Numerical investigation on the use of Dimethyl Ether (DME) as an alternative fuel for compression-ignition engines

Dimethyl Ether (DME) is an oxygenated fuel that could favour the transition of the heavy-duty transportation sector to carbon neutrality thanks to its similarities in terms of thermophysical properties with diesel fuel, which will facilitate the retrofitting of existing architectures, and the possibility to achieve good trade-offs between NOx emissions, soot formation and overall combustion efficiency. The possibility of producing it from a multitude of carbon-neutral sources and the low hydrogen-to-carbon ratio would allow for an overall lower CO2 output, making an attractive option in limiting the global warming impact of the heavy-duty transportation sector. In the present work, a numerical analysis of the combustion process of DME is carried out. First, the numerical setup is validated against experimental data available for a constant volume vessel with an initial density of 14.8 kg/m3, discussing the capabilities of a chemistry-based combustion model using tabulated kinetics of homogeneous reactors: the Tabulated Well Mixed (TWM) model. Ignition delay times (IDT) are compared for a wide range of temperatures, from 750 K to 1100 K, and oxygen concentrations, from 15% to 21%. The same setup is then applied in the simulation of a heavy-duty internal combustion engine (ICE). A first validation was done to assess the performance of the numerical methodology in a traditional Mixing Controlled Compression Ignition (MCCI) scenario. Then, two other points were simulated: an MCCI condition with 35% of EGR and a Late-Premixed Charge Compression Ignition (L-PCCI) one, with 35% of EGR and an SOIe of 4 CAD aTDC. Local temperature distributions were compared, analyzing the effect of these technologies in NOx emission mitigation and their impact on gross indicated efficiency (& eta;g), showing the advantages that using DME can have on a real-world application

Ignition Diagnostics based on Spark-Induced Breakdown Spectroscopy for Gas-Engine Applications

Ottomotoren unterliegen zyklischen Variationen im Verbrennungsprozess. Ein vertieftes Verständnis der zugrunde liegenden Mechanismen, welche diese zyklischen Schwankungen antreiben, ist fundamental in Bezug auf die Optimierung von Emissionen und Kraftstoffverbrauch. Der Ort des Flammenkerns und dessen Wachstumsrate sind wichtige Faktoren, welche die Amplitude und den Zeitpunkt des maximalen Zylinderdrucks beeinflussen. Die Entwicklungsgeschwindigkeit des anfänglichen Flammenkerns wird wiederum hauptsächlich durch die lokale Brennstoffkonzentration und -homogenität in der Nähe der Funkenstrecke und die lokalen Strömungsbedingungen, wie turbulente Struktur und Größe, beeinflusst. Gegenwärtig werden Motoren für magere Gemische und für direkteinspritzende Verbrennungskonzepte entwickelt, um den thermischen Wirkungsgrad zu verbessern und die, im Vergleich zu Benzin und Diesel, geringeren CO2-Emissionen zu nutzen. Die höhere Klopffestigkeit von Methan erlaubt es, das Kompressionsverhältnis zu erhöhen. Dies führt jedoch auch zu erschwerten Bedingungen um eine Verbrennung erfolgreich einzuleiten, insbesondere im Mager- und Schichtladungsbetrieb sowie bei hohen Abgasrückführraten. All diese Faktoren erhöhen die Anforderungen an das Zündsystem und an die Steuerung des Gemischbildungsprozesses. Ein Diagnosewerkzeug zum Bereitstellen von Informationen über die Gemischzusammensetzung an der Zündkerze während des Zündzeitpunkts ist sehr wertvoll. Funkeninduzierte Plasmaspektroskopie ist eine Technik, bei der das durch Funkenbildung zwischen zwei Elektroden gebildete Plasma als Anregungsquelle für die optische Emissionsspektroskopie verwendet wird. Die Lichtemission der durch das Plasma angeregten Spezies hängt vom lokalen Gemisch ab und kann daher als ein Diagnosewerkzeug verwendet werden, um während des Zündversuchs Informationen über örtliche Bedingungen um die Zündkerze zu liefern. In Ottomotoren kann diese Methodik mit minimalen Motormodifikationen in der Brennkammer eingesetzt werden und ist daher eine vielversprechende Alternative zu anderen Untersuchungstechniken wie laserinduzierte Plasmaspektroskopie, bei welcher ein Laser in den Brennraum gekoppelt werden muss.In dieser Arbeit wurden verschiedene ruhende Gemischzusammensetzungen bei Umgebungstemperatur und erhöhten Druckbedingungen in einer Konstantvolumenzelle untersucht. Ein Spektrometer und eine intensivierte Kamera wurden verwendet um die Plasmaemission der elektrischen Entladung zu erfassen, um eine SIBS-basierte Messtechnik für die Untersuchung von Zündereignissen zu entwickeln. Die Rolle von Methan, Luft und Wasserstoff auf die elektrischen Entladungseigenschaften und die Plasmaemissionen wurden untersucht. Darüber hinaus wurden atomare und molekulare Spektralemissionen unter verschiedenen Umgebungsbedingungen und für verschiedene Gemische analysiert und verglichen, um ein besseres Verständnis der Spektren der komplexen physikalischen und chemischen Phänomene zu erlangen, die dem Zündvorgang zugrunde liegen. Die Ergebnisse zeigen, dass die erarbeitete Methodik ein kompaktes und vielseitiges Werkzeug für Anwendungen ist, bei denen atomare und molekulare Spezies unter verschiedenen Umgebungsbedingungen detektiert werden

Practical Aspects of Cylinder Deactivation and Reactivation

Cylinder deactivation is an effective measure to reduce the fuel consumption of internal combustion engines. This paper deals with several practical aspects of switching from conventional operation to operation with deactivated cylinders, i.e., gas spring operation with closed intake and exhaust valves. The focus of this paper lies on one particular quantity-controlled stoichiometrically-operated engine where the load is controlled using the valve timing. Nevertheless, the main results are transferable to other engines and engine types, including quality-controlled engines. The first aspect of this paper is an analysis of the transition from fired to gas spring operation, and vice versa, as well as the gas spring operation itself. This is essential for mode changes, such as cylinder deactivation or skip-firing operation. Simulation results show that optimizing the valve timing in the last cycle before deactivating/first cycle after reactivating a cylinder, respectively, is advantageous. We further show that steady-state gas spring operation is reached after approximately 6 s regardless of the initial conditions and the engine speed. The second aspect of this paper experimentally verifies the advantage of optimized valve timings. Furthermore, we show measurements that demonstrate the occurrence of an unavoidable torque ripple, especially when the transition to and from the deactivated cylinder operation is performed too quickly. We also confirm with our experiments that a more gradual mode transition reduces the torque drop.ISSN:1996-107

Efficient light-duty engine using turbulent jet ignition of lean methane mixtures

Diesel engines use diffusion-controlled combustion of a high-reactivity fuel and offer high efficiencies because they combine lean combustion with a high compression ratio. For low-reactivity fuels such as gasoline or natural gas, premixed combustion is used, which leads to lower efficiency levels as usually stoichiometric combustion is combined with lower compression ratios. Trying to apply diesel-like process parameters to low-reactivity fuels inevitably leads to problems with classical spark ignition systems as they are not able to establish robust flame propagation for such hard-to-ignite conditions. One possibility to enable fast combustion for diluted mixtures at high pressure levels is to establish ignition in a prechamber and ignite the charge of the main combustion chamber using the turbulent jets exiting the prechamber. In this study, the experimental results of a prechamber-equipped four-cylinder natural gas engine with 2 L displacement are discussed in detail. In the majority of the engine map, auxiliary fueling is used in the prechamber and a global air–fuel equivalence ratio λ is set to 1.7. At full load, a λ of 1.5 is applied without auxiliary prechamber fueling. The experiments show that such a setup is able to achieve brake efficiency levels of above 45% while maintaining peak brake mean effective pressure levels above 20 bar. At high load conditions, cylinder pressure levels at ignition timing achieve more than 80 bar and cylinder peak pressures of around 180 bars occur. The technology proved to enable robust and very fast combustion at comparably low NOx levels. A remaining challenge for the on-road use of such a technology is the reduction of the methane emissions at lean conditions.ISSN:1468-0874ISSN:2041-314

Increased Internal Combustion Engine Efficiency with Optimized Valve Timings in Extended Stroke Operation

Spark-ignited internal combustion engines are known to exhibit a decreased brake efficiency in part-load operation. Similarly to cylinder deactivation, the x-stroke operation presented in this paper is an adjustable form of skip-cycle operation. It is an effective measure to increase the efficiency of an internal combustion engine, which has to be equipped with a variable valve train to enable this feature. This paper presents an optimization procedure for the exhaust valve timings applicable to any valid stroke operation number greater than four. In the first part, the gas spring operation, during which all gas exchange valves are closed, is explained, as well as how it affects the indicated efficiency and the blow-by mass flow. In the second part, a simulation model with variable valve timings, parameterized with measurement data obtained on the engine test, is used to find the optimal valve timings. We show that in 12-stroke operation and with a cylinder load of 5 Nm, an indicated efficiency of 34.3% is achieved. Preloading the gas spring with residual gas prevents oil suction and thus helps to reduce hydrocarbon emissions. Measurements of load variations in 4-, 8-, and 12-stroke operations show that by applying an x-stroke operation, the indicated efficiency remains high and the center of combustion remains optimal in the range of significantly lower torque outputs.ISSN:1996-107

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition. After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary. This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.ISSN:0196-8904ISSN:1879-222

Spark-induced breakdown spectroscopy for fuel-air equivalence ratio measurements at internal combustion engine-relevant conditions

ISSN:0584-854

Optimal Degree of Hybridization for Spark-Ignited Engines with Optional Variable Valve Timings

The electric hybridization of vehicles with an internal combustion engine is an effective measure to reduce CO2 emissions. However, the identification of the dimension and the sufficient complexity of the powertrain parts such as the engine, electric machine, and battery is not trivial. This paper investigates the influence of the technological advancement of an internal combustion engine and the sizing of all propulsion components on the optimal degree of hybridization and the corresponding fuel consumption reduction. Thus, a turbocharged and a naturally aspirated engine are both modeled with the additional option of either a fixed camshaft or a fully variable valve train. All models are based on data obtained from measurements on engine test benches. We apply dynamic programming to find the globally optimal operating strategy for the driving cycle chosen. Depending on the engine type, a reduction in fuel consumption by up to 32% is achieved with a degree of hybridization of 45%. Depending on the degree of hybridization, a fully variable valve train reduces the fuel consumption additionally by up to 9% and advances the optimal degree of hybridization to 50%. Furthermore, a sufficiently high degree of hybridization renders the gearbox obsolete, which permits simpler vehicle concepts to be derived. A degree of hybridization of 65% is found to be fuel optimal for a vehicle with a fixed transmission ratio. Its fuel economy diverges less than 4% from the optimal fuel economy of a hybrid electric vehicle equipped with a gearbox.ISSN:1996-107

Experimental comparison of efficiency and emission levels of four-cylinder lean-burn passenger car-sized CNG engines with different ignition concepts

Today’s passenger car CNG engines are based on petrol engines which typically have restrictions preventing the exploitation of the full potential of methane based fuels, especially if they have to be operated also on petrol as a second fuel. Additionally, the use of threeway-catalysis limits the engine operation to λ = 1. Here, we present the efficiency potential and the raw emission characteristics for a dedicated four cylinder passenger car CNG engine without sticking to the usual combustion peak pressure and λ limitations. Lean combustion reduces the knocking tendency but, because of the higher pressure levels, increases the ignition energy demand. Therefore, different ignition systems (spark plug, prechamber, Diesel pilot) have been used