Does high octane "suck" in the prius?

<div class='quotetop'>QUOTE(keydiver @ Jun 6 2006, 03:27 PM) [snapback]266806[/snapback]</div>

And that's exactly why I really dislike ethanol in my gas.. It may pollute marginally less, but it costs a fair bit more per gallon, *and* my car has to burn more of it, so in effect it hits my wallet twice..

I hope that guy promoting butanol manages to get somewhere with his campaign, because if you believe what he's saying, butanol is a far better vehicle fuel additive than ethanol.

 

<div class='quotetop'>QUOTE(windstrings @ Jun 6 2006, 06:36 PM) [snapback]266914[/snapback]</div>

The problem, as I see it, is you have no way to harness those extra BTU's. You are forced to inject a richer mixture, wasting the extra fuel, even though it has more energy content. Now, as someone else said, if your engine was designed from the ground up to burn ethanol, with a higher compression ratio, different cam timing, etc, you may be able to use some of that extra energy. But, otherwise, as I see it, it is just an extra safety margin, like running high octane fuel in your Prius, with no added benefits.
Now, on my turbocharged car.......hmmmm B)

 

Where you go wrong in your logic is that an engine doesn't care about how many BTUs it gets when it ignites an air-fuel mixture, it only cares about how fast it burns (via the knock sensor) and whether the mixture was stoichiometric (via the oxygen sensors). In other words, how much fuel is actually burned is *NOT* dependent on the BTU output, but on the volume of the engine cylinder and the chemistry of the fuel... Read Keydiver's description again carefully as he's got it exactly right..

I'll try my own description as well in hopes that it might make things clearer: Let's start at the beginning of the intake cycle- the intake valve opens, and the piston starts descending, drawing in a volume of intake air that is dependent on the size of the cylinder (assuming no turbo/supercharger or other compression device that can artificially increase the air charge, but we'll talk about turbos a bit later). Now as the piston nears the end of its intake stroke, depending on all the timing related programming, the ECU activates the injector and it shoots in the quanity of fuel that is needed to consume all the oxygen in the air just taken in. <- Note that you're always getting almost the same amount of air on every intake cycle (there is some control due to throttle opening/closing and intake valve timing, but they don't change the point of the argument so we'll ignore them for now). How much fuel is injected is thus calculated depending on the volume of air and the chemistry of the fuel.. The ratio for gasoline is 12.5 air to 1 gas; the ratio for ethanol is 6.5 air to 1 ethanol, but to make the math simpler, for this example, I'm going to use 12:1 gas and 6:1 ethanol..

Now think about what these ratios mean.. Let's for sake of argument assume that the displacement of one cylinder is 1200CC because it's a nice, round number that makes the math easy, so opening the intake valve and pulling down the piston will fill the cylinder with 1200CC of air. If you are burning gas, you will to inject 100cc of gasoline to get a 12:1 ratio for most efficient burn. If you are burning ethanol, you need to inject 200cc instead in order to get a 6:1 ratio.. In other words, you've used twice as much fuel to move the engine drive shaft the same amount.. Here's where a bit of understanding about the engine is helpful- burning that charge of air and fuel, no matter what the energy content, still only moves the drive shaft the same amount, it's how much *push* or torque you get that changes with the fuel. It's the transmission that will turn this fixed distance/variable torque motion of the driveshaft into forward motion of the vehicle..

So, going back to the chemistry: the burn of ethanol may result in more energy being released (26% according to Keydiver) so that there's more torque available with that same movement of the driveshaft and thus the engine has the *potential* to do more work. The key point here is that this extra energy is all *potential*- in most cases, this potential is wasted: although a more potent fuel may result in fewer revolutions (or lower RPMs) needed to get the vehicle moving than a less potent fuel, if the vehicle is not under full load- ie, the engine is idling or the car is cruising at a steady speed, you never get the full benefit of the extra power. In other words, you may see the benefit of ethanol as better acceleration and pick-up, but once you get up to speed, all that extra energy is wasted..

The upshot of all this is that you had to burn *twice as much fuel* by volume to get a maximum of 26% more power.. On average though, you don't get anywhere near 26% extra power, but you're still burning twice the fuel just to keep the driveshaft turning.. Burning 2 to get 1/4 is not a very good trade-off in my books...

At this stage, I think the point is made, so I'll return to our mention of turbochargers/superchargers- by compressing the intake air, you cram more air into the engine cylinder on each intake cycle. The vehicle ECU then compensates for the greater volume of air by increasing the amount of fuel it injects. This higher pressure mixture releases more energy than usual and again, it still moves the driveshaft the same amount, but there's more torque behind the movement. This basically reiterates the point that it's the amount of air that controls how much fuel must be injected; the BTUs you get from burning doesn't really have any impact on the fuel volume required except indirectly through the number of revolutions needed to get the car moving.

I'll close with the observation that the Prius, with it's effectively smaller intake volume due to the Atkinson cycle, may actually be able to make more efficient use of ethanol than the almost-fixed volume of a conventional Otto-cycle engine, but again, it's still the intake volume that controls fuel consumption, and *NOT* the amount of energy in the fuel, so a vehicle with smaller engine displacement would seem to be more optimal for burning ethanol.

 

The bottom line is:

You can only get so many MPG's using ethanol.

In fact it goes like this as far as MPG's are concerned.

with Diesel you can get the highest MPG's.
Then comes Gasoline.
Then at the bottom is Methanol/Ethanol.

However, many HIGH PERFORMANCE cars are built around Methanol/Ethanol.

The reason is Ethanol/Methanol has a very high octane rating.

This means the gas burns and doesn't detonate.

This means you can have very high compression ratios in the engine. Great for performance cars.

So, as long as your engine isn't ping'ing or pre-detonating, higher octane generally means slightly less MPGs. However, this slightly less MPG is insignificant compared to the amount of damage the pre-detonations are causing the engine.

Also, most modern engines sense this knocking, and they will de-tune themselves. So with higher octane rating, there's less chance that the engine de-tunes itself. If/when the engine detunes itself to protect itself, you get really really crappy mileage.

(leaving the scientific details out, so more people can understand)

Jimmy