Free Piston - Generator for the 3rd World
Modeling a Two-Stroke, Free-Piston HCCI-Like Engine for Hybrid Applications
3 November 2006
|Basic design of a free-piston engine as generator (not the engine modeled). Click to enlarge. Source: Prof. Valeri Golovitchev, Chalmers|
Researchers at Chalmers University of Technology, Sweden, are exploring the development of a two-stroke, free–piston compression-ignited engine employing uniflow scavenging as a genset in hybrid applications.
Free-piston engines (engines without a crank) are under consideration by a number of developers as a potential high-efficiency engine for use in hybrids. The EPA, for example, is exploring the use of a free-piston engine in its heavy-duty hydraulic hybrid system.
The Chalmers researchers, as described in a paper presented at FISITA 2006, have developed a numerical analysis for their free-piston engine. The engine under exploration consists of a block with a double-ended cylinder and four exhaust valves at each end. Air flow is supplied through two belts with twelve intake ports in each. The researchers used a diesel fuel model in their calculations.
Combustion at alternating cylinder ends drives the piston assembly back and forth, thereby driving permanent magnets fixed on the connecting rod back and forth through the coils of the linear generator. The engine uses and HCCI-like combustion mode—Partially Premixed Compression Ignition (PPCI)—combustion to deliver high efficiency and low emissions.
The researchers varied engine design and spray parameters such as fuel injection timings and spray included angles to optimize the engine operation in terms of indicated efficiency and exhaust mixture composition.
To select high efficiency low emissions operating conditions for Diesel and HCCI combustion modes the parametric map analysis has been employed. Based on the map analysis, the optimal one- and multiple-pulse injection timings and profiles and external EGR levels were found for the engine under study. Two-cycle free – piston engine modeling demonstrated nearly similar pressure vs CAD histories in each cycle indicating the engine stable operation.
Hydraulic Hybrids (EPA)
On the graphic, notice the air assist fuel injector. One could have a diesel, w/variable cylinder compression, like the one Caterpillar is developing. The engine would compress the charge in the cyinder to 99.9-99.99% of HCCI pressure. Then a blast of high pressure air, maybe w/H2+O2 assist, would ignite the charge.
___It might be similar to another design that involved 2 cylinders (2cycle) side by side (one air compressor and one fuel air charge cylinder). That design might work with 3 cylinders (4cycle), with 2 fuel air and 1 air (2cycle). Another mod may be 3 cylinders, w/2 fuel air running on 2 cycle mode, 1 air @ 2x RPM via gears.
PemPek Systems of Australia (http://www.freepistionpower.com) has another free piston design with a linear generator - 100KW in a 280x280x660mm design - approx 100kg from what I can tell. Not as simple a design as Swedish one. Incorporates 4 pistons, which I assume are sychronized to reduce vibration. 100KW of electrical power in a 100kg/220lb package would be quite nice.
This reminds me of a free piston Stirling design. The way they counter acted vibration was to have another inline engine of the same configuration running in an opposing phase, to cancel vibration.
This engineer from Scandia created a start-up company, which attracted considerable attention here on West Coast. It was even highly praised by Lee Iacocca. Their design used loop scavenging and piston bottom forced induction. Also they flirted with HCCI.The beauty of this concept is that tangential forces on piston are non-existed, and hence cylinder/piston skirt friction is minimal. It opens possibility to use low-friction ceramic coating and get rid of lubrication completely (and as a consequence to eliminate blow-by of oil on intake/exhaust ports). Simple stratified charge fuel injection, currently well established by a novelty at the time, could be easier to achieve then HCCI. Simple phasing mechanism like Scotch Yoke or better yet planetary Scotch Yoke certainly would work in such design, simplifying all thing dramatically
The main problems, as I understand, are high reciprocating mass, vibration, and troubles to compensate it in multi-cylinder design.
Take a look at the technical papers at sunpower.com to see the problems and fixes with stirling free piston linear alternators. Linear alternators are HEAVY and there is no way to get around it- basic physics gets in the way. The thing shown in the sketch would shake down the GM center.
On the other hand, free pistons can be balanced, and can have very high power, and can run on gas bearings with no wear for very long times. Long ago I tried a balanced, opposed IC free piston design with elecronically controlled valves and it looked good to me as a gas pumper driving a turbine. No money to pursue it.
I attended the SAE Commercial Vehicle Engineering Congress and Exhibition this last week. I visited with Peter Achten who was instrumental in developing the Chiron free piston engine (SAE paper 2000-01-2545). The Chiron, a hydraulic pumping free piston engine was licensed to Caterpillar for more development, which according to one Cat engineer that development project has been stopped.
The Pattakon PRE or Pulling Rod Engine presented at
seems a better and realistic approach for absolutely vibration free power plants with top overall thermal efficiency, power concentration and reliability.
It uses only tested, today technology.
Free Piston Power Pack
The FP3 is a most efficient, low cost, elegant electric generator module of exceptional power density. Operation is based on 'free pistons' (unrestrained by con-rods or crank shaft) being driven back and forth inside cylinders by controlled internal combustion. Permanent magnets are attached to these pistons and move through stator coils, thus generating electric power.
Compression ratio and stroke, as well as exhaust valve timing and lift, are software controlled rather than through conventional mechanical means. The result is mechanical simplicity, unequalled efficiency and extremely low harmful emissions.
(Figure 1: 100kW generator module)
Salient features of a 100kW FP3 module:
- Size: 660x280x280mm
- Mass: 100kg
- Two-stroke operation
- Integral charge compressor
- Power Density: 1kW/kg; 2 kW/litre
- Fuelled by Gasoline, Diesel (Bio, JP8), LPG, Ethanol, Hydrogen, etc.
- Direct, high pressure fuel injection
- Variable exhaust valve lift and timing
- Variable compression ratio and stroke
- Mechanical simplicity (software ‘replaces’ conventional con-rods, cam and crankshaft)
- Readily scalable from 25 to 500kW output power
- Efficiency 50%
What problem does our technology solve?
The FP3 has the potential to make a greater single contribution to the reduction of greenhouse gas emissions than any other technology currently available. Installed in a Series Hybrid Electric vehicle, it offers a significant reduction in fuel consumption and harmful emissions when compared with a conventional power train. It provides a means for oil to continue to be the primary automotive energy source but in an economically viable and environmentally responsible manner. The FP3 is a sterling solution to the land transportation fossil fuel waste-and-pollution conundrum.
The exceptional power density of the FP3 makes it possible to construct a Series Hybrid Electric Motor Vehicle capable of regenerating, storing and subsequently re-using the kinetic energy, presently wasted, when slowing down and braking. This is achieved without any compromise in vehicle mass, performance, handling and payload capacity. The up-shot is a substantial reduction in fuel consumption and harmful emissions.
(Figure 2: FP3 installed in a Series Hybrid Electric Family Sedan)
- Kerb weight 1300kg (2860lb)
- 100kW FP3 complete with power electronics and DSP control system
- Installed lithium-Ion battery power 164kW
- Electric wheel motors 4x50=200kW
- 200kW wheel motor control
- Wheel motor gear ration 6.75:1
- Torque on wheels:
- 2122Nm (1563ft/lb) 0-100km/h (62mph)
- 1415Nm (1042ft/lb) 150km/h (93mph)
- Acceleration 0-100km/h in 5.4 seconds
- Full recovery and storage of kinetic energy when slowing down and braking
- Fuel consumption (in accordance with NEDC): City= 2.4ltr/100km; Highway= 3.6ltr/100km
Status quo of the FP3 R&D program
The FP3 project was launched in February 2001. Prototype electronic control and power modules, sub-systems and internal combustion engine components as well as associated control software were developed.
We first ran a quarter section spark ignition prototype in October, 2004. Mover sensor-less position control performed well. Shortcomings in the scavenging process and linear generator efficiency were identified. Novel, tilted passive intake valves have been developed which significantly improve scavenging efficiency.
The linear generator has been completely redesigned and optimised. Electrical efficiency is expected to increase to 95%.
A Diesel head, with central injection and a novel quad exhaust valve with a total power consumption of less than 300W has been designed and constructed and is currently undergoing test with promising results.
A means of applying a ceramic coating to the Linear Generator Stator Assembly has been developed and is currently being applied.
We expect to be able to assemble and test our second-generation, Diesel prototype by December, 2007.
(Figure 3: 25kW FP3 spark ignition prototype module under test)
Four passive intake valves are located in the crown of each piston. They are seated poppet valves and form the intake ports through which fresh air enters the combustion chamber. The intake valves operate in a completely passive manner, i.e. there are no external controls. The opening and closing of the intake valves is governed by the pressure differential that exists between the compressor and the combustion chamber. The passive intake valves provide three major benefits:
- Uni-flow gas exchange
- Elimination of intake and exhaust slots in the cylinder wall resulting in reduced piston ring wear
- Axial module length reduction by allowing the generator to overlap the combustion chambers
Development of a linear alternator-engine for hybrid electricvehicle applications
Cawthorne, W.R.; Famouri, P.; Jingdong Chen; Clark, N.N.; McDaniel, T.I.; Atkinson, R.J.; Nandkumar, S.; Atkinson, C.M.; Petreanu, S.
Vehicular Technology, IEEE Transactions on
Volume 48, Issue 6, Nov 1999 Page(s):1797 - 1802
Digital Object Identifier 10.1109/25.806772
Summary:This paper examines the design and operation of a generation system that utilizes a linear crankless internal combustion engine in conjunction with a linear alternator. This system directly utilizes the linear motion of the piston to drive the alternator rather than first converting to rotary motion. The result is a more compact, reliable, and efficient unit as the system has only one moving part, making the system ideal for use in series hybrid electric vehicles. This paper describes the overall system design as well as the subsystems including the engine and alternator. A dynamic simulation is then presented which utilizes the model developed to determine the output characteristics of the system. The prototype system was successfully tested, and experimental results are also included