Engineers from Honda R&D Co., Ltd. presented a pair of papers at the SAE 2009 World Congress describing the development of a variable compression ratio (VCR) engine enabled by a dual piston mechanism. The compact VCR uses the inertia force of the piston and external hydraulic pressure to raise and to lower an outer piston to switch the compression ratio between low and high stages.
In testing in an otherwise conventional production 2.0-liter, 4-cylinder engine, the dual piston mechanism was able to adjust the engine from a CR of 9.6 to 14.2 and back again. Combining the high compression ratio with the Atkinson cycle, the engineering team demonstrated a 7.4% improvement in fuel economy in operation over the Japanese 10-15 cycle. As part of the study, the team also demonstrated switching durability of the dual piston mechanism of more than one million cycles.
Honda R&D began developing the VCR mechanism due to what Masahsi Kato, who presented a paper on the control system of the dual piston mechanism, described as drawbacks to current approaches to improving fuel-efficiency in gasoline engines of downsizing and new combustion technology.
When downsizing, Kato said, using a turbocharger or supercharger to offset the decrease in output can result in a fall in the knocking limit as the boosting pressure rises, causing the thermal efficiency to fall as well.
And while the new combustion technologies such as lean burn, stratified combustion and homogeneous charge compression ignition (HCCI) have been made practicable within lower speed and lower load regions, more conventional technologies still have to be used to cover the whole operating range.
Regarding HCCI, it has been reported that a high compression ratio realizes more efficient combustion characteristics, and a VCR mechanism that can simultaneously realize improved fuel economy and power performance has been demanded. For this reason, a VCR mechanism has been focused to achieve further fuel economy and power performance in automobiles employing downsizing technology and/or new combustion technology.—Kadota et al. (2009)
[Separately, Honda engineers are making a number of HCCI presentations at the SAE 2009 World Congress, including the development of mechanisms to expand the HCCI operational range.]
In contrast to the complexity, size and power demands of earlier VCR concepts, Kato said, the new dual piston VCR could be installed without changing any of the major parts of the engine such as the engine block.
A sensing mechanism and control strategy for the concept is key: during the two-stage switching between high and low compression ratios, the compression ratio does not change continuously—there is a large gradient in the demanded ignition timing. In switching from low to high CR, knocking is likely; in switching from high to low, the torque characteristics and fuel economy can worsen. To accurately set the ignition timing to avoid those issues, the control system must be able to accurately distinguish between the CR conditions at the two stages.
The dual piston mechanism. The piston structure has an inner and an outer piston. The outer piston sits atop the inner piston, and constitutes the combustion chamber against a cylinder head; the inner piston has the function of a piston skirt, a lifter mechanism, and a lock mechanism. The outer piston can be raised a lowered by 3.5 mm with reference to the piston pin. This is turn changes the compression ratio by 4.6, i.e., from 9.6 to 14.2 and back again.
The switching mechanism uses the inertia force of the piston itself and hydraulic pressure as an external force. Required hydraulic pressure for the switching is 0.8 Mpa (8 bar) and the oil flow rate is 3L/min. For the study, Honda installed a small pump separate from that of engine lubricating oil pump. Oil passage was constructed from the external pump through the crankshaft and connecting rod to the piston.
To switch from low to high, the hydraulic pressure is turned on. This acts on the pin of a lock plate, freeing the outer piston. The outer piston rises the 3.5 mm by the resultant force of the piston inertia force and the cylinder pressure force. To go from high to low, the hydraulic pressure is turned off, the lifter plate rotates out, and the out piston lowers over the inner piston by the resultant force.
Controls. To set accurate ignition timing for smooth torque or preventing knocking, the control system needs to be able to judge the actual compression ration with high-speed and accuracy.
Honda rejected the use of a gap sensor as too expensive, and requiring modification of the engine. It rejected a pressure sensor as too limited in measurement and judgement calculation time.
For the study, Honda settled on the technique of judging the compression ration from the vibration measured by a mass-produced knock sensor. To analyze the compression ratio from the vibration measurement, the engineers used a Hidden Markov Model (HMM), generally used in voice recognition applications.
To deliver smooth torque when switching, the engineers built an ignition timing control system that separately controls each cylinder and simultaneously performs knocking control.
S. Ishikawa, M. Kadota, K. Yoshida, K. Takahashi and S. Kawajiri (2009) Advanced Design of Variable Compression Ratio Engine with Dual Piston Mechanism (SAE 2009-01-1046)
M. Kadota, S. Ishikawa, K. Yamamoto, M. Kato and S. Kawajiri (2009) Advanced Control System of Variable Compression Ratio (VCR) Engine with Dual Piston Mechanism (SAE 2009-01-1063)