Hello,

We are in our 2nd day of vacation. I have not driven much at high altitudes (5000-9000 ft.). It seems like I have heard the boost guage will not show as much boost the higher above sea level you go. Is that true? It seems like it's not running as high as it normally does around home. Power seems normal, not having any trouble pulling the hills, just wondering if I need to be looking for a leak somewhere.

RD

 

The waste gate opens at a certain pressure. Doesn't matter what the altitude is, the waste gate operates the same.

And that's the real advantage of a turbo over a belt driven blower. The turbo will not lose power with altitude, where the blower does.

 

bob's correct to a point. However before the WG opens you will see slightly lower boost per a given load @ high altitudes. Say if @ sea level you normally see 18# running up a given hill, then you may only see 16+/- A 9K due to atm. density loss. But once you get up to a point where the WG starts to work it will be the same.

 

Thanks guys. I looked around on the truck last evening looking for obvious signs of a leak and everything looked good and tight. I guess my next question would be does the computer open the wastegate at different points based on EGT, coolant temperature or other engine parameters that might be considerably different at altitude than what I'm used to seeing?

RD

 

Just got back from camping yesterday, and I noticed that I was only getting about 22# of boost climbing the Eisenhower Tunnel here in Colorado. I had the camper on and pulling my Jeep on a trailer. The truck was pulling good, just not much boost. I took the camper off and I was not pulling the Jeep and this morning coming to work I could get 30# of boost easy.

 

I live at 7K ft of altitude, I have noticed that my boost pressure is a little lower than when I am at lower altitudes. I cannot tell you the specific differences because I spend most all my time at higher altitudes but when I have been down to say 1K ft I have more boost. The difference becomes greater when towing.

I figured it had to do with air density gains/losses, didn't really think twice about it because my truck is running the same as it did when it was new.


CD

 

Yeah, about 22# was what I was seeing. I'm not to awfully worried about it, like I said the power seemed fine. I just like to know why things are doing what they're doing in case I can catch something early.

Thanks again for the help.

RD

 

Uhh, no. Both will loose efficiency at higher altitudes due there being less air density. The turbo work soff of exhaust flow so if you can't suck it in you can't exhaust it. A blower is belt driven, you need more air just spin it faster.



WG opening is based purely on boost pressure, nothing else. Your boost is down due to less oxygen in the air and a lower air density at higher altitudes.

 

With both systems, if you need more air it just spins faster. The belt driven blower will spin at the same rate regardless of the altitude. It's based on crank shaft speed and nothing else. Simply put, boost is then a function of air density and engine/crank rpm. As you climb in altitude, the air density will decrease, and so will boost and power. The only way to change that is to change the pulley size.

X air density x Y compressor rpm = Z manifold pressure. When air density drops, there's no compensatory increase in rpm's. Then the equation becomes X-2 x Y compressor rpm = Z-2 Manifold pressure. You've lost manifold pressure, cylinder pressure, and power.

A turbo will almost make the same amount of boost regardless of altitude. The turbine spins and produces boost. But, the system always produces more boost than you actually need. That's where the waste gate comes in. When boost hits the pre determined level (say 20psi), the valve opens and any excess boost is vented.

When a turbo rises in altitude, the turbine spins the same amount for the exhaust flow. But the waste gate doesn't open as quickly. You'll still get the same boost, it just takes a little longer to build. If the size of the turbine is too small, the waste gate almost never opens. In that case, it won't be able to compensate for loss of air density.

WG opening is purely based on air pressure after the turbo compared to ambient air pressure. 30psi on the boost gauge really means Ambient pressure + boost pressure. Just like measuring tire pressure. It has nothing at all to do with oxygen levels. 30psi of boost carries the same number of oxygen molecules regardless of altitude; see Boyles Laws of Gasses.

 

The turbo doesn't make as much boost at 10k feet as it does at sea level because it cannot compensate for the lack of air density. The turbo relies on air density in a given range to provide enough boost to adequately feed the compression cycle. When boost drops the charge density drops as does oxygen content and the exhaust proportionately decreases because we no longer have adequate charge density and oxygen to provide the same expansion of exhaust gasses.

When you go up in altitude its normal to see more smoke, higher EGT's and less boost. The only way to compensate is a VGT or change a housing. With a blower its a simple pulley change to bring charge density back to whats needed. You can spin a blower faster becaus its mechanical drive. Can't spin a turbine faster because unless you have some way to vary the drive mechanism.

 

It'll make the same boost. But there's less O2 at higher altitude per given volume of air regardless of boost pressure, thus more smoke, higher EGT.

 

That explains why my truck smokes so much more when I am in the mountains.

 

So, I think I understand it this way and maybe this will help others: (I could still be wrong, but here goes)

It's the difference between "relative" vs. "absolute". At sea level there is proximity 14.7psi of pressure from our atmosphere. So a 'standard' in-cab boost gauge must be preset to zero at sea level (using 14.7psi as a starting point). As a reference point 18,000ft is 1/2 the atmosphere than what is present at sea level, 9,000ft is 1/4, 4,500ft is 1/8, ect.

So, it would be expected to see a boost gauge that has it's 0psi reference point set at sea level to show 1/4 less @ 9,000ft or 1/8 less at 4,500ft. Remember your gauge is now starting out below 0psi at these elevations. Your gauge might of read 20psi at sea level, but @ 9,000ft it will only read 16psi or @ 4,500 ft it would only read 17.8psi.

Your turbo is still compressing the same volume of air at any of these elevations I've given examples too, it's just your gauge is zero'ed out based on sea level.

 

Boost gauge is reading the difference between ambient and manifold pressure, thats all. There is no zeroing it as it is at zero when there is no difference between ambient and manifold.

The turbo is compressing the same volume of air at varying levels of density which gives the boost reference. Compressing 1 cubic foot of air x times at 10k feet gives less pressure than doing the same at sea level.

Volume is not as important as density which is where the amount of oxygen is contained to give the combustion the expansion rates to drive the turbine to compress the air. Less density equals less oxygen and less driving force.

 

Aircraft that are turbocharged have what is called a "critical altitude." Critical altitude is the height the plane has to be at to no longer be able to make sea level horsepower. The whole point of turbocharging a piston powered aircraft is to be able to make that same power up high. Some examples I can think of have critical altitudes above 20,000 feet.

Whether or not our truck turbos are designed with a critical altitude is the question for a Cummins engineer. A turbo certainly CAN be made to produce the same power at some design altitude as sea level. It can also be designed NOT to, maybe for some emissions requirement.

If the wastegate is opening on a "gauge pressure" comparing the boost to ambient outside air, there will be power loss with altitude gain. If the wastegate is opening on a specified absolute pressure (gauge pressure plus sea level atmospheric pressure) then the power will be the same until the critical altitude is reached, at which point you need a different turbo to go any higher to make the same power. I am sure operation anywhere near critical altitude would take longer for the turbo to spool up, but that is not much of a problem on aircraft where you don't change the throttle much during cruise operation compared to an automobile.

Since there are engine computers involved, you could actually have an engine designed to operate anywhere between both extremes. The question becomes what was the engineer designing to acheive? Efficiency, emissions, longevity, ect...