Luigi Teodosio

A numerical procedure for the calibration of a turbocharged spark-ignition variable valve actuation engine at part load

Referring to spark-ignition engines, the downsizing, coupled to turbocharging and variable valve actuation systems are very common solutions to reduce the brake-specific fuel consumption at low-medium brake mean effective pressure. However, the adoption of such solutions increases the complexity of engine control and management because of the additional degrees of freedom, and hence results in a longer calibration time and higher experimental efforts. In this work, a twin-cylinder turbocharged variable valve actuation spark-ignition engine is numerically investigated by a one-dimensional model (GT-Power™). The considered engine is equipped with a fully flexible variable valve actuation system, realizing both a common full-lift strategy and a more advanced early intake valve closure strategy. Refined sub-models are used to describe turbulence and combustion processes. In the first stage, one-dimensional engine model is validated against the experimental data at full and part load. The validated model is then integrated in a multipurpose commercial optimizer (modeFRONTIER™) with the aim to identify the engine calibration that minimizes brake-specific fuel consumption at part load. In particular, the decision parameters of the optimization process are the early intake valve closure angle, the throttle valve opening, the turbocharger setting and the spark timing. Proper constraints are posed for intake pressure in order to limit the gas-dynamic noise radiated at the intake mouth. The adopted optimization approach shows the capability to reproduce with good accuracy the experimentally identified calibration. The latter corresponds to the numerically derived Pareto frontier in brake mean effective pressure–brake specific fuel consumption plane. The optimization also underlines the advantages of an engine calibration based on a combination of early intake valve closure strategy and intake throttling rather than a purely throttle-based calibration. The developed automatic procedure allows for a ‘virtual’ calibration of the considered engine on completely theoretical basis and proves to be very helpful in reducing the experimental costs and the engine time-to-market.

Numerical analysis of the transient operation of a turbocharged diesel engine including the compressor surge

The paper deals with the simulation of a multi-cylinder turbocharged diesel engine for automotive applications, employing a one-dimensional approach with the aim of refining the turbocharger modelling during transient manoeuvres. The proposed methodology is able to handle stable compressor behaviour and also compressor surge. In addition, a waste-gate model is introduced to account for the instantaneous variation in the valve section as a result of the control signal, which is provided by the engine control unit, and the engine state. Preliminarily, the engine model is tuned against experimental data in terms of both the global performance parameters and the in-cylinder pressure cycles. The compressor performance is described through an ‘extended’ map obtained using a one-dimensional turbocharger model; in this way, a refined surge analysis can be performed, accounting for both direct flow compressor operations and reverse flow compressor operations. The one-dimensional model is applied to analyse different transient manoeuvres. First, the vehicles maximum speed is predicted and compared with the manufacturers data, during an acceleration manoeuvre. Then, a sudden part-to-full-load step is described with the aim of analysing in detail the turbo-lag. Finally, a full-to-part-transient manoeuvre is also analysed to verify the capability of the model to represent the compressor surge phenomenon. The numerical results provided in this work qualitatively reproduce the experimental observations available in the literature for transient operation of engines. Thus, the developed computational tool can be successfully used to support the design process and the transient analysis of turbocharged internal-combustion engines.