Article by Doug Switzer, Moto/ology Publisher.
Here we go again! How about an engine with no crankshaft, flywheel, traditional valves or valve gear or rotating parts at all, and just has its piston(s) flying about on the loose?
Well, it turns out there is such a thing, and it’s called a Free-Piston Engine…
For this issue of Moto/ology we welcome back another installment of Manic Motors and look at another strange and innovative design.
So, we’re probably all reasonably familiar with the classic 4-stroke (or two-stroke) piston engine with its reciprocating pistons in their allotted cylinders going up and down or in and out whilst they push on their respective connecting rods and in turn spin a crankshaft. In order for these pistons and the rotation to take effect, there is a collection of ports, reed valves or camshafts, poppet valves, valve springs, shims, rocker shafts or posts, lifters and so on that must all be intricately and intimately set up with each other to allow breathing, combustion and exhaust to take place. This set up has pretty well become the standard for ICE engines for well over a century now and has been fine-tuned, tweaked and developed into an extremely reliable and for the most part, efficient configuration. That is, so we thought.
Now, back around the turn of the 19th to 20th centuries, some bright bulbs come along and say: “Hey! Here’s an idea… let’s do away with the connecting rods, crankshaft, the camshaft and most of the valve gear and just have a piston or two flying around in a cylinder!” Madness you say! How can you have a piston impart any motion or do any work without attaching it to something like a crankshaft or… something?
Well, in fact, it can. A very early patent for just such a device was filed in 1912 by a chap named Matricardi in Italy, however, it appears it wasn’t developed as it could or should have been until in the early 1920’s another fellow from Argentina, Mr. Raúl Pateras Pescara filed patent papers for a more definitive engine and built some prototypes that actually worked. The mechanicals of Pescara’s engine were set up thusly: you place the piston in a cylinder with a combustion chamber at one end then at the opposite end, attach another closed “bounce” chamber that’s filled with nothing but air, and perhaps another co-joined piston, or, to make things even simpler still, position the bounce cylinder on the backside of the drive piston, effectively making the piston a double-headed design. One side of the piston has a combustion chamber, and the other side forms the bounce cylinder/chamber. An air/fuel mix is introduced into the combustion chamber side of this contraption and ignited, shooting the piston against the bounce chamber side, compressing the air within. When the compressed air reaches a predetermined pressure, it forces the piston to bounce back toward the combustion chamber side of things, drawing in another fresh air/fuel charge as it goes by ports in the cylinder and compressing it for another ignition/combustion stroke. Couple two of these cylinder/piston arrangements together and you can have a double-ended affair, effectively doubling the power output. Now here’s the clicker: the old, combusted air isn’t simply exhausted and wasted, being pressurized, it can be used to spin a turbine, and this is attached to a shaft that can now be harnessed to do work-stuff! Follow us so far?
Good, because the turbine-attaching, work-generating part didn’t come first. The concept was originally envisioned as a simplified, self-propelled air compressor. The combustion piston (or pistons in a multi-piston or multi-cylinder configuration) would do they’re deed banging back and forth (or up and down) bouncing off their air-cushion chambers at the non-combustion end of their cylinders, all the while generating a compressed airflow through additional cylinders and pistons that were attached via connecting rods or some other simple linkage system. (Please see the accompanying diagrams!) Somewhat surprisingly, this system proved to be fairly quiet, and through its inherent reduction of moving parts, it was reliable and required much-reduced maintenance. These compressors found a home in applications that required quiet, low vibration settings, one being marine installations like submarines and other vessels. Due to the large scale required to get any useful power from these engines, along with their use in ships they were mostly used in stationary engine applications on land. Other vehicular applications like railway locomotive powerplants were attempted but proved impractical and were not developed to any great extent.
In the present day, there is renewed interest in free piston powerplants, and they are employed in hydraulic engines and some other uses. One of the more promising developments is the use of free-piston linear generators (or alternators) as range-extending motors for electric vehicles. Several automotive manufacturers are actively exploring these uses.
Raul Pateras Pescara/Pescara & Raymond Corp. Patent drawings c. 1927 for a free-piston engine/compressor design. Note the piston that flies back and forth in the cylinder which is actually a cylinder within a cylinder that slides at the same time, thus controlling the intake and exhaust functions, similar to the action of a sleeve-valve.
With all the variations and claims made for the free-piston engine, it only stands to reason that over time a rash of conspiracy theories would arise. While the designers of some versions have made wild claims about efficiencies and performance, others have soldiered on as they were intended, providing reliable service in sometimes unglamourous applications. Now, with that in mind, there are some thought-provoking instances where the so-called conspiracy theory may have a basis in fact and with that in mind, we present to you the Stelzer Engine.
The Stelzer Engine
In the 1970’s, German inventor Frank Stelzer proposed a free-piston engine of elegant simplicity. With basically one moving part, a double-ended three-step piston, the Stelzer engine promised to be a most efficient method of producing either compressed air or electricity as it could do so without any means of traditional transmission.
In the Stelzer engine, the three “stepped” pistons and their connecting rod form an assembly that moves back and forth in an open-ended cylinder with the combustion process taking place on the internal sides of the two end pistons. The central piston serves as an enabler to intake and compress the fuel/air mixture then exhaust the burned charge allowing scavenging as it passes valved ports in a two-stroke system. The outward ends of the cylinder and piston are open, allowing the attachment of various fittings to perform the actual work. As such, the engine can perform duties as a compressor, an electrical generator or a hydraulic pump. Great efficiencies and remarkable power outputs were claimed and predicted, and development was started. When Stelzer demonstrated his engine, he did manage to secure some funding for development but according to him and his supporters, he was ridiculed, dismissed and ultimately brought down by outside competitors with conflicting interests, specifically those with invested ties to traditional reciprocating engines and power systems. Although not completely forgotten, the Stelzer powerplant has largely faded into obscurity and disinterest.
To add final insult to injury, before his invention was given a fair shake, Stelzer was accused of fraud, brought up on charges and he ultimately died without seeing his invention vindicated. If you think this is merely an example of a crack-pot inventor’s “sour grapes” there is an interesting comment by a U.S. government agency that may make you think twice.
To quote the U.S. Department of Energy’s ETDE Web abstract: “The (Stelzer) engine is claimed to have a high performance/weight ratio, coupled with a fuel consumption of up to 30% less than existing engines and the ability to take almost any fuel. Engine makers are opposed to production of this engine because it uses few advanced production skills and no complicated castings. (SC)”
It seems that according to this opinion, the Stelzer engine did indeed have some excellent potential and fine qualities that were taken as a threat to some traditional engine manufacturers, hence its dismissal. I wonder if anyone will someday pick up the gauntlet and resurrect its development? Hmmm… food for thought, eh?