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Modeling a Two-Stroke, Free-Piston HCCI-Like Engine for Hybrid Applications

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.



Manolis Pattakos

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.
Just take a look.
Manolis Pattakos

Gary Peck

I love the concept. I think it will vibrate a lot. Use two sets of pistons and ignite in the middle and at both ends and it should run smooth as glass.

Henry Gibson

Noax and others have built free piston engines that pump diesel or hydraulic fluids. Pressure tanks and electric controlled valves have allowed the easy operation of these engines. They have been demonstrated in a hydraulic fork lift vehicle. They should be used in future hydraulic hybrids for UPS.

They also should be used any time water or any other fluid is pumped to high pressure by the use of engines, either directly or indirectly, especially for reverse osmosis. The over-all efficiency is much better than a rotary diesel and rotary piston or centrifugal pump. Counter acting pistons and pump-pistons will eliminate most vibration. The very high compression ratios available will allow the compression ignition of both natural gas and hydrogen as well as increase the efficiency substantially. The rapid piston motion available after ignition reduces substantially the formation of Nitrogen Oxides. Running the exhaust of several such machines, after cooling and the addition of a little oxygen and fuel, through a Capstone turbine will eliminate most particulates and nitrogen oxides and unburnt hydrocarbons. In reverse osmosis plants the heat of the cylinders and exhaust can be used to heat the water for more efficient and faster operation.

Hydraulic hybrid cars and trucks have most of the advantages of electric hybrids and at far lower cost and complication; there is almost no reason to build an electric hybrid except for a plug in hybrid, so a free-piston electric generator is not needed and free-piston hydraulic pumps should be used instead.

An electric powered, moderate-capacity, hydraulic pump can be used most effectively to even make a plug in electric hybrid that can use the high peak torque from a high-pressure air-hydraulic accumulator instead of an expensive battery or ultra-capacitor array. Proper design of pipes, valves and channels can eliminate most losses in the same way that Chapelon eliminated losses in steam locomotives in France and doubled their horsepower.

Frank Stelzer built a free piston electric generator with no moving valves or fuel injectors a few years ago. He also built a free piston compressor. Free Piston air compressors were built by the Germans for submarines and later by the Japanese for construction sites. Pescara and others built them with gas turbine outputs for rotary motion, and as such they have powered ships, power plants and locomotives.

Free piston compressors were already in service, over a century ago, for providing compressed air for tunnel building and mines. Modern portable versions should be available for painters and builders at far less weight than the small engine powered versions. They are a good idea but, until computers, were to hard to control. Computer controled free piston engines with electric valves and hydraulic syncronizing plus turbine outputs would have a big market if the research had kept up, because any fuel can be burnt at very high compression ratios; Compression ignition of hydrogen and natural gas is even possible. The ongoing development of rotary crank piston engines and very high-pressure gas turbine engines have made their price so cheap that strange technology cannot be adopted easily. "No body gets fired for buying Microsoft." "No body gets fired for buying rotary crank diesel engine-pumps."

A very simple pelton wheel turbine has very high efficiencies, and free piston hydraulic pumps also have very high efficiencies. Build hydraulic hybrids and if you need a little bit or a lot of elecricity, use a standard rotating pelton wheel powered generator at high speeds and very low oscillating mass. Combined with more high-pressure hydraulic accumulators and extra nozzels, such a generator can cope with the large power pulses needed to start electric motors where a free piston generator would just stop mid-cycle. Turgo style wheels can accomodate a lot of nozzels. Rotating generators can be overloaded severely without damage for a few seconds, but free piston or even ordinary gasoline engines cannot be. Even small low speed flywheels are good to use. Precise Power uses them, well, for uninterrupted power. Steam-piston and steam-turbine powered generators have been frequently overloaded in service as have steam locomotives.

The free piston sterling engines are a good idea. for very long life in small sizes, but ongoing government grants and contracts have not been any incentive to actually have-to make and sell a bunch of them at low cost to keep food on the table.. HG

Henry Gibson

Forget the high powered permanent magnets. Optimized, strong transformer steel can be used in a linear Switched Reluctance motor/generator configuration with much less weight and at lower cost with higher efficiency. The switching sequence is more complicated but the silicon transistors and logic weigh far less than the magnets and cost less. The output is DC right to the battery, and transistors were needed for starting the permanent magnet version anyway. HG...


I have been following 2-stroke engine development for years untill they stopped due to emmision laws, I belive they still have a future for small applications, fuel injection, variable length exhaust pipes and rotary valves would solve all of the emmision laws....their power to weight ratio and simplicity is still not matched to this day.

Dave R

Listen, some years back I met a brilliant guy who was a student in Physics and an avid automotive enthusiast such as myself. He had been planning in his mind to use simple magnets in the cylinders of a basic v-8 engine. By using the reversed polarity of magnets to repel i.e. push the cylinder down like a combustion engine you could get the same results as fossil fuels. He went on to explain his theory that the engine could be casted in carbon fiber, synthetic lubricants, and other methods to get the heat down were explained. He was designing it in autocad. He also mentioned about some type of way to amplify the rpm by an electrical method to which I dont remember or understand really. But the I understood the concept perfectly and frankly it made alot of sense - whether or not it could be pulled off is a different story. What do all you rocket scientists think?

D James

Dear Dave,
What all the rocket scientists think is here

Tom Hedger

I have long been interested in water injection as a method of increasing octane rating for fuels. There seem to be many problems associated with the idea and despite the fact that it has been around for a very long time not many people have actually used it much! I freely admit that my physics is only high school level and do not claim to know whether the idea would fly or not, but can anyone out there tell me if combining t he idea of a free piston engine with water injection might help to eliminate some of the problems associated with both. I wonder also if the cooling effect of injecting water shortly after the combustion point/time of the cycle (whilst the mixture is still under considerable compression) might actually result in a temperature drop steep enough to actually freeze some of the water onto the inside of the cylinder and provide lubrication second to none. I look forward to any comments on my ideas, however stupid you may think they are. Please feel free to rubbish my ideas but please explain to me the faults in my thinking. T.H.

Henry Gibson

In regards to water injection: Modern electric producing turbines use it for pollution reduction and power boost. Old commercial jet aircraft used it for power boost on take off. Direct injection of water can control polution in automobile engines and prevent knocking. A salad dressing type of diesel fuel mix with water has been tested for lower pollution. Exhaust gas recirculation can do much the same.

Lobbiests for ethanol would fight automobile water injection tooth and nail.

The OPOC engine is a very interesting balanced two stroke supercharged engine that I have been waiting for production of for years. Similar piston drive was used in engines with 24 inch diameter pistons.

The (very) large Wartsilla ship engine runs at about 100 rpm while running on #6 fuel oil. (The oil moves slightly faster than tar.)

Paired NOAX/Caterpiller/chiron engines would eliminate vibration and improve efficiency. Hydraulic hybrids are still the cheapest high torque vehicles. But I still would like to see a tiny linear electric piston generator in production. Attach a hydro dam turbine to the Wartsillas for high electric power. Buildings full of smaller ones are being used as the most versatile power stations of the world. One of them makes more profit by selling its capability to start up when needed than by running continuously. Long live windmills. HG....


type bourkeengines into google, you will see an emmisssions free 2 stroke. this motor uses less fuel and no oil in the combustion procession. clean exhaust. cool exhaust 200 to 240 degrees. it was developed by russel bourke, an airplane mechanic in the late 20's and early 30's. a 400 cubic engine built for amc at 2,000 rpm put out 200 horsepower and over 500 lbs of torque. the future is backwards.


This engine concept is very nice,by removing parts that reduce efficiency.The linear induction for electricty is even more simple.This is all good stuff until we get to the lubrication part on the 2cycle concept.There is a total oil loss system going on.Why not try an adaptation of the Stirling motor.That solves the total oil loss parodox on the pollution and simplifies even more problems dealing with NO2 emissions.Weight should not be much of an issue since the motor is based on a diesel fuel and requires heavy duty parts already.

Ken Galileo

Re: Free Piston Engine

I've worked on the Bourke engine in the early and mid 1980's. Still have an original prototype..anyone interested?

Working on the Bourke engine led me to reinvent the Free Piston Engine in the late 80's.

I submitted a SBIR to the US DOD in 1993. They took it called a conference and handed out funds for development to Fed Labs, Universities and Defense contractors, leaving me with no help, only roadblocks.

I built a Free Piston Engine in 1995-96 and got it running in 1997. Quite a machine, able to vary stroke (compression) on the fly and output ac power directly with a linear alternator. Couldn't get funding and had to close operations in 2002. Breaks my heart to see others benefiting from work I did in the 80's and 90's.

Free Piston Engine is for sale. 250W output.
Uses 2 stroke weed-wacker heads and a stirling engine alternator. Data aquisiton system shows dynamic spark ignition timing, stroke, cylinder pressure and power output.

The technology has the best promise of all powerplants. HCCI combustion = ultra low emissions and if using Hydrogen, no carbon emissions. Oilless rings have much benefit here, and are being developed.

My health is bad and I could no longer continue the research.


Fox - send me an email. I'd like to talk to you about your Sterling engine. Thanks.



Dani Kostov

This is a step to make the use of fuels more efficient.
I hope soon there will be method for converting 99% from the energy in bio-diesel to electricity. We should not forget that our inventions must be practical and feasible NOW. Yet I like those sophisticated ideas that tickle my brain. Cheers, Dani. www.oavp.org

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