How Lawn Mowers Work

No matter how big or complicated, every rotary lawn mower needs a motor to make the blade spin fast enough to cut the blades of grass cleanly.

Most rotary lawn mowers are powered by an internal combustion engine, which runs on either two cycles or four. Two-cycle motors burn a mixture of gasoline and oil, while four-cycle motors burn gasoline and have a separate lubrication system. However, rotary lawnmowers that use electric motors powered by a cord, a rechargeable battery or even solar power are also popular.

Attached to the motor is a throttle, often mounted on the handlebar, which keeps fuel flowing to the motor. To prevent injury, the motor shuts off if the throttle is released. The throttle on some rotary lawn mowers allows adjustment of the motor's speed.

The motor turns a vertical axle that's attached to a large blade, which is usually made of a single piece of steel that lies suspended horizontally a few inches above the ground. The blade typically spins at 3,000 revolutions per minute and slices the grass as it passes over it. Blades can be either straight or curved, but it's the tip of the blade at either end that does all the cutting.

Some blades are specially designed to cut the grass clippings into tiny pieces that can be used in mulch. The deck on a mulching mower is designed to contain the clippings until the mulching blade can shred them. Whether the clippings are mulched or not, they move through the deck into a side- or rear-mounted bag that collects them, or the mower returns them to the ground.

Some mowers are called 2-in-1 mowers and can be used as mulching mowers or as bagging mowers. A 3-in-1 mower allows bagging, mulching or side-discharge of grass clippings.

If the mower is self-propelled, its motor turns a drive shaft connected to a belt or chain. That connects to a gearbox attached to an axle that spins, making the lawn mower's wheels move.

Those are the basics. Next, we'll see how the evolution of rotary lawn mowers has led to a bigger and more powerful species -- the riding mower.

For small lawns of a quarter-acre or less on a flat surface, a rotary push lawn mower will get the job done. For hilly lawns of up to one-half acre, a self-propelled walk-behind mower will do. But for lawns a half-acre or larger in size, a riding lawn mower can make the task much easier.

Riding mowers come in different sizes and types, with different horsepower engines and blade sizes. One the small end, a riding mower with an engine of about 14 horsepower that can cut up to an acre starts at around $1,000.

For larger lawns, a garden tractor is a heavy-duty version of a riding mower that can pull attachments behind it to plow or disc a garden, as well as cut grass. Equipped with an 18 to 24 horsepower engine, a garden tractor can cost $9,000, but popular varieties cost $2,500 to $4,500. Lawns of 5 acres or more can require a full-size farm tractor to tow a mowing apparatus behind it.

Another type of riding lawn mower is the zero radius mower. Popular for commercial landscaping and professional lawn care, these riding mowers use four-wheel steering to move precisely around trees and other obstacles.

Driving a riding mower can be similar to driving any other motor vehicle. The rider guides the mower while sitting on top of its deck. Some have pedals to control acceleration and braking, and a steering wheel to turn. Others use levers to control throttle and steering.

The engine powers both the wheels and the blade, just like a walk-behind self-propelled mower. But instead of being directly connected to it by an axle, the blade on most riding mowers is powered by a belt connected to the engine. In addition to other controls, the rider can also lower or raise the blade, speed it up or slow it down.

Before investing in a riding lawn mower, think hard about what purpose it'll serve. Most walk-behind rotary mowers cost less than $500, and the most basic riding mower starts at around $1,000 and goes up quickly from there.

There are other costs associated with operating a lawn mower. In the next part, we'll look at some of the safety and maintenance issues that come with keeping a neat lawn.

Every year, 75,000 Americans suffer injuries from mowing the lawn. Nearly 10,000 of the victims are children [source: University of Michigan]. A majority of those injuries come from flying rocks and debris hitting unprotected legs and faces.

Another 22 percent of injuries happen to the hands, fingers and arms and are caused when interacting with a mower blade that hasn't stopped spinning. Some of the most serious injuries are tied to riding mowers, which combine horsepower, weight and size into a potentially dangerous package.

Preventing injury to children involves following a few easy safety tips. First, make sure children aren't in the same area as a running lawn mower, whether it's a walk-behind or riding variety. Second, don't let anyone under age 16 operate a riding mower, and don't let children ride as passengers.

Before operating a lawn mower, make sure the area to be mowed is free of loose objects, such as sticks, stones and other yard debris. But just to be safe, never operate a lawn mower without wearing full-length pants and shoes. Some safety advocates recommend wearing steel-toed work shoes and safety goggles when mowing lawns.

Making sure that the engine and blade have come to a complete stop before making any adjustments to the lawn mower or inserting hands or tools inside the mower is another way to prevent injury.

And finally, keeping a lawn mower in good working condition helps promote safety and saves money by extending the machine's lifespan. Here are a few tips for proper maintenance:

Gas-powered lawn mowers don't have the same pollution controls as automobiles, and according to the Environmental Protection Agency (EPA), each one produces as much air pollution in a year as 43 new automobiles being driven 12,000 miles each [source: Environmental Protection Agency].

Electric-powered lawn mowers don't emit pollutants by themselves, but the electricity needed to power them is often generated in power plants that burn fossil fuels. Solar-powered electric mowers require no external source of electricity, but they can only run for a limited time on each charge. Since push reel mowers are powered exclusively by the person doing the pushing and are much cheaper than solar-powered mowers, they're the mower of choice if you're concerned about the environment and don't have a lot of money to spend.

On the other end of the spectrum, two-cycle gas lawn mowers are inefficient and release 25-30 percent of their mixture of oil and gas unburned into the air [source: People Powered Machines]. A four-cycle lawn mower engine burns its fuel more efficiently, but it still emits hydrocarbons, carbon dioxide and other pollutants linked to global climate change.

Mulching lawn mowers are more friendly to the environment because their blades cut the grass into pieces small enough to rapidly decompose and return vital nutrients to the soil. This reduces the need for adding fertilizer to the lawn and saves space in landfills that might otherwise be taken up with bagged clippings. However, bagged clippings can be sent to municipal compost sites instead of being taken to the dump.

Another consideration is the environmental impact of a rotary mower on the grass it cuts. While reel mowers cut with a scissor-like action that helps the grass retain moisture, rotary mowers tear the grass unless the blade is kept sharp. That tearing action can expose the grass to more diseases.

What about new lawnmowers? Have there been any advancements in the technology?

Modern rotary lawn mowers have made several advances over their predecessors, especially in safety features.

Today's mowers come with a blade brake control on the handlebar that stops the blade if the operator releases the control, or a kill switch that stops the motor entirely. They also come with a foot shield at the rear of the mower to prevent loose objects from being discharged and to prevent feet or other objects from entering the blade deck. Riding lawn mowers come with safety features that cut the motor if it tips over, and commercial models now routinely come with roll bars to prevent the operator from being pinned or crushed.

Riding lawn mowers also come with a wide variety of luxury options, including automatic transmissions, adjustable seats, cruise control, stereos and even beverage cup holders.

Many riding mowers can also tow attachments that allow them to plow snow, till the earth, spread fertilizer and do other duties that turn them into multipurpose machines.

Walk-behind mowers aren't without their options, though. Large rear wheels in newer models help turn the mower more easily, and push-button electric starters eliminate the tedious task of pulling a recoil line to get the mower going.

But the newest advance in lawn mowers is not an addition, but a subtraction. A new crop of lawn mowers doesn't need an operator, because it operates itself. Robotic lawn mowers can mow your lawn while you're sipping a cool drink on your hammock. Most robot lawn mowers run on cordless rechargeable batteries, and some are charged by solar power. So you can feel good about the environment while you're sipping that drink, too.

Many of the elements of future lawn care are already here today, and they're making it easier, greener and more efficient to keep the grass neat and trim. Robots are already available to cut your yard for you, but in the future larger, more powerful robotic lawn mowers could cut large fields and golf courses and otherwise be used for commercial landscaping.

Those lawn mowers could run on electricity powered by the sun, if the technology improves. Or they could run on biodiesel or hydrogen fuel cell technology already under development.

And researchers are always looking for ways to cut the grass more effectively, from improved mulching techniques to self-sharpening blades. For lots more information on lawn mowers and home improvement technology, see the links on the next page.