The concepts of engine downsizing and down-speeding offer reductions in CO2 emissions from passenger cars. These reductions are achieved by reducing pumping and friction losses at part-load operation. Conventionally, rated torque and power for downsized units are recovered by means of turbocharging. The transient response of such engines is, however, affected by the static and dynamic characteristics of the turbo-machinery. Recent advances in engine simulation and control tools have been employed for the purpose of the research reported in this thesis to identify and verify possible air-path enhancements. A systematic method for evaluating various turbocharger assistance concepts is proposed and discussed in this thesis. To ensure a fair comparison of selected candidate systems, an easily reconfigurable controller providing a close-to-optimal operation, while satisfying physical limits, is formulated. This controller is based on the Model Predictive Control framework and uses a linearised mean value model to optimise the predicted behaviour of the engine. Initially, the controller was applied to a 1D simulation model of a conventional light-duty Diesel engine, for which the desired closed-loop features were verified. This procedure was subsequently applied to various air-path enhancement systems. In this thesis, a turbocharger electric assistance and various concepts based on compressed gas injection were considered. The capability of these systems to improve engine response during third gear tip-in manoeuvre was quantified. This investigation was also complemented with a parametric study of how effectively each of the considered methods used its available resources. As a result, injecting compressed gas into the exhaust manifold was identified as an effective method, which to date has attracted limited attention from engine research community. The effectiveness of the exhaust manifold assistance was experimentally verified on a light-duty Diesel engine. The sensitivity of the improvements to compressed gas supply parameters was also investigated. This led to the development of the BREES system: a low component count, compressed gas based system for reducing turbo-lag. It was shown that during braking manoeuvres a tank can be charged to the level sufficient for a subsequent boost assistance event. Such a functionality was implemented with a very limited set of additional components and only minor changes to the standard engine control.

The scope of this thesis is the Miller engine cycle analysis and its practical application on a turbocharged spark ignited engine. Based on the sensitivity analysis of the limits affecting the ideal Miller cycle thermal efficiency a thermodynamic model of the engine with a prolonged expansion was set up in the GT-POWER software. The results of the analyses were used to evaluate the feasibility of the reference engine conversion for an operation with Miller cycle.

Cam phasers allow improvements to the efficiency and emissions of internal combustion engines. The usage of hydraulic vane type cam phasers for heavy-duty applications have shown problems with oscillations during testing at Scania CV. To investigate the root cause of the oscillations a detailed 1-d simulation model has been created. The 1-d simulation model was calibrated and compared against measurements of a cam phaser system mounted on a physical engine. The 1-d simulation model of the cam phaser system was shown to be able to reproduce the oscillatory behavior seen in engine tests. But there are some concerns regarding the model's reliability, due to its dependence on the integration time step. It was determined that free air in the phaser chambers is causing the excessive oscillations. The source of the free air present in physical phasers are not yet fully understood, but the movement of the phaser control valve, opening and closing of the phaser chamber vents, in conjunction with motion of the phase oscillations has a significant effect on the phaser's chamber pressure, resulting in reduce pressures. The reduced pressures result in air being sucked in to the phaser, form both the oil control valve vent and external chamber leakages. Another potential sources of free air is dissolved air in the oil supply being released due to low pressures from oil pressure fluctuations. A potential solution to increase the stability of the phaser is to reduce the sources of air, by having an oil accumulator at the vent, limiting leakage, having a high and steady oil supply pressure, modifying the oil control valve ports to have a more gradual initial opening area and controlling the oil control valve appropriately. How to implement these implementations and evaluate them are left as future work. Potential improvements to get rid of the 1-d simulation model's dependence on the time step, increasing the model's reliability, are discussed, but not tested.