Browse Topic: Low temperature combustion (LTC)
Electrification of transport, together with the decarbonization of energy production are suggested by the European Union for the future quality of air. However, in the medium period, propulsion systems will continue to dominate urban mobility, making mandatory the retrofitting of thermal engines by applying combustion modes able to reduce NOx and PM emissions while maintaining engine performances. Low Temperature Combustion (LTC) is an attractive process to meet this target. This mode relies on premixed mixture and fuel lean in-cylinder charge whatever the fuel type: from conventional through alternative fuels with a minimum carbon footprint. This combustion mode has been subject of numerous modelling approaches in the engine research community. This study provides a theoretical comparative analysis between multi-zone (MZ) and Transported probability density function (TPDF) models applied to LTC combustion process. The generic thermo-kinetic balances for both approaches have been analyzed in term of similarities. Only onion-skin for MZ models have been considered in this study. The governing assumptions linked to sub-models for each approach to describe mixing process for TPDF and interzonal heat and mass transport for MZ are discussed. This step identifies the calibrated model parameters for each approach and their effects on the accuracy in predicting LTC mode simulations. This work shows that the transported probability density function model has fewer parameters to calibrate compared to multi-zone model. Transported probability density function seems easier to use for LTC process.
With low-temperature combustion engine research reaching an applicable level, physics-based control-oriented models regain attention. For reactivity controlled combustion concepts, chemical kinetics-based multizone models have been proven to reproduce the governing physics for performance-oriented simulations. They offer accuracy levels similar to high-fidelity computational fluid dynamics (CFD) models but with a fraction of their computational effort. Nevertheless, state-of-the-art reactivity controlled compression ignition (RCCI) simulations with multizone model toolchains still face challenges related to predictivity and calculation speed. This study introduces a new multizone modelling framework that addresses these challenges. It includes a C++ code, deeply integrated with open-source, thermo-kinetic libraries, and coupled to an industry standard 1-D modelling framework. Incorporating a predictive turbulence mixing model, it aims to eliminate dependence on CFD-based initialisation, while applying a novel zonal configuration to achieve sensitivity to the combustion chamber´s geometrical features. Basic sensitivity simulations performed for zonal resolution and chemical kinetic mechanisms prove the approach is fit for purpose. Aiming for optimal trade-off between accuracy and simulation speed, the 12-zone model has a simulation time below three minutes per closed cycle. These achievements are validated against a medium-speed, large-bore, single-cylinder research engine, running in a dual-fuel mode with natural gas and light fuel oil. Using basic submodels, the framework reproduces measured in-cylinder pressure trace within an RMS error of 0.85 bar, and combustion performance indicators within a 5% error margin target. Ultimately, this is the first time the multi-zone kinetic framework has been proven suitable to reproduce RCCI combustion on a state-of-the-art marine engine geometry.
In the past years, stringent emission regulations for Internal Combustion (IC) engines produced a large amount of research aimed at the development of innovative combustion methodologies suitable to simultaneously reduce fuel consumption and engine-out emissions. Previous research demonstrates that the goal can be obtained through the so-called Low Temperature Combustions (LTC), which combine the benefits of compression-ignited engines, such as high compression ratio and unthrottled lean operation, with a properly premixed air-fuel mixture, usually obtained injecting gasoline-like fuels with high volatility and longer ignition delay. Gasoline Partially Premixed Combustion (PPC) is a promising LTC technique, mainly characterized by the high-pressure direct-injection of gasoline and the spontaneous ignition of the premixed air-fuel mixture through compression, which showed a good potential for the simultaneous reduction of fuel consumption and emissions in CI engines. Despite its potential, gasoline PPC might suffer from low combustion controllability and stability, because gasoline spontaneous ignition is significantly affected by slight variation of the local in-cylinder thermal conditions. This paper summarizes the work carried out to optimize gasoline PPC in a light-duty CI engine, operated in a test cell. The investigated system has been slightly modified to guarantee a stable operation, using gasoline instead of diesel, over a wide load range. The first part of the analysis has been focused on the study of gasoline auto-ignition, the goal being to define an injection strategy suitable to guarantee combustion stability. Then, further activity has been focused on performance investigation through a properly defined span of the main control parameters of interest, such as injection pressure and exhaust gas recirculation.
Premixed Charge Compression Ignition (PCCI) is a promising LTC strategy to reduce NOx and soot emissions without relying on after-treatment devices. One major drawback of PCCI is high HC and CO emissions resulting from fuel-wall impingement due to early injection of diesel. Narrow-angle direct injection (NADI) helps reduce the wall wetting of fuel. But it is effective only at lower loads. At mid and higher loads, it increases soot and CO emissions in small-bore engines due to the formation of fuel-rich pockets in the piston bowl region. This problem is addressed using a split injection strategy in the present work. A 3-D CFD model is developed and validated with experimental data at two load conditions. Simulations are performed using CONVERGE CFD software. Split injection strategies are explored using wide (148 deg) and narrow (88 deg) spray included angles. The investigations concluded that a main injection of 20 deg bTDC and 30 deg bTDC were optimal for wide and narrow spray included angles, respectively. For both cases, a dwell time of 15 deg CA was optimal. Compared to single injection, split injection resulted in 2% and 4% improvement in indicated thermal efficiency for wide and narrow-angle, respectively. Split injection results in a reduction of 83% and 80% in CO, 56% and 64% in soot and 48% and 60% in HC emissions for wide and narrow included angle respectively when compared with single-injection NADI-PCCI combustion.
Low Temperature Combustion (LTC) is an emerging technology that offers an alternative to conventional spark and compression ignition. A highly discussed LTC mode is homogeneous charge compression ignition (HCCI), which consists in a combustion of a highly diluted well-mixed charge at the end of compression stroke, when the charge reaches the auto-ignition state. Since HCCI is an LTC mode, it can result in low NOX emissions with an indicated efficiency comparable to a diesel engine. Otherwise, there are some challenges to overcome such as achieving high loads without knocking and combustion timing control. Several methods to control the combustion had been investigated, between them, the injection of water may be useful to extend HCCI knock free operation and to enable combustion phasing control. This work investigated the influence of water injection in the intake of an ethanol HCCI cylinder from a converted diesel generator set. The EGR, used in HCCI, was obtained via total recirculation of exhaust gas from a diesel cylinder. The results indicated that the combustion without water injection tended to start considerably before TDC with a very fast combustion and abrupt heat release rates causing knock. As consequence, the ringing intensity values were higher than the acceptable and the operational load was limited. It has been found that the water injection in the HCCI combustion retarded the combustion and increased its duration, preventing knock. The highest indicated mean effective pressure (IMEP) achieved was of 6.0 bar with a ringing intensity around 5.5 MW/m2 with indicated efficiency of about 40%. Finally, the water injection proved to be an efficient strategy to control the combustion timing and to expand the operation range by controlling mixture's reactivity and intake temperature.
Prior research studies have investigated a wide variety of gasoline compression ignition (GCI) injection strategies and the resulting fuel stratification levels to maintain control over the combustion phasing, duration, and heat release rate. Previous GCI research at the US Department of Energy’s Oak Ridge National Laboratory has shown that for a combustion mode with a low degree of fuel stratification, called “partial fuel stratification” (PFS), gasoline range fuels with anti-knock index values in the range of regular-grade gasoline (~87 anti-knock index or higher) provides very little controllability over the timing of combustion without significant boost pressures. On the contrary, heavy fuel stratification (HFS) provides control over combustion phasing but has challenges achieving low temperature combustion operation, which has the benefits of low NOX and soot emissions, because of the air handling burdens associated with the required high exhaust gas recirculation rates. This work investigates HFS and PFS combustion, efficiency, and emissions performance on a single-cylinder, medium-duty engine with a regular-grade gasoline (91 research octane number) at 1,200 rpm, 4.3 bar, and 3.0 nominal gross indicated mean effective pressure operating points with boost levels similar to those in a medium-duty diesel application. Authority of combustion phasing with main injection timing sweeps for HFS and second injection timing sweeps and fuel split sweeps for PFS are shown. In addition, this work is discussed in the context of previous findings with a light-duty diesel platform, and next steps and future direction for this work are presented1.
Premixed charged compression ignition (PCCI) is a promising low temperature combustion strategy for achieving a simultaneous reduction of oxides of nitrogen (NOx) and soot emissions in diesel engines. However, early direct injection results in a significant penalty in fuel economy, high unburned hydrocarbon (HC), and carbon monoxide (CO) emissions, especially in small-bore diesel engines. In the present work, computational fluid dynamic (CFD) investigations are carried out in a small-bore diesel engine using a commercial CFD software, CONVERGE. The computational models are validated with experimental results at two different load conditions, 20% and 40% of rated load. The validated models are used to carry out parametric investigations on the effects of fuel injection parameters, namely the start of fuel injection timing, injection pressure, and spray cone angle on PCCI combustion. The fuel-air equivalence ratio, temperature, and emission contours are used to get more insight into the effect of fuel injection parameters on the combustion process to reduce the spray wall wetting and the high HC and CO emissions. The results obtained show reduced NOx and soot emissions with advanced injection timings from 20 deg to 50 deg. CA bTDC with a penalty on the HC and CO emissions and the indicated thermal efficiency. Increasing injection pressure from 300 to 900 bar resulted in higher HC and CO emissions at both loads due to increased spray wall impingement. A narrow spray cone angle of 88 deg. results in a significant reduction in the HC and CO emissions by up to 77% and 80%, respectively, compared to a wider cone angle of 148 deg. at low load condition. At high load, using a narrow spray angle resulted in an increase in the CO and soot emissions with only a minor reduction in HC emissions due to poor air utilization in the current small-bore engine.
Extensive experimental investigations done over a decade in different engine types demonstrated the capability of achieving high efficiency along with low levels of oxides of nitrogen (NOx) and soot emissions with low temperature combustion (LTC) modes. However, the commercial application of LTC strategies requires several challenges to be addressed, including precise ignition timing control, reducing higher unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. The lower exhaust gas temperatures with LTC operation pose severe challenges for after-treatment control systems. Among the available LTC strategies, Reactivity Controlled Compression Ignition (RCCI) has emerged as the most promising strategy due to better ignition timing control with higher thermal efficiency. Nevertheless, the complexity of engine system hardware due to the dual fuel injection system and associated controls, high HC and CO emissions are the major limiting factors in RCCI. Homogeneous Charge with Direct Injection (HCDI) strategy is recently proposed to address the above limitations of RCCI. Unlike RCCI, HCDI is a single fuel LTC strategy with port and direct injection of diesel fuel, and thus, there is no reactivity stratification. However, the equivalence ratio and thermal stratification with direct-injected (DI) diesel fuel result in better combustion control lower HC and CO emissions in HCDI. The HCDI strategy is investigated with multiple injections of direct-injected (DI) fuel to examine the benefits in the present work. A production light-duty diesel engine used for agricultural water pumping applications is modified to run in HCDI mode through suitable changes in the intake manifold and fuel injection system. Experiments are conducted at the rated engine speed under varying load conditions in conventional diesel combustion (CDC), HCDI with single and double pulse DI modes. The results obtained show that at 4.6 bar imep, the indicated thermal efficiency is increased by 3.9% compared to CDC. However, NOx emissions are increased from 2.7 g/kW-hr to 13.41 g/kW-hr, CO increased from 2.8 g/kW-hr to 10.49 g/kW-hr, but UHC decreases drastically from 4.18 g/kW-hr to 0.10 g/kW-hr in single DI pulse HCDI. In the case of double-pulse HCDI at 3.5 bar imep, indicated thermal efficiency increases by 5.5%, CO decreases by 13%, UHC increase by 48%, smoke increase by three times, and NOx increase by 49.7% in comparison of single pulse HCDI.
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