What Grade Fuel do You Use?

Great info guys, thanks!
I'm am SO HAPPY I can purchase 87 octane instead of the 92 that my BMW REQUIRED. That might save me a few dollars:rockon:

 

Better not head to Denver Tom, your eyes will pop out of your head!!!

 

I was climbing out that way this fall. At 14,000 feet the air is a little thinner than it is around here.

Tom

 

For those of us that are scientifically challenged,,,,, is there a direct connection between drag and air density? (I never took physics!)

My intuition is that drag doesn't decrease at that rate,,, but I have been wrong about many things before,,, Maybe I am dense,,, or is it just the air.

Icarus

 

Direct Connection? Well, there is a correlation, though it isn't 1:1. For one look at drag as part of a study of aerodynamics related to automobiles, follow this link to Aerodynamics in Car Design.

 

So if my rough math is right,,, and the assumption that drag decreases by the same amount as pressure is correct, then the cost of drag is reduced ~6.5%, ergo my fuel mileage should be ~6.5% better at 3900' asl? That would translate to 50 mpg to ~53 mpg all things being equal. Interesting,,, not out of the range of credulity, but I wouldn't chalk it all up to elevation. I suspect that there are all kinds of things that would figure into any equation, drag losses only being part. Engine efficiency goes down with lower pressure,, combustion is more difficult etc.

Icarus

 

Really? Where did you see this in the manual? I have read that back to front and haven't seen it.

If so that is great news for me. They sell 85 here and it is 10 cents cheaper per gallon.

 

Your assumption is off by a little bit:

The equation for aerodynamic drag is:

D= CDA(ρ/2).V2

Where:
D = Drag
CD = Coefficient of Drag
A = Projected frontal area of the vehicle
ρ = Density of the ambient air
V = Velocity of the vehicle

Still, there is something behind it.

Don't forget, though, that the stoichiometry for "perfect" combustion isn't so perfect at altitude, where there isn't as much oxygen, by weight or volume, as at lower altitudes, while the gasoline is added at the same rate. I believe this is one reason the pistons "under compress" and why a lower octane could be more appropriate.

 

Just what we need,

some pointy headed intelectchul confusin' us with facts! Next thing they will be telling us that we came from apes, or the world is round!

Icarus

 

I don't have it in front of me, but 85 is "regular" at high altitude. 87 is regular at sea level. You should find that 85 works fine for you out where you live. It worked great for us when we were out there last fall.

Tom

 

Given that I'm Irish, I just don't know whether that was meant as a bad compliment, a good compliment or just something to say other than Sláinte Mhaith.

Round? That's a good one! I've had trouble staying on my porch; imagine what it would be like to fall of the edge of the world!

 

Fuel is not injected at a fixed rate. Doing so is incompatible with emissions and fuel economy requirements. Normally, the engine controller adjusts fuel flow to get the desired air:fuel ratio. The oxygen sensors provide the feedback needed to adjust this ratio.

Clean combustion at high altitude should be no problem at all. At low throttle settings on traditional Otto-cycle engines (fixed valve timing, always taking in a full cylinder of air on every intake stroke), the intake manifold vacuum is equivalent to air pressures at elevations far higher than Mt. Everest. Emission regulations force good fuel control over a wide range of throttle settings, and consequently, altitudes too.

The only problem here is that sucking that vacuum into the cylinder consumes a lot of power, called "pumping loss". Atkinson cycle engines (such as in Prius and Insight-II) consume less than a full cylinder of air, so they can run with less vacuum, therefore less pumping loss. Prius also uses other tools to reduce this pumping loss.

At higher altitude, Otto engines suffer less pumping loss, improving efficiency. Your Priuses, with Atkinson engines, start with less pumping loss, so they will see proportionately less fuel economy improvement at high altitude. But your external air drag component should be reduced at the same rate in that lower density air.

 

A simple way to think about the pumping loss post above is that at altitude your engine is less powerful. This is because of the smaller air/fuel charge from the thin air. A less powerful engine means that it gets run at higher throttle settings for the same power output. A higher throttle setting means a more open throttle plate, so less pumping loss.

Tom

 

Guys:

Thanks for bringing me into the 21st Century. I had forgotten all about the oxygen sensors and their purpose in adjusting fuel flow and air:fuel mixture.

As far as the pumping loss is concerned, I would think there would be less pumping loss in the Atkinson engine vs. the Otto engine. I thought the lower pumping loss would lead to more efficiency and could carry through to less efficiency loss at altitude.

I wouldn't mind being schooled a bit more on this topic . . .

1,000

 

Pumping loss is mostly a factor of pulling air through a small opening. When you run a gas engine close to idle, the throttle plate is nearly closed, so you are pulling the air in through a very small opening. As you increase power, the throttle plate opens which makes it easier to pull in air.

This is one of the reasons that diesel engines are more efficient that gasoline engines. Diesels don't use a throttle plate. A diesel engine always pulls in a full charge of air. Only the amount of injected fuel varies with power.

The Atkinson cycle engine still uses the same throttle system, so there is no advantage there. The advantage with the Prius is that the HSD allows the Prius to run the ICE at more optimal speeds. Instead of poking along at low throttle settings, the Prius can use the electric motors.

Tom

 

As Bob & Ray pointed out, that's a common misconception. In fact people are falling off all the time.

 

At high altitude, only the peak power of the engine is reduced. People driving for fuel economy don't use peak power (except for the pulse & glide fanatics). They are using partial power, which even at sea level is achieved by using the throttle plate to thin the air. The engine controller thins the fuel to match.

 

Atkinson engines keep the intake valve open during the first portion of the compression stroke, thereby taking in a smaller volume of air/vacuum. This reduces pumping loss. It also allows for less manifold vacuum, also helping reduce pumping loss.

While Atkinson uses the same throttle system as Otto, it can use less of it.

 

This post is a little misleading. Obviously the maximum power output of an engine occurs at peak power, by definition. Any power output below that level occurs because of the throttle setting, unless the engine is lugging below its torque curve (we can ignore that special case for this discussion).

As for the post being slightly misleading, as atmospheric pressure drops, the power output of the engine for any given throttle setting also drops. Looking at it this way you can say that the power of the engine is reduced across its entire operating range.

On the other hand, if you want to look at it this way, at anything less than full power you can always step on the gas and open the throttle a bit more, getting back to the original power before the atmospheric pressure dropped. In this respect the previous post is not wrong, only a bit misleading.

Tom

 

This is what I was thinking of, but I was not (probably still am not) sure of how that changed things for the pumping loss. I thought part of it was simply easing up on the actual compression so there was less dieseling going on. This would cut the power stroke, right? But it also would relieve some of the stress on the engine, reduce the vacuum and increase efficiency, except at the high-end, pedal-to-the-metal range, wouldn't it?

Of course, I could just as likely have this all wrong. :juggle: