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Concept: Rotary Shaft Power Extraction from an Efficient, Free-Piston HCCI Engine

The mechanism for rotary power extraction uses a pair of one-way clutches (one clockwise, CW; the other counter-clockwise, CCW) attached to the power output end of the pivot shaft. Click to enlarge.

Energy Transition Technology, Inc. (ETT) has devised a mechanism to extract direct rotary shaft power from its Free-Piston Floating Stroke (FPFS) engine—an Otto cycle, four-cylinder free-piston engine (FPE) featuring a continuously variable compression ratio and full-load-range homogeneous charge compression ignition (HCCI) combustion. ETT projects an FPFS engine simple cycle efficiency of 60% and, with turbo compounding, near Atkinson-cycle efficiency of ~65%.

Free piston engine (i.e., without a crank) power output is provided by an oscillating pivot shaft which can directly drive a compressor, hydraulic pump or electrical generator. However, with rotary shaft output, the free-piston engine is suitable for a broader range of applications than usually considered.

The rotary shaft solution is based on the use of a mechanical rectifier consisting of a pair of back-to-back one way (overrunning) clutches (OWC) and a synchronizing gear set assembled onto the power output end of the pivot shaft. After an evaluation of available OWC, ETT determined that the Epilogics Mechanical Diode (MD) met its requirements. The MD is a high-resolution planar ratchet, which uses low-mass rectangular struts instead of ratchet pawls. The struts are positioned between a plate with pockets for the retracted struts and a second plate with notches for strut engagement.

The notch plate is splined to the pivot shaft and oscillates with it. The pocket plates and integral gears rotate on plain bearings in opposite directions to each other, driven by the planar ratchet one-way clutch action. A pocket plate and gear turns ~90 degrees with each stroke and drives a pinion with one half the number of teeth to produce 180 degrees/stroke. The pinion gears intermesh providing 360 degrees of rotation for two strokes.

In a poster session presented at the 2008 Diesel Engine-Efficiency and Emissions Research (DEER 2008) research conference, EET principal John Fitzgerald said that the mechanical diode system as configured is rated at 723 ft-lb (980 Nm) continuous and 2,090 ft-lb (2,834 Nm) intermittent, at a maximum overrun speed of 5,000 rpm, with a strut of high strength alloy steel and oil lubrication.

A fatigue life of 10,000 operating hours is anticipated. With careful selection of design parameters, materials, surface treatments and lubrication, fatigue life can be significantly extended. The mechanical rectifier efficiency is ~ 98% (declines above rated speed due to lubricating oil pumping losses).

—John Fitzgerald

The Free-Piston Floating Stroke Engine. While an internal combustion reciprocating engine (ICRE) can attain a maximum theoretical efficiency equivalent to that of a fuel cell (Foster, 2004), in practice, implementation losses lower the actual efficiency. One approach to reduce those losses and increase the efficiency of the engine is to utilize very high compression ratios, constant volume combustion and HCCI.

Fitzgerald contends that the free-piston geometry, without the constraints of crankshaft and combustion timing, is better suited to deliver HCCI over the full load range and to attain meaningfully higher ICRE efficiency.

In order for a four-cycle free-piston engine to function, a minimum of four cylinders is required (due to the lack of energy storage, e.g. a flywheel). To assure HCCI/PCCI combustion, the FPFS engine is limited to four cylinders (only one cylinder in compression at any time).

A cutaway view of the FPFS engine. Click to enlarge.

The FPFS engine uses an adaptive control “floating stroke” algorithm. The pistons rapidly compress the premixed charge to the pressure required to cause charge ignition (CR = 40:1 to 50:1). Subsequent to the constant volume combustion, piston acceleration in the expansion stroke is high, thereby causing rapid gas expansion and cooling, which reduces heat transfer losses. As a result of a higher CR, combustion chamber wall area is less than in a conventional engine, which also reduces losses.

Without a crankshaft, piston side load and friction losses are also reduced. In addition, the lower heat transfer loss improves cycle compounding performance. Gas exchange valves are indirectly driven by an electric servo motor-powered camshaft or electro-hydraulic valve actuators (EHVA). The lube oil pump, coolant pump and cooling fan are also electrically driven.

Fitzgerald said he believed that with tailored fuels, the FPFS (which is fuel-flexible), could support compression ratios as high as 100:1.

EET is currently seeking a University partner so that we it can respond to an upcoming SBIR in November. The company has not yet built a prototype, but is seeking funds to do so.



John Fitzgerald

Real engine compression (not as high a speed as attained in RCEM) is polytropic and actual compression temperatures and pressures are considerably less than “near” adiabatic. Fuel/air ratio and the associated specific heat is also a major influence. Nor can the variability of charge specific heat with temperature and pressure be ignored in the compression process - particularly at increased compression ratios. There has been considerable experimental work over the last decade, using RCEM, to provide an adequate data set on a variety of fuels at low equivalence ratios for the estimation of polytropic coefficients and compression ignition properties at high compression ratios, in a real high compression ratio engine. Some of the findings are counterintuitive. Of particular interest in this regard is the work of Aichlmayr on micro HCCI.

HCCI allows operation at very low equivalence ratios. This can be utilized to control the engine down to idle - versus throttling. From idle, fuel/air ratio is increased to pick up load and subsequently supercharging is introduced to pick up additional load. Alternatively, from idle introduce supercharging while maintaining a low equivalence ratio then, to add load beyond the supercharging range, increase the fuel/air ratio. This will keep the NOx formation lower over a broader load range than the previous strategy.

Regardless of one’s views on this matter, we concur that the FP-FS engine offers significant potential for much higher efficiency and warrants further development effort. Time will tell. Appreciate the best wishes.



Thanks for your interesting contribution, but I am still skeptical that a design where there is no conservation of cinetic energy of the crankshaft can run smoothly. The cushion of the compressed air will be thin to stop all the mass of the moving system, and if your valves have the slightest leak ...ok. I can understand that the air cushion can work on a free piston engine where the moving mass is limited, but here... maybe is you use carbon fiber for the piston and crank, maybe...also the HHCI is quite an explosive type of combustion so your piston and cranck will hit a wall coming in the opposite direction, ok the shock wave will not be directed to the cranckshat like in a normal engine because of the balanacer type of configuration, well let see

John Fitzgerald


I understand your reservations. Although,I worry more about head gasket leakage than valves and rings. That aside, our engine is no different, in respect to compression issues, than the two-cycle free-piston linear engines that have been built and tested. I suggest you take a look at the work done by Innas, Volvo (FPEC engine) and Van Blarigan in partciular(as he is working towards equally high compression ratios as we are).


Do you have links for those documents you have referenced? I'm really interested in free-piston engines and its technology
Thanks in advance

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