Turbocharged engines utilize a turbocharger to compress intake air and increase the power output of the engine. A turbocharger may use an exhaust-driven turbine to drive a compressor which compresses intake air. As the speed of the compressor increases, increased boost is provided to the engine. Upon receiving an increased torque demand, it may take an amount of time for the turbine and compressor to speed up and provide the required boost. This delay in turbocharger response, termed turbo lag, may result in a delay in providing the demanded engine power.
Some attempts to address turbo lag and engine torque response delays include positioning a second compressor in series with the compressor of the turbocharger in the engine intake. In one example, the second compressor may be an electric assist compressor, and may provide extra boost to intake air while the compressor of the turbocharger speeds up. Another system to reduce engine torque response delays includes utilizing a dual turbocharger arrangement in which two turbochargers are arranged in series along the induction path.
However, the inventors herein have recognized potential issues with such systems that include two compressors in series along the induction passage. For example, the downstream compressor when turned on, may create a vacuum in the intake volume between the two compressors. Specifically, the downstream compressor may speed up more quickly than the first compressor, and as a result a vacuum may be produced between the upstream and downstream compressors. Thus, the pressure at an outlet of the upstream compressor may become so reduced due to the suction generated by the downstream compressor, that the pressure at an inlet of the upstream compressor (e.g., barometric pressure) may actually exceed the pressure at the outlet. Thus, a negative pressure differential may develop across the upstream compressor, which may lead to oil leakage through seals of the compressor, resulting in degradation of the compressor.
In one example, the issues described above may be addressed by a method comprising, in response to a desired engine torque increasing above a threshold: powering on a second compressor positioned downstream of a first compressor in an intake of an engine system, and opening a compressor recirculation valve (CRV) positioned in a bypass passage coupled across the first compressor for a duration, and routing a portion of intake gasses from upstream of the first compressor to the second compressor. In this way, by opening the CRV and providing a source of air to the second compressor from upstream of the first compressor, an amount of vacuum generated at an outlet of the first compressor by the second compressor may be reduced. Specifically, by opening the CRV, the second compressor may draw in air from upstream of the first compressor, thereby reducing an amount of air drawn from the outlet of the first compressor.
In some examples, the CRV may be maintained in an open position until a speed of the first compressor reaches a first threshold speed, where the first threshold speed corresponds to a speed of the first compressor sufficient to generate a pressure greater than barometric pressure at an outlet of the first compressor, with the second compressor powered on. Thus, by keeping the CRV open until the first compressor has reached the threshold speed, the pressure at the outlet of the first compressor may be maintained above a pressure at an inlet of the first compressor.
It yet further examples, a boost assist valve positioned in a passage coupled across the second compressor may be opened in response to a pressure in the intake downstream of the second compressor increasing above a pressure in a volume of the intake included upstream of the second compressor and downstream of the first compressor. Thus, by opening the boost assist valve when the second compressor is powered on, intake gasses compressed by the second compressor may be recirculated to upstream of the compressor, in a volume of the intake included between the first and second compressors. In this way, the pressure in the volume of the intake include between the first and second compressor may be increased, and as such vacuum at the outlet of the first compressor may be reduced. Thus, oil leakage across seals of the first compressor may be reduced.
In another representation, a method may comprise, in response to a tip-in from a vehicle operator: opening a compressor bypass valve positioned in a passage coupled across a first compressor, powering on a second compressor positioned downstream of the first compressor in an intake of an engine system, and closing the compressor bypass valve in response to a pressure at an outlet of the first compressor increasing above a threshold pressure.
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.