Imagine for a moment if an engineer had developed a powertrain solution that used existing tooling technologies, required no additional infrastructure development, yet could provide a 30% improvement in fuel economy and deliver a more pleasurable driving experience than traditional gasoline engines. You'd expect the world to flock to the innovation like kids to a candy store. Surprisingly (or not so surprisingly, depending on your viewpoint), this technology exists. It's called the diesel engine.
What makes the diesel engine efficient is its basic operating principle of using highly compressed air to ignite the fuel in the combustion chamber, as opposed to using a spark to ignite the fuel/air mixture in a traditional gasoline engine. The diesel requires high compression ratios (as high as 25:1) and precise delivery of fuel droplets into the combustion chamber at optimum operating conditions. Diesel also benefits from the fuel itself, which has a higher energy density than gasoline, which means less is required to operate. The drawback of using this combustion process is that it produces higher levels of nitrogen oxide (NOx), which has been classified as a toxic emission by most environmental regulatory bodies throughout the world.
NOx has been the major roadblock preventing diesel engines from gaining significant acceptance in the U.S. market. In 2004, U.S. regulators instituted a new round of diesel emission standards-Tier 2 BIN 5-that many thought would signal the end of diesel altogether. The regulation calls for diesel engines to produce no more than 0.07g of NOx, 4.2 g of carbon monoxide (CO) and 0.09 g of non-methane organic gases per mile. However, subsequent advancements in emissions after-treatment systems and advanced fuel injection technologies have allowed the diesel to persevere. What's on tap are a series of technologies that will continue to improve engine performance while optimizing overall operating efficiency that may determine whether the diesel engine remains largely niche or moves into the mainstream in its second century.
In Focus: Turbo Gets Hot
If there's one buzzword that permeates the diesel discussion it's "downsizing." Automakers are busy developing next-generation diesel engines that will use fewer cylinders to produce the same power output of bigger engines. This phenomenon is placing more emphasis on advanced turbocharging systems. The turbo challenge is handling higher temperature exhaust gases produced by the smaller displacement engines. "We are going to need to have higher temperature materials to handle this-moving to more nickel-based alloys, along with changes to the aerodynamic design of the turbine blades," says Craig Balis, vice president of product development for Honeywell's (www.honeywell.com
) turbo business, who predicts significant growth in the installation rate of diesel engines below 1.7 liters, jumping from by 27% to 50% of the global diesel market by 2013.
Balis predicts there will be three paths turbocharging systems will follow in the future. One is two-stage systems, which use a smaller and large turbocharger together to provide improved power delivery across low and high rpm ranges-BMW has used this for several years in its diesel powertrains and Mercedes recently followed. "These systems are the ultimate when it comes to a vehicle performance solution, but they are expensive and complex," Balis says. Single-stage boosting systems with variable geometry, which modifies the turbo aspect ratio to provide maximum boost at low and high rpm, will account for a majority of the turbo applications in the long-term, while "microturbos," those applied to engines less then 1-liter in displacement, will take root in emerging markets, particularly India. These are simple wastegate turbos that are a low cost solution to increasing power without increasing engine size.
All of these changes are going to require a laser-like focus on improving the operational efficiency of the turbocharger to accommodate higher levels of exhaust gas to reduce overall NOx emission levels. One solution is the adoption of high and low pressure EGR systems, which will cut NOx emissions even further, while improving the operating efficiency of the diesel engine. "The more efficient the turbo, the more EGR you can drive through it and the lower the engine out NOx emissions. That's why OEMs are moving toward low pressure, or long route, EGR systems to pump exhaust gas back through the turbocharger," Balis says. Increased use of exhaust gas will require more advanced engine control systems to assure accurate repeatability in all operating conditions.
In Focus: The Fuel
In addition to new technical systems for diesel engines, there is an on-going proliferation of diesel fuels. Biodiesel is been the latest to enter the fray. Commonly made from animal fat or vegetable oil, these fuels come in blends ranging from B5-5% biodiesel and 95% petroleum diesel-to the purest form, B100. Various blends and feedstocks can cause insurmountable problems for OEMs and suppliers who are required to develop robust, yet reliable fuel delivery systems that can dispense these various fuels to the engine through intricate high-pressure injectors, all while limiting any negative impact to engine-out emission performance that could violate air quality rules. "The feedstock is the main issue for us and we will need to get this right before the development of the next-generation biodiesel fuels-relying on vegetable oil-based fuels could help us maintain the same performance as petroleum diesel," says Richard Dorenkamp, head of Volkswagen's technical development for lowest emission engines and exhaust aftertreatment.
While concern is growing over the quality of biodiesel, OEMs and suppliers are hopeful the bio-based fuels will succeed because of their inherently better lubrication properties when compared to low-sulfur petroleum-based diesel, which has lower cetane. U.S. low-sulfur diesel has a cetane rating of between 38 and 45, while Europe runs up to 58-the higher level of cetane the better the fuel burn and lubricity. The added lubrication helps reduce injection system wear, while also reducing harmful emissions output. The push is on for U.S. fuel providers to increase the rate of cetane in low-sulfur diesel fuels to improve engine and emission performance.
In Focus: Injection Under Pressure
When it comes to efficiency and performance, one component in a diesel engine that can mean the difference between make or break: The fuel injector. In the early stages of diesel development, engineers deployed solenoid-based injectors to introduce the fuel into the combustion chamber. These mechanically controlled devices use magnets and hydraulic pressure to open and close the valve at precise moments during the combustion process, assuring the fuel burned at peak rate, while also providing improved operating and emissions performance through by injecting fuel into the chamber before and after engine operation. While solenoid systems have been relatively reliable and cost-effective, increased fuel economy and emissions standards required diesel system suppliers to develop new solutions that would introduce diesel fuel into the combustion process in an even more precise manner at much higher rail pressures-ranging from 1,600 to 2,000 bar-which required the use of piezoelectric technology. Piezo injectors use an electric charge to reshape crystals housed in the injectors to provide extreme customization of fuel delivery into the combustion chamber, down to the individual atom, with the ability to operate at extremely high pressure levels-upwards of 2,250 bar and beyond, in some cases.
There's little doubt the introduction of piezo injection systems has changed the way diesel engines perform for the better. But there are still a few issues on the horizon that could put added pressure on piezo injector development, while also risking the potential to turn solenoid injection systems obsolete. There's little doubt regulators will put continued pressure on OEMs and diesel system suppliers to develop even cleaner diesel engines in the future. Consumers, however, will require these engines to be more fuel efficient, yet more powerful. Likewise, these engines will have to maintain their eco-friendliness at all operating conditions across hundred of thousands of miles of operation-no passes will be given for non-compliance of U.S. Tier 2 BIN 5 or Euro 6 emission regulations.
These factors are causing many suppliers to look at innovative ways to manage fuel injection systems for next-generation diesel engines. One of the most-discussed ideas is rate shaping. In rate shaping, fuel isn't delivered in pre-prescribed increments during the combustion process, but in infinitely variable patterns based on operating cycle, similar to gasoline injection systems. "For the future, the question will not be the number of injections, but how you can modulate them. For this we'll need very innovative injection systems because this cannot be handled by solenoid injection systems, but no one has this technology on the market today," says Gunnar Lowack, vice president of Continental's engine systems product center, who expects rate shaping injection technology to enter the market in 2011 or 2012, in time for Euro 6 emission regulations. Rate shaping will require complex engine management systems with advanced processing power up to 64-bits, according to Lowack, which means an upgrading of control units: "We have to make sure we have the computing power."
Delivering fuel at these precise increments and at higher pressures will also require significant changes to the design of the combustion chamber and the piston crown, as well as the introduction of high-pressure fuel pumps. "That's the biggest challenge we're likely to face, developing these higher pressure products and keeping them cost competitive," says Doug Patton, senior vice president of engineering at Denso of America. These higher-pressure systems will require the use of advanced materials, including magnesium and aluminum, both of which are lighter and stronger than cast iron, although they come with significant cost penalties. Still, there's little advantage in avoiding the inevitable as regulators look at ways to cut greenhouse gas emissions while improving fuel economy.
Diesel Takes a Hit
"Given the current economic climate, GM has reviewed and updated its U.S. product portfolio and has decided to place on indefinite hold its previously announced plan to add a Duramax 4.5L V-8 diesel engine in 2010 to its Chevrolet Silverado and GMC Sierra light-duty trucks." Those words sent shivers down the spines of many diesel system suppliers when they hit the press in March, 2009. When it was revealed in October, 2006, this engine was heralded as a significant milestone in the progression of the image of the diesel engine from a loud, smelly and dirty powertrain alternative, into one that was fun, green and clean. But then the economic collapse occurred and the diesel was shelved.. "GM's decision has put a knife through the heart of diesel. But honestly, it just doesn't make sense, especially when you think about the Tier 2 BIN 5 emissions and what could happen in California in the future," a senior product development executive at a Japanese OEM recently told me. "I would have done the same thing."
What is the future for diesel in the U.S.? It seems that it will be written by the European manufacturers, as the U.S. OEMs are tepid and the Asians are pursuing gas-electric hybrids. According to Dr. Johannes-Joerg Rueger, vice president of diesel engineering at Bosch: "If you look at CAFE requirements alone, you come up with the quick conclusion that if you add diesels to your portfolio you make a huge step forward." Rueger concedes diesels do carry a cost premium over traditional gasoline engines, but the upfront expense is not nearly as high as a hybrid. That's why he thinks diesel has a future in the U.S.
So does Richard Dorenkamp, head of Volkswagen's technical development for lowest emission engines and exhaust aftertreatment, who predicts diesel sales will outpace hybrids in the U.S. in 2012. He admits the stringent Tier 2 BIN 5 emission regulations initially caused him to dismiss diesel's potential success in the U.S., but he's since changed his mind. "We thought it was impossible to meet, but we also thought the same thing when the Euro 4 standards came out," he says. VW sells a diesel-powered version of the Jetta sedan and the Touareg SUV in the U.S., with plans to add a diesel version of the Golf in 2009. The automaker says it believes diesel is the perfect solution for U.S. consumers who are looking for power and fuel economy.
At BMW, the feeling is mutual, although it took the automaker until 1984-nearly a decade after VW introduced its first diesel-to jump into the diesel market. "We said we would never come with a diesel unless we came to market with a diesel that fits our brand profile. Our engineers in Austria developed the details that made the diesel a BMW," says Wieland Bruch, spokesman and an engineer at BMW's powertrain group. He says BMW's diesel philosophy centers on performance, not just fuel economy, pointing to the introduction of the 2.0-liter 4-cylinder diesel housed under the hood of the 123d, which was the first 4-cylinder diesel in BMW's portfolio to use twin-turbo technology-a small turbo is used adjacent with a larger turbo to ensure constant delivery of boost across low and high rpms. "Our focus is much on the detail work, to make sure the diesel feels like a BMW," he says.
When it comes to future technologies, there's general consensus among OEMs and suppliers that one technology is of little benefit for the cost: diesel hybridization. The cost of a diesel hybrid system would add thousands of dollars onto the purchase price of the vehicle, while providing little fuel economy benefit compared to the standard diesel-in the range of 10% to 15% fuel economy improvement. "The more efficient your base engine is, the less you can gain with additional measures, like hybrids, so I think it will only be a niche market. Instead, we can use downsizing, turbocharging, start-stop systems and thermal management to maintain a 30% to 40% fuel economy benefit," says Bosch's Reuger.