Ferdinand Trenc

Analysis of Energy Conversion Efficiency in Parallel and Series Hybrid Powertrains

The aim of this paper is to present a simulation and analytical analysis of the energy conversion efficiency in parallel and series hybrid powertrains. The analytical approach is based on the energy balance equations, whereas the simulation approach is based on an accurate and fast forward-facing simulation model for simulating parallel and series hybrid powertrains. A very good agreement between simulation and analytical results gives confidence pertaining to the accuracy of the performed analysis and confirms the validity of the analytical framework. Thus, combined simulation and analytical analysis enables deep insight into the energy conversion phenomena in hybrid powertrains and reveals the advantages and disadvantages of both hybrid concepts running under different operating conditions. It is obvious from the presented results that the parallel hybrid powertrain features better fuel economy than the series one for the applied test cycles, whereas both hybrid powertrain concepts feature the best fuel economy at light-duty application.

A New Criterion to Determine the Start of Combustion in Diesel Engines

A new criterion for the determination of the start of combustion (SOC) from the diesel engine in-cylinder pressure diagram was developed. It is defined as the maximum of the third-order derivative of the cylinder pressure with respect to the crank angle. This criterion declares SOC more precisely than other previously published criteria based on pressure diagnostics. This fact was proven analytically and was discernable from the analysis of the experimental data. Besides its accuracy it is also robust enough to allow automatic evaluation of the SOC during processing of the pressure data for a large number of cycles. By applying the first law of thermodynamics analysis to the engine cylinder it was discovered that the third-order derivative of the in-cylinder pressure with respect to the crank angle is the most suitable criterion for determination of the SOC from the in-cylinder pressure diagram. Subsequently, the criterion was validated through experimental data analysis of the in-cylinder pressure diagrams for various engine speeds and loads. In order to evaluate the rate of heat release (ROHR), which formed the base for the experimental validation, in-cylinder pressure diagrams were processed with a computer code based on the first law of thermodynamics. The cylinder pressure was measured with an advanced piezoelectric sensor at the resolution 0.1 deg CA. Top dead center was determined with the capacitive top dead center sensor. Due to the analytic foundation of the developed method and its validation through highly accurate experimental data it can be concluded that new criterion is credible for the determination of the SOC.

Improvement of the dynamic characteristic of an automotive engine by a turbocharger assisted by an electric motor

Turbocharging and subsequent charge cooling of the working medium usually causes increase of the mean effective pressure in an automotive diesel engine. Poor performance during the engine load increase is attributed to the nature of energy exchange between the engine and the turbocharger. Filling of the intake and exhaust manifolds, as well as consequent increase of the pressure and acceleration of the rotating components of the turbocharger require a certain period of time. Dynamic performance of the turbocharger can be substantially improved by means of an electric motor attached directly to the turbo shaft. A new concept of asynchronous electric motor with a very thin rotor was applied to support the turbocharger during the transient operation of the engine. The experimental work of matching an electrically assisted turbocharger to an engine is rather expensive; it was therefore decided to determine general characteristic of the electric motor separately through experiments, whereas transient response of the turbocharged and intercooled diesel engine was simulated by a zero-dimensional filling and emptying computer simulation method. A lot of experimentally obtained data and empirical formulae for the compressor, gas turbine, flow coefficients of the engine valves, intercooler, high-pressure fuel pump with the pneumatic control device (LDA), combustion parameters, etc., were applied to overcome deficiency introduced by the zero-dimensional simulation model. As the result a reliable and accurate program compatible with the experimental results in steady and transient engine operation was developed and is presented in the work. Faster transient response, i.e., better load acceptance of the engine was obtained by applying an adequate electric motor to assist the turbocharger; three versions of electric motors with different torque to mass moment of inertia ratios and different operating regimes were introduced in the simulation program to investigate their influence on the transient behavior of the engine.

An Analysis of Turbocharged Diesel Engine Dynamic Response Improvement by Electric Assisting Systems

It is well known that turbocharged diesel engines suffer from an inadequate response to sudden load increase, this being a consequence of the nature of the energy exchange between the engine and the turbocharger. The dynamic response of turbocharged diesel engines could be improved by electric assisting systems, either by direct energy supply with an integrated starter-generator-booster (ISG) mounted on the engine flywheel, or indirect energy supply with an electrically assisted turbocharger. A previously verified zero dimensional computer simulation method was used for the analysis of both types of electrical assistance. The credibility of the data presented is further assured by the experimentally determined characteristics of the electric motors used as input parameters of the simulation. The paper offers an analysis of the interaction between a turbocharged diesel engine operating under various load conditions and electric assisting systems, as well as the requirements for supporting electric motors suitable for the improvement of an engine’ s dynamic response. It is evident that an electrically assisted turbocharger outperforms an integrated starter-generator-booster for vehicle application, however ISG is the preferred solution when instant power increase is demanded.

Pressure drop of laminar oil-flow in curved rectangular channels

The paper presents the results of experimental work performed on pressure losses in laminar oil-flow and curved rectangular channel-coils with different geometrical aspect ratios and different curvatures that are widely used for industrial application. Dean number was applied to describe the influence of the channel curvature. The results obtained are in accordance with those of other authors. An original formula that determines pressure drop in the curved rectangular channels is proposed as the result of these experiments.

Simultaneous study of pressure pulsation and structural fluctuations of a cavitated vortex core in the draft tube of a Francis turbine

The paper represents a simultaneous structural dynamics analysis and static pressure pulsation of a cavitated vortex core in the draft tube of a Francis turbine. Comparison of the static pressure oscillation signal and the dynamics of the cavitated vortex core structure, which were quantified by the computer-aided visualisation led to the experimental modelling of the cavitated vortex core. Through the wavelet analysis significant correlation of the observed processes and their interaction was confirmed. The obtained results of the experiments enables us to create a numerical prediction of the cavitated vortex core dynamics based on the well-known Fanelli differential equation [1]. Measured pressure and structure fluctuations of the cavitated vortex core were introduced into the differential equation. Consequently, using the method of least squares, the characteristic coefficients of the model at different operating conditions in a Francis model turbine could be determined. Appropriate regression coefficient with its typically high values (r2 > 0.7) confirms the correct choice of the applied type of differential equation and gives the opportunity to set up a phenomenological model that shows the interdependence between the complex cavitated vortex core dynamics and the performance characteristics of a turbine.

Flow kinematics in a rotating axial diffuser

Abstract A study of the flow velocity field inside a rotating axial diffuser with a circular cross-section is presented in the paper. Measurements of the flow velocity components were performed by the LDA system and confirmed the existence of two types of the flow: a rotating region near the spinning wall and a non-rotating region near the longitudinal axis of the diffuser. Significant increase of all three-velocity components was observed in a thin layer at the rotating diffuser wall. Influence of the diffuser rotation and its geometry on the radial outflow was observed as well. In addition, a phenomenological model for prediction of the distribution of the velocity components in the diffuser near-wall region was developed. Exponential law was applied to the phenomenological model, which included the dimensionless velocity component as a function of two Reynolds numbers––axial and tangential. The developed model confirmed experimental results and showed substantial influence of axial and tangential Reynolds numbers on all velocity components in the vicinity of the diffuser wall.

Innovative approach to air management strategy for turbocharged diesel aircraft engines

Theoretical study on performance optimization of a turbocharged diesel aircraft engine is presented in this article. It is based on a verified engine thermodynamics and fluid dynamics simulation model. An efficient air management strategy for increasing power-to-engine displacement ratios of a turbocharged diesel aircraft engines is introduced and analysed. Air management strategies generally applied to turbocharged spark ignition aircraft engines do not enable to take full advantage of potentials given by turbocharging diesel aircraft engines. A conceptually different air management strategy, where turbochargers waste-gate is fully closed up to the critical altitude, was therefore proposed and investigated in this article. Proposed air management strategy assures increased safety of engine operation through very simple and reliable engine controlling. Guidelines for selecting adequate air management components are also given in this article.

Determination of the Realistic Turbocharger Efficiency With Pulsating Gas-Flow Compared on a 4-Cylinder Engine

Small single or twin entry radial turbines are mostly used to drive compressors of the turbocharged internal combustion engines. There are two general possibilities to feed the turbine of a four-stroke, 4-cylinder turbocharged Diesel engine: 1) by preserving most of the available exhaust kinetic energy, or 2) by mixing exhaust pulses from all cylinders in one common manifold. In the first case, better utilization of the dynamic pulse energy increases efficiency of the turbine; highly unsteady mass flow of the exhaust gasses, on the other hand, and thus periods of partial exhaust flow admission at the turbine inlet simultaneously reduces this gain in the turbine efficiency. More steady mass-flow of the exhaust gasses is created in the case of the exhaust system 2), however some kinetic energy is lost during the mixing phase in the common exhaust manifold. Calculation of the overall turbocharger and turbine efficiency is normally based on average values of the measured pressures and temperatures. As the result apparent efficiencies are obtained; the more the flow is pulsating, the bigger is the difference between the real and the apparent efficiency. The ratio between these two efficiencies is known as the energy pulsation factor β. It depends generally on the “pulse intensity”-pressure deviation from its mean value, shape of the pressure pulse, number of the individual pulses feeding separate gas turbine inlets, turbocharger, and can be successfully used to determine real efficiency of a turbocharger and to define some working parameters of the engine.A field of β factors for different engine running conditions and for the 4-cylinder engine with 2-cylinder group pulse system (rarely applied), and the commonly applied exhaust system with 4-cylinder group and moderate pressure fluctuation is presented in the paper. Influence of the dynamic exhaust temperatures on the β value is discussed as well.Copyright

Influence of the Exhaust System Design on Scavenging Characteristic and Emissions of a Four-Cylinder Supercharged Engine

A four-stroke four-cylinder turbocharged engine can be fitted with two different types exhaust system: a simple common manifold fed by all cylinders, or a twin-branch manifold, where two selected cylinders, directed by the firing order, feed two separate turbine entries. In this case good utilization of the exhaust pressure pulse energy can be achieved at higher loads and lower engine speeds, leading to good overall turbocharger efficiency and favorable pressure distribution during the gas-exchange period. Improved engine scavenging capability affects quality and quantity of the fresh charge and consequently influences the exhaust gas emissions. If, in addition, valve overlap period is increased the benefit of this system is still more evident. Common manifold exhaust system shows its advantage through lower pumping losses at higher engine speeds and lower loads. Both systems were optimized and the results of numerical and experimental work are presented in the paper.