Mechanisms of the NO2 Formation in Diesel Engines

Diesel Engines

RESEARCH Diesel Engines

AUTHORS Mechanisms of the NO2 Formation in Diesel Engines

While the formation of nitric oxide in diesel engines has been extensively Dipl.-Ing. Michael Rößler investigated in the past, the processes of the engine-internal formation of is Research Associate at nitrogen dioxide are only hardly known. Although nitrogen dioxide represents the Institute of Internal Combustion Engines at the only a small proportion of the total nitric oxide emissions, the highly reactive Karlsruhe Institute of Technology (KIT) species plays an important role in exhaust gas aftertreatment. The current (Germany). FVV research work at the KIT aims to develop a basic understanding of the processes and mechanisms in the engine-internal formation of nitrogen dioxide.

Prof. Dr. sc. techn. Thomas Koch is Head of the Institute of Internal Combustion Engines at the Karlsruhe Institute of Technology (KIT) (Germany).

Dipl.-Chem. Corina Janzer is Research Associate at the Department of Molecular Physical Chemistry at the Institute of Physical Chemistry at the Karlsruhe Institute of Technology (KIT) (Germany).

Prof. Dr. rer. nat. Matthias Olzmann is Head of the Department of Molecular Physical Chemistry at the Institute of Physical Chemistry at the Karlsruhe Institute of Technology (KIT) (Germany).

70 Diesel Engines

1 NITROGEN DIOXIDE IN THE ENGINE EXHAUST the exhaust tract is also elementary for the efficiency of the entire 2 ANALYSIS OF ENGINE-INTERNAL FORMATION PROCESSES exhaust aftertreatment system. 3 TEST ENGINE AND BOUNDARY CONDITIONS 4 INFLUENCE OF ENGINE LOAD AND SPEED 2 ANALYSIS OF ENGINE-INTERNAL 5 PARAMETER VARIATIONS FORMATION PROCESSES 6 FAST GAS SAMPLING

7 KINETIC MODELING OF NO2 FORMATION During the combustion process, mainly NO is formed. From this, 8 SUMMARY NO2 can be formed subsequently. The formation mechanisms of NO have already been well investigated and comprehensively described. In contrast, the processes and mechanisms for the

engine-internal formation of NO2 are hardly known. Within the

FVV research project Formation/Modeling NO2, which was founded by the BMWi, the basic understanding for the formation

of NO2 under diesel engine conditions is to be developed using experimental and simulative tools. Extensive experimental investigations were performed on an engine test bench to identify and to analyse the impact of thermodynamic conditions and engine operating parameters on the engine-internal formation processes. Furthermore, a detailed chemical kinetic mechanism is developed for the description of the reaction pathways and the

formation processes of NO and NO2. For the determination of relevant concentration-time curves for the parameterisation and 1 NITROGEN DIOXIDE IN THE ENGINE EXHAUST validation of the mechanism, additional investigations on a shock tube were carried out. The latest discussions on pollutant emissions from diesel engines and the consequent urban emission load point out, in particular, 3 TEST ENGINE AND BOUNDARY CONDITIONS the relevance of nitrogen dioxide (NO2) emissions. Their harmful effects on humans and the environment are undisputed. In the The test engine used is a single-cylinder research aggregate,

case of strong solar irradiation, NO2 is a relevant precursor sub- based on the medium-heavy Euro VI commercial vehicle diesel

stance for the formation of ground-level ozone (O3) and thus, it is engine OM 936 from Mercedes-Benz. The technical data of the regarded as the main cause of photochemical smog. Furthermore, test engine are listed in TABLE 1. The test bench design allows

the compound of NO2 with moisture leads to the formation of individual and reproducible adjustment of all operating parame-

nitrous acid (HNO2) or directly to the formation of nitric acid ters. Coolant and engine oil are delivered via separate condition-

(HNO3). On the one hand, these are particularly detrimental to ing circuits and the fuel supply is carried out via an autarkical human respiratory organs and, moreover, as acid rain also damage driven high-pressure pump. A modified engine control unit is the ecosystem. used to specify the injection parameters. An external water-lu-

On the other hand, NO2 is very useful for exhaust aftertreatment bricated compressor is used to provide the demanded charge

of modern diesel engines. For example, the highly reactive NO2 pressure. Besides the pressure, also the temperature and the promotes soot oxidation and thus contributes significantly to the humidity of the charge can be set very precisely over large passive regeneration of diesel particulate filters. An increased pro- ranges. An exhaust throttle valve is used to adjust the exhaust

portion of NO2 is also advantageous in the aftertreatment of nitro- gas back pressure and thus to control the pressure gradient gen oxides using SCR technology because the catalytic efficiency between intake and exhaust gas. This allows adjusting near-se-

rises considerably with increasing NO2 concentration. NO2 that is ries operating conditions, corresponding to the turbocharged full formed in the engine is used up very quickly in the exhaust tract engine. Further, the increased exhaust gas back pressure is

and is usually not emitted into the atmosphere. But NO2 can also required to realise the high-pressure EGR. A gas-coolant heat be formed outside the combustion chamber. At an oxidation cat- exchanger with a mounted rotary valve is used for cooling and

alyst, nitric oxide (NO) may react easily with excess oxygen to NO2. dosing the recirculated exhaust gas. Those components corre- For this reason, the arrangement of the catalytic components in spond to the engine’s serial parts. An emission measurement system AVL AMA4000 and an FTIR of the type MKS MG2030 are used for the analysis of the compo- sition of the exhaust gas. The AMA4000 has a dual-channel TABLE 1 Specifications Engine data single-cylinder OM 936 of the single-cylinder chemiluminescence detector (CLD) that is capable of simultane- Stroke × bore [mm × mm] 135 × 110 test engine OM 936 ously measuring the concentrations of NO and NOx using a ther- (© KIT) mocatalytic NO converter. The concentration of NO then results Displacement [l] 1.283 2 2 from the difference between these two measured values. In addi- Compression ratio [-] 17.4 : 1 tion, the NO2 concentration is measured with a fast infrared Maximum torque [Nm] 220 analyser. DEGAS (Dynamic Exhaust Gas Analyser System) deter-

Maximum engine speed [rpm] 2600 mines the absorption of NO2 in the narrowband middle infrared range (wavelength = 6.135 μm) and allows the dynamic measure- Maximum rail pressure [bar] 2400 ment of NO2 with a sampling rate of up to 200 Hz [1].

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4 INFLUENCE OF ENGINE LOAD AND SPEED speeds. Here, the high combustion temperatures and the low process velocities ensure ideal conditions for the formation of thermal

Complying with current emissions directives requires the adaptation NO and are therefore decisive for the total NOx emissions. of various operating parameters such as EGR rate, air-fuel ratio λ For the formation of NO2, additionally, the oxygen concentration and also injection parameters (e.g. injection time, fuel pressure) for in the combustion chamber plays an important role. The maximum the entire engine operating range. As a consequence, the resulting NO2 concentration is recorded at medium engine load and low engine pollutant concentrations in the exhaust gas are significantly charac- speed. Under these conditions, an adequate amount of NO is formed terised by these specific application measures. But to consider the in the combustion chamber which, in combination with excess oxy- dependency of the nitrogen oxide emissions from engine load and gen and a sufficiently large time window, promotes NO2 formation. speed without any interfering effects, imposed by those applicational At higher loads and thus decreasing λ, the NO2 concentration also settings, the engine is operated without exhaust gas recirculation. drops significantly. And especially at high speeds, the NO2 emissions Further operating parameters are only adapted to a lesser extent, are reduced to a minimum. However, it is generally conspicuous that

FIGURE 1. Under these conditions, the NOx emissions reach their NO2, in this case, represents only a very small proportion of the total maximum concentration at the highest engine loads and low engine NOx emissions. In most cases, this is less than 4 %.

FIGURE 1 Engine maps of the concentrations of

NO and NO2 without EGR (© KIT)

FIGURE 2 Impacts of injection tim- ing and rail pressure on the NO und

NO2 emissions (partial load: 55 Nm at 1400 rpm) (© KIT) 72 FIGURE 4 Results of the in-cylinder measurement of the NO2 concentration by using fast gas sampling (partial load: 55 Nm at 1400 rpm) (© KIT)

rail pressure and start of injection. Although an increase in NO

concentration also causes an increase in the NO2 concentration,

the proportion of NO2 in the total NOx emissions decreases.

Accordingly, the highest NO2/NOx ratios are always recorded at the

lowest NOx emissions. Solely the increase in λ shows a deviating behaviour. While all other measures, particularly the exhaust gas recirculation, primarily influence the NO emissions, a change of λ acts more strongly on the nitrogen dioxide. Thus, an increase of λ also leads to an increase

in the NO concentration. However, the NO2 concentration increases even more. The comparison of the different dilution variations in FIGURE 3 Impacts of NO2 and EGR on the NO und NO2 emissions (partial load: 51 Nm at 1800 rpm) (© KIT) FIGURE 3 shows that the influence of the EGR rate on NO2, com- pared to the influence ofλ , is negligibly small.

6 FAST GAS SAMPLING 5 PARAMETER VARIATIONS In order to analyse engine-internal formation processes, exhaust The variation of the different operating parameters and influenc- gas samples are taken directly from the combustion chamber using ing variables shows that significantly higher NO2/NOx ratios can a gas sampling valve. A pressure sensor bore, which is located in also occur [2]. In the course of these investigations, the influences the cylinder head at the level of the piston bowl lip between the of different injection parameters, the charge dilution with fresh air inlet and outlet sides, serves as an access to the combustion and exhaust gas, as well as the charge state (temperature, humid- chamber. The gas sampling is applied at defined points of time ity) for different operating points are considered. Across all varia- during the working cycle. The solenoid valve opens in every tenth tions, it can be seen that any measurable effect on NOx emissions working cycle with a duration of 1.5 ms. By varying the time point always coincides with the change in both, NO and NO2 emissions. of the gas sampling, the course of the concentration of NO2 dur- Thus, an increase in NO also results in a simultaneous increase ing the phases of compression and expansion can be analysed, in NO2. FIGURE 2 illustrates this by an exemplary variation of the FIGURE 4. The measurement data show that the NO2 concentration

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increases immediately after the start of combustion. During com- tration does not seem to change, as the measured values before bustion, NO2 increases further but then decreases again during the start of the combustion show. expansion. During the later expansion, the measured values of NO2 correspond to the NO2 concentration of the emitted exhaust gas. 7 KINETIC MODELING OF NO2 FORMATION This leads to the assumption that the formation of NO2 appears to be completed at the end of combustion, and no further NO2 For the kinetic modelling, a reaction mechanism was developed that formation occurs during the later expansion and the charge cycle. is able to describe both fuel combustion and formation of NOx.

Even in the externally recirculated exhaust gas, the NO2 concen- n-heptane was selected as a model fuel due to its moderate molec-

FIGURE 5 Influence ofλ and NOX on the NO2 emissions (above) and the NO2/NOX ratio (below) – illustration of experimental results from a variation of λ and EGR (left) and the results from the kinetic simulations (right) (© KIT)

FIGURE 6 Correlation of NO2 or NO2/NOX with NO from various parameter variations: experimental results (left) and simulation results (right) (© KIT) 74 ular size and a cetane number (CN ≈ 54) [3] similar to that of diesel combustion chamber. The simulation points out that particularly fuel. The complete reaction mechanism with 668 chemical species the direct oxidation of NO with peroxy radicals plays an important and 6654 reactions consists of an n-heptane oxidation mechanism role. In addition, low engine speeds promote the NO2 formation,

[4] and a sub-mechanism for NOx formation [5-9]. The sub-mech- as more time is available for the formation process. Thus, all meas- anism was reduced for the actual range of our experimental condi- ures for influencing the NOx emissions also indirectly affect the tions. In this way, the number of species could be decreased by 17 NO2 formation. However, since those measures primarily influence and the number of reactions by 985. All simulations [10] were per- the NO formation, the proportion of NO2 usually decreases with formed with this reduced mechanism, where homogeneous reaction higher NOx concentrations. In contrast, any change in NO2 has a mixtures and adiabatic-isobaric conditions were assumed; transport stronger effect on the formation of NO2 than on NO. Therefore, an processes were not included. increase in the oxygen concentration leads to an increase in NO2

Extensive reaction path analyses revealed that NO2 is exclusively emissions with a simultaneous increase in the NO2/NOx ratio. The formed via NO, where the most important reaction is the direct oxi- results from the fast gas sample from the combustion chamber as dation of NO by peroxy radicals according to: well as from the chemical kinetic simulations support the assump-

tion that NO2 is formed directly during combustion. Accordingly, no further formation processes in the exhaust tract without any Eq. 1 NO + RO → NO + RO 2 2 interaction with catalytic components are to be expected.

Here, R represents a hydrocarbon radical or a hydrogen atom. REFERENCES Another reaction pathway proceeds via the reaction sequence: [1] Brunner, R.; Lambrecht, A.; Herbst, J.; Heubuch, A.; Jacob, E.: Quanten- kaskaden-Laserspektrometer für die schnelle und artefaktfreie Abgasanalyse.

7. Internationales Forum Abgas- und Partikelemissionen, Ludwigsburg, 2012 [2] Rößler, M.; Velji, A.; Janzer, C.; Koch, T.; Olzmann, M.: Formation of Engine Eq. 2 NO + OH + O2 → HNO2 + O2 → HO2 + NO2 Internal NO2: Measures to Control the NO2/NOx Ratio for Enhanced Exhaust After Treatment. SAE paper 2017-01-1017, SAE World Congress Experience, Detroit, 2017 [3] Yanowitz, J; Ratcliff, M. A.; McCormick, R. L.; Taylor, J. D.; Murphy, M. J.: As peroxy radicals are relatively stable intermediates, comparably Compendium of experimental cetane numbers. In: Technical Report, National high RO2 concentrations can be built up in the cooler regions of the Renewable Energy Laboratory (2014) flame. These peroxy radicals react with NO that is formed via the [4] Mehl, M.; Pitz, W. J.; Westbrook, C. K.; Curran, H. J.: Kinetic modelling Zeldovich mechanism in the hotter regions of the flame and trans- of gasoline surrogate components and mixtures under engine conditions. In: Proc. Combust. Inst. 33 (2011), pp. 193-200 ported by diffusion into the cooler regions [11]. In the simulations, [5] Faravelli, T.; Frassoldati, A.; Ranzi, E.: Kinetic modelling of the interactions we obtained significantly higher concentrations of HO2 than of RO2 between NO and hydrocarbons in the oxidation of hydrocarbons at low tempera- and consequently a dominance of the reaction: tures. In: Combustion Flame 132 (2003), pp. 188-207 [6] Contino, F.; Foucher, F.; Dagaut, P.; Lucchini, T.; D’Errico, G.; Mounaïm- Rousselle, C.: Experimental and numerical analysis of nitric oxide effect on the ignition of iso-octane in a single cylinder HCCI engine. In: Combustion Flame Eq. 3 NO + HO → NO + HO 2 2 160 (2013), pp. 1476-1483 [7] Dayma, G.; Ali, K. H.; Dagaut, P.: Experimental and detailed kinetic modelling study of the high pressure oxidation of methanol sensitized by nitric oxide and In the simulations, high absolute concentrations of NO2 were nitrogen dioxide. In: Proceeding Combustion Instution 31 (2007), pp. 411-418 obtained whenever a large amount of NOx was formed in a large [8] Glaude, P. A.; Marinov, N.; Koshiishi, Y.; Matsunaga, N.; Hori, M.: Kinetic excess of air (λ >> 1). High NO2/NOx ratios, however, were found Modeling of the Mutual Oxidation of NO and Larger Alkanes at Low Temperature. In: Energy Fuels 19 (2005), pp. 1839-1849 for rather low absolute concentrations of NOx. These results are [9] Manion, J. A. et al.: National Institute of Standard and Technology. consistent with the findings of the experimental investigations as NIST Chemical Kinetics Database, http://kinetics.nist.gov/kinetics/ is shown in FIGURE 5. Concordant parameter variations in the exper- [10] Cuoci, A.; Frassoldati, A.; Faravelli, T.; Ranzi, E.: OpenSmoke++: imental studies and the kinetic simulations also revealed a very An object-oriented framework for the numerical modelling of reactive systems good agreement between the respective amounts of NO and the with detailed kinetic mechanisms. In: Computer Physical Communication 192 2 (2015), pp. 237-264 NO /NO ratio as a function of the NO amount, FIGURE 6. 2 x [11] Bowman, C.T.: NOx Formation and Models. In: Encyclopedia of Automotive It turned out that the parameters with the strongest influence on Engineering. Chichester, John Wiley & Sons Ltd. (2014), pp. 83-90 the simulation results were the EGR rate, the temperature at the beginning of the combustion, and λ. The maximum amount of NO2 varies with λ and is located in the range λ = 1.3 to 1.8 depending on the burning conditions. Furthermore, a re-increase occurs for a large excess of oxygen. High temperatures at the start of combustion and low EGR rates lead to enhanced NO2 formation due to the higher resulting temperatures in the combustion chamber and the correspondingly enhanced NO formation. THANKS

The research project Bildung/Modellierung NO2 (project number 1173) was funded by the Bundesministerium für Wirtschaft und Energie (BMWi) and 8 SUMMARY supervised by the Forschungsvereinigung Verbrennungskraftmaschinen e.V. The presented investigations work out the relevant influence (FVV) . The authors would like to thank for the financial funding and the support parameter and process variables for the engine-internal NO2 for- of the supervising working group under the chairman Dr. Frank P. Zimmermann, mation. Both the experimental and the reaction kinetic studies Daimler AG. We would also like to thank Dr.-Ing. Amin Velji, Institute of Internal show that the NO2 formation in diesel engines is mainly influenced Combustion Engines at the Karlsruhe Institute of Technology, for his contribution by the NO concentration and the quantity of excess oxygen in the to the research project as well as to this publication.

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