What is a wasted spark ignition system?

Created by Jonathan Fry, Modified on Tue, 1 Apr at 10:12 PM by Jonathan Fry

A wasted spark ignition system is a type of ignition system used in internal combustion engines, particularly in some gasoline-powered engines with multiple cylinders. In this setup, the ignition coil fires two spark plugs simultaneously—one in the cylinder that's on its compression stroke (where the air-fuel mixture is ready to ignite) and another in a cylinder on its exhaust stroke (where there's no combustible mixture). The spark in the exhaust stroke cylinder is "wasted" because it doesn't contribute to combustion, hence the name.

This system typically uses one ignition coil to serve two cylinders, paired based on their positions in the engine's firing order (e.g., cylinders that are 360 degrees apart in a four-stroke cycle). It’s a simpler and more cost-effective design compared to systems with a dedicated coil for each cylinder (like coil-on-plug systems). Wasted spark setups don’t require a distributor or complex timing sensors to differentiate between compression and exhaust strokes, relying instead on the engine’s mechanical cycle.

It’s commonly found in older vehicles, motorcycles, or small engines, though modern engines often favor more precise systems for better efficiency and emissions control. The trade-off is that it’s less efficient than individual coil systems, but it’s robust and reliable for simpler applications.