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Concept: Honda R&D Developing Variable Compression Ratio Engine with Dual Piston Mechanism

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)



So the piston itself is like a fist with a glove; with the glove allowed to extend by 3.5 mm.
And they detect the glove position by the knock vibration sensor..

If anyone but Honda suggested this I would just laugh.

Apparently each piston switches from low to high, more or less independently since they cannot rely on the command without feedback.

This cleverly avoids another set of piston rings or other seals but it is like the caveman that changed from a square wheel to a triangular one; “one less bump”.

If this is just a design for a research engine –why not just move the crank centerline up and down with some ganged jack screws? So....

Nick Lyons

So presumably under the typical duty cycle, the engine would run at partial load with the higher compression ratio to compensate for the throttle-restricted airflow into the cylinder, thus improving thermodynamic efficiency. As the throttle opens at high load the engine would switch back to the lower compression ratio to accommodate the increased fuel-air charge.

The hydraulically switched, dual-mode nature of this concept is reminiscent of Honda's well-developed VTEC dual-mode valve train. Interesting--Honda-think.


With an engine like this, I wonder how much more efficiency they could squeeze out ethanol fuels to bring them closer to gasoline MPGs?


I wonder how much better an engine with VCR added, is likely to be compared to one with all the other, less challenging measures (VVT, Atkinson, HCCI etc).

I get the idea that it allows other improvements to be better optimized, rather than just replaced - but I don't know.

Roger Pham

Ah ha, very clever...the piston acts as if it's on V*agra...getting an er$ction of about 3.5mm long...well, it matters not the size of the dog in the fight...

Though, the increase in piston mass and hence reciprocating mass will limit top-end rpm somewhat, but not a big deal in a turbocharged engine having a lot of torque.


Jack-screw mechanism for moving the crankshaft have been tried, but the response is too slow for a high performance engine like a Honda to be used in a "fast-and-furious" rice burner...

VVT, Atkinson and HCCI will likely be used in conjunction with this Honda's new VCR mechanism to produce and engine capable of very high output, high response rate, and very fuel thrifty at the same time...something that is not possible before.


There are other methods, like putting the engine crank bearings in an eccentric and moving the crank and block relative to each other by rotating it, but I suspect that Honda isn't using it because it doesn't have that patent.

Back in the early 90's, Ford was using a hydraulic piston to automatically control compression for an experimental Otto-Atkinson cycle engine.  I gather that the piston's position was controlled by peak cylinder pressure pushing oil through a relief valve.


EMC5 has shown a concept that not only solve this control of fast adjustment in VCR but also suppresses the lateral friction component between the piston and the cylinder .

What Honda is proposing here looks pretty academic, but Honda has a track of record in term of efficient engine design so let see.


The piston STILL seems like a poor place for the PRODUCTION VCR mechanism.

The feedback and balancing of higher piston mass is problematic and the compression ratios between the 4 independent pistons is poorly synchronized.

But hydraulic lifters (and balancing shafts) seem to be doing OK and my daughter’s ’93 Civic with VTEC is fine at 240K miles (but we do change the oil religiously every 50K mi).

And yes, maybe this is useful in patent or licensing negotiations.

We'll see. I need to read up on the synergism (or lack thereof) between VCR, VVT, HCCI, lean burn, Atkinson, low cholesterol etc.

I am sure all the auto companies know this down to a fine point.

But it’s like OHVs and HCCI and even BEVs.
The challenge for the big auto makers is almost always in the engineering (making it affordable - to pay in the market place) not the science.

Account Deleted

My patents presents 17 stroke variation mechanisms realized by modification of the basic configuration of the mechanism, keeping constant the crank radius with the change of TDC position and displacement during operation, and with the position adjustment of BDC (and the volume of the chamber in relation to the adjusted displacement to maintain the prescribed compression ratio and the optimum conditions for combustion.
My solutions could be realized by classical technologies, with the variation of the compression ratio and the displacement (0-100%), generating high power and low consumption !
An industrial version of an engine with variable displacement mechanism (pending patent - 9 mechanisms) is under development, with perfect dynamic balance realized by classical crankshaft, rods, rotational or translational joints.

Other projects


If you are interested in any of the projects, please contact me in order to discuss details.

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