Understand Compression and Power Systems in Small Engines
Every engine ever made (including your lawnmower engine) requires three things: Ignition, which is a source of spark controlled by timing, compression, which means that air must be compressed to create rotation, and carburetion, which involves mixing fuel and air in the proper ratio for combustion. An internal combustion engine is nothing more than an air pump. Torque and horsepower are a function of how much air is pushed through the cylinders. The more air, the greater the output of power.
Contents
Steps
- Understand the basic function of the engine. The engine is an air pump. The more air, the more power.
- Become familiar with how compression works. The engine has a stroke, which is the travel of the piston up and down the cylinder. The amount of room in that cylinder is described in terms of cubic inches (or centimeters) with the piston at the bottom of the cylinder. For example, a 351 Ford has 350 cubic inches. The compression ratio is a comparison of the volume (with the piston at the bottom) and the space left with the piston at the top of its stroke. Most lawnmowers are low compression engines, with ratios of 8:1 or less. This is why they run well on cheap gas (87 octane).
- Diagnosing problems: If you have a compression problem, the engine won't run (in 99% of cases). The best indicator of low compression is the engine pulling through too easily. If you hit the starter and the engine spins like crazy (unusually fast), you will need to check the compression with a tester. This is a simple air gauge that screws into the spark plug hole and traps the air inside as the piston runs up and down. If you're getting less than 80 psi, you've got a problem. The most common causes are a blown head gasket, or a stuck or bent valve.
Tips
- Your fuel has got to be clean and fresh. Fuel begins to lose its octane in approximately 90 days.
- The carburetor is a mixing chamber for air and fuel.
- The ignition is controlled by the flywheel, key, and coil (properly timed to the engine).
- The engine is a pump. It must have compression to run and produce power.
Warnings
- Disconnect the battery before beginning any engine work!
- Disconnect the negative battery terminal first, then the positive. When connecting the terminals again do it in reverse order, making the negative connection last. Check on you make/model car before disconnecting the battery.
