Driving at higher altitudes

[larrydk]

First road trip to the Rockies with a tdi.....
I assume the on board computer adjusts for atmospheric pressure when dosing fuel mixture.....since I'm a low lander from the Midwest, any driving tips from high altitude dwellers?

I can't tell, but it does seem I have to rev the engine a bit higher to get moving, also my stick driving skills needed to be reminded on drinks in very hilly areas

 

[oilhammer]

Your car has this little device called a "turbocharger". It essentially "makes" its own atmosphere. You really will not notice much difference because of this. It is just that the turbo will have to work a little harder because of the less dense air it has to work with in the first place.

The engine management doesn't really care what atmospheric pressure is, only what the intake manifold pressure is. And it is using that value (from the MAP sensor) to work the VNT controls (to work the turbo) to maintain the boost pressure as required.

 

[larrydk]

Turbo charger

Thanks for the reply....I,m definitely not an "engine" guy, nor a tech....but at altitude isn't there a reduced concentration of O2 which would affect fuel mixture? Even if the turbocharger equalizes the reduced pressure issue, the O2 concentration is less so if fuel/air mixture isn't changed the mixture would be too rich??

Let me have it, like I said, not a mechanic

 

[ZippyNH]

Thanks for the reply....I,m definitely not an "engine" guy, nor a tech....but at altitude isn't there a reduced concentration of O2 which would affect fuel mixture? Even if the turbocharger equalizes the reduced pressure issue, the O2 concentration is less so if fuel/air mixture isn't changed the mixture would be too rich??
Let me have it, like I said, not a mechanic

Nope
The % of oxygen is the same...all the way up...just lower pressure...that's why an airplane can just pressurize the cabin, and does NOT NEED TO CARRY AN O2 TANK.
What effects people is "lower partial pressure" but that is for a biology or a flying course.
Drive more, think less!!
That and drink lots of water, and realize that the first beer will feel MUCH stronger....when I lived out in Colorado it was fun to have friends visit.

 

[oilhammer]

Yep, oxygen ratio will always be the same. Just the atmospheric pressure you are starting with. And the air/fuel ratio thing does not technically apply to a diesel in the same way you may be thinking it does with a gas engine. In a gas engine, the engine management (yes, even a carburetor is a crude form of this ) is always trying to balance the engine out to run at or close to a stoich level of air/fuel ratio. Whether it is at idle, light load, or full load.

A diesel engine always runs on the lean side of stoich, but varies widely throughout its load/speed ranges. So a ratio of let's say 50:1 air/fuel, a gas engine would lean misfire and stall long before it could even get there. It may only be able to get halfway there under most circumstances and even that would require some pretty smart engine management to do so, and would really be hard to keep emissions in check. A diesel would have no trouble running that lean, or even leaner. Because they vary the fuel more than they do the air. At least in the most simplistic ways of explanations. Of course, once to consider EGR, turbocharging, and throttle flaps, it gets a little complicated. But the basic idea is the same. Diesels are a variable ratio engine.

 

[KERMA]

The engine management doesn't really care what atmospheric pressure is, only what the intake manifold pressure is.

Not true. Every TDI ECU has a barometric pressure sensor built in. This effects almost every aspect of the ecu's engine control strategy. For example, boost and therefore power is derated at altitude to avoid overspeeding the turbo. Timing, glow plugs, starting quantity, N75 duty cycle, boost set point, fuel pressure, name almost any operating parameter and it is affected and modified in some way according to input from the baro sensor.

The good news here is that you don't need a special "high altitude tune" because the compensation is automatic and baked into the software from the factory. Likewise, there is no special "high altitude driving method" required and no special concerns when driving at high altitude. Power will be less but that's about it.

 

[VeeDubTDI]

Expect engine power to be reduced a bit at high altitude due to the decrease in atmospheric pressure. The ECU will main boost pressure above ambient, but the ambient will be lower than at sea level, for an overall power loss (although less than in a naturally aspirated vehicle). You may notice slightly more turbo lag, as well.

For example, manifold absolute pressure on our CKRA Passat with its stock engine tune is about 23 psi at full throttle at 2000 RPM at sea level, but only 20 psi or just under in the Denver area (corrected for sea level atmospheric pressure) with a noticeable and corresponding loss of power.

 

[nayr]

My TDI Loves the Rockies, it'll pull a trailer up to the tunnels @ 65mph with ease and pass priuses, and suv's that are beating the **** out of there mule to maintain 50mph.

The only problem I've ever had was the hill-hold feature in line to see the Hill Climb, 1 car at a time stop and go traffic up the side of Pikes Peak caused this feature to smoke my clutch out after a few miles of torture.. its been disabled and now I can get stuck in traffic up a mountain without worry, just alot of cursing.. if someone pulls up right on my ass I dont need the hill hold, I have a trailer hitch I can rest on their front bumper

Flat landers never learn to engine brake, when going down a mountain use a lower gear and dont ride your brakes all the way to the bottom.. a small adjustment in speed before entering a corner is all the brakes you need.. Its obvious your not from around here when your brake lights are on for 20mins straight.. for example coming down Pikes Peak there is a ranger half way down that stops everyone and uses a laser thermometer to read brake temps, he makes everyone with out of state plates pull over for a cool down; then us locals get the rest of the road to our selves Its awesome when 25 minivans all operated by terrified dads have to move out of your way

Always come with good brakes and tires, we have an 'unsafe mountain driving' violation they can slap you with if you impede traffic in any way thanks to a poorly maintained vehicle.

 

[abaron]

Flat landers never learn to engine brake, when going down a mountain use a lower gear and dont ride your brakes all the way to the bottom.. a small adjustment in speed before entering a corner is all the brakes you need.. Its obvious your not from around here when your brake lights are on for 20mins straight.. for example coming down Pikes Peak there is a ranger half way down that stops everyone and uses a laser thermometer to read brake temps, he makes everyone with out of state plates pull over for a cool down; then us locals get the rest of the road to our selves Its awesome when 25 minivans full of terrified dads have to move out of your way

HA. I've never heard of police doing something like that but it makes sense. This is, of course, one reason I like having a manual; engine braking is much more effective.

I drove this road a couple of years ago in my Corolla, was a very interesting experience. Oh, should I mention it's mostly about a lane and a half wide, switch backs and gravel? Grade Warnings

More info here: https://en.wikipedia.org/wiki/British_Columbia_Highway_20

 

[oilhammer]

Not true. Every TDI ECU has a barometric pressure sensor built in. This effects almost every aspect of the ecu's engine control strategy. For example, boost and therefore power is derated at altitude to avoid overspeeding the turbo. Timing, glow plugs, starting quantity, N75 duty cycle, boost set point, fuel pressure, name almost any operating parameter and it is affected and modified in some way according to input from the baro sensor.
The good news here is that you don't need a special "high altitude tune" because the compensation is automatic and baked into the software from the factory. Likewise, there is no special "high altitude driving method" required and no special concerns when driving at high altitude. Power will be less but that's about it.

Interesting, thanks for the info. I always thought Bosch just took a snapshot of the MAP value at that split second before startup and based their maps off of that, and that once running, the ECU was controlling the boost so it didn't really matter much what the atmosphere going into the turbo was, just that is was sufficient coming out. I learned something today! Well, two things... the other is that Honda made a bunch of Civic engines that have blocks that crack and leak coolant all over. Oops.

 

[larrydk]

Here's another altitude quit on to ponder.....
So with bike rack off and unloaded wagen, fuel milage seems to be better than I've ever experienced. At about 70% hwy driving in the keystone, vail, Leadville areas I averaged 52+ mpg?!

Does the lowered air resistance at altitudes have to do with this? Perhaps the long long descents?

I find this very interesting, in the past with previous gasoline vehicles in the area, performance was just diminished.

 

[nayr]

Taking the bike rack off is a HUGE help..

its the long downhill decent that screw with you, my wife sent me this photo when she went to go get coffee one morning while camping, basically rolled it into town.

your also going alot slower, the highways are not 80mph up there.. your average speed limits are 55-65mph and thats basically hypermiling.

 

[ZippyNH]

Naww....I always got better than EPA mpg in Colorado...hills and all, even in a gas car...
Bet it is a combination of things...roof rack is the biggest item...good for maybe 2+ mpg...
Engines do make LESS POWER at elevation (even a turbo will make less power at a "critical" altitude).... so you are using less fuel....and less wind resistance is a factor...in a plane we typically try to get to 8,000-10,000 asap...thinner air is the reason in normally aspirated pistol plane, about 16,000-18,000 in most planes with a turbo, and 22,000-28,000 for most turboprops.....jets even higher, comes down to the the combination of thinner air for efficiency and the engines outputs...go too high, and you start loosing engine hp...there are other factors at play, but efficiency is a HUGE player.

 

[nayr]

yeah not only is the air thinner there is almost never a headwind/tailwind unless your at the summit of a pass.. and thats very brief.

drafting on the highway also seems more effective, and rarely are you lucky enough to not be waiting behind a big camper or suv on the highways.. waiting for a passing lane.

lots of factors at play... anything with a Turbo will perform quite well up here.. just hotter because the thinner air + slower speeds makes cooling harder, then the steep inclines that make your engine work harder; ironically alot of engines get more efficient with the extra heat

 

[eav]

Did a 7000 mile road trip in a heavily loaded 2015 Sportwagen that crossed the Rockies many times, and never had the slightest problem on any hill, in fact this was the best car I've ever had for hills. I easily passed other cars going uphill at elevations of 7000 feet or more. Even my wife loved driving it and she is a bigger horsepower freak than I am.

 

[VeeDubTDI]

Did a 7000 mile road trip in a heavily loaded 2015 Sportwagen that crossed the Rockies many times, and never had the slightest problem on any hill, in fact this was the best car I've ever had for hills. I easily passed other cars going uphill at elevations of 7000 feet or more. Even my wife loved driving it and she is a bigger horsepower freak than I am.

Few vehicles are more fun than a TDI on a mountain interstate.