World's first: domestic connecting rod-less engines
Computer concept of the OM-127RN connecting rodless motor
A masterpiece of mechanics
Why remove the connecting rod from the engine, exactly? The internal combustion engine in its current form is over 160 years old, and no one is planning to interfere with its established design. But it wasn't always this way. Soviet engineer Sergei Stepanovich Balandin from OKB No. 2 of the People's Commissariat aviation In the 30s, the industry made a small technical revolution and deprived the engine of connecting rods.
The crank mechanism itself is good in every way, but it has its drawbacks. For example, the lateral tilt of the connecting rod during operation presses the piston into the cylinder wall. This leads to increased wear—everyone is familiar with the elliptical shape of the cylinder at high mileage, which compromises the combustion chamber's seal. The mechanical movement of the connecting rod causes parasitic vibrations, which require special balance shafts to compensate. An engine with connecting rods and a crankshaft is large and heavy. Due to the high inertial loads from the heavy connecting rods and pistons, when changing direction at top and bottom dead centers, colossal tensile loads occur, limiting maximum RPM. Removing the connecting rod eliminates most of these problems.
The author of the development, Sergei Balandin, writes in his book:
The connecting rod-less mechanism allows for a simple design, even with small dimensions and high-speed engines, to carry out a two-way working process in the cylinders, thereby obtaining almost double the liter and overall power.
Sergei Stepanovich Balandin
Balandin OMB connecting rodless engine with a power of 80-140 hp depending on the version
What is a Balandin engine, or a connecting rod-less internal combustion engine? On the one hand, it's simple: two opposing pistons are rigidly connected by a piston rod. The engine can be configured either as a boxer or a radial engine. No V- or in-line engines with a connecting rod design are possible. The absence of connecting rods significantly lightens the engine's design, reduces lateral forces acting on the piston to zero, and therefore virtually eliminates wear on the cylinder skirt and cylinder wall. Total power losses due to friction are reduced severalfold, efficiency increases, and fuel consumption is more efficient. All other things being equal, a connecting rod-less engine has a higher power per liter and the potential for increased revs. A clear advantage of a connecting rod-less engine is the ability to organize a bidirectional combustion process in the cylinders.
Designs of connecting rodless motors with a two-way working process
What is this? It shouldn't be confused with the two-stroke process. Double-acting combustion in internal combustion engines (or the use of double-acting cylinders) is an operating principle in which the combustion of the fuel-air mixture and the expansion of gases occur on both sides of the piston, rather than just on one (the top), as in conventional engines. This is a very significant engine improvement, providing a dramatic increase in power per liter, all other things being equal. For example, the experimental OM-127RN 8-cylinder, connecting rodless, double-acting combustion engine produced 3200–3500 hp. The unit's displacement was only 22 liters, and its weight was 2,1 tons. For the 1940s, these were record-breaking figures.
Motors of the (no)future
As with any technical innovation, connecting rodless designs presented challenges. For example, the mechanism for transmitting torque from the pistons to the shaft is very complex to manufacture. Currently, three design options are known: planetary gear, eccentric assembly, and sliders. Unlike a traditional crankshaft, these mechanisms require the highest level of manufacturing excellence. Manufacturing tolerances are minimal, which creates the problem of thermal expansion—with uneven heating, the connecting rodless mechanism could easily seize. Organizing lubrication for the highly loaded crankshaft bearings, eccentrics, and planetary gear was challenging.
Connecting rodless engines were extremely difficult to maintain for their time—balancing equipment was required for adjustment. Operating experience showed that the lateral force, which in a classic crank mechanism is transmitted by the piston to the cylinder wall, does not physically disappear in a connecting rodless engine—it is transferred to the sliders, piston rod guides, or gear teeth. These components have a smaller contact area and experience enormous contact stress, causing rapid wear. Repairing such complex components required a much higher level of technical expertise from mechanics.
Fundamental options for transmitting torque from the pistons to the shaft in a connecting rodless internal combustion engine
It's safe to say that Balandin's idea was significantly ahead of its time. But the prospects for a connecting rod-less engine were undeniable. In the relatively short period from 1938 to 1948, under Balandin's leadership, a number of experimental connecting rod-less engines were developed, built, and tested: the OMB, MB-4, MB-4b, MB-8, MB-8b, OM-127, and OM-127RN, varying in design and power output. Aviation was intended to be the primary application—only here could the high cost and complexity of connecting rod-less engines be tolerated. But jet propulsion systems entered the picture, and Balandin's design remained a technological curiosity.
Cross-section of the MK-127K engine
The pinnacle of "connecting-rodless creativity" can be considered the gigantic M-127K, created in the late 1940s. If not for the jet age, it would have become the primary engine of Soviet aviation. The full name of the machine is the M-127K X-shaped supercharged, twenty-four-cylinder, double-acting, connecting-rodless engine with 10,000 hp. It makes you want to say: they really knew how to make something like that back then! Its displacement is 82 liters, and its weight is remarkably light for such power—just 3450 kg. An even bigger monster was built on the basis of this engine: a connecting-rodless diesel engine with 14,000 hp.
Although the connecting rod-less aircraft engine proved unviable, this did not spell the end of the development. A unique exhibit—the ZIL-BD-1800 engine—can be seen at the Special Purpose Garage Museum at VDNKh. Naturally, this engine was created based on Sergei Balandin's ideas. The idea arose in late 1979, when engineer V.G. Tishakov proposed to the designers of the Moscow Automobile Plant's diesel design bureau that they create a fundamentally new type of power plant. The concept attracted significant interest from management: leading specialists from ZIL's Design and Experimental Work Department (DEDW)—chief designer V.K. Koshkin and his deputy A.V. Bragin—personally oversaw the development.
ZIL-BD-1800
The journey from theoretical calculations to actual metal construction took several years: first, in 1985, an experimental two-cylinder opposed engine model was built to test the mechanism's viability. The accumulated experience allowed for the assembly of two full-scale ZIL-BD-1800 prototypes with four cylinders by 1989. Reportedly, the second prototype ran on diesel fuel. The engine featured a compact opposed engine architecture and unique kinematics: the crankshaft performed a complex double rotation (around its own axis and around the crankshaft axis). The piston-rod assembly's motion followed a strict sinusoidal motion, eliminating parasitic higher-order inertial forces, and the use of linear plain bearings completely eliminated lateral friction forces on the cylinders and pistons, guaranteeing high efficiency and smooth operation.
Precise engine data is not publicly available, so we can only speculate. If we extrapolate Balandin's engine specifications to the ZIL development, the 1,8-liter engine could have produced over 110–120 hp. The engine weighed no more than 70–80 kg. Even for today's engine technology, these are remarkable specifications, let alone for the late 80s. However, the ZIL-BD-1800 remained an experimental design, eventually finding its place in a museum. The 90s in the new Russia spared no one, and the connecting rod-less engine was no exception.
Piston rod and crankshaft of a connecting rodless OMB
A rebirth of Balandin's idea in mass production is out of the question. The costs of converting automobiles and piston aircraft to connecting rod-less internal combustion engines are too high, and the dividends don't inspire investors. Meanwhile, modern manufacturing practices allow for the production of engines with acceptable levels of quality and reliability. Domestic engine enthusiasts, working in their garages, rarely, but methodically demonstrate working versions of connecting rod-less engines. Balandin's engine concept has gained unexpected relevance in unmanned aircraft. A compact connecting rod-less engine is almost ideal as a power plant for medium- and long-range UAVs. Sergey Stepanovich described the nuances of engine development in great detail, and this is clearly worth taking advantage of—or at least conducting an expert assessment of the feasibility of introducing connecting rod-less engines into unmanned aircraft.
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