I saw this and thought I would post it. I live at near 4000 elevation and use 85 Octane and it is fine. In locations with higher altitudes there is less available oxygen in the air. So for a given compressed combustion chamber, the total oxygen content being less limits the amount of fuel that can be burned. This is controlled by a fuel flow decrease as determined by O2 sensor feedback and can be further controlled by limiting octane to that which can be effectively burned. Think about this in reverse performance car terms. Higher compression means higher octane is required. Higher altitude means lower barametric pressure which lowers the effective compression ratio achieved. You therefore need less octane. 85 octane is more than adequate for Colorado Springs. I used to do powertrain development in that area, and some engineers debated the octane requirements. Try using 89 octane (premium in that area) before going up Pikes Peak. As you pass 8500 feet, the poor engine is doing everything it can to burn the mixture of low atmospheric pressure, high octane, and lower oxygen content. The engines ran much better with fuel of an octane that could be effectively fired by the spark plug, which was lower octane fuel. Octane is the measure of resistance to self ignition. When there is reduced effective compression and lower oxygen content, the ability to self ignite is reduced. The 85 octane is what you want to use in that area. Anything more results in inefficient combustion. Carburetors had more issues because while fuel flow was reduced as a result of lower engine vacuum at altitude, the amount of reduced fuel flow was not controlled as a function of the combustion process. With closed loop fuel control (fuel injection and an O2 sensor) the amount of fuel used can be more closely controlled to that which can be consumed during the combustion process.
I agree, use 85 octane at higher altitudes. It's all about absolute pressure in the combustion chamber and when you start with less pressure you have less when it's compressed. The only vehicles that should not use lower octane at high altitudes are those that are not normally aspirated, i.e. supercharged or turbocharged. I use 85 octane in my Prius anytime I am in a state that sells it. I think the requirement is 3500 ft or higher above sea level. I can't tell the difference between 85 and 87.
Will the Prius adjust its Atkinson cycle to increase compression ratio at high altitude? If so, you may get a bit more power by using gas rated higher than 85 Octane .
No and no. Altitude compensation is automatic in that the car is already adjusting mixture and spark. Higher octane will get you nothing. You can't make thin air thicker.
It's maximum ratio is already available at sea level, and is not out of the norm for Otto cycle engines that do just fine with 85 octane at similar altitudes.
It seems to me that making thin air thicker is exactly what the compression stroke does. With an Atkinson cycle, the compression ratio is lower than the expansion ratio and if intake valve timing can be controlled independently from exhaust valve timing then this compression ratio can be reset upward toward a maximum that is approximately the same as the expansion ratio. Depending upon the control method and within the limits of the available adjustment: You have the possibility of maintaining the same cylinder pressure at high altitude as you normally have at sea level and thus get the same power capability at high altitude that is available with 87 octane at sea level. You have the possibility of running high octane fuel and automatically adjusting the compression ratio to increase cylinder pressures to a higher level and thus increase power If the control is based on sensing incipient preignition and dynamically adjusting compression ratio to be as high as possible without causing preignition then you would achieve both 1 and 2. There may be other strategies that only achieve 1 or neither of these possibilities. IIRC, I read somewhere that the Prius has preignition sensing but I don't recall control details.
Toyota's current mechanical intake valve adjustment system does not have that much adjustment range. At some future time, electric valves should be able to do this.
My TSX calls for 91 PON, and says that use of anything below 87 PON will cause engine damage... While I was in Bozeman, MT(4400 ft), I filled with regular 85 and noticed no knocking, although I still wonder if it was unwise to run it.
