Evaluation of the Potential Benefits of an Automotive, Gasoline, 2-Stroke Engine 2015-01-1261
In the present paper, the use of a 2-stroke (2S) concept in an automotive gasoline engine is evaluated. In a first stage, the engine architecture chosen is discussed. Taking into account the requirements in gas exchange processes, a uniflow scavenging design was retained (intake ports in the cylinder, controlled by the piston; exhaust valves in the cylinder head, controlled by a Variable Valve Timing, VVT, system), performed by an external blower driven by the crankshaft. To avoid any fuel short-circuiting and to keep an acceptable cost, a direct injection (DI) air-assisted fuel injection system was selected. Since the engine behavior is much more complex compared to a classical 4-stroke engine, some complexity in the engine design needs to be added to allow engine optimization at the different operating conditions. This is the main reason why a VVT system, as well as a flexible fuel injection system were selected.
In a second stage, the chosen engine concept is evaluated. At high loads, because of the high quality of the scavenging process, the combustion initiation is controlled by a spark as in any standard spark ignition (SI) engine. However, at low load, the scavenging process is incomplete, and the high amount of residual gases leads, in some cases, to a controlled autoignition (CAI) combustion process. These two completely different scenarios are analyzed in the paper. In summary, on the one hand, the operation under SI conditions is basically similar to that of any classical 4-stroke, SI engine, but with higher knock risk. On the other hand, the operation in CAI leads to a faster combustion process, which might lead to higher fuel efficiency. However the control of the combustion process is more complex, since it is more sensitive to the operating parameters (air temperature, combustion chamber walls temperature…), and not fully controlled by the spark anymore. A significant effort has been carried out in the paper to understand how the combustion process can be controlled, and some ideas for such control are proposed and discussed.