Best type of gas for Prime?

One accidental touch of the windshield defrost button and you'll wish there was gas in the tank. Especially easy to do on the upper two trims with the 11.6" display.

 

Yes.
No. As noted above, modern cars have the fuel pump submerged in the gas tank. This cools and lubricates the pump. Never run any modern gas tank dry if you can avoid doing so.

 

Most PHEVs are designed to work with both the engine and electric side working together. The prime can run on electric most of the time for many trips, but you can't predict when it might want to use the engine. On a hot day, the battery can reach high temperatures. To reduce heat generation, the car can use the engine to reduce the draw on the battery. If there isn't gas in the tank, the car will go into a low speed limp mode, basically stranding you, or it will overheat the battery, possibly damaging it.

 

With the Volt, the owner's manual recommended keeping a few gallons in the tank.

 

No. As per user manual.

 

When I received my Prime in Utah, the dealer told me I could fuel the car with 85 Octane gas as long as I drove it at high altitude (i.e., Utah, Colorado.)
The user manual has no such indication and it says to fuel the Prime with 87 Octane or above (and no more than 10% Ethanol, if I remember correctly.)
Is there any hard science to support the fact that lower Octane fuel can be used at higher altitudes?

Regarding E10:
I have used 10% Ethanol in both my Gen 3 (7 years) and Gen 4 (for 3 years) and I have never had a problem with their engines.
I am conflicted about E10. I'd rather see more tree planted, better protection and management of forests, more varies crops for feeding good quality food to people, more investments in renewables to charge PHEV and BEV cars. Instead of using it for fueling cars that don't seem to have better fuel consumption and get bigger each year (the Prius is one of the few "mid-size" cars sold in the USA - otherwise the choice is between SUV and trucks and their EPA combined ratings are often well below 40, hybrids included!)
More info on E10 in the EU and in general here

 

It is OK to run 85 AKI gasoline at high elevation. You'll notice a loss of power.

In olden times when cars had carburetors 85 octane worked well at high elevation due to the thin air--less oxygen in the high altitude air reduced the tendency to knock so cheaper 85 could be used. All modern cars with computerized ignition use some sort of mass air flow sensor that adjusts the fuel delivery for the mass of the air and adjusts the timing with knock sensors. Every modern engine will run better at high altitude with the gasoline specified for that engine...87 AKI for our Primes.

 

It looks like it was an American Petroleum Institute recommendation and is currently an ASTM standard

Should state’s minimum fuel octane go up?
ASTM D4814 - 21 Standard Specification for Automotive Spark-Ignition Engine Fuel

 

While technically not hard science, there is the experience of tens of millions of drivers using it for decades and not noticing any problem with it. Have closer looks been finding problems that many lay people have missed?

I had two carbureted Fords before switching to fuel injected cars of other brands. Both developed octane- and altitude-related knock as they aged. Possibly from carbon buildup raising the effective compression ratio? Despite both being rated for 87, the first eventually needed higher octane anywhere below ~2000 feet elevation, while the second, kept to well over twice the mileage of the first, ended its life needing higher octane below ~1000 feet. Neither had problems with 85 octane in brief visits to Montana at 4000+ feet, though likely did not have enough fillups there to get the tank mixture all the way down to 85.

My subsequent fuel injected cars have spent far more time and distance in the 85-octane zones of MT, WY, CO, and UT, and never displayed any problems resembling what those older carbureted cars experienced with 87 at low altitudes.

My understanding is that minimum octane need is a function of absolute chamber pressure before ignition. Since these engines don't increase their compression ratios as ambient air pressure decreases, and ambient pressure at 5000 feet is just 83% of sea level pressure, that means the chamber pressure also decreases to just 83% of sea level operations. Less octane is needed, though I don't have any handy chart of needed octane vs. altitude.

That lower pressure is also why power decreases, and earlier downshifts and higher RPMs are needed when climbing mountain grades at 10k+ feet than on similar grades close to sea level.

One issue to watch is that these modern cars have greater fuel ranges than the old ones. This means that when leaving an 85-octane market, you can reach lower elevations than with the old cars of 40-50 years ago before needing a refill. You might want to put higher octane into the tank before actually needing additional fuel.

 

Hmmm... Seems like the circumstances when the ‘Prime uses the ICE are pretty well-documented. Short version: Indicated EV miles run out, above 84MPH, EV Auto mode (I don’t use it), the conventional windshield defrost hit (for me, 95% of the time accidentally when I’m intending to hit the temperature-down button — %£@€!), outside temperature below ... way the heck colder than it gets here in Austin (other than this past February!), plus a few other rarely-occurring circumstances (e.g., allegedly including “sacrificial burns” to clear the tank of ancient gas).

 

The manual has a line that basically means the engine can turn on for any reason not stated here. Just because someone hasn't reported it happening under some other condition, doesn't mean it can't.

Octane is a rating of how well the fuel resists knocking. Knocking is when the expanding flame front of the ignited fuel increases the preasure of the unburnt fuel air mix to the point that secondary ignitions happen.

At higher elevations there is literally less air within a given volume. Less air in the cylinders means everything is starting at a lower pressure. Lower pressure in the cylinder means lower octane will be fine.

The exception to that are engines with turbo or super chargers. These push more air into the cylinder, so the pressure will be elevated over ambient before the piston starts its compression stroke.

I though all new cars had to accept E15 by this time. My 2016 Camry can.

Using 87 when 85 will do can mean better running engine in the same way higher octanes can help at sea level. Even then the engine will make less power as the engine is injecting less fuel to match the amount of air coming in.

Carbon deposits can also become hot points within the cylinder.

The ignition timing range of the car should be able to adjust for octane a little bit below the recommended octane to allow for substandard gas somehow getting into the tank.

 

Yes, I have seen it turn on the ICE for reasons other than the usual: three times that I can recall, since May of 2017 when I bought it. Anything other than those stated circumstances is exceptionally rare, in my experience at least.

 

Same here: just yesterday, I had EV Auto turned on. Although it can do what it wants, it turned on the ICE while starting at a flat grade traffic light, with normal acceleration and with 30% battery left. I was surprised. Like:"Why is she turning on the ICE, of all times, now?"

I think that some Prius users forget that just because this car has a battery, hybrid or plug-in versions doesn't matter, it is still a car with a GAS engine. The ICE can need to turn on for whatever reason at any moment. Even at stand-still. Running out of gas is a bad idea, even on a Prius. And unless there is some foreseen emergency, I don't see why one would ever want to run with an empty tank. The damage far outweighs the feeling of "ohhh, I am driving an electric car!"

 

I'm saying that it is best to keep gas in the tank in case of those rare times. As opposed to trying to make the car a very short range BEV.

 

I agree, but you don’t need to keep very much gas in the tank.

 

EV Auto is one of the well-known cases.

I don’t think I’ve ever used EV Auto...

 

I think the original post I replied to was someone considering keeping zero in the tank.