Key characteristics of e-fuels for lean burn in passive pre-chamber ignition

Lambda load control in spark ignition engines, a new application of prechamber ignition systems

Pre-chamber jet ignition technology can effectively improve flame propagation speed and in-cylinder combustion rate, so it is conducive to the improvement of thermal efficiency and fuel economy. While, studies on the key parameters of the influence of active pre-chamber on combustion and emissions are not systematic. The influence of the key parameters of injection control in active pre-chamber on combustion and emission is not clear at present. Thus, in this paper, an active pre-chamber jet ignition system was designed and used in a single cylinder GDI engine, and the effects of compression ratio and pre-chamber injection strategy on pre-chamber jet ignition were experimentally studied, as well as the comparison of pre-chamber jet ignition and conventional spark ignition. The results show that, at 2750 r/min, IMEP 11 bar operation point, lean burn can effectively reduce the fuel consumption and increase the thermal efficiency of gasoline engine. Adopting traditional spark plug system can extend the lean burn limit to excess air ratio of 1.5 with the gross indicated thermal efficiency (GITE) of 45% limited by unstable combustion, while resembling active pre-chamber system can achieve GITE of 46.5% with the excess air ratio of 2.0 with the help of much more stable combustion. And the NOx emission of active-pre-chamber system has been reduced by 78% compared by conventional spark plug system. Increasing the compression ratio to 14.8 can further reduce the indicated fuel consumption to 177 g/kWh, and increase the GITE to 48.5% and further reduce the NOx emission to lowest 0.53 g/kWh with the excess air ratio of 2.1. With the increase of pre-chamber injection pressure, the ignition stability increases, the combustion duration decrases, and thermal efficiency increases. With the increase of the pre-chamber injection duration, the ignition delay first decreases and then increases. When the injection duration increases to 800 μs, COV is greater than 3%. If the pre-chamber injection duration further increase, the COV increases, the combustion phasing retard, and the thermal efficiency decreases. With the increase of pre-chamber injection duration and pressure, the wetting wall fuel increases, which leads to the increase of PN emission.

A review of prechamber ignition systems as lean combustion technology for SI engines

Development of a homogeneous charge pre-chamber torch ignition system for an SI engine fuelled with hydrous ethanol

The operation of a cogeneration internal combustion engine with unscavenged prechamber ignition was investigated. The objective was to evaluate the potential to reduce the exhaust gas emissions, particularly the CO emissions, below the Swiss limits (NOx and CO emissions: 250 and 650 mg/m3N, 5% O2 , respectively), without exhaust gas after treatment. The investigation was carried out on a small size gas engine (6 cylinders, 122 mm bore, 142 mm stroke) and required the development of cooled prechambers and the modification of the engine cylinder heads. The approach was essentially experimental, but included a numerical simulation based on the CFD-code KIVA-3V in order to assist and guide the experimentation. The numerical simulation was carried out in order to evaluate the differences in flow characteristics at the location of the spark plug electrodes between direct and prechamber ignition. Further, the influence of the prechamber geometrical configuration was investigated through variations of the nozzle orifice diameter, number and orientation, as well as prechamber volume and internal shape. Based on the results of the numerical simulation, the most promising prechamber configuration parameters were selected for experimentation. Then, variations of the selected prechamber configuration parameters, as well as of the piston geometry, of the turbocharger characteristics and of the engine operating parameters, were carried out in order to determine their influence on the engine performance and emissions. Through the generation of gas jets in the main chamber, the use of a prechamber strongly intensifies and accelerates the combustion process. However, this advantage is conditioned by a significant delay of the spark timing in order to generate substantial gas jets. This results in a large decrease in peak cylinder pressure and in an important reduction of NOx, CO and THC emissions. Minimum emissions are achieved at a spark timing of about 8°CABTDC. The prechamber geometrical parametric study indicates that trends which increase the penetration of the gas jets and/or promote an early arrival of the flame front at the piston top land crevice entrance are beneficial to reduce the CO and THC emissions. In comparison with the direct ignition, the prechamber ignition yields approximately 40% and 55% less CO and THC emissions, respectively. However, this also leads to about 2%-point lower fuel conversion efficiency. The optimisation of the turbocharger results in a recovery of about 1%-point in fuel conversion efficiency, but a consequent change in the exhaust manifold gas dynamics causes an increase in THC emissions. At the rated power output (150 kW), the prechamber ignition operation fulfils the Swiss requirements for exhaust gas emissions and still achieves a fuel conversion efficiency higher than 36.5%.

<div class="section abstract"><div class="htmlview paragraph">The current trend in automobiles is towards electrical vehicles, but for the most part these vehicles still require an internal combustion engine to provide additional range and flexibility. These engines are under stringent emissions regulations, in particular, for the reduction of CO2. Gas engines which run lean burn combustion systems provide a viable route to these emission reductions, however designing these engines to provide sustainable and controlled combustion under lean conditions at λ=2.0 is challenging. To address this challenge, it is possible to use a scavenged Pre-Chamber Ignition (PCI) system which can deliver favorable conditions for ignition close to the spark plug. The lean charge in the main combustion chamber is then ignited by flame jets emanating from the pre-chamber nozzles. Accurate prediction of flame kernel development and propagation is essential for the analysis of PCI systems. A modelling approach is proposed based on the Dynamic Discrete Particle Ignition Kernel model coupled with the G-equation combustion model. The model is validated for an air/methane academic benchmark. The approach is then applied to the investigation of performance of three pre-chamber designs developed within Horizon 2020 GASON project in conjunction with the experimental investigation of these pre-chambers mounted on Rapid Compression Expansion Machine (RCEM). The investigated pre-chamber designs vary with respect to the tangential nozzle angle and volume. The study focusses on a lean limit of the proposed system’s operation with the main charge at λ=2.0 and a variation of pre-chamber design and scavenging level. The comparison of the simulation results with the experimental observations demonstrates good accuracy of the developed model. In addition, the combined experimental and modelling provides insights into the effect of pre-chamber geometry on potential performance.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Pre-chamber ignition is a method to simultaneously increase the thermal efficiency and to meet ever more stringent emission regulations at the same time. In this study, a single cylinder research engine is equipped with a tailored pre-chamber ignition system and operated at two different compression ratios, namely 10.5 and 14.2. While most studies on gasoline pre-chamber ignition employ port fuel injection, in this work, the main fuel quantity is introduced by side direct injection into the combustion chamber to fully exploit the knock mitigation effect. Different pre-chamber design variants are evaluated considering both unfueled and gasoline-fueled operation. As for the latter, the influence of the fuel amount supplied to the pre-chamber is discussed. Due to its principle, the pre-chamber ignition system increases combustion speeds by generating enhanced in-cylinder turbulence and multiple ignition sites. This property proves to be an effective measure to mitigate knocking effects. It is shown that less spark retard compared to conventional spark ignition allows to exploit the efficiency benefit of elevated compression ratios also in high load operation for stoichiometric mixtures. Furthermore, auxiliary fueling of the pre-chamber enables ultra-lean combustion (λ=2.0) with very low NO<sub>x</sub> emission levels. Apart from decreased throttling losses lean burn assists to further suppress knocking in case of elevated compression ratios. Finally, it is demonstrated that the combination of ultra-lean combustion and elevated compression ratio leads to a significant improvement of the indicated efficiency with peak values of about 43 %. Higher PN emissions in low load and increased heat losses remain challenges of the used pre-chamber ignition system.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Pre-chamber ignition systems enable the combustion of homogeneous lean mixtures in internal combustion engines with significantly increased thermal efficiency. Such ignition systems provide a much higher ignition energy compared to a common spark ignition by burning a small portion of the charge in a separate chamber, generating multiple ignition sites in the main combustion chamber and increasing the turbulent flame speed. Pre-chamber ignition systems are commonly used in large natural gas engines but the integration in automotive engines is not feasible so far due to the lack of suitable fuelling systems needed to keep the pre-chamber mixture stoichiometric at lean operation of the engine. Based on preliminary investigations we developed an ignition system with fuelled pre-chamber for automotive engines utilizing the available space for the conventional spark plug. These investigations proved the thermal stability and function of the system up to a specific power of 100 kW/l and a speed of 12500 rpm. The fuelling system saturates air with fuel vapour and injects this mixture time-controlled into the pre-chamber. The already available fuel tank ventilation system in Otto-engine powered cars can provide such an air-gasoline-vapour mixture. Further, the system uses an inexpensive low pressure solenoid valve to control the pre-chamber enrichment. The prototype of the pre-chamber incorporates a spark plug, fuelling valve, thermocouple and pressure transducer and was measured in a full engine at characteristic operating points regarding thermal efficiency, combustion process and emissions. The ignition system showed the ability to ignite highly diluted mixtures with <i>λ</i> = 1.6 and an efficiency gain of 14.8% compared to stoichiometric spark plug operation at 4.5 bar IMEP and 1500 rpm with a NO<i><sub>x</sub></i> emission below 100 ppm.</div></div>

Numerical simulation of lean premixed combustion characteristics and emissions of natural gas-ammonia dual-fuel marine engine with the pre-chamber ignition system

A common solution in reducing NOx emissions to meet new emission regulations has been lean burn combustion. However, with very lean air∕fuel (A∕F) ratios, both carbon monoxide and hydrocarbon emissions become unacceptably high due to the spark misfiring and combustion instabilities. In order to mitigate this, a prechamber ignition system is often used to stabilize combustion at very lean A∕F ratios. In this paper, the heat release in a retrofit prechamber system installed on a large bore natural gas engine is examined. The heat release analysis is based on dynamic pressure measurements both in the main chamber and prechamber. The Woschni correlation is utilized to model heat transfer. Based on heat release modeling and test data analysis, the following observations are made. Main chamber heat release rates are much more rapid for prechamber ignition compared to spark ignition. During combustion in the prechamber, much of the fuel flows into the main chamber unreacted. About 52% of the mass in the prechamber, at ignition, flows into the main chamber during prechamber combustion. Prechamber total heat release, pressure rise, and maximum jet velocity all increase with increasing prechamber equivalence ratio. Prechamber combustion duration and coefficient of variation of peak pressure are minimized at a prechamber equivalence ratio of about 1.09.

Pre-chamber ignition systems are one way to enable homogeneous lean or dilute combustion. Both strategies can significantly increase the efficiency of spark ignition engines. Spark initiated combustion in the pre-chamber produces hot gases that rapidly enter the cylinder and ignite the diluted charge at multiple points. Active ignition systems with fuel in the pre-chamber can directly influence the composition of the pre-chamber to ensure good ignition properties and sufficient ignition energy. This paper shows results from a novel test facility that enables investigations on the jet propagation of an active pre-chamber inside a constant pressure vessel. Thereby, the background mixture inside the vessel is variated between λ = 1.0 and λ = 2.0. With a special designed single-hole pre-chamber the flame propagation of a single pre-chamber jet is measured both in direction of the transfer port and perpendicular to this direction. Compared to spark ignition under equal ambient conditions (350°C, 10 bar), the pre-chamber combustion propagates around eight times faster in direction of the transfer port and 60% faster in radial direction. Until λ = 1.6, the flame propagation speed in radial direction can be kept on the level of a stoichiometric spark ignition. The simultaneous record of OH*-chemiluminescence and high-speed Schlieren imaging shows that the dominant ignition mechanism of a passenger-car sized pre-chamber is jet ignition according to the classification of Biswas. In stoichiometric cylinder conditions, a rich pre-chamber hampers the ignition in the main chamber whereas in lean operation points the scavenging of the pre-chamber with a rich λ = 0.8 mixture is beneficial. These measurements indicate how the disrupted flame front of a pre-chamber jet increases the turbulence and enhances the flame propagation. The highest recorded propagation speed occurs in stoichiometric conditions both in the pre-chamber and in the combustion module.

A common solution to reducing NOX emissions to meet new emissions regulations has been lean burn combustion. However, with very lean air/fuel (A/F) ratios, both carbon monoxide and hydrocarbon emissions become unacceptably high due to spark misfiring and combustion instabilities. In order to mitigate this, a prechamber ignition system is often used to stabilize combustion at very lean A/F ratios. In this paper, the heat release in a retrofit prechamber system installed on a large bore natural gas engine is examined. The heat release analysis is based on dynamic pressure measurements both in the main chamber and prechamber. The Woschni correlation is utilized to model heat transfer. Based on heat release modeling and test data analysis the following observations are made. Main chamber heat release rates are much more rapid for prechamber ignition compared to spark ignition. During combustion in the prechamber much of the fuel flows into the main chamber un-reacted. About 52% of the mass in the prechamber, at ignition, flows into the main chamber during prechamber combustion. Prechamber total heat release, pressure rise, and maximum jet velocity all increase with increasing prechamber equivalence ratio. Prechamber combustion duration and coefficient of variation of peak pressure are minimized at a prechamber equivalence ratio of about 1.09, which corresponds roughly to the equivalence ratio of highest laminar flame speed. The above performance optimum does not correspond to the equivalence ratio where the most prechamber energy is released.

<div class="section abstract"><div class="htmlview paragraph">Environmental policies and fuel costs have driven the development of new technologies for internal combustion engines. In this sense, the use of mixtures with small portions of fuel allows lower fuel consumption and pollutants emissions, emerging as a promising strategy. Despite the advantages, lean burn requires a larger energy source to provide satisfactory flame propagation speed and consequently a stable combustion. The use of pre-chamber ignition systems (PCIS) has been used in SI engines to assist the start of combustion of lean mixtures, in which a supplementary fuel system can stratify the amount of either liquid or gaseous fuels supplied to the pre-chamber. In this context, this paper aims to evaluate combustion characteristics of a commercial engine with the use of stratified PCIS operating with impoverished mixtures of ethanol-air in main-chamber and hydrogen assistance in pre-chamber. For this analysis bench dynamometer tests were performed evaluating cylinder pressure, exhaust gas temperature, fuel conversion efficiency, cycle thermal efficiency, coefficient of variation of the indicated mean effective pressure, apparent heat release rate, flame development angle and burn duration. As expected, the high amount of energy supplied by the PCIS favored combustion characteristics and exhaust gas temperatures that points to favorable conditions to reduce emissions and fuel consumption. Improvement on combustion stability can also enable the extension of lean limit, making the system even more attractive.</div></div>

<div class="section abstract"><div class="htmlview paragraph">In order to achieve the climate targets, a mix of different powertrain technologies must be pursued to effectively reduce emissions. By producing hydrogen based on renewable energy sources, it becomes a reasonable choice for fueling internal combustion engines. The specific molecular properties of hydrogen thereby open up new possibilities for favorably influencing the combustion process of engines. The present paper deals with the analysis of a single-cylinder engine with passive pre-chamber ignition and a port fuel injection system, which was adapted for lean hydrogen operation. In this way, the test unit was operated in various load and speed ranges with lambda values from 1.5 to 2.5 and achieved up to 23 bar indicated mean effective pressure. The focus of this work is on the numerical investigation of the hydrogen combustion and its effects on the engine system. Special attention is hereby paid to the influence of different lambda operations. Simulations were carried out to evaluate the heat transfer towards the cooling system and to determine energy losses dependent on the gas temperatures. The validated 3D-CFD simulation illustrates the thermodynamic properties, as well as the interaction of injection strategies and mixture formation inside the cylinder and pre-chamber. The analysis points out that lean operation across all loads is advantageous in terms of indicated efficiency and particularly in lower loads up to 6 %-pts can be achieved by applying a de-throttling strategy. By going beyond test bench limitations, the virtual environment shows that the engine is knock-limited with lambda 1.5 and increased combustion temperatures imply high NO<sub>x</sub> emissions at high loads. In contrast, with lambda 2.5, the pressure gradient and turbulence level decrease sharply, so that a boost pressure of 4.5 bar is required to achieve the maximum load with a peak cylinder pressure of 180 bar.</div></div>

To ensure that private cars can continue to be used in the future, they must become significantly more efficient and at the same time emit considerably less pollutants. In addition to pure electric drives, further optimized gasoline engines in hybrid powertrain configurations still offer major potentials in this respect. A major step toward increasing efficiency can be achieved by extremely lean burn combustion. If, in addition to low fuel consumption, this operation should also simultaneously reduce NOx raw emissions, lean-burn operation with relative air/fuel ratios of λ≥ 2 must be enabled in wide ranges of the engine operation map. Within the scope of this publication, results of experimental investigations with a lean burn pre-chamber ignition system applied to a small gasoline engine with 75 mm bore and 90.5 mm stroke are presented. In this context, the effects of the pre-chamber design on emissions and fuel consumption are examined. By comparing different pre-chamber enrichments with natural gas and conventional RON98 gasoline, it can be shown that with the direct liquid injection of gasoline into the pre-chamber similar good thermodynamic results as with natural gas can be achieved with the advantage of easier integration of a single fuel system. Due to its significantly improved lean burn capability with relative air/fuel rations of up to λ = 3, combined with low specific indicated NOx raw emissions of less than 0.1 g/kWh, the presented lean-burn combustion system offers excellent conditions for further efficiency improvements of electrified powertrains. WLTP cycle simulations based on measured engine maps for the developed combustion process resulted in a fuel consumption reduction of up to 10%. At the same time, NOx raw emissions below the Euro 6d limit of 60 mg/km can be achieved.