The following is the forth in a series of articles exploring all facets of model engine fuel. The writer is Don Nix, founder and former owner of POWERMASTER FUEL. Readers are invited to contact Don directly via e-mail - FLYERDON@aol.com and to visit his website at http://www.rcauctioneer.com.).
Well, what do you think? Is there really a difference, or is this merely a big hype by the fuel manufacturers to sell more products? Let's see a show of hands…..ah, yes…about evenly divided. Well, let's explore the facts.
Fact: Most 4-stroke model fuels contain less oil than comparable 2-stroke fuels.
The most common response to this is, "But 4-stroke engines have more moving parts….they should need more oil, not less!" Well, that sounds reasonable, but it doesn't stand up under close examination. The number of moving parts has nothing to do with it. What is important? Think about it.
Fact: With rare exceptions, 4-stroke engines run at substantially slower rpms than a comparable 2-stroke engine…most in the under-10,000 rpm range vs. 12,000, 13,000 or more for a typical 2-stroke of the same size. They are engineered to deliver maximum power at slower rpms, with bigger props. What does this have to do with it? One of the main factors used in determining the proper oil content of fuel is heat. To use the well-worn term, it doesn't take a rocket scientist to figure out that the more slowly an engine turns, the less heat it generates from friction. If you don't believe that, rub your palms together slowly, then as fast as you can.
So….lower rpms = less heat = less need for oil.
Fact: 4-stroke engines only fire every other stroke, vs. every stroke by a 2-stroke engine. Firing, or combustion, burns fuel, which creates heat. Logically, it may be deduced that if there is fire in the chamber only every other stroke, the engine has time to cool off a bit between combustion cycles. Let's take that a little further: Using a hypothetical 4-stroke engine turning 10,000 rpm = 5,000 combustion cycles per minute, vs. a hypothetical 2-stroker turning 13,000 rpm…with the same number of combustion cycles per minute….the gap widens. The 2-stroker has 160% more combustion cycles than the 4-stroker. Even though this is