I've seen the terms here regarding the engine but can't find them in a search. What do they mean and when do they occur?

thanks

 

Closed loop: the computer watches the O2 sensor to monitor the air/fuel ratio and adjusts the injector pulse width as necessary.

Open loop: computer determines injector pulse from built-in data and from sensor readings (e.g. MAF)

Hope this helps!

Ben

Whoops, just noticed you also asked when they occur. Open loop is when the O2 sensors aren't hot enough to register (e.g. engine was just started and is warming up), and WOT. Closed loop otherwise.

 

just off the top of my head.......

Open loop
When you fire the beast up, whether the engine is stone cold or fully warmed up, the PCM starts out in open loop. This is because cold (or just warm) O2 sensors don't work very well, and they take a little while to get good and hot. Most LT1 O2 sensors have built-in heaters to speed up the process, which is why they have 4 wires coming out of them. Also, cold engines are happier when they are run richer than the fixed closed loop air/fuel ratio target. Since the O2 sensor signal is not reliable right after the engine is started, the PCM has no way to find out whether the engine is running too rich, too lean, or about right. But it still needs to take a stab at the right injector pulsewidth.

When the engine is running in open loop at light to moderate loads (idling, city driving, highway cruising), the PCM uses the AFR table for the pulsewidth calculation. The AFR table uses the ECT (colder engines need to be richer than warm engines) and MAP (higher loads require a richer mix than light loads) sensor inputs to point to the desired AFR for those temperature and load conditions. As always, the MAF sensor tells the PCM how much air the engine is inhaling. The PCM knows how big the injectors are (the amount of fuel is delivered per millisecond of pulsewidth), so with the MAF sensor output (the amount of air inhaled by the engine) and the AFR from the table (the desired air/fuel ratio), it can estimate its best-guess injector pulsewidth. Remember that the PCM has no idea whether or not this guess-timated pulsewidth is anywhere near right. It could be causing the engine to be very rich, or very lean, or anywhere in between.

Remember that engines like a richer air/fuel mixture when they are cold (richer than the fixed 14.7:1 ratio used in closed loop), and when they are asked to make more than just a cruising level of power. So the AFR table values for high MAP (heavy load) and low ECT (cold engine) are lower (richer) than the values for low MAP and normal operating ECT. This means that "cold engine enrichment" and a bit of "power enrichment" are built right in to the table. This quasi-power enrichment may be used in open loop when you maintain your freeway speed up a steep hill in high gear. But see the last section on this page for what happens when you really lean on the throttle.

When is the engine ready to switch from open loop to closed loop? The current theory is that the PCM decides to switch based on some combination of ECT and engine running time (3 minutes?).

Closed loop
When the engine temperature and time-since-the-engine-started timer agree that the O2 sensors are warmed up and ready, the PCM switches to closed loop (at least this how I think it works). Just as it did for open loop, the PCM calculates the closed loop base pulsewidth using numbers programmed in lookup tables.

While the target open loop air/fuel ratios are contained in the AFR table, the closed loop air/fuel ratio is hard-coded in the PCM to be 14.7:1. The MAF sensor tells the PCM how much air the engine is inhaling, the injector constant and injector-offset vs. voltage values tell the PCM how much fuel the injector will flow for any particular pulsewidth, so the PCM just does the math to calculate the appropriate injector pulsewidth. Well, it may be a bit more complicated than that...

(Note: The following 2 paragraphs presents a controversial topic: the use of VE tables in closed loop. Some folks assert that the VE tables are not used unless the MAF sensor is not functioning. They claim that the VE tables are in the PCM only for speed/density operation, which is a backup mode the PCM can use if the MAF sensor fails. Speed/density is covered below.)

Closed loop operation may refine the injector pulsewidth calculation using the 2-dimensional VE tables, which use MAP and RPM as the 2 inputs. In the 94-95 f-body (OBD-I) PCM there are 3 different VE tables, one for cranking/starting (0-340 rpm), one for low rpm (400-2000 rpm), and the third for high rpm (2000-7000 rpm). If used, these tables would account for the fact that an engine does not necessarily use all the air it inhales for combustion. For example, engines with long duration cams pass some of the inhaled air right out the exhaust valve during overlap.

I think the values in the VE tables are used as multiplication factors that are less than 1 (e.g. 0.93). Or you can think of them as percentages (e.g. 93%). For example, if the pulsewidth based on the current MAF value was 5.0 milliseconds, and the current MAP and RPM values point to a VE table value of 80, the final pulsewidth would be 0.80 * 5 = 4.0 milliseconds. this means that the engine is only using 80% of the measured air for combustion, the rest is blowing straight out the exhaust port.

Even in closed loop, the injector pulsewidth calculation is really just a guess based on the MAF sensor, the injector constant and offset, and (possibly) those VE table numbers. If the pulsewidth is not right, the fuel/air mixture will be too rich or too lean. It's very important to remember that the PCM tries to keep the AFR at the "ideal" ratio of 14.7 while it's in closed loop.

This is where the O2 sensors come in. They generate an electrical signal for the PCM that represents the amount of oxygen in the exhaust stream, which is proportional to the air/fuel ratio. This signal, which is the feedback that "closes the loop", tells the PCM how good a job it's doing on the fuel mixture. The PCM uses this feedback to calculate a correction number, sometimes called the integrator, or INT for short, or (more accurately IMHO) the short term fuel offset. The PCM uses the short term fuel offset to adjust the pulsewidth calculation so that the air/fuel ratio homes in on 14.7:1. Plus, and this is important, it stores the offset so it can be used later.

....top of my head....my ****....credit goes to:
http://para.noid.org/~lj/PCM%20Tutorial/PCMtutorial.htm

 

Only things you really need to know are that in Open Loop, the ECM only runs off other sensors to determine the A/F ratio, while in closed loop its reading the O2 sensor in the exhaust. The primary criteria is essentially the exhaust temperature, because the O2 has to be at about 600 C to read correctly. A heated O2 sensor usually comes into range at 350 C. There are also timers in the computer that must go off, which vary with engine temperature.

If you are running in open loop all the time, which can be because of a bad O2 sensor or using headers which move the sensor further away (less heat), you will be running rich.

 

thanks

mike

 

I learned a lot from this L98 Start Sequence link.

Jeff