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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.

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 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.