نشریه سوخت و احتراق
سال چهارم شماره 1 (پیاپی 7، بهار و تابستان 1390)

Thermodynamic models based on measured cylinder pressure versus crank angle are used to analyze the combustion characteristics in the internal combustion engine. The results of the thermodynamic models are inevitably error-prone. Therefore, in this paper, a new thermodynamic model under motoring conditions is developed for determination of parameters such as TDC position, offset in measured cylinder pressure, cylinder volume deformation and clearance volume. The model is validated using TDC position and offset in measured cylinder pressure obtained from experimental data. Accordingly, a satisfactory agreement is yielded between the results.

Homogenous Charge Compression Ignition (HCCI) engine is a promising idea to reduce fuel consumption and engine emissions. Natural gas، usually referred as clean fuel، is an appropriate choice for HCCI engines due to its suitable capability of making homogenous mixture with air. However، composition of natural gas strongly affects the auto-ignition characteristics of in-cylinder mixture and the performance of the HCCI engine. This paper has focused on the influence of natural gas composition on engine operation in HCCI mode. To this end، six different compositions of natural gas (including pure methane) have been considered to study the engine performance via a thermo-kinetic zero-dimensional model. The simulation code covers the detailed chemical kinetics of natural gas combustion and includes Zeldovich extended mechanism to evaluate NOx emission. Validations have been made using experimental data from other works to ensure the accuracy needed for comparison study. The equivalence ratio and the compression ratio were held constant but the engine speed and mixture initial temperature were changed for comparison study. The results show that the operational parameters of the engine such as gross indicated work، gross mean effective pressure، and NOx are dependent to natural gas composition. SOC is highly dependent to the amount of heavier hydrocarbons existing in the fuel composition، which leads to noticeable changes in the time and value of in-cylinder pressure/temperature peak.

In this work، the unsteady response of a laminar diffusion flame to harmonic mass fraction oscillations is investigated. Flame-sheet assumption is utilized to model the laminar unsteady two-dimensional co-flow diffusion flame. The flow is assumed to be subsonic، inviscid، and uniform. The convection-diffusion equation for conserved scalar with appropriate boundary conditions is solved. Considering the stoichiometric mass fraction surface to be the flame surface، it is possible to obtain the flame zone. Assuming that unburnt species cannot pass across the flame surface and that the diffusion coefficient is constant، heat release rate can be related to the flame area. To best of our knowledge، this is the first time this approach has been applied to a diffusion flame to calculate heat release rate. Flame response function is acquired as oscillations of heat release rate to fluctuations of fuel mass fraction. At each Peclet number، frequency domain is divided into three regions، namely diffusion-dominated region، convection-diffusion region، and convection-dominated region. Our results indicate that the magnitude of response function decreases as excitation frequency increases، while phase difference approaches a constant value. Also، as Peclet number increases، the amplitude of oscillations in heat release rate increases in diffusion-dominated and convection-diffusion regions، but it does not change significantly in convection-dominated region.

In this study، the biodiesel (ethyl ester) from sunflower oil was prepared by a transesterification method. Biodiesel has properties different from those of diesel fuel. The effects of biodiesel addition to diesel fuel were examined on the performance and emissions of a DI diesel engine equipped with turbocharger at full engine load. Increase in brake specific fuel consumption (BSFC) and decrease in brake power for biodiesel and its blends were observed compared with diesel fuel at 1400 rpm. Also، CO، NOx and UHC were increased while smoke was decreased. At 2000 rpm، the results indicated that engine fueled with biodiesel and its blends increased the brake power and BSFC. Also، CO، CO2، UHC and smoke emission were reduced، while NOx emission was increased.

In this paper، behavior of the non-ideal detonation wave is studied. Simulation of detonation is performed based on the one-dimensional Euler equations، taking into consideration the friction effects by additional momentum source term in the momentum conservation equation. One-step Arrhenius reaction model is utilized for modeling of the chemical reactions. The yielded results indicated that in the very low activation energy (equal to 8)، considering the friction caused a reduction in detonation velocity and an extension in length of the reaction zone up to a specific value. Nevertheless، stable behavior of detonation wave is observed even at higher values of friction factor. The results also show that in the activation energy of 22 (which is close to stability limit in the ideal detonation) and with low values of the friction factor، detonation wave retains its stability; but with increasing the friction factor، detonation wave leads to unstable and pulsating detonation. For higher values of the friction factor، at first، double oscillation detonation is appeared and finally، when friction factor exceeds the critical value، separation of the reaction zone and precursor shock wave occurs and then the detonation will be attenuated. Indeed، the behavior of detonation waves is controlled by the competition between the rate of energy release from chemical reaction and the rate of momentum dissipation by friction effect.

In this study، the diesel engine fuel system was modified and the use of alternative fuels، i. e. biodiesel and biogas، was investigated in biological dual-fuel diesel engines. Biodiesel fuel from waste cooking oil was produced by transesterification method and biogas fuel was produced through anaerobic fermentation of livestock waste. Biogas comprised primarily methane (70%) and carbon dioxide (30%). In the present study، 90% of the diesel was replaced by biogas، while the remaining 10% was used as a pilot fuel. Using equipment such as mixer، regulator، filter، valve and flow meter، the basic diesel engine (Lister M8/1) was converted into dual-fuel diesel engine. Afterwards، the engine performance and emission tests at full load and at fixed speed of 750 rpm were investigated for fossil fuels (diesel and natural gas) and biofuel (biodiesel and biogas). The experimental results indicated that the simultaneous use of biofuels (biodiesel and biogas) in dual-fuel engines is possible، though it is accomponied by the loss of engine power. In these conditions، the NOx emission decreased، unlike the enhanced emission of CO، CO2 and UHC.

In gas-fired furnaces، the flame radiation enhancement significantly improves the radiative heat transfer، which in turn reduces flame temperature and NO emission. In this paper، the effect of inlet natural gas preheating on the flame luminosity and overall efficiency in a 120KW boiler are investigated experimentally. Flame radiation is measured by use of laboratory pyranometer with photovoltaic sensor. A Testo350XL gas analyzer is also used for measuring the temperature and combustion species. The experimental measurements show that the effect of increasing inlet gas temperature on flame luminosity is increased abruptly for temperatures over 240C. This luminosity increase has considerable effects on flame temperature and NO emission reduction and enhances the boiler efficiency. The results also show that increasing the inlet gas temperature from 240C up to 300◦C increases the flame luminous radiation by 60%. The obtained results are in desirable accordance with finding of other studies regarding fuel preheating.