An opposed piston two stroke diesel engine may have advantages over a four stroke diesel including reduced mechanical complexity, reduced displacement for equivalent power, and reduced fuel consumption. However, a four stroke diesel engine may provide useful engine braking to reduce or maintain vehicle speed when driver demand torque is low. The four stroke engine provides engine braking torque (e.g., a torque that acts to slow vehicle powertrain or driveline rotation, which may be referred to as a negative torque) when exhaust manifold pressure is increased to a greater pressure than engine intake manifold pressure, thereby creating a positive pressure change in pressure across the engine (e.g., between the cylinder intake valves and exhaust valves). The positive pressure across the engine increases engine pumping work to provide engine braking. However, a two stroke engine requires intake manifold pressure to be greater than exhaust manifold pressure to ensure air flow through the engine. Further, engine pumping work of a two stroke engine is not affected by boosting a two-stroke engine. Therefore, engine braking from engine pumping work of a two stroke engine is not increased or decreased when the two stroke engine is boosted. Yet, engine braking may be desirable to control vehicle speed whether the vehicle includes a four stroke or two stroke engine. Therefore, it may be desirable to provide a way of engine braking for a two stroke opposed piston engine.
The inventor herein has recognized the above-mentioned disadvantages and has developed a two stroke diesel engine braking method, comprising: completing cycles of all engine cylinders in two revolutions of an engine; and continuously adjusting engine braking torque of the engine without increasing engine pumping work of the engine via continuously adjusting a drive ratio of the mechanically driven supercharger in response to a request for engine braking.
By continuously adjusting flow through a mechanically driven supercharger, it may be possible to provide the technical result of providing engine braking via a two stroke engine without increasing engine pumping work. In particular, work performed by the mechanically driven supercharger may be adjusted via adjusting a drive ratio (e.g., a ratio of an actual total number of rotations of an input shaft of a supercharger compressor to an actual total number of rotations of an output shaft of the supercharger compressor. And, since the mechanically driven supercharger is part of the engine and coupled to the engine crankshaft, engine braking may be increased or decreased via adjusting the drive ratio of the mechanically driven supercharger. The drive ratio of the mechanically driven supercharger may be adjusted via a hydraulic actuator responsive to desired engine braking torque. In addition, an exhaust valve may be closed to further increase engine braking torque if engine braking torque is less than desired engine braking torque and desired engine braking torque may not be achieve via adjusting the supercharger drive ratio alone.
The present description may provide several advantages. Specifically, the approach may allow a two stroke diesel engine to supply engine braking. Further, the amount or level of engine braking may be adjusted proportionately in response to an engine braking request. Further still, the approach may be realized with more than a single hardware configuration. Additionally, by not increasing engine pumping work while performing engine braking, it may be possible to more precisely control engine braking since changes in engine pumping work may not have to be determined to control engine braking.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.