[cfg83]

delslo -

Originally Posted by delslo

Well, hot air is less dense, that's for sure. Less dense air means more throttle and rpms to create the same output; ie more load. Additionally, less dense air reduces your combustion chamber pressures similar to lower compression ratio. This is the reason you can run lower octane at higher altitudes.

Yeah, I've heard of some people at higher altitudes (i.e. Colorado) getting great MPG, because they are "running lean" as a function of their altitude. They are not actually "lean" per se, but they are using less fuel than they would be at sea level.

Neither of these lend themselves to efficiency. Further, all things being equal, cold air, which is denser, will lean a/f, as in a carburetor situation.

For OBDII controlled cars (1996 to present), the AFR is controlled by the 02 sensor, which shoots for the magic 14.7 stoichiometric ratio. If the cold air is denser, there is more "air" being drawn into the car. The 02 sensor will report this to the ECU/PCM, which will increase the amount of fuel injected.

I lost the "perfect URL" for this, but here is a good explanation :

Tips on Reading Gauges; Air-Fuel Ratio Monitor

The stoichiometric (STOICH) air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer or Powertrain Control Module (PCM) monitors the voltage from the oxygen sensor. If the PCM sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the PCM immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The PCM is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a stoichiometric A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further.

Since the oxygen sensor output is non-linear and very sensitive at the stoichiometric A/F ratio it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This causes the A/F ratio meter LED's to rapidly cycle back and forth, and is normal operation when the PCM is in closed loop and trying to maintain a stoichiometric A/F ratio. The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.

During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the PCM for maximum power. During this time the oxygen sensor outputs a voltage that corresponds to a rich A/F ratio. But the PCM ignores the oxygen sensor signal because it is not accurate for indicating exactly what the A/F ratio is in this range. The PCM is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.

The reason I asked for the source is you'd think it'd be fairly easy for an OEM to integrate a load or tps based flap that would alternate between hot and cold intakes when needed if there was in fact merit to the claim. Excuse my cynicism that is borne from a performance background/culture abundant with wild claims. My Del Sol is slow, so may as well go for mpg , but I really would like to understand this further.

Other people have posted that this was done on some full size trucks in the 1970's. It used the WAI when the engine was cold, and switched to the CAI (with a flap, just like you said) once the engine was all warmed up. All mechanical stuff in those days, no carputer. I think that some Hondas use WAI by placing the intake deliberately close to "hot stuff", but I don't know the details.

CarloSW2