prius climbing to 9,000 feet questions

The amount of power you can get out of an engine is proportional (not exact, but close enough) to the mass of air you can move through it.

The ECU can only open the throttle all they way, so the amount of air that goes through the engine at full throttle is directly proportional to the air density.

This sound pedantic, but it isn't. The engine doesn't pull the air in, the atmospheric pressure pushes it in. If there is less atmospheric pressure, less air (in terms of mass) gets pushed into the engine. There is nothing you can do about that except add a supercharger.

 

What about a controlled Atkinson cycle?

1. Atmospheric pressure pushes the air-fuel charge into the cylinder
2. Because of the high expansion ratio, compressing this much charge at the full expansion ratio at sea level would overheat the charge and cause knocking (premature ignition). The Atkinson cycle solves this by delaying closure of the inlet valve so that the excess charge is driven back into the inlet manifold and the effective compression ratio is reduced enough to prevent knocking.
3. If the inlet valve timing is controlled, then at high altitude the inlet valve can be closed earlier to try to maintain the same mass intake as at sea level.

This would seem to mean no loss of power up to a fairly high altitude and a reduced loss above that.

Does the Prius have this kind of charge control? I know that I didn't notice any shortage of power at 8000 ft and without it I would have expected to.

 

I don't believe so. Conventional wisdom believes that its maximum compression ratio is in the mid to upper 8s, too low to need to hold back at sea level.

And if it did have this feature, it likely would have been rated for higher horsepower at sea level with high octane fuel.

 

yes it would put less fuel for the same amount of air to keep ratio

yes, less pumping losses is one of the reasons, the other ones are reduced power (all equal 100HP car will be more economic then 200HP version of it) and reduced aerodynamic drag. Last one is a main contributor.

Some anecdotal evidence.. when we were at Yellowstone on motorcycles my EFIed was getting ~64MPG, and the carbed 80s Honda was in upper 20s. When we got down to interstate I was getting ~51-53MPG and he was getting 45-47MPG.

 

That's a thought provoker. It sounds like it would work to some extent if the VVT is flexible enough but I don't think it is. I've noticed less power at 7,000 feet.

 

I think the dealer told me the odd squeak with depression of the brake pedal is a problem of a "stroke simulator valve". Does that sound like the same thing?

 

My Prius putters around 7000ft to 10000ft daily. Climbs to 11,000ft is fairly normal and it works just fine. I also drive it cross country and down to sea level often. It drives and performs the same.

I do run 85 octane at altitude. No problem maintaining 80mph up steep I-70 grades.

 

People used to drive 36 hp VWs to the top of Pikes Peak and Mt Evens all the time. Both over 14,000 feet.

 

Yes but you won't be going 80mph up Pikes Peak or up steep grades at speed with 36hp.

The Prius which is deemed slow and underpowered by those who don't know don't think it can make it. It can.

 

IMHO Toyota went to Atkinson cycle not to improve power at altitude (which potentially they could have) but to improve economy.

Would be interesting to see if Gen IV will have DFI and will go away with throttle just like the BMW or Mazda SkyActiv and use intake VVT instead, and if the range on VVT will be sufficient to cover 30% air density loss.

 

Been on 3 road trips in my 2010, including crossing the Rockies a couple of times. Heading east out of Laraimie, Wyoming, you go over the highest point on all of Interstate 80. We went up the hill with the cruise control set on 70 and never had a problem.

The battery never runs down in the mountains ... at least ours never has. You will need to learn to use the "B" shifter setting when goind DOWN big hills or long steep grades.

On a recent trip to San Diego, we went over a pass in Interstate 8 which had a sign saying 6% down grade for 13 miles. In "B", I never had to touch the brake pedal for the entire stretch and actually had to use the gas pedal to keep from slowing down TOO much.

You need to use "B" to save the brakes. It is actually a "jake brake" if you know what that is. It's what truckers use to slow down their big rigs by turning the motor into an air compressor.

You can go here John's Stuff - Toyota Prius and more for lot's of infor on the Prius.

 

Visited the Sierra Buttes last weekend. Started at 2,400 ft elevation and went to 6,500 ft. Air conditioner all the way. Got 46 MPG. Woo!!

 

I took the air travel route.

 

I can see the other side of the Buttes from my house!

 

I flew over your house! You live in a really cool area.

 

I have driven several round-trips on Highway 395 between San Bernardino, CA and Reno, NV in our Gen 3 Prius. This includes hundreds of miles of high elevation and several passes around 8,000 feet. The combination of ICE and electric motor torque made the Gen 3 Prius a dream when climbing to the summits. Early during each trip, I stopped worrying about depleting the battery during the climbs and power was sufficient to maintain 60 MPH. In spite of the substantial elevation change, I averaged 50.2 MPG indicated going north and 51.2 for the return. The only odd observation was the hum during long down-grades when the battery was fully charged. I have yet to drive a Prius over a 9,000 foot pass, but don't think it would be much different from my experience at 8,000+ feet.