Dual Fuel Research Articles

A three-stage heating system of dual-fuel controlled by negative feedback for fluidized bed fast pyrolysis

To ensure the reduction of unnecessary consumption of thermal energy, precise control of the pyrolysis temperature, reasonable use of pyrolysis by-products, and the self-supply of thermal energy in the pyrolysis process, a novel three-stage heating system for pyrolysis was designed, which used liquefied petroleum gas (LPG) and non-condensable gas (NCG) as dual fuels. The programmable logic controller (PLC) adopted the negative feedback to individually control the four key temperatures in the heating system, and finally realized the precise control of pyrolysis temperature. In the case of only LPG to combust, the heating and reaction characteristics of the experimental apparatus were obtained in the initial experiment. The results showed that the heating system had the basis for NCG to replace LPG to combust. From the perspective of saving energy and reducing consumption energy, when LPG and NCG combusted simultaneously and the yield of liquid product was 59%, the self-supply of thermal energy can be realized, that was, reaching the point of energy balance. When the pyrolysis temperature was 500 °C and the packing height was 100 mm/50 mm, the peak value of the combustion gas replacement rate reached 78.2%, and the ideal working conditions were realized at this time.

CFD study of a CI engine powered in the dual-fuel mode with syngas and waste vegetable oil

Syngas production from biomass gasification and its use in combined heat and power (CHP) generation systems is a feasible alternative to traditional fuels in Internal Combustion Engines (ICEs), although its quality is poorer in terms of calorific value and energy density. Therefore, a viable option for its exploitation can be assured if ICEs operate in dual-fuel (DF) mode, where employing non-edible oils (as Waste Vegetable Oils, WVO) as a further residual material may solve issues related both to the utilization of diesel fuels and to the high costs for biodiesel production. The practice promotes a virtuous circle to enhance material recovery without disposal. A combined experimental-numerical activity for the analysis of the performances of a Compression Ignition (CI) engine in the DF mode with syngas and WVO is here presented. An appropriate pre-heating system is mounted on the engine injection line to reduce the WVO viscosity. Experimental data collected in terms of in-cylinder pressure cycle, heat release rates and main pollutants at different loads are used to validate a properly developed three-dimensional (3D) Computational Fluid Dynamics (CFD) model, that helps investigating the detail of the syngas fuelling on power output and emissions.

Estimating the urban environmental impact of gasoline-ethanol blended fuels in a passenger vehicle engine.

A large portion of urban emissions in developing countries come from old gasoline vehicles driven in metropolitan areas. The present study aimed to develop models to estimate the environmental impact of different contents of gasoline and ethanol mixtures (pure gasoline; 25, 50, 75% ethanol blended to gasoline; and 100% ethanol) in a flex-fuel engine. We tested the blended fuel using three different speeds and recorded the GHG emissions and engine output data. The data mining approach was used to develop environmental impact predictive models. The ethanol content in gasoline; the engine rotational speed 900, 2000, and 3000 rpm; and λ were used as attributes. The classification target was the environmental impact concerning the CO2 emission ("low," "average," and "high"). We employed the Random forest algorithm to develop predictive models. The mean values of CO2 concentrations for all studied fuel content were above 2.47% of the volume. The trees' models (accuracy 73%, κ =0.61) showed three alternatives for predicting the environmental impact based on the ethanol blend, the engine rotation, λ, and the air-fuel ratio. Such models might help policymakers develop educational campaigns to reduce short- and medium-term urban commuter traffic pollution in countries that lack suitable urban transportation.

Integration of Fluidic Nozzles in the New Low Emission Dual Fuel Combustion System for MGT Gas Turbines

Fluidic oscillators have proven their capabilities and advantages in terms of the generation of oscillating jets without moving parts for many years, mainly in experimental studies. In this paper, the design, development, and integration of fluidic atomizers into the liquid-fuel system of the dual-fuel low NOX Advanced Can Combustion (ACC) system of the MAN Gas Turbines (MGT) are presented. The two-stage system comprises a pressure-swirl nozzle as a pilot stage and an assembly of four main premixed nozzles, based on fluidic technology. The design and the features of the pilot nozzle are briefly presented, whereas the focus lies on the functionality and layout of the fluidic nozzles. The complete integration, validation, and verification of this innovative liquid-fuel injection unit are presented. The final system features fast fuel-switchovers, low complexity, high reliability, and dry low emissions in liquid-fuel operation.

Performance and emission analysis of a dual-fuel engine operating on high natural gas substitution rates ignited by aqueous carbon nanoparticles-laden diesel/biodiesel emulsions

The dual-fuel (DF) combustion process is a promising engineering solution to achieve clean combustion and high thermodynamic efficiency. The composition of pilot fuel (PF) can be considered as one of the effective parameters on the combustion quality of the DF process. In general, employing biodiesel blended fuel samples as PF can reduce brake power (BP) and increase fuel consumption. Therefore, developing fuel formulations harboring additives that are capable of boosting brake power (BP) and reducing fuel consumption of DF diesel engines (DFDEs) while also mitigating their harmful emissions, is highly considered. To this end, the present study was set to investigate emission and performance characteristics of a natural gas (NG) DFDE ignited by water-emulsified (WE) diesel/biodiesel (D/B) blends (5 vol% biodiesel and 3 wt% water) containing 30 and 60 µM aqueous carbon nanoparticle (CNP). Neat diesel and D/B blend containing 5 vol% biodiesel were used as control fuel. The engine was fueled with high NG substitution rates (50–80% of the total fuel energy) at engine loads (25–100% of full load). Overall, emissions and performance characteristics of the engine were significantly affected by engine load and NG substitution rate. On average, the WE D/B blend doped with 30 µM could provide acceptable results in terms of engine emissions and performance. This fuel formulation could mitigate unburned hydrocarbon emissions by about 8 to 40% at different loads, even though increased NOx emissions. The power generation cost of the selected fuel blend was also about 5 to 23% lower than the control fuels.

Ignition delay time measurements and kinetic modeling of methane/diesel mixtures at elevated pressures

Natural gas/diesel dual-fuel (DF) combustion technology has attracted substantial attention in terms of thermal-efficiency improvement and emission reduction in advanced compression ignition engines. As known, autoignition behavior of DF mixtures plays a significant role in controlling the ignition timing and combustion performance in engines. In the present study, ignition delay time (IDT) measurements of methane/diesel mixtures with three varying diesel substitution ratios (denoted as DSR30, DSR50, and DSR70) were conducted on both a heated rapid compression machine and a heated shock tube under wide-range conditions (T = 640–1450 K, p = 6–20 bar, ϕ = 0.7, 1.0, and 2.0 in ‘air’ mixtures). Experimental results show that DF blends exhibit typical two-stage autoignition characteristics along with the negative temperature coefficient (NTC) response at the temperature region currently investigated, owing to the addition of high-reactivity diesel fuel. Moreover, both the total and first-stage IDTs decrease with the rise of pressure and equivalence ratio as well as diesel substitution ratio. Additionally, a crossover of IDTs occurs at a high temperature (~1400 K) for varying equivalence ratios, and there exists a non-linear promoting effect of diesel contents on IDTs. The simulation results performed using a detailed chemical kinetic mechanism (POLIMI_1412) in conjunction with a well-validated tri-component diesel surrogate show generally good agreement with the experimental results at the current test conditions. Furthermore, species evolution assisted with brute-force sensitivity analysis was carried out to further understand the autoignition chemistry of the DF mixture, particularly the chemical interplay between methane and diesel during the low-temperature ignition processes. It is found that methane hardly generates OH radicals, which are mainly produced via the low-temperature oxidation pathways of diesel fuel. The competition between methane and diesel for OH radicals inhibits the consumption of diesel while promotes the depletion of methane, resulting in an inhibiting impact on the overall reactivity of the reaction system. What's more, the experimental data reported herein provide a foundation for the development of DF kinetic models with high accuracy and robustness.

Molecular dynamics simulation of sub/supercritical evaporation with n-butanol/n-heptane blended fuel

Sub/supercritical evaporations of single-component fuels have been developed well so far. In the last decade, dual fuels, even multiple fuels have received more attention. However, sub/supercritical evaporations of dual fuels are more complicated than that of single-component fuels, as dual fuels have uncertain critical points, and single-component fuels have exact critical points. Therefore, in this study, we focused on the functional relationship between the critical temperatures and the blending ratios (n-heptane to n-butanol) of the n-butanol/n-heptane blends by molecular dynamics simulations. A liquid film of the blends was constructed in nitrogen environment. The sub/supercritical evaporations processes of the blends were simulated in different ambient temperatures and pressures and in different blending ratios of the blends. The density profile, surface temperature, evaporation mass ratio and evaporation rate were estimated and compared under sub/supercritical states. The critical temperature of the blends was determined by the surface temperature and the surface tension. Results indicate that under ambient low supercritical condition, a transition from subcritical to supercritical state does not happen in the evaporation process of blends. The evaporation mass ratio is not always equal to the blending ratio as the way used in current evaporation model, but a function over time. A fitting formula for calculating the critical temperatures of the blends is provided.

Fuel Economy and Emissions of E85 in Passenger Cars - A Move towards Flex Fuel Vehicle

<div class="section abstract"><div class="htmlview paragraph">Many countries are developing strategies to curb the consumption of fossil fuels, and to increase the share of alternative fuels such as alcohols, natural gas, fuel cell and electricity in the energy pool in order to improve energy security and reduce atmospheric pollution. Alcohol fuels are promising one and it has been widely used in many countries as blending component for gasoline. Ethanol has a high-octane number but it has a lower calorific value than gasoline. The performance of engine may be affected with higher percentage of ethanol in gasoline due to demand for larger quantity of fuel that could not be supplied by vehicles which are tuned to run on gasoline only. In this study, a second electronic control unit (ECU) was installed in series with the existing commercial or primary ECU and an ethanol sensor was installed in the fuel line. This secondary ECU modulates the fuel injection pulse width of the primary ECU depending on ethanol concentration in the fuel. The vehicle studies were carried out using Indian automotive gasoline meeting IS:2796 specification and 85% ethanol blended gasoline (E85) in a climatically controlled chassis dynamometer lab in which test cell temperature maintained at 25±1 °C throughout the test. Modified Indian Driving Cycle tests (MIDC) and constant speed tests were carried out using E85 and commercial gasoline to investigate the fuel economy, gaseous mass emissions, particulate emission in terms number and size characteristics. Gaseous emission was reduced up to 30% with the penalty of reduction in fuel economy by approximately 30% over the MIDC. The vehicle power was maintained while using both fuels at respective constant speeds during the road load simulation cycles however an increase in fuel consumption was observed with E85. The tailpipe exhaust particle emission measurement had shown up to 50% reduction in exhaust particulate emissions with E85. This study demonstrates the potential of using higher blends of ethanol up to E85 and meeting power or speed demand at constant speeds and driving cycle tests.</div></div>

Application of Response Surface Methodology for Optimization of Fuel Injection Parameters of a Dual Fuel Engine Fuelled with Producer Gas-Biodiesel blends

ABSTRACT The present work reports the development of a mathematical model to establish a correlation between controllable engine input parameters and their dependent responses over a varied range of engine loadings. Brake thermal efficiency (BTE), peak combustion pressure, and exhaust gas temperature (EGT) are chosen as a response while engine load, pilot fuel injection timing (FIT), and pilot fuel injection pressure (FIP) are taken as controllable input. The response surface methodology (RSM) has been used for the model development based on multilinear regression analysis and the design of experiments techniques. The experimental data for model development was collected through lab-based experiments carried out using diesel engines in dual-fuel mode. The dual-fuel engine was powered with producer gas as the main fuel and waste cooking oil biodiesel as pilot fuel. The best possible combination of input parameters optimized by RSM helps in the improved and stable operation of the dual-fuel engine. Furthermore, the developed model can predict the output responses very close to experimental results. The model robustness is verified with high values of correlation coefficients R2 for all three response variables.

Intervention of Artificial Neural Network with an Improved Activation Function to Predict the Performance and Emission Characteristics of a Biogas Powered Dual Fuel Engine

Biogas is a significant renewable fuel derived by sources of biological origin. One of today’s research issues is the effect of biofuels on engine efficiency. The experiments on the engine are complicated, time consuming and expensive. Furthermore, the evaluation cannot be carried out beyond the permissible limit. The purpose of this research is to build an artificial neural network successfully for dual fuel diesel engine with a view to overcoming experimental difficulties. Authors used engine load, bio-gas flow rate and n-butanol concentration as input parameters to forecast target variables in this analysis, i.e., smoke, brake thermal efficiency (BTE), carbon monoxide (CO), hydrocarbon (HC), nitrous-oxide (NOx). Estimated values and results of experiments were compared. The error analysis showed that the built model has quite accurately predicted the experimental results. This has been described by the value of Coefficient of determination (R2), which varies between 0.8493 and 0.9863 with the value of normalized mean square error (NMSE) between 0.0071 and 0.1182. The potency of the Nash-Sutcliffe coefficient of efficiency (NSCE) ranges from 0.821 to 0.8898 for BTE, HC, NOx and Smoke. This research has effectively emulated the on-board efficiency, emission, and combustion features of a dual-fuel biogas diesel engine taking the Swish activation mechanism in artificial neural network (ANN) model.

Developing of Emergency Safety Device Module Under Vessel Integrated Automation System for Dual Fuel Diesel Engine

Using gas as a fuel of the ship’s main engines give challenges in term of efficiency, fuel prices, low emission, and safety behavior. Gas characteristics need particular attention due to higher flammable, explosive, and hazard gas leakages. Adopting gas as fuel onboard a ship insists on a higher level of safety as guided by the IGF Code. Conventional safety devices installed in the conventional diesel engines only aware of its combustion sensors. This study intended to develop a new PLC module that is capable of an automatic stop of the dual fuel diesel engine under any programmable emergencies. The developed module is integrated under the Vessel Integrated Automation System. The logical program manages the system to assure the continuous fuel service and stop the system when pre-defined hazards happened. Hazards are predicted from excessive pressure and temperature, flow rate, and gas leakages. The module in a form of software and hardware will be built using industrial tools.

Technical Analysis of Diesel Engine Convert to Dual Fuel Engine for the Ship

Fossil fuel is the high-cost price. This trend of price is increasing and not stable. For the future is need alternative low cost and stable price fuel. Natural gas has low cost and stable price for alternative fuel. The objective of this paper is to analyze the effect of diesel engine converted to dual fuel engines using. The method of this paper is a mixture of HSD with CNG conducted on the ship diesel engine. The conversion is determining additional components required. Then, the reduction of the cargo for two cars for cylinder placement used. After the engine has been changed to dual fuel, the performance investigated. The performance investigation is based on the variation of HSD and CNG fuel composition in the engine. The result has shown that the best performance is when the 30: 70 of CNG: HSD mixture composition.

Large-eddy simulation of tri-fuel ignition: diesel spray-assisted ignition of lean hydrogen–methane–air mixtures

We present 3D numerical results on tri-fuel (TF) combustion using large-eddy simulation and finite rate chemistry. The TF concept was recently introduced by Karimkashi et al. (Int. J. Hydrogen Energy, 2020) in 0D. Here, the focus is on spray-assisted ignition of methane–hydrogen blends. The spray acts as a high-reactivity fuel (HRF) while the ambient premixed methane-hydrogen blend acts as a low-reactivity fuel (LRF) mixture. Better understanding on such a TF process could enable and motivate more extensive hydrogen usage in e.g. compression ignition marine engines where spray-assisted dual-fuel (DF) combustion is already utilised. The studied spray set-up is based on the modified ECN Spray A case, see Kahila et al. (Combustion and Flame, 2019) for DF combustion. The ambient pressure and temperature are 900 K and 60 bar. The hydrogen content of the LRF blend is varied systematically by changing the molar fraction , . The main added value of the study is that we extend the TF concept to 3D. The particular findings of the study are as follows: 1) Consistent with Karimkashi et al. 2020, hydrogen delays ignition also in 3D and the effect becomes significant for . 2) The ratio between the first- and second-stage ignition delay times and . Furthermore, the ratio between 3D and 0D ignition delay times is given as for all TF cases. 3) Finally, consistent with Karimkashi et al. 2020, also in 3D the high-temperature combustion heat release mode is shown to appear stronger in TF than the low-temperature combustion mode compared to DF methane–diesel combustion.

Pre-bent Flow-Field Plates for Enhanced Performance in Flexible Polymer Electrolyte Membrane Fuel Cells in Curved Shape

This article reports on a flexible polymer electrolyte membrane fuel cell (PEMFC) with a pre-bent flow field. The performances in the flat and bent positions were lower and higher, respectively than that of the traditional flexible fuel cells. The low performance in the flat position was attributed to the void space induced incomplete contact at the interface of the membrane electrode assembly (MEA) and flow-field plates, which resulted in poor performance due to the high ohmic resistance (5.85 Ω cm2) and faradaic resistance (4.17 Ω cm2). However, when bending stress was applied to the MEA, a decreased ohmic resistance (0.954 Ω cm2), faradaic resistance (0.737 Ω cm2), and an enhanced power density (88.7 mW/cm2) were observed because of the improved interfacial contact property between the MEA and the flow-field plates from the increased compressive stress. These experimental results were further analyzed and visualized with aid of the finite element analysis. Despite the relatively low performance in the flat shape, the proposed pre-bent design of the flexible PEMFC possesses promising applicability especially in flexible electronics where the curved shapes are highly required.

OPERATIONAL AND ENERGY USE PROFILE CHARACTERISATION OF LNG CARRIERS WITH DFDE PROPULSION PLANTS

Traditionally, the LNG fleet has been dominated by ships powered by steam plants however, new Four-stroke Dual Fuel Diesel Electric (DFDE) or Tri-Fuel Diesel Electric (TFDE) propulsive plants offer fuel flexibility while operating at a higher thermal efficiency. This propulsion option has become a relevant alternative to steam-powered plants representing about 32% of the total LNG fleet in 2018. This study seeks to establish the operational and energy use profile of LNG carriers based on the analysis of high-frequency in-service data. It focuses on LNG carriers with DFDE propulsion systems using data collected between 2016 and 2018 from 11 LNG carriers – representing about 7% of the DFDE and TFDE powered LNG fleet. The continuous monitoring data covers many performance parameters allowing for a reliable analysis of operation and energy use while reducing uncertainty by measuring essential parameters rather than assuming or approximating.

Exploring the efficiency and pollutant emission of a dual fuel CI engine using biodiesel and producer gas: An optimization approach using response surface methodology

The present study focuses on optimizing the engine operating parameters of a dual-fuel (DF) engine. Producer gas (PG) and Honge oil methyl ester (HOME) are used as primary fuel and pilot fuel respectively for the operation. An experimental design matrix of 20 different combinations was considered using Design of Experiments (DoE), based on the central composite design (CCD) of response surface methodology (RSM). The effects of these combinations were experimentally investigated to calculate the performance and emission characteristics of the engine. The objective of the work is to maximize the Brake thermal efficiency (BTE) and minimize the exhaust gas temperature (EGT), nitrogen oxide (NOx), hydrocarbon (HC), and carbon monoxide (CO) emissions. The RSM model is developed using the experimental data and further, the operating parameters were optimized using the desirability approach. The optimized combination of operating parameters was obtained at 61.10% engine load, compression ratio (CR) of 18, and injection timing (IT) of 23.30° before top dead center (BTDC). The optimum responses corresponding to these operating conditions were found as 14.23%, 354.29 °C, 52.18 ppm, 39.53 ppm, and 0.51% for BTE, EGT, NOx, HC, and CO respectively with an overall desirability of 0.962. The optimized responses were validated experimentally at optimum input conditions and found to be within acceptable error levels. Further, an economic analysis of the optimized DF system is also carried out.

Effect of Compression Ratio on Performance and Emission Characteristics of Dual Spark Plug Ignition Engine Fueled With n-Butanol as Additive Fuel

Renewable energy called normal-butanol is a possible alternative fuel for automobile vehicles like some other possible fuel such as compressed natural gas (CNG), liquid petroleum gas (LPG), ethanol, and methanol. Bio-butanol or normal-butanol is also a meritable energy source to substitute for regular fossil fuels. The normal-butanol has recently started to use as a possible substitute fuel to regular fuels for internal combustion engines to attain eco-friendly and capital benefits. As compared to regular energy sources in internal combustion engines, normal-butanol has some benefits, so it shows the potential to decrease tailpipe emission andan increase in positive network delivery. The current work carried out to investigate the performance and emission characteristics of dual spark plug ignition engine fuelled with normal-butanol as additive fuel by adopting 10:1 and 10.5:1compression ratios. The experimental results reveal that when compared between 10:1 and 10.5:1 compression ratios, brake power (BP) is increased by 3.5% and 3.2% for normal-Butanol 35 (nB35) blend and energy efficiency increased by 2.72% and 2.14% for nB35 blend at a part and full load for 10.5:1 compression ratio. The n-butanol create a greater impact on tailpipe emissions that the carbon monoxide (CO) decreased by 32%, 29%, and hydrocarbon (HC) reduced by 2.38% and 2.22% for nB35 blend at a part and full load condition respectively. The experimental results on dual spark ignition engine using n-butanol as additive fuel by varying compression ratioreveals that n-butanol can be a suitable replacement energy source for the automobile sector in the nearest future.

An Experimental Investigation on Performance and Emission Characteristics of PCCI Engine Using BiodieselEthanol Blends in Dual Fuel Mode

The global automotive industry is faced with the task of reducing global greenhouse gas emissions generated by vehicle exhaust. Heavy machinery like trucks, trailers, off-road vehicles, etc. powered by the diesel engines. These vehicles emit large quantities of NOx and smoke emissions. The simultaneous reduction of both emissions is the key challenge for the automotive industry. Burning a homogeneous charge at relatively low temperature seems to be the means to control both the emissions simultaneously. In this experimental study, the combustion, performance and emissions characteristics of a premixed charge compression Ignition Engine (PCCI) with ethanol injected in the intake manifold of the engine along with biodiesel (Palm oil methyl ester) injected directly into the combustion chamber. The experiments were carried out in a four-stroke, single cylinder vertical water cooled, constant speed diesel engine with the range of 10- 30% premixed ethanol fuel from no load to full load condition is studied. The experiments were conducted with an engine operating in PCCI mode with biodiesel – ethanol blends in dual fuel mode. The experimental results showed that there was a slight increase in Hydrocarbon (HC) and Carbon monoxide (CO) but there was a reduction in Nitrous-oxide (NOX) and Smoke emissions for the blend containing 70% biodiesel and 30% ethanol.

Characterization of combustion process and emissions in a natural gas/diesel dual-fuel compression-ignition engine

Dual-fuel (DF) combustion using natural gas and diesel has received considerable attention in the on- and off-road freight transportation sectors owing to its potential use in achieving better fuel economy and cleaner combustion. To determine the effect of the natural gas/diesel mixture quality on the combustion process and pollutant emission, high-speed flame visualization was used to investigate the phenomena of natural gas (NG)/diesel DF combustion in a 1.0 L optically-accessible single-cylinder engine. The diesel injection timing and natural gas substitution ratio (NGSR) were varied to implement diverse in-cylinder blending conditions under constant fuel energy input. A novel flame regime separation method based on color image segmentation in a hue-saturation-value (HSV) color space was used to quantitatively compare the spatial distributions of premixed and diffusion flame regimes. Because NG has a lower carbon content and higher auto-ignition resistance compared with diesel, the natural luminosity images for larger NGSRs revealed a significant reduction in the diffusion flame regime accompanied by retarded flame development. An earlier injection of the liquid diesel shifted the location of the early flame growth toward the piston bowl wall and created a rapid influx of propagating flame, while effectively suppressing the formation of intense soot radiation through longer ignition delay. These observations were verified by the exhaust emissions measured using the full-metal version of the engine and the same fuel supply parameters, specifically regarding the behavior of the smoke and nitrogen oxide emissions.

Functionalization of Doped ZnO through Solution Route Synthesis: A study Using Spectroscopy & Density of States

ZnO is one of the widely studied materials for multidimensional applications, viz. semiconductor material, catalysts, solid-state devices, etc. The primary functionalization is carried out by doping the required element (s) within the ZnO matrix, which can exist in either zinc blend or the wurtzite form. The present research reports synthesis of ZnO doped by Cr, Y, and Eu at two dopant concentrations. The synthesis technique is optimized using dual fuels during solution auto combustion synthesis. Detailed analysis of X-ray diffraction study reveals a comparative analysis of the peak area and FWHM magnitude. The influence of the doping element on the ZnO is studied in terms of UV and photoluminescence spectra. The highest bandgap of 3.08 eV is reported with Eu as the dopant within ZnO compared to Y, which shows lower bandgap energy of 2.44 eV. The density of states study of ZnO is found to be continuous with a significant nodal region within -3.4 to -2.4 eV. However, in the doped systems, irrespective of the dopant, nodal regions are more with specific band regions in the ZnO-Y/ZnO-Eu system. Irrespective of dopant type, doping within ZnO significantly influences the states in the conduction band.