Experimental investigation of prechamber autoignition in a natural gas engine for cogeneration

A novel ignition concept based on autoignition in an unscavanged prechamber is currently being developed at the Laboratory for Industrial Energy Systems (LENI). On a single cylinder test engine a series of experimental runs (CR = 8.5-14, λ = 1 − 1.6, RPM = 1150/1500 min⁻¹) have been realized with natural gas as fuel, comparing the new ignition concept to standard spark ignition. The comparison is based on fuel efficiency and exhaust emissions (CO, THC, NO_x). The feasibility of operating the engine in autoignition mode has been demonstrated, and the potential of prechamber autoignition, in particular in the lean combustion regime, is indicated by the trends in fuel efficiency and emission concentration. The resistive heating of the prechamber walls has been shown to be an effective mean to trigger ignition. The prechamber could clearly be identified as primary ignition location. A reduction of the cycle-by-cycle variations - due to mixture fluctuations - is necessary to exploit the full potential of this engine concept.     

First and second generation biodiesels spray characterization in a diesel engine

Potential improvement on exhaust emissions, biodegradability and the possibility to reduce dependence on fossil fuel resources has led to an increasing interest on the use of biofuels for transport application. In this work, the analysis of the spray behaviour of first and second generation biodiesel in a Euro 5, common rail transparent diesel engine has been performed. GTL, SME and RME fuels have been used in blends at 100% and 50% in volume; while reference fuel consisted of commercial diesel. Two engine operating conditions of the NEDC have been selected: 1500 rpm at 2 bar of brake mean effective pressure (BMEP) and 2000 rpm at 5 bar BMEP. The injection process has been accurately studied, and the influence of the combustion process on the spray behaviour has been taken into account. Typical jets parameters such as penetration and cone angles have been detected and a comparison with theoretical models of Hiroyasu and Siebers has been performed. A new correlation for the forecasting of the jet penetration has been obtained starting from Hiroyasu equations. An image-based method has been applied for the identification of the phenomena that control the spray behaviour during its evolution in the combustion chamber.First generation biodiesels, pure and blends, show longer penetration with respect to the reference fuel at both the engine speed analysed. Moreover, they penetrate for a longer time in the combustion chamber, because of the longer energizing time set, so impingement phenomena can be observed. On the other hand, the second generation biodiesels penetrate less than reference one, due to its lower density, but also because the combustion of the pilot injection causes an increase of pressure that obstructs the penetration in the combustion chamber. Finally, a good agreement between the breakup times computed by means of the Hiroyasu and Siebers correlations and the ones from the experimental data has been found.     

Experimental study of n-butanol additive and multi-injection on HD diesel engine performance and emissions

Experimental study was conducted to investigate the influence of the diesel fuel n-butanol content on the performance and emissions of a heavy duty direct injection diesel engine with multi-injection capability. At fixed engine speed and load, exhaust gas recirculation rates were adjusted to keep NO_x emission at 2.0 g/kW h. Diesel fuels with different amounts (0%, 5%, 10% and 15% by volume) of n-butanol were used. The results show that the n-butanol addition can significantly improve soot and CO emissions at constant specific NO_x emission without a serious impact on the break specific fuel consumption and NO_x. The impacts of pilot and post injection on engine characteristics by using blended fuels are similar to that found by using pure diesel. Early pilot injection reduces soot emission, but results in a dramatic increase of CO. Post injection reduces soot and CO emissions effectively. Under each injection strategy, the increase of fuel n-butanol content leads to further reduction of soot. A triple-injection strategy with the highest n-butanol fraction used in this study offers the lowest soot emission.     

Effect of nozzle orifice geometry on spray, combustion, and emission characteristics under diesel engine conditions

Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Modern engines employ micro-orifices with different orifice designs. It is critical to characterize the effects of various designs on engine performance and emissions. In this study, a recently developed primary breakup model (KH-ACT), which accounts for the effects of cavitation and turbulence generated inside the injector nozzle is incorporated into a CFD software CONVERGE for comprehensive engine simulations. The effects of orifice geometry on inner nozzle flow, spray, and combustion processes are examined by coupling the injector flow and spray simulations. Results indicate that conicity and hydrogrinding reduce cavitation and turbulence inside the nozzle orifice, which slows down primary breakup, increasing spray penetration, and reducing dispersion. Consequently, with conical and hydroground nozzles, the vaporization rate and fuel air mixing are reduced, and ignition occurs further downstream. The flame lift-off lengths are the highest and lowest for the hydroground and conical nozzles, respectively. This can be related to the rate of fuel injection, which is higher for the hydroground nozzle, leading to richer mixtures and lower flame base speeds. A modified flame index is employed to resolve the flame structure, which indicates a dual combustion mode. For the conical nozzle, the relative role of rich premixed combustion is enhanced and that of diffusion combustion reduced compared to the other two nozzles. In contrast, for the hydroground nozzle, the role of rich premixed combustion is reduced and that of non-premixed combustion is enhanced. Consequently, the amount of soot produced is the highest for the conical nozzle, while the amount of NO_x produced is the highest for the hydroground nozzle, indicating the classical tradeoff between them.     

Liquid penetration length of heptamethylnonane and trimethylpentane under unsteady in-cylinder conditions

While strategies employing early or late direct-injection of fuel can improve emissions, they also can lead to impingement of liquid-phase fuel on the piston and/or cylinder wall due to low in-cylinder temperatures and densities during the injection event. Previous work has shown that liquid-phase fuel films formed in this way can lead to pronounced degradations in efficiency and emissions. To avoid these problems, a quantitative understanding of fuel-property effects on the liquid penetration length is needed, and this understanding must include conditions where in-cylinder thermodynamic conditions and the injection rate vary with time. This work reports liquid penetration lengths measured in an optical engine under such time-varying conditions. Diagnostics included laser light scattering for measurement of the liquid length and conventional pressure-data acquisition for heat-release analysis. Unsteady liquid penetration was characterized for different injection timings, injection pressures, intake-manifold pressures, and fuel volatilities to gain an understanding of the relative importance of these factors. Fuel volatility was studied by using two fuels, 2,2,4,4,6,8,8-heptamethylnonane (HMN) and 2,2,4-trimethylpentane (TMP), which have very different volatility characteristics. Measured liquid lengths changed as in-cylinder conditions changed, with increasing temperature and density during the compression stroke causing a decrease in liquid length, and decreasing temperature and density during the expansion stroke causing an increase in liquid length. Intake-manifold pressure and fuel volatility were found to be primary factors governing liquid length. Heat loss from the charge gas to the engine and local charge cooling due to fuel vaporization were found to have a secondary influence on liquid length. Injection pressure was found to have little effect.     

Combustion performance of bio-ethanol at various blend ratios in a gasoline direct injection engine

Bio-ethanol has the potential to be used as an alternative to petroleum gasoline for the purpose of reducing the total CO₂ emissions from internal combustion engines and this paper is devoted to the investigation of using different blending-ratios of bio-ethanol/gasoline with respect to spark timing and injection strategies. The experimental work has been carried out on a direct injection spark ignition engine at a part load and speed condition. It is shown that the benefits of adding ethanol into gasoline are reduced engine-out emissions and increased efficiency, and the impact changes with the blend ratio following a certain pattern. These benefits are attributed to the fact that the addition of ethanol modifies the evaporation properties of the fuel blend which increases the vapour pressure for low blends and reduces the heavy fractions for high blends. This is furthermore coupled with the presence of oxygen within the ethanol fuel molecule and the contribution of its faster flame speed, leading to enhanced combustion initiation and stability and improved engine efficiency.     

Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NO_x emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO₂/O₂, and O₂ enriched recirculated flue gas. NO_x emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O₂ concentration on NO_x formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NO_x emissions has been obtained for air and CO₂/O₂ in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NO_x emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO₂/O₂. No significant impact of the oxidiser O₂ concentration in the CO₂/O₂ mixture on NO_x emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NO_x formation and reduction mechanisms showed, that the observed reduction of NO_x emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NO_x alone, but also to a reduced conversion of fuel-N to NO.     

Combustion of n-butanol in a spark-ignition IC engine

Alcohols, because of their potential to be produced from renewable sources and because of their high quality characteristics for spark-ignition (SI) engines, are considered quality fuels which can be blended with fossil-based gasoline for use in internal combustion engines. They enable the transformation of our energy basis in transportation to reduce dependence on fossil fuels as an energy source for vehicles. The research presented in this work is focused on applying n-butanol as a blending agent additive to gasoline to reduce the fossil part in the fuel mixture and in this way to reduce life cycle CO₂ emissions. The impact on combustion processes in a spark-ignited internal combustion engine is also detailed. Blends of n-butanol to gasoline with ratios of 0%, 20%, and 60% in addition to near n-butanol have been studied in a single cylinder cooperative fuels research engine (CFR) SI engine with variable compression ratio manufactured by Waukesha Engine Company. The engine is modified to provide air control and port fuel injection. Engine control and monitoring was performed using a target-based rapid-prototyping system with electronic sensors and actuators installed on the engine [1]. A real-time combustion analysis system was applied for data acquisition and online analysis of combustion quantities. Tests were performed under stoichiometric air-to-fuel ratios, fixed engine torque, and compression ratios of 8:1 and 10:1 with spark timing sweeps from 18° to 4° before top dead center (BTDC). On the basis of the experimental data, combustion characteristics for these fuels have been determined as follows: mass fraction burned (MFB) profile, rate of MFB, combustion duration and location of 50% MFB. Analysis of these data gives conclusions about combustion phasing for optimal spark timing for maximum break torque (MBT) and normalized rate for heat release. Additionally, susceptibility of 20% and 60% butanol-gasoline blends on combustion knock was investigated. Simultaneously, comparison between these fuels and pure gasoline in the above areas was investigated. Finally, on the basis of these conclusions, characteristic of these fuel blends as substitutes of gasoline for a series production engine were discussed.     

Effects of dimethyl-ether (DME) spray behavior in the cylinder on the combustion and exhaust emissions characteristics of a high speed diesel engine

The purpose of this study was to analyze the exhaust emissions of DME fuel through experimental and numerical analyses of in-cylinder spray behavior. To investigate this behavior, spray characteristics such as the spray tip penetration, spray cone angle, and spray targeting point were studied in a re-entrant cylinder shape under real combustion chamber conditions. The combustion performance and exhaust emissions of the DME-fueled diesel engine were calculated using KIVA-3V. The numerical results were validated with experimental results from a DME direct injection compression ignition engine with a single cylinder.The combustion pressure and IMEP have their peak values at an injection timing of around BTDC 30°, and the peak combustion temperature, exhaust emissions (soot, NO_x), and ISFC had a lower value. The HC and CO emissions from DME fuel showed lower values and distributions in the range from BTDC 25° to BTDC 10° at which a major part of the injected DME spray was distributed into the piston bowl area. When the injection timing advanced to before BTDC 30°, the HC and CO emissions showed a rapid increase. When the equivalence ratio increased, the combustion pressure and peak combustion temperature decreased, and the peak IMEP was retarded from BTDC 25° to BTDC 20°. In addition, NO_x emissions were largely decreased by the low combustion temperature, but the soot emissions increased slightly.     

Effect of HHO gas on combustion emissions in gasoline engines

Reducing the emission pollution associated with oil combustion is gaining an increasing interest worldwide. Recently, Brown’s gas (HHO gas) has been introduced as an alternative clean source of energy. A system to generate HHO gas has been built and integrated with Honda G 200 (197 cc single cylinder engine). The results show that a mixture of HHO, air, and gasoline cause a reduction in the concentration of emission pollutant constituents and an enhancement in engine efficiency. The emission tests have been done with varying the engine speed. The results show that nitrogen monoxide (NO) and nitrogen oxides (NO_X) have been reduced to about 50% when a mixture of HHO, air, and fuel was used. Moreover, the carbon monoxide concentration has been reduced to about 20%. Also a reduction in fuel consumption has been noticed and it ranges between 20% and 30%.     

Performance and emission characteristics of an SI engine operated with DME blended LPG fuel

In this study, a spark ignition engine operated with DME blended LPG fuel was experimentally investigated. In particular, performance, emissions characteristics (including hydrocarbon, CO, and NO_x emissions), and combustion stability of an SI engine fuelled with DME blended LPG fuel were examined at 1800 and 3600 rpm.Results showed that stable engine operation was possible for a wide range of engine loads up to 20% by mass DME fuel. Further, we demonstrated that, up to 10% DME, output engine power was comparable to that of pure LPG fuel. Exhaust emissions measurements showed that hydrocarbon and NO_x emissions were slightly increased when using the blended fuel at low engine speeds. However, engine power output was decreased and break specific fuel consumption (BSFC) severely deteriorated with the blended fuel since the energy content of DME is much lower than that of LPG. Furthermore, due to the high cetane number of DME fuel, knocking was significantly increased with DME.Considering the results of the engine power output and exhaust emissions, blended fuel up to 10% DME by mass can be used as an alternative to LPG, and DME blended LPG fuel is expected to have potential for enlarging the DME market.     

Optimization of a heavy-duty compression-ignition engine fueled with diesel and gasoline-like fuels

Optimal injection strategies for a heavy-duty compression-ignition engine fueled with diesel and gasoline-like fuels (#91 gasoline and E10) and operated under mid- and high-load conditions are investigated. A state-of-the-art engine CFD tool with detailed fuel chemistry was used to evaluate the engine performance and pollutant emissions. The CFD tools feature a recently developed efficient chemistry solver that allowed the optimization tasks to be completed in practical computer times. A Non-dominated Sorting Genetic Algorithm II (NSGA II) was coupled with the CFD tool to seek optimal combinations of injection system variables to achieve clean and efficient combustion. The optimization study identified several key parameters that influence engine performance. It was found that the fuel volatility and reactivity both play important roles at the mid-load condition, while the high-load condition is less sensitive to the fuel reactivity. However, high volatility fuels, such as gasoline and E10, were found to be beneficial to fuel economy at high-load. The study indicates that with an optimized injection system gasoline-like fuels are promising for heavy-duty CI engines due to their lower NOx and soot emissions and higher fuel economy compared to conventional diesel fuels. However, the high in-cylinder gas pressure rise rate associated with Partially Premixed Combustion of gasoline-like fuels can become problematic at high-load and the low-load operating limit is also a challenge. Potential solutions are discussed based on the present optimization results.     

Experimental measurements of the NOx and CO concentrations operating in oscillatory and non-oscillatory burning conditions

The effects of combustion driven acoustic oscillations in carbon monoxide and nitrogen oxides emission rates of a combustor operated with liquefied petroleum gas (LPG) were investigated. Because the fuel does not contain nitrogen, tests were also conducted with ammonia injected in the fuel, in order to study the formation of fuel NO_x. The main conclusions were: (a) the pulsating combustion process is more efficient than the non-pulsating one and (b) the pulsating combustion process generates higher rates of NO_x, with and without ammonia injection, as shown by CO and NO concentrations as function of the O₂ concentration. An increase in the LPG flow rate, keeping constant the air to fuel ratio, increased the acoustic pressure amplitude and the frequency of oscillation. The injection of ammonia had no influence on either pressure amplitude or frequency.     

Correlations for dependence of NOx emissions on heat loss in premixed CH4/air combustion

The present study represents an effort to correlate the dependence of NO_x emissions on heat losses to the atmospheric environment in a CH₄/air fueled combustor. To this end, the numerical analysis was performed over a wide range of residence times, equivalence ratios and heat losses using a perfectly stirred reactor (PSR) code. The numerical results showed that the calculated NO_x concentration initially increased, reached a maximum value and then decreased with increasing residence time when the heat loss was present. The similar variation was observed in changes in the thermal NO concentration that was evaluated by only considering the reactions associated with the thermal (Zeldovich) NO mechanism. With the heat loss increased, the calculated NO_x concentration was substantially reduced for all equivalence ratios investigated. In addition, the reductions in the NO_x concentration with respect to residence time became faster with increasing the equivalence ratio particularly for fuel rich conditions. The observed variations in the calculated NO_x concentration over the residence time (NO_x/τ) were found to fit well to the following correlation:ln(NO_x/τ)=a(HLI)+b. In the correlation, HLI is the dimensionless heat loss parameter and coefficients a and b are constants expressed as a function of adiabatic flame temperature (for a given equivalence ratio) and equivalence ratio, respectively.     

Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion

In chemical-looping combustion (CLC) a gaseous fuel is burnt with inherent separation of the greenhouse gas carbon dioxide. The oxygen is transported from the combustion air to the fuel by means of metal oxide particles acting as oxygen carriers. A CLC system can be designed similar to a circulating fluidized bed, but with the addition of a bubbling fluidized bed on the return side. Thus, the system consists of a riser (fast fluidized bed) acting as the air reactor. This is connected to a cyclone, where the particles and the gas from the air reactor are separated. The particles fall down into a second fluidized bed, the fuel reactor, and are via a fluidized pot-seal transported back into the riser. The gas leaving the air reactor consists of nitrogen and unreacted oxygen, while the reaction products, carbon dioxide and water, come out from the fuel reactor. The water can easily be condensed and removed, and the remaining carbon dioxide can be liquefied for subsequent sequestration.The gas leakage between the reactors must be minimized to prevent the carbon dioxide from being diluted with nitrogen, or to prevent carbon dioxide from leaking to the air reactor decreasing the efficiency of carbon dioxide capture. In this system, the possible gas leakages are: (i) from the fuel reactor to the cyclone and to the pot-seal, (ii) from the cyclone down to the fuel reactor, (iii) from the pot-seal to the fuel reactor. These gas leakages were investigated in a scaled cold model. A typical leakage from the fuel reactor was 2%, i.e. a CO₂ capture efficiency of 98%. No leakage was detected from the cyclone to the fuel reactor. Thus, all product gas from the air reactor leaves the system from the cyclone. A typical leakage from the pot-seal into the fuel reactor was 6%, which corresponds to 0.3% of the total air added to the system, and would give a dilution of the CO₂ produced by approximately 6% air. However, this gas leakage can be avoided by using steam, instead of air, to fluidize the whole, or part of, the pot-seal. The disadvantages of diluting the CO₂ are likely to motivate the use of steam.     

Nox Gene Expression and Cytochemical Localization of Hydrogen Peroxide in Polyporus umbellatus Sclerotial Formation

The effect of temperature shift on Polyporus umbellatus sclerotial development was investigated. Micromorphology of the sclerotia was observed by using scanning electron microscopy (SEM). The cytochemical localization of H₂O₂ expressed as CeCl₃ deposition at the subcellular level was observed by using transmission electron microscopy (TEM). Nox gene expression in sclerotia and mycelia was detected by quantitative real-time PCR (qRT-PCR) analysis. In addition, superoxide dismutase (SOD) and catalase (CAT) specific activities increased during sclerotial development and decreased after the antioxidant diphenyleneiodonium (DPI) was used. Results indicated that the temperature shift treatment induced P. umbellatus sclerotial formation. Compared with the mycelia, the Nox gene was respectively upregulated by 10.577-, 30.984- and 25.469-fold in the sclerotia of SI, SD and SM stages respectively. During the sclerotial formation, H₂O₂ accumulation was observed in the cell walls or around the organelle membranes of the mycelial cells. The antioxidant DPI decreased the generation of H₂O₂ in mycelial cells. The specific activity of SOD and CAT levels was decreased significantly by DPI. The activity of the two antioxidant enzymes in the mycelia increased much more during sclerotial formation (p < 0.05). Oxidative stress was closely associated with sclerotial development in P. umbellatus induced by temperature shift treatment.     

Numerical investigation on the combustion behaviour of pre-dried Greek lignite

Dry coal firing is expected to play an increasingly important role in future lignite power plants. The planned evolution from the conventional lignite drying concept with hot recirculated flue gas to the “fluidized bed drying with internal heat utilization, WTA”, technology in the next generation of lignite power plants is estimated to bring an additional efficiency increase of 2–4% points compared to the today’s state of the art. In this framework NTUA/LSB and CERTH/ISFTA has performed experimental investigations at a semi industrial scale 1 MWth facility on the characterization of Greek pre-dried lignite’s combustion behaviour in terms of temperature fields, heat transfer, emissions, slagging and fouling tendency and residues quality. The present work focuses on the numerical investigation of Greek dry lignite combustion firstly by post-processing and evaluation of available experimental data and secondly by combustion simulations. Q–T plots describing the heat transfer in the experimental facility are derived and specific cases of the performed tests are simulated with a commercial CFD tool, in order to estimate flow, temperature fields, NOx emissions and compare with the available experimental data. A good agreement between simulated and experimental results will support the further work on large scale boiler simulations in raw and dry coal co-firing mode, where the possibility of validation with experimental data is limited. The obtained Q–T diagrams are used to evaluate the influence of co-firing on the heat transfer in the facility and to further extrapolate the conclusions of the performed semi industrial tests on the large scale. The overall results of the CFD simulations, including predictions of temperature and NOx profiles, are in good agreement with the available experimental data at the reference case, while at the dry coal co-firing cases succeed on reproducing the basic trends of the performed experiments.     

Influence of two-stage injection and exhaust gas recirculation on the emissions reduction in an ethanol-blended diesel-fueled four-cylinder diesel engine

The purpose of this study is to investigate the effects of two-stage injection and exhaust gas recirculation (EGR) on the spray behavior and exhaust emission characteristics in diesel-ethanol fuel blends fueled four-cylinder diesel engine. The spray behavior is analyzed from the spray development process, spray tip penetration, and spray cone angle, which are obtained from the spray images. The combustion and exhaust emission characteristics are measured from the four-cylinder diesel engine with a common-rail injection system.The experimental results revealed that the increase of the pilot injection amount causes the fast development of the injected pilot spray, and the penetration difference among the main sprays is less than that among the pilot sprays. An increase in the ethanol blending ratio causes an increase in the ignition delay in the pilot combustion, but the main combustion is little influenced by the ethanol blending. The increase in the pilot injection amount shows the reduction effects of NO_x emissions when the pilot injection timing is advanced beyond BTDC 20°. The concentration of soot emissions shows a decreasing pattern according to the advance of the pilot injection and the decrease in the pilot injection amount. The CO emissions increase with the advance of the pilot injection timing, the increase in the pilot injection amount, and the ethanol blending ratio. In addition, the increase in the ethanol blending ratio and the advance of the pilot injection timing induce an increase in the HC emissions. The increase in the pilot injection amount induces a slight increase in the HC emissions.     

Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine

The aim of this work was to investigate the effect of narrow fuel spray angle injection and dual injection strategy on the exhaust emissions of a common-rail diesel engine. To achieve successful homogeneous charge compression ignition by an early timing injection, a narrowed spray cone angle injector and a reduced compression ratio were employed. The combination of homogeneous charge compression ignition (HCCI) combustion and conventional diesel combustion was studied to examine the exhaust emission and combustion characteristics of the engine under various fuel injection parameters, such as injection timings of the first and second spray.The results showed that a dual injection strategy consisting of an early timing for the first injection for HCCI combustion and a late timing for the second injection was effective to reduce the NO_x emissions while it suppress the deterioration of the combustion efficiency caused by the HCCI combustion.