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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.




At first glance this looks wonderful. I like how the electric generator is completely integrated with the engine itself.

I do have to think that vibration could be a serious issue though. Also, I assume that a turbo or supercharger is needed - logically a turbo would seem to make more sense since it doesn't require external power.

Roger Pham

I can foresee high reciprocating mass, hence restriction in engine uh...rpm...uh...I mean engine cycles per minute (cpm?) and thus lower maximum power available. Also, higher vibration level will also be a problem at higher power output, unless a number of these cylinders are bundled together.

Engine crankcase splash lubrication is not possible, forcing a total-loss lubrication? This will also cause a higher exhaust emission level.

What is the calculated advantage of this engine vs. a two-stroke Orbital engine with similar air-fuel direct injection?


This type of engine was invented and patented by a scientist at Sandia. He quit there and went to California. Haven't heard if he's done anything with his idea.

We can all see problems but - I'll wait and see. If it works, it could really be a giant leap forward.


Makes a lot of sense to develop a high efficiency integrated engine-generator for series hybrids, the other route being toroid combustion chambers and a classic spinning generator. One would expect the same type of multi-cylinder balancing for this engine as is done with current engines with regard to vibration. HCCI or close to it is well suited for series hybrid applications where the engine is not required to change load suddenly.


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.

John W.

Interesting! But can any magnet material withstand the high internal temperatures of a combustion engine?


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. Didn't check to see how efficient it is.


These guys at Chalmers are fantasizing that their IC engine's piston and rings will function without lubrication. Van Blarigan (at SNL) tried the same thing years ago. He dropped the concept because it was inefficient and had high levels of vibration in a single cylinder.

Van Blarigan even proposed running his engine on anhydrous ammonia fuel at 40:1 CR.



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 item and the resulting discussion illustrate how this web site stands above so many others. Thanks to all for constructive, reasoned comment.



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. I wrote a couple of letters to the company with these and some another thought, but never got answer.

Apparently company got busted, and together with their quite graphic web site vanished without trace. Pity for such an elegant design.

The main problems, as I understand, are high reciprocating mass, vibration, and troubles to compensate it in multi-cylinder design.

Rafael Seidl

The graph suggests the scavenging mode is either cross-flow or loop, not the more efficient uniflow mode. Note also that linear electric generators are harder to control and generally less efficient than rotary types.


The graphic does show a loop scavenged configuration. But if you read the .pdf file that the article references, you'll see that the research effort was primarily focused on using the KIVA-3 CFD code to optimize scavenging and swirl in a uniflow configuration.

Rafael Seidl

Terry -

they'd have to add valves in the cylinder heads for that, and actuators for them. Major design change.


“Major design change” – which will eliminate half of the beautiful simplicity and potential efficiency of this engine.


So now the race is on to figure out how to mass produce the super low-mass magnet or moving coil, then the super-hard low-friction coatings which work at high temperature in a moving vehicle, then the......probably not.


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.


Aside from the vibration issues, wouldn't this thing generate AC rather than DC needed to charge batteries or run an electric motor?

With a frequency that varried as the RPMs changed?

Wouldn't that pose problems whose solutions would reduce overall systems efficiency substantially (this is not my area of expertise).


BBM, you rectify the AC and use a charge controller to the batteries. That is a simplification, but in essence it is no problem.

Ed Danzer

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.
Peter is working with a car manufacture to make an electrical generating unit. One problem is reducing moving mass so permanent magnets are being used. They have had difficulties with the permanent magnets handing the start-stop shocks of operation.
I personally don’t think any 2 stroke will ever be more thermally efficient than a 4 stroke. First, the power stroke is the same length as the intake and compression (think Atkinson, or Miller cycle engines). This limits the time to convert the heat to work. Second, intake and exhaust must happen over a shorter time, and the intake air must be compressed and stored, then flowed at very high speeds with a pressure drop to complete intake and exhaust. Basic fluid power losses happen at high flow rates and with any pressure drop. Storing compressed air can have significant energy costs as heat is added to the air during compression.
Another problem with free piston or linear engines is the way the energy is made available for extraction. It starts out very high energy and fast moving, and becomes lower and slower as time increases. Converting ever changing levels of power to a high energy storage level efficiently seems to be the major problem for both, electric or hydraulic free piston engines.
The December 2002 thesis paper by Hans Thomas Aichlmayr, has some good background on the subject.


No mention of thermal efficiency of CR here - does anyone have any claims for this or similar designs?


Ed Danzer,
Regarding thermal efficiency of 2 and 4 stroke engines, I would like to let you know this:
The efficiency attained in large 4-stroke (low speed) diesel engines is about 48.5 %
The efficiency attained in large 2-stroke (low speed) diesel engines has reached 52 % !!!

These machines can also operate using fuel oil instead of diesel.
The huge 2-stroke engines work at about 100 RPM or slightly lower.


Cummings developed 2007 emission compliant heavy-duty truck engine with thermal efficiency of 45%:


Looks like four strokes catching up with these two-stroke monsters.

Tom Thorp

Erick, Could you elaborate on this please? "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."

Is this the process using Orbital's OCP with EGR being mixed with injection air(fluids) to the OCP injector? Cat had a patent app. in for this recently?

Thanks Tom

Tom Thorp

Sorry, Meant to address to Allen Z.

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