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Why Do Aircraft Engines Have Two Spark Plugs Per Cylinder?

Boldmethod

When it comes to spark plugs, are two really better than one? Most car engines have just one spark plug per cylinder, and that seems to work just fine. But when it comes to airplanes, dual spark plugs are important for 3 major reasons.

1) Reliability

First off, reliability plays a major role as to why your aircraft's engine has two spark plugs per cylinder. Picture this: you're flying along in cruise, and your magneto fails. If you had a single ignition system with only one spark plug per cylinder, your engine would stop running. And your prop? Depending on your airspeed, it would either slowly windmill as you started descending toward Earth, or it would stop completely.

Remember the saying that a propeller is a big fan that keeps pilots cool, and when it stops, pilots start to sweat? There could be some major sweating from that kind of failure.

But that's not the only problem. Let's say you had a spark plug stop working in-flight. If you only had one spark plug per cylinder, you would lose more than 25% of your power, if you were flying a 4-cylinder engine. And if you've ever flown a small single-engine plane, you know that a power decrease like that could mean the difference between cruising and not being able to maintain altitude.

By having a dual ignition system with two spark plugs, none of this is a problem. If you have a magneto fail, you'll probably see a slight drop in engine RPM, but the other magneto, and the spark plugs that it's connected to, will keep firing on all cylinders. And if you simply have a spark plug fail? The same thing happens.

2) More Power

There's another major benefit to having two spark plugs per engine, and that is power. When two spark plugs fire at the same time in a cylinder, you get something called a "twin flame front". That means that instead of your fuel-air mixture igniting from one point in the cylinder, it ignites from two separate locations on opposite sides of the cylinder.

This is important for two reasons: 1) the fuel air mixture burns more quickly, and 2) the fuel air mixture burns more completely. And when you have both of those things happening in your cylinder, you get more power, which is something almost every pilot is looking for.

3) Even Combustion

Combustion is the last major factor in a dual spark plug configuration. When you only have one spark plug per cylinder, the fuel-air mixture burns top-down from the center, moving outward.

But when you have two spark plugs on opposite sides of the cylinder, the fuel-air mixture burns from the edges of the cylinder, moving inward.

This even burn helps prevent hot-spots in the cylinder, and distributes pressure more evenly across the cylinder, which is important with the big, high-compression, air cooled engines found in most airplanes general aviation airplanes.

https://www.flickr.com/photos/scsibug/

Spark Plug Fouling

There's one last benefit to a dual spark plug configuration, and it has to do with spark plug fouling. 100LL Avgas contains lead. And to be more specific, it contains "TEL", or tetraethyllead.

TEL is added to avgas to prevent engine knocking (also known as detonation). TEL helps prevent engine knock, which is an uncontrolled combustion of pockets of fuel-air mixture in your cylinder, by raising the temperature and pressure where auto-ignition occurs.

But while TEL helps with knock, it introduces another problem: lead deposits. If the TEL doesn't completely vaporize in the combustion cycle of your engine, which can happen if your engine is running too cool, lead deposits can accumulate on your spark plugs, causing them to mis-fire.

AA1Car.com

That's where the dual ignition comes into play again. If a spark plug fouls, you have a backup spark plug that keeps firing.

Two Really Is Better Than One

By having two spark plugs per cylinder, your aircraft engine is more reliable, produces more power, and burns fuel more evenly. And when you combine all three, you get an ultra-reliable engine that keeps you aloft until you decide it's time to land.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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