Tuesday, December 3, 2019

2 Stroke or 4?

Most gas engines are fairly simple beasts: you fill the tank, you keep up on the oil changes, and everything is happy. Sometimes, though, you'll find an engine that doesn't have an oil reservoir, or has something like 50:1 on the fuel cap. These engines still require oil, but use it mixed with their gasoline.

Engines that require mixed gas are referred to as 2 cycle engines. Ones that don't, like the one in your car, are referred to as 4 cycle engines. A 4 cycle engine uses 4 cycles or "strokes" to make power:
  1. The intake stroke pulls air and atomized fuel into the cylinder. 
  2. The compression stroke applies pressure to the fuel/air mix, making it far easier to ignite. 
  3. The ignition or power stroke is where the spark plug ignites the fuel, driving the piston down and generating power.
  4. Finally, the exhaust stroke moves hot air out of the cylinder and engine and into the exhaust system.

A 2 cycle engine consolidates those cycles by combining intake and compression, and then ignition with exhaust. This makes a very simple, lightweight engine, but at the cost of power and fuel efficiency. Some off-road motorcycles also employ these engines, but most two cycle engines today are seen in yard tools like weed eaters and leaf blowers, and notably in chain saws. This is a very good set of animations demonstrating the 2 and 4 cycles in action.

Where a 4 cycle engine uses a separate oil reservoir for lubrication, a 2 cycle engine uses the oil mixed into the fuel. The owner's manual (and usually a sticker or engraving on the engine) will tell you the proper mix ratio. Using too little oil will under-lubricate the engine, causing premature wear and failure, but too much oil will burn very dirty, fouling the spark plug and generally running horribly. Stihl (a popular power equipment company) has a great set of instructions on how best to actually mix your oil, and AMSOIL has a comprehensive chart listing all the most common mixing ratios and the actual quantities of oil and gas needed (saves me the math!).

Mix your gas right to keep your equipment running long and strong. Your snowblower will keep your driveway clear, and your chainsaw will rapidly clear debris after a storm.



  1. I have been using a product called Opti-2 for years. It is a universal 2-stroke oil mix that comes in a pour-able pouch. You mix one pouch of Opti-2 to one gallon of gas, and it includes a fuel stabilizer. I just use one container of mix for all my 2-stroke power equipment. When storing for the off season, I drain all the fuel out and put them away. Every year when I get ready to use my equipment, I fuel them up, they start on the first pull, and run with no trouble all season. A mixed container doesn't sit around unused for very long.

  2. Two stroke engines are more powerful than a four stroke of the same capacity. That's because it makes power twice as often as a four stroke at the same RPM. A two stroke is also lighter and smaller. Emissions are the reason it's fallen out of favor.
    When Motocross motorcycles switched from two to four strokes, the 250cc two stroke was equal in power to a 425cc four stroke.


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