Im working on a high altitude turbo project where Im trying to maintain power to about 15k ft. To verify the turbo choice I ran several operating points through boost adviser. The engine is a 1.2L 4 valve per cylinder max power at 5500rpm. Running 94UL fuel. I used the values for air temperature based on altitude ending up at 23.7F at 15k.

The results from sea level to 15k are summarized below. I believe that the increase in boost pressure in the table is based on the boost gauge being correct at sea level and then reading higher and higher as the altitude increases and air pressure decreases. Fundamentally I believe that for the engine to produce a constant output the absolute manifold pressure would be constant and the turbo is perfectly capable of supporting that.

What I find completely irrational is the mass flow rate of the engine increasing by nearly 80% from sea level to 15k ft. This cant possibly be correct. The engine would not stay together if the mass flow increased by that amount. It would have a nearly 18:1 compression ratio. I have the impression that they are calculating the mass flow using the boost gauge pressure and not the MAP and this is what is creating the erroneous mass flow value. If this is the case its a serious flaw in their predictive tool.

Does anyone agree that the MAP should be constant and the mass flow should also be constant in order for the engine HP to be constant at the different altitudes ?

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Folks, apologize, the mass flow rate should in all cases be in lb/min not lb/hr. Im a metric guy, mass flow rate is almost always expressed in kg/hr to the /hr denominator is stuck in my mind.

 

Engines are volume pumps, so are turbos, they'll breathe the same volume of air whether they're at sea level or 40k feet. What changes is the mass (weight) of air. As you go up, you have to increase boost in order to increase the volume of air to keep the mass flow the same.

Atmospheric pressure at 15k is 8.3psi, which is 56% of sea level pressure. So in order to get the same mass flow at sea level, means the turbo needs to shove 56% more air in.

 

If you take the 10.36lb/min of air the turbo will be sucking in at sea level, and divide it by the 18.77lbs/min at 15,000. You get .55 or 55%, same as the difference in atmospheric pressure from sea level to 15,000.

If you do the same to the pressure ratio, you get the same 55%

 

if measuring in volume(cfm)that is correct, not for mass. it almost seems as if the calculator uses volume but labels it as mass.

 

It's calculating the needed extra volume of air at altitude, and correcting it to lbs/min, because all your compressor maps are corrected.

Whatever turbo you pick out needs to be capable of 18lbs/min @ 2.5pr in order to make your 118hp at 15,000'

 

The engine will not accept a mass flow of 18lb/min. It would detonate. The Garret tool has a software bug that has been introduced at some point.

 

It's not.
At altitude it'll be moving 10.36lbs/min of air.
10.36lbs/min = 136cfm at sea level.
To get the same 10.36lbs/min at 15,000' the compressor has to suck in 247cfm (which is equal to 18lbs @ sea level)

Matchbot verified those numbers as well.

 

A pound is a pound brother. We are not talking CFM.

 

A pound of air at 15,000' takes up more space than a pound at sea level.

 

So ? When they run the turbo to make that map, I can assure you the MAF on the inlet is reading 20lb/min when it says 20lb/min on the X axis. Anything else would be a complete and utter farce. When you correct for a change in air density, it does not imply that the meaning of 10lb/min has changed.

 

Seems like a discussion of air density. Air is less dense with added altitude. No matter the density of air, a compressed pound of air is the same amount of that measurement. If you add altitude you will need to remove fuel, if you remove altitude you will need to add more fuel. This is because of the simple factor of desired combustion outcome type and the fact that you need the correct af ratio. If you live in the mountains your desired tune is different from your desired tune if you live at sea level.

 

Nope, it could be 19.7lbs/min, or 20.15 ect, that's why they correct the map.
Turbos don't move mass, they move volume.

It's also pretty clear you didn't come here for an answer, you came to argue because you don't like to be wrong.
2 seperate programs give the same result, it's not a bug in the software, it's a bug with the user.
Good luck on your project.

 

so in essence turbo maps are made at sea level. to altitude compensate they have to use a "fudge factor" of cfm for inlet density.

 

The maps are made wherever, but they're corrected to standard day temp and pressure, so that anybody, anywhere, can use them.

Maps used to be scaled in cfm, this is why they switched to lbs/min. Engines and turbos move the same cfm of air no matter what they're altitude is, the density changes so then the power also changes. They needed a way to correct for lost density and so switched to lbs/min.

At 15,000' the op's turbo won't be sucking in 18lbs/min of air, rather it'll be 10lbs/min in weight of air, but that takes up the space as if it was 18lbs/min.

 

Turbos don't push mass, they push volume. BBI is trying to explain to you how the manufacturers are compensating for that.

It is industry standard. That is why Garret and Borg software are giving you the same outputs. Learn to read the maps and work the compensation math.

 

@Westcliffe01, is this turbo application going into some type of aircraft? That you mentioned maintaining the same power through altitude, and refence 94UL fuel is leading me in that direction. If so, then I assume you have manual control of the wastegate, is that correct?

 

Its converting cfm to lb/hr at sea level.

In other words, convert 18lb/hr back to CFM and that is the wheel speed and pressure island necessary at altitude to match the original flow rate for the original power.

In other other words, the power column (118hp column) is showing you the actual mass flow rate in lb/hr. The lb/hr column is showing you the (converted from cfm) wheel speed requirement to meet the actual mass flow rate (power column) at the new altitude. Just like the boost pressure column is doing.

 

What's interesting about shaft speed, is that it changes with temp. It's COLD at 15,000', 5°F/258K cold.

This changes the shaft speed of the turbo, if we divide standard absolute temp/ ambient absolute temp, square root it, and divide it by shaft speed on the map, you get corrected shaft speed.

So 288k/258k= 1.116
Sq/rt 1.116= 1.05
Pick whatever turbo you want, check out the rpms, now divide that by the 1.05 and you have corrected shaft speed.

 

Well. First, how do we know the actual temperature of the turbo. Isn't it being warmed by 200*F Oil and 1500*F exhaust? Will this not contribute. And what about the air inlet- it could pass by exhaust manifolds like in some cars or unique with insulation. There might be some vaporization issue with the fuel that requires heating. Furthermore the act of compressing air will heat the air and heat the compressor wheel as well. There are many ways for heat to get in there and no way to know what steady state would be like.

Next, your difference appears to be roughly 5%. Which, if sizing a turbo properly, becomes negligible since we always assume some reduced flow for aging, debris, wear, filter pressure drop, etc... I can take or leave 5 or even 10% off the end of a turbo when the bulk of the math is done with the right kind of safety factor.

Finally what exactly about cold temp influences the shaft speed? Where is this correction coming from? 5% because of friction when oil is too cold to warm the bearing? 5% because the blades shrink slightly and require a bit more wheel speed to... I don't see it yet