1,721,054 research outputs found
Experimental Activity on a Hydrogen Fuelled S.I. Engine with Two-Step D.I
No full textAn innovative hydrogen DI system was conceived, realized and tested that requires only 12 bar rail pressure, typical value of
PFI systems, and does not need special injectors. The purpose is to combine the well known benefits of DI with the ones of
PFI. The injection is accomplished in two steps: at first hydrogen, metered by an electroinjector (a conventional one for CNG
application), enters a small intermediate chamber; then it is injected into the cylinder by means of a mechanically-actuated
valve that allows very high flow rate (compared with the one of electroinjectors). In-cylinder injection starts at intake valve
closing (an earlier injection start could lead to backfire) and stops early enough to allow proper charge homogeneity and, in
any case, before cylinder pressure rise constrains hydrogen admission.
The prototype engine was realised modifying a production single-cylinder 650 cm3 engine with three intake valves. The central
one was modified and properly timed to inject hydrogen into the cylinder from the intermediate chamber.
The experimental results satisfied the expectations. The prototype engine ran properly at full load, without pre-ignition,
knocking or roughness even with stoichiometric or slightly rich mixtures, providing higher maximum power than with
gasoline. Also at part load the engine proved to work correctly even with very lean mixtures (till λ~4), reaching its maximum
brake thermal efficiency with λ ~ 2.4
Comparison of different system layouts to generate a substitute of natural gas from biomass and electrolytic hydrogen
No full textThe production of electrolytic hydrogen is considered among the best solution to mitigate the grid instability problems which arise from the widespread distribution of renewable energy sources, such as wind and solar. However, hydrogen is not easy to stock and distribute. Possible solutions are represented by its direct injection into the existing pipeline for natural gas distribution or its utilisation for the production of a substitute of natural gas. In this last case, which follows the so called approach of “power to gas”, a source of carbon is required. Preferably the carbon should come from biomass, since it can be considered “renewable carbon”. Starting from this idea, this study analyses two different approaches, depending on the grid power demand. In a first layout, biomass is gasified with electrolytic hydrogen to generate directly a methane rich syngas. After water condensation, the syngas is fed to a methanation process to convert almost completely carbon in methane. In the second layouts the biomass is gasified with electrolytic oxygen and the syngas is fed, together with other electrolytic oxygen, to a power unit, such as an internal combustion engine, a gas turbine or a high temperature fuel cells (SOFC). The exhaust gas from these power units is composed almost exclusively by carbon dioxide and water vapour. After water condensation, the carbon dioxide is fed together electrolytic hydrogen to a methanation process to obtain the substitute of natural gas. An overall best efficiency of roughly 74% is obtained when the plant is not connected to the grid. On the contrary, when electricity can be absorbed by the grid, best efficiency of 59.4% is reached utilising, as power unit, a SOFC fed at 6 bars. In all cases the input is low value energy (biomass and unstable electric power) and the output is high value energy constituted by a substitute of natural gas and stable electric power
What is the Future of the Two-Stroke S.I. Engine?
No full textThe conventional two-stroke S.I. engine is simple and low-cost, however its scavenging process is unfavourably characterised by the loss of a great part of the fresh charge from the exhaust port. Besides, bad combustion and/or misfire occur at light loads, because of the excessive ratio of residual gas to fresh gas within the cylinder. These drawbacks, because of the oncoming new limits for exhaust emission, might cause the death of this engine in few years, at least in Europe and in the other developed countries.
However, innovative solutions are expected saving this engine. The main one is direct fuel injection, which allows to avoid fuel short circuiting to the exhaust port during scavenging, even in two stroke engines of simple construction with loop scavenging, since the cylinder is scavenged with air alone. Moreover, some fuel injection systems can generate charge stratification, thus solving the problems occurring at light loads or, in the case of air-assisted fuel injection which is not able to perform late injection, the adoption of ATAC combustion should be the solution for light loads problems.
Thanks to direct fuel injection and to their intrinsic internal EGR (which gives rise to low CO and NOX emissions), the small two-stroke engines might satisfy the regulation ECE 2002 even without catalytic converter (the four-stroke ones need the converter).
The increasing diffusion of direct-injection, stratified-charge induces to consider evolved two-stroke engines in the automobile field too, since, on the one hand, direct injection and charge stratification equalise four-stroke and two-stroke-engines as regards exhaust emissions and, on the other hand, two-stroke engines have remarkable advantages of torque regularity and availability at low engine speeds. Of course, in this case, valved two-stroke engines without cylinder ports should be considered, to reduce cylinder wear and thermal deformation and to decrease lubricant
HYDROGEN GENERATION SYSTEM FOR AMMONIA-HYDROGEN FUELED INTERNAL COMBUSTION ENGINES
No full textAmmonia is a well known hydrogen carrier which can be effectively utilized as a fuel in internal combustion engines (ICEs) when a small percentage of other fuels are added as combustion promoters. Among them, hydrogen is certainly the most valuable since it is carbon free and has opposed and complementary characteristics to those of ammonia.
In this work, a Hydrogen Generation System (HGS) capable of supplying up to 1.4 Nm3 h-1 of H2 from ammonia was successfully developed and coupled to an ICE fuelled with ammonia. The main component of the HGS is a cracking reactor housing a ruthenium based catalyst. This system is capable of working both in a stand-alone mode (as required in vehicular applications for the cold start) and in combination with a spark ignited (SI) ICE (i.e. using the combusted gases exhausted by the engine). Beside the cracking reactor, an integrated system was designed and realized in order to allow system cold start and increase overall efficiency during steady state operations.
The engine experimental activity confirmed the reactor performance, which was previously verified on a dedicated test bench. Although a lower hydrogen flow rate could be used to achieve satisfactory engine operation, a greater value was used during engine experimentation with benefits for fuel economy and engine cyclic variability. On the other hand, this choice led to higher NOx emissions
STUDIO DI UN SISTEMA DI CAPTAZIONE SOLARE A CONCENTRAZIONE CON TORRE ED ELIOSTATI IN TOSCANA
No full textNell’ambito delle attività di ricerca volte a valutare le effettive possibilità di diffusione delle fonti rinnovabili, e in particolare del solare a concentrazione (CSP), il presente lavoro descrive lo studio di un sistema di captazione solare a concentrazione basato su torre ed eliostati da installare nel territorio toscano. Il sistema è costituito da un campo di eliostati con inseguimento su due assi che concentrano tramite specchi la radiazione solare su un ricevitore posto alla sommità di una torre così da riscaldare un appropriato fluido termovettore. Lo studio, svolto avvalendosi di un strumento informatico appositamente sviluppato in ambiente Matlab, ha avuto come scopo ultimo la definizione della configurazione che massimizza l’efficienza di captazione.
A tale scopo, dopo aver caratterizzato il sito prescelto in termini di disponibilità solare e aver definito i movimenti orari di inseguimento biassale di ciascun eliostato così da garantire la concentrazione della radiazione solare diretta sul ricevitore posto sulla sommità della torre, è stata valutata l’influenza sulle prestazioni del sistema di captazione dei suoi parametri geometrici caratteristici, quali l’altezza della torre, il numero di eliostati, il posizionamento di ciascuno di essi all’interno del campo di captazione (layout). L’efficienza mostra un massimo rispetto all’altezza della torre ed è maggiore per un numero minore di eliostati. I campi specchi con le prestazioni migliori sono risultati quelli con rapporto tra la dimensione nord-sud e quella est-ovest prossimo all’unità.
E’ stato infine effettuato il dimensionamento di un sistema di captazione per il sito prescelto con un’efficienza superiore al 60% e con una quantità di energia termica fornita annualmente al fluido termovettore pari a circa 200 kWh per ogni metro quadrato di superficie occupata dal campo. Se fosse ubicato in Sicilia, tale sistema di captazione risulterebbe possedere prestazioni superiori di circa il 50%
Progress in Two-Stroke S.I. Engines
No full textThanks to its lightness, small dimensions and attractive simplicity, the two-stroke, S.I. engine may be potentially preferable to the four-stroke one for city cars and other small vehicles.
However, the loss of fresh mixture from the exhaust port, together with bad combustion and/or misfire at light loads constitute great limitations.
Our researches directed to solve these problems concern the development of an air-assisted, low-pressure, pumpless injection systems, which bypasses the trapping losses, and the solution of combustion problems at light loads by means of charge stratification and innovative ignition systems
Numerical and Experimental Analysis on a Small GDI, Stratified Charge, Motorcycle Engine
No full textIn the field of engines for light motorcycles, two-stroke
cycle survival is submitted to the application of direct fuel
injection and charge stratification, even in the case of low-cost
small engines. However, charge stratification is a difficult
target in two-stroke engines, chiefly because timings of late
injection (necessary for charge stratification) and of early
injection (necessary for homogeneous charge) are much closer
than in four-stroke engines. The compatibility between
stratified and homogeneous charge operations needs a thorough
CFD study of injection and mixing processes, with the support
of techniques of spray visualization. Results strongly depend on
the possibility of optimising the interaction between in-cylinder
gas-dynamic field and spray; experimental activity is necessary
as data source and verification of computational prediction.
This paper shows the latest CFD investigation, experimental
tests and results concerning a 50 cm3 engine for light
motorcycles. The injection is of the liquid type with wall-andair
guided spray produced by a swirl injector. The research has
been focused on the attainment of charge stratification at every
engine speed. Spray actual characteristics have been
investigated, attesting suitable repeatability and proper
variation versus backpressure. Engine satisfactory behaviour
even at light loads in unthrottled condition is proved by good
fuel economy and engine stability in dynamometric bench tests.
Exhaust gas analysis and indicated pressure behaviour confirm
stratification and combustion correctness
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