Verbesserung der Verbrennungen im Dieselmotor durch thermische Isolierung der Kolbenmulde (English shop)

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The objective of this research project was to confirm and extend, by means of measurements, the predicted advantages of a partially insulated piston with regard to fuel consumption and in-cylinder pollutant formation.
In particular, the influence of the surface temperature of the combustion chamber walls on soot oxidation was determined. Numerical investigations in the part-load range predicted that soot oxidation is suppressed especially in the cool zones close to the wall.
In order to verify these theoretical results experimentally, experiments were carried out on a small-stroke, direct-injection diesel engine equipped with a common-rail injection system.
The focus was on tests involving variation of the start of injection, the injected mass, the injection pressure, the nozzle geometry, the exhaust gas recirculation rate and the bowl insert. With regard to the bowl insert variation, the wall heat losses were reduced by using a piston bowl made of a metal with poor thermal conductivity together with an air gap.
As a result of this partial combustion chamber insulation, it was expected that soot oxidation close to the wall would increase significantly.
To characterise the combustion process and the exhaust gas components, various measurement techniques were employed on engine test benches which, in addition to investigations of the overall engine behaviour, provide insight into in-cylinder pollutant formation, such as soot formation and soot burn-out.
In order to reliably capture the very low soot concentrations in the part-load range, an opacimeter with high measurement accuracy was developed and calibrated.
The influence of insulation measures on the bowl wall temperature was measured contactlessly by means of an infrared single-colour pyrometer.
The pyrometric temperature measurements on a moving piston showed that in the tests with an insulated piston bowl, an increase in temperature is achieved compared with the non-insulated piston bowl.
At the upper load point, the temperature difference amounts to 250 K.
The thermodynamic analysis at the same injection quantity and without exhaust gas recirculation showed that combustion in a partially insulated engine exhibited a shorter ignition delay, a reduced premixed fraction and an extended combustion duration.
This can be explained by higher combustion chamber temperatures, which lead to accelerated mixture preparation and ignition kinetics.
The consequence is a lower proportion of premixed combustion, accompanied by a sootier combustion and low NOx formation.
With partial insulation of the piston bowl, an increase in soot formation is initially observed.
However, soot emission is generally lower for the insulated piston before the drop in mean pressure.
This is due to improved oxidation, which is responsible for a reduction in the measured soot mass concentration in the exhaust gas.
At the same injection quantity and the same fuel-air ratio, the indicated mean effective pressure of the insulated piston increased by 5 %.
This increase cannot be attributed to better combustion, but to a reduction in heat flux.
The negative effect of the combustion chamber temperature on the ignition delay and on soot formation can be compensated both by an early shift of the start of injection and by a simultaneous variation of the start of injection and the EGR rate, such that the position of the peak cylinder pressure is kept constant at 10° CA after TDC.
Subsequently, three nozzles with different hydraulic flow rates and Ks factors were used in order to investigate the effects of the different injection rate profiles on the engine conditions and exhaust emissions.
The nozzles with a low flow rate generally exhibited a reduction in soot emissions, accompanied by an increase in NOx emissions.
Within the scope of these tests, the partial insulation of the piston bowl proved to be a measure for reducing soot emissions and fuel consumption and for increasing the exhaust gas temperature.
The reduction in soot emissions with the partially insulated piston is explained by soot-wall interaction mechanisms.
That is, if the fuel is injected at a high injection pressure, combustion will take place to a greater extent in the vicinity of the combustion chamber walls and will promote soot oxidation.
In order to confirm this statement, cycle-resolved emission spectroscopy was applied to measure soot concentrations during the combustion process in the combustion chamber.
This provided useful information on soot formation and oxidation. The measured soot profiles agree very well with the results of the thermodynamic investigation.
They confirm that increasing the surface temperature of the piston bowl promotes soot oxidation.