What is a major disadvantage of a magneto ignition system?

This simple, mechanically driven power source is nearly 200 years old and still has a well-defined, albeit limited, role in today’s systems.

Most of the historical and present-day applications for magnetos are for spark-plug ignition systems. This part looks at that aspect in more detail.

Q: What’s the structure and operation of a basic magneto-based ignition arrangement for a system with a single spark plug?
A: As discussed previously, the rotating magnets induce an electrical current and associated voltage in the coil’s primary windings (Figure 1) by morphing the raw magneto output into a more AC-like waveform suitable for a transformer. This voltage is multiplied through the coil’s secondary windings with its higher turn count.

Figure 1. This simple yet effective magneto-based scheme creates the high-voltage spark to ignite the fuel in a single-cylinder gasoline engine (Image: MechnicalJungle).

This high voltage is created on the secondary winding when the breaker points connected to the primary winding open, causing the primary coil’s magnetic field to collapse and prompting a large change in magnetic flux linkages. This spike in electricity crosses over to the secondary winding, which greatly amplifies it, creating anywhere from 10,000 to 30,000 volts. Thus, the voltage is high enough to cause the spark plug to flash over the plug’s gap and ignite the fuel.

A set of contact-breaker points travels over a cam lobe and regulates the timing of the electrical impulse and spark. The points interrupt the magnetic circuit, while the capacitor (called a condenser in those early days) absorbs the back EMF from the magnetic field in the coil to minimize point contact burning and maximize point life. The spark plug wire is attached directly to the coil on a single-cylinder engine such as a lawnmower or chain saw.

Q: What about engines with multiple cylinders?
A: On multiple-cylinder engines such as tractors, motorcycles, or cars, the magneto assembly is placed in a camshaft-driven housing. A rotor distributes the voltage to the appropriate cylinder via individual spark plug wires (Figure 2).

Figure 2. Adding a distributor and mechanical linkage allows the magneto-based system to drive an engine with multiple cylinders (Image: MechnicalJungle).

Some designs use a separate magneto coil for each cylinder and spark plug, which eliminates the need for a distributor but obviously requires more magneto “hardware” and brings other complexity, although it adds flexibility.

Q: Are magnetos still appropriate or viable for use in standard cars and large engines as the primary or sole source of spark-plug ignition energy?
A: No, there are several drawbacks to their use in this role. The most obvious drawback of the magneto-based system is that it is not “self-starting,” and you have to turn the engine “over” to get it started; it is not a self-starting system. You may have seen the movies with old airplanes, where the pilot says “contact” and closes a switch, then someone pulls on the propellor to get the magneto system started. That was done using a crank for cars, which can take a lot of muscle power. Lawnmowers and chainsaws use a manual pull chain for this purpose.

The development of the battery-powered electric starter in the 1920s eliminated the need for hand cranking. Also, it meant there was a battery in the car (6 V at the time, upgraded to 12 V in the early 1950s). That same starter battery could also be used for the ignition system, thus eliminating the need for the battery-free magneto.

Q: Is that the only drawback?
A: No, there are several others. The voltage from the magneto and, thus, the spark intensity is weak at low rpm; some early cars had a small supplemental battery to provide the power, used only at low rpm. The driver would connect the battery at starting and lower speeds, then manually switch to “magneto” as the car’s speed and engine rpm increase. Driving a car in those days was truly an interactive experience.

Q: Are there any other drawbacks?
A: Yes, there is an important one related to timing. The optimum point or timing (corresponding to piston position) at which the igniting spark should be created is a function of engine speed. This variable relationship has been known since the late 1800s. However, this timing is fixed constantly in a basic magneto system. Note that the need to shift timing versus speed is a minor and ignored issue for applications where the engine operates primarily at one speed, such as lawnmowers or chain saws.

To overcome this drawback, some early cars had manually operated “rods,” which the driver used to slightly rotate the magneto-mechanical arrangement to adjust timing — again, something for the driver to do. This manual adjustment was eventually automated, but the result was still sub-optimal.

Note that the same problem arises with the mechanically driven battery-spark and coil-distribution arrangement that replaced the magneto-based system, but it was easier to work out a self-adjusting scheme to adjust the timing; of course, the modern computer-controlled ignition system overcame this problem in firmware by using algorithmic-based “smarts” in the engine-control computer to maximize performance and minimize emissions.

The first reliable battery-operated ignition was developed by the Dayton Engineering Laboratories Co. (Delco) and introduced in the 1910 Cadillac; within a few years, it was standard in most cars as starters based on electric motors also came into use (along with electric lighting). Charles Kettering developed this ignition, which became the primary ignition system for many years, admittedly with refinements, in the automotive industry due to its lower cost, higher reliability, and relative simplicity.

Q: It sounds like the magneto is both obsolete and modern – is this the case?
A: Yes, due to their simplicity, reliability, and avoidance of a battery, they are still manufactured and have been highly refined in design (Figure 3) and overall assembly (Figure 4).

Figure 3. This complete Lucas K2F magneto ignition assembly is designed for twin-cylinder motorcycle engines (Image: MechnicalJungle).

Figure 4. This magneto system has a highly refined design that is suitable for aircraft use (Image: Flying Media).

Conclusion
The magneto is both an ancient and a modern way to generate a modest voltage and current reliably; it is both obsolete for some applications and necessary for others. Despite its efficiency and performance limitations, it still has a vital role as the source of the spark voltage for small, constant-speed internal combustion engines and a battery-free redundant spark source for mission-critical applications.

References
Hemmings Motor News, “Magneto Ignition Systems”
National MagLab Magnet Academy, “Magneto – 1832”
Mechanical Jungle, “What Is Magneto Ignition System?”
Monroe Aerospace, “What is a magneto, and why do airplanes use them?”
AOPA News, “How it works: Magneto”
Flying, “How It Works: Magneto”
Antique Auto Electrics (Australia), “Magneto History”
Wikipedia, “Magneto”
Wikipedia, “Magneto Ignition”
Buzz Pound, “Model T Ford Magnetos and Timing”
Model T Ford Club of America, “Magneto Output Waveform”
Model T Ford Club of America, “The Model T Ford Ignition System & Spark Timing”
Gas Engine Magazine, “Magneto Voltage Value: How to Use a Voltmeter”
Vern’s Old Phones, “Magneto Telephone Circuits”
Marvel Publications, “Max Eisenhardt & Magneto”