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worstmechanic · 01-19-2009, 09:24 PM

This is going to be long, but the bottom line is that I may have figured out why some folks are seeing better mpg after installing k&n filters.

I'm going to assume that we are dealing with vehicles that have an electronic engine management system. In addition, I'm assuming that the engine management control program's goal is to hold the air/fuel mixture at a constant target value as measured by the O2 sensors. I'm also assuming that the program is written as a closed loop feedback loop with feed forward. A regular feedback control would wait for the O2 sensors to move away from the target O2, then start changing fuel rate to get the O2 back to target. This control is relatively slow. Feed forward is provided by using a mass air flow sensor. The feed forward speeds up control. In fact, if the feed forward were perfect, there would be no reason for feedback.

So, I think it's supposed to work like this. The manufacturers want a certain target level of richness/leanness in the combustion chamber. They use O2 sensors to measure this, and decide on a target O2 level for the control program. You also have a starting air/fuel ratio target. So, you step on the throttle, the throttle plate opens, and the MAFS sees air flow, and uses the ratio target to set the fuel flow. The O2 sensor reading should be on target, but if it's not, it will change the air/fuel ratio setpoint.

Knowing this, as most of y'all have figured out, a high flow filter like the K&N should not change mpgs. There would be better performance at or near full throttle. But otherwise, all you're doing is taking less pressure drop across the filter, which is being compensated by more pressure drop across the throttle plate.

One of the Chevrolet truck forums is full of claims of improving mpgs by adding a K&N filter, and also, by cleaning the MAFS. Neither one of these SHOULD affect mpgs at STEADY STATE. But while driving, everything is constantly changing. It's possible that the control program just doesn't work as well when a K&N filter is installed, and that the engine may run lean, on average. In the same manner, a dirty MAFS may cause the proram to run rich, on average.

What do y'all think?

01-19-2009, 09:24 PM	#27 (permalink)
worstmechanic EcoModding Lurker Join Date: Jan 2009 Location: Houston Posts: 3 Thanks: 0 Thanked 0 Times in 0 Posts	This is going to be long, but the bottom line is that I may have figured out why some folks are seeing better mpg after installing k&n filters. I'm going to assume that we are dealing with vehicles that have an electronic engine management system. In addition, I'm assuming that the engine management control program's goal is to hold the air/fuel mixture at a constant target value as measured by the O2 sensors. I'm also assuming that the program is written as a closed loop feedback loop with feed forward. A regular feedback control would wait for the O2 sensors to move away from the target O2, then start changing fuel rate to get the O2 back to target. This control is relatively slow. Feed forward is provided by using a mass air flow sensor. The feed forward speeds up control. In fact, if the feed forward were perfect, there would be no reason for feedback. So, I think it's supposed to work like this. The manufacturers want a certain target level of richness/leanness in the combustion chamber. They use O2 sensors to measure this, and decide on a target O2 level for the control program. You also have a starting air/fuel ratio target. So, you step on the throttle, the throttle plate opens, and the MAFS sees air flow, and uses the ratio target to set the fuel flow. The O2 sensor reading should be on target, but if it's not, it will change the air/fuel ratio setpoint. Knowing this, as most of y'all have figured out, a high flow filter like the K&N should not change mpgs. There would be better performance at or near full throttle. But otherwise, all you're doing is taking less pressure drop across the filter, which is being compensated by more pressure drop across the throttle plate. One of the Chevrolet truck forums is full of claims of improving mpgs by adding a K&N filter, and also, by cleaning the MAFS. Neither one of these SHOULD affect mpgs at STEADY STATE. But while driving, everything is constantly changing. It's possible that the control program just doesn't work as well when a K&N filter is installed, and that the engine may run lean, on average. In the same manner, a dirty MAFS may cause the proram to run rich, on average. What do y'all think?