Despite the 2-ton (1.8-t) behemoth of a diesel engine that regularly runs at full power in a nearby test chamber, visitors to Georg Wachtmeister’s mechanical engineering lab at the Technical University of Munich (TUM) rarely smell exhaust fumes. That’s because the chaired professor, together with his grad students and staff colleagues, have spent the past three years figuring out just how to make the big, single-cylinder research engine burn fuel as cleanly as possible.
The TUM team’s ultimate goal was to get the heavily instrumented diesel to produce exhaust that is pristine enough to meet the stringent Euro 6 emissions standards that are forthcoming in 2014 without resorting to tailpipe aftertreatment. Diesel engines will be restricted to no more than a mere 5 mg of soot particles and 80 mg of nitrogen oxides per kilometer driven. Those threshold numbers are a fifth of the maximum soot levels and a quarter of the nitrogen oxides output permitted by the recently expired Euro 4 directive and less than half of the nitrogen oxides emissions allowed by the current Euro 5 standards.
Not surprisingly, only by resorting to relatively extreme injection and combustion pressures has the Munich group succeeded in cutting emissions to levels near to the specified limits. But even though some of these measures are likely to be impractical, too inefficient or too costly to use on the road in the foreseeable future, the researchers believe that the group’s fundamental investigation has given them deeper insights into how diesel engine combustion can best be optimized—a basic understanding that they expect will assist them in modifying conventional diesel technology to comply with the coming Euro 6 strictures.
TUM’s modular, common-rail laboratory engine, which can operate at 300-bar (4350-psi) combustion pressures and produce 3000-bar (43,500-psi) injection pressures, is the centerpiece of a research project called NEMo, a German acronym for “lowest emission truck diesel engine.” “With Euro 6 limits getting closer and closer, my students wanted to see what could be done to reach those targets,” Wachtmeister recalled. “They calculated the extreme pressure parameters for the engine and designed it,” he said. Although the project began as a student project, the project soon received technical support from industry partners, truck-maker MAN and Bavaria-based injection system specialists GSH as well as about a million Euros in funding from the Bavarian Research Foundation.
Thus commenced a step-by-step investigation of how best to trim emissions from the diesel combustion process, an effort that is complicated by the difficulty in moderating production of nitrogen oxides without effecting that of soot particles. “First, we used exhaust gas recirculation [EGR] to keep the formation of nitrogen oxides in check by lowering the combustion temperature, but that, of course, increased the output of carbon particles,” Wachtmeister began. The latter results because the lower proportion of oxygen in the air-exhaust mixture leads to less complete burning of the fuel. So to cut soot production, he continued, “we needed to increase the injection pressure.”
To accomplish that task, the researchers developed a high-pressure injection needle that atomizes the fuel into microscopic droplets, which allows them to burn more completely, and therefore retards soot formation. The NEMo injector nozzle operates at pressures greater than 3000 bar (43,500-psi). (Standard maximum pressures extend to approximately 1800 bar (26,100-psi). The injection needle is specially engineered to cope with the high pressures. “We did computer fluid dynamics calculations to optimize the shape [conicity] and number of the bore holes on the injection needle, and to develop geometries that coincide with the air movement—vortex and tumble—in the cylinder,” he said. After three design iterations, GSH specialists manufactured the final high-precision needle.
But using the new injector lowered power output, “so we needed to increase the air supply by boosting the charging pressure,” Wachtmeister continued. When more heavily compressed, the air-exhaust mixture contains enough oxygen for the diesel fuel to burn more completely. Achieving this last step was straightforward as the strongly constructed test engine is designed so that a high-performance screw compressor can inject the air-exhaust mixture into the combustion chamber under high pressure (up 9 bar [130 psi])—more than double the maximum pressure conventional diesel engines can handle.
Finding the right balance between the three key parameters—EGR, boost pressure, and nozzle configuration—proved to be challenging. “We did many different optimization loops,” he reported, but eventually the engine met the Euro 6 nitrogen-oxide limits with “little difficulty” and approached the specified soot limits.
“In the last stage of the project, we tried to transfer our findings to a conventional engine,” Wachtmeister said. “We used a similar process as we had with the research engine, but this time we had a lot more knowledge about how to proceed.” The engineers modified a six-cylinder production truck diesel, installing a high-pressure injection needle and other improvements, “but gains were limited because present turbochargers cannot produce the necessary pressures,” he noted, and added that perhaps future vehicles would use a high-pressure, two-stage turbocharger. “We got the turbocharging pressure up to about 4 bar and supported it for testing with screw-compressor air.”
Realizing that their results pointed toward technologies that are too costly for mass-produced vehicles, they are now considering how best to run diesels with exhaust aftertreatment. The TUM researchers are now “trying to control the combustion process in such a way that the particles that are produced can be easily filtered out by an aftertreatment system,” he explained. The engineers have installed a special probe that allows them to take samples directly from the combustion chamber while the engine is running. “A small piston shoots into the flame, takes a sample, and retracts within a millisecond without disturbing the combustion process,” he said. “It lets us study the particles that form at different successive stages as the combustion progresses” to discover precisely how soot forms.
They found that at certain times in the process very hard, compact particles of carbon form “that are unwilling to react, whereas others, which have a different shape and structure are highly likely to react,” Wachtmeister said. “So we’re targeting operating parameters that avoid the production of the nonreactive carbon particles and promote the formation of the reactive ones. This strategy would allow the latter to be easily oxidized in a particulate filter that doesn’t need to be continuously regenerated with engine heat, which cuts fuel economy.”