Numerical Investigations Of Piston Cooling Using Oil Jet

Abstract

Thermal loading o f diesel engine pistons has increased dramatically in recent years due to applications of various technologies to meet low emission and high power requirements. Control of piston temperatures by cooling of these pistons has become one of the determining factors in a successful engine design. The pistons are cooled by oil jets fired at the underside from the crankcase. Any undesirable piston temperature rise may lead to engine seizure due to piston warping. However, if the temperature at the underside of the piston, where the oil jet strikes the piston, is above the boiling point of the oil being used, it may contribute to the mist generation. This mist may significantly contribute to the non-tail pipe emissions in the form of unburnt hydrocarbons (UBHC). The problem of non-tail pipe emissions has unfortunately not been looked into so seriously, as the current stress of all the automobile manufacturers is on meeting the tail -pipe emission legislative limits. A numerical model has been developed using computational fluid dynamics (CFD) tools, such as finite elements methods for studying the oil jet cooling of pistons. Using the numerical model developed by Stevens and Webb (1991), the heat transfer coefficient (h) required at the underside of the piston is predicted. This predicted value of heat transfer coefficient significantly helps in selecting right oil type, jet velocity, jet diameter and distance of the jet from the underside of the piston. It also helps to predict whether the oil selected will contribute to mist generation or not and if it contributes to mist generation then it helps in selecting the oil which does not contribute to mist generation. Grid generation for a production grade M & M DI 2500 engine piston has been done using GNUPLOT. Isotherms of the predicted temperature profiles in the piston have been plotted using TECPLOT. Introduction Direct Injection (DI) Diesel engines have an advantage in fuel economy compared with gasoline engines. Diesel engines are ecofriendly and have high potential to meet future exhaust emission regulations because of their lower carbon dioxide (CO2) emissions. Diesel engines however suffer from the problem of the emission of nitrogen oxide (NOx) and particle matter (PM). Requirements of higher power and lower fuel consumption still remain in the case of diesel engines installed on commercial vehicles such as trucks and buses. In order to meet these requirements, current diesel engines are required to have boosted turbo -charging, high pressure fuel injection and improvement of airflow in the piston combustion bowl. The current trend in the automobile industry is towards increasing the power density of the engine and making lighter engines. These requirements lead to higher thermal load on the engine, especially on the pistons. In internal combustion engines, the thermal energy is released in the combustion chamber by combustion of the fuels. The combustion gases supply work to the piston, and the residual heat is transferred to the piston body, cylinder liner, valves, cooling fluid, lubricating oil, and exhaust gases. In the piston, the heat flux traverses the rings zone, oil gallery and under crown. Typical experimental values assigned to the rings zone 34.0% of the piston heat transfer, 44.9% to the oil gallery and the remaining to the under crown [1]. The piston is usually cooled by oil jets fired to the underside from the crankcase, in a heavy-duty diesel engine as shown in figure 1. The oil jets hit the hot piston at a very high relative velocity ranging from 5 m/s to 50 m/s. The oil jet breaks into mist, if the temperature at the underside of the piston is above the boiling point of the oil being used to cool the piston. This piston cooling generated mist contributes significantly towards the non-tail pipe emissions from the engine. Figure1: Oil Jet Cooled Piston Historical Perspective The problem of air pollution created by automotive engines in metropolitan cities has become very severe and requires urgent corrective action. Unburnt hydrocarbons (UBHC) are important pollutants, which are contributed by the following three sources in a petrol engine. Evaporative Losses 15-25% of HC Crankcase blow -by 20-35% of HC Tailpipe exhaust 50-60% of HC In diesel engines , evaporative losses do not exist, crankcase blow -by is present but contribution made by it is not clearly known. It should be quite significant as pressures developed during combustion and power stroke is quite high. Also, in general, diesel engines are poorly maintained as compared to petrol engines. Most of the big engines are diesel powered, therefore the contribution of hydrocarbon emission from diesel engines is quite large. The control of blow-by emission is quite simple and inexpensive and results in 15-35% reduction in total UBHC emission together with increased lubricating oil change period and decreased deterioration of lubricating oil [2]. UBHC emissions from the diesel engines are mainly contributed by blow-by and the mist generated by oil jet cooling in modern high powered internal combustion engines. The oil jet cooling is an effective way of keeping the piston under-crown surface temperatures under control. The studies of surface cooling by means of jets were originally conducted aiming the thermal protection of stator and rotor blades of gas turbines. Thus, extensive reviews presented in the literature such as by Martin (1977) refer to gas jets or air jet cooling in air surroundings [3]. Besides the problem of single jet, this study showed results with array of jets, discrete hole injection, and slot injection. Down and James (1987) presented experimental correlations for liquid jets in quiescent air. They presented results from different works for many jet and flow conditions (Reynolds and Prandtl numbers), heating or cooling, liquid or gaseous medium, plane or concave surfaces, and circular array or slot jet. Hrycak (1988) presented studies on the impingement of round jets on flat and concave surfaces with models for turbine blades [4]. Beltaos (1976) analyzed the fluid dynamic behaviour of circular turbulent jet impingement [5]. Sparrow and Lovell (1980) obtained experimental data on jet impingement on surfaces at oblique angles (90° – 30°) [6]. They have observed that the point of maximum Nusselt number (Nu) moves upwards against the flow. However the mean value of the heat transfer coefficient (h) is not affected significantly. Chang H. Oh et. al designed liquid jet array cooling modules for operation at very high load fluxes and used them to remove fluxes as high as 17 MW/m 2 [7]. The cooling was entirely convective, without boiling. Wen et. al used impingement cooling on a flat surface by using circular jet with longitudinal swirl strips for cooling [8]. Smoke-flow visualization is also used to investigate the behaviour of the complicated flow phenomenon under the swirling-flow jet for this impingement cooling. Oliphant et. al compared liquid jet array and spray impingement cooling in the non-boiling regime experimentally [9]. Cornaro et. al used jet impingement cooling for convex semi-cylindrical surface [10]. On the other hand, studies on cooling of internal combustion engine started in 1960’s. Bush has worked for Prof. London at Stanford University (USA) and introduced the term “cocktail shaker” [11]. His interest was on reciprocating pistons with partially filled cavities. After long tests, he presented heat transfer models and governing parameters. His experimental correlations were obtained for liquids with Pr > 0.5 and Pr << 1. Further results were presented by French with several different ri g and engine test configurations, and an expression for the heat transfer coefficient was presented [12]. Evans (1977) conducted a more thorough study of the “cocktail shaker” piston cooling concept [13]. Movies of a flow visuali zation apparatus were taken. This time, an open gallery was used. His main observation was the detection of different flow regimes in the off gallery. Considering the full 360 cycle of the piston (crank angle) six regimes were identified. He has modelled the six regimes using known correlations and a numerical method is presented to evaluate the average value of h for each cycle. Kajiwara et. al calculated the heat transfer coefficient in the cooling gallery of the oil jet cooled piston directly using CFD code [14]. Piston temperature distribution has also been predicted quite accurately by this approach. In order to realize the clean exhaust emission and the customer’s requirements, such as higher power and fuel economy, one of the most effective designs in combustion bowl optimization is the re-entrant shape design. The active airflow and the lower thermal capacity together increase the bowl edge temperature. Therefore, it becomes difficult to secure sufficient reliability and durability of the pistons that have the reentrant combustion bowl. Spray impingement cooling research is still being used to a great extent in achieving high heat transfer rates from heating surfaces and are not being extensively used in automobiles currently. Martins et. al (1993) analyzed the cooling conditions of articulated piston and their impact on the piston performance in an effort to optimize articulate piston cooling [12]. Pimenta et. al used numerical simulation (finite element method) temperature profiles and heat fluxes to study cooling of automotive pistons by investigating liquid cooling jets [1]. Dhariwal investigated blow-by emission and lubricating oil consumption in I. C. engine and tried to control blow -by losses using Positive Crankcase Ventilation (PCV) [2].