Fuel Requirement

Question:

I think I understand why lower octane at higher altitudes is OK with older carburated engines. The mass of air sucked in is less at higher altitudes so the absolute pressure in the combustion chamber is less allowing a lower octane. At least that's how it's been explained to me.

But now we have mass airflow sensors and EMS computers that inject exactly the correct volume of fuel for the amount of air in the cylinder. So wouldn't the octane requirement remain the same regardless of altitude?

 

Using lower grade gas in engines designed for higher grade.

99% of the time, you will be OK with the sensors.

There is a few areas in driving where the sensors are too slow. Like if you start a car on a cold morning, on the bottom of a hill, put it in drive and stomp on the gas to climb uphill. It takes a while for the sensors to heat up and even for the exhaust gas to heat up the muffler/catalytic converter. Or you are driving and suddenly stomp on the gas to get away from a crash.

The system has hysteresis, a delay from the time you started the action (stomping) to the time the system senses that more gas in added to the combustion in the exhaust. During the hysteresis, there is no (or insufficient) feedback.

So during these unusual circumstances, the control program goes on a pre-determined curve set at the factory based on certain assumptions about the engine and the fuel. During these times, when the sensor is not providing feedback as fast as desired, if the engine or the fuel is not up to spec, there may be problems (knocking is a problem) which - hopefully the antiknock sensor will kick it and fix it but it depends.

I am just pointing it out to you. If you don't drive like a maniac, it would probably be OK.

This does not apply to the Prius because the Prius is approve for using the lowest grade of fuel sold in the U.S. anyway. So you cannot disappoint the predetermined curve. It already assume the worse - that you are using the lowest grade of gas.

 

It's the same with carburetion or fuel injection. Either way the compression stroke starts with less air in the cylinder, so there is less compression. Lower compression calls for lower octane.

The only way to get around the lower air pressure at altitude is to use some sort of compressor: turbocharging or supercharging.

Edit: Sophisticated variable valve timing can also be used to control compression, assuming the engine normally operates at less than full compression.

Tom

 

Thanks Tom, I had been using 87 at high altitude when I could get it because I wasn't sure. Of course is only pennies a gallon but I was curious.

 

Yep, the manual does say 87.

 

87 won't hurt anything. Most mountain gas stations sell 85.

Tom

 

From Toyota G3 Spec sheet (handy data sheet on the G3):

Powered by Google Docs Half way down on the 1st page, "Recommended Fuel"

- 87 Octane is what's recommended by Toyota.

 

I generally run Shell-87 due to its low cost and generally acceptable fuel economy. However, like others, I've noticed a 20 - 25% reduction of fuel economy during the winter months. Then I remembered that I'd had some of my best mileage last summer ~66 mpg, measured, after I'd filled up with a tank of Shell-91 fuel. So I decided to try running with a tank of Shell-89 during winter to see what would happen.

Based on my results, I'm not convinced that 87 is the only choice. Here's my reasoning:

87 Pros:
1. Lowest cost.
2. Acceptable performance.

87 Cons:
1. Typically contains 10% ethanol which can reduce engine performance.
2. Most gas stations will tell you that there are minimal additives in 87 gasoline. Since these additives are used to help keep your engine clean, this could lead to potentially more carbon buildup as the engine ages.
3. When the engine is gently accelerating at low speeds (1280 RPM's or lower) in stop-and-go traffic, I frequently hear it lugging (low-pitched engine resonance). I suspect this is due to the reduced power provided by the 10% ethanol requirement.
4. There is tremendous competitive pressure on a car manufacturer to make his car run on 87 fuel due to its reduced cost (up to 10%). It has to run acceptably but it doesn't have to run optimally.

89 Pros:
1. Typically contains only 5% ethanol so more power is available at all RPMs.
2. More additives for a cleaner-running engine.
3. Virtually no lugging with 89 fuel.
4. My fuel economy jumped up by about 5% (although this is not entirely definitive since it has been a bit warmer up here lately).

89 cons:
1. Higher fuel cost (up to 10%).

So is it wise to go with the the bottom-grade, lowest cost fuel available and save maybe $4.00 per tank? I'm not so sure anymore...

 

Is this specific to Canada?

After running my '86 Honda for nearly 23 years on 87 octane, I'm not feeling any reason to be concerned about carbon buildup. And according to relatives in the refinery industry, even the cheapest name brands now have better detergents than the best available when I bought that Honda.

 

Or, if anyone is concerned, a periodic bottle of Redline SI-1 or Chevron Techron costs only about $6 or $7, and is cheap insurance for a fuel system.

 

The difference in gas price in our area from Reg. to Mid Grade is 10 cents, the Prius has an 11.4 gal capacity, so even if you were empty the money you save would be less than the cost of a cup of coffee.
If you live in an area that has Texaco stations you will get Techron with every fill up and as the previous poster said it is cheap insurance for a clean fuel system even if you have to buy a can.

 

Just got my 2010 a week ago, filled up the first tank with 87. It's a daily driver for me and I don't race it reving the engine high all the time. I think it also depends how each one of us drive the car and our driving habits. If you are a spirited driver or angry about how slow the car is by pegging the gas pedal down hard all the time, during hot summer days with 87 octane and high rpm rev engine will knock (though you won't be able to hear it), it would be cheap insurance to pump 89 or so during summer time.

As for me I buy this car to save gas and I tend to drive relatively normal to get a good mpg, so 87 is good enough for me. The car will outlast me owning it than having a drastic engine failure due to pumping 87.

On the side note, our family have owned many BMW leases, all called for 91 octane, but guess what most individuals I bet only put 87 or 89 simply because they don't own that car and will return the car to the bank after the lease is over. So for those high end cars it's probably not a good idea to buy a used lease return where I bet most owners put cheap gas in the car during their lease period. Even then we have had no issues with the car with engine troubles based on the 3 year lease term. After 3 years? Well I don't know but by that time people have had already returned their lease.

 

Good question. The price difference between regular and mid-range gas up here in Canada is somewhere between 8 and 10 cents per liter, so this would equate to about 35 cents US per gallon. I've always assumed that the additive package would be the same but maybe there are some legal requirements that dictate otherwise.

It seems that some engines carbon up more than others. Perhaps it depends on whether or not the engine is burning oil or maybe it depends on how much the engine is idled during the warm-up period. I had a Mazda Protege a few years back that I added a remote starter to for winter warm-ups, and it started fouling spark plugs before three years had passed. Needless to say, I won't be adding remote starters to any of my future automobiles.

I don't know if the 2010 Prius is susceptible to carbon-related issues, but I rented a 2008 Prius with 34K miles a few years back on a trip from Los Angeles to San Diego (and back). While leaving Los Angeles, I hit a major traffic jam and the traction battery soon dropped into the two-bar region. Then the engine started cycling on and off -- touch the gas and the engine would start, touch the brakes and the engine would stop. More than a few times when the engine started, it ran rough for about the first 1 - 2 seconds, enough to give me pause. I suspect that this might be due to carbon buildup, perhaps on one of the spark plugs or perhaps in one of the fuel injectors.

I speculate that the Prius engine might be a bit more susceptible to carbon buildup than an ordinary car due to the Atkinson-cycle, where the intake valves are kept open at the beginning of the compression cycle, forcing the fuel-air mixture back into the intake chamber -- but again, that's just speculation.

 

Some of you may find the following informative:

http://www.state.mn.us/mn/externalDocs/Commerce/Gasoline_Octane_Facts_102902052227_OctaneFacts.pdf

http://www.ridelust.com/octane-ratings-premium-vs-regular-gas-whats-the-difference/

Higher octane gases are recommended for higher compression ratio and higher reving cars than the prius. The octane rating is a measure of pinging (or pre-detonation) in the cylinder.

The prius is NOT a high compression-ration car. Nor is it high reving. Save your money and stick with 87 octane, as per Toyota's recommendation. But if it makes you feel better, then choose mid-grasde....but you're wasting your money. But know this - many of the additives in the gas to boost octane, and "clean sludge" are also more harmful to the environment.

 

This on-off behavior of the ICE has nothing to do with fouling: it's simply the HSD control system doing its job. The ICE comes on when you call for power and goes off when it isn't needed.

The Prius ICE lives an easy life. It never races at high RPM, nor does it ever lug at low RPMs. Even when it sounds like it is racing, the RPMs are still low for a small ICE.

Tom

 

I don't think most people realize that the ICE is spun up to about 1000RPMs (by the elecric motor) before the ICE begins to fire and burn fuel. Every cold start is this way. Yes, the ICE lives a "good" life. I really would not worry about sludge/carbon build-up in the prius. I also think running (and changing) synthetic oil regularly certainly mitigates sludging issues...with any car.

 

I don't think how you drive the car has very much to do with whether it knocks or not. Cars also don't knock as much at high rpm's as they do when they are lugging. Cars that are designed for 87 octane gas will not knock on 87 octane gas even if you were to enter it in the LEMANS 24 hour race, unless it were broken in some way.

You would have one heck of a hard time lugging the engine in a prius given the CVT. Use whatever you want, just be aware that any penny you spend on higher octane than 87 is money down the toilet. It won't even increase your credentials as a green guy!

 

That is certainly true. Both of my carbureted Fords seemed to have this problem, prompting me to buy mid-grade gas when driving at low elevation. The second also had some oil fouling later in life. But I've yet to experience these problem in the fuel injection era, despite running odometers to much high numbers.

No experience here with remote start. I don't think they are made for manual transmissions.

 

The only pre-2010 Prius I've ever driven in was that rental. I had no real problem with it turning on and off (although I would have preferred that it would have stayed on long enough to charge the battery a bit before turning off, like the 2010 I now drive does). It was the stumbling on startup that I didn't like. Perhaps this is a normal trait with the previous generation cars, and maybe it has nothing at all to do with carbon fouling of any kind. All I know is that it sounded to me like an engine miss during the first few seconds of startup, and it was getting bad enough that I was relieved when the traffic jam cleared about 35 minutes later. So far, my 2010 Prius has not yet exhibited any of those symptoms -- it starts smoothly and it turns off equally smoothly.

It sure sounds like lugging to me. Take a standard transmission car and put it in a higher gear while moving at a slow speed so that the engine spins at about 1000RPM. Then accelerate gently and you will usually hear this low-frequency exhaust rumble that I call lugging. When I hear lugging in the 2010, the Scangauge shows something between 1088 and 1280RPM. If you push the gas pedal down a bit further, the RPMs jump up to about 1400RPM and the lugging mostly vanishes. However, I note that nobody else has reported anything like this, so perhaps it's just a quirk that's unique to my Prius. Certainly it gives excellent fuel economy (65MPG during the summer and about 50MPG during the winter), and it runs very well, with the possible exception of this admittedly fairly minor issue. Also, I'm on my second tank of Shell-89 gas, and this lugging noise has yet to reappear.

 

Just to explain where I am coming from. My Prius is purely a tool for me to get on the HOV and to save money on gas. It is a tool. While it is my only car (wife has her car) it is a daily driver or beater if you want to call it that. My main concern is low cost of ownership.

I will probably trade it in in less than 100,000 miles if Toyota comes out with Generation 4 which has more safety features. Safety is even more important than saving money - assuming I can afford it. Of course, there is a limit to what I can pay for safety so there is some compromise there too.

This Prius is only going to be with me for a few years and I want to get my money's worth - exploit the beep out of it and sell the thing. How it performs after I get rid of it is not my concern - someone else's problem. I am not going to cheat anyone - I'll just trade it in at the dealer.

So I am not going to spend extra on gas, oil changes etc unless I think it will affect the car in the first 100,000 miles or affect the trade-in value (I am trading it in through the dealer).

Anything more than the absolute minimum to avert a repair bill in the 100,000 mile or the absolute minimum to affect the trade-in value is a waste of money. Thing like more frequent oil changes, better gas, frequent washes are really money down the drain. The money I save can be used on my kids, my wife, or myself - especially now that times are tough.

So it is 87 octane for me all the way until I sell it. Instead of thinking mpg, I think miles per dollar which is more realistic. If you can get 60 mpg but have to use expensive gas and frequent oil changes, in the end, your miles per dollar may be worse than someone who gets 50 mpg.

I am not ranting and raving - just letting you see my motivation and to urge other people who may be similar to me to see the big picture - and not waste money. I humbly respect all the other viewpoints and understand that you have your reasons for doing what you recommend. I see that if you plan to keep the car for 200,000 miles, collect it as a classic, use it for dating or to project an image at work, etc, you may think differently.