LiquidPiston announced a U.S. Army Small Business Innovation Research (SBIR) contract on December 15, 2020, to develop its X-Engine platform for compact 2–5 kW tactical generators. The award and an Army-tested 2 kW prototype show development progress—not a production purchase, a fielded replacement, or proof of current deployment. The “inside-out” name refers to the engine’s reversed rotor-and-housing arrangement compared with a Wankel rotary engine.
What the Army contract covered
LiquidPiston’s December 15, 2020 announcement described an SBIR development contract to mature the X-Engine for small tactical generators in the 2–5 kW range. The company said the award followed delivery of a Compact Artillery Power System (CAPS) prototype for Army field testing.
LiquidPiston described CAPS as a 2 kW diesel generator prototype intended to power the M777 Howitzer’s Digital Fire Control System. The announcement quoted Christopher Hatch, Deputy Program Manager for the U.S. Army PM for Towed Artillery Systems: “Initial testing of the CAPS generator with the M777 Howitzer to power the Digital Fire Control System was successful.” This is Hatch’s statement as quoted by the company, not a separately retrieved Army statement.
The announcement establishes a research and development award and prototype testing. It does not establish that the Army procured a production system or fielded it as standard equipment.
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What “inside-out” means
A conventional Wankel engine has a triangular rotor moving inside an oval or trochoidal housing. LiquidPiston describes its X-Engine as reversing that geometry: an oval or peanut-shaped rotor moves within a three-lobed housing. Aviation Week’s October 2020 description notes that the X-Engine places apex and face seals on the stationary housing; LiquidPiston says this arrangement allows direct lubrication of those seals.
The company’s High Efficiency Hybrid Cycle (HEHC) is intended to combine features associated with several combustion cycles: high compression associated with the Diesel cycle, constant-volume combustion associated with the Otto cycle, and over-expansion associated with the Atkinson cycle. LiquidPiston presents this as an efficiency-oriented design approach. Its technical-paper summaries include a 45% brake thermal efficiency figure as a target or potential result for a particular development program—not as a general measured result for every X-Engine.
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What the published generator figures show
The following figures are from LiquidPiston’s December 2020 announcement. They are company-reported comparisons, not independently verified results. The 5 kW LiquidPiston configuration was described as a concept.
| Configuration | Output | Weight | Volume | Status in the announcement |
|---|---|---|---|---|
| LiquidPiston CAPS | 2 kW | 41 lb | 1.5 cu. ft. | Diesel prototype described as delivered for Army testing |
| Army MEP-501A | 2 kW | 123 lb | 6 cu. ft. | Comparison unit |
| LiquidPiston small tactical generator | 5 kW | About 100 lb | 4–6 cu. ft. | Concept, not a fielded replacement |
| Army MEP-1030 | Not stated in the announcement comparison | 764 lb | 30 cu. ft. | Comparison unit |
Those figures offer a snapshot of the company’s stated size and weight goals in 2020. They do not establish equivalent performance across duty cycles, fuel use, durability, or operational conditions; the announcement does not provide those comparative test details.
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Battery support and silent operation
LiquidPiston said CAPS could operate as part of a battery-supported hybrid system, with the battery providing power during silent periods. Its announcement claimed that CAPS paired with a 6T lithium-ion battery could provide up to five hours of silent operation for each hour of generator operation. That is a company-reported claim for the described system, not a universal runtime figure for X-Engine generators.
Other applications were development work, not deployment proof
Aviation Week reported in October 2020 on Army-backed work involving hybrid-electric propulsion for unmanned aircraft and auxiliary power for rotorcraft, alongside prior demonstrations reported by the company. These are historical demonstrations and development aims; they do not establish current service deployment.
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What remains unconfirmed
The cited material does not establish the Army contract’s current status, production procurement, present-day durability or endurance results, or consumer availability. New Atlas reported in December 2020 that CEO Alec Shkolnik described testing at that time as measured in tens of hours rather than hundreds; that is historical context, not a statement of the engine’s current test status. See New Atlas’s December 2020 report.
A U.S. SBIR topic page lists corrosion, abrasive wear, seal degradation, heat management, and lubrication as general challenges for rotary engines in naval environments. Those are broad environmental concerns, not LiquidPiston test results or qualification findings. The page is available at SBIR.gov.
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The cited sources describe military prototypes, research awards, technical papers, and possible applications. They do not establish a consumer X-Engine or CAPS generator for sale.
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