DIY CDI Unit for Motor-Assisted Bicycles
This article outlines the creation of a custom Capacitor Discharge Ignition (CDI) unit designed specifically for Spartamet and Saxonette motor-assisted bicycles, commonly referred to as mopeds. The motivation behind this project stemmed from the author’s personal experience of relying on a Spartamet for commuting between home and work. During this period, it became evident that although the moped functioned well under regular conditions, issues arose when operating at maximum throttle and top speed (15 mph).
Notably, the ignition system started misfiring, leading to reduced performance and increased fuel consumption. The fuel efficiency dropped significantly, from 118 miles per gallon (mpg) at 3/4 throttle to 71 mpg at full throttle. Subsequent investigation, including examining the spark plug and exhaust after multiple rides, confirmed the suspicion that the ignition misfires were directly linked to the heightened fuel consumption.
Overcoming Ignition Limitations: DIY CDI Unit Modification
The original design of the 30 cc two-stroke engine’s CDI (Capacitive Discharge Ignition) unit intentionally causes ignition to skip sparks when the engine reaches full throttle and top speed. This design choice serves as an electronic speed limiter, ensuring the moped adheres to legal road speed limits. However, this method results in unburnt fuel accumulating in the exhaust, impacting both fuel efficiency and the exhaust system’s longevity. Modifying the existing CDI unit is challenging due to its potting compound casing. To address this, an alternative circuit was developed, focusing on generating sparks without limiting the engine’s rpm. The resulting schematic efficiently manages a capacitor discharge into the ignition coil at the precise moment, ensuring optimal engine performance.
Compact and Essential PCB Design
A single-sided PCB layout has been devised for the circuit, available for download . Components are mounted on both sides to maintain the original CDI unit’s size (59x38x24 mm). Diodes D1, D2, DI1, and C2 are first soldered onto the component side, followed by D3 and thyristor TH1. These components are strategically placed to align with the board. MKP capacitor (C1) is positioned alongside the board, while varistor (VR1) and resistor (R1) are mounted on the solder side. Additionally, three spade terminals are soldered onto the board. Enclosure options include a small Hammond box (001100) or a homemade acrylic box (cassette or CD case). Proper encapsulation with potting compound is crucial to withstand engine vibrations and ensure long-term functionality.
Effective Implementation and Performance Boost
Upon installation and usage, the positive impact of the modified CDI unit becomes immediately apparent. The engine operates much smoother at full throttle, eliminating misfires. The average fuel consumption significantly improves to 166 miles per gallon (mpg). While the mechanical limitations of the engine restrict substantial speed enhancements, a modest increase of 2 to 2.5 mph is observed. The primary advantages lie in the engine’s enhanced performance and notably improved fuel efficiency.