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Journal Article

Computing Optimal Heat Release Rates in Combustion Engines

The combustion process has a high impact on the engine efficiency, and in the search for efficient engines it is of interest to study the combustion. Optimization and optimal control theory is used to compute the most efficient combustion profiles for single zone model with heat transfer and crevice effects. A model is first developed and tuned to experimental data, the model is a modification of the well known Gatowski et al.-model [1]. This model is selected since it gives a very good description of the in-cylinder pressure, and thus the produced work, and achieves this with a low computational complexity. This enables an efficient search method that can maximize the work to be developed. First, smooth combustion profiles are studied where the combustion is modeled using the Vibe function, and parametric optimization is used to search for the optimal profile.
Technical Paper

Methods for Cylinder Pressure Based Compression Ratio Estimation

Three methods for compression ratio estimation based on cylinder pressure traces are developed and evaluated for both motored and fired cycles. Two methods rely upon models of polytropic compression and expansion for the cylinder pressure. It is shown that they give a good estimate of the compression ratio, although the estimates are biased. A method based on a variable projection algorithm with a logarithmic norm of the cylinder pressure, which uses interpolation of polytropic models of the expansion and compression asymptotes, is recommended when computational time is an important issue. For motored cycles it yields the smallest bias and confidence intervals for these two methods. For firing cycles a user-specified weighting factor is needed during the combustion phase, which pays off in a smaller estimation bias but also a higher variance. The third method includes heat transfer, crevice effects, and a commonly used heat release model for firing cycles.
Technical Paper

CHEPP - A Chemical Equilibrium Program Package for Matlab

A program package, that calculates chemical equilibrium and thermodynamic properties of reactants and products of a combustion reaction between fuel and air, has been developed and validated. The package consists of the following four parts: 1) A program for calculating chemical equilibrium. 2) A database that contains thermochemical information about the molecules, which comes from the GRI-Mech tables. 3) A GUI that allows the user to easily select fuels, fuel/air ratio for the reaction, and combustion products. 4) A set of functions designed to access the thermochemical database and the chemical equilibrium programs. Results are validated against both the NASA equilibrium program (Gordon and McBride, 1994) and the program developed by Olikara and Borman (1975). It is shown that the new method gives results identical to those well recognized Fortran programs.
Technical Paper

Determining TDC Position Using Symmetry and Other Methods

It is important to determine the phasing of a measured cylinder pressure trace and crank angle with high accuracy. The reason is that erroneous determination of the position of TDC is a major error source when calculating properties such as heat release etc. A common way to determine the TDC position is to study motored cycles. Heat transfer makes the task more complicated, since it shifts the position of the maximum pressure away from TDC. In this paper a new method for determining the TDC position is proposed that does not require any additional sensors other than a cylinder pressure sensor and an incremental encoder. The idea is to find a point that the cylinder pressure from a motored cycle is symmetric around, since the volume is close to symmetric on either side of TDC. The new method and four published methods are tested and evaluated. Cylinder pressure data used for comparison are from simulations of a SAAB Variable Compression engine.
Technical Paper

Compression Estimation from Simulated and Measured Cylinder Pressure

Three methods for estimating the compression from measured cylinder pressure traces are described and evaluated for both motored and fired cycles against simulated and measured cylinder pressure. The first two rely upon a model of polytropic compression, and it is shown that they give a good estimate of the compression ratio for simulated cycles for low compression ratios. For high compression ratios, these simple models lack the information about heat transfer. The third method includes a standard heat transfer and crevice effect model, together with a heat release model and is able to estimate the compression ratio more accurately.
Technical Paper

Mean Value Models for Exhaust System Temperatures

Exhaust temperatures are important for turbine and catalyst performance. A set of exhaust temperature models suitable for turbo matching as well as design and analysis of engine control systems are developed and investigated. The models are lumped parameter heat transfer models, that fall within the category of mean value engine models. The model is developed for describing exit temperatures from the exhaust manifold and temperature drops in pipe sections in the exhaust system. The components used to model the exhaust temperature are: engine out temperature, temperature drop in a straight pipe, and a set of heat transfer modes. The model is validated using data from three different engines. It is shown that, for a spark ignited engine operating at MBT and stoichiometric conditions, it is sufficient to model the engine out temperature as a linear function of mass flow. Recommendations for tuning the model are also given.
Journal Article

Calculation of Optimal Heat Release Rates under Constrained Conditions

The work extends a methodology, for searching for optimal heat release profiles, by adding complex constraints on states. To find the optimum heat release profile a methodology, that uses available theory and methods, was developed that enables the use of state of the art optimal control software to find the optimum combustion trace for a model. The methodology is here extended to include constraints and the method is then applied to study how sensitive the solution is to different effects such as heat transfer, crevice flow, maximum rate of pressure rise, maximum pressure, knock and NO generation. The Gatowski single zone model is extended to a pseudo two zone model, to get an unburned zone that is used to describe the knocking and a burned zone for NO generation. A modification of the extended Zeldovich mechanism that makes it continuously differentiable, is used for NO generation.
Journal Article

Control-Oriented Compressor Model with Adiabatic Efficiency Extrapolation

Downsizing and turbocharging with single or multiple stages has been one of the main solutions to decrease fuel consumption and harmful exhaust emissions, while keeping a sufficient power output. An accurate and reliable control-oriented compressor model can be very helpful during the development phase, as well as for engine calibration, control design, diagnostic purposes or observer design. A complete compressor model consisting of mass flow and efficiency models is developed and motivated. The proposed model is not only able to represent accurately the normal region measured in a compressor map but also it is capable to extrapolate to low compressor speeds. Moreover, the efficiency extrapolation is studied by analyzing the known problem with heat transfer from the hot turbine side, which introduces errors in the measurements done in standard gas stands.
Technical Paper

Optimal Aftertreatment Pre-Heat Strategy for Minimum Tailpipe NOx Around Green Zones

Green zones are challenging problems for the thermal management systems of hybrid vehicles. This is because within the green zone the engine is turned off, and the only way to keep the aftertreatment system warm is lost. This means that there is a risk of leaving the green zone with a cold and ineffective aftertreatment system, resulting in high emissions. A thermal management strategy that heats the aftertreatment system prior to turning off the engine, in an optimal way, to reduce the NOx emissions when the engine is restarted, is developed. The strategy is also used to evaluate under what conditions pre-heating is a suitable strategy, by evaluating the performance in simulations using a model of a heavy-duty diesel powertrain and scenario designed for this purpose.