Transient heat transfer studies on a diesel engine valve

Numerical calculations based on finite difference approximations are carried out to assess the transient thermal response of the inlet and exhaust valves of a Tata-Mercedes, six cylinder, four stroke, water cooled diesel engine with a compression ratio of 19.5 and a rated power output of 110 hp at 3000 rpm for three different engine loadings. A detailed analysis has been given for estimating the boundary conditions of the inlet and exhaust valves of an internal combustion engine. The problem has been extended by applying a thin ceramic insulation coating of 2 mm thickness at the valve plate. The isothermal distribution in the valve bodies and the heat flow rates through the various cooling media for three different engine loadings have been depicted for each of the cases with and without insulation coating. The results indicate a reduction in heat loss through valves by use of an insulation coating on the valve plate.     

Piston Assembly Design for Improved Thermal-Tribological Performance

The tribological system in the piston assembly of an internal combustion engine includes contacts at interfaces of piston/piston ring/cylinder liner, piston skirt/cylinder wall, and piston/piston pin/ connecting rod. The thermal and tribological properties of the piston, piston rings, and cylinder wall are critical to the life and quality of the engine. Severe wear and scuffing failure, especially at the ring/ring groove and ring/liner interfaces, may present a major problem if the piston temperature is too high. Temperature considerations for the piston often limit the effort to increase the engine power.

A new engine piston incorporating the heat pipe cooling technology has been developed for reducing the piston temperature, especially in the ring land and along the piston wall. The current work aims at investigating the effect of reciprocating heat pipes on heat conduction in the piston, and thus the tribological behavior of the piston assembly. Due to the high thermal conductance of the reciprocating heat pipe, a considerably large amount of combustion heat, which is conventionally conducted through the piston wall, is transferred through heat pipes. This new design will result in a lower temperature on the piston wall and a reasonably low temperature distribution in the piston.     

Performance and emission characteristics of a low heat rejection spark ignited engine fuelled with E20

In internal combustion engines, the concept of low heat rejection (LHR) using thermal barrier coating on the surface of combustion chamber is gaining attention. Thermal barrier coating reduces the heat transfer to the cooling system, protects engine components from peak heat flux and fluctuating temperature produced during combustion and improves the performance of the engine. Information in the literature is plentiful for LHR diesel engine and only few studies exist on LHR spark ignited engine. The application of thermal barrier coating in spark ignited engine is limited by pre-ignition and knocking due to elevated combustion chamber temperature. A spark ignited engine with moderate insulation on the combustion chamber and higher octane fuel can overcome this difficulty. The objective of the present experimental study is to quantify the changes in performance and emission characteristics brought by partial thermal insulation on the combustion chamber of a four stroke spark ignited engine fueled with E20 blend. Partial thermal insulation was created by coating 0.3 mm thick Alumina (Al₂O₃) on the cylinder head, inlet and exhaust valves. The changes are quantified with respect to unmodified engine fueled with gasoline. The combustion parameters such as flame development and rapid burn duration are also estimated and compared. The results indicate that partially insulated SI engine when fueled with E20 improves performance and reduces emission. A maximum of 48% reduction in THC and 50% reduction in CO emission at part load was achieved.

     

Experimental and numerical study of piston thermal management using piston cooling jet

This research investigates the effects of piston cooling jet (PCJ) on the temperature and heat transfer of a piston. A numerical model was developed by using the computational fluid dynamic approach in which the fluid and solid domains of the piston were coupled in a three-dimensional space. Two-phase flow of oil and air was also simulated. This method was used to analyze the effects of oil velocity and piston position on the heat transfer coefficient at the bottom of the piston as the new outcomes of this study. For the experiment, combustion heat flux on the piston was simulated in a test rig, and numerical results were validated. The results showed a linear relation between the oil jet velocity and the average of heat transfer coefficient at the bottom of the piston, and a periodic correlation between the piston’s vertical position and the average of heat transfer coefficient. The average of the piston crown temperature could be reduced to about 70 K by using the PCJ system, but this cooling method could create 50 K temperature gradient in the piston.     

Performance and combustion characteristics of a diesel engine with titanium oxide coated piston using Pongamia methyl ester

Owing to the increasing cost of petroleum products, fast depletion of fossil fuel, environmental consideration and stringent emission norms, it is necessary to search for alternative fuels for diesel engines. The alternative fuel can be produced from materials available within the country. Though the vegetable oils can be fuelled for diesel engines, their high viscosities and low volatilities have led to the investigation of its various derivatives such as monoesters, known as bio diesel. It is derived from triglycerides (vegetable oil and animal fates) by transesterification process. It is biodegradable and renewable in nature. Biodiesel can be used more efficiently in semi adiabatic engines (Semi LHR), in which the temperature of the combustion chamber is increased by thermal barrier coating on the piston crown. In this study, the piston crown was coated with ceramic material (TiO₂) of about 0.5 mm, by plasma spray method. In this present work, the experiments were carried out with of Pongamia oil methyl (PME) ester and diesel blends (B20 & B100) in a four stroke direct injection diesel engine with and without coated piston at different load conditions. The results revealed 100% bio diesel, an improvement in brake thermal efficiency (BTE) and the brake specific fuel consumption decreased by about 10 % at full load. The exhaust emissions like carbon monoxide (CO) and hydrocarbon (HC) were decreased and the nitrogen oxide (NO) emission increased by 15% with coated engine compared with the uncoated engine with diesel fuel. The peak pressure and heat release rate were increased for the coated engine compared with the standard engine.     

气动发动机排气冷量在内燃机冷却系统中的回收利用研究

针对气动发动机排气的低温特性,提出了利用气动发动机排气冷却内燃机散热器的技术方案。通过初步试验,获得了气动发动机在不同转速下的排气流量和温度特性。基于初步试验结果,建立了气动发动机排气与内燃机冷却水的热交换模型,并进行了仿真计算,得到了不同水泵流量下,热水流经换热器的进出口温差以及换热量。研究结果表明,换热器进出口热水温差、换热量随着气动发动机转速的升高而增大;随水泵流量的增加,热水流经换热器进出口的温差逐渐减小,换热量增大,但各个水泵流量下的换热量相差较小,当发动机转速为700r／min时,温差增加值及换热量的增加值均为最小;随着气动发动机转速增加,气动发动机排气的冷量炯变化范围很小,回收指数逐渐上升,冷量回收效果变好。     

活塞内冷油腔的振荡传热特性及位置的研究

以一款非道路用高压共轨柴油机铝合金活塞为研究对象,在试验测试的基础上建立了活塞-内冷油腔耦合传热数值仿真模型,研究了发动机循环过程中内冷油腔的振荡传热特性,分析了内冷油腔位置对活塞温度场和热应力场的影响规律。研究结果表明：在发动机循环过程中,内冷油腔壁面换热系数随油腔中机油分布规律周期性变化;内冷油腔靠近活塞顶面和环槽底面时,能显著降低活塞最高温度及一环槽温度,内冷油腔设计时应在结构强度允许范围内,尽量靠近活塞顶面和环槽底面;内冷油腔过于靠近活塞壁面时,活塞第三环槽及内腔顶面区域出现较大的局部热应力。     

防爆柴油机活塞的热载荷响应特性研究

为提升矿用防爆柴油机的换热能力,提出水冷代替油冷方案,对燃烧室内活塞的热载荷响应特性进行了研究与分析。基于CFD方法,建立活塞冷却腔内的气液两相流模型,选用SST k-ω湍流模型,结合VOF 模型和 Level Set模型得出不同曲轴转角下的液相比例和对流换热系数变化规律,分析了冷却腔结构对传热效果的影响。构建活塞热机耦合模型,计算了极限条件下活塞的热应力和热变形。研究结果表明:冷却腔内的液相比例并非决定换热效率的唯一要素,液相的振荡作用可显著增大对流换热系数;活塞头部的薄壁变形主要受热载荷影响,受腔内压力和惯性力的影响较小。     

固体火箭发动机抗烧蚀防热涂层的研究

直径及开口都较大的固体火箭发动机燃烧室大都采用橡胶基绝热层,但是,对于长细比大,或者是开口较小的客体,采用橡胶基绝热层在工艺上难以实现.为了寻找同时具备隔热效果良好,且针对小口径壳体工艺可行性高的内防热材料,我们开展了以环氧树脂、橡胶为基体,云母粉等耐高温无机填料组成的防热材料.通过试件测试及产品验证,证明该防热涂层也是固发燃烧室一种较为适宜的烧蚀防热材料,它不受被保护产品的几何形状限制,烧蚀率较小.     

Frictional evaluation of thermally sprayed coatings applied on the cylinder liner of a heavy duty diesel engine: Pilot tribometer analysis and full scale engine test

The piston system accounts for roughly half of the mechanical friction of an internal combustion engine, thus it is important to optimize. Different thermally sprayed cylinder liners were investigated in order to optimize the frictional impact of the contact between cylinder liner and piston ring/piston. A novel tribometer test setup was used to scan through different materials at different running conditions. Two cylinder liner materials showed significantly lower friction than the other tested materials, CrC–NiCr and MMC. All the thermally sprayed cylinder liners were worn significantly less than the reference material. Based on these results a full-scale single cylinder test was performed to validate the results from the rig. Comparing the thermally sprayed cylinder liner MMC with reference cylinder liner the test showed higher friction torque for the MMC cylinder liner except in one case; at low speed and high pressure. An analysis of the results between the tribometer and the engine points at the importance of the ratio between viscous and mechanical friction losses. The most probable cause of higher friction torque for the thermally sprayed coating (MMC) is that the functional surface of the cylinder liner promotes an increase in viscous friction.     

车用柴油机燃烧和热流分析的数值研究

利用商用软件STAR—CD及ES—ICE对某D6114柴油机在的缸内燃烧过程进行了数值模拟计算,分析和比较了不同喷油提前角对缸内燃烧过程和燃烧室表面热流的影响。研究结果表明：喷油提前角提前,柴油机缸内的燃烧效果优于喷油提前角推迟,燃烧过程中缸内的压力和温度比推迟喷油提前角时要大,同时缸内的最高燃烧压力和最高温度也高;喷油提前角对缸盖和活塞顶壁面平均热流的影响与其对缸内平均温度的影响相似,对缸套壁面的影响是喷油提前角提前越早,传给缸套的热流越小。数值计算结果为高功率、高强化和低热损的柴油机设计提供理论依据。．     

Effect of piston bowl shape on the in-cylinder flow characteristics of IC engines

Numerical simulation of the in-cylinder flow for internal combustion (IC) engine with different bowl shapes has been performed. The LES models are applied to a piston-cylinder assembly with a stationary valve and a harmonically moving piston. Gas motion inside the engine cylinder determines the thermal efficiency of an IC engine, and combustion chamber geometry affects the performance of the IC engine. Comparison of the flow characteristics inside the engine cylinder equipped with different piston geometries shows that the squish flow affects the turbulence generation process near the top dead center during compression stroke. The A-type combustion chamber with reentrant shape is shown to have higher radial velocity and turbulence intensity in the piston bowl compared with other types. Results of these simulations aid in the improved understanding of the effect of intake and compression process of piston geometry on the in-cylinder flow. The detailed flow characteristics inside the in-cylinder for different piston bowl shapes can offer basic guidelines to improve the combustion process.     

Piston ring friction loss behaviour for motored and fired reciprocating engines

A previously developed piston ring lubrication model has been further extended so that the piston ring frictional losses can be predicted in both hydrodynamic lubrication and metal-to-metal contact regions for various engine operating conditions. Ring friction results for two engine types are presented for both hot motoring and engine firing conditions. The hot motoring predictions were found to be in good agreement with tests. Results show that when the engine is motored, piston ring friction losses in the hydrodynamic lubrication region predominate. If the engine is fired, the losses in the metal-to-metal contact region become dominant due to high gas pressure and temperature effects. Ring friction loss can be significantly reduced by using low tension rings with a correct ring sliding face profile.     

A new approach to the prediction of heat flow and temperature in engine pistons with special reference to thermal barriers

Finite element techniques for the prediction of heat flow and temperature in pistons and other engine components are well established, although they rely invariably on initially assumed boundary conditions, particularly with respect to mean gas temperature during the cycle.Cycle analysis techniques for Diesel engines are equally well established, but these rely on much wider heat flow models for the combustion chamber walls, and generally are an assumed mean wall temperature.The paper describes a technique for one-dimensionalizing the results of finite element piston heat flow analysis and of incorporating this simplified piston model directly in the cycle analysis, using the concept of equivalent conductance.The two techniques are thus brought into much closer relationship than hitherto, allowing predictions of the effect of piston design changes on thermodynamic performance to be made much more readily.     

Thermal flow analysis of vehicle engine cooling system

This paper deals with theoretical model developed for analyzing the heat transfer of automotive cooling systems. The model has a modular structure which links various cooling system submodels. From the model, heat transfer rate of automotive cooling systems can be predicted, providing useful information at the early stages of the design and development. The aim of the study is to develop a simulation program for automotive cooling system analysis and a performance analysis program for analyzing heat exchanger. Heat release rate from combustion gas to coolant through the cylinder wall in engine cylinder was analyzed by using an engine cycle simulation program. In this paper, details of each submodel are described together with the overall structure of the vehicle model.     

基于Fluent的柴油机冷却环道流动特性分析

 为缓解柴油机活塞的过热问题,对冷却环道内的气液两相流动状态及传热特性进行了研究。根据计算流体力学方法确立柴油机冷却环道流动的湍流模型和相界面模型。采用有限元方法,建立冷却环道两相流仿真模型,针对柴油机不同的转速、油压和油温条件分别进行仿真计算,得出油液的体积率和平均换热系数随曲轴转角的变化规律。通过发动机喷流试验台的验证可知,冷却环道内两相流动状态的实验与仿真结果具有良好的匹配性。研究结果表明：喷油压力对油液体积率和平均换热系数的影响非常小;油体积率和平均换热系数的变化趋势不存在一致性;冷却环道的换热能力主要由油液黏度和振荡强度决定。     

Development of an evaporative cooling system for small four-stroke engines

A new evaporative cooling system for small four-stroke engines has been developed. Performance tests of a four-stoke single cylinder engine equipped with this evaporative cooling system have been carried out in the laboratory. Air removal rates from the closed coolant loop during the starting stage of the engine have been monitored for various engine operating conditions. In addition, data of the brake horsepower, the specific fuel consumption, heat loss to the coolant, cylinder-liner wall temperature and the wall heat flux have been obtained and were compared to those of the identical engine equipped with a conventional liquid cooling system. At a fixed air fuel ratio and under the MBT condition, the brake horsepower of the engine for the evaporative cooling system is enhanced compared to that for the liquid cooling system. The heat loss through the cylinder liner is decreased when the evaporative cooling system is adopted. The test result indicates several benefits of the evaporative cooling system such as faster warm up, better fuel economy and greater engine durability.     

液压自由活塞发动机动态特性的仿真研究

液压自由活塞发动机(hydrau lic free p iston engine,以下简称HFPE)是将内燃机和液压泵集成为一体,以液体为工作介质实现动力非刚性传输的一种特种发动机。在左、右动力腔的交替驱动下,HFPE的活塞组件在腔体中作往复直线运动,同时泵出液压油驱动负载工作。与曲轴式发动机不同,由于不受曲柄连杆机构的约束,HFPE活塞组件的运动规律完全取决于其本身的质量及所受到的作用力。本文通过建立所研制的HFPE样机各子系统的数学模型,重点研究了HFPE样机的动态特性,以期为系统的结构及控制系统提供理论依据。     

Temperature–friction characteristics of used lubricant from two-stroke cross-head marine diesel engines

It is now quite apparent that, in a two-stroke, cross-head marine diesel engine, the lubrication regime in the contact between piston ring and cylinder liner at the top dead centre (TDC) is of a boundary type. Therefore, the tribological performance of a system to simulate the real contact should be assessed under conditions closely resembling the operating engine environment. In the reality of engine operation, the lubricant is often contaminated by fuel and products of combustion, hence the need to study the temperature–friction characteristics of this actual lubricant under the conditions of boundary lubrication.In this paper, an oil taken from the drainage system of the engine was used. A five times heating and cooling test methodology was employed to assess tribological performance of a model contact lubricated with the actual oil. The model contact was formed by a pin sliding over a plate both made of materials used in two-stroke, cross-head marine diesel engines.Experiments showed that the general trend in temperature–friction characteristics of the used oil is similar to that of a new oil. However, the level of friction in the contact lubricated with an used oil is significantly higher than that for a new oil.     

Friction and wear processes in piston rings

It has been recognised that a large part of the top piston ring wear of an ic engine takes place in boundary lubrication around top dead centre (tdc) position. A quantitative assessment of the friction behaviour using actual piston ring and cylinder liner under conditions close to tdc has been made. The factors responsible for wear under these conditions have been identified as surface temperature, peak combustion pressure, total energy on the wearing surfaces and other physical properties of the material under sliding