new government regulations, which, in the past, have actually hindered diesel sales. The government wants all automakers (import and domestic) to significantly increase their Corporate Average Fuel Economy (CAFE) numbers over the next four years.

In 2011, the average combined fuel economy for passenger cars and light trucks was 27.3 mpg, a number that has changed little over the previous decade. The actual breakdown was 30.2 mpg for cars and 24.1 mpg for trucks. The new regulations require automakers to achieve an average fuel economy of 39 mpg for cars by 2016, and 30 mpg for trucks, with a combined overall average of 35.5 mpg.

Import vehicle manufacturers are currently taking two different approaches to meeting the new government fuel economy standards. Many of the European automakers that sell diesel-powered cars in their home markets are offering diesels here (Audi, BMW, Mercedes-Benz and VW). They see a booming market for Clean Diesel technology in the U.S.

Yet many Asian car makers who sell diesels in ­Europe (Honda, Hyundai, Mazda, Mitsubishi, ­Nissan, Subaru and Toyota) currently do not offer any diesel-powered cars in the U.S. That stance may change in the years ahead, but, for now, the Asian car makers are banking on hybrid vehicles and new direct injection gasoline engines to help them boost their fuel economy numbers. For example, Mazda’s new Skyactiv-G technology combines direct injection with ultra-high compression ratios (13:1) to boost fuel economy and performance up to 15%.

The hybrid market has shown steady growth in the U.S. in recent years, with more than 100 hybrid models now available from both import and domestic automakers. But, the overall numbers are still relatively small, accounting for only about 2.5% of all new vehicle sales (and most of those have been Toyota Prius models). Hybrid production has been very limited due to the high cost of the technology (an extra $3,000 per vehicle on average) and limited battery production capacity here and abroad.

In 2011, approximately half a million hybrid vehicles were sold in the U.S. By 2016, the sale of hybrid and plug-in electric vehicles is expected to grow threefold, to an estimated 1.5 million vehicles per year.*

By comparison, the sale of new Clean Diesel cars and light trucks is expected to skyrocket in the U.S., going from 680,000 in 2011, to over 900,000 by the end of this year (2012), to more than 9 million a year by 2016!*

Current Clean Diesel vehicles include the Audi A3 and Q7 TDI models, BMW 335D and X5 xDrive35d, Mercedes-Benz E350, ML350, GL350 and R350 BlueTEC diesels, and VW Beetle, Golf, Jetta, Passat and Touareg TDI models.

*Source: JD Power and Associates.

 

CLEAN DIESEL ADVANCEMENTS

Compared to the diesel engines that were offered 20 years ago, today’s Clean Diesel engines produce 99% fewer NOx emissions, and 98% less particulate matter (soot) in the exhaust! That’s a huge difference. Fuel economy and performance are also much better. Some of today’s Clean Diesel cars are rated at over 40 mpg, such as the Audi A3, which gets 42 mpg on the highway with a 2.0L turbo diesel engine (50% better than the exact same car with a gasoline engine).

The technology that makes all of this possible is common rail fuel injection at very high pressures (up to 26,000 psi!) and frequencies. Clean Diesels typically use piezo injectors that can cycle as many as five to eight times during an injection event. This allows very precise control over the combustion to reduce noise and improve efficiency.

Today’s Clean Diesel engines are engineered to run on ultra-low sulfur diesel fuel, and most can also run on B5 (a 5% blend of biodiesel with diesel fuel). ­Research is also being conducted to determine the suitability of higher biodiesel blends, different types of biofuels, including diesel-ethanol blends, and the use of diesel-butanol blends (butanol is a different type of alcohol that can also be made from corn). Future diesel fuels may even include tiny nanoparticles of metals, oxides, carbides, nitrides or nanotubes to improve combustion efficiency and fuel economy even more.

 

CLEAN DIESEL DRAWBACKS

One of the drawbacks of current Clean Diesel technology is the need for some type of exhaust after-treatment to clean up soot and other pollutants. Conventional “three-way” catalytic converters that work well with gasoline engines do not work as efficiently with the oxygen-rich combustion fumes that lean-burn engines produce. The catalyst usually requires more platinum (which is expensive) and other additives (an alkaline oxide such as barium oxide) to handle the pollutants. What’s more, a separate Diesel Particulate Trap (DPT) is usually required to catch soot.

Many diesels also add an after-treatment system (Selective Catalytic Reduction or SCR) that ­injects a water-urea solution (Diesel Exhaust Fluid or DEF) into the exhaust. On Mercedes-Benz BlueTEC diesels, the fluid is called “AdBlue.” At gas stations and truck stops, the fluid is usually marketed as DEF.

Diesel Exhaust Fluid is a solution of 67.5% purified water and 32.5% urea (ammonia). When this type of after-treatment system is used, it reduces the amount of Exhaust Gas Recirculation (EGR) that the engine needs to control NOx. This, in turn, allows the ­engine to be calibrated for slightly better overall fuel economy.

On vehicles that use a DEF after-treatment system, the fluid is stored in an on-board tank and is sprayed into the exhaust (after the diesel particulate trap) to break down oxides of nitrogen. The storage capacity will vary depending on the vehicle, and usage will depend on how the vehicle is driven. Typically, a gallon of fluid will last about 2,500 miles. The storage tank may hold enough fluid to go the distance of an oil change (7,500 miles up to 15,000 miles). EPA rules require the DEF level to be monitored, and that a warning light be used to signal the driver when the fluid level is low. Furthermore, the engine must power down to a limp-in mode if the after-treatment system runs out of DEF — which can create a real problem for the unwary driver who allows his fluid level to run out.

Another concern with DEF is that the liquid freezes at 12° F

(-11° C). This requires a tank heater to keep the reservoir warm during cold weather, and to thaw frozen fluid following a cold start if the fluid has turned to ice.

Next-generation after-treatment systems may eliminate the need for DEF and the periodic maintenance requirements to keep the fluid level filled. Automakers are developing and testing new diesel catalysts that use no platinum and do not require urea injection to control oxides of nitrogen. General Motors says it has a diesel converter that uses “perovskite oxide” (metal oxide), instead of platinum, to treat diesel exhaust.

Mazda’s Sky-D technology for diesel engines also promises to eliminate the need for NOx after-treatment. By lowering the compression ratio of a diesel engine, and injecting fuel into the combustion chamber earlier in the compression stroke, the engine can burn cooler and cleaner, allowing it to meet current and future diesel emission standards.

Variable-lift exhaust valves are also used to retain more heat during the compression stroke and to aid cold engine starting. The engine still requires a large oxidation catalyst and a particulate trap, but no DEF after-treatment or other bandaids. Mazda says its Sky-D technology boosts fuel efficiency 20% over current Clean Diesel engines. There’s no word yet as to which models Mazda will put its new Sky-D 2.2L diesel in, but some say it may be offered in the 2013 CX-5 later this year.

 

CLEAN DIESEL RECALLS

Volkswagen’s TDI engines have long set the benchmark for Clean Diesel technology. However, the ­National Highway Traffic Safety Administration (NHTSA) has recently launched an investigation into fuel system problems on 2011 Jetta TDI models. Some vehicle owners have experienced fuel ­injector clogging issues at relatively low mileages (sometimes under 10,000 miles), as well as fuel leaks in the fuel lines. The problem may be due to the high-pressure injector’s sensitivity to fuel quality. Earlier Jetta models (2009 and 2010) have also been investigated for premature fuel pump failures and fuel line leaks.

VW did issue a recall notice (23J9/V5 and 23J8/K5) on certain 2009 to 2012 Jetta and Jetta Sportwagon models with 2.0L TDI ­engines to check for possible fuel line cracks. The recall says the fuel injection pulses coincide with the natural frequency of the injector line #2 at certain engine speeds and loads, causing the line to ­vibrate. The vibration may lead to metal fatigue and cracks in the fuel line. The fix is to replace the #2 injector fuel line with an improved design, and to install vibration dampers on all of the fuel lines to control flexing.

Mercedes-Benz issued a recall (2010110005) covering 2009 to 2011 BlueTEC diesel models regarding the heater tank lines for the AdBlue reservoir. Over time, changes in resistance may set a false fault code indicating a problem in the heater circuit. The SCR module and PCM reflash recalibrates the fault detection software so it won’t set false codes.

BMW recalled 120,000 diesel models worldwide for a problem with the diesel fuel filter heater. On some vehicles, the heater would remain on after the ignition was switched off, causing the filter to overheat and start a fire. Other BMW diesel-related recalls include one to replace a missing protective hose on the Selective Catalyst Reduction (SCR) line on 335D models built from Feb. 18, 2010 to Oct. 1, 2010. The SCR line runs next to the driver’s-side fuel tank strap. If a dealer has already made the repair, the 585 code on the B-pillar label will be punched out.

 

SERVICE OPPORTUNITIES

One thing to keep in mind about Clean Diesel technology is that it requires special tools and know-how to service these engines. Bosch is the original equipment supplier of most of the Clean Diesel injection systems on the European makes, so it’s a primary source of training and technical information. For more information, go to www.boschtechinfo.com. 

Clean Diesels also require the latest scan tool updates for diagnostics, as well as special injector service tools on many applications. Most of these ­vehicles are still under factory warranty. Powertrain warranties on Audi, BMW, Mercedes-Benz and VW typically run four or five years and 40,000 to 50,000 miles. Consequently, as these vehicles come out of warranty, it will create new diagnostic and repair opportunities for the aftermarket.