Press Release
Dr. Leopold Mikulic
- The diesel must become even cleaner and the gasoline engine even more economical
- New combustion processes car for injection systems with innovative piezo injectors for diesel engines
- Only jet-controlled direct-injection systems fully exploit the savings potential of gasoline engines
- Paper presented by Dr. Leopold Mikulic, Vice President, Powertrain Development, Mercedes Car Group, DaimlerChrysler AG
DaimlerChrysler has always stood for target-oriented research and development, transferring innovative technologies into production applications in a pioneering approach. Where propulsion systems are concerned, the engineers are working on visions which, however, are to become reality in just a few years' time. At this point in time, 57 million cars are annually produced, virtually all of them using internal combustion engines. And since this will not change in the longer term, the most prominent task of this decade is to improve the fuel consumption and emissions of internal combustion engines by means of new technologies with a lasting effect.
The crude oil resources and the fuels produced from them are finite but for the next 20 or 30 years, they will continue to be instrumental for our mobility, and so will be the conventional internal combustion engine. However, the latter no longer has much in common with Nikolaus Otto's explosion engine from the pioneering age or Rudolf Diesel's compression-ignition engine. After Gottlieb Daimler succeeded in raising the power-to-weight ratio to such an extent that the combustion engine turned the carriage into a motor vehicle, the engine developed into what it is today: a modern, economical and high-performance propulsion system. And in the course of continuously tightened emission legislation within the last 20 years, the engine has developed into a component that burdens man and environment only to a minor extent. Ever since the first European emission legislation, emissions have been reduced by 97 percent on average.
With the EURO IV limits which will come into force in 2005, a highly level of emission quality will be achieved. This norm has already been met by all Mercedes-Benz passenger cars with gasoline engines since 2002 and by all Mercedes-Benz passenger cars with four- and five-cylinder diesel engines since October 2003.
In spite of this, the next, even more stringent norm, EURO V, is already being discussed. At the same time, the engine developers are called upon to work intensively on new concepts for additional reductions in fuel consumption, not least due to the voluntary commitment on the part of the European association of motor manufacturers, ACEA, to reduce consumption to a level that is equivalent to an average emission of 140 g CO2/km. Under these framework conditions, gasoline and diesel engines are subject to very different targets.
The gasoline engine can reach the current and forthcoming emission standards comparatively easily by means of presently available methods of emission control. Its drawback, however, is that it consumes between 20 and 25 percent more fuel than a diesel engine. Its further development is therefore primarily determined by the goal of reducing fuel consumption.
By contrast, the diesel engine represents the ideal consumption concept, being the internal combustion engine with the highest degree of efficiency. On the other hand, thermodynamic conditions cause higher nitrogen oxide (NOx) emissions. What's more, the "inhomogeneous" compression-ignition process as applied to date is the reason for the specific emission of diesel particulates. Further development of the diesel engine must therefore concentrate first and foremost on the reduction of these emission components.
The diesel engine may have been able to raise its market share among passenger car drive systems especially in Europe, but the gasoline engine continues to be clearly ahead on a global scale. Technical development of the conventional passenger car drive system must therefore concentrate on securing the gasoline engine's position in its traditional markets and improving the diesel engine's starting position especially in the expected growth markets, the USA and Asia.
Quo vadis diesel engine?
The progress in terms of comfort and performance made by modern diesel engines such as the cdi models of Mercedes-Benz in recent years is remarkable. And all this has been achieved while retaining the diesel engine's proverbial economy in fuel consumption.
This is why diesel-engined passenger cars enjoy continuously growing popularity. In Germany, diesel cars account for some 39 percent, in West Europe even 43 percent of new registrations. In West Europe, 57 percent of our customers currently opt for diesel-engined cars.
In the USA, the diesel engine continues to play a subordinate role in the passenger car market. Nevertheless, DaimlerChrysler considers the USA to be one of the major growth markets in the future. This is also verified by the recent step taken with the introduction of the E 320 CDI in the North American market - this model does, after all, record fuel consumption figures of 7.3 liter/100 km in the NEDC cycle, equivalent to some 40 mpg. By comparison, the E 320 with gasoline engine has a fuel consumption of 9.9 liters/100 km, equivalent to some 30 mpg. DaimlerChrysler considers the efficient diesel combustion process with its low consumption particularly at part-load to be ideal for both stop-and-go traffic and the typical cruising. This even applies to the Japanese market with its high share of part-load operating conditions. And yet, the diesel is still finding it difficult to gain a foothold in the Japanese market for traditional reasons.
The long-term viability of the diesel engine will therefore primarily depend on how the forthcoming emission limits can be met. DaimlerChrysler will accept this challenge. In our view, however, this challenge can no longer be mastered in the evolutionary development of the present-day diesel engine merely by fine-tuning the well-known levers of injection pressure, nozzle holes and swirl.
The clean diesel of the future needs new combustion processes
To be prepared for future emission limits and to avoid a significant deterioration of fuel economy at the same time, DaimlerChrysler is intensively engaged in the research and development of new technologies and combustion processes. The primary goal in technology development is to reduce the raw emissions by means of engine modifications to such an extent that the diesel engine's exhaust gas purification - if at all required - can be effected at reasonable cost. A particularly crucial role will be played in this context by questions of handling ability and service life.
DaimlerChrysler continues to use common-rail injection technology. Even greater significance will in future be attributed to the highly effective options for reducing pollutant and noise emissions. These include the possibility to adjust injection pressure to the most diverse operating conditions in the wide range between idle and full-load operation. Above all, they include the possibility to inject the fuel in several minute charges into the combustion chamber during the combustion stroke which lasts only a few milliseconds, with the aim of making precisely the quantity available that is required during the combustion process.
However, development work focuses on innovations which go far beyond this. To prevent undesired emissions from being generated in the first place, the engineers are, in a way, attacking the fundamentals of the venerable diesel principle. Up until now, diesel engines have always been operating with a more or less excess air. This means the fuel is injected into the existing charge of compressed intake air. In future, however, the imponderabilities inherent in this process must be ruled out and combustion must be controlled even more effectively. This means that in the part-load range with its high share of emissions (nitrogen oxides!), the diesel should be operated on a homogeneous air/fuel mixture, very much like the gasoline engine. The aim is to expand this homogeneous range as far as possible. However, when approaching the full-load range, the engine has to be switched back to conventional, injection-controlled combustion.
This also means that exhaust gas aftertreatment systems like particulate traps, NOx catalysts or SCR systems have to be further developed where engine modifications no longer suffice to reduce raw emissions drastically. This will necessitate five or more individual injections over a much wider range than before, for instance during the regeneration stages. It goes without saying that this extremely complex interplay of operating strategies must be staged so discreetly that it goes unnoticed by the driver, in terms of both torque and noise.
Such goals place high demands on the flexibility of future injectors. The latter have to operate with extremely short response times of just a few micro-seconds, that is at maximum needle opening speed and with reliable (controlled) reproducibility. The engineers therefore dream of direct nozzle needle control by means of an extremely fast electro-mechanical actuator, for instance a piezo stack. This is because the piezo injectors already used today still control the nozzle needle movement by means of a hydraulic servo system.
Higher injection pressure ratings of up to 2,000 bar will be just as useful in realizing the different combustion processes as the concept of the vario-nozzle which is currently being intensively developed. A second, separately controlled row of nozzle holes would open up new degrees of freedom with respect to both different load conditions and injection patterns.
The work on future, lower-emission diesel engines can therefore be summarized as follows:
- Implementation of new operating strategies with a wide homogeneous range
- Development of a new generation of even faster piezo injectors which permit the implementation of such strategies
- Incorporation of exhaust gas aftertreatment systems in these operating strategies
The direct way: gasoline engines
The gasoline engine, too, still has great potential when it comes to further development. Unlike the diesel engine, however, the relevant work focuses first and foremost on improving fuel economy. Alongside measures such as variable camshaft and valve adjustment, major potential lies above all in direct fuel injection.
In terms of physics, direct injection permits the greatest savings in the part-load range, that is in urban stop-and-go traffic when the engine rarely comes anywhere near its minimum-consumption range. In a first step, DaimlerChrysler introduced direct injection in a CGI version of the company's M 271 four-cylinder engine.
With modifications to piston shape and, above all, cylinder head, the injection nozzle is set at an extreme angle, making a stratified charge possible, with a partly homogeneous mixture concentrating around the spark plug at the decisive moment. The formation of an ideal mixture cloud is strongly dependent on the guidance of the injection jet through piston crown and cylinder wall. With this "wall-controlled" injection, the fuel consumption of the 200 CGI engine can be reduced by up to five percent, depending on operating conditions.
However, DaimlerChrysler Research has established that the complete economy-improving potential of direct injection can only be exploited in combination with advanced technologies. The ideal is the formation of a mixture cloud around the spark plug, solely created by the injection nozzle.
Such an "injection-controlled" combustion process opens up the opportunity to double the current reduction in the CGI engine's fuel consumption in the New European Driving Cycle. DaimlerChrysler is intensively working on such a process. The greatest technical challenge lies in ensuring stable, injection-controlled combustion over the engine's entire operating range. However, direct-injection engines with stratified charging can initially only be used in countries where the required sulfur-free fuels are generally available, that is in the EU and perhaps also in Japan. In the USA, where reductions in fuel consumption are much less at the center of attention than in Europe, engines with manifold injection are expected to continue to prevail, especially since they make ends meet with conventional exhaust gas purification by means of the less expensive three-way catalytic converter.
Nevertheless, DaimlerChrysler is currently developing engines with manifold injection and a number of related technologies with the potential of enhancing fuel economy, one example being the new 3.5-liter V6 which made its debut in the new SLK.
In this engine, the wide range of intake and exhaust camshaft adjustment permits an overlapping of valve opening times, which goes a long way towards highly efficient exhaust gas recirculation. This not only curbs the generation of nitrogen oxides but also has positive effects on fuel consumption. The same applies to the separately engaged tumble flap in the intake manifold. This intensifies the swirl of the fresh air taken in in the part-load range and thereby improves combustion quality. Electronically controlled heat management is another module in using the fuel more efficiently.
DaimlerChrysler pursues two approaches to reduce the fuel consumption of gasoline engines:
- Optimization of detail features of engines with manifold injection, as demonstrated on the new 3.5-liter V6
- Further development of economy-enhancing technologies, for instance variable valve control and direct injection.
Engine and transmission: Economical only when coordinated
It goes without saying that the transmission must not be ignored when considering the possibilities of optimizing fuel economy. Alongside the engine, the transmission is the second, crucial element of the powertrain. With the 7G-Tronic, the most recent generation of automatic transmissions with seven speeds, we are now able to find a better compromise between optimum performance and optimum fuel economy than before. In terms of consumption, and compared with the previous five-speed automatic transmission, the most important critera are the clearly wider ratio range and torque converter lockup to the greatest possible extent. This means that the ratio range between first and seventh gear is larger and permits even more sensitive engine-speed adjustments to every road-speed and load spectrum. With these features alone, fuel consumption in the NEDC can be reduced by some 0.6 liters per 100 km. What's more, the seven-speed automatic transmission tangibly increases agility.
The smaller gear gradations and wider ratio range of the 7G-Tronic point the way towards further improvement. The theoretical ideal, in terms of both acceleration and consumption, would, after all, be the continuously variable adjustment of transmission ratios to the engine's operating conditions. However, a further increase in the number of gears would make little sense from today's point of view. Instead, a continuously variable automatic transmission (CVT) would be the next logical step.
The third approach: hybrid drive
Fuel economy is also the principal goal of hybrid technology which has been offered by several competitors in the USA and in Japan since recently. With hybrid drive, the greatest reductions in fuel consumption are achieved where stop-and-go phases and the proportion of low-speed driving in city traffic prevail. It is under these operating conditions that acceleration, which would raise the fuel consumption of the combustion engine, is delegated to an electric motor which is known to develop its maximum torque from standstill. Intelligently controlled, these two types of engine can be combined so as to complement each other advantageously.
DaimlerChrysler is systematically working on hybrid drive concepts. With the concept introduced at the Frankfurt International Motor Show in 2003, a combination of two electric motors with a 3.2-liter V6, fuel consumption can be reduced by up to 20 percent, depending on the driving cycle. The most recent research car, the F 500 MIND, is the first to be powered by a hybrid system consisting of diesel engine and electric motor.
However, a hybrid drive system is significantly more expensive, due to the additional installation space required and the necessary storage battery. What's more, the unavoidable extra weight offsets part of the advantages again. DaimlerChrysler nevertheless makes out opportunities for hybrid drive under specific framework conditions, first and foremost motoring with a high stop-and-go proportion. These may also include legal framework conditions as in the USA and in Japan where such concepts are tax-privileged. Over and above this, however, DaimlerChrysler sees hybrid drive as a conceivable intermediate step on the way to the fuel cell car.
The characteristic advantages of the latest hybrid concept can be summarized as follows:
- Clearly reduced exhaust emissions and a reduction in fuel consumption by up to 20 percent
- Automatic stop/start system
- Energy recovery during braking
- Operation by means of the electric motor alone is possible
- Holistic onboard energy management
Summary
The medium-term perspective for passenger-car propulsion systems can be described as follows: the goals and the relevant technologies are also influenced by the political and social framework conditions. While US legislation first and foremost specifies a strict limitation of pollutant emissions, European legislation aims at further reducing CO2 emissions - and thus fuel consumption - as well, for reasons of climate protection. Since the combustion engine will remain the dominating drive concept in the foreseeable future, we need to develop its strengths and eliminate its weaknesses to the greatest possible extent - with different consequences. The low-consumption diesel engine must retain its economy and reduce its nitrogen-oxide and particulate emissions, whereas engineering concepts must be developed to further reduce the fuel consumption of gasoline engines. However, whether diesel or gasoline, the combustion engine must always been seen in combination with the transmission.
During the transitional period leading on to the alternative propulsion concepts of the 21st century, hybrid systems may well come into their own under specific conditions, preferably in Japan and the USA. In this partnership, too, the combustion engine will play a crucial role.
