Pierre Podevin

Some aspects concerning the combination of downsizing with turbocharging, variable compression ratio, and variable intake valve lift

Abstract The inefficient running of the spark ignition engine at part loads due to the load control method but, mostly, their major weighting in the vehicles operation time justifies the interest in the technical solutions, which act in this particular operating range. These drawbacks encountered at low part loads are even more amplified when considering larger engines. For instance, it is well known that, at the same engine load, a larger engine is more throttled than a smaller engine; therefore the concerns are the higher pumping work, the lower real compression ratio, and the overall mechanical efficiency, which is also lower. One solution is a reduction in the displacement without affecting the power output. This is what is now commonly known as the downsizing technique. The combination of downsizing and uploading an engine has been known for a long time. However, the conversion, in an acceptable way, of this potential to actual practice is very challenging. On the one hand, the degree of the downsizing is related to the boost pressure. In order to cope with the knocking phenomenon, the downsized high-pressure turbocharged gasoline engine requires a lower volumetric compression ratio that limits the efficiency on part loads. Therefore, the degree of the downsizing has been limited and, thus, the possible fuel consumption reduction has not yet been fully achieved. On the other hand, other problems are encountered when considering a downsized turbocharged gasoline engine: insufficient low-end torque, poor starting performance, and turbo lag. In order to solve these problems an effective combination of the downsized turbocharged gasoline engine with additional technologies is needed. Thus, the paper will present a so-called adaptive thermal engine, which has at the same time a variable compression ratio and a variable intake valve lift. It will then be demonstrated that it is highly suitable for turbocharging, thus resulting in a high downsizing factor.

Experimental Study of Turbocharger’s Performances at Low Speeds

One of the most efficient ways to reduce the pollution and fuel consumption of an automotive engine is to downsize the engine, whilst maintaining a high level of power and torque. This is achieved by using turbochargers. In urban, and often in suburban, traffic conditions the engine power demand is weak in relation to the maximum power available, so the turbocharger runs at low speed. To appreciate and improve engine performance, it is necessary to know the characteristics of the turbomachinery in this functioning area, characteristics which are not given by turbocharger manufacturer. The reason for this lack of information will be explained and the experiments we are currently conducting at low turbocharger speed are presented. Experimentally, it has been demonstrated that the measured performances of the compressor are dependent on heat exchange (convection and conduction) and are also linked to the pressure and temperature of the lubricating oil. At the CNAM laboratory, the turbocharger test rig has been equipped with a special torquemeter, allowing rotation speeds of up to 120000 rpm, set up between the turbine and the compressor. The turbine is thus separated from the compressor and could be considered as a drive which provides mechanical power to the turbocharger (torquemeter + compressor + bearing unit). Temperature and pressure of the lubricating oil can be adjusted to an experiment’s requirements. The test bench lay out is described. To achieve accurate measurements and evaluate the influence of heat exchanges, tests have been carried out with the whole compressor thermally isolated and with preheated inlet air. The compressor can be assumed to be adiabatic, and the power given to the air flow can be calculated using the first law of thermodynamics. Mechanical bearing losses can be deduced from this calculation and torquemeter power, but also from measurements of lubricating oil flow, and oil temperature at inlet and outlet. The results of experiments for different lubricating oil temperatures and pressures and turbocharger speeds are presented. Real compressor characteristics curves are set up and a comparison of experimental mechanical power losses with a journal bearing CFD model is presented.Copyright

Numerical study of the waste heat recovery potential of the exhaust gases from a tractor engine

The paper presents an analysis of the possibilities of exhaust gas heat recovery for a tractor engine with an output power of 110 kW. On the basis of a literature review, the Rankine cycle seems to be the most effective way to recover the exhaust gas energy. This approach reduces the fuel consumption and allows engines to meet future restrictions on carbon dioxide emissions. A simulation model of the engine by means of a one-dimensional approach and a zero-dimensional approach was built into the simulation code AVL BOOST, and a model of the Rankine cycle was implemented. The experimental values of the effective power of the engine, the mass flow and the exhaust gas temperature were used to validate the engine model. The energy balance of the engine shows that more than 28.9% of the fuel energy is rejected by exhaust gases. Using the engine model, the energy and the exergy of the exhaust gases were studied. An experimental study of the real working cycle of a tractor engine revealed that the engine operates most of the time at a constant speed (n = 1650 r/min) and a constant load (brake mean effective pressure, 10 bar). Finally, Rankine cycle simulations with four working fluids were carried out at the most typical operating point of the engine. The simulation results reveal that the output power of the engine and the efficiency of the engine increase within the range 3.9–7.5%. The highest value was achieved with water as the working fluid while the lowest value was obtained with the organic fluid R134a. The power obtained with water as the working fluid was 6.69 kW, which corresponds to a Rankine cycle efficiency of 15.8%. The results show good prospects for further development of the Rankine cycle.

Optimization of automotive Rankine cycle waste heat recovery under various engine operating condition

An optimization study of the Rankine cycle as a function of diesel engine operating mode is presented. The Rankine cycle here, is studied as a waste heat recovery system which uses the engine exhaust gases as heat source. The engine exhaust gases parameters (temperature, mass flow and composition) were defined by means of numerical simulation in advanced simulation software AVL Boost. Previously, the engine simulation model was validated and the Vibe function parameters were defined as a function of engine load. The Rankine cycle output power and efficiency was numerically estimated by means of a simulation code in Python(x,y). This code includes discretized heat exchanger model and simplified model of the pump and the expander based on their isentropic efficiency. The Rankine cycle simulation revealed the optimum value of working fluid mass flow and evaporation pressure according to the heat source. Thus, the optimal Rankine cycle performance was obtained over the engine operating map.

Computational Fluid Dynamics Calculations of Turbocharger's Bearing Losses

Fuel consumption in internal combustion engines and their associated CO2 emissions have become one of the major issues facing car manufacturers ev eryday for various reasons: the Kyoto protocol, the upcoming European regulation concerning CO2 emi ssions requiring emissions of less than 130g CO2/km before 2012, and customer demand. One of the most efficient solutions to reduce fuel consumption is to downsize the engine and increase its specific power and torque by using turbochargers. The engine and the turbocharger have to b chosen carefully and be finely tuned. It is essential to understand and characterise the turboc harger’s behaviour precisely and on its whole operating range, especially at low engine speeds. T he characteristics at low speed are not provided by manufacturers of turbochargers because compresso r maps cannot be achieve on usual test bench. Experiments conducted in our laboratory on a specia l test rig equipped with a high-precision torquemeter, demonstrate that compressor performanc es in this area cannot be deduced from adiabatic assumption. Nevertheless, our study sugge sts that as long as torque at the shaft end is measured and mechanical power losses are known, the effective power provided to the air flow can be calculated. Tests and calculations reveal that t hese mechanical power losses cannot be evaluated by general physical laws. A better knowledge of the se losses is required. In this paper, a CFD model of a turbocharger journal bearing system is propose d. The real behaviour of what occurs in the bearing system (such as leakage flow, heat transfer from the inner film to the outer through the brass bearing material) has been computed with the en rgy equation. The bearing system performance is presented against the rotational spe ed at various oil inlet temperatures and pressures. The impact of those parameters has been studied in detail and presented in this paper. It is demonstrated that the oil temperature rise decrease s the friction torque along the rotational speed by making the viscosity drop. Moreover, an increase of the oil inlet pressure results into a higher friction torque. This paper provides an analysis of this trend showing the link between oil inlet pressure, oil mass flow and thermal exchange inside the bearing. Results also present the variation of oil mass flow along the entire speed range and i ts distribution between the inner and outer clearances.

Some aspects concerning the geometry of a hinged engine with a variable compression ratio

Abstract An improvement in automotive fuel consumption has been for many years the most important challenge in engine development. In a century of automotive spark ignition engine development, only two parameters were subject to automatic control: the air-fuel ratio and spark advance. Ever since, a wide range of technologies has been developed in order to improve fuel economy further. The following strategy was used: the identification of the sources of losses in the conventional spark ignition engine and then the attempt to reduce them one by one. The potential of these technologies needs to be evaluated by a cost and consumption benefit trade-off and all these, of course, without affecting the power level. In this context, the authors are about to develop a variable geometrical compression ratio engine, in order to overcome the main drawback of the spark ignition engine: the continuous variation of the real rate of compression, caused by the load control. The concept of the variable geometrical compression ratio (VCR) engine is the subject of a patent and consists of a hinged mechanism. It is an intrinsic, automatic self-regulation system with a fast response time and at the same time is a natural development of the classic engine. So far, two prototypes have been designed and tested. The paper presents some specific features regarding hinge position choice. The following are taken into consideration: the upper block rotational angle, added angle of the piston, tipping torque, and relative motion of the camshaft with respect to the crankshaft.

Surge detection on an automotive turbocharger during transient phases

The surge limit on automotive turbocharger needs to be avoided to prevent operations with pressure and mass flow oscillations. Mild surge is accompanied by noise which is disturbing. Deep surge can cause significant loss of engine power and severe drivability issues. It is necessary to know the stationary limit in order to match a turbocharger with an engine, ensuring enough surge margin. However, this choice does not guarantee surge free operation during transient functioning. In this paper, the surge onset of a compressor while closing a downstream valve is studied. Various tests have been carried out varying the closing time, the position of the initial operating point and the volume of the circuit. The inlet and outlet signals of physical parameters are analyzed with spectral and temporal methods in order to define the instant of the surge occurrence.

Study on the combustion process in a modern diesel engine controlled by pre-injection strategy

The paper aims to study the combustion process in a modern diesel engine over the engine operating map. In order to study the rate of heat release (ROHR), an automotive diesel engine was experimentally tested using the injection parameters factory defined. The experimental test was conducted over the engine operating map as the engine speed was limited to 2400 rpm. Then, an engine simulation model was developed in AVL Boost. By means of that model the ROHR was estimated and approximated by means of double Vibe function. In all engine operating points we found two peaks at the ROHR. The first is a result of the pilot injection as the second corresponds to the main injection. There was not found an overlap between both peaks. It was found that the first peak of ROHR occurs closely before top dead center (BTDC) at partial load than full load. The ROHR peak as a result of main injection begins from 4°BTDC to 18°ATDC. It starts earlier with increasing engine speed and load. The combustion duration varies from 30 oCA to 70 °CA. In order to verify the results pressure curve was estimated by means of defined Vibe function parameters and combustion duration. As a result, we observed small deviation between measured and simulated pressure curves.

Surge Limit Prediction of Centrifugal Compressor Using Semi Classical Signal Analysis

This paper presents the use of the recently developed semi classical signal analysis (SCSA) method for surge limit detection of a centrifugal compressor. The SCSA is based on the fact that the studied temporal signal is considered as a trap-potential for a semi-classical particle and is represented by discrete energy levels given by the discrete spectrum of a Schrodinger operator. This approach provides new spectral parameters whom variability might be interesting for signals analysis. Inlet and outlet pressure signals recorded on a turbocharger test bench while reducing the compressor mass flow rate are analysed. The spectral parameters provided by the SCSA show interesting behaviour especially around the transition zone between stable operation and surge. Following these observations, a new criterion for surge limit detection is proposed. The sensitivity of the detection may be adjusted by the threshold value. The implementation of this new method combined with active regulation may extend the usable zone of the compressor map.Copyright