Peter Obrecht

Temporal soot evolution and diesel engine combustion: Influence of fuel composition, injection parameters, and exhaust gas recirculation

Abstract This study deals with the investigation of the influence of fuel composition, injection parameters, and exhaust gas recirculation on not only engine-out, NOx, and particulate matter (PM) emissions but also on the temporal behaviour of soot formation and oxidation during combustion. Based on accurate measurements of exhaust emissions (gaseous components, particle number, and size distributions), in-cylinder temporal evolution of soot and temperature (computed from multicolour-pyrometry data), and analysis of the relevant parameters in direct-injection diesel engine combustion (calculation of heat release rates and different zone temperatures in the cylinder), insight has been gained with respect to the underlying mechanisms of the observed effects. Crank-angle-resolved data of the in-cylinder soot concentration have revealed the positive influence of the addition of water or of oxygenated diesel additives in diesel fuel in terms of reducing the soot formation and enhancing the soot oxidation rate. A clear linear correlation was found between the filter smoke number and KL signal at the end of the oxidation process. With respect to the particulate number or mass an exponential fit with KL at the end of main oxidation is shown.

INFLUENCE OF POST-INJECTION PARAMETERS ON SOOT FORMATION AND OXIDATION IN A COMMON-RAIL-DIESEL ENGINE USING MULTI-COLOR- PYROMETRY

The emission trade-off between soot and NOx is an issue of major concern in automotive diesel applications. Measures need to be taken both on the engine and on the aftertreatment sides in order to optimize the engine emissions while maintaining the highest possible efficiency. It is known that post injections have a potential for exhaust soot reduction without any significant influence in the NOx emissions. However, an accurate and general rule of how to parameterize a post injection such that it provides a maximum reduction of soot emissions does not exist. Moreover, the underlying mechanisms are not understood in detail. The experimental investigation presented here provides insight into the fundamental mechanisms of soot formation and reduction due to post injections under different turbulence and reaction kinetic conditions. In parallel to the measurement of soot elementary carbon in the exhaust (using a Photo Acoustic Soot Sensor), the in-cylinder soot formation and oxidation process have been investigated with an Optical Light Probe (OLP). This sensor provides crank angle resolved information about the in-cylinder soot evolution The experiments confirm conclusions of earlier works that soot reduction due to a post injection is mainly based on two reasons: increased turbulence (from the post injection) during soot oxidation and lower soot formation due to lower amount of fuel in the main combustion at similar load conditions. A third effect of heat addition during the soot oxidation, which was often mentioned in the literature, could not be confirmed. In addition, the experiments show that variations of turbulence (from swirl) and reaction kinetics have a minor influence on the diffusion controlled heat release rate. However, the time phasing of the soot evolution is highly influenced by these variations with only small changes in the peak soot concentration. It is shown that the soot reduction of a post injection depends on the timing. More precisely, the soot reduction capability of a post injection decreases rapidly as soon as its timing is late in the soot oxidation phase. The soot oxidation rate can only be improved by increased turbulence and heat addition from the post injection in a time window before the in-cylinder soot peak occurs. Depending on EGR and swirl level, a maximum dwell time can be defined after which the post injection effect becomes counterproductive with respect to the soot oxidation rate.

Apparent effects of in-cylinder pressure oscillations and cycle-to-cycle variability on heat release rate and soot concentration under long ignition delay conditions in diesel engines

In diesel engines, long ignition delay due to cold in-cylinder conditions has been shown to lead to high cycle-to-cycle variability, as well as result in pressure oscillations due to rapid localised pressure rise rates from the resulting premixed combustion. These pressure oscillations appear as superimposed pressure waves on the engine indication graph, with an oscillation frequency corresponding to the first radial vibration mode. In the current study, the influences of pressure oscillations on heat release rate and the progress of in-cylinder soot concentration are investigated. Results showed that cycles where pressure oscillations occur reach a higher peak pressure than average or low pressure oscillation cycles, as a result of increased diffusion combustion rate and apparent mixing rate. Additionally, using in-cylinder soot pyrometry, cycles with high pressure oscillations were shown to exhibit increased soot oxidation rates. The combination of the two above-mentioned observed effects leads to the conclusion that pressure oscillations in direct injection diesel engines result in more rapid mixing due to increased turbulent intensity.

Development and validation of a virtual soot sensor: Part 1: steady-state engine operation

The reduction of the emission limits has lead to an increased complexity of the ECU calibration process and to the need for expensive aftertreatment methods in order to fulfil the legislated limits. Integrating feedback of the Particulate Matter (PM) and NO x emissions into the engine management could make fulfilment of legislation easier and reduce the complexity of the necessary calibration process. Since production type PM sensors for raw emission feedback are not available, a virtual soot sensor (VSS) has been developed. The VSS is a mean value soot model which provides predictions for the PM in real-time (cycle resolved). Its inputs are ECU variables and characteristic values of the heat release rate which are obtained on-line from in-cylinder pressure measurement. The structure of the VSS has been derived from optical kL-measurement data, i.e. from representative, crank angle resolved evolutions of the in-cylinder PM (3-color pyrometry). The model is structured into three consecutive phases which represent the in-cylinder PM evolution and is calibrated with measurements of the exhaust PM concentration of a standard engine operating map only. The three phases correspond to an initial phase where formation of PM dominates, a phase when formation and oxidation are roughly in balance, and a phase during which oxidation dominates. For steady state experiments, the VSS shows an excellent correlation with the exhaust gas soot (respectively elementary carbon) concentration that has been measured with a photo-acoustic soot sensor (PASS). In addition a reasonable ratio between soot formation and soot oxidation is reproduced.

Predictive Phenomenological C.I. Combustion Modeling Optimization on the Basis of Bio-Inspired Algorithms

A new approach within the well-known trade-off in combustion process simulations between computational efforts (and thus the capability for engine operating map calculations) on the one hand, and accuracy of predictions on the other, has been developed and applied successfully to diesel combustion, in particular to energy release and pollutant formation. Using phenomenological models in combination with bio-inspired algorithms (for parameter identification), it is now possible to predict thermal, chemical and injection related engine characteristics over an entire operating map including different types of fuel (e.g. diesel, water-in-diesel emulsions and oxygenated diesel).

Exhaust-Stream and In-Cylinder Measurements and Analysis of the Soot Emissions From a Common Rail Diesel Engine Using Two Fuels

The operation and emissions of a four cylinder, passenger car common-rail diesel engine operating with two different fuels was investigated on the basis of exhaust-stream and in-cylinder soot measurements, as well as a thermodynamic analysis of the combustion process. The two fuels considered were a standard diesel fuel and a synthetic diesel (fuel two) with a lower aromatic content, evaporation temperature, and cetane number than the standard diesel. The exhaust-stream soot emissions, measured using a filter smoke number system, as well as a photo-acoustic soot sensor (AVL Micro Soot Sensor), were lower with the second fuel throughout the entire engine operating map. To elucidate the cause of the reduced exhaust-stream soot emissions, the in-cylinder soot temperature and the KL factor (proportional to concentration) were measured using miniature, three-color pyrometers mounted in the glow plug bores. Using the maximum KL factor value to quantify the soot formation process, it was seen that for all operating points, less soot was formed in the combustion chamber using the second fuel. The oxidation of the soot, however, was not strongly influenced by the fuel, as the relative oxidized soot fraction was not significantly different for the two fuels. The reduced soot formation of fuel two was attributed to the lower aromatic content of the fuel. The soot cloud temperatures for operation with the two fuels were not seen differ significantly. Similar correlations between the cylinder-out soot emissions, characterized using the pyrometers, and the exhaust-stream soot emissions were seen for both fuels. The combustion process itself was only seen to differ between the two fuels to a much lesser degree than the soot formation process. The predominant differences were seen as higher maximum fuel conversion rates during premixed combustion at several operating points, when fuel two was used. This was attributed to the lower evaporation temperatures and longer ignition delays (characterized by the lower cetane number) leading to larger premixed combustion fractions.

Development and validation of a virtual soot sensor: Part 2: Transient engine operation

The reduction of the legislative emission limits has led to an increased complexity of the engine control unit (ECU) calibration. Integrating feedback of the Particulate Matter (PM) and NOx emissions into the engine management could make fulfilment of legislation easier and reduce the complexity of the necessary calibration process. Due to the fact that production type PM sensors for raw emission feedback will not be available, or will be exceedingly expensive in the near future, in part 1 of this work, a virtual soot sensor (VSS) has been developed. Along with the very good steady state behaviour, the VSS is able to predict PM emissions in transient engine operation with a sufficient precision. This has been approved with different changes in torque demand at constant engine speed. Though an overestimation of the soot occurs during the first cycles of the step in torque demand, the behaviour of the engine out soot was well captured and compared with measurements from a photo-acoustic soot sensor (PASS) and characteristic end values of the representative in-cylinder soot trace (measured by multi-colour pyrometry). The performance of the control structure with integrated VSS is demonstrated on the new European driving cycle and an Urban Dynamometer Driving Schedule. Furthermore, a change in set point value demonstrates the opportunity of changing the raw emission strategy on-line. These results offer the opportunity to expand this cylinder-pressure-based VSS approach to other pollutants (primarily NOx) as well.

Integration of a Cool-Flame Heat Release Rate Model into a 3-Stage Ignition Model for HCCI Applications and Different Fuels

The heat release of the low temperature reactions (LTR or cool-flame ) under Homogeneous Charge Compression Ignition ( HCCI ) combustion has been quantified for five candidate fuels in an optically accessible Rapid Compression Expansion Machine (RCEM). Two technical fuels (Naphthas) and three primary reference fuels (PRF), (n-heptane, PRF25 and PRF50) were examined. The Cetane Numbers (CN) of the fuels range from 35 to 56. Variation of the operating parameters has been performed, in regard to initial charge temperature of 383, 408, and 433K, exhaust gas recirculation (EGR) rate of 0%, 25%, and 50%, and equivalence ratio of 0.29, 0.38, 0.4, 0.53, 0.57, and 0.8. Pressure indication measurements, OH-chemiluminescence imaging, and passive spectroscopy were simultaneously implemented. In our previous work, an empirical, three-stage, Arrhenius-type ignition delay model , parameterized on shock tube data, was found to be applicable also in a transient, engine-relevant environment. The pressure rise due to cool-flame heat release , which is crucial for the induction of main ignition , was included in the experimental pressure traces that have been used. To fully predict the ignition delay in HCCI -engine applications however, the cool-flame heat release characteristics need to be known in advance. In this work, the cool-flame heat release characteristics have been investigated with regard to operating parameters. A simplified, cool-flame heat release model is proposed, that is mathematically independent from the three-stage ignition delay model . It provides the cool-flame heat release profile that is used to reconstruct a pressure/temperature trace including the effect of the cool-flame . The reconstructed trace is the input to the three-stage model , and thus both cool and hot-flame ignition delays can be predicted. The overall performance of the combined three-stage/ cool-flame heat release model was assessed. Very good agreement was observed between the experimental ignition delay and the combined cool-flame /three-stage ignition model computations.

Kennfeldtaugliche Vorausberechnungen beim Dieselmotor

Im klassischen Zielkonflikt der Vorausberechnung von Verbrennungsvorgangen zwischen Rechenzeit einerseits sowie Exaktheit der Ergebnisse andererseits wurde am Laboratorium fur Aerothermochemie und Verbrennungssysteme (LAV) der ETH Zurich am Beispiel der Energieumsetzung und Schadstoffentstehung in der dieselmotorischen Verbrennung ein neuer Ansatz zur stochaistischen Modell-optimierung in Wechselwirkungen mit Messungen entwickelt und erfolgreich angewandt.

Models for engine-map-wide prediction of diesel engine combustion and emissions

At the Aerothermochemistry and Combustion Systems Laboratory (LAV) at ETH Zurich, a new approach within the well-known trade-off in combustion process simulations between computational costs (and thus the capability for engine operating map calculations) on the one hand, and accuracy of predictions on the other, has been developed and applied successfully to diesel combustion, in particular to energy release and pollutant formation. Bio-inspired algorithms have been thereby used to identify values of critical model parameters.