Sangyul Lee

Improvement of the Low-Speed Friction Characteristics of a Hydraulic Piston Pump by PVD-Coating of TiN

The hydraulic pump of an Electro-hydrostatic Actuator should be able to quickly feed large ume of oil into hydraulic cylinder in order to reduce the response time. On the other hand, it should be also able to precisely dispense small amount of oil through low-speed operation so t the steady state position control error of the actuator can be accurately compensated. Within the scope of axial piston type hydraulic pumps, this paper is focused on the investigation how the surface treatment of their cylinder barrel with TiN plasma coating can contribute to the reduction of the friction and wear rate of valve plate in the low-speed range with mixed rication. The results showed that the friction torque of the valve plate mated with a TiN-coated cylinder barrel could be reduced to 22% of that with an uncoated original one when load pressure was 300 bar and rotational speed 100 rpm. It means that the torque efficiency of the test p was expected to increase more than 1.3% under the same working condition. At the same time, the wear rate of the valve plate could be reduced to 40–50%.

On the structure of hydrogen diffusion flames with reduced kinetic mechanisms

Abstract Structure of hydrogen/air diffusion flames has been analyzed by adopting a stagnant diffusion layer as a model problem. Starting from a twelve-step hydrogen oxidation mechanism, a two-step reduced mechanism has been deduced by applying appropriate steady-state and partial equilibrium assumptions. Various flame structures have been identified depending on the strain rate which affects the relative rate of the recombination and branching reactions. In the small strain rates, all the radicals can be assumed in steady-state that a one-step reduced mechanism can describe the flame structure in which the branching and recombination reactions occur simultaneously in a thin reaction zone. Radical profiles and their relative ratios have been derived and the results agree well with the numerical calculations In the moderate strain rates, the steady-state assumption for the hydrogen radical breaks down that the two-step mechanism should be used. The flame has a structure of a thin branching zone imbedded in...

Combustion Modeling of Split Injection in HSDI Diesel Engines

The second-injection ignition delay of HSDI diesel engines is generally shorter than that of the first injection. C. Hasse and N. Peters (2005) estimated the ignition delay of the second injection using 2-dimensional flamelet equations. However, simulation of 2-dimensional flamelet equations requires enormous computational time. Thus, to analyze the combustion phenomena of the split injection mode in HSDI diesel engines effectively, the 2-dimensional flamelet combustion model was modified. To reduce the calculation time, 2-dimensional flamelet equations were only applied near the stoichiometric region. If this region was ignited, the species and temperatures of other regions were changed to the steady-state solutions of 1-dimensional flamelet equations. Using this method, the calculation time for solving flamelet equations was reduced by 80% without any loss of calculation accuracy. The simulation results of the second-injection ignition delay were well matched with the experimental results in a precombusted vessel.

A characteristic study and optimization of in-cylinder configuration for particulate matters reduction in an off-highway diesel engine with mechanical fuel injection system

This paper shows development challenges for 11-liter heavy-duty off-highway diesel engines to meet Tier 3 emission regulations with a base diesel engine compliant with Tier 2 emission regulations. In the case of the installation of an exhaust gas recirculation (EGR) system for reduction of NOx emissions, there exists a risk of increased particulate matters (PM) emissions. An in-cylinder PM reduction is still necessary since a diesel particulate filter (DPF) after-treatment system is not under the consideration. The objective of this research is to see whether the base engine has a potential to meet Tier 3 emission regulations by changing in-cylinder configuration parameters including the bowl shape, injector position, the number of intake and exhaust valves, injector tip protrusion, and injector tip specifications such as nozzle spray angle and nozzle flow rate. These parameters are very important parts which enhance the air and fuel mixing process that helps the combustion process. Thus, the optimization of these design variables is essential to improve combustion efficiency and emissions reduction. In this study, the multi-dimensional computational fluid dynamics (CFD) code KIVA-3V is used to perform combustion simulations. The Kelvin–Helmholtz/Rayleigh–Taylor (KH-RT) model is employed for spray breakup and a reduced chemical mechanism for n-heptane is employed to simulate ignition delay and combustion of diesel fuel. To verify the simulation results, engine bench tests were performed with installations of the final version of in-cylinder geometry in C1-8 mode which is one of main test cycles to meet Tier 3 emission regulations. Finally, Tier 3 emission regulations have been met with the currently optimized in-cylinder configuration parameters.

Development of Reduced Normal Dodecane Chemical Kinetics

Generally, a reduced chemical mechanism of n-heptane is used as chemical fuel of a 3-D diesel engine simulation because diesel fuel consists of hundreds of chemical components and various chemical classes so that it is very complex and large to use for the calculation. However, the importance of fuel in a 3-D simulation increases because detailed fuel characteristics are the key factor in the recent engine research such as homogeneous charged compression ignition engine. In this study, normal paraffin, iso paraffin and aromatics were selected to represent diesel characteristics and n-dodecane was used as a representative normal paraffin to describe the heavy molecular weight of diesel oil (C10~C20). Reduced kinetics of iso-octane and toluene which are representative species of iso paraffin and aromatics respectively were developed in the previous study. Some species were selected based on the sensitivity analysis and a mechanism was developed based on the general oxidation scheme. The ignition delay times, maximum pressure and temperature of the new reduced n-dodecane chemical mechanisms were well matched to the detailed mechanism data.

Fundamental Study on the Chemical Ignition Delay Time of Diesel Surrogate Components

Due to its accuracy and efficiency, reduced kinetic mechanism of diesel surrogate is widely used as fuel model when applying 3-D diesel engine simulation. But for the well-developed prediction of diesel surrogate reduced kinetic mechanism, it is important to know some meaningful factors which affect to ignition delay time. Meanwhile, ignition delay time consists of two parts. One is the chemical ignition delay time related with the chemical reaction, and the other is the physical ignition delay time which is affected by physical behavior of the fuel droplet. Especially for chemical ignition delay time, chemical properties of each fuel were studied for a long time, but researches on their mixtures have not been done widely. So it is necessary to understand the chemical characteristics of their mixtures for more precise and detailed modeling of surrogate diesel oil. And it shows same ignition trend of paraffin mixture with those of single component, and shorter ignition delay at low/high initial temperature when mixing paraffin and toluene.