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A low-pressure hydrogen direct-injection solution is presented that allows some typical benefits of direct injection, such as high specific power and backfire prevention, plus low residual storage pressure, that improves vehicle range and is a typical advantage of external mixture formation. Since the injection must end early enough to allow good charge homogeneity and, in any case, before in-cylinder pressure rise constraints hydrogen admission, especially at heavy loads hydrogen flow to the cylinder is higher than present electro-injectors allow. The injection is realised in two steps: hydrogen flow rate is simply controlled by a conventional CNG electro-injector that feeds a small intermediate chamber. From this chamber hydrogen next enters the cylinder in a short crank angle period by means of a mechanically-actuated valve that opens at the intake valve closure to avoid backfire.

Direct fuel injection combined with charge stratification represents an important upgrading for S.I. engines. In the case of two-stroke engines, it prevents fuel loss from the exhaust port and incomplete combustion or misfire at light loads. In the case of four-stroke engines, it increases power and, especially at light loads, fuel economy. To obtain the best results, stratification should be kept stable when engine operating conditions change, fuel dilution and spreading in consequence of burnt gas expansion should be prevented especially at light loads and fuel impingement on combustion chamber walls should be avoided. In the case of two-stroke engine, air-and-wall guided spray represents a suitable solution owing to its affordable cost. For four-stroke engine, self-guided spray is preferable, since it avoids fuel impingement on piston surface.

This paper shows a numerical and experimental study concerning liquid high-pressure direct injection with charge stratification in a 200 cm3 two-stroke motorcycle engine. Numerical predictions have been performed using KIVA-3v code with different combustion-chamber shapes and spray characteristics. Experimental outcomes have substantially confirmed the predictions of computation and proved that injector characteristics are of the utmost importance to achieve satisfactory results.

In two-stroke S.I. engines, direct fuel injection allows avoiding fuel loss from the exhaust port, since the cylinder is scavenged only with air. However, only if injection produces charge stratification, also combustion difficulties at light loads, due to the excessive presence of residual in the cylinder, can be removed. An alternative solution for this problem is ATAC (Active Thermo Atmosphere Combustion), which turns the effect of residual gas from negative to positive, since its thermal energy is used to prime the combustion of fresh gas. ATAC leads to very good combustion stability with small cycle-to-cycle variation and to good fuel economy and low exhaust emissions of unburned hydrocarbons. This paper deals with ATAC combined with direct fuel injection, both of the air-assisted medium-pressure type and of the liquid high-pressure type.

Direct fuel injection is becoming mandatory for two-stroke S.I. engines, since it allows using only air in the scavenging process, preventing fuel loss from the exhaust port. However, also combustion irregularity at light loads is a problem of these engines, due to excessive presence of residual gas in the charge. For its solution, additional strategies must be adopted. The two most suitable ones are charge stratification and ATAC (Active Thermo Atmosphere Combustion), which makes use of residual gas energy to ignite fresh gas, turning the effect of residual gas from negative to positive. In previous papers, the possibility of combining ATAC with different injection systems was proved. This paper provides further insight on ATAC combined with liquid high-pressure direct injection and shows the studies and the first experimental results relative to charge stratification.

In two-stroke S.I. engines, direct fuel injection prevents fuel loss from the exhaust port, since only air is employed for the scavenging process. However, to solve the problem of combustion irregularity at light loads due to excessive presence of residual gas in the charge, fuel injection should also produce charge stratification. An alternative to stratification is ATAC (Active Thermo Atmosphere Combustion), which turns the effect of residual gas from negative to positive, since residual gas energy is exploited to prime the combustion of fresh gas. For the first time, the feasibility of ATAC combined with liquid high-pressure direct injection is proved in this paper. To illustrate the compatibility of ATAC with fuel injection, ATAC behaviours are shown in the cases of liquid high-pressure direct injection, air-assisted medium-pressure direct injection and indirect injection.

In two-stroke S.I. engines, direct fuel injection prevents fuel short-circuiting from the exhaust port, however it does not solve per se combustion problems at light loads due to excessive ratio of residual-to-fresh gas. These problems can be solved by ATAC (Active Thermo Atmosphere Combustion), since residual-gas thermal energy is used to prime the combustion of fresh gas. Experimental results of a small two-stroke S.I. engine with medium-pressure air-assisted fuel injection, operating on ATAC at light loads are shown and prove the possibility to combine the two solutions.

A study was performed that takes into account both turbulence and chemical kinetic effects in the numerical simulation of diesel engine combustion in order to better understand the importance of their respective roles at changing operating conditions. An approach was developed which combines the simplicity and low computational and storage requests of the laminar-and-turbulent characteristic-time model with a detailed combustion chemistry model based on well-known simplified mechanisms. Assuming appropriate simplifications such as steady state or equilibrium for most of the radicals and intermediate species, the kinetics of hydrocarbons can be described by means of three overall steps. This approach was integrated in the KIVA-II code. The concept was validated and applied to a single-cylinder, heavy-duty engine. The simulation covers a wide range of operating conditions.

The paper describes an experimental study concerning the feasibility of using bio-oil obtained from flash pyrolysis of wood for fuelling diesel power plants. The research is based on various tests aimed at verifying relevant operative characteristics of the fuel: spray analyses, engine tests, thermogravimetric analyses (TGA), single-drop reactor tests and corrosion tests. The spray analyses show that the achievement of a satisfactory atomisation with flash-pyrolysis oil is problematic. The engine experimentation shows that flash-pyrolysis oil needs to be modified or mixed (e.g. with alcohol) to make self ignition possible. Besides, unacceptable build-up of carbonaceous deposits, injection system clamping and engine seizure occur. Very large char generation is the main finding of the tests in the TGA apparatus and in the single-drop atmospheric reactor (“drop-tube”). The corrosion tests demonstrate that steel undergoes fast erosion by contact of flash-pyrolysis oil.

The study and the evolution of a simple air-assisted injection system for two-cycle, S.I. engines is described. In the first stage of the research the validity of the system was investigated. During this time the system underwent several improvements and modifications. Later, the research was carried on with the following aims: a) to distinguish the spray characteristics by means of a high speed photographic technique; b) to investigate the dependence of such characteristics on some parameters of the injection system, and their influence on engine performance, with special regard to fuel economy and exhaust emissions. Finally, the case of fuel vaporisation at the outlet of the electroinjector was investigated. In fact, vaporisation and ideal atomisation are similar under some aspects and the knowledge of b.s.f.c. and HC emission in the limit case of perfect atomisation would give a precise idea of the improvements obtainable through betterments in spray characteristics.

The study and the evolution of a simple air-assisted injection system for two-cycle, S.I. engines is described. In the first stage of the research the validity of the system was investigated. During this time the system underwent several improvements and modifications. Later, the research was carried on with the following aims: a) to distinguish the spray characteristics by means of a high speed photographic technique; b) to investigate the dependence of such characteristics on some parameters of the injection system, and their influence on engine performance, with special regard to fuel economy and exhaust emissions. Finally, the case of fuel vaporisation at the outlet of the electroinjector was investigated. In fact, vaporisation and ideal atomisation are similar under some aspects and the knowledge of b.s.f.c. and HC emission in the limit case of perfect atomisation would give a precise idea of the improvements obtainable through betterments in spray characteristics.