How a Small Engine Works

Home » How a Small Engine Works

A small engine is “small” in name only as you depend on it to be the heart of all your gas-run outdoor power equipment, whether you’re operating a lawn mower, snowblower, pressure washer, or wood chipper, or utilizing hand-held equipment such as a chainsaw, string trimmer, or leaf blower. If the engine is having trouble starting, runs rough, or keeps stalling, you’re going to have great difficulty completing your lawn and garden task. In this article, we’re going to talk about how two-cycle and four-cycle engines actually work and what you can do to keep those engines running in peak condition.

What’s the difference between two-cycle and four-cycle engines?

Larger outdoor power equipment, such as lawn mowers, snowblowers, pressure washers, wood chippers, and more heavy-duty tillers will depend on four-cycle engines to provide the necessary torque, whereas smaller hand-held equipment, such as chainsaws, pole pruners, and leaf blowers, can get by with the lesser amount of torque provided by a two-cycle engine. The main difference between the two types of engines is that the four-cycle engine requires two revolutions of the crankshaft and piston to draw the fuel and air into the cylinder, ignite it, then exhaust the combustion gases; the two-cycle engine requires just one revolution to do the same thing.

Similar to an automobile engine, a four-cycle engine draws gasoline from its fuel tank and has a separate sump for oil. A two-cycle engine requires the gasoline to be mixed with the oil for the engine to operate properly. This can be tricky since attempting to run a two-cycle engine without a suitable oil and gas mix will cause the engine to seize due to insufficient lubrication. In this instance, a four-cycle engine is the easier engine to maintain (just pour the gasoline into the tank as you would for your car and remember to change the oil regularly). If you’re concerned about getting that gas/oil balance right for your two-cycle engines, you can use a pre-mixed fuel and oil product like TRUFUEL 50:1 Mix to fill the tank.

Additionally, a larger four-cycle engine may utilize an electric start feature that allows the engine to be turned over by using an ignition key or a start button. Smaller four-cycle engines, and all two-cycle engines, rely on the operator to pull a starter rope to fire the engine up. Apart from these differences, the basics of two-cycle and four-cycle engine operation are quite similar.

Basics of engine operation

So, how does a small engine work? When the start button is pressed, the ignition key turned, or the rope is pulled, the starter engages the starter cup on the flywheel and rotates the crankshaft. The rotating crankshaft connects to the piston which moves up-and-down within the cylinder and ignition process begins. The flywheel has permanent magnets built into it, and as it rotates past the ignition coil, a magnetic field is created. The magnetic field induces electricity, allowing the ignition coil to send voltage to the spark plug.

At the same time on a four-cycle engine, the piston will travel down the cylinder creating a vacuum that draws fuel and air through the carburetor where they mix before entering the cylinder through an intake port. This action is called the “intake stroke”. Next, the “compression stroke” takes place as the piston travels back up to the top of the cylinder and the intake valve closes. The spark plug now fires and ignites the fuel and air mixture which forces the piston down (known as the “power stroke”). The momentum of the spinning flywheel provides enough force to push the piston back up. The exhaust valve will then open, and the combustion gasses will exit through the muffler in what is the fourth and final cycle – the “exhaust stroke”. A four-cycle engine will continue to run and repeat these four cycles until it is turned off.

Similarly, as a two-cycle engine’s piston travels down the cylinder, it exposes an intake port and pumps gas, oil, and air into the cylinder. As the piston travels back up the cylinder, a vacuum is created and the gas, oil, and air is drawn through the carburetor into the crankcase. When the piston reaches the top of the cylinder, the energized spark plug will ignite the fuel and air mixture, driving the piston down, exposing an exhaust port where the spent fuel exits. A split-second later, the intake port is exposed again. Fresh fuel is drawn in and the process continues with every revolution of the crankshaft.

The exhaust from both two-cycle and four-cycle engines will travel through the muffler and, often, a spark arrestor. The muffler reduces engine noise while the spark arrestor prevents burning carbon deposits from exiting the engine and, potentially, starting a fire.

Engines will have a choke, primer bulb, or both to assist with starting, especially when the engine is cold. The choke will temporarily restrict air flow through the carburetor so more fuel can enter the cylinder. Likewise, a primer bulb draws additional fuel through the carburetor before the engine is started.

The 6 “must dos” to keep the engine tuned-up

Keeping both four-cycle and two-cycle small engines in peak condition requires regular maintenance. Before using your lawn mower, string trimmer, or tiller in the spring, your wood chipper or leaf blower in the fall, or your snowblower during the winter months, these are the six “must dos” to keep the engine tuned-up:

  1. Install a new air filter – A dirty air filter inhibits good airflow, preventing the proper ratio of fuel and air from entering the engine cylinder. The air filter is easy to replace and should be done annually or whenever the filter appears soiled.  
  2. Change the spark plug – Due to carbon build-up and a weakened electrode, a spark plug will degrade over time. You can inspect the spark plug for damage and test the component using an ignition tester to determine whether it’s defective or not, but this is another part that should simply be replaced once a year.
  3. Replace the fuel filter – The fuel filter helps to remove contaminants from the fuel before it leaves the tank and enters the carburetor. Old or bad fuel can leave a gummy residue in the filter, significantly weakening the fuel flow. For this reason, it’s a good idea to replace the filter annually as a preventative measure (and keep the fuel fresh, making sure it contains no more than 10% ethanol).
  4. Change the oil – To keep a four-cycle engine’s crankshaft, piston, and gears working smoothly, you need clean oil. While the procedure for replacing the oil will differ depending on the engine model, you can usually unthread the oil tank cap, remove the dipstick, if applicable, and carefully tip the engine to empty the old oil into an approved container. Outdoor power equipment engine oil, like car engine oil, can be recycled at most auto parts stores. Once the oil tank is emptied, return the engine to its upright position, and pour the new oil into the oil fill tube. To avoid overfilling, pour in approximately three-quarters of the bottle, then check the tank with the dipstick to determine if the oil level is at full. You can then add more oil, as necessary.
  5. Keep the carburetor clean – As with the fuel filter, old or bad fuel can leave a gummy residue inside the carburetor as well which can create a restriction. This will prevent the proper ratio of fuel and air from entering the cylinder which can cause the engine to have trouble starting, or may cause it to run rough, or stall. You can try cleaning the carburetor ports with a dedicated carburetor cleaner, or WD-40, to clear out the restriction, but if the clog is particularly bad, you may need to replace the carburetor with a new one.
  6. Use a fuel stabilizer – While you should only use fresh fuel when filling your outdoor power equipment’s fuel tank, you can use a fuel stabilizer to help maintain the quality of that fuel. The stabilizer can be added to a fuel can or the equipment’s fuel tank, but you should refer to the manufacturer’s instructions to determine the proper amount for both. If adding the stabilizer to the equipment’s fuel tank, be sure to run the engine for at least two minutes to circulate the stabilizer throughout the fuel system.                                                                    

Repair Clinic has the parts to fix and maintain small engines

Whether you’re looking to repair your small engine or just keep it tuned-up, Repair Clinic makes it easy to find the parts you need. Just type the full model number of the engine in the Repair Clinic website search bar to identify the specific parts associated with the engine, from the right spark plug and air filter to the compatible carburetor, fuel filter, ignition coil, flywheel, and recoil starter. Repair Clinic stocks genuine manufacturer parts for all the big names in small engines, such as Briggs & Stratton, Craftsman, Honda, Husqvarna, Kawasaki, Kohler, MTD, Snapper, Tecumseh, Toro, Walbro, and Zama, but you’ll want to make sure you’re purchasing the exact item that matches your engine model.

Repair Clinic VIP Email

Join the Repair Clinic VIP email list for updates and special offers!

Scroll to Top