International Journal of Engine Research

A new diagnostic for the quantification of Sauter mean diameter in high-pressure fuel sprays has been recently developed using combined optical and X-ray measurements at the Georgia Institute of Technology and Argonne National Laboratory, respectively. This diagnostic utilizes liquid scattering extinction measurements from diffuse back-illumination imaging, conducted at Georgia Tech, and liquid absorption measurements from X-ray radiography, conducted at Argonne’s Advanced Photon Source. The new diagnostic, entitled the scattering–absorption measurement ratio, quantifies two-dimensional distributions of path-integrated Sauter mean diameter, enabling the construction of the spatial history of drop size development within practical fuel sprays. This technique offers unique benefits over conventional drop-sizing methods in that it can be more robust in optically dense regions of the spray, while also providing high spatial resolution of the corresponding droplet field. The methodology for quantification of Sauter mean diameter distributions using the scattering–absorption measurement ratio technique has been previously introduced and demonstrated in diesel sprays using the Engine Combustion Network Spray D injector; however, a more detailed treatment of measurement uncertainties has been needed. In this work, we present a summary of the various sources of measurement uncertainty in the scattering–absorption measurement ratio diagnostic, like those due to the experimental setup, data processing methods, and theoretical assumptions, and assess how these sources of uncertainty affect the quantified Sauter mean diameter. The spatially resolved Sauter mean diameter measurements that result from the scattering–absorption measurement ratio diagnostic will be especially valuable to the engine modeling community for the quantitative validation of spray submodels in engine computational fluid dynamics codes. Careful evaluation and quantification of measurement uncertainties are important to support accurate model validation and to ensure the development of more predictive spray models.

A parametric study was conducted to predict the conditions leading to spray collapse in multi-hole gasoline direct-injection fuel injectors using computational fluid dynamics simulations. The computational fluid dynamics simulations were performed using an in-house multi-dimensional code that accounts for thermal non-equilibrium and entrainment of the non-condensable gas and coupled with primary atomization. The simulations were performed for a fixed injection pressure and fuel temperature on nine different six-hole injectors. The parameters were varied to include the effects of the ratio of the ambient pressure to the saturation pressure (Pa/Ps), the drill angle, and the diameters of the nozzle and the counter bore, respectively, on the spray. The findings indicate that spray collapse results from a combination of the nozzle geometry, the thermodynamic conditions of the fuel, and the ambient pressure. Spray collapse was observed in injectors with a narrow arrangement of the nozzle holes under extreme flash-boiling conditions with very low ambient pressures and in the case of non-flash-boiling conditions with very high ambient pressures.

A new diffusion-based combustion concept (named it as Actively Controlled Rate of Diesel Combustion) for the confirmation of brake thermal efficiency optimum heat release rate profile based on multiple fuel injectors has been investigated. The outstanding results are; it is possible to achieve desired heat release rate profile only by the independent control of injection timing and duration of three injectors installed to a cylinder. The optimum brake thermal efficiency was not achieved with the Otto-like cycle but with the Sabathe-like cycle as predicted by a zero-dimensional thermodynamic model. Furthermore, smoke emissions were concurrently reduced with NOx emissions by increasing fuel amount from the side injectors without any deterioration in CO and total hydrocarbon emissions. On the other hand, brake thermal efficiency itself was not so improved than expected, because of lower heat release in the late part of combustion and unexpected less heat loss reduction. To solve these issues, combustion visualization and numerical simulation analysis were carried out. The results suggested that the adjacent sprays with narrower angle from each side injector deteriorated air entrainment and mixture formation, which might also result in the deterioration in wall heat loss in the expansion stroke. To solve both issues simultaneously, modified nozzle to inject against the swirl from the side injectors was utilized and achieved an improvement in both brake thermal efficiency and heat loss. That is the interdependent and reciprocal control of in-cylinder flow and fuel injection will be one of the breakthrough technologies for current trade-offs by the temporal and spatial spray flame optimization. Furthermore, the nozzle having higher flow rate with less number of orifice was utilized for the side injectors. Even though the smoke emissions were not optimized yet, brake thermal efficiency was much improved with higher heat release rate in the late part of combustion.

This article describes the main results of an investigation about counter-bore injector nozzle impact on the combustion process in a modern Euro 6 diesel engine. First, hydraulic and spray visualization tests have been performed, showing a potential advantage of such nozzle design in fuel–air mixing efficiency. Then, combustion performance has been assessed on a GM-designed 1.6-L four-cylinder engine. The engine has been installed on a dynamometric test bench and instrumented with an AVL cylinder pressure transducer for heat release rate analysis, as well as HORIBA MEXA gas analyzer for exhaust emissions and AVL 415 Smoke Meter. Engine efficiency and emissions have been analyzed on four different part-load steady-state points, representative of New European Driving Cycle and Worldwide harmonized Light duty Test Cycle certification cycles, and covering engine speeds from 1250 to 2000 r/min and brake mean effective pressure between 0.2 and 1.4 MPa. Results of indicated analysis show that counter-bore nozzles have significant differences in terms of pilot injection combustion at low load points, which in turn lead to a better ignition and shorter combustion of the main injection. In addition, an improvement of diffusive combustion is observed as load increases. Because of both, fuel consumption is reduced by approximately 1% with respect to a standard nozzle. Finally, an appreciable decrease in engine exhaust emissions has been recorded, especially in terms of particulate matter and hydrocarbon emissions. This reduction has been linked to the improvement of fuel–air mixing promoted by the counter-bore nozzle previously observed.

Large eddy simulations of transcritical injection and auto-ignition of n-dodecane in a combustion chamber are performed. To this end, a diffuse-interface method is employed that solves the compressible multi-species conservation equations, and a cubic state equation together with real-fluid transport properties is employed to describe the transcritical fluid state. The reaction chemistry is represented by a finite-rate chemistry model involving a 33-species reduced mechanism for n-dodecane. Compared to commonly employed two-phase approaches, the method presented in this work does not introduce tunable parameters for spray-breakup. Large eddy simulation calculations are performed by considering the Spray A single-hole injector at non-reacting and reacting conditions at a pressure of 60 bar and temperatures between 800 and 1200 K. Quantitative comparisons with measurements for liquid and vapor penetration lengths are performed for non-reacting conditions, and sensitivity to threshold values on mixture fraction are examined. The analysis of reacting flow simulations focuses on comparisons of the instantaneous temperature and species fields for OH and CH₂O at 800 and 900 K, respectively. Quantitative comparisons with measurements for ignition delay and lift-off heights as a function of ambient temperature are performed. To examine the transient ignition phase, comparisons of radially integrated OH profiles obtained from the simulations with reported measurements for OH* are performed, showing good agreement. These results show that the large eddy simulation modeling framework adequately reproduces the corresponding ignition processes, which are relevant to realistic diesel-fuel injection systems.

In this research, there are two major sections for analysis: the characteristics of gasoline and diesel dual-fuel combustion and their application to operating load extension with high thermal efficiency and low emissions. All the experiments were completed using a single-cylinder compression ignition engine with 395 cc displacement. In the first section, the dual-fuel combustion modes were classified into three cases by their heat release rate shapes. Staying at 1500 r/min with a total value of 580 J of low heat for each cycle condition, the diesel injection timing was varied from before top dead center with a 6–46 °crank angle with 70% of gasoline fraction based on the low heating value. Among the modes were two suitable dual-fuel combustion modes for a premixed compression ignition. The first suitable mode shows multiple peaks in the heat release rate (mode 2) and the second suitable mode shows a single peak with a “bell-shaped” heat release rate (mode 3). These two dual-fuel combustion types showed a high gross indicated thermal efficiency of up to 46%. Based on the results in the first section, the practical application of dual-fuel premixed compression ignition combustion was investigated considering a high thermal efficiency and a high-load condition. At a 1500 r/min/gross indicated mean effective pressure of 6.5 bar, 48% of the gross indicated thermal efficiency was obtained by using dual-fuel premixed compression ignition combustion mode 3. This mode was typical of a “reactivity controlled compression ignition,” while the nitrogen oxides and the particulate matter emissions satisfied the EURO-6 regulation (0.21 g/kW h and 2.8 mg/m³, respectively). In addition, a high thermal efficiency (45%) with low maximum pressure rise rate, NO_x (nitrogen oxides), and particulate matter emissions was obtained at 2000 r/min/gross indicated mean effective pressure 14 bar condition by the adjustment of dual-fuel premixed compression ignition combustion mode 2. As a result, this research contributes to the understanding and practical application of dual-fuel combustion for a light-duty compression ignition engine.

Thermal efficiency–related parameters in semi-premixed diesel combustion with a twin peak shaped heat release were experimentally investigated in a 0.55-L single-cylinder diesel engine. Here, the first heat release peak is realized with the premixed combustion at top dead center after the end of the first fuel injection with a sufficient ignition delay. The fuel injection quantity for the first combustion was maximized in a range to limit the rate of pressure rise below 0.6 MPa/°CA at 0.4 MPa IMEP, 0.8 MPa/°CA at 0.8 MPa IMEP, and 1.0 MPa/°CA at 1.3 MPa IMEP to ensure the large degree of constant volume heat release and to suppress smoke emissions. The second heat release peak is formed from the rate-controlled combustion with the second fuel injection immediately after the end of the first combustion. The influence of the intake oxygen concentration and the intake gas pressure on the thermal efficiency and the exhaust gas emissions was systematically examined at three load conditions (indicated mean effective pressure ≈0.4, 0.8, and 1.3 MPa). The results with two types of combustion chambers, a toroidal chamber expecting smaller cooling losses with weaker in-cylinder gas motion, and with a re-entrant chamber expecting better air utilization with stronger in-cylinder gas motion are compared. At the medium load, a significantly high indicated thermal efficiency exceeding 50% is established with a reduction in the intake oxygen concentration due to the smaller cooling loss. The indicated thermal efficiency improves with a decrease in the intake oxygen concentration as the reduction in the cooling loss is more significant than the increase in the exhaust loss. However, an excessive reduction in the intake oxygen concentration results in a deterioration in the indicated thermal efficiency due to a reduction in the combustion efficiency. At low load conditions, the indicated thermal efficiency is lower than at the medium load due to lower combustion efficiency and the improvement in the indicated thermal efficiency with reductions in the intake oxygen concentration is not significant as the combustion efficiency decreases with the decrease in the intake oxygen concentration. At the high load condition, the indicated thermal efficiency is lower due to a larger exhaust loss than at the low and medium load conditions and the indicated thermal efficiency decreases with the decrease in the intake oxygen concentration. With an increase in the intake gas pressure, the indicated thermal efficiency increases consistently mainly due to the reducing cooling loss. In comparison with the re-entrant combustion chamber, the indicated thermal efficiency with the toroidal combustion chamber is 1% higher due to a smaller cooling loss at the low load, almost comparable at the medium load and 1.2% lower at the high load due to the larger exhaust loss.

Heat losses through combustion chamber walls are a well-known limiting factor for the overall efficiency of internal combustion engines. Thermal insulation of the walls has the potential to decrease substantially these heat losses. However, evaluating numerically the effect of coating and of its location in the combustion chamber and then design an optimized combustion system require the use of high-fidelity engine models. The objective of this article is to present the whole workflow implying the use of three-dimensional computational fluid dynamics techniques with conjugate heat transfer (CHT) models to investigate the potential benefits of a coating on a passenger car Diesel engine. First, the baseline combustion system is modeled, using CHT models to solve in a coupled simulation the heat transfers between the fluid in the intake and exhaust lines and in the combustion chamber, on one hand, and the solid piston, head and valves, on the other hand. Based on this setup, a second simulation is performed, modeling a thermo-swing insulation on all combustion chamber walls by a contact resistance, neglecting its thermal inertia to keep a manageable computational cost. Results show a decrease of 3.3% in fuel consumption with an increase in volumetric efficiency. However, decoupled one-dimensional/three-dimensional simulations highlight the inaccuracy of these results and the necessity to model the coating thermal inertia, as they show an overestimation of the heat insulation rate and, consequently, of the gain in fuel consumption (−2.1% instead of −1.6%), for a coating on the piston with no thermal inertia.

The primary objective of this work is a more complete characterization of the sources of diesel unburned hydrocarbon and a better understanding of their minimization over a range of combustion-system-relevant parameters, so as to optimize engine system performance. Measurements of unburned hydrocarbon emissions from a single-cylinder heavy-duty research engine (2.53 L) were acquired with slow and fast flame ionization detector exhaust instrumentation. Temporal lift-off-length correlation analysis coupled with a one-dimensional jet-mixing model was used to interpret and understand the spray-driven mixing-controlled unburned hydrocarbon sources. Separate experiments were conducted in a cold-spray vessel with a newly developed high-speed injector dribble imaging diagnostic, which enables quantification of the dribble event, including rate shape, velocity, duration, and probability. To supplement our understanding of injector dribble, a fuel injector nozzle flow volume-of-fluid computational fluid dynamics simulation methodology and study were conducted to illuminate nozzle geometry, injection and cylinder boundary condition influences on the dribble event. Engine combustion computational fluid dynamics simulations were also completed, incorporating the newly acquired dribble behavioral knowledge. Prior works indicate that fuel injector dribble sources of unburned hydrocarbon can dominate; however, the spray-driven unburned hydrocarbon sources were found to have significance for these engine conditions. Engine unburned hydrocarbon emissions increased with factors that promote overmixing: longer lift-off-length, higher injection pressure, reduced nozzle steady flow, and reduced dilution. Unburned hydrocarbon emissions also increase for lower in-cylinder temperatures (reduced load) and shorter charge residence times (higher engine speed). Dribble quantities were found to increase with injection pressure and nozzle sac volume, with a mean value of 18% the sac volume. The dribble event duration was found to be longer than expected at 1–2 ms, and stochastic in nature regarding hole-to-hole observation. Incorporation of these dribble phenomena into engine combustion simulations was necessary for reproducing measured engine-out unburned hydrocarbon trends. Without modeling effects of dribble, computational fluid dynamics predictions of unburned hydrocarbon levels were between 2 and 20 times low.

This work investigates nanoparticles formation process in light-duty gasoline direct-injection engines operated in homogeneous combustion. The analysis specifically focuses on the contribution of particles in the range of 10–23 nm, which are expected to be taken into account in future emission regulations. Experiments were carried out on a single-cylinder 0.4-L displacement gasoline direct-injection optical engine. Exhaust gases were analyzed by means of a commercial device (DMS500) to obtain a quantitative measurement of the particulates number and size. Optical diagnostics (broadband color imaging and liquid phase laser-induced fluorescence) were employed to correlate the exhaust measurements to the in-cylinder physical phenomena. The main leverages to control soot particulates formation were investigated. The engine temperature was found to have a significant impact during the entire warm-up phase on the global particulates number and also on the relative contribution of the 10–23 nm range to the total particulates number. Injection phasing has also a primary role in the formation of particulates in the small range of the spectrum when operating in mild stratified combustion mode. On the other hand, an increased in-cylinder aerodynamic (e.g. combined swirl-tumble motion) has a positive impact in reducing global particulates number but also causes an increase of the 10–23 nm relative contribution. Optical diagnostics helped establishing a relationship between the detection of liquid film, the consequent pool fires and the exhaust gas measurements. The evaporation of the liquid film detected on the injector nozzle and the intake valve right after the injection appears to be decisive for the formation of nanoparticles in pool-fire mode. The experimental results indicate that when pool fires were detected, the particulates number increased drastically in the whole range of the spectrum including the 10–23 nm range. Authors relate this effect to in-homogeneities in the fuel-mixing field induced by the evaporation of the liquid film.

A manipulation of the charge exchange allows controlling the amount of residual gas during engine warm-up. The residual gas during the warm-up phase leads to an increase of the exhaust gas temperature and supports to reach the exhaust after-treatment system operating temperature faster. In addition, the warm residual gas increases the combustion chamber temperature, which reduces the HC and CO emissions. However, fuel consumption increases. For that reason, such heating measures should be the best compromise of both, exhaust gas temperature increase and engine efficiency, in order to provide efficient heating strategies for passenger car diesel engines. Therefore, simulative and experimental investigations are carried out at the Institute of Internal Combustion Engines of the Karlsruhe Institute of Technology to establish a reliable cam design methodology. For the experimental investigations, a modern research single-cylinder diesel engine was set up on a test bench. In addition, a one-dimensional simulation model of the experimental setup was created in order to simulate characteristics of valve lift curves and to investigate their effects on the exhaust gas temperature and the exhaust gas enthalpy flow. These simulations were based on design of experiments (DoE), so that all characteristics can be used sustainably for modeling and explaining their influences on the engine operation. This methodology allows numerically investigating promising configurations and deriving cam contours which are manufactured for testing. To assess the potential of these individual configurations, the results obtained were compared with each other as well as with the series configuration. Results show that the combination of DoE and one-dimensional simulation for the design of camshaft contours is well suited which was also validated with experimental results. Furthermore, the potential of residual gas retention by favorable configurations with a second event already revealed in various publications could be confirmed with respect to exhaust gas temperature increase and engine efficiency.

Exhaust gas recirculation can be achieved by means of two different routes: the high-pressure route (high-pressure exhaust gas recirculation), where exhaust gas is conducted from upstream of the turbine to downstream of the compressor, and the low-pressure one (low-pressure exhaust gas recirculation), where exhaust gas is recirculated from downstream of the turbine and of the aftertreatment system to upstream of the compressor. In this study, the effectiveness of both exhaust gas recirculation systems on the improvement of the NOx-particulate matter emission trade-off has been compared on a Euro 6 turbocharged diesel engine equipped with a diesel oxidation catalyst, a lean-NOx trap, and a diesel particulate filter. Emissions were measured both upstream and downstream of the aftertreatment system, at different combinations of engine speed and torque (corresponding to different vehicle speeds), at transient and steady conditions, and at different coolant temperatures as switch points to change from high-pressure exhaust gas recirculation to low-pressure exhaust gas recirculation. It was shown that low-pressure exhaust gas recirculation was more efficient than high-pressure exhaust gas recirculation to reduce NOx emissions, mainly due to the higher recirculation potential and the lower temperature of the recirculated gas. However, such a differential benefit decreased as the coolant temperature decreased, which suggests the use of high-pressure exhaust gas recirculation during the engine warm-up. It was also shown that the lean-NOx trap storage efficiency decreased more rapidly at high engine load than at medium load and that such reduction in efficiency was much faster when high-pressure exhaust gas recirculation was used than when low-pressure exhaust gas recirculation was used.