Is it impossible to run a type IV in closed loop?, Megasquirt Options

[DNHunt]

After about a month of tuning I have a fueling map that seems close. Most of the map yields air fuel ratios of about 13.8. From what I have read this is about where the motor was designed to run and it runs very well.

However, I continue to have problems with overrun. Between 2600 and 3600 rpms under very light load the air fuel ratio is much richer. This is quite disappointing as I would hope to gain some economy here. The solution would seem to be closed loop with the oxygen sensor leaning out the air fuel mix. The problem is that the pulse widths are so short that O2 correction leads to large changes percentage wise in pulsewidth and fuel mix and the car bucks. I have one more idea to try. I am waiting on some new injectors since it feels as if one cylinder goes dry.

If that doesn't help, it appears to be run open loop and send fuel ou the exhaust or run closed loop and suffer some loss of smoothness.

I'd appreciate any advice,

Dave

[Mark Henry]

I don’t have a clue if this will help you, it’s off the SDS site:

Closed Loop Feedback Control (CLOSE LP or CL)

CLOSED LOOP is in the OFF when shipped from the factory. CLOSED LOOP should be OFF if you are trying to program the system. When CLOSED LOOP is OFF then the SDS system does not need an oxygen sensor connected to the gray wire. When CLOSED LOOP is OFF the system runs from the values that are programmed.

CL1.JPG

To turn CLOSED LOOP ON push the +1 button. The gray wire must be connected to an oxygen sensor for proper closed loop operation. To turn CLOSED LOOP OFF push the +1 button.

When CLOSED LOOP is ON the EM-4 reads the oxygen sensor voltage and adjusts the injector pulses to attempt to hold the air/fuel ratio around stoichiometric (14.7 to 1 AFR for gasoline). Closed loop does not change any of the programmed values that you see in the programmer. Since there is a time delay time between sensing and correction, the air to fuel ratio will continuously fluctuate slightly to either side of stoichiometry. This condition can be seen when using a mixture meter for setup. Holding the mixture close to the stoichiometric range is essential for the lowest possible emissions when the engine is fitted with a catalytic converter. Closed loop operation is not possible with leaded fuels, since leaded fuel will damage the O2 sensor.


Many engines will not tolerate being idled at the relatively lean mixtures associated with closed loop operation nor will they safely tolerate these lean mixtures at full throttle, therefore the closed loop mode has rpm and manifold pressure limits, outside of which the EM-4 will switch out of closed loop into open loop (programmed values) mode. The EM-4 will also switch out of closed loop when the throttle is opened quickly. Closed loop will not be engaged by the EM-4 until the engine temperature exceeds approximately 35C (95F) and the sensor voltage first exceeds .625 volts. The rpm and MAP must also fall within the programmed limits.

CL2.JPG CL3.JPG

CL4.JPG CL5.JPG

The following limits are programmable: CL LO RPM LIMIT and CL HI RPM LIMIT, which determine where the EM-4 will discontinue closed loop control at. If you select 1500 and 4500 RPM respectively in these slots, closed loop operation will be discontinued whenever the rpm goes above or below these limits. The same thing applies for the manifold pressure limits, CL MAP LO and CL MAP HI. The engine must operate within these 4 limits or the EM-4 will revert back to open loop.


The O2 sensor will not supply reliable information to the EM-4 when cold (below 600 degrees F). A 3 wire heated sensor will give better results over an unheated one. Software limits prevent the EM-4 from adding or subtracting more than 25% to the primary pulse width in closed loop so it is important that the open loop fuel values are reasonably close to correct for proper functioning of the closed loop control.


Closed loop operation in most OE applications is generally limited to cruising conditions. Never program in limits corresponding to high power, high rpm conditions, since this will cause a loss in power and possible engine damage due to lean mixtures. A lean stumble is often apparent when the engine is unhappy about running in closed loop. Limits should be set to avoid these conditions. A mixture meter is highly recommended for system setup.


As stated before, many engines will not idle smoothly in closed loop mode. We recommend as a rough guideline, setting the CL LO RPM LIMIT no lower than 1500 rpm and the CL HI RPM LIMIT no higher than 70% of the redline rpm limit. Likewise, closed loop manifold pressure limits should preclude the low vacuum idle conditions as well as the high throttle ones. Set the CL MAP LO limit 3 to 5 inches above the normal idle MAP value and the HI limit around -5 inches for naturally aspirated engines and from -5 to 2 psi boost on turbo/super charged engines.


In CLOSED LOOP ON/OFF mode, ON/OFF is selected with the +1 and -1 buttons. In the other 4 CL modes, the +1,-1 buttons are used to select the HI/LO points which will be in the same graduations as your RPM and MAP ranges. Always leave the closed loop off if no O2 sensor is connected and always leave closed loop off until all normal programming is completed.