Numerical 1D-3D CFD Assessments of FVVT Technology in Poppet Valve Four-Stroke and Reverse Loop-scavenged Two-Stroke Engines: A Physics-based Approach

: (Alternative Format Thesis)

Abstract

The global rise in temperatures has necessitated the implementation of stringent CO2 emission standards worldwide, prompting Original Equipment Manufacturers (OEMs) to innovate propulsion technologies that balance efficiency and environmental responsibility. Among these innovations, fully variable valvetrain technology has emerged as a critical advancement, offering unprecedented control over air intake into engine cylinders. This is particularly important for optimizing the performance and efficiency of downsized, electrified internal combustion engine (ICE) powertrains. Such innovations are vital in the face of rising environmental pressures, as recent studies suggest that ICEs, particularly when using renewable fuel blends, can be more environmentally sustainable over their entire lifecycle than battery electric vehicles (BEVs).

However, while efforts to reduce CO2 emissions have garnered significant attention, ICEs also produce pollutant emissions that contribute to air quality deterioration and human health risks. Of these, nitrogen oxides (NOx) are particularly severe and pose the most pressing challenge for ICE technologies. NOx emissions, which include nitric oxide (NO) and nitrogen dioxide (NO2), are formed during high-temperature combustion processes. As combustion temperatures rise, nitrogen in the air reacts with oxygen, resulting in NOx formation. These emissions are highly problematic because NOx contributes to smog formation, ground-level ozone, and fine particulate matter, all of which are linked to respiratory diseases, environmental degradation, and the broader issue of acid rain.

One of the primary factors influencing NOx emissions is the air-fuel ratio within the engine. Under lean-burn conditions, where excess oxygen is present, the combustion process can achieve greater thermal efficiency, thus reducing CO2 emissions. However, lean mixtures tend to increase combustion temperatures, which accelerates NOx formation. This presents a significant dilemma: while lean-burn strategies can improve fuel efficiency and lower greenhouse gas emissions, they exacerbate the production of NOx, a pollutant heavily regulated under emissions standards due to its harmful effects.

The challenge of NOx mitigation is further complicated by the limitations of aftertreatment technologies. In spark ignition engines, three-way catalysts (TWCs) are commonly used to control emissions of NOx, carbon monoxide (CO), and unburned hydrocarbons (HC). However, TWCs function optimally only near the stoichiometric air-fuel ratio (where fuel is burned with just enough oxygen). In lean-burn modes, the excess oxygen overwhelms the TWC’s ability to reduce NOx, as the reduction of NOx into nitrogen requires a lower oxygen environment. This effectively prevents the widespread adoption of extra-lean combustion strategies in spark ignition engines, as TWCs cannot cope with the NOx emissions under such conditions.

In diesel engines, which inherently operate under lean conditions, the challenge is even greater. Diesel engines are more thermally efficient but tend to produce higher NOx emissions due to their high combustion temperatures. To meet stringent emission standards, diesel engines rely on advanced systems such as selective catalytic reduction (SCR), where a urea-based solution (AdBlue) is injected into the exhaust to convert NOx into nitrogen and water. While effective, SCR systems add complexity and cost to diesel engine architectures.

In this context, fully variable valvetrain technologies, such as the Freevalve system, provide a pathway for reducing NOx emissions by enabling more precise control over in-cylinder gas exchange and combustion dynamics. Freevalve technology allows for optimized exhaust gas recirculation (EGR), a technique that reintroduces a portion of exhaust gases back into the combustion chamber to lower combustion temperatures. By reducing the peak combustion temperatures responsible for NOx formation, Freevalve systems can mitigate NOx emissions even in lean-burn scenarios without the efficiency penalty traditionally associated with rich combustion.

The ability to dynamically adjust valve timing, lift, and duration with Freevalve also enables more effective management of in-cylinder turbulence, particularly swirl and tumble, which enhance fuel-air mixing and combustion stability. This leads to more complete combustion and reduced formation of NOx, CO, and unburned hydrocarbons. The combined benefits of improved combustion control, enhanced gas exchange, and reduced NOx emissions make Freevalve technology a significant contributor to meeting future emissions standards while maintaining high efficiency.

This study, by employing physical one-dimensional and three-dimensional computational fluid dynamics (CFD) tools used in conjunction with supercomputing resources, further illustrates how fully variable valvetrain systems can be fine-tuned to achieve optimal performance across various engine architectures, including both four-stroke and two-stroke configurations. Through the detailed evaluation of scavenging and trapping efficiencies, this research provides valuable insights into the potential for NOx reduction in ICEs equipped with Freevalve technology. The findings highlight how these advanced control strategies can help address the dual challenge of reducing CO2 and NOx emissions through efficiency metrics, ensuring that ICEs remain a viable and environmentally responsible option in the evolving energy landscape.

Date of Award	13 Nov 2024
Original language	English
Awarding Institution	University of Bath
Sponsors	Koenigsegg Automotive AB & Freevalve AB
Supervisor	Nic Zhang (Supervisor), Sam Akehurst (Supervisor) & Chris Brace (Supervisor)