Search Results

With growing concern about global warming, alternatives to fossil fuels in internal combustion engines are searched. In this context, hydrogen is one of the most interesting fuels as it shows excellent combustion properties such as laminar flame speed and energy density. In this work a CFD methodology for 3D-CFD in-cylinder simulations of engine combustion is proposed and its predictive capabilities are validated against test-bench data from a direct injection spark-ignition (DISI) prototype. The original engine is a naturally aspirated, single cylinder compression ignition (Diesel fueled) unit. It is modified substituting the Diesel injector with a spark plug, adding two direct gas injectors, and lowering the compression ratio to run with hydrogen fuel. A 3D-CFD model is built, embedding in-house developed ignition and heat transfer models besides G-equation one for combustion.

Despite the continuous efficiency increase, traditional vehicles still dissipate large amounts of energy. While actual hybrid powertrains allow to recover kinetic energy during the braking phases, exhaust gas energy can be recovered through the electric turbo compound (ETC). A turbocharger (TC) match with an electric machine (EM) appears to be the best solution for passenger cars since it also permits turbo lag reduction with restricted system complexity increase. The goal of this paper is to investigate the TC electrification impact on fuel consumption, considering transient behavior during a whole drive cycle simulation. This is obtained coupling a detailed one-dimensional (1-D) engine model with vehicle dynamics and signal processing by means of sole GT-Suite software. A baseline engine refers to a 1.3 L, SI, 3-cylinder turbocharged engine while a class C vehicle is modelled.

To downsize a spark ignited (SI) internal combustion engine (ICE), keeping suitable power levels, the application of turbocharging is mandatory. The possibility to couple an electric drive to the turbocharger (electric turbo compound, ETC) can be considered, as demonstrated by a number of studies and the current application in the F1 Championship, since it allows to extend the boost region to the lowest ICE rotational speeds and to reduce the turbo lag. As well, some recovery of the exhaust gas residual energy to produce electrical energy is possible. The present paper shows the first numerical results of a research program under way in collaboration between the Universities of Pisa and Genoa. The study is focused on the evaluation of the benefits resulting from the application of ETC to a twin-cylinder small SI engine (900 cm3).

Storing hydrogen is one of the major issues concerning its utilization on board vehicles. A promising solution is storing hydrogen in the form of ammonia that contains almost 18% hydrogen by mass and is liquid at roughly 9 bar at environmental temperature. As a matter of fact, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen itself, thus involving relatively small volumes and light and low-cost tanks. It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support and speed up combustion especially in the case of high-speed S.I. engines. The best promoter is hydrogen, due to its opposed and complementary characteristics to those of ammonia, Hydrogen has high combustion velocity, low ignition energy and wide flammability range, whereas ammonia has low flame speed, narrow flammability range, high ignition energy and high self-ignition temperature.

Current energy policies are encouraging the near-term use of fuels derived from civil and industrial waste residues, giving new perspectives for their disposal. The possibility of using, in Diesel engines, a liquid fuel derived from waste synthetic polymeric matrices, such as scrap tyres, is evaluated in this paper. The fuel is obtained by means of an innovative technology based on a thermo-mechanical cracking process at moderate temperatures and pressures. A preliminary investigation was carried out on a 440 cm3 single-cylinder Diesel engine for stationary applications using a commercial automotive Diesel fuel (UNI-EN 590:2010) and two mixtures of automotive Diesel fuel and tyre pyrolysis oil (TPO): the first one containing 20% TPO by volume, the other containing 40% TPO.

Due to concerns regarding pollutant and CO2 emissions, advanced combustion modes that can simultaneously reduce exhaust emissions and improve thermal efficiency have been widely investigated. The main characteristic of the new combustion strategies, such as HCCI and LTC, is that the formation of a homogenous mixture or a controllable stratified mixture is required prior to ignition. The major issue with these approaches is the lack of a direct method for the control of ignition timing and combustion rate, which can be only indirectly controlled using high EGR rates and/or lean mixtures. Homogeneous Charge Progressive Combustion (HCPC) is based on the split-cycle principle. Intake and compression phases are performed in a reciprocating external compressor, which drives the air into the combustor cylinder during the combustion process, through a transfer duct. A transfer valve is positioned between the compressor cylinder and the transfer duct.

A partnership of Research and Industry entities has developed a fully working hybrid electric vehicle equipped with a 15 kW IC engine fuelled with liquid ammonia as range extender of the lithium batteries pack on board. All the vehicle powertrain, i.e., the IC engine, the electric generator coupled with the engine, the electric motor, the electronics, etc., has been studied and designed. Regarding the IC engine, as known, ammonia combustion is characterized by a high activation energy and a low flame velocity, therefore a small amount of hydrogen is requested as igniter and combustion promoter. The necessary amount of hydrogen is coming from the thermal decomposition of the same ammonia by cracking it at approximately 400°C inside a special catalyst (purposely realized within the project) heated by the engine exhaust gasses. The Nitrogen and Hydrogen mixture coming from the cracker is introduced, by an indirect injection, into the intake manifold together with the ammonia.

Storing hydrogen is one of the major problems concerning its utilization on board vehicles. Today hydrogen can be compressed and stored at 200 or 350 bar (it is foreseen that in a near future storage pressure will reach 700 bar, according to new expected regulations and using tanks in composite materials) or cryogenically liquefied. An alternative solution is storing hydrogen in the form of ammonia that is liquid at roughly 9 bar at environmental temperature and therefore involves relatively small masses and volumes and requires light and low-cost tanks. Moreover, ammonia contains almost 18% hydrogen by mass and, by volume, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen. It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support combustion especially in the case of high-speed S.I. engines.

An 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 realized modifying a production single-cylinder 650 cm₃ engine with three intake valves.

The paper describes the CFD analysis, the arrangement and the first experimental results of a single-cylinder engine that employs an innovative low-pressure hydrogen direct-injection system, characterized by low fuel rail pressure (12 bar) and consequent low residual storage pressure. The injection is split in two steps: at first hydrogen is metered and admitted into a small intermediate chamber by an electroinjector (a conventional one usually employed for CNG), next a mechanically actuated poppet valve, that allows high volumetric flow rates, times hydrogen injection from the intermediate chamber to the cylinder within a short time, despite the high hydrogen volume due to the low injection pressure. Injection must be properly timed to maintain pressure below 6 bar (or little more) in the intermediate chamber and thus keep sonic flow through the electroinjector, to maximize volumetric efficiency and to avoid backfire in the intake pipe.

Two-stroke S.I. engine survival is submitted to direct fuel injection and charge stratification. An exhaustive activity concerning a 50 cm3 two-stroke S.I. engine with liquid direct injection and charge stratification has given really satisfactory results as regards engine aptitude to operate unthrottled at every speed and load. However, unthrottled operation does not necessarily lead to the best overall result. By CFD investigation and experimental tests, this paper proves that some throttling reduces HC and NOx emissions as well as pumping loss and increases exhaust gas temperature at light loads, with evident advantage for catalytic converter efficiency.

High-pressure liquid fuel injection is a suitable means to get either stratified charge or homogeneous charge for two-stroke engines. This paper shows the development of this solution for a small 50 cm3 engine for light motorcycles. By means of computational fluid dynamics, a combustion chamber suitable for proper fuel distribution in every engine operating condition has been designed. It has been realized, and experimental results confirm its fairly satisfactory behaviour, with good fuel economy, low exhaust emissions and small cycle-to-cycle variation even at light loads. Recent CFD studies indicate how to improve engine geometry to achieve a better stratification stability at partial loads independently on engine speed.

Direct fuel injection is becoming mandatory in two-stroke S.I. engines, since it prevents one of the major problems of these engines, that is fuel loss from the exhaust port. Another important problem is combustion irregularity at light loads, due to excessive presence of residual gas in the charge, and can be solved by charge stratification. High-pressure liquid fuel injection is able to control the mixing process inside the cylinder for getting either stratified charge at partial loads or quasi-stoichiometric conditions, as it is required at full load. This paper shows the development of this solution for a small engine for moped and light scooter, using numeric and experimental tools. In order to obtain the best charge characteristics at every load and engine speed, different combustion chambers have been conceived and studied, examining the effects of combustion chamber geometry, together with injector position and injection timing

The paper deals with experimental activity concerning ATAC, which, in two-stroke gasoline engines, helps solving the crucial problem of combustion instability at light loads. ATAC consists of employing the energy of residual gas to prime an efficient combustion. The research is aimed to give further insight into ATAC mechanism both by visualisation of the combustion process and by examination of the influence which relevant parameters like air-fuel ratio, engine speed, compression ratio, scavenging passage design have on ATAC operation. Several results have been acquired and collected hitherto. A part of them are shown and discussed in this paper.