Steam engine | EPFL Graph Search

A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be transformed, by a connecting rod and crank, into rotational force for work. The term "steam engine" is generally applied only to reciprocating engines as just described, not to the steam turbine. Steam engines are external combustion engines, where the working fluid is separated from the combustion products. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In general usage, the term steam engine can refer to either complete steam plants (including boilers etc.), such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine. Although steam-driven devices were known as early as the aeolipile in the first century AD, wi

New slip synthesis and theoretical approach of CVT slip control

Yves Rothenbühler

Today's vehicle must be efficient in terms of gas (CO2, NOx) emissions and fuel consumption. Due to improvements in material and oil, the continuous variable transmission (CVT) is now making a breakthrough in the automotive market. The CVT decouples the engine from the wheel speed. CVT enables significant fuel gains by shifting the engine operating point for specific power demands. This optimization of the operating point enables a reduction of this fuel consumption. A CVT is constituted of two pulley sheaves, one fixed and the other one movable in its axial direction when subjected to an external axial force, in general hydraulic. The transition from the minimum to the maximum speed ratio is continuous and an infinite numbers of ratio is available between these two limits. An intermediate element (a metallic belt or chain) transmits the power from the input (the primary) to the exit (the secondary) of the CVT or variator. Further improvements of the fuel consumption and gas emission are still required for example by improving the variator efficiency. Increasing hydraulic performance or decreasing mechanical losses by reducing the axial forces are some solutions. The latter method is not without risks. The diminution of the clamping forces increases the slip between pulley sheaves and the intermediate element. If the axial forces decrease too much, high slip values can be reached and cause damage to the pulleys and the intermediate element. Control of the slip is an attractive solution to decrease the clamping forces in order to safely improve the variator efficiency. The objective of this thesis is to understand and model the slip of each pulley and establish analytic tools dedicate to the variator control. The slip study and the theoretical approach of the CVT variator is applied to the slip control of the variator with a chain. The contribution of this work is threefold. Firstly, the slip and the traction coefficient are analyzed for each pulley. The slip analysis of each pulley is then used to define a new slip synthesis as the summation of the slip of each pulley. It is demonstrated that the slip and the traction coefficient are different for each pulley and depend on the speed ratio of the variator. In low ratios, both the secondary pulley and the primary pulley slip, but only the primary reaches macro slip. For middle or higher ratios, only the secondary pulley slips and reaches high values of slip. Experiments show that the pulley with the smallest clamping force limits the system. Secondly, based on kinematics, force equilibrium, elastic deformations of the pulleys and the intermediate element, a detail model of the variator is proposed. The principal results are the estimation of the clamping forces, of the traction curve for each pulley and of the chain efficiency. These results are implemented in a simpler model that describes the variator dynamics. This last model considers the two pulleys and the intermediate element as free bodies. The hydraulic circuit and the actuators, which are important to take into account for control, are also modeled. Thirdly, the new slip synthesis and the results of the dynamic models are applied to the slip control of the variator in order to improve the efficiency. A pole placement law is applied to the actuators to control the flow that enters or exits the pulleys. With this law, the actuators are decoupled and the bandwidth is increased sufficiently for actuators dynamics to be neglected. The primary and the secondary pressures are decoupled and linearized by an input-output feedback linearization. The resulting system is linear and linear control theory can be applied to control the two pressures. The speed ratio is controlled by the primary clamping force. The secondary pressure is chosen as a function of the control mode of the variator: standard mode or slip mode. In standard mode, the intermediate element is overclamped by 30%, whereas in slip mode, the secondary clamping force is set as a function of the desired slip. By controlling the slip at 2%, the mechanical efficiency was increased by more than 2% and the clamping forces reduced by more than 30%. For the slip control, a proportional-integrator law and a model reference adaptive control (MRAC) are presented and the performances compared. The MRAC gives slightly better results.

EPFL2009

Modelling vapour expansion of extruded cereals

Laurent Lach

The vapour expansion of extruded cereals is a versatile technique used in the food processing industry to produce a wide variety of light, crisp & crunchy products such as snacks, breakfast cereals and pet foods. The range of textures that can be produced depends on a complex interaction of a many parameters controlling the expansion phase making the development and optimisation of the process a difficult task involving many trials. This thesis is aimed at developing a numerical model of vapour expansion of extruded cereal to improve our understanding of the physical process and to help speed up the development of new products. Rather than try to model the growth of each bubble explicitly a more economical "micro-macro" approach was developed involving the coupling of a 1D model for single bubble growth with a CFD code for modeling the bulk fluid flow. Several bubble growth models have previously been developed and coupled with simplified models for the flow inside the die, but none of them have succeeded in predicting even the right trend in the observed dependence of the expansion with operating conditions. A microscopic model was first developed along similar lines to those described in the literature with some improvements and then coupled to different macroscopic flow models. Although the process requires at least a 2D coupling to properly capture the full behaviour of vapour expansion, considerable insight was gained by coupling with a 1D compressible macroscopic flow model assuming isotropic expansion in order to predict the evolution of the extrudate outside the die. In particular the 1D model predicted the growth and maximum extrudate diameter in qualitative agreement with experimental measurements and showed for the first time why the expansion is observed to be stronger with lower water content or lower temperature. It showed for example that increasing the water content influences the expansion more by changing the partition between lateral and longitudinal expansion than by changing the degree of volume expansion itself. In other words, the increased water content decreases the viscosity and increases the nucleation pressure so the expansion occurs sooner and more rapidly and hence spends more time expanding inside the die where it can only accelerate. There is thus less bubble growth remaining for lateral expansion by the time it reaches the die outlet. For very low moisture content the model predicts a weak reduction in lateral expansion due to the reduced availability of water for vaporization. The resulting peak was also observed experimentally. Two types of 2&3D coupling with the Fluent CFD code were developed and tested: 1) where the equations for the microscopic model are solved in Eulerian form as User Defined Scalar (UDS) equations and 2) where the microscopic model equations are solved in Lagrangian form along stream lines computed using the Discrete Phase Model (DPM). In both cases the free-surface expansion of the extrudate from the die is simulated with the Volume of Fluid (VOF) capability in Fluent. Unfortunately neither approach could be made to converge stably due to numerical problems that could not be rectified due to the closed source nature of Fluent. A similar problem with the 1D model was solved by deriving a more intimate coupling between the micro and macro equations to reduce their order.

EPFL2006

NUMERICAL FLOW SIMULATION OF A NATURAL GAS ENGINE EQUIPPED WITH AN UNSCAVANGED AUTO-IGNITION PRECHAMBER

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos, Dirk Wunsch

This work is part of ongoing research on stationary natural gas engines equipped with unscavanged prechambers. Spark ignition inside the prechamber has been successfully proven with natural gas as well as biogas, reducing emissions while keeping good efficiencies. The conversion of the prechamber system from spark ignition to auto-ignition will lead to longer maintenance intervals in the case of stationary natural gas engines and to a probable more efficient and cleaner combustion. The latter expectation is based on the combustion mode: HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front, compared to prechamber spark ignition. To the authors knowledge no work following this approach has been conducted. From this new approach one of the major difficulties arises. While for standard prechamber systems either the spark plugs or injecting gas directly into the prechamber creating rich conditions guarantee to well know the point of ignition, auto-ignition of a premixed gas does not do so. The basic idea of this prechamber is to create as homogeneous conditions as possible. Then, the reacting mixture streams through the orifices into the main chamber. The difficulty here is to create a state that triggers auto-ignition only inside the prechamber. As temperature is the main influencing factor on the reaction chemistry the prechamber walls are heated, aiming at higher gas temperatures inside the prechamber. However, auto-ignition will take place wherever the energy level is sufficiently elevated. In particular, this causes difficulties in the region of the exhaust valve. In this work the three- dimensional flow conditions in a mono-cylinder test engine were numerically simulated in order to better understand the conditions inside the engine, with the new prechamber configuration. Therefore, a geometric model was created, disregarding intake and exhaust valves to simplify matters. A suitable multi-block mesh for parallel computing was generated and models for the initial and boundary conditions derived. The chemical reactions were incorporated to the flow field, using a 54 species mechanism developed for natural gas combustion by two approaches. In a first step the flow field was superposed with zero-dimensional Chemkin calculations whereas in a second step the species conservation equations were solved together with the Navier Stokes equations, which is much more expensive in calculation time. Finally, the numerical results were compared to experimentally gained data. Results show the sensitivity of the computations to varied initial and boundary conditions and provide a guideline for modifications in order to enhance the prechamber configuration. Numerical results show the risk of auto-ignition in the main chamber. Experimental results could be reproduced numerically in regard to the time dependency of first ignition.

2007