A New Jersey company is working on some atypical heat engine prototypes, with architecture at odds with conventional blueprints. LiquidPiston, this is the name of the American company, offers a particularly compact and light combustion engine, which in mobility can become relevant for hybrid vehicles in the role of a range extender, but if necessary it can be raised to a power of a thousand horsepower. Starting from the basic slewing structure, the impeller inherits other advantages, including the absence of valves and additional moving parts.
At the same time, LiquidPiston addresses the typical problems of rotary engines, proposing a new architecture that the company itself presents as an “inverted Wankel”. The first subversion lies in the geometry: the engine is actually equipped with Triangular stator and two-lobe rotor, unlike a Wankel where the stator has a two-lobed shape instead, while the rotor has a triangular outline. Another coup lies in the air exchange processes. In fact, on Wankel the intake and exhaust ports are obtained inside the stator, while LiquidPiston moves everything inside the rotor.
This is a fundamental difference that leads to another big news. The combustion chambers of Wankel units rotate with the rotor, while in the American rotor they remain limited to the three “vertices” of the triangle obtained in the stator. This gives you more freedom to control the different stages, Extension of the stretching phase in relation to the compression phase To maximize the beneficial work of combustion, based on the principle that governs the Atkinson cycle.
In the LiquidPiston prototype, the air flow passes the engine along its axis. Much of this flow passes the engine from one side to the other and is useful for cooling it, But part of the air gets inside the shaft into the engine. The intake air is thus delivered from time to time to the individual combustion chambers which are encountered during rotor rotation.
At this point, in a manner similar to what happens in Wankel units, the movement of the rotor compresses and expands the combustion chambers, carrying out the stages of compression, combustion and expansion. at this point Burnt gases flow into the second exhaust duct formed in the rotorwhen this meets the single combustion chamber as it rotates.
LiquidPiston stresses that it is its own engine Maintain the advantages of compactness, lightness, simplicity and balance typical of rotary machines. However, compared to the Wankel engine, the American engine has combustion chambers with a better volume-to-surface ratio, which improves flame spread and reduces heat loss to the walls. The ultimate benefit is increased engine efficiency. Moreover, thanks to the bi-lobe shape of the rotor, the seals between the different combustion chambers are positioned directly in the stator. This makes it possible to locally lubricate the area with access from the stator, without having to introduce oil into the chamber, reducing its consumption compared to a conventional Wankel and reducing gas leakage from one chamber to another by more than three times.
Prototypes on the bench: petrol and diesel
Two different models were tested in the LiquidPiston Labs. One of these is the X-MiniFour-stroke petrol unit with indirect injectionwith a displacement of 70 cubic centimeters and a weight of only 2 kilograms. With a 9:1 compression ratio, maximum power is 3.6hp at 9,000rpm, with a thermal efficiency of 22%. In its mature version, however, the engine is estimated to reach 5 bhp at 140,000 rpm, with an efficiency of 25% and specific power of about 71 bhp/l.
Then there Diesel unit tests, which operates according to the patented thermodynamic cycle HEHC, short for High Efficiency Hybrid Cycle, which has typical characteristics of Otto, Atkinson and Diesel cycles. The ignition is automatic, but unlike diesel engines, combustion is rounded off at a constant volume, followed by a prolonged expansion phase compared to the compression phase. At present, the 750 cc, 27 kg prototype delivers 40 bhp at 7,000 rpm, with a compression ratio between 16 and 18:1. However, the ultimate goal is to reduce weight to 18 kg and raising the compression ratio to 26:1. This should make it possible to increase power to 50 hp, Raise thermal efficiency from 33 to 45%.
For both petrol and diesel units, there is the possibility to expand the power, by varying the thickness and diameter of the rotor, as well as increasing the number of rotors, as also seen on Mazda’s Wankel powertrains. It is also present for partial downloads Control strategies for ignition deactivation in individual chambers in alternating order. This logic is similar to the principle of cylinder deactivation, which aims to reduce the number of active chambers to increase load and efficiency. Finally, LiquidPiston provides direct injection of water into the combustion chamber, both with a cooling action and with the aim of increasing the internal pressure by means of steam.
Among the advantages mentioned by LiquidPiston, in addition to compactness and lightness, the absence of valves and the distribution system stands out, which increases mechanical efficiency, simplifies the structure, and contains production costs. Furthermore, thanks to the extended intake stage, turbulence in the exhaust flow is greatly reduced, eliminating the need for a muffler. Finally, like other rotary engines, vibrations are mitigated by the excellent overall balance, however In contrast to Wankels, oil consumption and blowing phenomena between chambers have been significantly reduced.
The potential applications posited by the American company are numerous: Range extender on hybrid vehicles, portable generators, drones, garden vehicles, go-karts, marine propulsion and auxiliary power units for boats and aircraft. Prevailing electrification in the automotive industry discourages major manufacturers from investing in technologies that undermine more than 100 years of heat engine architecture, and whose future looks increasingly uncertain. However, LiquidPiston continues to test its prototypes, waiting to find a commercial outlet for its invention.
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