building a most fuel economic ram 1500 5.2L
Banned Goober Smoocher
Joined: Jun 2010
Posts: 8,916
Likes: 1
What'd I do? I coulda sworn that because it burned slower, in a sense it acted like a small timing retard. I'm not trying to get a beef with ya in the least.
Grand Champion
Joined: Apr 2010
Posts: 5,011
Likes: 6
From: Meeker, CO
Octane numbers indicate how much you can compress the fuel before it ignites due to compression alone.
Advancing ignition timing increases combustion chamber temperature, which has the effect from the fuel's perspective of upping compression. What happens there, in a nutshell, is that the vapor pressure increases due to heating when the fuel enters that hot environment, so the vapor pressure is already increased above what it would be in a cooler engine even before the compression stroke begins. If you're running right on the edge of compression detonation, then you don't need that much more energy to initiate ignition... so a hot spark plug, an overheated portion of the cylinder wall that's not cooling because there's some nucleate boiling going on, whatever, is all it takes to ignite the fuel. Ping ping bang bang.
Where running too high an octane for the engine, the fuel doesn't get as close to that edge of compression ignition as it needs to be for effective flame front propagation. Instead of a relatively cohesive flame front like a balloon expanding, you get a raggedy thing more like the top of a campfire. Unless you've gone way over the edge a flame front will eventually form and progress at the same rate give or take a hoogivsa5h17 as the lower octane stuff. It's just happening after the piston has traveled further down the cylinder so it's not going to make as much horsepower.
In short: Higher octane = harder to ignite. Once lit, it's pretty much same:same.
Advancing ignition timing increases combustion chamber temperature, which has the effect from the fuel's perspective of upping compression. What happens there, in a nutshell, is that the vapor pressure increases due to heating when the fuel enters that hot environment, so the vapor pressure is already increased above what it would be in a cooler engine even before the compression stroke begins. If you're running right on the edge of compression detonation, then you don't need that much more energy to initiate ignition... so a hot spark plug, an overheated portion of the cylinder wall that's not cooling because there's some nucleate boiling going on, whatever, is all it takes to ignite the fuel. Ping ping bang bang.
Where running too high an octane for the engine, the fuel doesn't get as close to that edge of compression ignition as it needs to be for effective flame front propagation. Instead of a relatively cohesive flame front like a balloon expanding, you get a raggedy thing more like the top of a campfire. Unless you've gone way over the edge a flame front will eventually form and progress at the same rate give or take a hoogivsa5h17 as the lower octane stuff. It's just happening after the piston has traveled further down the cylinder so it's not going to make as much horsepower.
In short: Higher octane = harder to ignite. Once lit, it's pretty much same:same.
