Engines and Fuels for Future Transport

Global transport is almost entirely powered by internal combustion engines (ICEs) and petroleum-derived liquid fuels. Currently, there are many efforts to move transport away from this energy system to reduce its carbon footprint. In addition, tailpipe emissions from ICEs have to meet increasingly stringent standards. However, all the alternatives, such as battery electric vehicles (BEVs), start from a very low base and face very serious barriers to unlimited and fast expansion and cannot completely displace ICEs for several decades to come. Hence it is also important to continue to improve ICEs to ensure the sustainability of transport. This volume considers some of these approaches, sustainable fuels, and methodologies like life cycle analysis (LCA) needed to assess the true impact of alternative approaches to power transport. This first chapter provides brief descriptions of the fourteen chapters that follow.

The past century has seen a remarkable rise in personal mobility and heavy goods transport. The development of the internal combustion engine has played a pivotal role in this development. Significant progress has been made in improving engine efficiency and reducing emissions. However, further improvements are necessary in order to meet local zero emission regulation as well as global climate goals. A rapid transition to renewable energy sources is key, enabling clean electricity generation and widespread deployment of sustainable fuels. Every country has a role to play. Developing nations must learn to become less dependent on fossil fuels as they grow their economies and industrialized nations must continue their sustainability journey and quickly transfer critical knowledge and lessons learned. Technologies should be assessed in terms of their life cycle impact and not simply their tailpipe emissions. As we consider the wide range of disparate applications across the transportation sector, we would be wise to embrace a fact-driven approach, keeping multiple options open and to build on past successes. Rather that betting it all on a single technology, a diverse mix of low-carbon technologies should be pursued.

The year 2020 marked the 50th anniversary of the passage of the Clean Air Act in the United States. The subsequent creation of the Environmental Protection Agency (EPA) and establishment of national ambient air quality standards (NAAQS) for criteria air pollutants paved the way for increasingly stringent tailpipe emission limits for passenger cars and trucks. These limits have been met through significant advances in both engine hardware and controls, and after-treatment systems, the latter of which will be the focus of this chapter. Modern powertrains using advanced after-treatment systems can practically eliminate the harmful gases and particulates from entering the atmosphere. Three-way catalysts (TWC) can address NO_x, CO and hydrocarbons (HC) emissions from stoichiometric gasoline vehicles with near 100% efficiency when operating above the light-off temperatures. Diesel vehicles, despite the bad press in recent years, can be emitting well below the regulated limits with the adoption of the latest technologies such as selective catalytic reduction of NO_x (SCR). The key challenge in both technologies is addressing the “cold-start” emissions, which is the combination of high emissions following an engine start and after-treatment temperatures below light-off. We will review the latest options that are being pursued to address this challenge and lead to “zero-impact” emitting vehicles. Particulate filters are ubiquitous on diesel vehicles and are also making their way on to gasoline vehicles with the recent particle number regulations in Europe and China. Filtration efficiency is very high and only improves with vehicle age due to the accumulated ash layer. There is evidence that in highly polluted urban environments, the tailpipe particulate emissions can in fact be lower than the ambient concentrations. Emissions control technology is mature but still there is much more work to be done. New regulations such as Euro 7, LEV 4 and the heavy-duty low NO_x regulations in US and Europe are looking for further deep cuts in pollutant limits, regulating new species and particulates down to 10 nm, and extending the testing to include “all” driving conditions. We will review the new component technologies and the after-treatment system layouts being developed to address these upcoming regulations.

Opposed-piston (OP) engines were used in many applications in the 1930s–1960s, largely because their high thermal efficiency was important in applications that valued long-range, like aviation, marine vessels, and military vehicles. The engine architecture largely fell into disuse when modern emissions standards began in the 1960s.

Recently, OP engines have enjoyed a renaissance, driven by three primary factors:

These factors together suggest that the OP engine is a superior architecture for providing near-zero criteria emissions and lower CO₂ in a cost-effective and robust manner.

This chapter provides an overview of OP engine design characteristics and summarizes recent test results for heavy-duty diesel engines. Results include:

Fossil-fuel-based transportation has raised several global concerns such as ever-increasing demand for petroleum oils, their high prices and exhaustible reserves, and the associated environmental (global warming, climate change) and health degradation. Hence, the world is facing challenges to find sustainable solutions to these major issues. Adaptation of efficient, and cleaner alternative transportation systems with low to zero carbon footprint has been emphasized among the different strategies. Hybrid electric vehicle (HEV) technology has exhibited promising solutions for ensuring improvements in fuel consumption and emission rates with a performance comparable to the conventional vehicles. The performance of an HEV largely depends on the powertrain type, components configuration and its architecture, and energy management strategy (EMS). This chapter presents an overview on essential components used in HEVs including the energy storage system (i.e. the battery, super-capacitor, and fuel cell), electric motors, and dc-dc/dc-ac converters and their size/ capacity optimization. Development of an efficient EMS to harvest the benefits of HEVs without compromising vehicle performance remains a major challenge. The state-of-the-art in EMSs for efficient HEVs is presented. Furthermore, factors/ issues affecting the performance of the EMS are discussed. It is quite crucial to apply a real time control strategy for an HEV capable of coordinating the on-board power sources ensuring enhanced fuel economy and reduced emissions. However, challenges exist in developing an efficient control strategy satisfying conflicting control constraints involving fuel consumption, emissions and drivability, without over-consuming battery power at the end of a given drive cycle. A section is devoted to explore emissions performance of HEVs under actual operating conditions and the emissions management of an HEV. Finally, key issues and challenges of HEV technology are identified and discussed.

Regulators, policymakers, environmental groups, and investors promote battery electric vehicles (BEVs) as zero-emission vehicles (ZEV). Under current regulations, emissions are measured at the vehicle tailpipe. This definition is widely accepted. However, it is misleading as emissions are still created elsewhere during the life cycle of a battery electric vehicle. For example, mining the earth for raw materials and then manufacturing them into vehicle and powertrain components create embedded emissions. In addition, the production of fuel or electricity generates emissions, and the vehicle end-of-life also impacts total emissions, depending on scrappage or recycling processes. To accurately account for these stages, a life-cycle analysis is required. In a proper life-cycle analysis, it is possible to quantify emissions at each stage of the vehicle’s life to better understand cumulative emissions. Understanding cumulative emissions for different decarbonization pathways, it is possible to correctly choose the best strategy to move towards lower overall GHG emissions from transportation. LCA studies show that BEVs can lower GHG emissions, especially when charged from a renewable grid. However, alternative fuels and hybrid powertrains can have even lower life cycle GHG emissions. Therefore, LCA should be adopted as part of future policy and regulations. The main challenge with the adoption of LCA is the vast number of assumptions used. The final GHG prediction is sensitive to these assumptions and can alter the conclusion of any LCA study. This chapter will review several LCA studies and identify commonly used assumptions, variations on assumptions, and their impact on conclusions. Finally, a set of recommendations for future LCA work is presented.

In spark ignited engines, the efficiency of the engine is strongly influenced by the quality and duration of the combustion process as initiated by the ignition system. Jet Ignition is a concept initiated by pilot combustion in a pre-chamber connected to the main cylinder, where pilot combustion products are introduced as reactive high velocity jets. These jets initiate auto-ignition of the main fuel-air mixture, resulting in multiple distributed points of ignition. In passive jet ignition systems, this distribution leads to rapid burning, helping to mitigate knock likelihood. In active jet ignition systems, which contain auxiliary fuelling in the pre-chamber, the increased ignition energy results in the ability to operate the engine with air dilution beyond the capabilities of typical ignition systems, enabling large increases in compression ratio. Active jet ignition engines have demonstrated increases in peak and drive-cycle average thermal efficiencies of 15–25% over modern engines, representing a step change in efficiency. Pre-chamber combustors are a well-research technology, with a lineage dating back over 100 years. A wealth of research has established pre-chamber combustors as an enabling technology for high efficiency engine operation. This study will examine recent applications and benefits of pre-chambers, including knock reduction in high specific output downsized engines and lean limit extension in high efficiency engines. Fundamental barriers to pre-chamber engine adoption will be discussed: pre-chamber geometry optimization that spans the full engine map, in-pre-chamber mixture preparation, low load and idle stability, and ensuring sufficient spark retard for catalyst light-off. Pre-chamber engines can also be ideal platforms for future fuels, including low carbon fuels such as hydrogen that have highly specific operating requirements. Recent work by the authors in this area is presented. Finally, the role of the pre-chamber engine in future transport and its potential for facilitating significant de-carbonization of the sector is discussed.

Stable internal combustion (IC) engine operation with a lean mixture allows improved thermal efficiency and reduced engine-out emissions. However, lean limits in IC engines are challenging due to poor ignitibility. Narrow throat pre-chamber as an ignition source allows extending the lean limit through a robust multi-reactive jet ignition and in-cylinder turbulence generation. These benefits have revived the research interest in such narrow throat configurations of pre-chamber. Metal engine studies offer limited insights into the physics of pre-chamber combustion (PCC). However, when coupled with recent optical engine studies involving high-speed visualization and laser diagnostics, a better understanding of this combustion mode is unlocked. This work attempts to evaluate and summarise the recent advancement in PCC research.

Pre-chamber combustion systems combined with bio-hybrid fuels allow for an efficiency increase of internal combustion engines and a reduction of engine-out emissions. Experimental and numerical investigations are required to understand the combustion process and in particular the phenomena inside the pre-chamber. To this end, this study presents experimental investigations of an active pre-chamber combustion system in generic optical experiments and on a thermodynamic single-cylinder research engine. Moreover, the generic experiments are accompanied with large-eddy simulations of the single-cylinder flow field. Additional optical experiments on a rapid compression machine provide insights into both the ignition inside the pre-chamber and the subsequent combustion in the main chamber (MC) by use of the bio-hybrid fuels DEM and ethyl acetate. These fuels were also investigated on the thermodynamic single-cylinder research engine. A maximum indicated efficiency of 43% is achieved at an engine speed of 2000 1/min, an indicated mean effective pressure of 6 bar, and a relative air/fuel ratio of 2.2. A maximum relative air/fuel ratio of 2.25 is realized in the same operating point. In ongoing research, numerical investigations of the mixture behavior after the direct fuel injection will contribute to the understanding of the combustion process in the thermodynamic single-cylinder research engine. Moreover, the findings from the numerical investigations will be validated with those of a motored optically accessible single-cylinder research engine.

In the past decades, combustion engines have played an important role in addressing the transportation demands. However, emissions of greenhouse gases such as carbon dioxide (CO₂) emitted by combustion engines rapidly increased, leading to climatic issues like global warming. This adverse environmental impact has been the main motivation to search for clean solutions for fossil fuel replacement. Ammonia is recognized as a promising carbon-free fuel to meet the requirement in CO₂ reduction. Specifically, ammonia has a higher volumetric energy density compared with Hydrogen, and it is easier to produce, store and transport in comparison to other carbon-free fuels. However, ammonia-fueled applications exhibit lower efficiency and stability compared with conventional-fueled systems, due to the lower flame speed and combustion temperatures of ammonia. In this review paper, the previous and ongoing studies exploring the potential of ammonia as an engine fuel are highlighted. The strategies to realize the usage of ammonia in combustion engines are summarized, and the future pathway to use ammonia as an engine fuel is discussed.

The idea of using ammonia as fuel is nothing new as the first well-known Belgium use of buses fleet during World War II. Even if several studies performed during the mid-60’s investigated the possibility to consider ammonia as fuel for internal combustion engines, mainly by means of CFR experiments or 0D modelling, ammonia-based combustion engine fueling methods are not ready to be marketed as the use of this toxic molecule still poses major problems not only because of supply and safety issues but also because of its physical characteristics compared to conventional fuels. As function of the target, i.e. to supply ammonia either partially in standard engines to limit carbon footprint or to employ it mainly in dedicated engines to reach zero footprint, the technological challenges (dual fuel or unique fuel, SI or CI engines …) could be different if it is as main power or auxiliary power unit (to extend battery vehicles) and as a function of the transportation type (mean duty or heavy duty engines for freight, construction, marine transportation, …). In this chapter new results of advanced researches will be discussed in order to highlight the potential of this future green fuel.

Methanol has attracted broad attention over the past few decades as an alternative fuel for diesel and gasoline for internal combustion engines. Renewable methanol has helped methanol engines achieve almost zero carbon dioxide emissions in recent years, showing greater advantages than other fuels. The development of methanol engines has undergone a long history. The revolution of methanol engines has been closely linked to the application of energies, including oil resources, and the environment. Over the last few years, a variety of proposals on the applications of methanol engines have been put forward. Methanol engines can operate with pure methanol, blended fuel, or dual fuel. In a general sense, methanol has its unique properties, such as its high oxygen content, fast laminar burning velocity, great latent heat of vaporization, and having no C–C bond, which favors its use in advanced engine technology. A detailed discussion will be made on the in-cylinder combustion process and emissions characteristics of methanol engines, as well as some typical issues found in the application in this chapter. It is hoped that some useful points can be provided for the development and study of methanol engines.

Design trend in spark ignition (SI) engines aims to improve power density and efficiency to ensure that regulated emissions levels are minimized. From a thermodynamic point of view this involves increased in-cylinder pressure and local mixture conditions that promote autoignition of the end gas, leading to knock. In order to prevent knock, the commonly used technique consists in retarding spark timing and combustion phasing, which results in a lower thermodynamic efficiency and higher exhaust temperature. A delayed spark timing is frequently coupled with over-fueling that allows to reduce the exhaust temperatures as well as knock tendency. This strategy clearly results in worsened fuel economy and increased emissions levels. At present time knock represents a limiting factor for a further development of new generation of high performance spark ignition engines and a number of technological solutions have been proposed by the scientific community and OEMs to overcome the limitations connected to this phenomenon. New engine architectures with optimized combustion chamber geometry shapes and combination of methodologies have been proposed for a reduced combustion time to enhance high-performance engines. Changes in the engine cooling systems have been applied to enhance end gas cooling leading to knock suppression effect. Other favorable options proposed for knock mitigation are water injection and cooled exhaust gas recirculation. Moreover, attention is also paid on renewable alcohol fuels with a research octane number higher than gasoline. This contribution aims to review some of the recent progresses in this topic with detailed analyses of current and promising future technologies to avoid knock and enable the development of future high efficiency SI powertrains with low impact on emissions.

Simple surrogate formulations for gasoline are useful for modelling purposes and for comparing experimental results using a carefully designed fuel. The most common approach is to start with Primary Reference Fuels (PRF)—i.e., isooctane and n-heptane. These have the disadvantage that they cannot replicate the octane sensitivity (RON-MON) of a real fuel, and so it is common to add toluene to make a Toluene Primary Reference Fuel (TPRF) surrogate. The vapour pressure of TPRFs is much lower than a real fuel, and so the flash boiling behaviour of a real fuel cannot be properly replicated. To overcome the volatility challenges, an alternative to TPRF is advocated which involves replacing some or all the isooctane by isopentane. In the event of total replacement, a three-component “THIP” (Toluene, n-Heptane, Iso-Pentane) surrogate fuel is produced. It is shown that by adding isopentane it is possible to have a simple surrogate that can reproduce the lower part of the distillation curve of a real fuel. Explicit equations are presented that allow THIP surrogates to be created based on a desired RON, MON and DVPE (Research Octane Number, Motor Octane Number, Dry Vapour Pressure Equivalent).

A theoretical prediction of ignition modes in shock tubes relevant to engine conditions is proposed and validated with a wide range of shock tube experiment data. The predictive Sankaran number, \(\mathrm{{Sa}_p}\), is adapted to distinguish between the weak and strong ignition modes. The non-ideal temperature and pressure rise inherently occurring in combustion devices is considered in the formulation of \(\mathrm{{Sa}_p}\). The \(\mathrm{{Sa}_p}\) criterion is then validated by the experimental data in shock tubes for a number of fuels exhibiting negative temperature coefficient (NTC) and non-NTC behavior. It is demonstrated that the \(\mathrm{{Sa}_p}\) criterion can accurately predict the weak and strong ignition modes regardless of the NTC and non-NTC fuels over a wide range of pressure and temperature. \(\mathrm{{Sa}_p}\) \(= 1\) serves as a reliable marker to delineate the boundary between the strong ignition (\(\mathrm{{Sa}_p}\) \(< 1\)) and weak ignition (\(\mathrm{{Sa}_p}\) \(> 1\)). As inspired by the newly-developed \(\mathrm{{Sa}_p}\) criterion in shock tube, it strongly suggests that the sensitivity of ignition delay variation in non-constant volume reactors such as the polytropic compression/expansion heating effect in an internal combustion engine and in a rapid compression machine (RCM) should be incorporated in evaluating an ignition criterion to better predict the ignition modes.