The compression ratio of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine. In conventional engines, the compression ratio is fixed and thus the engine efficiency cannot be optimized during operating conditions to improve fuel economy and engine power performance.
Various technologies have been developed to enable the compression ratio of an engine to be varied with engine operating conditions. One example approach is shown by Yoshida et al. in U.S. Pat. No. 7,258,099. Therein, cam timing adjustments are used to vary the effective compression ratio. For example, a late intake valve closing is used to reduce the effective compression ratio. Still other approaches, such as shown by Kamada et al. in US20130055990, rely on a piston displacement changing mechanism that moves the pistons closer to or further from the cylinder head, thereby changing the size of the combustion chambers.
However the inventors herein have recognized potential issues with such approaches. As one example, the optimal fuel economy gain associated with adjusting a compression ratio may not be realized due to the fixed gear ratio of the transmission. In particular, at a given driver demand, for each compression ratio of the engine, there may be an associated fixed engine speed and load range that meets the driver demand. An engine controller may transition to a more fuel efficient compression ratio for the driver demand. However, upon changing compression ratios, there may be engine limitations experienced at the associated engine speed-load that may reduce the fuel economy benefit of the compression ratio transition. As an example, upon transitioning to a higher compression ratio, the engine may become more knock-limited at high loads. The fuel penalty associated with the knock mitigation may outweigh the fuel economy benefit of the compression ratio transition. As another example, upon transitioning to a lower compression ratio, the engine may become more friction limited at low loads. Another issue is that frequent changes in operator pedal demand may cause the engine load to move back and forth, leading to frequent switching between compression ratios. Excessive compression ratio switches can degrade fuel economy due to losses incurred during transitions.
The inventors herein have recognized that the fuel economy benefits of a variable compression ratio (VCR) engine may be better leveraged through integration with a continuously variable transmission (CVT). In particular, the CVT may enable the engine speed and load to be adjusted while maintaining the fuel efficient compression ratio and while maintaining the power output of the engine. In one example, fuel economy may be improved by a method for an engine coupled to a CVT comprising, for a desired power level, comparing engine efficiency at a current compression ratio to engine efficiency at a modified compression ratio with an adjusted engine speed-load; and in response to a higher than threshold improvement in the engine efficiency at the modified compression ratio with the adjusted engine speed-load, transitioning to the modified compression ratio and adjusting to the adjusted engine speed-load. In this way, an engine can be operated with a compression ratio that provides an improved fuel economy for a given driver demand without being excessively knock limited at higher loads. In addition, the need for frequent compression ratio switching can be reduced.
As an example, an engine system may be configured with a VCR engine coupled to vehicle wheels via a CVT transmission. The VCR engine may be configured with a piston position changing mechanism that enables the compression ratio (CR) to be varied between at least a lower value and a higher value. For a given driver demanded power level, an engine controller may compare the fuel efficiency for each of the higher CR and the lower CR. Then, for the more fuel efficient compression ratio, the controller may predict if there are any limitations, such as knock limitations, associated with the corresponding engine speed-load. If so, the controller may further determine if the engine speed-load can be changed while maintaining the selected CR and while maintaining the demanded engine power output, and any fuel penalties associated therewith. If the engine speed-load can be changed after transitioning the CR with a net fuel economy improvement, the controller may proceed with the CR transition. Else, the original CR may be maintained. As an example, upon transitioning to a higher compression ratio, for a given driver demand, the engine speed may increase while the engine load decreases. To address knock anticipated at the higher compression ratio, an engine controller may actuate the CVT to increase the engine speed while decreasing the engine load so as to maintain the demanded engine power output while providing a net fuel benefit. Likewise, when transitioning to a lower compression ratio, the engine speed may be lowered (from the previous engine speed for the higher CR) while load is increased (as compared to the previous load for the higher CR).
In this way, fuel economy benefits can be improved. The technical effect of integrating VCR engine technology in a vehicle having a CVT transmission is that for a given driver demanded power, the benefits of a variable compression ratio can be better leveraged. In particular, the engine speed and torque for a given driver demanded power can be adjusted to reduce knock limitations at higher loads and friction losses at lower loads, while accounting for changes in compression ratio. The technical effect of assessing the fuel economy benefit of changing the compression ratio with the fuel penalty associated with operating at the engine speed-load profile corresponding to the selected compression ratio is that frequent CR switching can be reduced. In addition, engine operation in a more fuel efficient compression ratio can be extended despite changes in driver or wheel power demand.
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.