Heavy-duty Compression Ignition Engine Research Articles

Modeling the impact of the fuel injection strategy on the combustion and performance characteristics of a heavy-duty GCI engine

Gasoline compression ignition (GCI) is a promising strategy to achieve high thermal efficiency and low emissions with limited modifications to the conventional diesel engine hardware. It is a partially premixed concept, which derives its superiority from higher volatility and longer ignition delay of gasoline-like fuels combined with higher compression ratio typical of diesel engines. The present study investigates the combustion process in a GCI engine operating with different injection strategies using computational fluid dynamics (CFD). Simulations are carried out on a single cylinder of a multi cylinder heavy-duty compression ignition engine, which operates at a compression ratio of 17:1 and an engine speed of 1038 rev/min. Two different injection strategies viz., late injection (LI), and early pilot injection (EP) are investigated to understand their impact on combustion and performance of the engine. Renormalized group (RNG) k-ε model is used to describe in-cylinder turbulence and KH RT model is used to simulate the fuel spray breakup. The developed CFD methodology is validated against relevant experimental data under a wide range of operating conditions for each injection strategy. The developed CFD methodology was found to capture the engine combustion behavior quite well. Based on the validated CFD model, the differences in the progress of combustion event for the two injection strategies is highlighted. It was found that a larger pilot fuel mass fraction results in a steeper rise in the initial heat release rate which in turn influences the transition to mixing controlled combustion. In line with the experimental data, the study showed that the late pilot injection strategy with three injection pulses, results in higher performance compared to the other conditions.

Investigation of the drop-in capabilities of a renewable 1-Octanol based E-fuel for heavy-duty engine applications

This article presents the results of an investigation of neat 1-Octanol and its blends with Diesel, which can be potential drop-in fuels for heavy-duty compression ignition engines. Fuel blends consisting of 10 %, 20 %, and 30 % 1-Octanol by volume in Diesel, including neat 1-Octanol, were investigated for ignition and combustion performances. The ignition behavior was assessed using a constant-volume combustion-chamber-based ignition tester. The experimental investigation was conducted on a heavy-duty single-cylinder engine by varying the combustion center. In addition, injection-rate shaping was performed to understand the effects of various injection-rate shapes, namely square, ramp, and boot rate shapes, on the performance of neat 1-Octanol.The ignition tester results indicated an increased ignition sensitivity of neat 1-Octanol towards the chamber pressure and temperature compared to that of Diesel. At an elevated chamber temperature (650 °C) and pressure (20 bar), the ignition delay of neat 1-Octanol approaches closer to that of Diesel. Engine experiments revealed an improved thermal efficiency with neat 1-Octanol and its blends by up to 1.2 %, owing to their improved mixture homogenization capabilities. Furthermore, neat 1-Octanol and its blend led to a drastic reduction in smoke and carbon monoxide. In addition, it results in the highest peak during the premixed heat release, and with a square injection-rate shape, an additional minor improvement in thermal efficiency was observed. The relative changes in carbon monoxide and hydrocarbon emissions between the injection-rate shapes were negligible. At higher load, the boot-rate shape results in lower NOx formation than the square-rate shape at an almost identical thermal efficiency. Overall, the 10 % 1-Octanol, 20 % 1-Octanol and 30 % 1-Octanol blends, which fulfill the Cetane number requirements for the given fuel norms, can be used as drop-in fuels, as they exhibit a comparable or even better performance than Diesel.

The potential of dimethyl ether (DME) to meet current and future emissions standards in heavy-duty compression-ignition engines

Dimethyl Ether (DME, H3C-O-CH3) is regarded as a meaningful alternative fuel as it has a comparably high energy density, can easily be stored on vehicles, can be produced via different renewable pathways, has a high Cetane number and is therefore suitable for use in efficient and durable compression ignition engines. Moreover, the DME molecule lacks carbon bonds and due to the inbuilt oxygen atom enables low-sooting diffusion combustion. This study reports on the performance and emission characteristics of a state-of-the-art heavy-duty compression ignition engine with 11 L displacement which was optimized specifically for DME combustion. This includes the adaptation of the fuel supply and injection system as well as the combustion chamber geometry. In addition, the engine is equipped with an electrically driven volumetric pump in the exhaust gas recirculation (EGR) path which gives the freedom to set any desired EGR rate, independently from the pressure ratios across the turbocharger. The experimental results show that DME operation retains the performance on the base Diesel engine (338 kW peak power) while significantly reducing the engine out emissions. The NOx-soot tradeoff, typical of Diesel applications, basically does not exist anymore so that a desired NOx level can basically be set without the restrictions of sooting combustion. In addition, the combustion is very complete and very low levels of regulated and non-regulated pollutant emissions can be detected (e.g. for carbon monoxide, DME, methane, benzene, toluene, acetaldehyde, formaldehyde, formic acid). Transient operation is unproblematic and very low emissions can be achieved using the same exhaust gas treatment system as for diesel. Actually, total hydrocarbon, carbon monoxide and particle emission levels of the actual EuroVI and possibly even for the proposed EuroVII on-road legislation could be met without any exhaust gas purification. In terms of tailpipe CO2 emissions, a reduction of around 11% is possible compared with a diesel-fueled engine.

Numerical Investigation of a Heavy-Duty Compression Ignition Engine Converted to Ammonia Spark-Ignition Operation

Abstract Global decarbonization requires the increased use of zero-carbon fuels. Compared to hydrogen, ammonia is easier to store, transport, and produce. In addition, products of complete combustion of ammonia are water and nitrogen. Therefore, ammonia is an ideal green fuel for internal combustion engines. Drawbacks relate to the high ignition energy and low laminar flame speed of ammonia. This three-dimensional numerical study investigated the potential of converting existing diesel engines to ammonia spark ignition operation. Results indicated a slower kernel inception process, but the speed of the fully developed turbulent flame was enough to complete the bulk combustion process despite the lower laminar flame speed. The problem with pure ammonia operation was the reduced combustion efficiency and the high level of unburned ammonia emissions since the slow spark inception process can be compensated by a larger compression ratio. The results also suggested that emissions formation and subsequent oxidation were a more complex phenomenon. For example, lean ammonia combustion produced higher nitrogen oxides (NOX) concentrations due to the nitrogen in the fuel, despite the lower combustion temperature. Moreover, advancing spark timing reduced the NOX emissions, which was opposite to the traditional spark ignition engines. Additionally, the ammonia in engine crevices that escaped the late oxidation process was the main source of nitrous oxide (N2O) species in the exhaust gas that usually do not appear in traditional engines. Overall, all these results suggested that more fundamental research on ammonia combustion is needed to understand its use in efficient, decarbonized engines.

Hierarchical multi-time-scale predictive thermal management and fuel optimization for heavy-duty compression ignition engines

For heavy-duty diesel engines, NOX emissions reduction is strongly constrained by fuel efficiency. This paper presents a hierarchical model predictive controller (H-MPC) for coordinated control of tailpipe NOX emissions and fuel consumption. The H-MPC uses the separation of slow and fast dynamics that exist in the engine and its aftertreatment system. The controller is synthesized with an architecture in which a high-level MPC uses a longer prediction horizon compared to the low-level predictive controller which tracks the high-level controller command and manages the thermal dynamics of the aftertreatment system. Engine load preview enables the high-level controller to estimate the desired catalyst temperature ahead of time and addresses the selective catalytic reduction (SCR) slow thermal dynamics. Calculated by the high-level controller, the intake manifold pressure, and the start of injection (SOI) crank angle is used as reference trajectories in the low-level controller that regulates fast dynamical behaviors such as engine out NOX emissions. Hardware-in-the-loop (HIL) validation of this integrated H-MPC on a rapid prototype controller shows that when the SCR catalyst temperature is above light-off temperature (warmed-up condition), the engine operation is shifted to operate with the best fuel economy since the warmed-up SCR can efficiently reduce the engine-out NOX emissions. Results indicate that up to 0.8% benefit in cycle averaged BSFC along with a 13% reduction in tailpipe NOX compared to a stock engine calibration can be achieved with the coordinated engine and aftertreatment system through H-MPC.

Experimental and numerical investigation of an innovative complex piston with enhanced free spray length for the heavy-duty engine applications

The combustion and emission formation process in heavy-duty compression ignition engines are strongly influenced by the design of the piston bowl, the cylinder charge motion and the performance of the injection system. This paper proposes an innovative complex piston bowl design approach called as ’flower piston’ to improve the combustion process. It can be manufactured using the conventional machining approaches without use of still expensive additive manufacturing techniques. The flower piston features a scooped cavity with an intention of increasing the free fuel spray penetration. The potential of this innovative flower piston is evaluated using a validated 3D CFD model and compared with the results of single cylinder engine experiments at operating points with minimum fuel consumption. The results of the experimental investigations with the flower piston show an improvement in thermal efficiency in comparison to the conventional piston by up to 0.5%. This is due to a faster initial heat release and increased high pressure cycle work. However, the smoke emissions increased threefold due to fuel entrapment into the flower cavities, leading to formation of fuel-rich pockets. Also, the wall heat losses increased twofold with an increase in NOx raw emissions. With increased spray targeting angle, there was a drastic reduction in smoke emissions and an additional improvement in the thermal efficiency.

Performance and emission characteristics of diesel-biodiesel-ethanol blends in a heavy-duty compression ignition engine at simulated high altitudes

The effects of anhydrous ethanol (E) and castor oil (R) mixed with diesel fuel (D), commercially containing 5% biodiesel by volume, were researched experimentally to evaluate the combustion process and specific fuel consumption in a compression ignition engine, supercharged, six cylinders. In total, three mixtures were used, varying the diesel content from 95% (D95) to 80% v/v (D85), obtaining the following compositions: D95B0E0, D85B6E9 and D80B6.5E13.5. The experimental tests were performed in stationary conditions (1000 and 1800 rpm), two values ​​of effective torque (80, and 160N.m) and two conditions of intake air pressure (100 and 80 kPa). The results obtained show that the mixtures containing ethanol and biodiesel lead to an early start of combustion at 1000 rpm and a delay at the start of combustion at 1800 rpm. The increase in ethanol and biodiesel over commercial fuel does not have any influence on specific fuel consumption; however, the altitude effect generates an increase in specific fuel consumption up to 11% and 7% at 1000 rpm and 1800 rpm respectively. The specific emissions of NOX, CO and CO2are increased with higher specific fuel consumption; in addition, the effect of simulated altitude allowed to verify that at higher altitudes there is a slight increase in sfc and EE of CO and CO2, together with a reduction in EE NOX. At 2000 m of simulated altitude (APCI = 80 kPa) the maximum increases of sfc, CO and CO2, with the mixture D80B6.5E13.5, were 5%, 122% and 18%, respectively. And the maximum reductions in NOX EE were also obtained with the D80B6.5E13.5 mixture, reaching decreases of up to 6%.

Investigation of combustion characteristics, physical and chemical ignition delay of methanol fuel in a heavy-duty turbo-charged compression ignition engine

In previous research, there have been more investigations on methanol blended with other fuels such as diesel, biodiesel, gasoline, etc., but fewer investigations on methanol with ignition additives as a mono-fuel. To better understand the methanol mono-fuel combustion characteristics and to further apply them, a combined experimental and simulation study of methanol in a Scania heavy-duty compression ignition (CI) engine was carried out in this work. The experiments consisted of four groups with variable injection timings, variable fraction of ignition additives, variable charge air temperatures, and variable overall excess air ratios/power sweeps. Heat release rate (HRR), cylinder pressure, ignition delay and indicated efficiency were analyzed for each case. The analysis showed that the combustion type was partially premixed combustion (PPC) in some cases and diesel-like combustion in the rest. By observing all cases, the shortest ignition delay was 14.1°, and the longest was 22.8°. The indicated efficiencies were in the range of 0.35 to 0.43. Simulations and validation analyses were performed for all cases by a multi-packets model. The physical and chemical ignition delays were predicted. The physical ignition delays were in the range of 4.25 to 8.10°, and the chemical ignition delays were in the range of 6.66 to 17.1°. The chemical ignition delay was always longer than the physical one. This indicates that chemical ignition delay has to be prioritized to improve the ignition performance of methanol fuel.

Comprehensive Correlation for the Prediction of the Heat Release Characteristics of Diesel/CNG Mixtures in a Single-Zone Combustion Model

Fuel combinations with substantial differences in reactivity, such as diesel/CNG, represent one of the most promising alternative combustion strategies these days. In general, the conversion from diesel to dual-fuel operation can be performed in existing in-use heavy-duty compression-ignition engines with minimum modifications, which guarantee very little particles, less nitrogen oxide (NOx), and reduced noise by half compared to diesel. These factors make it feasible to retrofit a CNG fuel system on an existing diesel engine to operate it in dual fuel mode. However, the single-zone combustion models using the traditional single-Wiebe function are exceptionally adopted to assess the dedicated dual fuel engines, whereas the heat loss to the walls is estimated by using the Woschni heat loss formulation. It means that the fast and preliminary analysis of the unmodified engine performance by 1-zone models becomes complicated due to the obvious deterioration of the energy parameters, which, in turn, was predetermined from the deviation in the thermodynamic cycle variables as the calculation outcome. In this study, the main novelty lies in the fact that we propose a novel composition-considered Woschni correlation for the prediction of the heat release duration characteristics of diesel/CNG mixtures for the unmodified diesel engine. The elimination of former deficiencies distinctive to a single-zone thermodynamic model by applying the interim steps described became the core of the research presented in this paper. It led to successful derivation of the necessary correlation for modelling the heat release duration characteristics of an ICE operated in the dual fuel mode.

Autoignition of sustainable fuels under dual operation with H2-carriers in a constant volume combustion chamber

The expected increase in the use of environmentally friendly liquid fuels in medium and heavy-duty compression ignition engines (for both off-road and transport applications), together with the well-documented benefits of hydrogen-carriers to decrease energy dependence and to achieve a neutral-carbon economy (internal combustion engines highly contributing to the global CO2 emissions), have motivated this work. The autoignition characteristics of different alternative diesel-type fuels (hydrotreated vegetable oil, advanced biodiesel and blends of conventional diesel fuel with polyoxymethylene dimethyl ether and 1-butanol) under dual-fuel operation with H2, NH3, and CH4 were studied in a constant volume combustion chamber at 535 and 600 °C. The high reactivity fuel was replaced by the gaseous fuel up to 40 % by energy. The main ignition delay time was significantly affected not only by the type of low reactivity fuel, with ammonia considerably retarding autoignition, but also by the nature of the liquid fuel, biodiesel being the less sensitive to the presence of the H2-carrier fuel. Results also proved that the higher the reactivity of the diesel-type fuel (quantified through the cetane number), the lower the influence of the gaseous fuel on the autoignition time.

Hydrogen or hydrogen-derived methanol for dual-fuel compression-ignition combustion: An engine perspective

Synthetic fuels or e-fuels, produced from captured CO2 and renewable hydrogen, are envisaged as a feasible path towards a climate-neutral transportation in medium/heavy-duty and maritime sectors. EU is presently debating energy targets by 2030 for these fuels. As their production involves chemical processing of hydrogen, it must be evaluated if the extra cost is worthy, at least in applications where hydrogen use is possible. This manuscript focuses on the performance and environmental impact when hydrogen and methanol are fed to a heavy-duty compression-ignition engine working under dual-fuel combustion. The trade-off thermal efficiency-NOx emissions is primary considered in the assessment by combining both variables in an own defined function. During the work, engine operating settings were adjusted to exploit the potential of methanol and hydrogen. Compared to conventional combustion, methanol required centering the combustion towards TDC and doubling the EGR rate, resulting in a low temperature, highly premixed combustion, almost soot-free and with extremely low NOx emissions. The best settings for hydrogen were in the middle of those for methanol and conventional combustion. Results showed great dependance with the engine load, but methanol proved superior to hydrogen for all conditions. At high load, 20–60 % methanol even improved the efficiency and reduced the NOx emissions obtained by conventional combustion. However, at low load hydrogen could substitute 90 % of the diesel fuel, while methanol failed at substitutions higher than 55 %.

The effects of diesel-OME1 blended fuel on combustion characteristics of a heavy-duty compression ignition engine by a numerical study

Diesel engines are playing an important role in the transportation system and regardless to their acceptable efficiency (near 40 %), the main problems are the amounts of CO2 and soot production of these engines. In this research, Oxymethylene ether is used as a blend with diesel by a numerical study, and engine performance, fuel economy, and emission analyses have been investigated. The OM355 engine with Euro II emission standard is selected as the base engine and the percentages of Soot, NOx, CO2, flame speed, flame surface density, brake specific fuel consumption, specific power, and specific torque has been studied from 0 to 60% of diesel-OME1 compounds. Overall results of this numerical investigation indicate that OME1 can be a suitable component due to positive effects on the exhaust emissions. Initial estimates show that engine soot and NOx emissions can be reduced more than 80% and CO2 also near to 66% reduced compared to pure paraffinic diesel. In fact, due to a lower combustion temperature of OME1, soot and NOx emissions would barely produce due to their formation limit temperature, hence the engine operates much cleaner. The main disadvantage of this synthetic fuel is the low amount of heating value and lower cetane number in comparison to diesel, which in turn leads to a performance loss and a higher amount of fuel consumption for producing a specific amount of power and torque output.

Computational study of the multi-injector isobaric combustion concept in a heavy-duty compression ignition engine

Compared to the conventional diesel combustion engine that is equipped with a single centrally installed injector, the multi-injector spray-combustion concept has the potential to achieve higher thermal efficiency. In this work, a numerical study to explore the combustion characteristics of the multi-injector concept on an isobaric combustion compression ignition engine was conducted. The simulation results reveal that, compared to the single-injector case, a significantly lower heat transfer loss was induced using two side injectors owing to the less intense flame-piston interaction. In addition, good fuel flexibility was achieved using multiple injections, which was attributed to the initially generated hot regions downstream of the injection trajectory. To further optimize the engine combustion performance, the effects of several major design parameters such as the injector location and rotation angle, hole number and gap angle, spray angle, and piston geometry were investigated. The results showed that the spray/flame-wall interaction had a significant effect on the development of combustion heat release. Various design parameters showed a significant impact on the combustion heat release during the late combustion period when an intense flame-wall interaction was occurring. To attain a better engine performance, the injector location and nozzle design should be adequately adjusted according to the inner piston radius and depth. The reduction of intake temperature and introduction of an appropriate amount of isochoric combustion process were found to further promote the engine work, making it possible to achieve a thermal efficiency of over 50%.

Effect of multi-angle diesel injector nozzle on emission and efficiency of natural gas/diesel dual-fuel combustion in compression ignition engine

In this study, natural gas and diesel dual-fuel combustion in a heavy-duty compression ignition engine is experimentally evaluated by adopting a modified multi-angle nozzle of a diesel injector to reduce the global warming impact (GWI) factor while maintaining high thermal efficiency.The original injector has a single layer with an umbrella angle of 150° and eight holes, whereas the modified injector has an additional layer below with an injection angle of 90° and four holes. The engine speed is 1,200 rpm under low load conditions (intake pressure of 0.11 MPa and overall equivalence ratio of 0.5, such that the torque is varied); meanwhile, the natural gas fraction is varied from 45% to 71% while considering the combustion stability. The diesel injection timing is varied from the top dead center to 70° before top dead center at intervals of 10° for the injector nozzle conditions. Results show that the modified injector improves combustion efficiency under the reactivity-controlled compression ignition (RCCI) regime. In addition, the GWI factor is reduced by 5.6% compared with that of the original injector. Hence, RCCI combustion by early diesel injection using a multi-angle nozzle affords low emissions while considering the GWI factor, as well as maintains a high gross indicated thermal efficiency of 48.6%.

Artificial Neural Network Modeling and Numerical Simulation of Syngas Fuel and Injection Timing Effects on the Performance and Emissions of a Heavy-Duty Compression Ignition Engine.

The use of syngas as an alternative fuel in compression ignition engines is a potential way to curb the emission of pollutants and optimize the performance of these engines. The present research studied the effect of changing fuel injection timing and adding syngas on the output of a heavy-duty diesel engine. The optimal mode of the turbulence model, fuel spray model, combustion model, and pollutant emission model was used to solve computational fluid dynamics. Changing fuel injection timing from 70 to 10° before top dead center (BTDC) and diesel variants, including conventional diesel, diesel + 20% syngas, and diesel + 40% syngas, is the strategy studied here. It was found that the use of syngas could reduce emissions significantly. The in-cylinder mean effective pressure was the highest for diesel + 20% syngas. On the other hand, increasing the rate of syngas and retarding injection timing reduced the ignition period and in-cylinder temperature. The lowest rate of CO emission was obtained from diesel + 40% syngas at a fuel injection timing of 70° BTDC, whereas the lowest particulate matter emission was related to diesel + 40% syngas at 40° BTDC injection timing. The lowest rate of CO2 emission was related to diesel + 40% syngas at a fuel injection timing of 10° BTDC, while the same timing for conventional diesel exhibited the lowest NOX emission rate. Finally, the performance parameters of the engine including indicated power, indicated fuel consumption, and indicated thermal efficiency were decreased with the increase in syngas fraction, so that their highest values were obtained from the conventional diesel at the injection timing of 40° BTDC. In addition, an artificial neural network (ANN) model based on a standard back-propagation learning algorithm was developed for modeling the performance and emissions of the engine. The results for the optimum ANN model showed that the optimal ANN has two hidden layers with 20–25 neurons and the transfer function of logsig–logsig for hidden layers 1 and 2, respectively, and can predict different parameters of the engine for different modes. The correlation coefficients (R-value) of optimal topology for training, validation, and testing are 0.99992, 0.96612, and 0.93424, respectively. The results for the optimum ANN model showed that the constructed model sufficiently predicts the performance and emissions of the CI diesel engine.

Multiple-objective optimization of heavy-duty compression ignition engine fueled by gasoline/hydrogenated catalytic biodiesel blends at low loads

Multiple-objective optimization of a heavy-duty compression ignition engine fueled by gasoline/hydrogenated catalytic biodiesel (HCB) blends at low loads was performed by employing the KIVA-3V code and genetic algorithm. In addition, the mechanism of multiple-injection and sensitivity of operating parameters on engine performance of the optimal cases were also explored. The results indicated that efficient combustions for G70H30 (70% gasoline and 30% HCB) and G100 (pure gasoline) with ultra-low nitrogen oxides (NOx) and soot emissions could be obtained after optimization. As HCB fraction increases, the ranges of operating parameters become more extensive, and the required initial temperature for optimal cases can be effectively reduced. When the main injection occurs after the ignition caused by pilot injection, main injection moderates the heat release rate (HRR) by creating concentration and temperature stratifications in the spray area simultaneously, and the exhaust gas recirculation (EGR) rate, pilot, and main start of injections and pilot fraction play dominant roles on engine performance. Moreover, when main injection is much more advanced than the ignition timing, main injection controls the HRR only through the concentration stratification in the reaction zone, and the EGR rate, initial temperature, and pilot faction have dominated effects on engine performance.

Performance and emissions of renewable blends with OME3-5 and HVO in heavy duty and light duty compression ignition engines

Interest in poly(oxymethylene)dimethyl ether (OME3-5) as an alternative to fossil fuels in compression ignition engines has increased owing to its potential for soot reduction. The high oxygen content of the polymer and lack of carbon–carbon bonds and aromatic structures can help to reduce engine out soot emissions. However, OME3-5 is potentially damaging to engine components, and thus engine modifications are required when using neat OME3-5.In the present study, OME3-5 was blended with hydrotreated vegetable oil (HVO), rapeseed methyl ester and the C8-alcohol 2-ethylhexanol (an isomer of n-octanol) to ensure miscibility. Three blends were designed with an oxygen content of 6.4, 12.8 and 17.8% by mass. Performance and emissions were compared to the reference fuels fossil diesel and HVO in a single cylinder light duty and heavy duty compression ignition engine at different loads.Evaluation of the combustion in both engines showed similar trends: The indicated thermal efficiency was slightly higher for the oxygenated fuel and the combustion duration shorter compared to diesel. Due to the lower heating value of the blends, the indicated specific fuel combustion increased with increasing share of OME3-5 in the blend.For both engines, engine out soot emissions were decreased strongly, whereas NOx emissions were slightly increased. Analysis of the particle size distribution showed a decrease in the particle number of agglomerated particles (>30 nm) for the blends. For the heavy duty engine, an increase in nucleation mode particles (<30 nm) was measured.

Effect of Spark Timing on the Combustion Stages Seen in a Heavy-Duty Compression-Ignition Engine Retrofitted to Natural Gas Spark-Ignition Operation

Effect of Spark Timing on the Combustion Stages Seen in a Heavy-Duty Compression-Ignition Engine Retrofitted to Natural Gas Spark-Ignition Operation

Computational study of ECN Spray A and Spray D combustion at different ambient temperature conditions

The combustion process of the Engine Combustion Network (ECN) Spray A and Spray D is studied over a wide range of ambient temperatures from 750 K to 900 K. With nominal diameters of 89.4 μm for the Spray A and 190.3 μm for the Spray D, these n-dodecane sprays are representative for light- and heavy-duty compression ignition engine applications, respectively. Computational Fluid Dynamics calculations are carried out using a Lagrangian parcel Eulerian fluid approach in a Reynolds averaged Navier–Stokes framework. For the Spray D reference condition, two sub-grid flame structure assumptions and the effect of turbulence chemistry interaction (TCI) on autoignition and flame structure at quasi-steady state are assessed in the context of a well-mixed and an Unsteady Flamelet Progress Variable (UFPV) combustion model. After that, UFPV approach is used to evaluate combustion behavior for the different ambient temperature conditions. Reference condition results show that both well-mixed and flamelet assumptions lead to a similar autoignition sequence. In terms of ignition delay time, TCI plays an important role, within the UFPV model, in reproducing the experimental trend observed for the increase in nozzle diameter. In terms of lift-off length, the well-mixed model is observed to predict a longer value compared to the flamelet-based sub-grid assumption. Lastly, the analysis of the autoignition sequence and flame structure at quasi-steady state is also extended over the whole range of ambient temperature conditions.

Effect of piston bowl geometry and compression ratio on in-cylinder combustion and engine performance in a gasoline direct-injection compression ignition engine under different injection conditions

Low temperature combustion (LTC) of high-octane number fuels in compression ignition engines offers an opportunity to simultaneously achieve high engine thermal efficiency and low emissions of NOx and particulate matter without using expensive after-treatment technologies. LTC engines are known to be sensitive to the operation conditions and combustor geometry. It is important to understand the fundamental flow and combustion physics in order to develop the technology further for commercial application. A joint numerical and experimental investigation was conducted in a heavy-duty compression ignition engine using a primary reference fuel with an octane number of 81 to investigate the effects of injection timing, piston geometry, and compression ratio (CR) on the fuel/air mixing and combustion covering different regimes of LTC engines, homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and the transition regime from HCCI to PPC. The results show that with the same combustion timing, a higher CR leads to a lower NOx, but a higher emission of UHC and CO. The piston geometry shows a significant impact on the combustion and emission process in the transition regime while it has minor influence in the HCCI and PPC regimes. It is found that high engine efficiency and low emissions of NOx, CO and UHC can be achieved in the earlier PPC regime and later transition regime. The fundamental reason behind this is the stratification of the mixture in composition, temperature and reactivity, which is dictated by the interaction between the spray and the cylinder/piston walls.