Absolute vs Gauge Boost

[DanaT]

I believe I know the answer to this question.

I think the boost control (stock) on the 2G GSX controls boost to gauge pressure and not to absolute pressure. What does this mean.

Normal sea level pressure is 1000mbar (or about 14.7 psi). At sea level (standard conditions) absolute pressure is 1000mbar but gauge pressure is 0 mbar. Now add 14.7 pounds of boost (1000mbar) and the absilute pressure is 2000mbar and gauge pressure is 1000mbar.

Now move up in altitude (lets say 5000 feet). The absolute pressure is about 800mbar. add 1 bar gauge of boost onto that and you have 1800mbar of boost absolute but 1000 mbar gauge. But in absolute boost the boost is down about 20%. This means that about 3psi more boost is needed at 5000ft than at sea level.

I know MBC run off of gauge boost, but an electronic system can run abolsute (meaning it would add the 3psi back automatically) but must compare atmosphere pressure and manifold pressure for a differntial.

So, how is the boost controlled on DSM?

-Dana

 

[jrohner]

The stock ecu has no idea how much boost you are actually running, so it can't target a specific boost pressure. The boost is only lowered with the BCS for knock (crappy gas).

 

[DanaT]

No. 800mbar atmosphere pressure plus 1bar boost (1000mbar) is 1800mbar.

The big difference is that "0" on a typical gauge is referenced to atmosphere instead of absolute (there are absolute gauges, but they are spendy and normally used on industrial applications).

On my other car, with a MBC, I run about 3psi more to try and compensate for the loss of atmospheric pressure.

Just curious how the stock boost selenoid works.

-Dana

 

[delta448]

No. 800mbar atmosphere pressure plus 1bar boost (1000mbar) is 1800mbar.

Okay so I looked it up, and you're technically right. Absolute pressure is defined at absolute zero for engineering/scientific purposes. One bar equals one atmosphere, well actually it's 0.9869 atm if you want to get technical, but I didn't realize "absolute" meant something other than atmospheric plus gauge. Absolute pressure does not rely on the measure of relative atmospheric pressure.

First post deleted.

 

[Defiant]

Automotive stuff uses PSIG, which calls atmospheric pressure around sea level "zero". Airplanes use absolute pressure to work the altimeter, which you correct to barometric pressure before takeoff.
However, it's pretty much a moot point. The whole car is using the same methods, and is built to automatically compensate for altitude and other pressure differences. It's kind of like lap times when it rains~ everybody's running on the same track.
At altitude, a turbocharger has less air to push- however, it also has less air to fight. It self-compensates for the thinner air by being able to spin faster in the lower drag.
DSMs control boost by feeding back to the wastegate. More boost, more wastegate opening, less energy through the exhaust turbine, less boost. It doesn't give a crap about how the air pressure's measured.

 

[TunaTalon]

As Defiant explained the Boost controls and boost gauge use Pounds per Square Inch Gauge (PSIG) which is pressure measured relative to the ambient air. However, once inside the combustion chamber, compressed by the turbo, heated by the compression process, cooled by the intercooler, and squeezed down again by the piston with more heat of compression, the air has no idea about the ambient air pressure.

Measuring the intake air directly in mass flow units closes the loop around ambient pressure and temperature and simplifies the calculation of the fuel needed for the desired F/A ratio.

Measuring boost in PSI seems intuitively obvious and is handy as a rule of thumb but not very useful in calculating engine operation.

 

[1gentalonfan]

this is a little off topic but i get what your saying with even though theres less atmospheric pressure the air is thinner and its still able to suck in as much, but when you use a compound turbo set up and you pump say 10 psi out of the first turbo into the second and the second is effectively set to 10 psi then its actually putting out 20. wouldnt this be the same as having more or less atmospheric pressure?

 

[DanaT]

I dont think I am explaining so that people understand what I am asking.

Lets say I decide to run 15psi boost (close enough to 1 standard atmosphere for arguements sake) at sea level. The wastegate has a spring to keep help keep it closed plus some pressure against the valve in the wastegate. On the other side it should have a pressure signal to force it open at a certain boost level (say 15psi). It is simply a force balance equation where the air pressure opening the wastegate is greater than the spring force and pressure on the valve. So in effect I have 30psi absolute pressure. Basically what this means is that in a perfect world (lets assume no heating here because I dont know what the temp rise and intercooler efficiency is to cool) I have doubled the mass of air in the system and now have 2 moles of air instead instead of 1 mole of air that would go into the engine under non-boosted conditions.

Now I take the car to 5000ft. If the system works on gauge presure, the wastegate still opens with 15psi pressure *differentail*. I now have about 27psi absolute boost (12 +15). I now have 1.8 moles of in the system instead of the 2 moles at sea level.

With a "fixed" amount of air (which means a fixed amount of O2) I can only burn so much fuel. This means the mass system will cut back fuel because the mass of the air is measured less and less power is made.

With an absolute system, the car would boost to 30psi and then 2 moles would be present once again.

Now, again, this is simplified because as a turbo compresses the thin air more, it runs at a different pressure ration and therefore a different efficiency, compressing the air more generates more heat which decreases the quanitity of air (moles) for a given pressure/volume, etc.

What I am interested in knowing is does the engine management system have a amtospheric pressure measuring sensor to adjust bases upon altitude or only a manifold style pressure sensor (or none?).

-Dana

 

[delta448]

The ECU uses MAF, IAT and Baro sensors to look up a pre-defined table to calculate mass airflow.

You will notice a performance decrease in much higher elevations. The turbo and your intercooler will both be less efficient the lower that atmospheric pressure becomes. The ECU will compensate for A/F ratio only, it does not control boost -unless as mentioned you are still using the BCS, in which case it cuts boost pressure when knock occurs. Knock will occur more easily with an overspinning turbo and inefficient intercooler.

Your wastegate will still open when it sees the amount of pressure that overcomes it's spring. A spring does not care what the atmospheric pressure is, -unless for some reason the outside case is wimpy enough to expand under lower prerssure, causing less pressure on the spring. Your wastegate spring will still collapse at the same absolute pressure at sea level or at 5000ft.

 

[TunaTalon]

I dont think I am explaining so that people understand what I am asking.

Communications is a wonderful illusion.

Lets say I decide to run 15psi boost (close enough to 1 standard atmosphere for arguements sake) at sea level. The wastegate has a spring to keep help keep it closed plus some pressure against the valve in the wastegate. On the other side it should have a pressure signal to force it open at a certain boost level (say 15psi). It is simply a force balance equation where the air pressure opening the wastegate is greater than the spring force and pressure on the valve. So in effect I have 30psi absolute pressure.

You have this right. The wastegate operates on gauge pressure. In your example it will open when the pressure on the diaphragm is 15 PSI higher than the ambient air pressure.

Basically what this means is that in a perfect world (lets assume no heating here because I dont know what the temp rise and intercooler efficiency is to cool) I have doubled the mass of air in the system and now have 2 moles of air instead instead of 1 mole of air that would go into the engine under non-boosted conditions.

Well ok for now but tuning for pressure ratios and ignoring the heat of compression will be hard on the top of your pistons. But for this discussion yes, doubling the absolute pressure without changing the temperature will double the mass of the intake air.

Now I take the car to 5000ft. If the system works on gauge presure, the wastegate still opens with 15psi pressure *differentail*. I now have about 27psi absolute boost (12 +15). I now have 1.8 moles of in the system instead of the 2 moles at sea level.

Without comment on the actual values, you have the direction right. If the temperature at 5000 feet is the same as before at sea level the engine will make considerably less power.

With a "fixed" amount of air (which means a fixed amount of O2) I can only burn so much fuel. This means the mass system will cut back fuel because the mass of the air is measured less and less power is made.

Right again still assuming that the temperatures are the same. At higher altitudes the temperature is usually lower meaning more moles of oxygen in a cubic foot of air at standard pressure. But you’re right that the pressure change is the major effect.

With an absolute system, the car would boost to 30psi and then 2 moles would be present once again.

There is another unspoken assumption here that the turbo has enough extra flow potential to reach the higher boost required to reach the same PSIA level. Remember that the turbo also works on differential pressure, not absolute pressure. This is probably a good assumption for only 5000 feet. The point where a turbo is no longer capable of boosting enough to reach sea level mass flow is called the critical altitude in the aviation world. My Talon seemed to reach critical altitude at about 6000 feet with the original T25.

Now, again, this is simplified because as a turbo compresses the thin air more, it runs at a different pressure ration and therefore a different efficiency, compressing the air more generates more heat which decreases the quanitity of air (moles) for a given pressure/volume, etc.

Ah, I should have read this paragraph before answering the one above. Yes the turbo works on differential pressure and compressor maps are in differential pressure.

What I am interested in knowing is does the engine management system have a amtospheric pressure measuring sensor to adjust bases upon altitude or only a manifold style pressure sensor (or none?).

No. The engine management system uses mass flow to calculate fuel requirements and that compensates for differences in absolute pressure and temperature.

You can indeed turn up your boost at higher altitudes but be careful. The oil companies know about this issue and don’t offer 93 octane at higher altitudes. The lower PSIA in the compressed charge gets along fine with the 91 octane. If you do turn the boost up at altitude be sure to set it back down before getting down to the flat lands.