Pramod S. Mehta

Predictions of tumble and turbulence in four-valve pentroof spark ignition engines

Abstract A predictive model for tumble charge motion and turbulence in four-valve pentroof engines has been developed. The model formulation is based on a mean flow analysis of tumble motion in conjunction with k-∊ turbulence equations. All major phenomena, including fluid shear, affecting mean vortex behaviour and turbulence generation are included. The predictions are made for both intake and compression periods of the engine cycle. The model predictions have been compared with earlier analytical investigations in two engines and are found to be in good qualitative and quantitative agreement. The distinct stages characterizing mean vortex and its turbulence have been identified in this work. Further, the mechanism responsible for turbulence enhancement through tumble has been synthesized and related to these stages. A preliminary parametric study with intake valve lifts and pentroof angles is carried out to demonstrate the capability of the model in design studies. It is revealed that a large-angled pentroof retains significant vortex structures even at top dead centre. In an optimized geometry, these structures may play a role in improving overall mass burn rates. The chamber geometry is found to have a significant influence on bulk motion and turbulence levels at ignition. The intake system, however, influences the formation of tumble vortices during the intake stroke.

Accurate prediction of the rate of heat release in a modern direct injection diesel engine

Abstract An accurate model for the heat release rate in a modern direct injection (DI) diesel engine is newly evolved from the known mixing controlled combustion model. The combustion rate could be precisely described by relating the mixing rate to the turbulent energy created at the exit of the nozzle as a function of the injection velocity and by considering the dissipation of energy in free air and along the wall. The complete absence of tuning constants distinguishes the model from the other zero-dimensional or pseudomultidimensional models, at the same time retaining the simplicity. Successful prediction of the history of heat release in engines widely varying in bores, rated speeds and types of aspirations, at all operating conditions, validated the model.

A correlation for soot concentration in diesel exhaust based on fuel-air mixing parameters

Abstract The acceptable performance of direct injection diesel engines is critically influenced by exhaust soot concentrations. Fuel-air mixing is a central process for diesel engine combustion and soot emission. The fuel injection and in-cylinder air motion have a pronounced effect on this controlling process. Considering this important parameter and other engine parameters, such as engine speed, fuelling rate, injection timing and swirl ratio, a regression analysis has been made to determine the parameters that are likely to interact significantly and those which may safely be ignored. In the present study, a correlation has been proposed between soot emissions and various parameters of a direct injection diesel engine.

Experimental investigations on combustion of jatropha methyl ester in a turbocharged direct-injection diesel engine

Suitability of vegetable oil as an alternative to diesel fuel in compression ignition engines has become attractive, and research in this area has gained momentum because of concerns on energy security, high oil prices, and increased emphasis on clean environment. The experimental work reported here has been carried out on a turbocharged direct-injection multicylinder truck diesel engine using diesel fuel and jatropha methyl ester (JME)-diesel blends. The results of the experimental investigation indicate that an increase in JME quantity in the blend slightly advances the dynamic fuel injection timing and lowers the ignition delay compared with the diesel fuel. A maximum rise in peak pressure limited to 6.5 per cent is observed for fuel blends up to 40 per cent JME for part-load (up to about 50 per cent load) operations. However, for a higher-JME blend, the peak pressures decrease at higher loads remained within 4.5 per cent. With increasing proportion of JME in the blend, the peak pressure occurrence slightly advances and the maximum rate of pressure rise, combustion duration, and exhaust gas temperature decrease by 9 per cent, 15 per cent and 17 per cent respectively. Although the changes in brake thermal efficiencies for 20 per cent and 40 per cent JME blends compared with diesel fuel remain insignificant, the 60 per cent JME blend showed about 2.7 per cent improvement in the brake thermal efficiency. In general, it is observed that the overall performance and combustion characteristics of the engine do not alter significantly for 20 per cent and 40 per cent JME blends but show an improvement over diesel performance when fuelled with 60 per cent JME blend.

Experimental investigations on the increase in nitric oxide emissions using biodiesels and their mitigation

One of the major shortcomings to be addressed in the widespread applications of biodiesel fuel for compression ignition engines is the formation of higher nitric oxide emissions. It is well established in the literature that thermal nitric oxide is a dominant source for nitric oxide formation in engines. Thermal nitric oxide formation increases by any in-cylinder combustion strategy that alters the in-cylinder temperatures, the oxygen fraction or the residence time of high-temperature post-flame burned gases. The differences between the properties of biodiesel in terms of a higher bulk modulus, a higher cetane number and the presence of a fuel-bound oxygen fraction and the properties of diesel are found to affect the in-cylinder charge conditions and thus the nitric oxide formation. The present work aims to understand the major contributor to the higher nitric oxide formation with biodiesel based on experimental investigations in two different engine configurations: one with a conventional mechanical-type injection system and the other with a modern common-rail direct-injection system. The experimental results highlight that the dynamic injection timing advanced up to a maximum of 2.6° crank angle owing to the higher bulk modulus of biodiesel. This factor contributes to specific nitric oxide emissions which are 7.5% higher in an engine having a mechanical-type injection system. The increase in the nitric oxide is neutralized on restoring the injection timing to that of the diesel injection time setting. In the case of an engine with a modern common-rail direct-injection system, the injection timings remain unaltered, and the nitric oxide concentrations for diesel and for biodiesel–diesel blends also remains the same.

Phenomenological modeling of combustion and emissions for multiple-injection common rail direct injection engines

The high-pressure multiple injections in common rail direct injection diesel engines offer a possibility of simultaneous reduction of exhaust smoke and oxides of nitrogen. The purpose of the present work is to develop a phenomenological model to enable parametric understanding of the combustion and emission characteristics of multiple-injection common rail direct injection engines. The model is based on a two-zone formulation comprising of fuel–air spray and the surrounding air. The model predictions for combustion and emissions are validated with measured results of different multiple-injection schedules available in the published literature. The effect of parametric variations of multiple-injection scheduling on emission characteristics are predicted using the proposed model. It is observed that the simultaneous reduction of oxides of nitrogen and smoke is possible with an optimized pilot fuel quantity and dwell between the injection pulses.

Predicting surface tension for vegetable oil and biodiesel fuels

Vegetable oil and biodiesel are considered as alternatives to diesel fuel due to their favorable engine characteristics and renewable nature. Estimates of their surface tension values are essential in understanding fuel spray behavior. This study proposes an approach for predicting the surface tension of vegetable oil and biodiesel based on their composition. In the proposed methodology, the surface tension of fatty acids and methyl esters are first estimated using suitable property correlations available in the literature. The suitability of correlations is adjudged based on validation with the measured data. Further, the correlations are also modified to improve the predictions. A weighted average mixing rule is then employed to determine the surface tension of the vegetable oil and biodiesel from their measured composition. The predicted and measured surface tension values of karanja, palmolein and coconut are compared and found to agree within 7 percent over a useful temperature range of up to 353 K. The effects of transesterification and compositional variations on the surface tension of biodiesel fuels are also discussed in this paper.

Analysis of Multimode Burning Characteristics of Isolated Droplets of Biodiesel–Diesel Blends

Blended fuels such as biodiesel–diesel blends are being extensively used in practical devises such as engines. The burning characteristics of blended fuels are quite different than that of the individual fuels and need to be understood. In this study, a semiempirical analysis concerning the mass burning rate characteristics of biodiesel–diesel blends is presented based on the data measured using porous sphere experiments. Finally, a correlation for evaluating instantaneous burning rate of biodiesel–diesel blended fuels has been proposed for practical applications. Further, using this correlation, transient burning characteristics of blended biodiesel–diesel droplet in suspended mode have been studied for different blend compositions. Multiple modes of burning regimes are identified for the blended fuels.

Second-law analysis of fuel–air mixing and combustion subprocesses in a heterogeneous charge engine:

Fuel–air mixing and combustion in engines are complex phenomena involving several subprocesses, wherein the availability (exergy) of the working fluid could be destroyed owing to irreversibilities or could be lost during energy exchanges. In the present work, a quasi-dimensional multi-zone engine model is used to quantify the availability destructions and losses associated with various mixing and combustion subprocesses in a heterogeneous charge compression ignition engine. This second-law analysis helps us to estimate the contributions of individual subprocesses towards the overall availability deficit. The results indicate that most of the availability destruction prior to ignition occurs because of fuel vaporization and mixing, whereas chemical reaction accounts for up to half of the total availability deficit during combustion. In-cylinder pressure equilibration and mixing of the combustion products with spray gases have major roles in availability destruction during the premixed combustion phase and the mixing-controlled combustion phase respectively. The contributions of wall heat transfer, heat transfer from the spray to the surrounding air and mixing of entrained air with spray gases assume significance in the later stages of combustion. Parametric studies indicate that reducing the in-cylinder air swirl and retarding the injection timing effectively curtail the availability deficit, thus improving the exergetic efficiency.

Predicting Mixing Rates in Multiple Injection CRDI Engines

The possibility of multiple-injection in Common Rail Direct Injection (CRDI) engine allows achieving improved combination of oxides of nitrogen (NOx) and smoke emissions. In CRDI engines, the turbulent kinetic energy due to high pressure fuel injection is primarily responsible for fuel air mixing and hence the in-cylinder mixture formation. The air fuel mixing characteristics in the case of multiple-injection are quite different from that of single injection schedule. In this work a zero-dimensional model is proposed for mixing rate calculations with multiple-injection scheduling. The model considers generation and dissipation of in-cylinder turbulence through processes namely fuel injection, air swirl and combustion. The model constants are fine tuned with respect to the data available in existing literature. The model predictions are validated with the available data for the cylinder pressure and heat release rate histories on known single and multiple-injection schedules. These comparisons show good agreement to establish the role of mixing rate variations with multiple-injection. A single set of constants were found to match the cylinder pressure and heat release rate histories for single and multiple-injection from different sources in the literature. Further, the mixing rate and peak temperature predictions of the model are found to relate with the possible effect of specific injection scheduling on emission reductions reported in CRDI engine investigations.© 2009 ASME