Future Engine Technology Research Articles

A Comprehensive Review of the Free Piston Engine Generator and Its Control

Review A Comprehensive Review of the Free Piston Engine Generator and Its Control Patrick Brosnan 1, Guohong Tian 1, * , Umberto Montanaro 1, and Sam Cockerill 2 1 School of Mechanical Engineering Sciences, University of Surrey, Surrey GU2 7XH, UK 2 Libertine FPE Ltd., Sheffield S9 1DA, UK * Correspondence: g.tian@surrey.ac.uk; Tel.: +44-1483-300-800 Received: 2 February 2023 Accepted: 10 March 2023 Published: 23 March 2023 Abstract: The Free Piston Engine (FPE) is a unique machine with a higher thermal efficiency than its counterpart, the Conventional Reciprocating Piston Engine (CPE). The unique piston motion of the FPE is not constrained kinematically like the CPE with its connecting crankshaft and rotational inertial masses. Moreover, when directly coupled to the Linear Electric Machine (LEM) to harness electric energy production, the Free Piston Engine Generator’s (FPEG) characteristic motion, now being dynamically constrained, permits an extensive range of piston trajectory profiles to be exploited during operation. In addition, exploring varied piston trajectories during the development stages may be vital in reducing in-cylinder combustion emissions through strategies such as Low-Temperature Combustion (LTC) and Homogeneous Charge Compression Ignition (HCCI). This review paper will focus on the key motivations and drivers for continued FPEG development. It will also highlight and review its distinct advantages and challenges in being a viable solution as a future zero-carbon engine technology. Finally, FPE fundamentals, alongside its rich history, will be introduced, clearly presenting how academia and industry have described and controlled its intrinsic non-linear dynamics.

Investigation of aircraft engine performance utilizing various alternative fuels

The airlines are subjected to the energy crisis and have raised environmental issues at the same time. Future engine technology advances could decrease the effect on the environment and energy consumption. Alternative fuelspotentially assist in the reduction of engine emissions and hence lower the energy-related issues. This study presents analysis of the efficiency of aircraft engines as a function of thrust force, flow of the and specific fuel consumption (SFC) at distinct mixing ratios (40% and 100%) of African natural gas, Algae, Camelina, Jatropa, Diesel, Hydrogen, Synthetic paraffinic kerosene, UK natural gas at cruising altitude. In – house Cranfield University simulation codes, PYTHIA & TURBOMATCH have been used to investigate and model a three-shaft high bypass engine analogous to RB211 - 524. The engine model has been certified and authenticated in commercial aircraft with open works found in the Bio - Synthetic Paraffinic Kerosene test program.Blended fuel of Kerosene & hydrogen (KE+HY) fuels gives values of 331.6 KN,1.2577KG/S, and 6.9512 kg/kwh for net thrust force,the flow of fuel and specific fuel feastingat mingling ratio of 40 % respectively. However, at mixing ratio of 100% Blended fuel of Kerosene & hydrogen (KE+HY) fuels gives values of 339.01 KN, 0.800KG/S, and 4.333 kg/kwh for net thrust, fuel flow, and specific fuel consumption respectively. It is found that blended fuel of Kerosene &hydrogen (KE+HY) fuels give better engine performances as compared to other alternative fuels. However, Kerosene &diesel (KE+DI) fuels have shown a slight reduction in engine performance.

Assessment on critical technologies for conceptual design of blended-wing-body civil aircraft

Civil aviation faces great challenges because of its robust projected future growth and potential adverse environmental effects. The classical Tube-And-Wing (TAW) configuration following the Cayley’s design principles has been optimized to the architecture’s limit, which can hardly satisfy the further requirements on green aviation. By past decades’ investigations the Blended-Wing-Body (BWB) concept has emerged as a potential solution, which can simultaneously fulfill metrics of noise, emission and fuel burn. The purpose of the present work is to analyze the developments of critical technologies for BWB conceptual design from a historical perspective of technology progress. It was found that the high aerodynamic efficiency of BWB aircraft can be well scaled by the mean aerodynamic chord and wetted aspect ratio, and should be realized with the trade-offs among stability and control and low-speed performance. The structure concepts of non-cylinder pressurized cabin are of high risks on weight prediction and weight penalty. A static stability criterion is recommended and further clear and adequate criteria are required by the evaluations of flying and handling qualities. The difficulties of propulsion and airframe integration are analyzed. The energy to revenue work ratios of well-developed BWB configurations are compared, which are 31.5% and 40% better than that of TAW, using state-of-art engine technology and future engine technology, respectively. Finally, further study aspects are advocated.

Engine Thermal Efficiency Gain and Well-to-Wheel Greenhouse Gas Savings When Using Bioethanol as a Gasoline-Blending Component in Future Spark-Ignition Engines: A China Case Study

In 2017, the Chinese government issued a strategic policy of nationwide use of bioethanol as a gasoline-blending component by 2020 for the consideration of reducing smog and greenhouse gas (GHG) emissions. It is highly relevant to estimate the benefits of well-to-wheel (WTW) GHG emission savings using future engine technologies. However, literature about the WTW GHG emissions for ethanol blends did not cover the engine efficiency gains in engines with future technologies. In a previous publication from the authors’ group, an empirical model was developed to predict the anti-knock property and engine thermal efficiency gains of ethanol blends in spark-ignition (SI) engines. This paper is a follow-up study, looking at not only the potential engine thermal efficiency gains but also WTW GHG emissions in future engine technologies. More specifically, a case study of adding bioethanol to two representative E10 fuels (main- and premium-octane-grade fuels) from China was conducted. It is assumed that future engin...

Ignition characterization of F-76 and algae-derived HRD-76 at elevated temperatures and pressures

This study investigates the impact of fuel structure and chemistry on the combustion dynamics of a high-pressure fuel spray under realistic diesel engine conditions. The focus of the research was to better understand the combustion characteristics of a bio-derived hydroreformed Navy diesel, HRD-76, which is populated with a higher percentage of paraffins and expected to exhibit more rapid ignition behavior in comparison to the conventional Navy diesel, F-76. Specific attention was paid to intermediate temperature ignition conditions, which are of particular interest due to unique ignition behavior and the potential for reducing common oxide pollutants in this region. Experiments were conducted in a constant pressure flow chamber using dry air at ambient temperatures of 800K≤T≤975K, under various compressed pressures within the standard engine operating envelope (20–95bar), and under a moderate to high fuel loading condition when compared to realistic engine applications. The ignition delays, combustion duration, and ignition dwell of the fuels at various temperatures and pressures were measured utilizing a photodiode to capture the OH* emission signature. In addition, high-speed broadband chemiluminescence imaging and intensified high-speed OH* chemiluminescence imaging were utilized to understand the fuel's ignition behavior at frame-rates of 60kHz. The optical diagnostics employed showed that the HRD-76 has a higher propensity to ignite in comparison to F-76 under the same ambient conditions. The results were correlated with shock tube data and chemical kinetic simulations to determine the impact of the fuels’ physical delay time on the overall ignition delay. These results will supply fundamental combustion information for the use of alternative fuels in current and future engine technologies.

Experimental and numerical assessment of methods to reduce warm up time of engine lubricant oil

Carbon dioxide emission reduction is the most important challenge concerning on the road transportation. Stringent quantitative commitments have been set, both for automotive and for light and heavy duty vehicles. Future engine (and vehicle) technologies will consider a portfolio of new components, engine layouts, control strategies, integrated functions which will match also new comfort standards and fun to drive options, and complying fuel consumption savings.Engine thermal management is an area of intervention aimed at reducing the warm up time: in most part of the homologation cycle, both in passenger cars and in light duty vehicles, engines do not reach a stabilized thermal state. This produces a significant pollutants and fuel consumption increase. Engine thermal management can match also more effective cabin heating requirements, improving comfort, which is an important market advantage.Basic idea of engine thermal management is to reduce or make nil the cooling fluid circulation inside the engine.In this paper, the oil warm up is considered as the main focus: a faster temperature grow up decreases oil viscosity and improves mechanical efficiency and organic efficiency, during engine cold state. An experimental activity has been conducted on a dynamic test bench, testing the cooling fluid and oil dynamics of a widely known commercial engine, in fixed engine points and during the New European Driving Cycle (NEDC).A comprehensive mathematical model reproducing both the integrated circuits has been developed and validated: this required an overall engine and vehicle modeling whose input data was determined by the vehicle mission profile. By means of such model, three new technologies have been proposed and verified in terms of warm up time. They are: (a) by pass of the oil cooling, (b) oil heating using exhaust gases, and (c) a partial reduction of the oil quantity inside the sump.All of them were applied during transients, which revealed they are able to reduce warm up time of about 65–70% with a significant benefit in terms of CO2 reduction.

A novel engine cooling system with two circuits operating at different temperatures

The driver of the future engine technology will be the reduction of the CO2 emissions which is strictly related to fuel consumption saving. Recent regulations on pollutant emissions and greenhouse gases require a technological effort on engine efficiency which can be made with a different set of options. It is important to consider those ones that have lower costs and a suitable ratio between CO2 reduction and cost. Relevant importance is, so, assumed by a revision of the engine cooling system, which remained substantially unchanged for several years or decades.If the engine and vehicle thermal needs are considered in a whole way, the cooling system can be rearranged realizing the integration of conventional thermal needs (engine, oil and EGR cooling) with unconventional ones (cabin conditioning, charge air cooling) realizing benefits in terms of engine efficiency and comfort.In this paper, a mathematical model of an internal combustion engine is presented. It is a lumped parameter model, physically consistent and it is used to appraise the performances of conventional and unconventional engine cooling systems and the integration in it of vehicle thermal needs. In particular, a double cooling circuit has been considered, with two different temperature levels, that allows important improvements in terms of engine warm up, fuel consumption saving and air boosting.The model has been applied to an existing engine whose experimental characterization was done concerning the heat rejection toward the cooling fluid. On this engine, a double circuit at two temperature levels has been proposed, according to a layout which redistribute in an optimal way the engine and vehicle thermal requirements.

Predicting Fuel Performance for Future HCCI Engines

The purpose of this research is to investigate the impact of fuel composition on auto-ignition in homogeneous charge compression ignition (HCCI) engines in order to develop a future metric for predicting fuel performance in future HCCI engine technology. A single-cylinder, variable compression ratio engine operating as an HCCI engine was used to test reference fuels and gasoline blends with octane numbers (ON) ranging from 60 to 88. Correlations between fuel composition, ON, and two existing methods for predicting fuel auto-ignition in HCCI engines (Kalghatgi's octane index and Shibata and Urushihara's HCCI index) are investigated. Results show that octane index and HCCI index poorly predict the impact of fuel composition on auto-ignition for fuels with the same ON. The effect of ethanol in delaying auto-ignition depends on the composition of the original gasoline blend; the same is true for the addition of naphthenes. Low-temperature heat release (LTHR) correlates well with auto-ignition for gasoline fuels exhibiting LTHR.

Head and block split cooling in ICE

The driver of the future engine technology will be the reduction of the CO2 emissions which looks straightforward to fuel consumption saving. The fulfillment of primary pollutants emission levels requested by EURO 6, LEV III or Japanese standards would still add a new innovation stimulus, but the main concept is a more efficient vehicle.Among the more interesting technologies, those related to a revision of the engine cooling, as well as, more generally, those oriented to fulfill the overall vehicle thermal needs appear very promising in terms of expected fuel saving performances. They have an additional advantage related to the low vehicle's cost increase per unit of CO2 saved (or fuel saved). Thermal management is the common way to present such a technological interest and it resumes many innovations based on the idea that a vehicle is a system and all the thermal needs (of the engine and of the vehicle) should be optimized looking at the overall system and not at specific aspects.One of the main goal of thermal management is to decrease engine warm up time considering the weight of the cold phase inside the most common approval cycles. Benefits on primary pollutants emission reduction are matched with those on fuel saving, with a double positive contribution. Many technologies are already available to reach this aim, but thermal management invites to consider a number of other innovations which have to be exploited.The paper makes use of an engine mathematical model previously developed by the Authors to discuss the performances of a traditional cooling flow passage inside an existing engine and two novel circuits which try to implement a split cooling between engine head and block. If head mean temperature is kept colder and engine block warmer, engine performances can be increased reducing friction losses, warm up time and having the possibility to increase volumetric compression ratio.

The impact of microstructural alterations at spray coated cylinder running surfaces of diesel engines – Findings from motor and laboratory benchmark tests

Future motor engine design and technology demand new materials that meet the requirements in terms of friction and wear properties. In order to reduce weight of the engine block and further more reduce the emissions thermal spray coatings are options to manufacture wear resistant and low-friction cylinder running surfaces. A motor cylinder with a thermally sprayed mild steel coating after a road test is prepared for transmission electron microscopy (TEM) investigations in the positions top dead center (TDC) and bottom dead center (BDC). In the dead centers of engines mixed/boundary lubrication conditions prevail. Within this study a new nanocrystalline thermal spray coating is investigated using two different laboratory tests where mixed/boundary lubrication regime is presumed. Conventional mild steel coatings and the new high alloyed Fe-base coatings are tested against two different piston rings in a reciprocating sliding wear test and an oscillating sliding wear test, known as SRV © test. TEM studies of the surface near microstructure demonstrate the difference in the microstructural alterations and wear mechanisms. Findings from the motor cylinder in surface and subsurface modifications are considered when discussing the results from laboratory tests in terms of initial grain size, influence of lattice defects and chemical influences due to oil additives.

Influence of nozzle geometry on ignition and combustion for high-speed direct injection diesel engines under cold start conditions

Starting at low temperatures (below 0°C) is an important issue for current and near future diesel engine technology. Low ambient temperature causes long cranking periods or complete misfiring in small diesel engines and, as a consequence, an increased amount of pollutant emissions. This paper is devoted to study the influence of nozzle geometry on ignition and combustion progression under glow-plug aided cold start conditions. This study has been carried out in an optically accessible engine adapted to reproduce in-cylinder conditions corresponding to those of a real engine during start at low ambient temperature. The cold start problem can be divided in two parts in which nozzle geometry has influence: ignition and main combustion progress. Ignition probability decreases if fuel injection velocity is increased or if the amount of injected mass per orifice is reduced, which is induced by nozzles with smaller hole diameter or higher orifice number, respectively. Combustion rates increase when using nozzles which induce a higher momentum, improving mixture conditions. For these reasons, the solution under these conditions necessarily involves a trade-off between ignition and combustion progress.

Systems development for future passenger car diesel engines

By setting up a fully indicated, modern passenger car diesel engine, Mahle has created the basis for systematically testing current and future engine technologies. The experimental set-up has both an integrated high- and low-pressure EGR circuit and an open engine control unit, thus enabling efficient development of new systems under real-world conditions. The advantages of this comprehensive approach are illustrated below, using the example of different EGR technologies tested.

Analytical and Numerical Simulations of the Two-Phase Flow Heat Transfer in the Vent and Scavenge Pipes of the CLEAN Engine Demonstrator

Advanced aircraft engine development dictates high standards of reliability for the lubrication systems, not only in terms of the proper lubrication of the bearings and the gears, but also in terms of the removal of the large amounts of the generated heat. Heat is introduced both internally through the rotating hardware and externally through radiation, conduction, and convection. In case where the bearing chamber is in close proximity to the engine’s hot section, the external heat flux may be significant. This is, for example, the case when oil pipes pass through the turbine struts and vanes on their way to the bearing chamber. There, the thermal impact is extremely high, not only because of the hot turbine gases flowing around the vanes, but also because of the hot cooling air, which is ingested into the vanes. The impact of this excessive heat on the oil may lead to severe engine safety and reliability problems, which can range from oil coking with blockage of the oil tubes to oil fires with loss of part integrity, damage, or even failure of the engine. It is therefore of great importance that the oil system designer is capable of predicting the system’s functionality. As part of the European Research program efficient and environmentally friendly aero-engine, the project component validator for environmentally friendly aero-engine (Wilfert, et al., 2005, “CLEAN–Validation of a GTF High Speed Turbine and Integration of Heat Exchanger Technology in an Environmental Friendly Engine Concept,” International Symposium on Air Breathing Engines, Paper No. ISABE-2005-1156;Gerlach et al., 2005, “CLEAN–Bench Adaptation and Test for a Complex Demo Engine Concept at ILA Stuttgart,” International Symposium on Air Breathing Engines, Paper No. ISABE-2005-1134) was initiated with the goal to develop future engine technologies. Within the scope of this program, MTU Aero Engines has designed the lubrication system and has initiated an investigation of the heat transfer in the scavenge and vent tubes passing through the high thermally loaded turbine center frame (TCF). The objective was to evaluate analytical and numerical models for the heat transfer into the air and oil mixtures and benchmark them. Three analytical models were investigated. A model that was based on the assumption that the flow of air and oil is a homogeneous mixture, which was applied on the scavenge flow. The other two models assumed annular two-phase flows and were applied on the vent flows. Additionally, the two-phase flow in the scavenge and vent pipes was simulated numerically using the ANSYS CFX package. The evaluation of the models was accomplished with test data from the heavily instrumented test engine with special emphasis on the TCF. Both the analytical and the numerical models have demonstrated strengths and weaknesses. The homogeneous flow model correlation and the most recent correlation by Busam for vent flows have demonstrated very good agreement between test and computed results. On the other hand the numerical analysis produced remarkable results, however, at the expense of significant modeling and computing efforts. This particular work is unique compared with published investigations since it was conducted in a real engine environment and not in a simulating rig. Nevertheless, research in two-phase flow heat transfer will continue in order to mitigate any deficiencies and to further improve the correlations and the CFD tools.

Insights relating to the autoignition characteristics of alcohol fuels

A detailed chemical kinetic modeling study was undertaken to characterize the auto-ignition behaviour of a full range of blends of both methanol and ethanol with a PRF 80 base fuel. The overall results were encapsulated in a computationally efficient semi-empirical formulation. The auto-ignition model was combined with an ASTM (CFR) engine simulation to explore the underlying causes for the octane number value exhibited by alcohol fuels. The model predictions were validated against experimental pressure traces obtained with a range of alcohol-PRF blends. These results confirmed the model’s general validity and provide valuable insights relating to the controlling role of the cool flame in the octane number determination and the use of alcohol fuels as octane blending components for use in modern and future gasoline engine technologies.

KS-1: Development Trends of Heavy Duty Diesel Engines in View of Future European Legislative Requirements(Keynote Papers)

The proposed new European emission standards (EURO VI) to be in force by 2013/2014 are considered very severe, having a NO_x-standard of 0.4 g/kWh in the European Transient Cycle (ETC) together with a low particulate limit of 0.01 g/kWh. Above limits require significant in-cylinder reductions of pollutants as well as exhaust gas aftertreatment for both, nitrogen oxides and particulates. Furthermore, due to the much discussed climate change, the emissions of greenhouse gases (GHG) are to be reduced. For EURO VI, the first step will be to reduce engine-out emissions by means of advanced combustion process development using improved fuel injection equipment, supercharging, exhaust gas recirculation, flexible engine systems, extended engine control and by the mechanical development. Then it would appear that exhaust gas aftertreatment systems are needed. Since approximately 30 % of the fuel input is lost in the exhaust gas, an exhaust gas energy recuperation might be a step forward to reduce fuel consumption and GHG-emissions. In a second part an overview of the EU R&D-project GREEN will be given. In this project, European HD engine manufacturers join forces with suppliers, academia and leading engineering institutes. The common goal is to promote future advanced engine technologies to achieve lower emissions, lower fuel consumption and improved sustainability for future fuels. The main objective of GREEN is to perform research, which will lead to innovative sub-systems for heavy duty engines. The project puts emphasis on diesel engines for trucks and rail applications and on natural gas engines for city transport applications.

Heavy Duty Diesel Engine Coolant Technology: Past, Present, and Future

Abstract Significant advances have been made in heavy duty diesel engine technology to meet increasingly stringent environmental regulations for emissions. Today's heavy duty diesel engines are being designed with lighter and softer metals, greater turbocharging, increased combustion controls, and new emission reduction equipment. The cooling systems contained in these vehicles are similarly being impacted by smaller designs, new cooling system configurations, and increased usage of lighter, softer metals. Vehicle thermal loads have significantly increased due to increased power densities, higher engine temperatures, and greater metal-coolant fluxes which places greater emphasis on oxidation/thermal stability, and high temperature corrosion protection performance of the coolant. Other operating conditions (coolant flow rates, turbulence, pressure drops, deaeration) are also becoming more severe calling for improved erosion-corrosion protection, cavitation protection, and elastomer, seal, hose compatibility. This paper reviews the changes in heavy duty diesel engine technology and provides information on coolant performance in 2002-4 emission compliant engines. Predictions are also made on future engine technology and next generation engine coolants.

Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors: Near Wall Reaction Effects on Film-Cooled Backward-Facing Step Heat Transfer

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions to occur downstream of the combustor is increased. This may take place when partially reacted species leave the combustor and encounter film-cooled surfaces. One common feature on turbine endwalls is a step between various engine components and seals. Such step features produce recirculating flows which when in the vicinity of film-cooled surfaces may lead to particularly severe reaction zones due to long fluid residence times. The objective of this paper is to study and quantify the surface heat transfer implications of such reacting regions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm) flows over a film-cooled backward-facing step. The test article contained two sets of 35 deg film cooling holes located downstream of a step. The film-cooling holes could be supplied with different gases, one side using air and the other nitrogen allowing for simultaneous testing of reacting and inert cooling gases. A mixture of ethylene and argon provided a fuel-rich free stream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5–2.0), and Damköhler numbers (ratio of characteristic flow time to chemical time) from near zero to 30. The experimental results show that for conditions relevant for future engine technology, adiabatic flame temperatures can be approached along the wall downstream of the step leading to potentially significant increases in surface heat flux. A computational study was also performed to investigate the effects of cooling-jet blowing ratio on chemical reactions behind the film-cooled step. The blowing ratio was found to be an important parameter governing the flow structure behind the backward-facing step, and controlling the characteristics of chemical-reactions by altering the local equivalence ratio.

Potential of interactions between fuels and future powertrains

A large part of the world’s natural gas reserves is located in areas where domestic demand is small. These remote gas fields can be developed by converting the natural gas to synthetic diesel fuel via the Shell Middle Distillate Synthesis (SMDS) process, a modern version of the Fischer-Tropsch process. This process produces high quality, environmentally friendly diesel fuel. Simultaneous exhaust emission reductions of 50 % for particulates and 20 % for NOx have been found in Volkswagen Bora diesel vehicles with SMDS fuel compared to standard diesel. Due to the flexibility of the SMDS process, fuels can be tailored to meet the specific needs of future engine technologies like the Combined Combustion System, a homogeneous combustion technology developed by Volkswagen.

Future engine technologies for passenger cars

Due to environmental concerns, exhaust emissions and fuel consumption of vehicles will have to improve considerably in the next 10 years. New technologies for gasoline engines such as downsizing with high boosting, direct injection and fully variable valve train systems are being developed. For diesel engines, improved components including piezoelectric injectors, particle filters and DeNOx catalyst systems are expected.

Honda’s current and future engine technologies

Serious air pollution problems in major cities and global warming concerns have led to proposals to replace the internal combustion engine in vehicles. However, in order to commercialise new power units, major issues such as fuel infrastructure and cost must be resolved.