A fixed crankshaft engine has pistons, connecting rods, rod and main bearings that all closely resemble parts of a conventional reciprocating piston engine, but it does not have a rotating crankshaft. A conventional reciprocating piston engine has a crankshaft rotating inside the engine to provide the reciprocating motion of the pistons. A fixed crankshaft engine on the other hand has a cylinder and cylinder head assembly that rotates around the engine to provide for reciprocating motion relative to the pistons. With the pistons and rods simply swinging around the fixed crankshaft there is not much in the way of reciprocating losses, and this is the big advantage of a fixed crankshaft engine.
History of the Fixed Crank Engine
Mean Piston Speed
Undersquare Fixed Crank Engines
There is some reason to believe that fixed crankshaft engines may have seen limited use in airplanes sometime in the early 20th century, but facts on the subject appear to be few and far between. Radial engines with a large number of cylinders arraigned symmetrically around a rotating crankshaft were quite commonly used in airplanes through the mid 20th century, but these are conventional reciprocating piston engines that happen to have a symmetric cylinder layout. A fixed crankshaft engine sort of resembles a radial engine when it is not running simply because it's overall shape is also round when viewed from the front or back.
The first point that should be made about the engine speed of a fixed crankshaft engine is that the same relationship between the temperature of combustion potential of the fuel and the mean piston speed of the engine that applies to conventional engines also applies to fixed crankshaft engines. See
Combustion Properties of Fuel. In other words there is some upper limit for piston speed (the speed of the piston relative to the cylinder) for any internal combustion piston engine while remaining efficient. Removing the reciprocating losses simply means that this upper limit can more easily be attained under reduced engine loads. Said another way fixed crankshaft engines would be able to run over a wider range of engine speeds and a wider range of engine loads while attaining high efficiency. Particularly on carbureted and port injected gasoline engines that have to run at rather high engine speeds to be efficient eliminating the reciprocating losses translates into radically improved operational efficiency.
Just how much of an advantage a fixed crankshaft engine would be depends on just how high the temperature of combustion potential of the fuel in fact is. The fact that the highest performance reciprocating piston internal combustion engines operate at rather high mean piston speeds seems to indicate that the temperature of combustion potential of commonly available fuels is in fact so high that a fixed crankshaft engine would be an enormous advantage. To put some numbers to this idea a more or less standard automotive gasoline engine of three and a half inches of stroke can be considered. This three and a half inch stroke gasoline engine has been the main motive power plant for automobiles for approximately six decades. Longer stroke gasoline engines have been used, particularly before the 1950's, and slightly shorter three inch stroke engines have sporadically been used in smaller quantities over the decades. The three and a half inch stroke engine can however be considered standard because of it's prevalence over such a long period of time. Most of these three and a half inch stroke gasoline engines have been severely hampered in their performance and efficiency by heavy rods and heavy pistons that lead to exorbitant reciprocating losses up at more than 4,000RPM where carbureted or port injected engines need to spin to be efficient. The heavy rods are in large part due to the split bearing design with rod bolts, but for any type of multiple cylinder engine the split rod bearings are nearly impossible to eliminate. The heavy pistons result from a combination of sloppy design and a need for large skirts to hold up to long hours of severe service. What these heavy pistons and rods end up meaning is that the standard automotive engine can only run at speeds above 3,000RPM under a heavy load and at speeds above 5,000RPM even a full load is not sufficient. In order to get these heavy pistons and rods running on a three and a half inch stroke to work at all up to the 6,000 to 8,000RPM required for a good running carbureted or port injected engine requires four valves per cylinder or two large canted valves running on a quite aggressive camshaft along with large and well designed intake and exhaust systems so that near 100% volumetric efficiency can be attained at high engine speed. The three and a half inch stroke gasoline engine certainly can be made to make power, but it is then only good for running under a full load and is in fact really just a race engine.
If the pistons and rods are made reasonably light then a high performance three and a half inch stroke engine can make big power up to about 9,000RPM and even this upper limit is due to reciprocating losses becoming extremely large. The point here is that it does appear that the temperature of combustion potential of combustion fuels is in fact very high.
The other way to get some idea of what the temperature of combustion potential of combustion fuels is, would be to consider what the lower limit on mean piston speed for reasonably efficient operation would be. Six inch stroke diesel engines are able to attain quite high efficiency down to 1400RPM and even 1200RPM does not appear all that much too slow. This mean piston speed would be only 2000 to 2400RPM for a three and a half inch stroke engine, which is in fact far too slow for a carbureted or port injected engine to run well. This appears to be a dead end way of determining what the combustion potential of combustion fuels in fact is. Ultimately the only thing that can be said is that there is some variation in the temperature of combustion potential of various fuels and the range of mean piston speeds that any one fuel can run over tends to be substantially wide.
The thing that eliminating reciprocating losses certainly does do is remove the limit on how radically undersquare an engine can be made. This is extremely significant when an efficient low power output high cylinder count engine is required. Keeping the stroke long while reducing displacement is extremely desirable for building small engines, but this does not work out well for conventional reciprocating piston carbureted or port injected engines. The problem of course is that carbureted or port injected engines have to spin up to more than 4,000 or 6,000RPM to run efficiently, and radically undersquare engines have difficulty flowing well enough up at these rather high engine speeds. The fixed crankshaft engine solves this problem because removing reciprocating losses means the engine can run efficiently down to much lower power output levels at reduced volumetric efficiency.
A three and a half inch stroke fixed crankshaft engine would have no difficulty running efficiently at 4,000 to 6,000RPM under medium loads even with very small bore sizes. The advantage of keeping the stroke up at three and a half inches for a carbureted or port injected engine would be that it would run as well as it possibly could at the low end of the engine speed range for carbureted or port injected engines. A three and a half inch stroke carbureted or port injected engine is able to do somewhat better down at 3,500 to 5,000RPM than a two inch stroke carbureted or port injected conventional reciprocating piston engine. If both engines are run up at sufficiently high engine speeds for carburetion or port injection to work then a three and a half inch stroke engine is actually a smaller engine than a two inch stroke engine of the same displacement. Another way of saying this is that the three and a half inch stroke used on most automotive engines is in fact probably the correct stroke length for carburetion or port injection, it is just that the reciprocating losses associated with heavy pistons and rods in a conventional reciprocating piston engine severely cut into heavy load efficiency and totally preclude light load operation.
A radically undersquare fixed crankshaft engine might have a three and a half inch stroke with inch and a half bores. The cylinder count might be anywhere from two to twelve depending on the application. A six cylinder engine with these bore and stroke dimensions would displace only 600cc, but could be expected to produce 60hp maximum output at 7,000RPM. Because of thermal problems associated with the extremely small bore sizes continuous output would need to be somewhat lower, and the engine would probably normally be run between about 4,000 and 6,000RPM. With essentially no reciprocating losses this lower 30 or 40hp continuous output could also be supported quite efficiently all the way up to 8,000RPM. Ultimately it is this ability to run under somewhat reduced loads over a very wide range of engine speeds that is so appealing about the fixed crankshaft engine. With somewhat reduced peak efficiency even much higher engine speeds would be possible, and the three and a half inch stroke fixed crankshaft engine could run rather well up to about 10,000RPM for a very wide range of operable engine speeds. With this extremely radically undersquare configuration a power output would however not go up at higher engine speeds as the small valves just would not flow very well. For a performance engine a shorter two and a half inch stroke with a two inch bore would yield a slightly larger 800cc displacement for the six cylinder engine, but power output would be more like 120hp at 10,000RPM. The less radically undersquare fixed crank engine would also have the ability to rev all the way out to about 13,000RPM where slightly more power could be produced. What the two and a half inch stroke performance engine would not do quite as well would be to run along at the minimum engine speed of 3,500 or 4,000RPM. The shorter stroke engine would make more power over a wider range of engine speeds while being lighter and more compact, but it would also use somewhat more fuel in low speed cruising situations.
For propeller driven aircraft applications a fixed crankshaft engine seems to fit quite well , with the propeller bolted directly to the rotating cylinder assembly. Even for this simplest application though there are a few significant challenges that have to be overcome in actually building a fixed crankshaft engine. Probably the most daunting challenge is the fact that the fixed crankshaft can be supported only at the ends, and in the case of the propeller plane application the fixed crankshaft could only be supported at one end. What this means is that there can be no main bearings between the rod bearings on the fixed crankshaft, and this tends to place large demands on the strength of the fixed crankshaft. There are however some mitigating factors that make the small number of main bearings less of a problem than would at first be expected. Since reciprocating losses are not an issue the diameter of the rod bearings can be quite large. The large diameter rod bearings allows the crankshaft to be large and strong to be able to support the loads of a large number of cylinders while being supported only on the ends. In a propeller aircraft the crankshaft only has to support the static load of the engine, and the power output is in the form of forward thrust that does not present structural problems for the crankshaft. Low cylinder counts are also not any kind of a problem for driving a propeller with no reduction gear. For other applications there has to be some way of getting the power off of the rotating cylinder assembly, and this would normally be in the form of a gear set. With a gear driven power take off the fixed crankshaft would be able to be supported at both ends, which would make high cylinder count engines easier to build.
The other main challenge for building a fixed crankshaft engine is getting the intake air and fuel to the valves and getting the exhaust to an exhaust system. The simplest way of dealing with this would be for the intake air to be drawn directly into the rotating cylinder assembly and for the exhaust to exit directly from the other side of the rotating cylinder assembly. The problem with this would be that any air cleaners on the intake would have to be on the rotating cylinder assembly and any exhaust mufflers would also have to be located directly on the rotating cylinder assembly. A more likely arrangement would be to use seals on the rotating cylinder assembly down close to the crankshaft so that intake air could flow in one side and exhaust could flow out the other side. Regardless of how the intake air was provided to the cylinders it would be quite easy to design the intake system to provide some forced induction effect at high engine speed. If the air was introduced directly into the cylinders then scoops could be used to "ram" the intake air in at elevated pressure when the engine was running at higher engine speed. If the intake air was introduced through seal rings near the fixed crankshaft then the passages out to the intake valves could be shaped so that the intake tract would function as a centrifical super charger, which would also provide intake boost at high engine speed.
A fixed crankshaft engine could be either a two stroke or a four stroke. Because intake air would have to be forced down into the cylinders against centrifical force for a four stroke engine a two stroke fixed crankshaft engine tends to look very appealing. Because a fixed crankshaft engine would be able to run well over such a wide range of engine speeds though cylinder ports tend to look like a bad idea. Possibly the ideal fixed crankshaft engine from the perspective of flow would be a two stroke with intake ports in the cylinder walls and exhaust valves in the heads. The intake air would then flow up into the cylinders with centrifical force and exhaust would flow up and out of the cylinders with centrifical force. This configuration would eliminate the hot exhaust ports which are responsible for such radically reduced engine life in most two strokes.
From the perspective of the intake tract being able to provide a forced induction effect at high engine speeds though just a regular four stroke could in all likelihood be made to work quite well up to very high engine speeds. Additional forced induction could also be provided with turbocharger units located directly on the rotating cylinder assembly. Any fixed crankshaft engine would probably have one camshaft for each cylinder, and these camshafts would probably be gear driven off of the side of the rotating cylinder assembly. Pushrods, rocker arms and lifters riding directly on cam lobes on the fixed crankshaft might also be used for a two stroke engine.
The final challenge for constructing a fixed crankshaft engine is that the fuel, lubricating oil and coolant also have to be conveyed out to the rotating cylinder assembly. This could be as simple as two additional sets of seal rings front and back on the rotating cylinder assembly. Air cooling of the cylinders and heads would be an option for simplifying the cooling system, but air cooling would of course limit maximum power production potential. Lubricating oil for the rod bearings would be pumped into passages in the fixed crankshaft, but there is also the problem of getting the oil back into the sump. If the lubricating oil was simply allowed to run out of the rod bearings it would pool up under the pistons and the engine would not be functional. The solution of course is the use of seals on each rod bearing to contain the oil, and a second set of passages in the fixed crankshaft for the oil to return to the sump through. Some small amount of lubricating oil would have to be allowed to escape into the engine to lubricate the cylinders, rings and small end bearings. Some of this "upper cylinder lubricant" could be allowed to escape past the rod bearing seals, but it would probably be beneficial for at least part of oil to be metered out by an injector. All of the oil allowed to escape up into the engine would be forced past the rings by centrifical force and burned in the engine, so the oil flow would have to be carefully metered to provide sufficient lubrication without causing excessively high lubricating oil consumption.
A carburetor could be used on a fixed crankshaft engine, but it would have to be located off of the engine so that the fuel/air mixture would travel through the entire intake tract. This would be a bit problematic for cold starting, but it could still be made to work. Carburetors would not really be able to be located directly on the rotating cylinder assembly because the changing centrifical forces at different engine speeds would be enormously difficult to compensate for. The obvious solution is some form of fuel injection.
A fixed crankshaft engine could have any type of mechanical or electronically controlled fuel injection system, but electronic controls would be somewhat easier to convey out to the rotating cylinder assembly. With an electronically controlled injection system just two additional sets of seals would be required. A large number of contacts for a bundle of control wires would not work out well, because each wire would require it's own contact race. A better solution would be radio control, or digital control over a single set of wires.
A fixed crankshaft engine could run as any type of internal combustion reciprocating piston engine. It could be port injected to run as a full flame front travel engine, but there would be little reason to build this type of engine. It could also be a full diesel engine, but the advantages over a conventional reciprocating piston diesel engine would be rather modest. The most likely type of fixed crankshaft engine would be a port injected or direct injected gasoline engine. The high engine speeds attainable by eliminating the reciprocating losses would be of great benefit for a port injected gasoline engine as the biggest problem with most port injected engines is simply that they are not capable of spinning fast enough to run well. Even for a diesel engine though there would be some significant advantages of the fixed crankshaft configuration. Removing the reciprocating losses would allow long and heavy pistons that would hold up well to heavy use over long periods of time, and being able to make the rod bearings a larger diameter would also contribute to durability and longevity.