Knock with Exhaust Gas at Full Load

Knocking combustion limits the efficiency of turbocharged SI engines at high loads.

This phenomenon can be inhibited by deploying recirculation of cooled exhaust gas.

However, the development support for full load exhaust gas combustion systems with

0-D/1-D simulations is not yet expedient, as no reliable knock model exists which

accounts for exhaust gas. In order to develop such a model, a new kinetic mechanism

for gasoline surrogate fuels was developed and in parallel the effects of exhaust gas have

been investigated experimentally with a single-cylinder research engine. Based on the

measurement database and the new kinetic mechanisms, at the RWTH Aachen University

and the University of Stuttgart a predictive knock model for 0-D simulation applications

was developed and validated jointly as part of an FVV research project.

AUTHORS

Max Mally, M. Sc.is Research Associate

at the Institute for Combustion Engines of

RWTH Aachen University (Germany).

Alexander Fandakov, M. Sc.

is Research Associate at the Institute of Internal

Combustion Engines and Automotive Engineering of

University of Stuttgart (Germany).

Dr.-Ing. Liming Caiis Research Associate at

the Institute for Technical Combustion of RWTH

Aachen University (Germany).

Dipl.-Ing. Heiko Minwegen

is Research Associate at the Junior Professorship

for Physico-Chemical Fundamentals of

Combustion of RWTH Aachen University

(Germany).

© RWTH Aachen University

Gasoline Engines

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1 MOTIVATION

In order to optimize the efficiency of internal combustion engines, today many variations such as the compression ratio are used. In the gasoline engine, the efficiency generally increases with increas-ing compression. At high loads, high compression leads, however, to the tendency to knock. This phenomenon can be prevented by means of a cooled Exhaust Gas Recirculation (EGR). The develop-ment of a full-load exhaust gas recirculation combustion method in the 0-D/1-D simu lation is possible only to a limited extent, since there is no reliable knock model to date that reflects the influence of exhaust gas recirculation. At RWTH Aachen University and the University of Stuttgart, a novel, predictive knocking model for work process accounting has now been developed.

2 KINETIC MECHANISM DEVELOPMENT

Ignition delay times of a gasoline fuel with 10-% ethanol content (RON95E10) were measured with two experimental setups, a rapid compression machine and a shock tube [1]. Measurements were carried out for two equivalence ratios of 0.77 and 1.18 at pres-sures from 20 to 40 bar in a temperature range of 700 to 1250 K and, particularly, for two EGR ratios of 0 and 25 %. As shown in FIGURE 1, the addition of 25 % EGR prolongs the ignition delay time by up to 100 %.

Since real petroleum based fuels are composed of a large vari-ety of hydrocarbon components, surrogate mixtures of only a few

representative species are typically employed in computational research. For this study, surrogate mixtures of n-heptane, iso- octane, toluene and ethanol have been used. The composition of the surrogate fuels was blended in order to match the properties of the real fuel, TABLE 1. A chemical mechanism was developed for the combustion of the surrogate fuels based on the published mechanism from [2]. The sub-models for n-heptane and iso- octane of the reference mechanism in their work [2] have been revised according to the latest state-of the-art kinetic knowledge. The aim is to improve the model prediction ability of ignition delay times, which is important for the accurate reproduction of knocking behavior. For good model performance, the reaction rates were further calibrated using an advanced uncertainty quantification framework [2]. The reaction mechanism was vali-dated successfully against the data obtained in this study, as shown in FIGURE 1.

3 EXPERIMENTAL INVESTIGATIONS

The experimental investigations were performed on a direct injec-tion spark ignition single-cylinder research engine with homoge-neous charge formation. The engine features external boosting, low pressure exhaust gas recirculation, a three-way-catalyst and a tumble generation device. Information about the level of charge motion and the flow performance of the intake ports can be found in [3]. Further technical data of the engine are summarized in TABLE 2.

1 MOTIVATION

2 KINETIC MECHANISM DEVELOPMENT

3 EXPERIMENTAL INVESTIGATIONS

4 TWO-STAGE KNOCK MODEL

5 SUMMARY

FIGURE 1 Comparison of measured and computed ignition delay times (closed lines λ = 1.30 and xEGR = 0 %, dashed lines λ = 0.85 and xEGR = 25 %) (© RWTH Aachen University)

Gasoline Engines

TABLE 1 Fuel properties and properties of blended surrogate fuel (© RWTH Aachen University)

Property RON95E10 Surrogate1 Surrogate2

ROn [-] 96.5 95.0 96.2

MOn [-] 85.2 86.6 88.7

ROn-MOn [-] 11.3 8.4 7.5

H/C ratio [1] 1.94 1.93 1.93

Density [kg/l] 0.7417 0.7443 0.7493

Composition [mass-%]

10.3 % ethanol

13.6 % n-heptane45.7 % iso-octane 30.5 % toluene10.3 % ethanol

1 = According to mixing rules, 2 = measurement

TABLE 2 Engine technical data (© RWTH Aachen University)

Stroke (s) [mm]

90.5

Bore (D) [mm]

75

Engine displacement [cm³]

399

Compression ratio [1] 10.9

Valve train DOHC (4V)

Fuel pressure [bar] 200

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During baseline testing, the engine was operated with non-cat-alyzed exhaust gas. In order to cover a wide range of thermody-namic states, the intake temperature, the engine speed and the EGR rate have been varied comprehensively. A summary of the results with EGR is given in FIGURE 2. EGR proved to inhibit knock at all investigated operating points when the intake temperature is kept on a constant level. With increased EGR rates, the spark

timing had to be advanced in order to compensate the longer com-bustion duration. In all cases an earlier center of combustion could be achieved with EGR, and the engine efficiency was increased by up to 4 % at the highest EGR rate. The measurements were used as a base for the knock model development.

The recirculated exhaust gases contain various reactive species. Especially Nitric Oxide (NO) is considered to be a species which

FIGURE 2 Experimental results of the influence of EGR (© RWTH Aachen University)

FIGURE 3 Influence of nitric oxide on engine knock (© RWTH Aachen University)

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can strongly affect the auto-ignition in the cylinder, as summarized in [4]. With this in mind, measurements with external NO feed into the intake system were performed while running the engine with catalyzed exhaust gas. Due to the size of the catalyst, the NO con-centration could be reduced to less than 10 ppm in the recircu-lated exhaust gas.

The experimental results of the NO addition in the intake sys-tem are depicted in FIGURE 3. The average combustion and the combustion stability are not affected by the addition of NO. How-ever, the number of knocking cycles changes with the addition of NO. At 0 % EGR, the number of knocking cycles was reduced with the addition of NO. At 10 % EGR, no significant difference between 0 and 100 ppm external NO was observed. With further increase of NO, the number of knocking cycles decreases again. Surprisingly, no knock promoting effect was observed at these two operating points. At 20 % EGR, the number of knocking cycles is drastically increasing at 100 ppm external NO. Again, a further increase of external NO reduces the number of knocking cycles. Without knowledge of the internal EGR rate and the cylinder NO concentration, this behavior seems to be inconclusive. Taking the remaining NO concentration in the cylinder into account, it can be concluded that the maximum knock promoting effect of NO is observed at about 100 ppm cylinder NO concentration. Higher NO concentrations lead to a reduced knock tendency. At 20 % EGR, the internal NO concentration is about 20 ppm NO. Here, external addition of NO leads to a strong increase of knocking cycles. Fur-ther increase of cylinder NO concentration leads to a reduction of knocking cycles.

Measurements comparing pre- and post-catalyst extraction of EGR have been carried out as well, FIGURE 4. At EGR rates above 15 %, and about 2 °CA advanced center of combustion with cat-alyzed exhaust gas could be achieved. In order to understand this

advantage, the concentration of NO in the cylinder was calculated for pre- and post-catalyst EGR extraction. With pre-catalyst extraction, the cylinder NO concentration cannot be reduced below 100 ppm. Only with post catalyst extraction, the cylinder NO con-centration can be reduced with increasing external EGR rate. Due to the very low NO concentration at high EGR rates, a lower knock tendency is observed for post catalyst extraction.

For validation of the knock model, additional experimental results have been obtained using another cylinder head combined with an increased compression ratio of 11.84. Engine loads have been varied between 12 to 20 bar IMEP in order to cover a wide range of the engine operation map.

4 TWO-STAGE KNOCK MODEL

All commonly used 0-D/1-D knock models are based on the cal-culation of a pre-reaction state of the unburnt mixture with the so-called knock integral which represents a simplified approach for the evaluation of the reaction progress in the end gas. Simu-lations with the newly developed kinetic reaction model deter-mined that the self-ignition in the unburnt zone, which precedes a knocking event can occur in two stages. In this case, a low tem-perature heat release occurs in the unburnt mixture which is cov-ered by the main combustion. This phenomenon significantly influ-ences the auto-ignition behavior of the unburnt mixture, which severely impairs the auto-ignition prediction capabilities of the commonly used knock integral. Hence, for the accurate prediction of the knock boundary, an improved approach for modeling the progress of the two-stage chemical reactions is needed.

Based on these findings, a new two-stage approach for the pre-diction of the auto-ignition behavior was developed. The occur-rence of each of the two ignition events is predicted by each one

FIGURE 4 Comparison of pre- and post-catalyst extraction (© RWTH Aachen University)

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single integral. The inputs of the two coupled integrals are the values of the ignition delay for the corresponding ignition stage as a function of the current boundary conditions. For this purpose, an enhanced three-zone approach for modeling the influence of various parameters (pressure, temperature, EGR, Air-fuel Equiva-lence Ratio (AFR), ethanol content and surrogate composition) on the auto-ignition delay times of the mixture was developed [5]. Furthermore, models for the delay of the low-temperature ignition as well as the temperature increase resulting from the first ignition stage as a function of the boundary conditions were formulated [5].

The newly developed two-stage auto-ignition model predicts the occurrence of the two-stage ignition and very accurately qualifies the significant influence of the low-temperature heat release on the mixture’s auto-ignition behavior at various operating condi-tions, FIGURE 5.

However, the correct prediction of local auto-ignition is not suf-ficient for the reliable estimation of the knock boundary, as the occurrence of this phenomenon does not necessarily result in knock. The commonly used knock models assume that no knock can occur after a pre-defined, constant Mass Fraction Burnt (MFB)

FIGURE 6 Measured and predicted center of combustion at the KLSA at various operating conditions (© RWTH Aachen University)

FIGURE 5 Crank angles of auto-ignition of single cycles simulated with a detailed mechanism and predicted by the newly developed two-stage approach (© RWTH Aachen University)

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point. The evaluation of measured single knocking cycles however has shown that the latest possible MFB-point where knock can occur changes significantly with parameters such as engine speed, EGR rate and the AFR. Hence, a cycle-individual criterion for the occurrence of knock considering the current operating conditions is needed. To this end, an approach based on the unburnt mass fraction in the thermal boundary layer at the time of auto-ignition was developed which considers operational conditions, combus-tion chamber geometry and the flame propagation [6]. The bound-ary layer volume is estimated with a phenomenological model and, because of the cool cylinder walls, it has a temperature that is much lower than the mean unburned mass temperature. Hence, it is assumed that if the unburnt mass fraction in the boundary layer at the predicted time of auto-ignition is higher than a pre-de-fined threshold calibrated at the measured knock boundary, no knock can occur. This threshold is the only tuning parameter of the new knock model. It must be calibrated for an operation point at the experimentally determined knock limit.

The new knock model does not contain any empirical measure-ment data fits and has just one engine-specific calibration param-eter which is independent of the operating conditions. Therefore, the new model can be applied to different engines without any limitations. Finally, an extensive model validation against measure-ment data on different engines at various operating conditions was performed. FIGURE 6 demonstrates that the new, fully predictive model can estimate the knock boundary very accurately.

5 SUMMARY

Based on fundamental kinetic investigations and extensive engine testing, a predictive 0-D/1-D knock model was developed. The model was successfully validated against measurement data and thus contributes to an efficient engine development process.

REFERENCES[1] Cai, L.; et al.: Experimental and numerical study of a novel biofuel: 2-Butyl-tetrahydrofuran. In: Combustion and Flame 178 (2017), no. 4, pp. 257–267[2] Cai, L.; et al.: Optimized chemical mechanism for combustion of gasoline surrogate fuels. In: Combustion and Flame 162 (2015), no. 5, pp. 1623–1637[3] Adomeit, P.; et al.: Effect of Intake Port Design on the Flow Field Stability of a Gasoline DI Engine. SAE Technical Paper 2011-01-1284, 2011[4] Parsons, D.; et al.: The potential of catalysed exhaust gas recirculation to improve high-load operation in spark ignition engine. In: International Journal of Engine Research 27 (2015), no. 6, pp. 592–605[5] Fandakov, A.; et al.: Two-Stage Ignition Occurrence in the End Gas and Modeling Its Influence on Engine Knock. In: SAE International Journal Engines 10 (2017), no. 4, pp. 2109–2128[6] Fandakov, A.; et al.: Ein neues Modell zur Vorhersage der Klopfgrenze bei Volllast-AGR. MTZ Conference Ladungswechsel im Verbrennungsmotor, Stuttgart, Germany, 2017

THANKSThis report is the scientific result of a completed research task undertaken by the

Forschungsvereinigung Verbrennungskraftmaschinen e. V. (FVV, Frankfurt) under

the number 6301 and processed by the RWTH Aachen University and the Uni-

versity of Stuttgart. We would like to thank the FVV Working Group chaired by

Dr.-Ing. André Kulzer (Dr. Ing. h.c. F. Porsche AG) for their guidance.

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