Journal of Energy Engineering

The present investigation evaluates the combustion, performance, and exhaust emissions of diesel fuel in blends with either 5, 10, and 15% ethanol, or with 8, 16, and 24% $n$-butanol, or lastly with 8, 16, and 24% diethyl ether (DEE, an isomer ether of butanol) by volume, fueling a standard, experimental, single-cylinder, four-stroke, high-speed direct injection (HSDI), Hydra diesel engine. The tests were conducted using each of the above fuel blends, with the engine operating at three different loads. Fuel consumption, exhaust gas temperature, and exhaust smoke, nitrogen oxides (NOx), carbon monoxide (CO), and total unburned hydrocarbons (HC) were measured. The differences in combustion, performance, and exhaust emissions of those biofuel blends from the baseline operation of the diesel engine (when working with neat diesel fuel) and among themselves are compared. Fuel injection diagrams, combustion chamber pressure diagrams, and heat release rate (HRR) diagrams obtained thereof, reveal some interesting features of the combustion mechanisms. These results and the differing physical and chemical properties of those biofuels among themselves and against those for the neat diesel fuel are used to aid the correct interpretation of the observed engine behavior. With increasing percentage of all biofuels in the blends, significant reduction of smoke opacity (higher for the $n$-butanol case) is observed, reduction of NOx (higher for the diethyl ether case), decrease of CO, and increase of HC emissions, against the corresponding neat diesel fuel case.

Ambient oxygen concentration, a key variable directly related to exhaust gas recirculation (EGR) levels in diesel engines, plays a significant role in particulate matter (PM) and nitrogen oxides (${\mathrm{NO}}_x$) emissions. The utilization of biodiesel in diesel engines has been investigated over the last decades for its renewable characteristics and lower emissions compared to diesel. In an earlier work, we demonstrated that the soot temperature and concentration of biodiesel were lower than diesel under regular diesel engine conditions without EGR. Soot concentration was quantified by a parameter called KL factor. As a continuous effort, this paper presents an experimental investigation of the ambient oxygen concentration on soot temperature and KL factor during biodiesel and diesel spray combustion. The experiment was implemented in a constant volume chamber system, where the ambient oxygen concentration varied from 21 to 10% and the ambient temperature was kept to 1,000 K. A high speed two-color pyrometry technique was used to measure transient soot temperature and the KL factor of the spray flame. The soot temperature of biodiesel is found to be lower than that of diesel under the same conditions, which follows the same trend from our previous results found when the ambient temperature changes to 21% oxygen conditions. A reduction in ambient oxygen concentration generally reduces the soot temperature for both fuels. However, this is a complicated effect on soot processes as the change of oxygen concentration greatly affects the balance between soot formation and oxidation. The KL factor is observed to be the highest at 12% ${\mathrm{O}}_2$ for diesel and 18% ${\mathrm{O}}_2$ for biodiesel, respectively. On the other hand, the 10% ${\mathrm{O}}_2$ condition shows the lowest KL factor for both fuels. These results can provide quantitative experimental evidences to optimize the ambient oxygen concentration for diesel engines using different fuels for better emissions characteristics.

Oil-jet cooling of internal combustion engine pistons has risen in prominence with the need to improve fuel economy by increasing the power density of engines. Laboratory tests were performed on a production automotive piston under a range of conditions representative of engine conditions to develop a suitable area-average heat transfer correlation to be employed for numerical evaluation of oil-jet cooling of internal combustion engine pistons. In addition to the Reynolds and Prandtl numbers, the Nusselt number was found to be a function of the dimensionless nozzle diameter and viscosity, and a weak function of the dimensionless nozzle-to-wall spacing.

The feasibility of upcycling waste plastics into concurrent production of carbon nanotubes (CNTs) and of a hydrogen-enriched gaseous hydrocarbon stream was shown to be possible in a continuous steady-state steady-flow process. The feedstock was post-consumer (waste) low-density polyethylene (LDPE), which was pyrolyticaly gasified at 800°C, in a flow of nitrogen carrier gas. The evolving gaseous hydrocarbon pyrolyzates were first used as carbon growth agents for CNTs in a reactor, a process that lowered their carbon content and increased their hydrogen content. Thereafter, the unreacted hydrocarbon pyrolyzates and the generated hydrogen were mixed with air and burned. This conversion of solid waste plastics to gaseous fuels allowed thorough mixing with air and, upon ignition, the formation of environmentally benign fuel-lean premixed flames for subsequent power generation purposes. The CNT generation in the reactor took place by chemical vapor deposition (CVD) on untreated stainless steel mesh substrates, which also served as catalysts. Provisions were made for automatic motion and replenishment of the substrate/catalyst for continuous CNT production. The reactor was specifically designed to have a cold-wall configuration, where only the catalyst substrate was heated to a temperature of 800°C, in an otherwise unheated chamber. This cold-wall reactor has significantly lower energy consumption than conventional hot-wall reactors of similar dimensions and throughput. This work is part of an overall study that aims at illustrating the upcycling of common recycled post-consumer plastics simultaneously to gaseous fuels and value-added nanomaterials. Its specific goal was to produce the latter in a newly designed low energy input reactor.

The article presents the bench test results of a fully instrumented, four cylinder, naturally aspirated, (60 kW) direct injection (DI) diesel engine running on the normal (class C) diesel fuel (DF) and aviation-turbine (JP-8) fuel. The article addresses changes in engine performance and exhaust emissions resulting from the replacement of commercial diesel fuel with JP-8 fuel. Investigation and comparison of the effects of the cetane number improving additive on the autoignition delay, maximum heat release rate, maximum in-cylinder pressure, performance efficiency of an engine, and exhaust emissions were provided for sound analysis of the test results. The series of engine tests were conducted running alternately on the normal diesel fuel, JP-8 fuel, and JP-8 treated with 0.04, 0.08, 0.12, 0.16, and 0.24 vol% of 2-ethylhexyl nitrate. Studies on the operating characteristics of an engine were carried out for 15–10%, 50 and 100% loads, and the two ranges of speed: 1,400 revolutions per minute (rpm), at which maximum torque occurs, and a rated speed of 2,200 rpm. Use of JP-8 fuel suggested fuel energy savings and ecological benefits for medium (50%) and full (100%) loads mainly when running at a low speed of 1,400 rpm. Autoignition delay, maximum heat release rate, maximum in-cylinder pressure, and nitrogen oxide (${\mathrm{NO}}_{\mathrm{x}}$) emissions decreased; however, the carbon monoxide (CO), hydrocarbon (HC) emissions, and the smoke opacity of the exhaust increased to a certain extent responding to the addition of 2-ethylhexyl nitrate to the JP-8 fuel. The brake-specific fuel consumption and brake thermal efficiency of a fully (100%) loaded engine changed little when running on treated JP-8-12 fuel at a rated speed of 2,200 rpm. Analysis of the test results revealed that the cetane number improving additive can be considered as an effective but not the only measure to be applied for an intended use of aviation-turbine JP-8 fuel in ground-based diesel engines.

In dual-fuel engines, a combustible mixture of air and generally a gaseous fuel is ignited, thanks to the injection and autoignition of a small amount of liquid fuel. It is well-known that dual-fuel engines suffer from poor combustion when operated at low loads. This behavior, due mainly to the presence of an overlean mixture into the combustion chamber, leads to unacceptably high levels of carbon monoxide and unburned hydrocarbons emitted at the exhaust. In order to solve this problem a possible solution could be to split the pilot injection of liquid fuel into two split injections, the second having the function of boosting the combustion of gaseous fuel also during the late combustion phase. In this paper this solution has been implemented on a diesel common rail single cylinder research engine converted to operate in dual-fuel mode. The composition of the gaseous fuel, indirectly injected, simulated a typical producer gas. The liquid fuel used during the experiments was biodiesel, injected by means of a common rail injection system. The first section of results describes the tests run for comparison purposes, performing only one pilot biodiesel injection and varying its timing on a wide range. The second section of results then presents the tests run for different timings, varied on a wide range, of the first split injection, and different dwells between the first and the second injections. The engine behavior has been discussed in terms of heat release rate, fuel conversion efficiency, and nitric oxides, total hydrocarbons, and carbon monoxide emission levels at the exhaust. The results demonstrate that splitting the pilot injection leads to an increase of fuel conversion efficiency and a reduction of both total hydrocarbons and carbon monoxide. This final result allows to state that splitting the pilot injection is an effective way for sustaining the gaseous fuel combustion in dual-fuel engine late during the combustion phase.

This paper focuses on the effect of diesel injection timing, intake boost pressure, and diesel injection pressure on diesel-methane dual-fuel combustion performed in a single-cylinder research engine. The engine was operated at a constant speed of 1,500 revolutions per minute (rpm) while percentage of methane energy substitution (PES) and load were maintained at 80% and 5.1 bar net indicated mean effective pressure (IMEP), respectively. The start of injection (SOI) of diesel was varied from 250 crank angle degrees (CAD) to 350 CAD while keeping the injection pressure at 500 bar and intake boost pressure at 1.5 bar. Advancing SOI from 330° to 300° reduced indicated specific NOx (ISNOx) emissions from $11.9\mathrm{g}/\mathrm{kW}·\mathrm{h}$ to less than $0.02\mathrm{g}/\mathrm{kW}·\mathrm{h}$; further advancement of SOI did not yield any significant ISNOx reduction. Net indicated fuel conversion efficiency (IFCE) increases from 29.4% at 350 CAD SOI to 40.5% at 300 CAD SOI. Combustion efficiency trends are consistent with unburned hydrocarbon (HC) and carbon monoxide (CO) emission trends. Moreover, smoke emissions were lower than 0.1 filter smoke number (FSN) for all SOIs. A diesel injection pressure sweep from 200 to 1,300 bar at 300 CAD SOI showed that very low injection pressures lead to apparently more heterogeneous combustion and higher ISNOx, indicated specific CO (ISCO), and indicated specific HC (ISHC) emissions, whereas smoke, IFCE, and combustion efficiency remained unaffected. An injection pressure of approximately 500 bar appeared to be optimal for early SOIs. Finally, an intake boost pressure sweep from 1.1 to 1.8 bar at 300 CAD SOI and 500 bar injection pressure showed that ISNOx and smoke remained fairly low at all conditions ($\mathrm{ISNO}\mathrm{x}<0.15\mathrm{g}/\mathrm{kW}·\mathrm{h}$; $\mathrm{smoke}<0.1\mathrm{FSN}$); however, increasing boost pressure resulted in an increase in both ISHC and ISCO emissions while combustion efficiency and IFCE were reduced.

Nitric oxide (NO) emissions are practically the only ones emitted from spark-ignition (SI), hydrogen-fueled engines, and their reliable prediction is important in engine simulation codes. In this work, the reaction mechanisms of nitric oxide are investigated in such engines during load variation by using a very wide range of exhaust gas recirculation (EGR) rates, up to 47%. For that purpose, a three-dimensional computational fluid dynamics code is applied, which has been developed by the authors and validated for its main sub-models, such as the heat transfer and combustion. The latter one includes the thermal NO mechanism, widely known as “Zeldovich mechanism,” whereas two alternative production paths have been included, viz. through the NNH and ${\mathrm{N}}_2\mathrm{O}$ species formation in order to improve the numerical predictions. The NNH path has been shown to be favored under lean and low-temperature combustion conditions, especially for hydrogen flames, whereas the ${\mathrm{N}}_2\mathrm{O}$ path becomes important for lean flames irrespectively of the fuel used, whereas such flames have many similarities with highly-diluted mixtures by recirculated exhaust gases. The calculations are compared with available measurements concerning the NO exhaust emissions, in order to quantify the applicability of the alternative production paths at such conditions and applications. For the high load cases (low EGR rates) the NO predictions have good accuracy because the thermal NO is the main production path. However, at mid and low engine loads, when the combustion temperature is lower and higher EGR rates are used, a significant discrepancy exists between the calculations and the measured data, which is improved when the NNH route is taken into consideration.

This paper presents the performance, combustion, and emissions characteristics of a light-duty vehicle equipped with a Euro 4 diesel engine, operating with animal fat biodiesel, gas to liquid (GTL), hydrotreated vegetable oil (HVO), and a commercial diesel fuel (with 5.8% of biodiesel) as reference. Tests were carried out in on a chassis dynamometer under controlled ambient conditions following the New European Driving Cycle (NEDC). One of the engine cylinders was instrumented with a Kistler Kibox device combined with an opened electronic control unit (ECU) and ETAS-INCA PC system for online thermodynamic combustion diagnosis. Regulated emissions and thermodynamic diagnosis parameters have been compared at different time windows along the NEDC. Tested fuels exhibited significant reduction of CO and small variations in total hydrocarbons (THC) and NOx in comparison with commercial diesel fuel. Both specific particulate number and particulate mass were below Euro 5b directive for all tested fuels due to the use of diesel particle filter. In general, all parameters studied here were affected by both, fuel properties, and the evolution of the thermal conditions of the engine and the after-treatment devices.

This work presents a comparative study of the performance between fully premixed and stratified propane flames stabilized in a disk burner configuration operating with a swirl coflow over a range of stoichiometric to ultra-lean conditions. A selection of radial equivalence ratio gradients up to the fully premixed case are regulated by staged premixing of propane and air within a double cavity formed along three concentric disks. Flame stabilization is established at the recirculation region of the afterbody disk. Measurements of temperatures, flame chemiluminescence images of ${\mathrm{OH}}^{*}$ and ${\mathrm{CH}}^{*}$, and gas analysis provided information for the interpretation of the relative variations in flame structure and burner performance. The study elucidates some aspects regarding the influence of the variable inlet fuel–air profile and its interaction with the swirling coflow on the stabilization and disposition of the toroidal flame front. The results help to delineate important differences in the emission performance of each setup and provide useful information to allow better control or exploitation of their variable mixing characteristics and improve their modeling.

This paper examines the impact of biodiesel addition on density, viscosity, distillation profile, and cetane index of diesel fuels. Biodiesel has become an important component in the European diesel fuel market. Its addition to diesel alters the distillation profile, which in turn may impact on the cetane index; the latter remains an important parameter for the evaluation of ignition quality of diesel fuels, in combination with the cetane number. In this series of experiments, fifty diesel fuel samples were used (35 automotive diesel samples and 15 heating gasoil samples) as base fuels, to which sunflower oil biodiesel was added at a concentration of 5% volume in the fuel blend. The distillation profile, density, and kinematic viscosity were measured in all base fuels and blends, whereas the cetane indices were calculated from density and distillation measurements. As expected, the results showed increased density and kinematic viscosity due to the biodiesel addition, whereas the front end volatility (10% recovery) and midboiling point (50% recovery) were increased in the blends. On the other hand back end volatility (90% and 95% recovery and final boiling point) was in most cases reduced due to the biodiesel addition. Observed changes in cetane index were insignificant in most cases.

Combustion characteristics of pulverized coal were studied in a methane jet that entrained pulverized coal particles using the Venturi effect. The dependence of entrainment rate on the size of coal particles was studied as a function of the flow rate. Particle streak velocimetry performed on coal-laden jets provided valuable insight into the relative velocity of entrained coal particles with respect to the fluid velocity. High-resolution still images and high-speed videos of laser-sheet light scattered by the coal particles were used in order to determine the mode of interaction of entrained coal particles with the flame front. The effect of combustion on the entrained coal particles was analyzed both in terms of macrostructure and microstructure using a combination of loose density measurement, Fraunhofer-diffraction-based particle-size distribution measurements, and scanned electron microscopy. It was established that the combustion process did not have any significant effect on the macrostructure of the coal particles. At the same time, remarkable changes were observed in particle microstructure. Based on these findings, it was established that the coal particles underwent only partial devolatilization during their passage through the flame due to the small residence time. Hence, it was concluded that oxidation was basically a surface phenomenon. The effect of oxidizer composition on the combustion of coal particles was studied by comparing the measured particle-size distributions for ${\mathrm{CH}}_4/\mathrm{air}$, ${\mathrm{CH}}_4/{\mathrm{O}}_2/{\mathrm{CO}}_2$, and ${\mathrm{CH}}_4/{\mathrm{O}}_2$ flames.

Wood gas represents a viable energy source, particularly for stationary electric power generation, as it allows for a wide flexibility in fossil fuel sources and can be used as full supplement fuel in conventional heavy-duty (HD), turbocharged (T/C), spark-ignited (SI) engines. For such engines fuelled with wood gas, spark timing and boost pressure are critical parameters that affect both engine performance characteristics and NO and CO emissions. Thus, the main objective of this study is to investigate theoretically the effects of these parameters on the performance and exhaust emissions (NO and CO) of such an existing engine fueled with wood gas. The investigation is conducted by using a comprehensive two-zone phenomenological model. The predictive ability of model was tested against experimental measurements, which were obtained from the operation of such an engine fueled with wood-gas fuel under various operating conditions. The experimental results were found to be in good agreement with the respective computed ones obtained from the simulation model. From the comparative evaluation of the theoretical findings presented in this work, it is generally revealed that for a heavy-duty, turbocharged, SI wood-gas engine operating at 65% of (full) throttle valve opening (TVO), the increase of boost pressure accompanied by advanced spark timing could lead to a more efficient engine operation (i.e., decrease of the brake specific fuel consumption) with a slight improvement on its environmental behavior (i.e., decrease of specific NO and CO emissions). On the other hand, at 100% TVO engine operating point, the slight improvement of engine performance and environmental characteristics, which is achieved via the simultaneous increase of boost pressure and spark timing, is restricted by the occurrence of phenomena influencing its structural strength, because the maximum cylinder pressure becomes considerably higher compared to the respective one observed under normal spark timing and normal boost pressure operating mode.

Homogeneous charge compression ignition (HCCI) combustion has shown the potential to improve gasoline engine efficiency, by integration into future dual-mode spark ignition/HCCI engines. With the current trend of downsizing and boosting, it is critical to implement HCCI combustion under elevated pressure, and assess its potential for high load and efficient operation. Thermodynamic analysis of such engines provides insight into the energy flows of the systems, and enables optimal design for efficiency. This study is focused on modeling a 4-cylinder boosted HCCI engine, and utilizing second law to describe the exergy flows of the system in order to identify areas of irreversibilities and inefficiencies. The implications of boosting and variable valve actuation on exergy flows are discussed and analyzed. Results from this modeling study indicated that light-duty boosted HCCI can be utilized up to 10-bar brake mean effective pressure (BMEP) with peak brake efficiency of 39%.

The reactions responsible for the initial development of the time scale that characterizes the autoignition of a homogeneous stoichiometric ${\mathrm{CH}}_4/\mathrm{air}$ mixture are identified algorithmically using the computational singular perturbation method. The analysis considers a wide range of initial temperatures and pressures, located below and above the explosion limit. The major role of reactions ${\mathrm{CH}}_3+{\mathrm{O}}_2\to {\mathrm{CH}}_3\mathrm{O}+\mathrm{O}$ (at high temperatures), ${\mathrm{CH}}_3+{\mathrm{O}}_2\to {\mathrm{CH}}_2\mathrm{O}+\mathrm{OH}$ (at low temperatures), and ${\mathrm{CH}}_2\mathrm{O}+{\mathrm{O}}_2\to \mathrm{HCO}+{\mathrm{HO}}_2$ (at intermediate temperatures) is identified. The results provide some additional insights regarding the significance to the initiation of the process of some reactions that are generally considered to become active at later times.

An experimental investigation of premixed propane-air flames propagating in long, narrow tubes open at both ends was performed in order to establish flame phenomenology and make comparisons to theoretical predictions. Fuel-air equivalence ratios of 0.8, 1.0, 1.2, and 1.4 were studied, and the effect of tube length was investigated using lengths of 59.1 and 104 cm. Quantitative comparisons to theoretical studies provided support to the hypothesis that a pressure gradient is established in the tube because of the Poiseuille flow induced in the gas by the flame, leading to flow away from the flame in both directions. Several regimes of propagation were established, including initial smooth propagation at constant speed, regions of high-amplitude oscillation accompanied by flame acceleration, and regions of small-amplitude oscillation during which the mean propagation speed was nearly uniform. The most intense flame oscillations and consequent acceleration occurred in mixtures with equivalence ratios of 1.0 and 1.2. This oscillatory behavior is hypothesized to be the result of an acoustic pressure field set up in the gas by the outflow of gases at both open ends of the tube. These standing pressure waves excited flame oscillations and ultimately drove the flames to instability. The instabilities were particularly intense in near-stoichiometric rich flames, for which $Le<1$ and the flame was diffusionally unstable. Lean propane flames ($Le>1$) are diffusionally stable, and this tends to mitigate flame oscillations but does not always suppress them. The general conclusion is that smooth, laminar flame propagation in long, narrow, open tubes will almost never occur in practice, due to the inherent destabilizing effect of the acoustic pressure field set up by the flame.