How It Works: The 1-Stroke | Free Piston Engines | That Defy Physics (2026)

In this video, we will analyze absolutely every unique internal combustion engine development that has been introduced in recent years. And most importantly, I will tell you what the further development of each of these designs has been, and which of these internal combustion engines are already in serial production. So it will be especially interesting. Let us begin. But first, let us identify the problems that all these new internal combustion engine developments are trying to solve. Reciprocating motion must be converted into rotational motion. This requires connecting rods and a crankshaft. This leads to a number of other problems. Due to the angle of the connecting rod, friction occurs against one of the cylinder walls. There is also an uneven movement of the piston in the cylinder, which causes torque spikes within a single power stroke. And due to the crank pins on the crankshaft and connecting rod, the pistons move up and down with slightly different acceleration, which also creates imbalance. Second, the camshaft and valves. Energy is also consumed to rotate the camshafts, as they must be spun, and they push relatively stiff springs. In turn, the valves do not allow for high revolutions, because due to those same springs at high revolutions, they will not have time to close, and the piston will simply strike the valve. And third, engines are becoming increasingly complex in the pursuit of higher efficiency. And these new solutions can help with all of this. The Spherical Internal Combustion Engine. The Hüttlen Spherical Engine was developed by German engineer Herbert Hüttlen and introduced in 2011. Structurally, it features four pistons that are paired facing each other, and using guides, their motion is immediately converted into rotation. The goal of the design is to eliminate the crankshaft and connecting rods, thereby simplifying the design and increasing efficiency. The spherical shape also allows for better integration of the engine for energy generation and hybrids. It is claimed to have an efficiency exceeding 30%, uses two times fewer parts, only 62 in total, has an engine displacement of 1.2 liters, or 73.2 cubic inches, a weight of 62 kilograms, or 136.7 pounds, and should produce 100 horsepower. There are several designs, both for a pure internal combustion engine and a hybrid. However, it seems this development has remained at the prototype level, as nothing has been heard about the engine in the last 10 years. Moreover, the engine looks overly complex with its rotating rollers, which likely will not provide the necessary reliability in the long run. The Coaxial Engine. This design looks more promising, and perhaps it is even better than the engines we will discuss next. In this engine, as we can see, the classic crankshaft and connecting rods have been eliminated. The piston moves uniformly up and down with the help of rollers on guides. Additionally, the piston is held longer at the dead centers. Furthermore, the two-piston version is completely free of vibrations. Regarding the valves, they did not try to reinvent anything here, so we still have camshafts and valves. This results in better combustion of the fuel-air mixture. The peak torque is reached earlier and lasts longer, and all the energy is immediately converted into rotation, while reducing the reduction ratio by two times. It is claimed that, theoretically, the internal combustion engine's efficiency can be increased to 55% thanks to these improvements. Nothing has been heard about this development for more than 10 years either, and it is a shame that no working prototypes were presented. However, another company from Spain with a somewhat similar design has gone much further, made everything even better and more efficient, and is now very close to mass-producing this engine for automobiles. This is the one-stroke engine from In-Engine. The operating principle here is partially similar to the coaxial engine, as the piston's trajectory is set by the same principle, but plates are used. But they went even further. It was decided to position the pistons opposite each other, and instead of valves, ports are used, eliminating the need to spend energy on the camshaft and valves. As a result, when the pistons move apart, scavenging occurs, and the exhaust gases are replaced with air. Then the exhaust closes, enrichment occurs, and compression begins. Then the power stroke. This allowed them to eliminate the camshaft. The engine, in turn, operates like a two-stroke, but there is no need to add oil to the fuel. And here, there is also an active variable compression ratio system, achieved by shifting the plates relative to each other, allowing the engine to adapt to any fuel. The flagship E-REX engine, with a displacement of 0.7 liters, or 42.7 cubic inches, and a weight of 38 kilograms, or 83.8 pounds, produces 120 horsepower and 244 newton meters, or 180 pound-feet of torque. This is official and confirmed. And overall, they have demonstrated that their engines work on a dynamometer, in an unmanned aerial vehicle, and most importantly, in a car. But money is needed. Therefore, they first managed to come to an agreement with the horse company, which is mostly owned by China's Geely and France's Renault, and which develops and produces power units for many automakers. This is one of the largest companies in the world for internal combustion engine production, and they are taking the In-Engine for testing. This will be done in the near future, and after which, if horse is satisfied, In-Engine will receive funding, and Geely will receive an internal combustion engine that is 55% smaller, 70% lighter, and 22% more efficient. And in very recent news, In-Engine has signed a contract with Airbus, and as I understand from the Spanish channels, it is with the military division of Airbus. Therefore, a simplified version of the Rex B engine, which is more suitable for drones, will likely be used here. But, since this is a military application, further development and implementation in unmanned aerial vehicles will likely be classified. Although, on the other hand, they are showing how they are installing this engine into a single-seat passenger quadcopter. So, In-Engine probably has a future, and reputable companies believe in this design. The Cat-Gen gas turbine engine, or what they call the catalytic generator. Here, instead of valves and pistons, Cosworth decided to go back to the origins, where none of this is used. Instead, a very compact gas turbine engine is employed for power generation, and the most interesting thing is that anything that burns can be used to generate energy. The power is stated as 48 horsepower, and the weight is 48 kilograms, or 105.8 pounds, with a fuel consumption of 5 liters. This gas turbine engine was tested in vans under real-world conditions. They then planned to implement it in the aerial hypercar, but Cosworth signed with the British military company Babcock, and all news about this development disappeared. This is because the military has big money, and unfortunately, it is always secret. But, on the other hand, it eventually reaches ordinary users. The development from the Center for Energy Research with a four-piston linear motor generator has faded, and nothing more is heard about it. There is also no news about the Toyota W piston engine with gas spring chambers. And the Aquarius engine, as we know, first received tens of millions in investment, and then they signed a contract with the United States Army, which, according to the latest information, will last until mid-2027, and when the military arrives, all news disappears again. But this means that this engine is most likely already in serial production, and according to the latest information, it is used in ultralight generators with a power of 32 kilowatts, or 42.9 horsepower. This engine attracted special attention because it is the simplest design possible, and probably the most efficient. Here, there is only one piston and one cylinder, but there are immediately two combustion chambers, and a power stroke occurs with absolutely every movement of the piston. Let us look. Air enters through the hollow piston rod, and the exhaust port is located in the middle of the cylinder wall. So, in the right chamber, air intake is now occurring, followed by compression. The injector adds fuel, and ignition occurs. And when the piston reaches the middle of the cylinder, the exhaust port in the wall opens, as does the intake in the hollow rod. Scavenging begins, and the exact same thing happens in the right combustion chamber. As a result, there are just over 20 parts here, the weight is only 10 kilograms, or 22.0 pounds, and it produces 32 kilowatts of energy. This is ideal for a portable generator in the military, but with mass production, its cost could possibly be reduced to $100. And perhaps this generator may reach hybrid cars, but for now, the military is hiding all the information. There is also progress in the development of rotary engines. Astron Aerospace initially developed the Omega-1 rotary engine, but all mentions of it have been deleted, and now they are working on a new H2 Starfire engine. This engine features two chambers, one in the front and one in the rear. And in each chamber, there is a rotating piston. Currently in the front, intake occurs on one side of the piston, and compression on the other. The compressed mixture then enters the chamber on the other side of the engine through a valve, where ignition and the power stroke occur, and exhaust gases exit on the other side of the piston. This means that four strokes occur simultaneously in just one revolution of the engine. And to my surprise, this project is actively developing. They have now shown that the engine is capable of running at 3,000, 4,000, and 5,000 revolutions per minute, and can operate for a long time with minimal heating. Now the bearings are warming up. They're over 100. Only the upper bearings. The weight of this engine is 54 kilograms, or 119.0 pounds. It produces 450 horsepower and 813 newton meters, or 600 pound feet of torque. The maximum revolutions are 25,000 revolutions per minute, which is unattainable for a conventional piston engine. And its size is only 30 by 40 centimeters, or 11.8 by 15.7 inches. So this engine is being tested. Now everything is leaking out of all the seams, and we see that it is truly capable of working. Another important rotary engine is the rotor from Liquid Piston. Here there is a piston where the intake and exhaust are located directly in the piston, and these open and close through the movement of the rotor. As a result, unlike the Wankel rotor, there are not one, but three power strokes per revolution, and additionally, better sealing is achieved. Many are skeptical of this engine, but Liquid Piston, at one time, were among the first developers to show that their engine was very much operational, and thanks to this, they received the most funding, by my estimates, over 65 million dollars. And no one hands out such large amounts of money just like that. But what happened next? That is right. The military arrived, specifically the United States, and a contract was signed. Therefore, this development has almost disappeared from sight. It is only mentioned that Liquid Piston produces ultralight diesel generators based on their rotor with a power output of 10 kilowatts, or 13.4 horsepower, and is also currently developing hybrid power systems for command centers. What conclusions can we draw from all of this? The only one of these engines that currently might enter serial production, thanks to the Chinese, is the E-REX engine, again thanks to horsepower train, and only if it passes all tests, then production is organized, and so on. As for all the other developments, as soon as any real tests appear, as soon as a company shows that its engine truly works, as soon as they demonstrate that it is even slightly more efficient than what is currently available, military companies arrive, sign a contract, where, of course, nothing can be disclosed, give tens of millions of dollars, and the company continues to develop in the shadows. And perhaps later, someday, these technologies will actually reach us. Write in the comments what you think about this.