great white
Well-Known Member
Why an Intercooler makes a difference
(Disclaimer; for the “Bill Nye science guy” types who loves to poke holes in everything- this is very simplified so go easy OK?)
We all know what an intercooler is and we all know it reduces temperature of the air charge. But why does it work? That’s and easy answer isn’t it?
It makes the air more dense.
Well, let’s talk about that a bit and see if we can’t gain a bit more than that rudimentary understanding….
First, some theory and definitions.
Definitions and resources;
PSIG = PSI gauge. This is gauge pressure displayed over 14.7 psi (1 atmosphere)
PSIA = PSI absolute. This is the pressure your engine actually sees. Boost plus 1 atmosphere if you prefer.
Pressure altitude (PA) = pressure altitude is the indicated altitude when an altimeter is set to a pressure setting. Standard is when set at 29.92 in/mg, and then adjusted for pressure of the day to give aircraft a representative value for altitude above sea level. Can be thought of as “altitude above sea level” once the correction factor for the day is added.
Density altitude (DA) = without getting too wordy, it’s how “thick” the air is. Temperature and humidity affect this value. Hotter is thinner, more humid is less dense.
Standard ICAO day = 29.92 in/mg at 15 Celsius.
Atmosphere is always 21% oxygen. That never changes. In a NA, supercharged or turbo engine, you never get more than 21% O2 in there. What you do with a turbo is shove more atmospheres in so you get more oxygen per volume, not %. Think about it a bit, it’ll start making sense.
Online DA Calculator - http://wahiduddin.net/calc/calc_da_rh.htm
Altitude chart - http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
Theory;
Engines don’t really care about pressure altitude, or what you see on the road sign; they only “see” density altitude. Whether you’re talking axial flow or centrifugal jet engines or reciprocating assemblies, an engine only cares about how much air it can pull (or in our case, blow) in during a set period of time.
(Well, all engines actually “blow” air into the cylinders, but that another discussion for another time)
That’s density.
In aircraft speak, it’s called density altitude and affects everything from lift to engine performance.
Now, density altitude is affected by a couple factors. Temperature is a big one. So is in/mg and humidity.
Let’s only look at temperature since that’s easier for us to get our heads around in an automotive application.
15 degrees Celsius and 29.92 in/mg is considered an ICAO standard day. For arguments sake, that’s what we’ll start with and 50% humidity.
Now, you’re rolling along by the ocean at 0 feet ASL and boosting your 10 psi because you’ve got your foot into it. It’s a pleasant day and the scenery is going by at a high rate of knots. Or, you’re rolling up the coast, sea breezes are blowing, sun’s out and have a hill to climb so your foot is “in to it”.
Life’s good.
Let’s say its 15 c outside the IAT at 10 psi is 140c. Humidity is hanging right around 50%. Not unrealistic numbers.
So, what’s the density altitude our engine is seeing?
A couple quick calculations later and you get 6124 feet.
Holy crap! My engine thinks it’s at 6124 feet but I’m staring at the ocean out my window!
So let’s move on to how this affects boost. (This is going to get a bit trickier to follow because we're jumping back and forth from density to pressure)
You’re making 10 psig. But what does the engine see in absolute?
Logic says to add 14.7 psi right? That’s 24.7 psia.
Hold on a second now. Your engine thinks it’s at 6000 feet remember?
Pressure at 6124 feet is around 11.3 psi. So you’re really boosting to 21.3 psia.
What the heck? I want my 3 lbs back!
So, let’s drop an intercooler into the equation taking the same variables except for temperature.
Let’s say, with a 15c (59F) outside air temp and 140f IAT we get a delta of 40F. A reduction in our IAT from 140f to 100f.
What does that do?
A quick whip through the Jeppson calculator says the engine now sees 2991 feet.
Wow!
A 40F IAT drop more than halved the density altitude.
What does this mean for our boost?
3000 feet shows 13.2 PSIA, so now our boost number is 23.2 psia.
Much closer to what can be had at ICAO and we grabbed 2 psia back from The Reaper.
Your gauge will still show 10 psi, but your engine sees a very different psia.
Add in altitude above sea level and things get even more wonky.
Lets look at 3000 ft on an ICAO day with our other variables the same.
3000 ft at 140 IAT = 9810 feet DA! Yipes!
That’s 20.1 psia now. Crap, 4 psia is just gone….
3000 ft at 100IAT = 6658 ft. Still yipes, but better than almost 10,000 ft DA and now our engine sees 21.8 psia. Not a huge difference at these altitudes, but significant enough.
So you can see how lowering your IAT’s can reap big benefits.
This is why an intercooler might give more power, as can a water injection system except that introduces the variable “humidity” into the equation.
Intercooling allows you to cram more air (density, remember) in the cylinder per “blow”.
Now, you need the fuel in order to take advantage of that added density as well….but that’s another story with our trucks….
So, what’s all this mean for our intercooler theory discussion at the bottom line?
It makes the air more dense……
(Disclaimer; for the “Bill Nye science guy” types who loves to poke holes in everything- this is very simplified so go easy OK?)
We all know what an intercooler is and we all know it reduces temperature of the air charge. But why does it work? That’s and easy answer isn’t it?
It makes the air more dense.
Well, let’s talk about that a bit and see if we can’t gain a bit more than that rudimentary understanding….
First, some theory and definitions.
Definitions and resources;
PSIG = PSI gauge. This is gauge pressure displayed over 14.7 psi (1 atmosphere)
PSIA = PSI absolute. This is the pressure your engine actually sees. Boost plus 1 atmosphere if you prefer.
Pressure altitude (PA) = pressure altitude is the indicated altitude when an altimeter is set to a pressure setting. Standard is when set at 29.92 in/mg, and then adjusted for pressure of the day to give aircraft a representative value for altitude above sea level. Can be thought of as “altitude above sea level” once the correction factor for the day is added.
Density altitude (DA) = without getting too wordy, it’s how “thick” the air is. Temperature and humidity affect this value. Hotter is thinner, more humid is less dense.
Standard ICAO day = 29.92 in/mg at 15 Celsius.
Atmosphere is always 21% oxygen. That never changes. In a NA, supercharged or turbo engine, you never get more than 21% O2 in there. What you do with a turbo is shove more atmospheres in so you get more oxygen per volume, not %. Think about it a bit, it’ll start making sense.
Online DA Calculator - http://wahiduddin.net/calc/calc_da_rh.htm
Altitude chart - http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
Theory;
Engines don’t really care about pressure altitude, or what you see on the road sign; they only “see” density altitude. Whether you’re talking axial flow or centrifugal jet engines or reciprocating assemblies, an engine only cares about how much air it can pull (or in our case, blow) in during a set period of time.
(Well, all engines actually “blow” air into the cylinders, but that another discussion for another time)
That’s density.
In aircraft speak, it’s called density altitude and affects everything from lift to engine performance.
Now, density altitude is affected by a couple factors. Temperature is a big one. So is in/mg and humidity.
Let’s only look at temperature since that’s easier for us to get our heads around in an automotive application.
15 degrees Celsius and 29.92 in/mg is considered an ICAO standard day. For arguments sake, that’s what we’ll start with and 50% humidity.
Now, you’re rolling along by the ocean at 0 feet ASL and boosting your 10 psi because you’ve got your foot into it. It’s a pleasant day and the scenery is going by at a high rate of knots. Or, you’re rolling up the coast, sea breezes are blowing, sun’s out and have a hill to climb so your foot is “in to it”.
Life’s good.
Let’s say its 15 c outside the IAT at 10 psi is 140c. Humidity is hanging right around 50%. Not unrealistic numbers.
So, what’s the density altitude our engine is seeing?
A couple quick calculations later and you get 6124 feet.
Holy crap! My engine thinks it’s at 6124 feet but I’m staring at the ocean out my window!
So let’s move on to how this affects boost. (This is going to get a bit trickier to follow because we're jumping back and forth from density to pressure)
You’re making 10 psig. But what does the engine see in absolute?
Logic says to add 14.7 psi right? That’s 24.7 psia.
Hold on a second now. Your engine thinks it’s at 6000 feet remember?
Pressure at 6124 feet is around 11.3 psi. So you’re really boosting to 21.3 psia.
What the heck? I want my 3 lbs back!
So, let’s drop an intercooler into the equation taking the same variables except for temperature.
Let’s say, with a 15c (59F) outside air temp and 140f IAT we get a delta of 40F. A reduction in our IAT from 140f to 100f.
What does that do?
A quick whip through the Jeppson calculator says the engine now sees 2991 feet.
Wow!
A 40F IAT drop more than halved the density altitude.
What does this mean for our boost?
3000 feet shows 13.2 PSIA, so now our boost number is 23.2 psia.
Much closer to what can be had at ICAO and we grabbed 2 psia back from The Reaper.
Your gauge will still show 10 psi, but your engine sees a very different psia.
Add in altitude above sea level and things get even more wonky.
Lets look at 3000 ft on an ICAO day with our other variables the same.
3000 ft at 140 IAT = 9810 feet DA! Yipes!
That’s 20.1 psia now. Crap, 4 psia is just gone….
3000 ft at 100IAT = 6658 ft. Still yipes, but better than almost 10,000 ft DA and now our engine sees 21.8 psia. Not a huge difference at these altitudes, but significant enough.
So you can see how lowering your IAT’s can reap big benefits.
This is why an intercooler might give more power, as can a water injection system except that introduces the variable “humidity” into the equation.
Intercooling allows you to cram more air (density, remember) in the cylinder per “blow”.
Now, you need the fuel in order to take advantage of that added density as well….but that’s another story with our trucks….
So, what’s all this mean for our intercooler theory discussion at the bottom line?
It makes the air more dense……
