This might be a stupid question, but i need some real clarification on this topic. What is an open loop and what is a closed loop? Please describe both engine conditions. Thanks alot!

 

Open loop is when you either start up a cold engine, or floor the accelerator pedal.

The ECM is pre-programmed to make the engine do certain things if the coolant is cold, or for a certain amount of time. Whichever comes first.

It basically bypasses the O2 sensor, and sets the air/fuel mixture at (from hear-say) 13.7:1. It also increases the idle speed a little.

It also does this when you floor it. These settings are written, and can't be bypassed from different driving conditions. The only way to change them is to burn your own chip.

Closed loop is when the coolant is warm enough, or a certain amount of time passes. Once in closed loop, all the sensors are used by the ECM. It's primary goal is to decrease emissions, and get maximum fuel economy. The air/fuel ratio is always kept (or at least attempted to be kept) at 14.7:1.

The term came from basic electricity. When you have a light switch in your room set to the "Off" position, it's an "open" circuit, or "loop". When you switch it to the "On" position, you have a "closed" circuit, or "loop".

Make sense?

 

To expound a little:

The terms come from control theory. Open loop means the control system makes adjustments to the system based on input only - the system doesn't check to see how the adjustment affected the system to futher refine the control.

Closed loop is also known as feedback control. Not only does the system make adjustments based on input, but also on the effect or "feedback" of the system after the change is made.

Example of open loop control: Old mechanical carbs. The mixture control system was the power piston or power valve. The input was manifold vacuum. When manifold vacuum dropped when the throttle was opened more, for instance, the mixture was enrichened. But, the actual fuel needed or used was not part of the equation.

An example of closed loop control is the computer controlled fuel mixture our 3rd gens all have (except Canadian, in some cases). The inputs include RPMs, throttle position, manifold vacuum, and barometric pressure. When manifold vacuum drops because the throttle is opened more, the mixture is enrichened by the ECM. But, the oxygen sensor reads how much oxygen is present in the exhaust, and will enrichen the mixture more if excess oxygen is present, lean out if oxygen content is too little.

The knock sensor is also part of a closed loop system in conjunction with the ECM spark timing curves. The ECM sets timing advance based on engine operating conditions (input), and the knock sensor will retard that if is senses detonation (feedback).

Another example of closed loop control is the steering wheel. The driver sees the car is drifting to the shoulder, and turns the wheel to the left. As the car heads back towards the centerline, the driver "feeds back" control to the car by turning the wheel back to the right to keep the car down the center of the lane. If driving was open loop, you'd hit the shoulder, turn the wheel to the left, hit the centerline, turn the wheel to the right, hit the shoulder, turn the wheel to the left, etc., etc., etc.