FYI: Cold Air Intake -> Bad for MPG

[IZwack]

So because my van is a moving cubic building and gets horrible MPG, I've been thinking about intakes and MPGs. Ive come to the realization that:

Since intercoolers and "cold air intakes" (CAI) reduce the average temperature of the incoming air volume and therefore increasing the air density, this means that for that much more oxygen in that air, the fuel injectors (driven by the ECU) must also compensate by increasing the fuel being injected -- the rate of which is proportonal to the increase in air density (and therefore temperature). What this means is that for those looking for MPG gains, CAIs are not the way to go.

So I looked this up and as it turns out, its true. If you really want high MPGs, dump the CAI because what you want is a warm air intake (WAI): http://en.wikipedia.org/wiki/Warm_air_intake

If you need proof, search around for WAI -- as it turns out, people on hypermiling sites have been utilizing WAIs (like ecoModder.com)

 

[kert0307]

While that is true, at hyway speeds a stock intake and CAI will be pulling the same temp air. Most see mpg gains from CAI because they are usually much less restrictive, so the engine doesn't have to be as much of an air "pump".

 

[Tbars4]

...I'm am glad you added this informative thread with easy to read info, including the short but to the point info in that link...

..Growing up with 4 barrel's, dual and triple carbed set up's, I totally understand the need for more fuel when adding more air...Now that the vehicles are computer controlled and fuel injected, I had to learn about WAI on my own...

..A quote from that link...

The net effect is for the engine to intake the same amount of oxygen (and thus burn the same amount of fuel, producing the same power) but with less friction losses, allowing for a gain in fuel economy, at the expense of top-end power.

...Living in the Southwest and it's climate, made a good testing ground to try this out...

...This is a perfect set up for my "Project X" and offroading...I drive hundreds of miles on the freeway to a trail while getting good gas mileage...On the trails you need low end torque so top end power is redundant...

..My setup...

...I hope those who read your post can appreciate the intent of this set up and the info included here...

...This is truly not a set up for speed or racing but it is great for low end power and fuel savings...

 

[2000StreetRod]

Many factors involved

There are many factors involved in fuel efficiency's dependency on inlet air temperature. There are no simple answers for complex questions.

The PCM decreases the pulse width of the fuel injector as the intake air temperature increases. This is because warm air is less dense than cool air so the amount of fuel injected must be decreased to maintain the same Air/Fuel ratio.

The PCM decreases the pulse width of the fuel injector as the engine coolant temperature increases during engine warm up. Less vaporized fuel collects on the intake port walls as the temperature of the walls increases.

Fuel injected into cold air does not disperse as quickly and evenly as fuel injected into warm air.

Warm air contains more water vapor than cold air.

A less restrictive air filter reduces air flow restriction. A larger diameter air intake tube, MAF sensor and throttle body lowers the airspeed of the intake air and reduces total friction. The lower airspeed can reduce the volume of air (charge) into the cylinders during the intake stroke of the piston.

The PCM relies on tables derived from extensive testing to determine how much fuel to inject based on mass of air flow, engine coolant temperature, engine speed, throttle position, intake air temperature, and oxygen content of exhaust. A MAF sensor measures the mass of air flowing instead of the volume. That is why a vehicle can run well at sea level and also 5,000 feet above sea level without making mechanical adjustments. The MAF also reduces engine sensitivity to air density variation due to temperature change.

A fuel injected engine does not get optimum fuel economy at temperature extremes (cold or warm). The first generation Explorer had a thermostatically controlled air intake to avoid cold air intake extremes.

It is my opinion that any perceived gains in fuel economy by increasing the intake air temperature above normal (non-extreme) ambient temperature is due to (1) the PCM leaning the mixture due to the IAT sensor, (2) the engine reaching operating temperature slightly sooner, and (3) less restriction from the slight increase in throttle plate opening.

Number 1 above can be achieved simply by providing a bogus IAT to the PCM and does not reduce the mass of air entering the cylinder.

Number 2 can be achieved by using a thermostatically controlled electric engine coolant fan.

The fuel economy gains from number 3 are probably insignificant. Note that number 3 is not the same as increasing the diameter of the throttle body.

My plan for better fuel economy is (a) to incorporate thermostatic air intake control to avoid temperature extremes, (b) to utilize a low restrictive air filter and inlet, (c) to utilize a large diameter air intake tube, MAF sensor and throttle body, (d) to increase airflow speed as it nears the intake manifold, (e) to replace standard lubricants with synthetic lubricants, (f) to maintain airflow speed as it exits the cylinder (low restriction exhaust), (g) to utilize the PCM to run leaner than stock without detonation, and (h) to reduce the drag of the accessories driven by the serpentine belt. I have completed b, c, d and part of a and e.

 

[IZwack]

Most see mpg gains from CAI because they are usually much less restrictive, so the engine doesn't have to be as much of an air "pump".

Thats probably true. So what we really need is a less restrictive intake tube that takes its air from around the exhaust manifold area, instead of near the hole behind the passenger headlight (where cool ambient air is available).

 

[kert0307]

Ya, it's kind of a tough balance between leaning out the engine and having an efficient engine (thermodynamically engine efficiency goes up when the inlet temp and outlet have a larger gap if I remember correctly). Imo I would rather remove parasitic losses like mechanical fans, and having a less restricted intakes and exhaust. I would rather have an efficient engine and run it at a lower rpm than try to lean out the mixture at a higher rpm. Unfortunately with an auto trans it really isn't an option as the shift points are more set.

 

[IZwack]

We're still removing parasitic losses - think of this as from the hypermiling perspective where we've already done as much as we could to remove parasitic loss. And also, look at this for vehicles in general, not just Explorers or RBVs.

I think the whole point is to move the P/V cycle down on the graph because we dont need that much energy to move a vehicle on the highway (lowering the final combustion pressure). For example, on my Civic, cruising at 60 mph, I can stomp on the gas and the vehicle accelerates at a moderate rate - thats not good. If the engine is running at peak hypermiling situations, then it should take forever to accelerate on the highway. Feeding the engine warm air would decrease the amount of oxygen being fed into the engine (and therefore decrease the amount of fuel per A/F ratio and also move the P/V cycle down a bit) - which would mean a decrease in fuel consumption on the highway, and should also reduce the acceleration rate on the highway.

Otto cycle P/V curve:

 

[Turdle]

true, however since a given amount of "work" must be achieved, you will be burning fuel "longer" to achieve the same work.

IE
accelerating a 2500lb vehicle to 60mph takes a given amount of energy. A warm air intake just increases the time to get there, which burns the same amount of fuel.

It is the accelerations which kill economy, not the cruise-

 

[IZwack]

Right and hypermilers are very good about managing acceleration and stretching out the acceleration time. But the point is, since we spend most of the time in cruise and very little time getting up to 60 mph, then lets put a little effort on the cruise part - so moving the P/V curve down is primarily focused on cruise. We know how to minimize fuel consumption during acceleration.

Isolate all the other factors and focus only on the warm air vs cold air issue and how that changes the rate at which fuel is being injected. We have to look at this from a scientific point of view and focus on only changing one variable and ignore the rest or otherwise the complexity increases and things become more difficult to comprehend and keep track of.

EDIT - to test this concept, maybe build a box (steel or aluminum), stick a small intercooler in there and pipe the exhaust around that intercooler. Basically its like sticking the intercooler (of which the intake air goes through) into an exhaust resonator/muffler. It would probably also work with a heater core warming up the intake flow but a heater core carries fluid and if theres a leak, that would be detrimental.

 

[2000StreetRod]

heat shroud

I'm enjoying this very interesting discussion!

The first generation Explorer had a vacuum actuated valve that selected between two air sources for the air filter enclosure opening. One source was from a duct above a heat shroud above the exhaust manifold on the passenger side to collect warm via radiation and convection (heat rises). I believe the other source was the opening behind the headlight. A thermostatic vacuum switch was mounted on the inside of the air filter enclosure and controlled the valve based on the temperature in the air filter enclosure. My Volvos of many years ago all had the same type of arrangement. I have purchased many of the components from salvaged first generation Explorers and plan to implement something similar on my 2000 Sport.

 

[Tbars4]

...Some info on the first gen stock CAI can be found here...
http://www.explorerforum.com/forums/showthread.php?t=241696&highlight=cai

...about the thermostatic air control....

the thermostatic air control is only used to warm up your engine a few minutes faster than normal.

...The 91 and a number of 92 X's did not come with the heat tube from the exhaust manifold...I have Borla headers on my 91X and when doing some research on their C.A.R.B. certificate, I found this out...It is in their cert documents for the 91...

 

[hazmat456]

my 85 isuzu had a water jacket around the intake plenum, that car got 32mpg.however it was only a 2.0, or maybe a 1.9(same motor as the chevy luv) with a manual, my wife gets 32mpg now in her 04 cavalier. it doesn't have that water jacket set-up, has a 2.2 with a manual, and about 75 more hp than that isuzu did, so dose anybody think that heating up the intake charge will help increase my wife's mileage? Go to the extreme and it would suffer greatly, a warmer charge means less horsepower which translates to more throttle input not only during accelerating but also to maintain cruising speed, it would also increase the amount of time you spend accelerating.

 

[hazmat456]

that being said I wouldn't use a aftermarket cold air intake either less restrictive filter just means more dirt, and the stock setup seems to be doing it's job until you start adding other power adders; headers, cam ext.

 

[Tbars4]

...My old Toyota pickup had the 20R motor with a similar set up...

..It had the water jacket in the upper intake but, it also ran through the carb...Every time it came up for smog I had to drain the radiator to pull the carb and work on it...

..I am not sure but I think Toyota came up with this idea..

 

[IZwack]

my 85 isuzu had a water jacket around the intake plenum, that car got 32mpg.however it was only a 2.0, or maybe a 1.9(same motor as the chevy luv) with a manual, my wife gets 32mpg now in her 04 cavalier. it doesn't have that water jacket set-up, has a 2.2 with a manual, and about 75 more hp than that isuzu did, so dose anybody think that heating up the intake charge will help increase my wife's mileage? Go to the extreme and it would suffer greatly, a warmer charge means less horsepower which translates to more throttle input not only during accelerating but also to maintain cruising speed, it would also increase the amount of time you spend accelerating.

You are comparing a vehicle from one manufacturer using 1985 technologies to a 2004 vehicle from another manufacturer. You think there might be some differences?

And this thread isnt about having more power, its about having just enough power to cruise on the highway. We're not concerned about acceleration since any modern car can accelerate to highway speed. Were concerned about minimizing horsepower to optimize cruising.

 

[2000StreetRod]

torque requirements

I agree that increasing the intake air temperature will cause the PCM to decrease the fuel injected in accordance with stored data tables. Since the resulting power will be less the throttle opening will have to increase to maintain a particular cruise speed. If the intake air temperature is increased enough the operating conditions will be outside of the index parameters in the stored data tables. The PCM would probably utilize the settings for the max stored condition and might even set an out of range DTC.

A warm air intake system does have the potential to increase fuel economy. However, leaning the mixture reduces low speed torque. An Explorer or Aerostar has a large frontal area and a high coefficient of drag. It requires significantly more engine torque to maintain highway cruise speeds than a Honda. At some point of leaning there will be insufficient torque to maintain cruise speed no matter what the throttle position. Combustion temperatures will increase and detonation may occur. If the vehicle is equipped with a knock sensor then the PCM will retard the timing to avoid engine damage.

I have run my SOHC in neutral with a simulated IAT sensor using a potentiometer. As long as the output value is within the stored range of the sensor the PCM does not complain. If the resistance is zero or infinity the PCM sets a DTC.

I would be cautious about utilizing a high temperature intake (>150 degrees) without having a wideband A/F ratio meter to determine how the engine is responding to the condition. I would be even more cautious if my vehicle were not equipped with a knock sensor.

Anyway, it sounds like an interesting experiment and I may pursue it in the future. I already have two 1.75 inch diameter cold air ducts connected to the lower section of my stock air filter enclosure in addition to an enlarged opening behind the headlight. I could run my standard 100 mile test drive and monitor my IAT with my scanner. Then I could block the cold air openings and route a warm air opening from the exhaust manifold area. At cruise speeds there is a lot of air passing thru the engine compartment so the IAT increase may not be significant. But it's worth a try. If the IATs are significantly different then I can compare the fuel economies.

 

[hazmat456]

You are comparing a vehicle from one manufacturer using 1985 technologies to a 2004 vehicle from another manufacturer. You think there might be some differences?

And this thread isnt about having more power, its about having just enough power to cruise on the highway. We're not concerned about acceleration since any modern car can accelerate to highway speed. Were concerned about minimizing horsepower to optimize cruising.

it is the same manufacturer, but there are reason for abandoning the technology;cost and complexity for little payback. Fuel injection has come a long way, while my isuzu was multi-point injection, pulse width and event timing were fixed, compare that to a new toyota where not only are pulse with and pulse event timing are variable as is ignition, intake and exhaust valves are variable.
BMW has added to that,they can also control cam lift and duration, and have added a variable intake runner lengthhttp://www.canadiandriver.com/2002/02/13/auto-tech-bmws-7-series-engine-technology.htm
so with all this technology at hand... I am curious to see if streetrod will see any real results

 

[n2epv]

I read through this thread with interest - you're talking about low or no-cost mods for improving MPG's. What about spark timing tweeks? Back in the old distributor days, you could "bump up" the timing to the point where it would start to ping and then back it off a little. That always gave a nice boost to both MPG and power over stock timing. And doing this with mid or premium-grade gas gives even better results.

Granted - you can trash a perfectly good engine with too much spark advance.

My guess is the "performance tuning" systems advance the spark past "stock", especially when they talk about an "87 octane tune" and a "93 octane tune". Can faking out the IAT sensor with a resistor advance the timing? I have considered hogging out the holes in the CPS (the sensor next to the harmonic balancer) so the timing could be adjusted, but it's in a real nasty location on my 5.0.

 

[mawrazen]

Wouldn't the O2 sensor signal to the PCM automatically adjust the Richness/Leaness of the mixture making changes to compensate for the A/F ratio????

I know in HHO applications they sell a module to trick the PCM. I wonder if manipulating all of the inputs to the PCM will make a difference.

Interesting thread! I'm subscribing.

 

[IZwack]

That's the idea - with a smaller density in oxygen, the computer will compensate and inject less fuel (per unit of time) - provided that the driver doesn't keep accelerating too rapidly (no 'jack rabbit' starts).
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