What type of Gas should Prius stay AWAY

Hi,

I can explain the methodology. The route I use is called "Brindley Mountain" but is actually a segment of "Dixie Highway". Brindley Mountain is about 20 miles from the house so the test vehicle has enough time to warm the engine and transmission. The climb route is shown below:

Approaching the climb course, there is a three mile run up on river bottom to the entrance at the lower left of the divided highway. The run up appears to be long enough to normalize the battery state of charge before the the climb since a previous descent puts the traction battery at maximum SOC. During the final approach, less than 0.25 mile, the cruise control is set to the climb speed.

The measured climb starts at the bottom by resetting a trip meter at a highway sign. Cruise control maintains the speed during the climb and the end is the first, right turn at the upper right, the top of the hill. GPS and map readings give:

• 1.2 miles (1.92 km)
• 525 ft climb (160 m)
• ~8.3% grade

So this is what I got on my ZVW30 hill climb tests:

The initial climb series was using Shell 89, E10 octane from the dealer. The second series used Shell 87, E10. As you can see, the second series showed the 87 octane fuel consumption flattened out at speeds in excess of 80 mph up the 8% grade. This is consistent with the engine reaching peak power and the extra being taken from the traction battery. In contrast, 89 octane fuel consumption continued as the engine was able to use it to power up the hill.

Here is equivalent data from our 2003, NHW11 Prius:

You'll notice at higher speeds, the fuel consumption goes down. This happens because the 1.5L NHW11 car draws the extra energy from the traction battery. I was surprised to discover the ZVW30 did not do the same except possibly when using 87 octane climbing at speeds over 80 mph. The engine rpm does not increase at these speeds.

Using a Graham scanner to record the data with our NHW11, I was able to read out the shaft power vs fuel consumption with one second resolution. I used this recorded data to compare different brands of fuel:

This was the first time I saw direct evidence that the 1.5L, NHW11 engine could tune itself for higher octane fuel. This is not enough to overcome the higher energy content of 87 octane fuel but it does mean more power going up the hill. I suspect it comes from the knock sensor tweaking the timing and valve advance setting but this has not been measured.

Near as I can tell, higher octane means the engine can carry more of the load when climbing a hill at high speed ... speeds that peg the engine rpm. I don't advocate high-speed climb since the 1.5L Prius does best at 55 mph when climbing a hill. But after all, it is your gas, time and vehicle.

GOOD LUCK!
Bob Wilson

Well this can't be right can it? Because here in England our minimum fuel is premium unleaded at 95 octane and 98 for super. Surely Toyota must have planned for this in their design? Can't imagine them designing a 'world' car that doesn't run properly on fuel available to half the markets.

Just use it and then use a lower octane after

Using 91 octane in place of 87 octane is like switching your size 10 running shoes for size 12 running shoes and then think you can run faster.

England has a different octane system. Just like your gallons are different from our gallons, your octane is different than ours.

Tom

There are two basic ways of measuring 'octane ratings': the Research Octane Number, which is what we (Brits) use for cars, and the Motor Octane Number, which was commonly used for aeroplane engines. The MON is a tougher test and usually comes out 8 to 10 points lower than RON.

The US and Canada use an 'anti-knock index' which is the average of RON and MON. This comes out about 4 to 5 points lower than RON.

Therefore, 95 RON is about equivalent to 90 - 91 AKI.

Why design the Prius for 87 AKI when Western Europe has standardised on 95 RON? Because Toyota sells so many more Prius in North America (over 60% of all cars ever built), and the extra octane gives very little benefit. A standard design for what is still quite a low-volume car also offers benefits, particularly when they're all built in the same factory or few factories in Japan.

Yes, Toyota could crank up the compression ratio a bit for higher-octane fuel and thereby get a little more energy per unit volume of fuel, but they don't have to, and the extra cost of 'premium' fuel outweighs the efficiency gain.

Hey guys--- (I am a newbie here!!)
I have a 2005 with 59K miles on it and I am seeing my mpg dropping steadily. Even when I do my best hypermiling, I cannot get into the 60s in the summer. I know ethanol is not good, but even adjusting for that this winter, I am struggling to get 52 or 53, when normally I do better. I keep all mileage stats, so I know I can substantiate this. Are my batteries fading or is there something wrong with the charging system?

Winter.

And if you are talking about a longer term drop in mileage (over the last year) then you should have your 12v battery checked out. If it is the stock unit in your 2005 then it may be dying.

Here is a thread on how to check it:

http://priuschat.com/forums/gen-ii-prius-technical-discussion/73415-2008-12v-acceptable-range.html

guess I'll switch back to 87. Started using 89 because the manual recommended 88.

It was a typo.

Is it okay to mix octanes? I have a half a tank of 89 so I'm wondering if I should fill it with 87 for a blend.

Yup. I used to mix 100 octane or 105 octane race gas with my regular fuel when running nitrous. I've even used methanol injection when racing my turbocharged truck. No problem with mixing some fuels just don't try it with diesel! Hahaha

Interesting read for sure. May I say that here in Canada Shell V Power (91) has no ethanol in it, on top of the fact that it contains a higher detergent level. I might try a few tanks of Shell 89 just to reduce the amount per volume of ethanol in the tank, to see if it makes any difference. I kow that in the past when Ontario switched to ethanol, I took a mileage hit that was measureable between 4-6%

Re: What type of Gas should Prius use?

The gas station here has two tanks: 87 and 91 octane. The 89 octane pump mixes the 87 and 91 together, and this is apparently standard practice; the tanker that delivers the fuel has two tanks, not three.

According to Toyota's website, the Prius engine has a 13:1 compression ratio. That is VERY high for 87 octane gas; it's even high for 91 octane. (I'm referring to American ratings). It's difficult for me to believe that 89 and 91 have no benefit. I wonder whether, after a few tanks of high octane, will the engine management advance the ignition timing?

The 13 to 1 compression ratio is misleading. The Prius uses an Atkinson cycle engine which features a very late closing intake valve, so charge leaks back into the intake tract during the compression stroke. The 13 to 1 does have full effect on the power stroke.

http://en.wikipedia.org/wiki/Atkinson_cycle#Modern_.22Atkinson-cycle.22_engines

It has been confirmed over and over that higher octane gas has no mileage improvement in the Gen 1 and Gen II engines, There is a very minor HP improvement. The Gen III engine is newer, so more testing could not hurt, but no one has found differences due to octane so far.

The Prius engine management seems to 'hate' alcohol and butane, treating them as near inert filler rather than fuel, so E10 and winter blend gas both have bad effects on mileage.
Petroleum - Wikipedia, the free encyclopedia

My layman's understanding is that high compression cars need slower burning gas (higher octane) in order to avoid premature ignition (an unwanted explosion which causes knocking) caused by the higher compression.

The Prius is not designed for the slower burn cycle. Stick with the 87 octane it is designed for if you are driving at normal altitudes.

Please correct me if I am off base here.

This post is pure win. You absolutely should not piss your money away above 87 in any vehicle unless the engine is specifically tuned to higher octane. Many engines are, generally higher performance motors but in this the recommended in the manual is 87. Please do not waste (and it is pure waste) the $.20/gallon to go to 91. Even if 87 and 91 are the same price there is no reason to put in 91.

Geeze, 3? You didn't need to call even one. 91 is fine. 95 is fine. It's just a waste of money. OK, if you got really high in octane your car would lack the compression for the spark to ignite it, so 100+ may certainly cause issues.

That's true. I remember watching one of the Die Hard movies when he takes a match to rocket fuel. My brother told me (granted, I don't know if this is actually true) that in real life you wouldn't get the heat from a match required to burn that fuel. That seems unlikely in retrospect, but the concept is there; greater octane reduces its combustibility at a given pressure. So, if your motor is tuned to a high octane and you put in a lower, easier to burn one at a given pressure, you'll get pre-ignition--the fuel/air mixture self-explodes before the spark from the plug has told it to.

I think

Re: What type of Gas should Prius use?

The Toyota spec sheets I collected a year ago very clearly label this as 'expansion ratio', not compression. The Atkinson cycle does not compress a full stroke of air.

There's a lot of interesting opinions here with charts and graphs. The bottom line is use quality fuels at the recommended octane (87) which usually translates to "regular" at the pump. I use Shell or Chevron even though it might be a bit more expensive because of the additives. One thing quality fuel will do is help prevent future fuel injector problems. With my prior vehicle, one of the senior forum members said:

Of course he was referring to N* engines. The Toyota in-line 4 cylinder engines have a totally different coolant (the pink stuff) so I doubt additives are needed for that coolant either.