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In the world of heavy-duty vehicles, the retarding capability is of high interest, as the natural retarding power and braking force per vehicle weight is only about one-tenth and a half of the passenger vehicles respectively. This is why such vehicles use an auxiliary braking device, either a drivetrain retarder or an engine brake. It brings not only the safety benefit, but also the economic one, as it prolongs service intervals of friction brakes and the vehicle can go downhill at higher speed with sufficient safety reserve. As the operational vehicle fuel economy improves, the natural vehicle retarding power decreases. For this reason, the performance and torque characteristic of the auxiliary braking systems need to be improved as well. The work focuses on the development of an advanced engine brake system, which works in two-stroke cycle. The development process utilized both the simulation and experimental techniques. A project partner simultaneously proposed a new valve train design, but this development is beyond the scope of this paper. The simulation phase started with a 1-D model of a serial production heavy-duty Diesel engine, at which both the experiment and new valve train design proposal aimed. The model underwent partial calibration, mainly in terms of discharge coefficient and engine and manifold geometry, as combustion is not of interest in the engine brake mode. Parameterized advanced valve actuation in the model enabled the optimizing tool to arbitrarily modify the valve lift profiles without colliding issues. The optimizer used several constraining parameters that are given either by requirements of the valve train mechanism or have an impact on stress and durability of engine components. Resulting valve lift profiles fulfilling the constraints showed braking performance which exceeds the nominal positive power output of the engine and outdoes the contemporary 4-stroke engine brake, particularly at low engine speed. The final valve lift profiles were then inputted into the mechanical model of the proposed valve train mechanism and checked for dynamics behavior. After that, the model kinematics produced final cam profiles and technical documentation was prepared for camshafts manufacturing. The camshafts were then mounted in the engine together with the newly designed valve train mechanism designed by a project partner. Due to cost and dynamometer possibilities, the experiment had to be performed only on selected cylinders. For this reason, a numerical torsional model of the engine crank-train coupled to the dynamometer helped determine proper engine layout and safety operating points of the engine. The engine was then equipped with fast and slow sensors and data acquisition system. The subsequent experiment yielded data for further re-calibration and validation of the 1-D model. The experiment confirmed correctly proposed valve lift profiles and performance prediction and also revealed unexpected phenomena during the gas exchange process.
Ing. Ondrej Bolehovsky, Faculty of Mechanical Engineering, Czech Technical University in Prague, CZECH REPUBLIC Dr. Radek Tichanek, Faculty of Mechanical Engineering, Czech Technical University in Prague, CZECH REPUBLIC Prof. Michal Takats, Faculty of Mechanical Engineering, Czech Technical University in Prague, CZECH REPUBLIC Ing. Zbynek Syrovatka, Faculty of Mechanical Engineering, Czech Technical University in Prague, CZECH REPUBLIC