|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.
US Patent 7,258,086: Four-cylinder, four-cycle, free piston, premixed charge compression ignition, internal combustion reciprocating piston engine with a variable piston stroke
John Fitzergald (2007) 50% Thermo-Mechanical Efficiency Utilizing a Free-Piston Engine in Hybrid Vehicles (DEER 2007)
John Fitzgerald (2008) Rotary Shaft Power Extraction From A Free-Piston Engine (DEER 2008) [not yet published on DEER site]
Chris Edwards, Shannon L. Miller, Matthew N. Svrcek, Sankaran Ramakrishnan (2006) Development of Low-Exergy-Loss, High-Efficiency Chemical Engines (GCEP report)
David Foster (2004) Are There Practical Approaches For Achieving the Theoretical Maximum Engine Efficiency? (DEER 2004)