Georges Descombes

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.

Association of Finite-Time Thermodynamics and a Bond-Graph Approach for Modeling an Endoreversible Heat Engine

In recent decades, the approach known as Finite-Time Thermodynamics has provided a fruitful theoretical framework for the optimization of heat engines operating between a heat source (at temperature ) and a heat sink (at temperature ). The aim of this paper is to propose a more complete approach based on the association of Finite-Time Thermodynamics and the Bond-Graph approach for modeling endoreversible heat engines. This approach makes it possible for example to find in a simple way the characteristics of the optimal operating point at which the maximum mechanical power of the endoreversible heat engine is obtained with entropy flow rate as control variable. Furthermore it provides the analytical expressions of the optimal operating point of an irreversible heat engine where the energy conversion is accompanied by irreversibilities related to internal heat transfer and heat dissipation phenomena. This original approach, applied to an analysis of the performance of a thermoelectric generator, will be the object of a future publication.

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

Frost Growth Investigation and Temperature Glide Refrigerants in a Fin-and-Tube Heat Exchanger

Biomethane is produced by removing undesirable components such as water vapor, carbon dioxide and other pollutants in a biogas upgrading process. Frosting the water vapor contained in the biogas is one of the dehydration processes used in a biogas upgrading process. In order to simulate a frost layer on a cold plate, many models have been developed. These models are valid for a limited temperature range. In this study, heat and mass transfer equations were used in a numerical approach to model the frost growth and its densification on the external side of a fin-and-tube heat exchanger. The model used in this study is valid for low temperatures from 0∘C to −40∘C and lower. The evaporation process of temperature glide refrigerants is also modeled from −50∘C to −20∘C. The results show a decreased heat transfer rate during frost mass growth on fins and rows. During its growth, frost layer thermal conductivity is relatively low leading to decrease the heat exchanger performance. On the other hand, frost layer thickness increases the external surface blockage, leading to higher pressure drop on the external side. This model has been validated by comparing numerical and experimental results for the biogas outlet temperature.

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.

Thermodynamic Modelling of an Ejector with Compressible Flow by a One-Dimensional Approach

The purpose of this study is the dimensioning of the cylindrical mixing chamber of a compressible fluid ejector used in particular in sugar refineries for degraded vapor re‑compression at the calandria exit, during the evaporation phase. The method used, known as the “integral” or “thermodynamic model”, is based on the model of the one‑dimensional isentropic flow of perfect gases with the addition of a model of losses. Characteristic curves and envelope curves are plotted. The latter are an interesting tool from which the characteristic dimensions of the ejector can be rapidly obtained for preliminary dimensioning (for an initial contact with a customer for example). These ejectors, which were specifically designed for the process rather than selected from a catalog of standard devices, will promote energy saving.

Progress in high performance, low emissions, and exergy recovery in internal combustion engines

Summary This article first gives a brief review of thermal engines designed for terrestrial transportation since the 1900s. We then outline the main developments in the state of the art and knowledge about internal combustion engines, focusing on the increasingly stringent pollution constraints imposed since the 1990s. The general concept of high-energy performance machines is analyzed from the energy, exergy, and public health point of view and illustrated with typical examples of clean energy production and zero emissions. Whereas the energy analysis revealed high potential of waste heat recovery from both exhaust and cooling system, the exergetic analysis revealed much higher recovery potential from exhaust gases. The exergy content of exhaust gases was observed to be within the range from 10.4% to 20.2% of the fuel energy. The cooling exergy is within the range from 1.2% to 3.4% of the fuel energy. The article concludes with some perspectives for the emergence of an economic model that could be applied to land-based transport systems in the framework of energy transition by 2030. Copyright

Influence of Heat Transfer on Gas Turbine Performance

In the current economic and environmental context dominated by the energy crisis and global warming due to the CO2 emissions produced by industry and road transportation, there is an urgent need to optimize the operation of thermal turbomachinery in general and of gas turbines in particular. This requires exact knowledge of their typical performance. The performance of gas turbines is usually calculated by assuming an adiabatic flow, and hence neglecting heat transfer. While this assumption is not accurate for high turbine inlet temperatures (above 800 K), it provides satisfactory results at the operating point of conventional machines because the amount of heat transferred is generally low (less than 0.5% of thermal energy available at the turbine inlet). Internal and external heat transfer are therefore neglected and their influence is not taken into account. However, current heating needs and the decentralized production of electrical energy involve micro Combined Heat and Power (CHP) using micro-gas turbines (20-250 kW). In aeronautics, the need for a power source with a high energy density also contributes to interest in the design of ultra-micro gas turbines. These ultra and micro machines, which operate on the same thermodynamic principles as large gas turbines, cannot be studied with the traditional adiabatic assumption, as has been underlined by many authors such as Ribaud (2004), Moreno (2006) and Verstraete et al. (2007). During operation, heat is transferred from the turbine to the outside, bearing oil, casing and compressor, thus heating the compressor and leading to a drop in turbine performance. Consequently, the performances reported on the maps developed under the adiabatic assumption are no longer accurate.

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.