Higher Ambient Temperature = More Boost is safe and good? (A scientific approach...)

[hsk8te2006]

Hi Everyone,

I have a theory that I believe is scientifically sound based on chemistry and thermodynamics, but I'd like the DSM experts with experience to tell me what they think...

The ideal gas law that approximates the relationship of air temprature, volume, pressure, and amount (number of molecules) is:

PV = nRT

where, P = pressure, V = volume, n = the amount of air (the # of moles, or # molecules x avagadro's #), R is the ideal gas constant, and T = Temperature.

The volume of my engine is a constant. (I know the EV can be altered by RPM, but that won't affect this doscussion).

I can use a MBC to control the amount of pressure in the intake manifold, and therefore the pressure of the air that enters the combustion chambers. Thus, intake pressure under full throttle is a constant with a MBC. (Ignoring boost creep or spike and shitty stock turbo high RPM performance for now, as they also dont affect this discussion).

However, when ambient air temperature goes up, the temeprature of the air that enters the combustion chamber will go up as well.

Since Pressure and Volume are constants in a properly MBC'd engine, then if the Temperatrue goes up, the only way for the ideal gas law to be satisfied is if fewer molecules of air enter the engine.

That being said, unless I'm wrong, the limits of any fuel system (such as my low out put stock fuel system) is based on balancing the number of molecules of fuel with the number of molecules of air, at an appropriate AFR for safety and performance.

If I am correct so far, then the fuel system being able to deliver a set max number of molecules of fuel to the combustion chambers, should mean that attaining the stoichiometrically equivalent maximum number of air molecules would be the desired result of the maximum boost of the turbo/intake system.

Therefore, I think that in order to match up the appropriate number of air molecules to match the maximum fuel system capacity (also a number of molecules), then at a higher ambient air temeprature, a slightly higher boost level (pressure level) should compensate for the lower number of molecules present in the engine's volume due to the higher temperature.

Thus, my theory is that at a higher ambient air temperature, a slightly higher boost level may be safe and beneficial for power.

Expert opinions?

Thanks,
Jonathan

BTW... using the ideal gas law for calculations for my theory above, it means that if an appropriate boost level was 15.5psi at 90 Degrees F, then at 0 degrees F it would be equivalent to run about 10.7 PSI.

OR, if running 15 psi at 70 Degrees F, then at 30 degrees F it would be equivalent to run about 12.8 PSI.

Similarly, 25psi at 90 Degrees F would = 18.7 psi at 0 Degrees F

and

25 psi at 70 Degrees F would = 22.1 psi at 30 degrees F

 

[jaday]

The goal of a turbocharged engine is to maximize "n". Therefore, in order to raise n as high as possible, you must attempt to lower T as much as possible and increase P.

If we hold V and R constant, for the sake of this balance, lets just examine this:
P = nT

What you are showing is that in order to compensate for an increased temperature and keep the amount of air constant, you must then increase the pressure. It's not that it will increase power and safety of your engine, it will just allow you to keep the same amount of airflow. Just remember that temperature and # of moles are always inversely related...

If you decrease the temperature, keep the same pressure, n will rise, and you can cram more air into your engine!

 

[TunaTalon]

Noooooooooooo!

That little piece of the ideal gas law you quoted does not account for the heat of compression. In the real world of the DSM the intercooler is still important.

Your underlying assumption that maximum boost is a function of cylinder pressure is flawed. Wrong it is.

Detonation limits max boost.
Detonation is more a function of charge temperature than charge pressure.

At low ambient temperatures the charge temperature goes down allowing HIGHER boost pressure without detonation.

Please see the “Boost is Good” section of http://www.kidzuku.com/StrokeOrNot.pdf and maybe even read about Adiabatic and Isothermal processes before turning up the boost on your 4G63 on a hot day.

 

[hsk8te2006]

I follow your statement that decreased temprature and equal pressure = more air and totally agree. However, as much as we want to keep the temperature down (for timing reasons if nothing else), regardless my intake charge will be hotter in the middle of summer than in the dead of winer (especialy in northern Vermont... high temps in the negatives are common in January).

Also, I diasgree that the goal of a turbo is to cram as much air as possible into the engine. Instead, I believe the goal of a properly tuned turbo setup will be to cram the appropriate amount of air into your engine to match the max amount that your fuel system and exhaust system can handle.

So, in order to try and keep cramming the exact same appropriate amount of air into the engine, then changing the Pressure (boost level) to compensate for the weather forced changes in temperature makes sense to me.

 

[silver bullit]

Knock and detonation would own you, its not possible, intake temps go up and the chance of you throwing your motor to pieces goes through the roof. Look at all the racers- and race teams they are trying to get the temps down not up up makes less power.

 

[hsk8te2006]

Tunatalon... I never stated that maximum boost was a function of cylinder pressure. I stated that maximum boost was a function of amount of air to match what the fuel system can handle.

However, I appreicate the thought that turning up the boost will increase the charge temp (especially if I heat saturate my stock SMIC), so I need to be careful in summer not to boost to hight by turning it up any.

However, I was more interested to learn if I need to dial my boost down in extremely cold winter mornings since the charge amount of air (at the same boost level I run in summer) will be higher.

 

[TunaTalon]

I follow your statement that decreased temprature and equal pressure = more air and totally agree. However, as much as we want to keep the temperature down (for timing reasons if nothing else), regardless my intake charge will be hotter in the middle of summer than in the dead of winer (especialy in northern Vermont... high temps in the negatives are common in January).

Also, I diasgree that the goal of a turbo is to cram as much air as possible into the engine. Instead, I believe the goal of a properly tuned turbo setup will be to cram the appropriate amount of air into your engine to match the max amount that your fuel system and exhaust system can handle.

So, in order to try and keep cramming the exact same appropriate amount of air into the engine, then changing the Pressure (boost level) to compensate for the weather forced changes in temperature makes sense to me.

No one is proposing that a turbo should cram as much air as possible into the engine.

One more time. Your underlying assumption about the “appropriate amount of air” is flawed.

Please try to think about the “appropriate peak pressure/temperature” at ignition.
With a colder intake charge from either a more efficient intercooler or lower ambient temperature the appropriate pressure will be higher for the appropriate temperature.

If the math doesn’t work for you then use the seat of your pants. The 4G63 in my Talon loves the cold winter air.

 

[DontStartModdin]

With methanol maybe.

 

[rrussell]

However, I was more interested to learn if I need to dial my boost down in extremely cold winter mornings since the charge amount of air (at the same boost level I run in summer) will be higher.

I would say yes you need to dial your boost down for the winter if your fuel system is max out during the warmer climates at the same boost levels.

The maf Temp sensor will see that temperature is colder and will increase the fuel
(pulse width) based on that. But you won't have enough injector to accommodate.

On the other side of it, when its hotter in the summer climate the maf will see warmer temperature and decrease the pulse width. You could increase your boost but now your limited to x amount of intake temperature before you start to go into detonation.

 

[kenamond]

I agree with what others have said already. You are forgetting about knock. If your charge temps are higher, your knock propensity goes up, and you lose timing advance and torque and power with it. And because knock has to exist in order for the ECU to be able to retard timing (how else would it know it has to retard timing?), you beat up the motor with knock in order to be able to retard timing in closed-loop mode.

The goal is to maximize power. Torque is energy per radian of engine revolution. Power is energy rate. So in order to maximize power at a given rpm you want the integral of torque over all four strokes of the cylinders to be maximized...but without damaging the engine in the process. That couples AFR, timing advance, and load (g/rev) into the equation. As you've mentioned, load and AFR are functions of boost level, intercooling, and fuel maps. Nevertheless, the solution to this "equation" is "a good tune" that also includes timing advance and engine survival.

 

[hsk8te2006]

No one is proposing that a turbo should cram as much air as possible into the engine.

Tuna... Check out the first line of the first post by jaday... thats' what I was responding to when you quoted me.

Ok, so rrussell is saying that yes it needs to be turned down in the winter, or my injecto's won't be able to match the pulse width change from my MAFT sensor.

I guess the real answer here is to log/tune the car both during hot and cold weather, to know how it will react best in each condition, right?

My problem... I have no ability to tune right now other than to adjust my boost level. So, what do I need to look for to tell if I need to turn my boost dwon while it's still colder weather, and conversely what's the indication it's boosting too high in the warmer summer weather? (All of this with no wideband yet...)

Thanks,

 

[MrBoxx]

Look for knock to tell whether to turn the boost up or down. Knock, a large number of degrees being pulled due to knock, or high IAT temps will tell you that there's too much hot air going in. Turning boost down lowers the heat, preventing knock. A good logger should tell you at least 2 out of 3 of those symptoms.

And yes, turbo DSM's LOOOOOVE winters in the northern states.

 

[Mikael SS]

Therefore, I think that in order to match up the appropriate number of air molecules to match the maximum fuel system capacity (also a number of molecules), then at a higher ambient air temeprature, a slightly higher boost level (pressure level) should compensate for the lower number of molecules present in the engine's volume due to the higher temperature.

Thus, my theory is that at a higher ambient air temperature, a slightly higher boost level may be safe and beneficial for power.



No gym for home?

Those last two paragraphs make no sens and are wrong. Combustion engines are already figured out, don't try and re write the book.

 

[kenamond]

Therefore, I think that in order to match up the appropriate number of air molecules to match the maximum fuel system capacity (also a number of molecules), then at a higher ambient air temeprature, a slightly higher boost level (pressure level) should compensate for the lower number of molecules present in the engine's volume due to the higher temperature.

As long as you don't knock as a result, sure.

Thus, my theory is that at a higher ambient air temperature, a slightly higher boost level may be safe and beneficial for power.

As long as you don't knock as a result, sure.

No gym for home?

Those last two paragraphs make no sens and are wrong. Combustion engines are already figured out, don't try and re write the book.

I have absolutely no clue what you're talking about or referencing here. But I can be clueless sometimes.