Mercedes Perspective: Bending the Diesel Principle

Press Release

Homogenous combustion at partial loads could cut out some diesel emissions before they even occur

Although the diesel engine continues to impress with low fuel consumption, it still needs to clean up its act. That's why engineers at DaimlerChrysler are now looking into new combustion processes and injection systems that feature innovative piezo technology.

On the European automobile market, diesel models have grown substantially in popularity in recent years. In Germany, for example, 39 percent of all new vehicles are now fitted with a diesel engine, while the proportion for Western Europe as a whole is even higher, at 44 percent.

European customers fell in love with the diesel as soon as it began to cast off its traditional image as slow and dirty. The diesel engines of today are powerful and dynamic and combine a genuinely sporty performance with a truly modest fuel consumption.

Of all the different types of internal-combustion engine, it is the diesel variety that boasts the highest combustion efficiency. In turn, this results in low fuel consumption, especially at partial loads, and makes it ideal for stop-and-go traffic in cities and long-distance travel, where the engine is rarely run at full throttle.

Such virtues are also starting to turn heads in Japan and the U.S., where diesel models have traditionally played a minor role. Indeed, since the beginning of this year, diesel technology has been making a spectacular comeback in the U.S., where the Mercedes-Benz E 320 CDI has created a sensation with its powerful torque, low consumption and huge range between filling stations.

Time for a radical rethink

However, with new restrictions looming on the horizon, the diesel won't be resting on its laurels. In fact, increasingly stringent emissions controls in Europe and the U.S. � along with the self-imposed obligation of the automobile industry to reduce CO2 � will pose a challenge for all types of internal-combustion engine, albeit in different regards. Gasoline engines, for example, need primarily to curb their thirst. The diesel, on the other hand, has to prove that it can combine its frugal use of fuel with low emissions. This could well mark a turning point in the development of the diesel engine, 112 years after its invention.

Leopold Mikulic, head of engine and power train development at the Mercedes Car Group, explains: "In our opinion, it will take more than just a fine-tuning of the usual variables such as injection pressure, jet apertures and adjustable swirl to cut the emissions of today's diesel engines."

In other words, instead of minor adjustments, a radical rethink is required. In turn, this means that the diesel engine of tomorrow needs new combustion processes. "Our main aim at DaimlerChrysler," says Mikulic, "is to reduce raw emissions by modifying the combustion process. The key is to make them so low that the costs of a supplementary exhaust-gas purification � to the extent that this is necessary � are acceptable."

The research and development activities of DaimlerChrysler to enhance fuel economy and reduce emissions of the diesel engine can be divided up into three main areas:

• new combustion processes to cut raw emissions
• new types of injector to enhance fuel injection
• optimization of the exhaust-treatment process

For engineers, such innovations also demand a departure from received wisdom and a willingness to travel along paths that point in new directions. "In the quest to prevent an engine from producing undesirable emissions in the first place, we've even begun to take a long hard look at what we currently regard as the fundamental principles of diesel combustion," says Mikulic. "To date, a conventional diesel engine has always had an excess of air inside the cylinders. In other words, the diesel fuel is injected into a predetermined amount of compressed intake air. However, this process makes combustion unpredictable. We therefore need to exclude this uncertainty in the future if we are to gain more control over the combustion process." In a conventional diesel engine, this excess air results in non-homogenous combustion, which in turn promotes the formation of soot particles. Moreover, for thermodynamic reasons, this process also leads to relatively high emissions of nitrogen oxides (NOx) when the engine is operating at partial loads. "However, at partial loads," says Mikulic, "it would be perfectly possible to run a diesel engine with a homogenous air/fuel mixture and thereby prevent certain emissions from occurring at all. That's what happens in a gasoline engine. Our objective is thus to develop an engine that will operate with a homogenous mixture across as broad a range of loads as possible."

A further objective is to enhance the fuel injection process, which was revolutionized in 1997 with the development of the common rail. Before then, diesel technology was dominated by mechanical inline or distributor injection pumps, which inject a predefined amount of fuel into the combustion chamber each time the crankshaft reaches a certain position, but do not permit the fuel to be flexibly dosed.

The development of the common rail made it possible to control each individual injection process in a flexible way and divide it up into a number of injection phases. This is possible because the fuel is transported to the fuel injectors via a single high-pressure line � the so-called common rail. Hydraulic servo-systems are used to open and close the jets, and the injection process itself is no longer dependent on the crankshaft position but instead is independently controlled by solenoid valves or piezo injectors.

At injection times of between one and two milliseconds, the injector needles have to open and close as many as 10,000 times a minute. The amount of fuel injected ranges from one cubic millimeter � the equivalent of half a pinhead � to around 50 cubic millimeters � two raindrops.

The ability to precisely dose the amount of fuel injected and to divide the injection process up into any number of partial injections has an influence on the shape of the flame front within the combustion chamber. In turn, this changes the temperature distribution, which has an influence on the formation of nitrogen oxides.

Limiting raw emissions

According to Mikulic, the ideal situation is to be able to control the injection process right across the full range of engine loads: "The engineer's dream is to be able to directly control the injector needles using an ultra-fast electromechanical actuator such as the piezo injector."

In the future, the use of higher injection pressures of up to 2,000 bar and the so-called vario nozzle, which features two rows of injection apertures instead of only one at the tip, will make it possible to control the injection process more precisely. "The incorporation of a second, separately controllable row of injection apertures will give us even greater freedom with respect to both engine load and jet profile," says Mikulic in reference to the vario nozzle, which is now being developed in close cooperation between DaimlerChrysler and automotive suppliers.

The new type of piezo injectors with electromechanically controlled jet needles increase the proportion of homogenous combustion at partial load and therefore have the potential to decrease substantially the amount of raw emissions produced by diesel engines of the future. However, this development will not completely eliminate raw emissions, as the engines still have to operate with conventional combustion processes when running at full load.

"Wherever we can't control the combustion process to achieve a drastic reduction in raw emissions, we will have to promote the development of exhaust-treatment systems such as particulate filters, NOx storage-type catalytic converters and the selective catalytic reduction (SCR) systems," says Mikulic.

Once again, DaimlerChrysler is blazing the trail for diesel. In October 2003, for example, Mercedes-Benz became the world's first automobile brand to offer a new system for its diesel cars that not only meets the EU-4 emissions limits, which come into force in 2005, but also almost completely eliminates particulate emissions. The new particulate filter, which functions without the need for any additives whatsoever, was specially developed by DaimlerChrysler with the following criteria in mind: long service life, everyday practicality and low fuel consumption. There was never any suggestion of designing a filter system that requires the use of fuel additives.

Although studies have shown that such fuel additives do improve the burn-off of the particles that have been collected in the filter, they also remain as non-regenerative ash in the filter channels, which they eventually tend to block. This increases exhaust gas back-pressure and, with it, fuel consumption and CO2 emissions, while the engine performance decreases continuously. The diesel remains versatile Thanks to its new particulate filter system and advanced CDI engines with common-rail direct injection, four-valve technology and exhaust-gas recirculation, Mercedes-Benz has succeeded in reducing particulate emissions by around 87 percent since 1995.

In other words, the good old diesel engine has greatly benefited from from a number of technological innovations in recent years. What's more, others will follow in the shape of piezo injectors and homogenous combustion processes.

In the 100 years and more since it was first developed, the diesel engine has proven itself to be extremely versatile. That is the main reason why it remains so important even today. And the way things are going at present, it is unlikely that the diesel will soon be losing any of its appeal.

The engineer's dream: Direct control of the jet needles

Voltage Creates Pressure

Piezo materials are special ceramics and metal alloys that undergo mechanical deformation when subjected to electrical or thermal energy. The converse effect also occurs. Piezo ceramics are therefore ideal for use as both sensors and actuators. For example, when such materials are subjected to high pressure, the negatively charged lattice points of the crystal structure are displaced relative to the positively charged points, producing a measurable voltage. Since the surface charge is proportional to the degree of deformation produced by the mechanical stress, piezo ceramics can be used as sensors.

Conversely, piezo ceramics can also be used as actuators: By applying a voltage to such a material, it is possible to change its charge and thus also its external form. Piezo actuators generate very large forces and execute very precise movements extremely rapidly.

In order to generate the degree of travel required to operate the injectors, stacks of piezo ceramic wafers are used to provide a cumulative displacement in the millimeter range.

Photo Captions:

Rigorous examination: Before series production, a new engine must undergo major tests. Engineers at the Untertürkheim Plant put a diesel engine through its paces on one of the world's most modern test stands.

Power pack: The V8 in the Mercedes-Benz S 400 CDI is one of the cleanest diesel engines around.

Bits and pieces: The parts of the V8 diesel engine in the S 400 CDI combine to provide large torque, low consumption and a long range.

Bits and pieces: The parts of the V8 diesel engine in the S 400 CDI combine to provide large torque, low consumption and a long range.