RAM AIR: FACT or FICTION Part I
03-11-2004, 11:06 AM
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RAM AIR: FACT or FICTION Part I Hey guys, I found this artice related to Ram Air. I couldn't get the link to work; therefore, I just copied it. The link however is: www.vetteguru.com/ramair/
Since this is a pretty long article, I'm going to have to break it into 2 e-mails.
What do you guys think?
Dispelling the Myth about "Ram Air" in Automobiles
There are many air intakes on the market today. Many claim "superior" performance over others.
Air intakes can be seperated into specific catagories:
- Those that take in warm engine bay air
- Those that are exposed to cooler/fresher air from the front.
The biggest bennefit of adding an aftermarket air intake is unshrouding the factory air box.
The last bennefit is exposing it to fresh air.
Ram Air is a myth, and many intake manufactures use the word Ram Air strictly for propaganda. They also try to show track results compared to other intakes that simply incur too many variables to make a meaningful and empirical determination. 60 foot times, atmospheric changes, shifting, etc, etc. So do not beleive anything you hear regarding such claims regarding air intakes.
Lets take a look at the "Ram Air" Myth in automobiles:
The Ram Air Myth by Dave Rodabaugh
The Ram Air Myth is the most mythical of them all. It differs from the other myths, in that the other myths are misinterpretations of physical phenomena, whereas ram air simply does not exist.
MYTH: Use of a scoop on the front of the vehicle to collect intake air, or provide “ram air” can raise engine performance.
TRUTH: At automobile velocities, there is no ram air effect.
SIMPLE EXPLANATION
The "Truth" statement says it all. How much simpler can it be? The Ram Air effect is a total myth because it simply does not exist. “But Pontiac uses it on the Trans Am, and they know more than you do.” To those who offer this, tsk tsk. Careful reading of Pontiac’s statements on the matter reveal that the HP increase of the WS6 package are a result of a less restrictive intake, and a freer-flowing exhaust, NOT any ram air effect.
So why does Pontiac use Ram Air? Easy! To make people buy their cars! And they are quite effective with this strategy.
DEEPER EXPLANATION
Of all of the applied sciences, fluid mechanics is among the most difficult for many people to comprehend. It is a relatively youthful applied science as well, meaning that it has not had two or three centuries of work to mature into an applied science on par, with say, chemical combustion. To make matters worse, it is mathematically defined almost entirely by experimentally-determined mathematics.
This last point is the true differentiator between those who only understand concepts, and those who can quantify what they are discussing. Truly, quantification is the real skill of the engineer. It is one thing to speak about qualitative issues (the “what” of the physical sciences); it is entirely another to quantify them (the “how much” and “to what extent” of the same). In grade school, students are first taught about “closed form mathematics” and then that these mathematics are typical of scientific expression. A good example of this is Newton’s famed “law of action and reaction”, the mathematical expression of which is a succinct F=MA. So straightforward. So simple. Three variables in perfectly-defined harmony. Given any two of them, the third is easy to nail down.
Unfortunately, a vast, vast majority of the mathematics used in engineering are NOT closed form. Instead, they are multi-variable correlations valid only for a narrow set of circumstances. Deviate from those narrow circumstances, and a new expression must be experimentally derived. Fluid mechanics is almost entirely defined by these experimentally-determined expressions, further muddying an applied science not well understood.
And if there ever were an applied science for which common sense is wholly inappropriate, it is fluid mechanics. Virtually nothing obeys the “common sense” rules of observation, explaining why those who believe in ram air have extreme difficulty in believing that is simply does not exist.
The Deeper Explanation begins with a basic explanation of engine principles. Air and fuel must be combusted at a specific ratio, namely, 14.7 parts air to 1 part fuel (this is a chemical ratio). Stuffing more fuel into the cylinders without increasing the amount of air they also swallow will get no gain whatsoever. So the hot rodder’s adage “more air = more power” is proven correct. Figure out a way to stuff more air into the cylinder at any given RPM and throttle setting, and you can burn more fuel. Since burning fuel is what makes power, more air truly does create more power.
The amount of air which is inducted into a cylinder is a function of the air’s density. As the air flows through the intake tract, it loses pressure, and as the pressure decreases, so does the air’s density. (Denisty is mass divided by volume. Since cylinders are a fixed volume, increasing the density will also increase the mass of the air in the cylinder.) There are two ways to increase the pressure and density of the air inducted into the cylinders:
- Decrease the pressure drop from the throttle plate to the cylinders
- Increase the starting pressure at the throttle plate.
Ram air is an attempt to do the second. Under normal circumstances, the air at the throttle plate is at atmospheric pressure, and this pressure drops until the air reaches the cylinders. Ram air would start the process at some pressure higher than atmospheric, and even though the drop is the same, the cylinder pressure is higher because of the increase at the start.
Just how would this increase in pressure at the throttle plate occur? The oft-wrong “common sense” says, “If a scoop is placed in the airstream flowing around the vehicle, the velocity of the air ‘rams’ the air into the scoop, thus increasing the pressure.”
Why is this incorrect? There are two types of pressure: static and dynamic. Placing of one’s hand in front of a fan, or out of a moving car’s window, clearly exerts a force on the hand as the air diverts its path to flow around it. Most people would say “See? This is a clear indication that ram air works. Clearly there is pressure from the velocity of the air.” Well, this is correct, but only to a point. This is an example of dynamic pressure, or the force any moving fluid exerts upon obstacles in its path as the gas is diverted around the obstacle.
What an engine needs is static pressure. This is the pressure the same fluid exerts on any vessel containing it at rest. For those who were physics/chemistry geeks, it is the pressure caused by the force of the molecules bouncing off of the walls of the container. The key to understanding the difference between static and dynamic pressure lies in the velocity of the gas. Dynamic pressure is only a momentum effect due to the bulk motion of the fluid around an obstacle. Static pressure is an intrinsic property of a gas or fluid just because the molecules of the fluid are moving around. Any fluid which is moving can have BOTH dynamic and static pressure, but a fluid at rest only has static pressure.
The point of ram air would be to increase the static pressure, which would correspond to an increase in the in-cylinder air density, and of course, more air. Superchargers and turbochargers do what the mythical ram air purports to do. A supercharger trades the power of the belt and uses it to compress the air in the intake tract. This energy trade-off results in an increase in intake air pressure, more air in the cylinders, more fuel burned, and more power. A turbocharger trades the power of the hot gases and uses it to compress the air in the intake. The overall effect is the same – an increase in intake static pressure.
For ram air to work, it would have to trade the energy of the air’s velocity (as the vehicle moves through the air) for an increase in static pressure (since static pressure is a part of a gas’s internal energy, we see this is TRULY a trade in kinetic energy for an increase in internal energy). Now for the true reasons why ram air is a myth:
- The way for air velocity to be traded for an increase in static pressure is to actually SLOW IT DOWN in a nozzle of some sort. This is easily the MOST counterintuitive part of fluid mechanics for most people. The “common sense” mind says “In order to increase the pressure of the intake, the velocity of the air needs to be increased, just as increasing the speed of a fan exerts more force upon the hand.” Not only does this confuse dynamic with static pressure, but is also misses the point, which is to trade the kinetic energy of the gas for an increase in internal energy. How can this trade occur if the kinetic energy of the gas is increased? It cannot, and in fact, the only way to trade it is to use the velocity of the gas to compress itself – by slowing it down.
- Below about Mach 0.5 (or about half the speed of sound), air is considered “incompressible”. That is, even if the correct nozzle is selected, and the air is slowed down (the official term is “stagnated”) there will be zero trade. No kinetic energy will be traded in as work capable of compressing the air. The reasons for this are not discussed here; the reader may consult any reputable fluid mechanics textbook for confirmation of this fact. In plain English, a car is just too slow for ram air to work.
Still not enough evidence? Here is a little test. For ram air to work, the nozzle must be of a specific shape. The “Holley Scoop” for the Fiero is the wrong shape, by the way. The fact that it has no net shape at all immediately means it cannot effect any kind of energy trade off, so it cannot possibly create ram air. This is also true for the hood scoops on the Pontiac Firebird WS6 package as well, by the way.
End of Part I
Since this is a pretty long article, I'm going to have to break it into 2 e-mails.
What do you guys think?
Dispelling the Myth about "Ram Air" in Automobiles
There are many air intakes on the market today. Many claim "superior" performance over others.
Air intakes can be seperated into specific catagories:
- Those that take in warm engine bay air
- Those that are exposed to cooler/fresher air from the front.
The biggest bennefit of adding an aftermarket air intake is unshrouding the factory air box.
The last bennefit is exposing it to fresh air.
Ram Air is a myth, and many intake manufactures use the word Ram Air strictly for propaganda. They also try to show track results compared to other intakes that simply incur too many variables to make a meaningful and empirical determination. 60 foot times, atmospheric changes, shifting, etc, etc. So do not beleive anything you hear regarding such claims regarding air intakes.
Lets take a look at the "Ram Air" Myth in automobiles:
The Ram Air Myth by Dave Rodabaugh
The Ram Air Myth is the most mythical of them all. It differs from the other myths, in that the other myths are misinterpretations of physical phenomena, whereas ram air simply does not exist.
MYTH: Use of a scoop on the front of the vehicle to collect intake air, or provide “ram air” can raise engine performance.
TRUTH: At automobile velocities, there is no ram air effect.
SIMPLE EXPLANATION
The "Truth" statement says it all. How much simpler can it be? The Ram Air effect is a total myth because it simply does not exist. “But Pontiac uses it on the Trans Am, and they know more than you do.” To those who offer this, tsk tsk. Careful reading of Pontiac’s statements on the matter reveal that the HP increase of the WS6 package are a result of a less restrictive intake, and a freer-flowing exhaust, NOT any ram air effect.
So why does Pontiac use Ram Air? Easy! To make people buy their cars! And they are quite effective with this strategy.
DEEPER EXPLANATION
Of all of the applied sciences, fluid mechanics is among the most difficult for many people to comprehend. It is a relatively youthful applied science as well, meaning that it has not had two or three centuries of work to mature into an applied science on par, with say, chemical combustion. To make matters worse, it is mathematically defined almost entirely by experimentally-determined mathematics.
This last point is the true differentiator between those who only understand concepts, and those who can quantify what they are discussing. Truly, quantification is the real skill of the engineer. It is one thing to speak about qualitative issues (the “what” of the physical sciences); it is entirely another to quantify them (the “how much” and “to what extent” of the same). In grade school, students are first taught about “closed form mathematics” and then that these mathematics are typical of scientific expression. A good example of this is Newton’s famed “law of action and reaction”, the mathematical expression of which is a succinct F=MA. So straightforward. So simple. Three variables in perfectly-defined harmony. Given any two of them, the third is easy to nail down.
Unfortunately, a vast, vast majority of the mathematics used in engineering are NOT closed form. Instead, they are multi-variable correlations valid only for a narrow set of circumstances. Deviate from those narrow circumstances, and a new expression must be experimentally derived. Fluid mechanics is almost entirely defined by these experimentally-determined expressions, further muddying an applied science not well understood.
And if there ever were an applied science for which common sense is wholly inappropriate, it is fluid mechanics. Virtually nothing obeys the “common sense” rules of observation, explaining why those who believe in ram air have extreme difficulty in believing that is simply does not exist.
The Deeper Explanation begins with a basic explanation of engine principles. Air and fuel must be combusted at a specific ratio, namely, 14.7 parts air to 1 part fuel (this is a chemical ratio). Stuffing more fuel into the cylinders without increasing the amount of air they also swallow will get no gain whatsoever. So the hot rodder’s adage “more air = more power” is proven correct. Figure out a way to stuff more air into the cylinder at any given RPM and throttle setting, and you can burn more fuel. Since burning fuel is what makes power, more air truly does create more power.
The amount of air which is inducted into a cylinder is a function of the air’s density. As the air flows through the intake tract, it loses pressure, and as the pressure decreases, so does the air’s density. (Denisty is mass divided by volume. Since cylinders are a fixed volume, increasing the density will also increase the mass of the air in the cylinder.) There are two ways to increase the pressure and density of the air inducted into the cylinders:
- Decrease the pressure drop from the throttle plate to the cylinders
- Increase the starting pressure at the throttle plate.
Ram air is an attempt to do the second. Under normal circumstances, the air at the throttle plate is at atmospheric pressure, and this pressure drops until the air reaches the cylinders. Ram air would start the process at some pressure higher than atmospheric, and even though the drop is the same, the cylinder pressure is higher because of the increase at the start.
Just how would this increase in pressure at the throttle plate occur? The oft-wrong “common sense” says, “If a scoop is placed in the airstream flowing around the vehicle, the velocity of the air ‘rams’ the air into the scoop, thus increasing the pressure.”
Why is this incorrect? There are two types of pressure: static and dynamic. Placing of one’s hand in front of a fan, or out of a moving car’s window, clearly exerts a force on the hand as the air diverts its path to flow around it. Most people would say “See? This is a clear indication that ram air works. Clearly there is pressure from the velocity of the air.” Well, this is correct, but only to a point. This is an example of dynamic pressure, or the force any moving fluid exerts upon obstacles in its path as the gas is diverted around the obstacle.
What an engine needs is static pressure. This is the pressure the same fluid exerts on any vessel containing it at rest. For those who were physics/chemistry geeks, it is the pressure caused by the force of the molecules bouncing off of the walls of the container. The key to understanding the difference between static and dynamic pressure lies in the velocity of the gas. Dynamic pressure is only a momentum effect due to the bulk motion of the fluid around an obstacle. Static pressure is an intrinsic property of a gas or fluid just because the molecules of the fluid are moving around. Any fluid which is moving can have BOTH dynamic and static pressure, but a fluid at rest only has static pressure.
The point of ram air would be to increase the static pressure, which would correspond to an increase in the in-cylinder air density, and of course, more air. Superchargers and turbochargers do what the mythical ram air purports to do. A supercharger trades the power of the belt and uses it to compress the air in the intake tract. This energy trade-off results in an increase in intake air pressure, more air in the cylinders, more fuel burned, and more power. A turbocharger trades the power of the hot gases and uses it to compress the air in the intake. The overall effect is the same – an increase in intake static pressure.
For ram air to work, it would have to trade the energy of the air’s velocity (as the vehicle moves through the air) for an increase in static pressure (since static pressure is a part of a gas’s internal energy, we see this is TRULY a trade in kinetic energy for an increase in internal energy). Now for the true reasons why ram air is a myth:
- The way for air velocity to be traded for an increase in static pressure is to actually SLOW IT DOWN in a nozzle of some sort. This is easily the MOST counterintuitive part of fluid mechanics for most people. The “common sense” mind says “In order to increase the pressure of the intake, the velocity of the air needs to be increased, just as increasing the speed of a fan exerts more force upon the hand.” Not only does this confuse dynamic with static pressure, but is also misses the point, which is to trade the kinetic energy of the gas for an increase in internal energy. How can this trade occur if the kinetic energy of the gas is increased? It cannot, and in fact, the only way to trade it is to use the velocity of the gas to compress itself – by slowing it down.
- Below about Mach 0.5 (or about half the speed of sound), air is considered “incompressible”. That is, even if the correct nozzle is selected, and the air is slowed down (the official term is “stagnated”) there will be zero trade. No kinetic energy will be traded in as work capable of compressing the air. The reasons for this are not discussed here; the reader may consult any reputable fluid mechanics textbook for confirmation of this fact. In plain English, a car is just too slow for ram air to work.
Still not enough evidence? Here is a little test. For ram air to work, the nozzle must be of a specific shape. The “Holley Scoop” for the Fiero is the wrong shape, by the way. The fact that it has no net shape at all immediately means it cannot effect any kind of energy trade off, so it cannot possibly create ram air. This is also true for the hood scoops on the Pontiac Firebird WS6 package as well, by the way.
End of Part I
03-11-2004, 12:52 PM
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Well, duh. I agree that "Ram-Air" is a misnomer and that "cold-air", or "fresh-air" induction would be more honest. However, Ram-Air sounds more sexy and powerful, so it is used as a marketing tool. I have a FTRA and while I consider it a good mod, I don't believe it is a true "ram-air" package at the velocity my vehicle usually sees...
