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Strict emissions legislations and continuous race for improvement of fuel economy urge to develop more efficient and cleaner IC engines for commercial and private use. Engine downsizing has been shown as an effective means to reduce the vehicle's fuel consumption but the full potential of engine downsizing is limited by the knocking combustion at boosted operations and the presence of pumping loss at part load conditions. A variable valve train system can be used to minimise the knocking combustion by implementing Miller and Atkinson cycles through alteration of the effective compression ratio (ECR) at high load via Early Intake Valve Closure (EIVC) or Late Intake Valve Closure (LIVC), as well as reducing the pumping loss at part load. In this work, a single cylinder direct injection Spark Ignition (SI) gasoline engine equipped with an electro-mechanical valvetrain system named iVT (intelligent Valve Technology) by Camcon was set up and used to investigate the potential benefits of the iVT. Engine experiments were carried out at 1500 and 3000rpm with various valve profiles including EIVC and LIVC with single and two intake valve mode as well as early intake Maximum Opening Position (MOP) at 35% and 40% MOP. Their effects on engine performance and emissions were measured and analysed at 4, 6, 9 and 12.6bar net IMEP. Results showed that EIVC and LIVC profiles were successful in reducing the fuel consumption with two valve mode at low loads thanks to lower pumping loss and at high loads where spark timing was knock limited. Those profiles also resulted in lower emissions. In particular, the LIVC profile reduced the NOX concentration by up to 20% at low loads due to lowest ECR. Single valve mode operations also provided improved fuel economy at 4bar and 12.6bar net IMEP when combined with EIVC profiles. However, the most significant reduction in ISFC was achieved with early MOP. With combination of LIVC and early MOP, ISFC was reduced by up to 5.4% at low load and by up to 7.1% at high load compared to the baseline profile at 1500rpm.

The simultaneously requirements of ultra-low emissions and lower operating cost are driving the technology development of heavy-duty (HD) diesel engines. In order to meet the stringent emission regulations and maximise the fuel efficiency and thus minimise the greenhouse gases (GHG), significant efforts have been made to develop highly efficient and clean diesel combustion. In addition to the challenge of the trade-off between exhaust emissions and fuel efficiency, HD diesel engines experience low exhaust gas temperature (EGT) at low-load operation, which significantly reduces the effectiveness of the exhaust aftertreatment systems. In response to these issues, variable valve actuation (VVA)-based advanced combustion control technologies have been researched to curb engine-out emissions and maximise fuel efficiency as well as improving the low-load thermal management, helping to minimise total engine operational cost (EOC). This research started with an investigation into the influence of injector nozzle designs on engine combustion, emissions, and performance. This was followed by the systematic study of Miller cycle operation over the engine speed-load map with and without exhaust gas recirculation (EGR). Experimental work was carried out in a single cylinder HD diesel engine equipped with a high pressure common rail fuel injection system, an external cooled EGR circuit, and a VVA system. The results showed that the injector geometry optimization is necessary to improve the trade-off between engine-out emissions and fuel efficiency. Special attention should be paid to the interaction of spray-bowl and spray-wall under different operating conditions. The introduction of residual gases via an intake valve re-opening during the exhaust stroke was demonstrated as an effective combustion control strategy for exhaust thermal management at the low load operation of 2.2 bar indicated mean effective pressure (IMEP). Additionally, the combined "Miller cycle + EGR + post injection" strategy was shown to beneficial for engine-out emission reduction and EGT control at 6 bar IMEP. Furthermore, the effectiveness of Miller cycle with or without EGR was examined from low to full engine loads at constant intake pressure and constant lambda conditions. A cost-benefit and the cycle-averaged results calculated over the World Harmonized Stationary Cycle (WHSC) were presented and analysed. The results showed that the "Miller cycle + EGR" strategy with a constant lambda attained the highest corrected net indicated efficiency over the WHSC by up to 8% in comparison to the baseline without EGR. Finally, the potentials of different combustion control strategies to meet the Euro VI emission regulation were assessed at different NOx aftertreatment efficiencies. Overall, this study has presented promising cost-effective emission control and fuel efficiency technologies that could be suitable for the "SCR-only" and "SCR + EGR" technical routes for the future HD diesel engines.

Natural gas is currently an attractive substitute for diesel fuel in the Heavy-Duty (HD) diesel transportation sector. This is primarily attributed to its cost effectiveness, but also its ability to reduce the amount of CO2 and harmful engine pollutants emitted into the atmosphere. Lean-burn dual-fuel engines substitute natural gas in place of diesel but typically suffer from high engine-out methane (CH4) emissions, particularly under low load operation. In response to this issue, this work set out to improve upon the efficiency and emissions of a lean-burn dual-fuel combustion system in an HD diesel/natural gas engine. Thermodynamic experimental engine testing was performed at various steady-state operating points in order to identify the most effective methods and technologies for improving emissions and efficiency. Low Temperature Combustion (LTC) along with several valvetrain and injection strategies were evaluated for benefits, with special attention paid to low load operating conditions. LTC was proven to be a useful method for decreasing methane emissions while simultaneously improving engine efficiency. The benefits of LTC were a function of load with the greatest advantages experienced under medium load operation. Additionally, the low load strategies tested were determined to be effective techniques for reducing methane emissions and could possibly extend the dual-fuel operating regime to lighter load conditions. Overall, no operating condition tested throughout the engine map resulted in a brake engine-out methane emissions level of less than 0.5 g/kWh at gas substitutions greater than approximately 75%. It is suggested that the limits of this particular lean-burn dual-fuel design were reached, and that it would likely require improvements to either the combustion system or exhaust after-treatment if Euro VI emissions levels for methane were to be achieved.

In response to the global concerns towards oil scarcity and climate change, the automotive industry is currently focusing on improving fuel economy and reducing exhaust emissions. Modern downsized gasoline engines that come with a package that includes a boosting system, variable valve train and direct fuel injection system is effective for fuel economy improvement and emission reduction. However, the knocking issue becomes severe at high load operations as a result of the high intake boosting pressure. In regard to the part load conditions, the gas exchange process requires extra work to draw in air into the cylinder due to a lower amount of pressure in the intake manifold caused by the restriction of the throttle plate. The Miller cycle is regarded as a potential strategy of knock control for downsized gasoline engines. Extensive works have sought to examine the performance improvement via the Miller cycle, yet only limited research has been conducted on the manner in which it can influence knock suppression. The focus of this thesis is to investigate early and late intake valve closing timings in terms of how they affect the compression process, the ability to suppress engine knock and meet the power output required at high loads for spark-ignited gasoline engines. Apart from that, this research also demonstrates the Miller cycle potential by utilising fully variable valve timing in controlling the load at the part load condition without using a throttle. The early intake valve closing with different valve lifts was tested in order to investigate the impact during the gas exchange process, particularly the pumping losses and the potential to improve fuel economy. This study includes both experimental and simulation studies. A Lotus single-cylinder research engine referred to as SCORE was mainly used for the experimental component of the study. The simulation work was conducted using a one-dimensional spark ignition engine model built in the Ricardo WAVE software for naturally-aspirated and downsized engines. The engine model values are validated against the experimental values from the Lotus SCORE and Lotus SABRE engines. The combustion model with a reduced kinetics mechanism was validated using a Rover K-series engine. A broad matrix of the engine operations has been investigated combining a variety of engine speeds and engine loads. The Miller cycle effects on knock suppression in a downsized engine environment have been investigated in three parts, namely the Miller cycle at different speed-load, knock suppression with extreme Miller cycle, and knock analysis with combustion kinetics. Through the works, the Miller cycle has demonstrated its capability to suppress engine knocking in a more efficient manner as opposed to the standard engine operation. This is contributed by the fact that early and late intake valve closings could affect the end gas condition at the end of the compression stroke, thus making it possible to suppress the engine knocking. The experimental study for controlling load without using throttle under the naturally-aspirated condition found that the Miller cycle with an early intake calve closing strategy able to improve fuel consumption by reducing pumping losses. The downsized engine condition, which has been evaluated via modelling work, also showed an improved performance trend using the unthrottled Miller cycle strategy. The open cycle and close cycle efficiencies have improved through the Miller cycle implementation. The contribution of this work is made in order to establish the comparison of the Miller cycle strategy in suppressing knocking between the early intake valve closing and late intake valve closing under a boosted environment. For the part load condition of the downsized engine, the research contributes to the existing body of knowledge by comparing the throttle-less Miller cycle and the standard throttled operation as a load control strategy.