Premium or Regular?

http://priuschat.com/forums/gen-iii-2010-prius-technical-discussion/67073-regular-plus-prem.html

Shell regular here.

 

It is not only unnecessary, but it will reduce your gas mileage. Premium gas is only an advantage in engines designed for premium. Why not try diesel fuel, as it has more energy per gallon? The answer to that is obvious: the Prius engine in not a diesel. While using premium gas isn't as silly as using diesel, the same concept holds true. Use the proper fuel for your car. In the case of the Prius, this is 87 octane (85 at altitude).

Tom

 

regular here

 

My dealer filled the tank with Shell 89, E10 and I was able to run a hill climb test with that tank and later, a tank of Shell 87:

The Prius can 'tune' in high power settings to use 89 octane and continue up the hill on gas power. In this case, a 6% grade hill at 80 mph. But above 80 mph, the 87 octane requires the engine to 'de-tune' and make up the energy deficit from the traction battery.

If my wife is driving and we are headed to the mountains, I'll top off the tank with 91 octane to mix with the 87. But if I have enough time, I'll burn down the 87 octane and fill with 89, a mid-grade.

Now if I'm driving the mountain segments, I'll run 87 octane and keep the speed at say 70 mph. It really depends upon how many mountains and what speed you want to climb at.

Bob Wilson

 

And how high the mountains. At altitude, 87 octane is more than the Prius needs.

Tom

 

Tom, In Denver, altitude 5,280 ft, give or take, 87 isn't available at any gas station I frequent. My choices are regular at 85 and premium at 91 octane rating on the pumps.

I hadn't thought about using premium when I head into the high country (across the rockies) on either I-70 or up to Wyoming and across I-80. --TK

 

You might want to consider doing some hill-climb tests like I did. What you are looking for is a plateau where the quantity of fuel needed to reach a fixed altitude levels out at higher speeds. Then use the next slower speed when running that octane when going up the mountain or hill. This will minimize drawing significant power from the traction battery and help it avoid excessive heating to hopefully last longer.

Our studies have shown that 'heat is the enemy' and the batteries get warmest from:

1. ohmic heating during discharge
2. ohmic heating during charge with exothermic reaction from charging
3. ohmic heating during discharge > endothermic discharge reaction

So we're finding a trend in the NHW11s that early traction battery losses tend to occur:

• hotter climates
• more hills (Texas hills qualify) and mountains
• higher speeds in the above areas

My hypothesis is if you can climb hills and mountains at a slow enough speed to avoid drawing traction battery power the traction battery will have time to dissipate the heat that will happen on the downgrade charging. Then using "B" on the downgrade shunts some of the excess energy out the ICE so the traction battery isn't charged as rapidly. This is at best a model based upon individual measurements and would take a substantial effort to validate. But the little data I have indicates this is likely to be the case.

Bob Wilson

 

85 octane is recommended at your altitude. As Bob points out, you may find situations where a higher octane may help with hill climbing, but only testing will tell. The Gen III is a new engine, and I don't have any data for it.

Tom

 

I use 87 what ever is cheapest.

 

The analogy I was given for premium vs regular was shoe size. A size 11 shoe won't make your size 9 feet run better. But wearing a size 9 shoe on your size 11 feet will be a problem.

Maybe running uphill in the mountains while wearing "Bozo the Clown shoes" (premium) WILL make you run faster....one for the mythbusters (or Bob).

 

Whatever size, as long as they're PF Flyers!

 

Not so much 'faster' as less likely to exhaust the traction battery. I've read several reports of 'I was going up the mountain at <some high rate of speed> when suddenly the car slowed down and ...' The traction battery Ahr capacity is still the same (for a new battery) and that is the only other source to maintain speed when climbing at peak ICE power settings.

Now if I've got 87 octane and I'm in West Virgina, I would keep my speed under 80 mph. But if I've got 89 octane, my lead-footed wife could drive and I would not be too worried. I don't see a need to test higher octanes unless I can find a really, really steep hill that I can climb really, really fast.

Bob Wilson

 

I love how people read their owners manual about what grade of fuel to burn. The 87 octane regular on the flat lands is reduced to 85 octane in the mountains because of the altitude. If you're unsure call your dealer. I drove my 87 octane car thru the Rockies using 85 octane and never had a problem. I alswys use Shell, BP, or Phillips 66 gas whenever I can get it on my trips. I had a rental car back in August in KS (a 2009 Yaris) and got 40 mpg running 70 mph on I-70 with the A/C on using Phillips 66.
UncleMark

 

Actually, the energy content of premium is slightly higher than regular. If anything, one should get better mileage with premium, however, the difference is small and hard to accurately measure--and probably not enough to offset the price difference.

The 2010 Prius has a high compression engine (13:1), but like all new engines these days, has knock senors that will cause the ecu to retard the timing when burning regular fuels. There is a performance hit when doing this, but it is only significant at or near wide open throttle.

 

That is odd, I usually hear the opposite.

Make that an expansion ratio. Due to the Atkinson cycle, the actual compression ratio is significantly less, and variable.

 

Energy density per unit volume tends to increase with increasing molecular weight. Regular and premium are blend dependent and, for most purposes, the same. The reality is that we are both right.

You are right about the compression ration. (Heck, I've only had this car three weeks and forgot about its Atkinson cycle.) The compression ratio is what it is, 13.0:1 in our case, but the effective compression ratio must be less.

My comments about high compression engines & premium gasoline are still correct, just not relevant to the Prius.