Chapter 3:  Closed-loop operation on DSMs

Relating the closed-loop system to the DSM ECUs, the newly-added feedback arrow represents the output of various sensors which are placed in the engine to monitor its operation. The primary sensor of concern is the oxygen sensor, a device which measured oxygen in the exhaust gases produced from the engine. Others include the RPM sensor and crankshaft angle sensor, which are also crucial elements in engine control.

[I suppose to be more accurate, you should divide the 'engine' box into two halves. The top half would represent the sensors used to monitor the engine, while the bottom half could represent the spark plugs, fuel injectors and other controlled components. You can see this here. Still, you get the idea.]

When people talk about open or closed-loop operation on DSM ECUs, they are invariably talking about the specific function of fuel delivery. The ECU has other functions as well, but the most prominent task is the requirement to inject the correct amount of fuel into the engine, based on the amount of air entering the cylinders. Too little fuel and the car won't run - too much and the car will not burn it all, leading to waste and poor fuel economy.

The ECU has several input sensors that help it determine the correct amount of fuel. The most widely recognized is the mass air flow sensor (MAS or MAF). The MAS consists of the air volume sensor, the intake air temperature sensor, and the barometric pressure sensor. These three sensors form a single component located directly behind the air filter, and it provides the ECU with the information required to calculate air mass.

[Why mass? Well, the ideal ratio of air to fuel depends on the number of molecules of each type of substance. Knowing the volume doesn't get you anywhere, since the number of air molecules in a given volume changes with temperature and pressure. If you know air volume, temperature and pressure, though, you can get air mass, which in turn can be used to determine the correct amount of fuel.]

So the input signals consist of the air volume, temperature and pressure from the MAS cluster. Feedback is provided by the oxygen sensor, located in the cars downpipe.

[Why oxygen? Measuring the unburned oxygen in the exhaust is a practical way of figuring out how much fuel was really burned in the engine. Very little oxygen in the exhaust indicates that too much fuel was added to the air/fuel mixture. This is known as a rich condition. On the other hand, lots of oxygen in the exhaust indicates that there is too much air in the mixture, a condition known as a lean mixture.

If the mixture is too rich, the engine is eating more fuel than it can use, which is a waste of gasoline. Also, unburned gas and soot from the incomplete burn will be expelled from the engine, coating exhaust parts and leading to increased air pollution. When the mixture is too lean, though, the mix will burn much hotter than normal, leading to the possibility of literally melting engine components. Bad bad bad.]

With the MAS providing the input and the oxygen sensor the feedback, our fuel control system can run in closed-loop mode. Note I said 'can', not 'will'. Just because the feedback sensor is present doesn't mean the ECU has to use it. More on this later.

Before we can continue, a little background on the oxygen sensor. The sensor output is inversely proportional to oxygen - the less oxygen, the higher the sensor output. Also, the sensor output does not increase steadily with decreasing oxygen. Instead, the sensor can be thought of as an 'oxygen thermostat', which tends to change a great deal at a certain oxygen level.

Not by coincidence, the DSM oxygen sensor is designed for a large change in output right around the oxygen level that results from the ideal 14.7:1 air/fuel ratio. Add a little bit of air, and the signal goes way down, while a little bit more fuel makes the signal go way up. Outside this 'magic' range, the oxygen sensor is roughly linear, although it is difficult to predict the actual output for any given air/fuel ratio because of variations in the sensors themselves.

Under normal operating conditions, the engine is running at a near-constant speed with the throttle in a near-constant position. This can be at idle, or while cruising down the street or highway.

Under these conditions, the engine is doing a steady amount of work, well within its power capabilities. The ECU, therefore, is programmed to maximize fuel economy and minimize emissions. To this end, it uses closed-loop operation to try to optimize the amount of fuel being delivered to each cylinder.

So what does this mean??

Well, the ideal air/fuel mixture is 14.7:1. From the MAS information, the ECU knows the mass of air entering each cylinder, and can determine the correct amount of fuel to deliver. It translates these into injector open and close times to control the fuel delivery.

Good enough? Well, not really. Variations in injectors, fuel pressure, and the sensors themselves (all of 'em) can lead to significant inaccuracies in this system. So the injector open time that the ECU has figured out is really not much more than an educated guess.

So what happens? Well, the ECU will fire the injectors, and then check the oxygen sensor output to see if the fuel delivered was too much, or too little. As described before, the oxygen sensor output changes sharply at the special 14.7:1 point. If the ECU is a little off in one direction or the other, the oxygen sensor feedback signal changes a lot, letting the ECU know that it missed the correct fuel amount.

Knowing this information, the ECU can change the next set of injector times to be a little closer to ideal. If too much fuel was delivered, the times are shortened; too little, and the injectors are fired a bit longer. The ECU will check the oxygen sensor output again after this round, readjust the injector times, and repeat over and over again. In this manner, the ECU is continually adjusting the fuel quantity to try to get it just right, and hold that oxygen sensor signal at the magic point of 500 mV.

Owners of air/fuel gauges will know, however, that the ECU never gets it 'just right'. In fact, the oxygen sensor signal changes so much around that perfect 14.7:1 point that the ECU cannot ever hit the 500 mV point and hold it steady. Instead, the ECU keeps hitting around that point, never quite getting it right.

In more detail, what happens is the ECU is a little off one way (too rich by a bit), and the oxygen sensor signal shoots up. The next round, the ECU trims the fuel down a little to fix it. This time, the ECU is off a little the other way (a tad too lean), and the oxygen sensor signal shoots down. The ECU trims the fuel up a bit in response, and keeps missing in this fashion all the time. This is what leads to the 'bouncy' oxygen sensor signal output that all A/F gauge users are familiar with. Far from being a sign of malfunction, this up-and-down signal behavior is perfectly normal and, in fact, is an excellent sign of a healthy oxygen sensor.

To sum up, closed-loop operation means that the ECU uses the MAS information to determine an initial amount of fuel to deliver. The oxygen sensor signal provides feedback to help correct this fuel amount for variations in the individual sensors and engine. This leads to better fuel economy and emissions, as there is very little wasted fuel. Closed-loop operation occurs when the engine is running at a near-constant speed and a near-constant throttle position.

So closed-loop operation works great! Why change it?