This video provides a technical deep-dive into the design, constraints, and thermodynamic calculations for a custom-built Stirling engine intended to power a bicycle. The creator explains how physical limitations, such as his CNC machine's 50mm cutting depth, influenced the engine's dimensions. Using a detailed spreadsheet, he models the engine's ideal performance based on the Carnot cycle, revealing that a critical design flaw—a non-airtight displacer—is currently halving its potential output. The video outlines a clear path to achieving the target 100-150 watts of power through key improvements: sealing the displacer, pressurizing the system to 8 atmospheres, implementing a heat-recycling regenerator to improve thermal efficiency, and adding a water-cooling system.
Dissecting the Stirling Engine Bike: Design, Flaws, and Path to Power
00:00:01 Project Goal: This video is a technical follow-up explaining the design of a Stirling engine built to power a bicycle, with a target output of 100-150 watts for a 15 mph cruise speed.
00:34:00 Key Design Constraints: The engine's size was dictated by the need to fit a burner and the limitations of the creator's CNC machine, which has a maximum cutting depth of 50mm, directly constraining the piston's stroke.
01:51:00 Inefficiency of "Dead Volume": The design places the hot and cold cylinders very close together to minimize the volume of air that is not actively participating in the heating and cooling cycle, as this "dead volume" is a primary source of inefficiency.
02:37:00 Thermodynamic Modeling: The creator uses a spreadsheet to run ideal-case calculations based on the Carnot cycle. It models variables like temperature, pressure, and volume to predict the engine's theoretical power output on a pressure-volume graph.
06:32:00 Initial Ideal Performance: Even in a 100% ideal scenario ignoring friction and leaks, the engine's current configuration is calculated to produce only 14.5 watts at 200 RPM.
07:25:00 Critical Flaw Identified: The creator realized the displacer was not made airtight. His calculations show that this single flaw is likely cutting the theoretical power output in half. Sealing it is the first major step for improvement.
07:59:00 Strategy 1: Pressurization: The engine was designed to run on high pressure. The model shows that pressurizing the engine to 8 atmospheres (approx. 100 PSI gauge) could theoretically boost the output to 116 watts.
09:00:00 Strategy 2: The Regenerator: Pressurizing the engine creates an enormous demand for heat (over 2.8 kW). To make this practical, a regenerator (e.g., high-surface-area steel wool) will be added to absorb and recycle heat as air moves between the hot and cold sides, significantly improving thermal efficiency.
11:03:00 Strategy 3: Enhanced Cooling: A water-cooling system for the engine's cold side is planned to lower the minimum operating temperature, which increases the overall temperature difference and thus improves efficiency and power.
11:58:00 Clear Next Steps: The creator's plan to increase power is to 1) make the displacer airtight, 2) add water cooling to the cold cylinder, 3) experiment with crank lengths, and 4) begin pressurizing the engine.