Quote:
Originally Posted by tahdizzle
Driving habits are mutually exclusive to octane. Octane has NO affect on MPG. Period. No discussion. This has been proven time and time again. You're engine retarding timing to prevent knock is because the nature of octane. And you may get a MARGINAL decrease in fuel efficiency, but its not the fault of the octane.
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You baffle me... We seem to agree on all the steps and then at the last moment you go right when I go left.
The combustion engine we know and love runs on the Otto cycle, essentially from a pure physics perspective the base amount of work an engine outputs is based entirely on compression ratio, a fluid is compressed and it's expansion generates work or power we know and love.
https://en.wikipedia.org/wiki/Otto_cycle
Now, if you don't feel like reading that we can jump down to the last paragraph in the 'cycle analysis' section where we see equation 6 which looks a bit like:
n= 1 - (1/r^(y-1))
Which translates to efficiency of an otto cycle is a function of compression ratio. Interestingly enough the paragraph right underneath is the next important thing. We both agree that knock is caused by a compression ratio that is too high for the fuel being burned. And we seemingly both agree that timing is pulled to artificially reduce the compression ratio to prevent knock.
Now the trick is we seem to tentatively agree that artificially reducing the compression ratio reduces engine efficiency, i.e. we burn the same amount of fuel and make less power which means that we need to burn more fuel to make the same amount of power. Now yes this is highly idealized but it is the basic principle by which combustion engines have been designed under for 150 years, there are heaps of variables to throw at these equations to be more accurate, but it is the physics behind which we use to understand engine performance, no ifs ands or buts.
Stop me here if we don't agree.
Now translating that to real world driving. Looking back at the post I've quoted, awesome, we agree that driving habits are mutually exclusive to octane, that means that no matter if you're driving a 5.0L V8 or a 1L I3 when you're cruising along at 65 mph on the freeway or merging up to speed from say 25 to 65 you have roughly speaking the same demands from either engine if you're not playing around.
So if you have two engines sitting there, one using the recommended octane running at it's designed state and the other using a lower octane and pulling timing, it would stand to reason that the engine pulling timing is less efficient right? It takes more fuel to produce the same amount of power, correct? So I take both cars, drive them up to 65 mph along the exact same profile of speed vs. time, the one producing less power will require me to open the throttle more, burn more air and fuel to keep up with the engine running at optimum tune, or do we disagree with that logic?
I will not argue the magnitude of the benefits of higher octane in a high compression engine designed for it, but mpg is undoubtedly one of them. You are correct the differences are small and unnoticeable to most but I have a problem when people try to contradict something I firmly believe to be true given my training in physics and engineering. I also have an abundance of time because I'm a loser.