**2.5.2 Turbocharged engines**

268 Computational Simulations and Applications

description of many physical phenomena, such as: wall friction and heat transfer, concentrated losses, flow through the valves, fuel injection, and so on. Thus, they are generally not able to describe the engine fluid-dynamics accurately, unless semi-empirical input data are provided by means of the model calibration, which is usually carried out by adjusting specific model coefficients so as to accurately reproduce the experimental

In order to calibrate the flow losses in the intake and exhaust pipelines of a SI engine, points at wide-open throttle are more significant, due to the very high mass-flow rate. However, part-load data should also be acquired, in view of using the model for predictive purposes over a wide range of operating conditions. In general, a 1-D model will unlikely be fully predictive (i.e., quantitative) outside the calibration region, though its results may still be significant as trends. If an engine is in the early stages of design, the experimental database may be incomplete, but measurements from other engines with common or similar

The accurate prediction of the in-cylinder pressure trace is mandatory for a good estimation of the engine indicated power, which in turn is fundamental to achieve the correct value of the brake power and torque. For this reason, the in-cylinder pressure is almost always acquired, on a crank angle basis, on engine test rigs. Experimental in-cylinder pressure is the main reference data for the calibration of combustion and in-cylinder heat transfer models. The xb profile versus crank angle can be extracted by means of a diagnostic analysis (also known as heat-release analysis), (Catania et al., 2003; Guezennec & Hamada, 1999; Heywood, 1988). Then, the 'experimental' xb profile can constitute the basis for the calibration of a predictive combustion model, can be used to set up specific lookup tables for the Wiebe parameters ('semi-predictive' approach), or it can be directly imposed in the

Some commercial codes, such as GT-Power, embed tools that are specifically developed for this purpose (Gamma Technologies, 2009). Using these embedded tools, most of the needed inputs for the burn rate calculation (such as trapped air-mass and residual fraction) can be worked out directly by running the simulation of the engine model. In addition, diagnostics is carried out with the same thermodynamics and chemistry as that used for the predictions. Sometimes, heat-release (HR) profiles from external diagnostic tools may be available. However, it should be pointed out that the different thermodynamic models between the diagnostic and the simulation codes can introduce additional uncertainties into the calculation of the gas thermodynamic properties as functions of temperature. For this reason, the tools provided by the adopted 1-D code for HR analysis should be used

As far as the calibration of the heat transfer within the cylinder is concerned, two

a. If the HR is worked out internally, the procedure allows the calibration of the heat transfer at the same time (Gamma Technologies, 2009). For example, the diagnostic tool embedded in GT-Power ('EngBurnRate' template) provides a few parameters that allow the heat-transfer calibration to be assessed. In particular, the heat transfer multiplier can be calibrated to minimize both the deviations of the so-called fuel 'lower heating value multiplier' (LHVM) from unity and the 'cumulative compression heat release'. The code

measurements taken at selected steady-state operating conditions.

**2.5.1 Combustion and in-cylinder heat-transfer model calibration** 

components may still be useful.

model.

whenever possible.

alternatives are possible.

The calibration procedure is more complex in the case of turbocharged engines, in which the compressor, the engine and the turbine are fluid-dynamically coupled. For turbocharged engine applications, it is not advisable to tune the entire model, consisting of the engine and the turbocharger, from the beginning. In fact, slight inaccuracies that are usually present before model tuning can make the compressor operation point (and, hence, the boost level) deviate to a great extent from the real one. In this case, it is very difficult to isolate the root cause of a performance problem (Baratta et al., 2010; Gamma Technologies, 2009; Westin & Ångström, 2003). For this reason, an engine model without the turbocharger should be built first. The intercooler outlet and the turbine inlet should be replaced by two environments in which pressure, temperature and fluid composition are properly set on the basis of measured data, and the model should be calibrated, as in the case of a naturally-aspirated engine.
