Free piston: free piston engine

Already in November this year, the Chevrolet Volt, an electric vehicle with an on-board power generator, will enter the US market. Volt will be equipped with a powerful electric motor that rotates the wheels, and a compact ICE that only recharges a depleted lithium-ion battery. This unit always works at the most efficient speed. The usual ICE, accustomed to a much heavier burden, can easily cope with this task. However, soon it can be replaced by much more compact, lightweight, efficient and cheap units, specially designed to work as an electric generator.

When it comes to fundamentally new designs of ICEs, skeptics start wrinkling their noses, nodding at hundreds of pseudo-revolutionary projects dusting on shelves and shaking the holy relics of four pots and a camshaft. One hundred years of domination of the classic internal combustion engine anyone you want to convince of the futility of innovation. But not professionals in the field of thermodynamics. These include Professor Peter Van Blarigan.

Energy locked up

One of the most radical ICE concepts in history is a free piston engine. The first mention of it in special literature dates back to the 1920s. Imagine a metal pipe with blind ends and a cylindrical piston sliding inside it. At each end of the pipe there is an injector for fuel injection, inlet and outlet ports. Depending on the type of fuel, spark plugs may be added to them. And that’s it: less than a dozen of the simplest parts and only one - moving. Later, more sophisticated models of the internal combustion engine with a free piston (FPE) appeared - with two or even four opposed pistons, but this did not change the essence. The principle of operation of such motors remained the same - the reciprocating linear motion of the piston in the cylinder between the two combustion chambers.

Where efficiency goes Peter Van Blarigan’s engine differs from a conventional ICE in significantly higher efficiency due to the absence of spurious losses. The design does not contain rotating masses whose inertia increases due to centrifugal force. The lateral forces that press them against the cylinder walls do not act on the pistons, thereby reducing friction. Bearings of the crankshaft and connecting rods, piston pins, camshaft, cams and valves - all those parts of the OTTO engine in which friction is rampant are absent in FPLA. In addition, for each cycle of the engine with a free piston there are two working cycles. At the same time, FPLA is much more compact, simpler and more reliable than a conventional ICE. A working prototype of Van Blarigan’s motor was already embodied in metal and successfully passed the first stage of testing.

Theoretically, the FPE efficiency exceeds 70%. They can work on any type of liquid or gaseous fuel, they are extremely reliable and perfectly balanced. In addition, their lightness, compactness and ease of production are obvious. The only problem: how to remove power from such a motor, which is mechanically a closed system? How to saddle a scurry piston with a frequency of up to 20, 000 cycles per minute? You can use the pressure of the exhaust gases, but the efficiency at the same time drops at times. This task remained unsolvable for a long time, although attempts were made regularly. General Motors engineers were the last to break their teeth in the 1960s while developing a compressor for an experimental gas turbine car. The existing samples of marine pumps based on FPE in the early 1980s were made by the French company Sigma and the British Alan Muntz, but they did not go into a series.

Bentley brings together 1, 321 cars

Perhaps FPE would not have been remembered for a long time, but chance helped. In 1994, the US Department of Energy commissioned Sandia National Laboratory scientists to study the efficiency of various types of hydrogen-powered onboard electric power generators. This work was entrusted to a group of Peter Van Blarigan. During the implementation of the project, Van Blarigan, who was well aware of the FPE concept, managed to find a witty solution to the problem of converting the mechanical energy of a piston into electricity. Instead of complicating the design, which means reducing the resulting efficiency, Van Blarigan went by subtraction, calling for help from the magnetic piston and copper winding on the cylinder. Despite its simplicity, such a solution would have been impossible neither in the 1960s nor in the 1970s. At that time, compact and powerful permanent magnets did not yet exist. Everything changed in the early 1980s after the invention of an alloy based on neodymium, iron and boron.

Shtelser's engine diagram A single part combines two pistons, a fuel pump and a valve system.

For this work, at the 1998 SAE World Congress of Automotive Engineers Society, Van Blarigan and his colleagues Nick Paradiso and Scott Goldsboro were awarded the Harry Lee Van Horning Honorary Prize. The obvious promise of a free piston linear generator (FPLA), as Van Blarigan called his invention, convinced the Department of Energy to continue financing the project right up to the experimental unit stage.

Electronic ping pong

The Blarigan push-pull linear generator is a pipe made of electrical silicon steel 30.5 cm long, 13.5 cm in diameter and weighing a little over 22 kg. The inner wall of the cylinder is a stator with 78 turns of square copper wire. Powerful neodymium magnets are integrated into the outer surface of the aluminum piston. The fuel charge and air enter the combustion chamber of the engine in the form of fog after preliminary homogenization. Ignition occurs in HCCI mode - in the chamber at the same time there are many microcenters of ignition. FPLA does not have a mechanical valve timing system - the piston itself performs its functions.


Frank Stelser pipe

In 1981, the German inventor Frank Shtelser demonstrated a two-stroke free piston motor that he had been developing in his garage since the early 1970s. According to his calculations, the engine was 30% more economical than a conventional ICE. The only moving part of the motor is a twin piston, scurrying at a frantic frequency inside the cylinder. A steel pipe 80 cm long, equipped with a low-pressure carburetor from a Harley-Davidson motorcycle and a block of ignition coils Honda, according to Stelser's rough estimates, could produce up to 200 hp. power at a frequency of up to 20, 000 cycles per minute. Shtelser argued that his motors can be made of simple steels, and they can be cooled with both air and liquid. In 1981, the inventor brought his motor to the Frankfurt International Motor Show, hoping to interest leading car companies. Initially, the idea aroused some interest from German auto manufacturers. According to Opel engineers, the prototype engine showed excellent thermal efficiency, and its reliability was completely obvious - there was practically nothing to break. Only eight parts, of which one is moving - a twin piston of complex shape with a system of o-rings with a total weight of 5 kg. In the Opel laboratory, several theoretical transmission models for the Stelser motor were developed, including mechanical, electromagnetic and hydraulic. But not one of them was recognized as sufficiently reliable and effective. After the Frankfurt Motor Show, Shtelser and his brainchild disappeared from the field of view of the auto industry. A couple of years after that, the press constantly reported on Shtelser's intentions to patent technology in 18 countries, to equip desalination plants with his own motors in Oman and Saudi Arabia, etc. Since the early 1990s, Shtelser has disappeared forever, although he a site on the Internet is still available.

The maximum power of the FPLA is 40 kW (55 horses) with an average fuel consumption of 140 g per 1 kWh. In terms of efficiency, the engine is not inferior to hydrogen fuel cells - the thermal efficiency of the generator when using hydrogen and a compression ratio of 30: 1 reaches 65%. Propane is slightly less - 56%. In addition to these two gases, FPLA with appetite digests diesel fuel, gasoline, ethanol, alcohol and even used vegetable oil.

However, nothing comes of little blood. If the problem of converting thermal energy into electrical energy by Van Blarigan was successfully solved, then controlling the capricious piston became a serious headache. The top dead center of the trajectory depends on the degree of compression and the rate of combustion of the fuel charge. In fact, piston braking occurs due to the creation of critical pressure in the chamber and subsequent spontaneous combustion of the mixture. In a conventional ICE, each subsequent cycle is analogous to the previous one due to the rigid mechanical bonds between the pistons and the crankshaft. In FPLA, the cycle time and top dead center are floating values. The slightest inaccuracy in the dosage of the fuel charge or the instability of the combustion mode causes the piston to stop or hit one of the side walls.

Green and flat The Ecomotors engine is not only modest in size and weight. Externally, the flat unit resembles the boxer engines Subaru and Porsche, which give special layout advantages in the form of a low center of gravity and the bonnet line. This means that the car will be not only dynamic, but also well-controlled.

Thus, an engine of this type requires a powerful and high-speed electronic control system. Creating it is not as easy as it seems. Many experts consider this task difficult. Harry Smythe, General Motors power plant science supervisor, says: “Free-piston internal combustion engines have a number of unique advantages. But in order to create a reliable production unit, you still need to learn a lot about the thermodynamics of FPE and learn how to control the combustion process of the mixture. ” Professor of the Massachusetts Institute of Technology John Heywood echoed him: “There are still many blank spots in this area. It’s not a fact that for FPE it will be possible to develop a simple and cheap control system. ”

Van Blarigan is more optimistic than his colleagues. He argues that the control of the position of the piston can be reliably ensured through the same pair - the stator and the magnetic shell of the piston. Moreover, he believes that a full-fledged prototype generator with a tuned control system and an efficiency of at least 50% will be ready by the end of 2010. An indirect confirmation of the progress in this project is the secretion in 2009 of many aspects of the Van Blarigan group's activities.

Who has a longer connecting rod A significant part of the friction loss in conventional ICEs is due to the connecting rod turns relative to the piston. Short cranks rotate at a greater angle than long ones. OPOC has very long and relatively heavy connecting rods that reduce friction losses. The unique design of OPOC rods does not require the use of piston pins for internal pistons. Instead, they use radial concave nests of large diameter, inside which the connecting rod head slides. Theoretically, such a design of the assembly allows to make the connecting rod 67% longer than usual. In a conventional ICE, severe friction losses occur in loaded crankshaft bearings during the operating cycle. In OPOC this problem does not exist at all - linear multidirectional loads on the internal and external pistons completely compensate each other. Therefore, instead of five crankshaft thrust bearings, only two are required for OPOC.

Constructive opposition

In January 2008, the famous venture investor Vinod Khosla declassified one of his latest projects - EcoMotors, created a year earlier by John Coletti and Peter Hoffbauer, two recognized motor industry gurus. Hoffbauer's track record has many breakthrough developments: the first turbo diesel for Volkswagen and Audi cars, the boxer engine for Beetle, the first 6-cylinder diesel for Volvo, the first 6-cylinder Inline-Compact-V diesel engine, first installed in the Golf, and its twin VR6 created for Mercedes. John Coletti is no less famous among automotive engineers. For a long time he led the Ford SVT division for the development of special series of charged cars.

The total assets of Hoffbauer and Coletti are more than 150 patents, participation in 30 projects for the development of new engines and in 25 projects of new production cars. EcoMotors was created specifically for the commercialization of the Hoffbauer invented modular two-cylinder two-stroke opposed turbodiesel with OPOC technology.

Small size, crazy power density of 3.25 hp per 1 kg of weight (250 hp per 1 liter of volume) and tank draft of 900 N • m with more than modest appetite, the ability to assemble 4-, 6- and 8-cylinder blocks from separate modules are the main advantages of the OPOC 100-kilogram module EM100. While modern diesel engines are 20–40% more efficient than gasoline ICEs, OPOC is 50% more efficient than the best turbo diesel engines. Its design efficiency is 57%. Despite its fantastic charge, the Hoffbauer engine is characterized by perfect balance and very soft work.

At OPOC, the pistons are connected to the central crankshaft by long connecting rods. The space between the two pistons serves as a combustion chamber. The fuel injector is located in the region of the top dead center, and the inlet air port and the exhaust port for exhaust gases are in the region of the bottom dead center. This arrangement, coupled with an electric turbocharger, ensures optimal cylinder purge - there are no valves or camshafts in OPOC.

A turbocharger is an integral part of the engine, without which its operation is impossible. Before starting the engine, the turbocharger heats a portion of air to a temperature of 100 ° C for one second and pumps it into the combustion chamber. OPOC diesel does not need glow plugs, and starting in cold weather does not cause problems. At the same time, Hoffbauer was able to reduce the compression ratio from the usual for diesel engines 19−22: 1 to a modest 15−16. All this, in turn, leads to a decrease in the operating temperature in the combustion chamber and fuel consumption.

Trojan horse

Already today, EcoMotors has three fully-engineered boxer units of various capacities: a 13.5 hp module. (dimensions - 95 mm / 155 mm / 410 mm, weight - 6 kg), 40 hp (95 mm / 245 mm / 410 mm, 18 kg) and 325hp module (400 mm / 890 mm / 1000 mm, 100 kg). Hoffbauer and Coletti intend to demonstrate an electric hybrid five-seater middle class sedan with an OPOC diesel generator based on one of the mass models this year. The average diesel fuel consumption of this car will not exceed 2 liters per hundred in combined electric and mixed modes. Recently EcoMotors has opened its own technical center in the city of Troy, Michigan, and is already looking for a suitable enterprise for the mass production of its motors. Despite the declassification of the project, extremely scarce information comes from the bowels of the company. Apparently, Vinod Khosla decided to keep the slaughter trumps for the time being.

The article was published in the journal Popular Mechanics (No. 4, April 2010). Everything for cars ...

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