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CAIs and Fuel Economy; what's the real truth?
From time to time I've run across forum posts with claims such as, “I added a Cold Air Intake on my Mazda3 and it improved my fuel economy from 28 mpg to 34 mpg.” As some of you might know due to a recent thread here on the Mazda Forum, I am highly skeptical of such claims.
Recent thread: https://www.mazdaforum.com/forum/new-member-area-5/3-5-a-20558/ Instead of continuing to hijack the thread the debate was in, I have created this new thread for the purpose of laying all of the cards on the table, and either demonstrating why CAIs are bad for fuel economy, or admitting I've got it all wrong and eating some serious crow. Over the next day or two I will lay out my arguments as to why it is my belief that CAIs cannot possibly enhance fuel economy, and why in all likelihood, the addition of a CAI kit to any given car will actually reduce the number of miles or kilometers that said car can drive on any given amount of fuel. First, an appetizer… http://www.ricksfreeautorepairadvice...%80%99s-pocket More soon. |
Terminology
A few terms and definitions that may be needed for this discussion:
Brake Specific Fuel Consumption (BSFC):Stoichiometry:Gasoline Stoichiometric Air/Fuel mixture: 14.7:1 (see the above two discussions) Standard Day:Density Altitude:Stirred Reactor (aka. Perfectly Stirred Reactor, Ideally Stirred Reactor, Partially Stirred Reactor, Well Stirred Reactor): Within combustion science the Stirred Reactor is heavily relied upon as one tool of many for the detailed study of the combustion process.
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The PSR...
The following dissertation was written by an occasional colleague of mine. He wrote this for a different forum in response to folks who were maintaining that things like the Tornado Gas Saver were highly effective at improving fuel economy and that the 200 mpg intake (i.e. a fuel vaporizing induction system) had long since been invented and squashed by the World's automobile companies. I have chosen to add his dissertation to this discussion because while he wasn't addressing CAI in any way, shape, or form, much of what he wrote is extremely relevant to the question at hand.
I *hate* to do what I'm about to do, and I've never done it on this forum, but it needs to be done: lay out my credentials. Credentials are an appeal to authority that leads participants in a discussion away from the merits of a case, but in this thread merits don't seem to be doing too well for the obvious reason that most of you participating in this discussion not only don't understand the science/engineering behind this, but you almost surely lack the education to understand that science/engineering. I'm slightly annoyed by the inability and/or unwillingness by some to be swayed by preponderance of evidence. So, my credentials: I hold a degree in mechanical engineering. (Ohio State, class of 1999.) As part of that degree I was required to take a certain number of "technical electives" and mine were (1) internal combustion, (2) heat exchanger design, and (3) a graduate course in combustion science. That last item is the big cannon here; while I am not an expert in combustion science, I'm a lot closer than most engineers will ever be, let alone people without technical education. Further disclaimer: I do not now, nor have I ever, worked as a mechanical engineer in the field of internal combustion or combustion science. I'm just an educated guy with more than a passing interest in cars, who was lucky enough to use his education to further his interest. Combustion science was hard. If you've got a college degree, especially a technical degree, you take dozens of courses in seemingly unrelated fields and you think "I'll never use these again, especially not together!" Then I sat down in the first day of combustion science and realized how wrong that was. To comprehend combustion science, you need detailed understanding of the following:
(1) How much better is fuel vaporization than fuel atomization? Key to Understanding #1: the Perfectly Stirred Reactor Combustion reactions thrive best in a vessel knows as a "perfectly stirred reactor" or "PSR". In a PSR, the fuel and oxidizer mixture is 100% uniform and also in perfect ratio. Literally, there would not be one extra molecule of fuel or oxidizer out of balance, and every fuel molecule would have the perfect number of oxidizers around it so that when the flame front swept through, the only thing that happened was pure chemical reaction at its maximum theoretical rate. This creates maximum process temperature for maximum useful work. Let's say that the PSR has the perfect ratio with the perfect distribution and that both are required. Key to Understanding #2: Heat of Reaction There is a maximum amount of heat energy that can be released by any combustion reaction. This amount is dictated only by the quantity of fuel involved; no other consideration is required. So for quantity X of gasoline, it can offer quantity Y of heat useful for work in an engine. Key to Understanding #3: The Second law of Thermodynamics The laws of thermodynamics are known to govern the world of Newtonian mechanics insofar as energy flows and balances are concerned. The first law says, "You can't win" meaning that you can't get more energy out of process than you put into it. The second law clamps down even more tightly, saying "You can't break even". This means that you can't get even 100% of the available energy out of a process; you are doomed to some efficiency below 100%, with the theoretical maximum efficiency being a factor of the temperature differences at the ends of the process. For internal combustion, this is in the 60's for gasoline. Practically, it won't even be half of that. This means that 10x gain is ridiculous; a 2x gain is probably the most for which you could ever hope, even that's ridiculous. So, how much better is vaporization than atomization? First, let's talk about why it's better. It's better because liquid fuel fails both the ratio and distribution requirements for the PSR. Liquid fuel, by definition, cannot locally be in the correct ratio with its oxidizers since liquid fuel contains kabillions of fuel molecules in a single drop that will have far too few oxygen molecules around it. And if it's out of local ratio, then it is also out of perfect global distribution. The net result is less total heat released by the reaction, released more slowly, and with incomplete combustion. Vaporized fuel could be much better. But before you way, "A ha! I knew it!" we aren't done. Let's suppose that you have vaporized 100% of the fuel. Is this sufficient to make a PSR? Not remotely! That vapor still has to be mixed with the oxidizer to make a perfectly uniform distribution. This is extremely difficult to do in a piston engine because the flow around the valves and down into the cylinder is difficult to control. So, how MUCH better is this legendary vaporization than atomization? Real-world answer: only marginally. It may be a few percent better than the old carburetor used to do; 10x gain is dismissed with raucous laughter by any knowledgeable engineer the instant that figure is quoted. Why only marginally? Consider that the difference between the two would be influenced by the following factors:
Quote: Originally Posted by DASander I have read that it was more efficient by a factor of ten. If this is true, why is it not being produced? The author then quotes a question posed by another member of the forum: I have read that it (fuel vaporization) was more efficient (than fuel injection) by a factor of ten. If this is true, why is it not being produced? The author then quotes another comment posed by the same member: I have often wondered why no one is furthering fuel vaporizing technology. Throttle body injection, or TBI. The fuel injector replaced the venturi in the carburetor and injected liquid fuel into the intake air stream near the throttle plate. Instead of being drawn by a vacuum, the fuel was injected in a spray pattern under pressure. The net result of this was twofold. First, air/fuel ratio control was far superior when electronic control when lambda feedback was employed. Second, the fuel droplet size could be easily controlled (read: probably smaller) and those smaller droplets could more quickly vaporize in the intake, if only partially. Result: Improvement. Multiport fuel injection, or MPFI. In this setup, there is one fuel injector aimed at each intake port. In most applications, MPFI was batch fire, firing once per crank revolution. This gave each intake valve TWO squirts of fuel for each intake event, and for most cylinders, the fuel spray was directed at a closed intake port. This represented a magnificent leap in fuel control. A major contribution was greatly improved fuel vaporization because the fuel was shot (a) under higher rail pressures for decrease droplet size, and (b) at a closed AND HOT intake port. Misted gasoline, when sprayed into a hot and closed intake port, vaporizes very rapidly. In fact, for most operating conditions, the degree of vaporization for MPFI is shockingly high. Sequential port fuel injection, or SPFI. Architecturally, SPFI is just like MPFI in that it has one injector per intake port. Operationally, it is different because each injector is fired on its own, with the timing of the firing relative to its own cylinder's position. If you shoot the fuel at the closed and hot intake port at the right time, you can get good, uniform vaporization for all the cylinders. Direct injection, or DI. DI injects gasoline straight into the cylinder. The pressures at which it does so are very high. Injecting the fuel into a very hot (because it's being compressed) and turbulent intake charge allows the gasoline to vaporize AND mix very quickly. It's a good way to get closer to the PSR ideal I described above. |
the only defense that i can make FOR cai's, is that it will reduce the amount of work that the motor is under to produce power. the negative side of that is this; you need a given amount of restriction to balance everything. bigger isn't always better and nominal is the name of the game.
i believe that you could gain fuel economy off of a cai, but to say that you can do that with all of them is completely ignorant of the process. the bottom line is that it's not what you drive, but how you drive it. if anything was going to allow you to get better mileage, my money would be on a header. that also comes down to the proper header, but that is a tangent that i am not going to go on. |
A quick summary...
What do the above posts and references tell us?
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Originally Posted by wsoape281
(Post 100873)
the only defense that i can make FOR cai's, is that it will reduce the amount of work that the motor is under to produce power. the negative side of that is this; you need a given amount of restriction to balance everything. bigger isn't always better and nominal is the name of the game.
i believe that you could gain fuel economy off of a cai, but to say that you can do that with all of them is completely ignorant of the process. the bottom line is that it's not what you drive, but how you drive it. if anything was going to allow you to get better mileage, my money would be on a header. that also comes down to the proper header, but that is a tangent that i am not going to go on. |
well, you can say that mother nature does the work, but mother nature only provides the initial pressure (atmosphere). once the air is taken, it has to be displaced. atmospheric pressure isn't enough to fill the intake on it's own. the high pressure on the outside of the box can't move at any rate other than the pressure of atmosphere, which is fine at idle, but simply cannot move fast enough when intake pulses increase. in the end, air does have mass and moves, like everything else that has mass, best in a straight line. the air box forces air to change directions a lot.
it's not that the restriction of a stock intake is immeasurable, it just simply has not been measured. a modern motor also measures all of the relevant characteristics of the air entering. MAF, MAP, and IAT. older motors simply had one or two of these things and they couldn't compute a change in an additional characteristic. bottom line, some motors don't even gain power from a cai. those motors won't benefit one bit of economy, rather, lose it severely. most motors are in the second class where they will gain power, but not gain or lose economy. there are a handful of motors that have the means to measure all aspects, but whether they gain economy depends on the intake. |
Originally Posted by wsoape281
(Post 100888)
well, you can say that mother nature does the work, but mother nature only provides the initial pressure (atmosphere). once the air is taken, it has to be displaced. atmospheric pressure isn't enough to fill the intake on it's own. the high pressure on the outside of the box can't move at any rate other than the pressure of atmosphere, which is fine at idle, but simply cannot move fast enough when intake pulses increase. in the end, air does have mass and moves, like everything else that has mass, best in a straight line. the air box forces air to change directions a lot.
Originally Posted by wsoape281
(Post 100888)
it's not that the restriction of a stock intake is immeasurable, it just simply has not been measured. a modern motor also measures all of the relevant characteristics of the air entering. MAF, MAP, and IAT. older motors simply had one or two of these things and they couldn't compute a change in an additional characteristic.
bottom line, some motors don't even gain power from a cai. those motors won't benefit one bit of economy, rather, lose it severely. most motors are in the second class where they will gain power, but not gain or lose economy. there are a handful of motors that have the means to measure all aspects, but whether they gain economy depends on the intake. Using a Mazda3 as an example, a little rough guesstimation yields a number that suggests a Gen 1 Mazda3 requires something less than 25 continous HP to sustain 70 mph on the above mentioned flat highway. Given that a Mazda3 of that generation produced something around 150 peak HP, the engine of our subject vehicle requires the throttle butterfly valve to be roughly 18.66% open (or said another way, 81.34% closed). At freeway speeds, it is the throttle body that provides the vast majority of the intake restriction, so much so that if you measured the barometric pressure immediately in front of the throttle body and compared it with the ambient barometric pressure, the two would be virtually identical. |
9/10, the fuel economy in debate isn't the highway fuel economy. personally, i feel that you couldn't improve highway fuel economy based on air or fuel, only spark. a clean and efficient spark is the only way that i feel highway numbers could improve in any way. based on that, it goes back to my original statement of work on the motor. if the motor has to work hard to burn all of the fuel, economy will be lost. neither here nor there, my point is that i am not debating on highway economy.
to me, the economy in question is city. back to my previous statement, the modern engine doesn't just base the fuel mixture on the temperature of the air. before, that was true. cars that didn't have an maf had a map. let's call the map speed/density. the maf, we will call speed/temp. speed density engines had the luxury of measuring the actual pressure inside the manifold. the achilles heel here is that they didn't measure temp and seldom measured atmospheric pressure, only assumed it. speed/temp cars often had pressure sensors, but it was to run vacuum systems rather than influence fuel mixture. they also used temp probes to combine with the maf readings for the most effective information. the benefit here is that they were much more accurate at measuring the volume of air moving. they didn't, however, measure density or actual pressure for fuel mixture influence. both of these systems have their place where they are the most nominal set-up. forced induction applications benefit the most from speed/density and stock naturally aspirated applications benefit from a speed/temp. one set-up will lose power with a cai and the other will not gain much, if anything. where am i going with all of this? glad that you have asked yourself that already. the way that all of this rolls together is that the modern engine CAN interpret the density of air, which means that cool air IS better. it also CAN interpret mass, temperature, and velocity in order to accurately supply fuel. that means that the first statement of cooler air is bad is only true for older vehicles which cannot interpret temperature for fuel application. moving on. the engine, itself, is a pump. a valve opens, a piston moves down, air is drawn in. compression, combustion, then the piston is forced down and moves up again. air is then pushed out through another open valve. this, basically, causes evacuation. that means that the air is forced out of an area and more air must displace it from the other end. that represents a full cycle of a 4 stroke engine. given as fact, rather than postulate or theory, that the engine pumps it's own air, we can begin to look at the key features of operation. if we looked at it on a very low speed and molecular level, we could see the pause between intake cycles of the different cylinders at idle or at low output situations. that means that the engine gets plenty of air at those points without working for it. the engine, however, cannot spend 100% of it's time between one of those two situations; there is also a time to accelerate. acceleration is where the most tax on the motor will take place. this is where you need to look the most for economy. intake moves in pulses. the pulses are invisible to us, but they affect the engine by producing currents in the intake manifold and intake itself. when the pulses are aligned perfectly, they draw their own air into the motor. when the pulses are randomly assorted, they create what is known as an eddy current. basically, the air finds any location possible to form a vortex. at low rpm's, the vortexes lock and don't have any effect on air flow. at accelerating rpm's, the vortexes break free and cause resistance. where am i going with this? once again, glad that you asked. if the engine is restricted on air, it will be restricted on power and possibly run rich. rich causes power loss and taxes the motor further. this comes down to far more factors than i have already stated. there are several kinds of drivers as well. if a driver is very light-footed, the stock intake will provide the most economy. if a driver is a little harder on the gas, but not heavy footed, they could benefit the most from either set-up depending on the car. THE MOST IMPORTANT STATEMENT ON THIS PAGE IS HERE; if a driver is heavy-footed, neither set-up will benefit them for more economy. heavy-footed drivers will always tax the motor too far in either situation. here is my conclusion to all of this. lots of science involved, but it boils down to this; cai's don't work for all cars. they can lose economy and power on some, but gain power across the band for others. not all cars gain economy and no car gains economy all of the way across the band. the name of the game is nominal. you don't increase your economy just by driving slow and shifting low. driving in the nominal power band is where you will make your gains. getting an intake that will reduce resistance in that range, but not reduce the nominal back-pressure for that range MIGHT gain a small amount of economy. i wouldn't recommend spending $200-300 for a gamble on whether or not you will gain economy. the odds are stacked against it. even if you gain economy, it won't be enough to recoup the cost of the intake any time soon. |
Originally Posted by wsoape281
(Post 100902)
where am i going with this? once again, glad that you asked. if the engine is restricted on air, it will be restricted on power and possibly run rich. rich causes power loss and taxes the motor further.
Once you understand that, then you'll understand why it is physically impossible for a CAI to improve fuel economy. Maybe you missed this link where this is discussed at length: http://www.ricksfreeautorepairadvice...%80%99s-pocket ;) |
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