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Mechanical Engineering 2022 • School Project

Two-Stroke Engine Prototype

Designed and machined a functional two-stroke engine from scratch in a school workshop, learning engine fundamentals and precision manufacturing through hands-on iteration and failure analysis.

Project Overview

During my school years, I spent approximately 2 months building a two-stroke engine from the ground up. Working about 4 hours per week with an intensive push of 20 hours in the final weeks, I designed the engine in CAD and machined every component in the school workshop.

The design targeted a theoretical output of 15 horsepower, and while the engine never achieved sustained operation due to a critical design flaw, it became one of my most valuable learning experiences in mechanical engineering and failure analysis.

Key Achievement: The cylinder and cylinder housing achieved excellent tolerances and compression without proper piston rings—a testament to precision machining fundamentals.

Technical Approach

Design & CAD Modeling

I modeled the entire engine assembly in CAD before beginning fabrication. The design included:

  • Cylinder and piston assembly with calculated displacement for target power output
  • Crankshaft and connecting rod system
  • Intake and exhaust port timing
  • Cooling considerations for sustained operation

Manufacturing Process

All components were machined in the school workshop using manual and CNC equipment:

  • Cylinder boring: Achieved tight tolerances for compression without traditional piston rings
  • Crankshaft fabrication: Turned and assembled with welded joints (later identified as the failure point)
  • Port placement: Precisely positioned intake and exhaust ports for proper two-stroke timing
  • Housing assembly: Fabricated and sealed the crankcase to contain combustion pressures

Technologies Used

CAD Design
CNC Machining
Manual Lathe Work
Welding
Engine Theory
Precision Measurement

Challenges & Failures

The Critical Flaw: Crankshaft Design

The engine's fatal weakness was the crankshaft assembly. The design relied on welded joints that, under operational stress, proved unable to handle the forces involved:

  • Bearing friction: The crankshaft didn't run smoothly enough, creating excessive resistance
  • Pressure amplification: When the crank would stick, combustion pressure had nowhere to go except into the housing and crank arm itself
  • Joint failure: The welded connections repeatedly failed under these concentrated forces, causing the engine to destroy itself during testing

What Worked: Cylinder Precision

Despite the crankshaft issues, the cylinder assembly was remarkably successful:

  • Achieved excellent compression without proper piston rings—something typically requiring very tight tolerances
  • Port timing and placement allowed proper air-fuel mixture flow
  • Demonstrated my ability to machine to demanding specifications with limited tooling

Key Learnings

Engineering Fundamentals

  • Two-stroke theory: Deep understanding of intake/exhaust timing, port design, and combustion cycles
  • Bearing design: The importance of proper bearing selection and crankshaft balance
  • Stress analysis: How forces propagate through mechanical assemblies under dynamic loads

Manufacturing Skills

  • Precision machining: Achieving sub-millimeter tolerances on manual equipment
  • Assembly techniques: Understanding how manufacturing tolerances compound in assemblies
  • Material joining: Learned the limitations of welding for high-stress applications

Failure Analysis & Iteration

  • How to diagnose failure modes through systematic testing and observation
  • The value of identifying root causes rather than treating symptoms
  • Why bearing friction and stress concentration matter more than raw material strength
The Real Success: While the engine never ran continuously, the project taught me more about mechanical design, manufacturing constraints, and failure analysis than any successful build could have. Understanding why something fails is often more valuable than building something that works.

Impact & Applications

The lessons from this project continue to inform my approach to mechanical design:

  • Design validation: I now prioritize testing critical stress points early in prototyping
  • Bearing selection: Proper bearing design is non-negotiable in rotating assemblies
  • Manufacturing constraints: Welded joints require careful engineering analysis for dynamic loads
  • Iterative testing: Build failure modes into early prototypes to identify weaknesses

The cylinder machining skills and precision measurement techniques I developed during this project have proven invaluable in subsequent builds, from the CNC mill to the e-bike motor mounting.

Related Projects

This engine project laid the groundwork for several later builds: