If you want to dip your toes into engine repair, fixing lawn mowers is a great place to start. They’re simple machines and make for great stepping stones toward tackling more complex projects like cars and trucks. But in order to repair a lawn mower, you have to know how it works.
The most common types of lawn mowers are riding mowers and walk-behind mowers. We’ll be focusing on gas-powered, walk-behind mowers for this explainer, but some points will also apply to riding mowers, as well as battery-powered and wired electric mowers.
Think about your lawn mower as a car, with a spinning blade mounted on the bottom that cuts the grass as you “drive” through your yard. That’s basically all it is, just on a much smaller and more manageable scale, which is why they’re so great for novice fixers.
That rope you have to pull on ten times to get the mower started manually spins the engine, which forces it to suck in air and fuel. This is done through a carburetor, and it’s responsible for mixing the right amount of air and fuel before it enters the engine.
This mixture is ignited by a spark plug, which gets its electricity from a flywheel that rotates along with the engine. As it spins, magnets attached to the flywheel pass over an ignition coil to create a magnetic field, which generates electricity and sends voltage to the spark plug. The flywheel also serves as a cooling fan (thanks to fins attached to it) and as a shock absorber (when the blade hits a rock or tree roots). It also provides momentum to keep the engine rotating between combustion cycles.
The ignition of the air and fuel mixture results in a controlled explosion that drives a piston down and rotates a crankshaft. It’s very similar to the engine in your car, only smaller. (A typical engine on a walk-behind mower is about 150 cubic centimeters in size, whereas the engine in a compact car is over ten times that.) On a car, the engine’s rotation is transferred to the wheels. But on a lawn mower, that rotation is directly transferred to a cutting blade.
The engine is the most important component on a lawn mower, and it needs to be treated as such with regular maintenance, like changing the oil and replacing the spark plug and air filter. And the carburetor is fairly low-maintenance with the proper care, like adding stabilizer to the fuel and never letting the fuel sit in the engine for more than a few months—old gas can decompose inside the carburetor and clog it up in no time.
All lawn mowers require you to press and hold down some sort of lever near the hand grips in order to start and run the engine. This is called the blade control handle or brake bail arm. The moment you let go of it, the engine will shut off and the blade will stop spinning.
It’s a convenient way to turn the mower off when you’re done mowing, but more importantly, it’s a safety feature that can prevent serious injury in the event that you trip or fall while mowing.
Here’s how it works: The handle is connected to a mechanism near the engine (via a cable) that does two things: It grounds out the ignition to act as a spark plug kill switch of sorts, and it applies a brake to the flywheel to stop the engine (and the blade) quickly.
There’s no maintenance involved with this safety system, but on the odd chance that the grounding wire comes loose, the brake pad wears down, or the cable breaks, it’s good to know how it all works before you tackle the repair.
All of the above components are working together to serve the lawn mower’s main purpose: spinning a blade that cuts the grass. But that’s not as simple as it sounds. There’s actually a lot of physics going on underneath your lawn mower, and a video from Smarter Every Day explains and demonstrates this in slow motion.
Think for a second how grass would react to a spinning blade. Grass doesn’t stay in position when you swipe something over it—it’s so light that it moves out of the way. So your lawn mower has to account for that.
If you’ve ever taken a peek at the cutting blade on your lawn mower, you might have noticed how it’s not just a straight, flat blade all the way across. Instead, there are tabs on the ends of the blade that curve up slightly, which produce lift and suction. This motion, plus the enclosed space of the mower deck, creates a vacuum, sucking up the grass as the mower passes over it. This forces the grass to stand up straight while the blade makes its way through.
But even if it’s mostly physics doing all the work, it’s still important that you inspect the blade periodically and sharpen it if it’s not cutting the grass very well. Or replace it if it’s completely worn out.
You probably didn’t expect to get a quick lesson on physics while reading about lawn mowers, but it’s what keeps your yard looking fresh during the summer months. And while lawn mowers have a lot of moving parts, they all come together to create a fairly simple piece of machinery that’s easy to repair and maintain.
Title image by Sarah Laval/Flickr