Simulation of turbocharged SI-engines – with focus on the turbine Simulation of turbocharged SI-engines - with focus on the turbine Fredrik Westin Doctoral thesis KTH School of Industrial Engineering and Management TRITA – MMK 2005:05 Royal Institute of Technology ISSN 1400-1179 SE-100 44 Stockholm ISRN/KTH/MMK/R-05/05-SE3 Fredrik Westin TRITA – MMK 2005:05 ISSN 1400-1179 ISRN/KTH/MMK/R-05/05-SE Simulation of turbocharged SI-engines – with focus on the turbine Fredrik Westin Doctoral thesis Academic thesis, which with the approval of Kungliga Tekniska Högskolan, will be presented for public review in fulfilment of the requirements for a Doctorate of Engineering in Machine Design. The public review is held at KTH - The Royal Institute of Technology, room M3, Brinellvägen 64 Stockholm, on 31 May 2005, 14.00. 4 Simulation of turbocharged SI-engines – with focus on the turbine ”On and on we're charging to the place so many seek In perfect synchronicity of which so many speak We feel so close to heaven in this roaring heavy load And then in sheer abandonment, we shatter and explode. I'm your turbo lover Tell me there's no other” Judas Priest 5 Fredrik Westin Introduction The research objective This PhD-thesis is the outcome of a project initiated by Saab Powertrain AB and started by Prof. Hans-Erik Ångström at KTH in 2000. The initial objective was to investigate a wide range of boosting systems in order to sort out the system most suitable for boosting downsized SI-engines. Since downsizing is a well-known method for reduction of the fuel consumption of SI-engines it is of all engine manufacturer’s interest to investigate the technology. Saab had at the start of the project developed the third development step of its heavily downsized engine concept SVC, Saab Variable Compression. The engine had half the displacement of its competitors and subsequently it needed approximately twice the normal boost pressure. These high levels of boosting, together with the small displacement, set very high demands on the boosting system in terms of transient response and efficiency. One single turbocharger was not enough for the task and the question was with what to replace it. The investigation should be done with 1D engine simulations. Unfortunately it was very soon detected that the current state of the simulation software was not good enough to fulfil the task with sufficient precision. The decision was made by both Saab Powertrain and KTH to change the objective to an investigation of the accuracy of the simulations instead, and that is what the main part of this thesis deals with. Turbocharging can both increase the power density of an engine as well as decrease the fuel consumption of a vehicle. However, to fully explore the potential, some problems must be overcome during the development of the engines. For low engine speeds, turbocharged engines often have less torque than N/A-engines with comparable peak torque and speed. For the highest speeds, the turbine inlet temperature limits the maximum power of the engine. In addition, especially for low engine speeds, the turbocharged engine has slower transient response than a N/A-engine. All of these issues are related to the gas exchange process of the engine and selection of turbocharger components. To optimise this process, virtually all engine manufacturers use 1D engine simulation codes. Unfortunately these simulation codes, for certain engine modes, have problems in accurately predicting the performance of the engine. For the mid-speed region, where the wastegate is open, the codes have reasonable prediction accuracy. The simulation controls the wastegate to the set boost-pressure, just as the real controller on the engine. Very seldom do measurement data of the wastegate massflow fraction, to correlate with the 6 Simulation of turbocharged SI-engines – with focus on the turbine simulation results, exist. However, for the low speed region, where the wastegate is closed, the simulation is much more sensitive to an error in predicted turbine power. The reason is that there is no control mechanism to adjust the turbine power. Therefore this region, up to where the wastegate is opened on the engine, normally cannot be simulated accurately without calibrating the simulation model against measurement data for the particular engine in question. The same also holds for transients, since the wastegate is closed up to when the desired boost pressure is reached. Thus, the most useful tool for removing two of the turbocharged engine’s largest drawbacks has drawbacks in itself exactly where it can be of largest use. The process For this project there existed funding for one full-time employed PhD-student to work both in a test-cell and with computer simulations in a commercial simulation code. At the start of the project there was little activity around 1D simulations both at KTH and at Saab. In addition a brand new test cell needed instrumentation and installation of the engine. To fulfil the task both the measurement technique needed further investigation and proper use of the simulation tool had to be learned. The thesis The thesis text describes the work done within the project. The aim is to share experience gained when simulating (and doing measurements on) the turbocharged SI-engine as well as describing the limits of current state of the technology. In addition an overview of current boosting systems is provided. The target readers of this text are engineers employed in the engine industry as well as academia who will get in contact, or is experienced, with 1D engine performance simulation and/or boosting systems. Therefore the text requires general knowledge about engines. The papers included in the thesis are, in reverse chronological order: [8] SAE 2005-XX-XXX Calculation accuracy of pulsating flow through the turbine of SI-engine turbochargers - Part 2 Measurements, simulation correlations and conclusions Westin & Ångström To be submitted to the 2005 SAE Powertrain and Fluid Systems Conference in San Antonio 7 Fredrik Westin [7] SAE 2005-01-2113 Optimization of Turbocharged Engines’ Transient Response with Application on a Formula SAE / Student engine Westin & Ångström Approved for publication at the 2005 SAE Spring Fuels and Lubricants Meeting in Rio de Janeiro [6] SAE 2005-01-0222 Calculation accuracy of pulsating flow through the turbine of SI-engine turbochargers - Part 1 Calculations for choice of turbines with different flow characteristics Westin & Ångström Published at the 2005 SAE World Congress in Detroit April 11-14, 2005 [5] SAE 2004-01-0996 Heat Losses from the Turbine of a Turbocharged SI-Engine – Measurements and Simulation Westin, Rosenqvist & Ångström Presented at the 2004 SAE World Congress in Detroit March 8-11, 2004 [4] SAE 2003-01-3124 Simulation of a turbocharged SI-engine with two software and comparison with measured data Westin & Ångström Presented at the 2003 SAE Powertrain and Fluid Systems Conference in Pittsburgh [3] SIA C06 Correlation between engine simulations and measured data - experiences gained with 1D-simulations of turbocharged SI-engines Westin, Elmqvist & Ångström Presented at the SIA International Congress SIMULATION, as essential tool for risk management in industrial product development in Poissy, Paris September 17-18 2003 [2] IMechE C602/029/2002 A method of investigating the on-engine turbine efficiency combining experiments and modelling Westin & Ångström Presented at the 7th International Conference on Turbochargers and Turbocharging in London 14-15 May, 2002 [1] SAE 2000-01-2840 The Influence of Residual Gases on Knock in Turbocharged SI-Engines Westin, Grandin & Ångström Presented at the SAE International Fall Fuels and Lubricants Meeting in Baltimore October 16-19, 2000 The first step in the investigation about the simulation accuracy was to model the engine as accurately as possible and to correlate it against as accurate 8 Simulation of turbocharged SI-engines – with focus on the turbine measurements as possible. That work is covered in the chapters 3 and 5 and in paper no. 3 in the list above. The scientific contribution here is to isolate the main inaccuracy to the simulation of turbine efficiency. In order to have anything to compare the simulated turbine efficiency against, a method was developed that enables calculation of the CA-resolved on-engine turbine efficiency from measured data, with a little support from a few simulated properties. That work was published in papers 2 and 8 and is the main scope of chapter 6 in the thesis. The scientific contributions here are several: • The application on a running SI-engine is a first • It was proven that CA-resolution is absolutely necessary in order to have a physically and mathematically valid expression for the turbine efficiency. A new definition of the time-varying efficiency is developed. • It tests an approach to cover possible mass accumulation in the turbine housing • It reveals that the common method for incorporating bearing losses, a constant mechanical efficiency, is too crude. The next step was to investigate if different commercial codes differ in the results, even though they use equal theoretical foundation. That work is presented in chapter 4, which corresponds to paper 4. This work has given useful input to the industry in the process of choosing simulation tools. The next theory to test was if heat losses were a major reason for the simulation accuracy. The scientific contribution in this part of the work was a model for the heat transport within the turbocharger that was developed, calibrated and incorporated in the simulations. It was concluded that heat losses only contributed to a minor part of the inaccuracy, but that is was a major reason for a common simulation error of the turbine outlet temperature, which is very important when trying to simulate catalyst light off.