Free Piston Stirling Engine Overstroke Limiting Mechanism

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Solution Overview

Problem

Free-piston Stirling engines are not tolerant to loss of load, leading to unstable operation and potential engine damage due to increased piston amplitude, which can result in collisions with internal components and debris generation.

Innovation Solution

The engine is modified by positioning the heat rejecter cylinder port to be covered by the piston sidewall during peak power, adding a leaker port to vent gas bearing cavity pressure, and incorporating a resilient bumper to limit displacer motion, thereby reducing engine power and preventing runaway piston amplitude.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If the engine is designed to operate at maximum power, then the power output is optimized, but the piston amplitude becomes unstable and increases beyond safe limits when load is reduced

Engineering Contradiction:

Improvepower outputVSAvoidpiston amplitude stability

Core Design Contradiction:

PowerVSStability of the object's composition

Solution Approach 1:

The heat rejecter cylinder port is positioned to be covered by the piston sidewall during peak power operation, proactively preventing excessive piston amplitude before it can cause damage. This preliminary positioning creates a mechanical constraint that counteracts the natural tendency of the piston to overstroke when load is reduced, thereby resolving the contradiction between maximizing power output and maintaining piston amplitude stability.

Inventive Principle:

Principle #9Preliminary anti-action

2Power

If the piston amplitude is increased to produce more power, then the power output increases, but the piston collides with internal components causing engine damage

Engineering Contradiction:

Improvepower outputVSAvoidpiston collision damage

Core Design Contradiction:

PowerVSObject-affected harmful factors

Solution Approach 1:

The resilient bumper is positioned to contact the piston during excessive amplitude excursions, providing a cushioning force that prevents the piston from colliding with harmful internal components. This beforehand cushioning mechanism absorbs the excess kinetic energy of the piston when amplitude becomes unstable, thereby protecting the engine from damage while allowing the piston to continue operating at high power outputs.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

3Speed

If the engine operates under reduced or zero load, then the engine runs at higher speed, but the piston and displacer overstroke and collide with physical structures

Engineering Contradiction:

Improveengine speedVSAvoidpiston and displacer collision

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

The heat rejecter cylinder port positioning creates a mechanical constraint that proactively prevents overstroke collisions before they occur during high-speed operation under reduced load. By covering the port during peak power, the system establishes a physical boundary that counteracts the increased piston and displacer excursions that would otherwise cause collisions at higher speeds.

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The resilient bumper provides beforehand cushioning by contacting the piston during excessive amplitude excursions that occur during high-speed operation. This cushioning mechanism absorbs the excess kinetic energy and prevents the piston and displacer from colliding with physical structures, thereby enabling safe high-speed operation under reduced or zero load conditions.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

4Reliability

If the piston amplitude is limited to prevent collisions, then engine damage is prevented, but the power output is reduced

Engineering Contradiction:

Improveengine safetyVSAvoidpower output

Core Design Contradiction:

ReliabilityVSPower

Solution Approach 1:

The heat rejecter cylinder port is positioned to be covered only during peak power operation, creating a localized constraint that limits piston amplitude only when necessary to prevent damage. This local quality approach allows the piston to operate at full amplitude during normal operation for maximum power output, while providing protection only during critical high-power excursions, thereby resolving the contradiction between engine safety and power output.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The resilient bumper provides beforehand cushioning that protects the engine from damage during excessive amplitude excursions without continuously limiting the piston amplitude during normal operation. This cushioning mechanism allows the piston to achieve maximum power output during steady-state operation while providing protection only when amplitude becomes excessive, thereby resolving the contradiction between reliability and power output.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

These modifications stabilize the engine by reducing power output and preventing engine damage from excessive piston amplitude, ensuring safe operation even under reduced or zero load conditions.

Implementation Method 1

a heat acceptor 14, which transfers externally applied heat into the working gas

Methodology Applied

Scientific EffectHeat transfer: Conduction (thermal)

Implementation Method 2

a heat acceptor 14, which transfers externally applied heat into the working gas

Methodology Applied

Scientific EffectConvection: Convection

Implementation Method 3

a heat rejecter 16, which transfers heat out of the working gas

Methodology Applied

Scientific EffectHeat transfer: Conduction (thermal)

Implementation Method 4

a heat rejecter 16, which transfers heat out of the working gas

Methodology Applied

Scientific EffectConvection: Convection

Implementation Method 5

The periodic increase and decrease of the pressure of the working gas in the working space 8 drive both the piston 28 and the displacer 30 in reciprocation

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 6

Gas that is in the compression space 12 and gas that is flowing into or out of the compression space 12 through the heat rejecter 16 rejects heat to surrounding surfaces

Methodology Applied

Scientific EffectGas compression: Compression

Implementation Method 7

The piston 28 also has an annular cutout or relieved portion to form a central cap or boss 36 that is unrelated to the invention

Methodology Applied

Scientific EffectGas lubrication: Air Lubrication

Data Source

PatentUS10323603B2Free piston stirling engine that limits overstroke

Publication Date: 2019.06.18 SUNPOWER INC

AI summary

A free-piston Stirling engine that limits piston amplitude and reduces engine power as the piston amplitude increases beyond its maximum power. The inward edge of the heat rejecter cylinder port is located outward of the most inward excursion of the inward end of the piston sidewall during a part of the piston's reciprocation cycle so that the heat rejecter cylinder port is entirely covered by the piston sidewall during an inward portion of the piston reciprocation when the engine is operating at the selected maximum engine power. A leaker port extends from a gas bearing cavity through the piston sidewall and is positioned axially outward from the gas bearing pads of the engine's gas bearing system and vents working gas to the engine's back space at a piston amplitude of reciprocation that exceeds the piston's amplitude of reciprocation at maximum engine power. A resilient damping bumper is attached to the outward end of the piston and a displacer gas cushion is disclosed.