Combined effect of injection timing and combustion chamber geometry on the performance of a biodiesel fueled diesel engine
Graphical abstract
Highlights
▸ Investigates the effect of injection timing and combustion chamber design for biodiesel operation. ▸ Tests were carried out using 20% Pongamia biodiesel blend with diesel and diesel. ▸ Toroidal Re-entrant (TRCC) and Hemispherical combustion chambers (HCC) were used. ▸ Tests were carried out at different injection timings of 20°, 21°, 22°, 23° and 24° bTDC. ▸ TRCC has shown improved performance and emission characteristics at 21° bTDC.
Introduction
Energy conservation and emissions have become of increasing concern over the past few decades. The growing vehicle population and the rapid rate of industrialization in recent decades have resulted on the fuel crisis. There is a severe shortage of fuel resources and the existing resources are getting depleted at an ever increasing rate. The search for alternative sources of fuel is gathering momentum. In this context, there has been growing interest on alternative fuels like biodiesel to provide a suitable diesel oil substitute for Compression Ignition (CI) engines [1].
Biodiesel has properties comparable to ULSD [2], [3], however certain properties of biodiesel such as viscosity, calorific value, density and volatility differ from ULSD. These properties strongly affect injection, air-fuel mixing and thereby combustion and performance characteristics of biodiesel in a diesel engine.
A large number of experimental investigations on the performance, emission and combustion characteristics of biodiesel fuel in diesel engines have been carried out without any modification to the diesel engine. These studies have reported that the use of biodiesel blends and neat biodiesel in diesel engine decreases carbon monoxide by about 3–15% [4], [5], [6], sunburnt hydrocarbons by about 6–40% [7], [8], [9], and smoke emission levels by up to 45% [10], [11] compared to ULSD. However, increases in NOx emission levels by up to 26% [12], [13], [14], increases in brake specific fuel consumption by about 6–15% [15], [16], decreases in brake thermal efficiency by upto 9% [17], [18], decreases in brake power by about 4–5% [19] and decreases in brake mean effective pressure [20] have been reported.
The poor performance of biodiesel operated diesel engine in comparison with ULSD fueled diesel engine is mainly due to change in fuel properties, engine design and operating parameters. Although transesterification reduces viscosity of biodiesel [21], [22], the viscosity of biodiesel was found to be 10–15% higher than ULSD [23]. The high viscosity of biodiesel affects injection characteristics, particularly atomization of the biodiesel fuel. The poor atomization, insufficient in-cylinder air motion and low volatility of biodiesel lead to difficulty in the air-fuel mixing. Inadequate air-biodiesel mixing and sluggish evaporation process significantly affect the combustion process [24] leading to poor performance of biodiesel fueled diesel engine. Further, increases in NOx emission with biodiesel operation is attributed to an inadvertent advance of fuel injection timing due to the higher bulk modulus of compressibility of the biodiesel fuel [25], [26]. The higher bulk density and viscosity transfers the pressure wave through fuel pipe lines faster and an earlier needle lift will lead to advanced injection [27], [28].
In addition in DI diesel engine, the combustion chamber has been optimized for combustion of ULSD, including improvement of mixing between injected fuel and in-cylinder air, and not for biodiesel. Apart from injection parameters, the shape of the combustion chamber can also help to form better mixtures. The shape of the combustion chamber and the fluid dynamics inside the chamber are of great importance in biodiesel combustion. As the piston moves upward, the gas is pushed into the piston bowl. The geometry of the piston bowl can be designed to produce a squish and swirling action which can improve the fuel/air mixture before ignition takes place. T. Saito et al. [29] compared conventional combustion chambers and re-entrant combustion chambers in terms of the combustion process, engine performance and NOx and smoke emissions for ULSD operated direct injection diesel engine. It has been reported that re-entrant chamber enhances combustion because of the higher in-cylinder velocity accompanied by increased turbulence. Montajir R et al. [30] have studied the Effect of Combustion Chamber Geometry on Fuel Spray Behavior in a Small DI Diesel Engine. It has been found that a re-entrant type combustion chamber with round lip and round bottom corners provides better air and fuel distribution than a simple cylindrical combustion chamber.
In order to obtain higher performance and lower emissions from biodiesel when used as a fuel in diesel engine certain modifications to engine design and injection parameters are required. The available energy in biodiesel should be utilized very efficiently and economically. Inefficient methods of utilizing the available energy should be curbed. Keeping all this in mind an effort was made in this study to evaluate experimentally the effect of varying the combustion chamber geometry and varying the injection timing on the combustion, performance and emissions of a DI diesel engine fueled with a blend of 20% Pongamia Oil Methyl Ester (POME) by volume in ULSD (B20).
Section snippets
Biodiesel production and properties
Usage of non-edible oils for the production of biodiesel is found to be best suited given the deficit supply of edible oils and their cost of production. Among the non-edible oils, Tree Borne Oil seeds (TBOs) like jatropha and pongamia gain importance. Pongamia Pinnata, an excellent shrub having natural spread across the globe, is one of the promising biofuel crops ideally suitable for growing in the wastelands. Pongamia seeds contain 30–40% oil. To prepare POME, the transesterification
Results and discussion
The performance, emission and combustion characteristics of the base engine with HCC and modified engine with TRCC and at different injection timings were determined, compared and analysed for brake specific fuel consumption, brake thermal efficiency, unburnt hydrocarbon, carbon monoxide, oxides of nitrogen, smoke emissions and combustion parameters such as ignition delay, cylinder peak pressure, exhaust gas temperature and heat release rate.
Conclusion
The fuel properties of biodiesel and their blends were determined and compared in relation with that of ULSD. Even though properties of POME are comparable with ULSD the viscosity of B20 was found to be about 14.5% higher and calorific value was 2.3% lower, when compared to ULSD. Experiments were performed using a DI diesel engine to investigate the combustion, performance and emission characteristics of biodiesel fuel derived from Pongamia oil using Hemispherical open type and Toroidal
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