Introduction to Closed Loop and the Lambda Sensor.
It was necessary to add a fourth correction module that could constantly measure the result of the combustion inside the engine and use this information to make a final adjustment of the fuel injection.
This feedback process is known as Closed Loop operation, and the air molecule content in the exhaust gasses is measured by the Lambda sensor that is located in the exhaust header – See the chart below.
If the lambda sensor is seeing any oxygen molecules in the exhaust fumes, it will inform the ECU of a “Lean” condition, and if not, it will let the ECU know that the conditions are “Rich” – the lambda sensor is very close to being an on/off switch.
The Lambda sensor is a very important and rather complex device, so I will deal with it in details in a separate chapter.
First it makes sense to discuss the constant switch between open loop and closed loop operation.
Open loop – Closed loop switch.
The signal feedback process from the Lambda sensor is not without problems, because there’s a certain delay from the time you change the engines conditions until you can measure the result reliably in the exhaust.
So the ECU must sense a “Stable” condition for a few seconds before it will take the lambda sensor input into consideration.
The only way the ECU can sense if the condition is stable is by looking at the fuel map and see if the active cell stays the same or not (If RPM and throttle positions are kept at a steady level).
• If the active cell in the fuel map stays the same for a few seconds or more, the ECU will see the engine conditions as stable – and make a final adjustment to the air/fuel mixture according to the signal from the Lambda sensor.
• Now the ECU is operating in Closed loop mode, and the Lambda sensor is effectively in charge of the air/fuel Ratio
• As soon as engine speed (RPM) moves up or down or the rider moves the throttle, another cell in the fuel map will become active. Now the ECU will see the condition as “Not stable” and ignore the Lambda sensor signal.
• The ECU is now operating in Open loop mode, and the air/fuel mixture to be injected to the engine is calculated from the information stored in the fuel map and input from the temperature and pressure sensors.
This process is repeating itself constantly where the ECU will switch to closed loop operation when the conditions have been stable for a few seconds, and go back to open loop operation instantly when another cell in the fuel map becomes active.
The term “a few seconds” is used deliberately, because the different manufactures are using slightly different numbers here, and the necessary delay will also change with the overall engine configuration (Displacement and bore/stroke ratio). But it’s usually in the range between 1,2-2,6 seconds.
It is obvious that closed loop is at steady speeds and open loop is during acceleration and engine braking, but there is one more important thing to consider.
The nature of the fuel map is that the cells in the lower left hand quarter is spaced much closer than in the rest of the map because it is very hard to control the AFR in the low RPM and low throttle opening area.
As a result, you will never stay long enough in the same cell in the fuel map at low speeds because even though you think you are riding at a steady low speed, the slightest movement of throttle or RPM will take you to a new cell in the fuel map – and the lambda sensor will be ignored for another few seconds. And in reality, you will never be able to keep the conditions stable enough in this part of the map to make the ECU go closed loop.
Open Loop / Closed Loop summary:
Open Loop
• In Open Loop mode, the injected fuel amount is preprogrammed by the information in the fuel map and the input from temperature and pressure sensors.
• The ECU is operating in Open Loop mode in these situations:
• Idle
• Low RPM
• Acceleration
• Deceleration (Engine braking)
Closed Loop
• In Closed Loop mode, the ECU will use the input from the lambda sensor in the exhaust to determine the correct amount of fuel to inject to the engine.
• This way the ECU is able to adjust or regulate the AFR based on the actual result of the combustion.
• The ECU is operating in Closed Loop mode when you maintain a constant speed on the open roads.
Open Loop / Closed Loop switch
• The ECU will switch from Closed Loop to Open Loop operation when a new cell in the fuel map becomes active (By shifting throttle position or engine RPM).
• The switch from Closed Loop to Open Loop is instant.
• The ECU will switch from Open Loop to Closed Loop mode when the engine conditions is considered stable (When throttle position and RPM is constant so the active cell in the fuel map stays the same)
• You need to keep the active cell in the fuel map constant for a few seconds before the ECU will consider the engine situation as being “Stable”, so the switch from Open Loop back to Closed Loop takes a few seconds.
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Bottom line of all this is that the addition of the Lambda sensor and Closed Loop operation makes it possible for the manufactures to control the AFR with a much higher precision (obviously only in Closed Loop mode)
Due to the tolerances in the involved components, they could never obtain the necessary control with an open loop system, so the manufacturer had to add the Closed Loop mode module in the ECU to make their bikes pass the emission requirements.
And as the manufactures are forced to run the bike very lean (very close to the limit of damaging the engine), it would mean that they would have a lot of bikes breaking down prematurely due to the bigger AFR range in a 100% Open Loop system (and some of them would be running really bad).
From a customer perspective, it is obvious that the Lambda sensor makes our bikes more complex, more expensive, and add another component that can fail.
But in reality, the Lambda sensor is a sturdy component and it does actually give us a few important advantages:
Fuel Economy
• Closed Loop operation fuel consumption will be excellent as the engine is kept at a lean, but safe AFR
• The average user is spending about 80% of the riding time in Closed Loop mode, so the Lambda sensor is a major contributor to the fuel economy on your bike.
Aftermarket exhaust and filters
• The addition of the Lambda sensor and Closed Loop operation means that you can now install aftermarket exhausts and air filters without the risk of damaging your engine.
• With engines fed by carburetor or an early “Open Loop only” fuel injection, you have nothing in place to compensate for the higher airflow found in most aftermarket exhausts and air filters, and you would have to re-jet the carb or re-program the ECU to avoid running the bike dangerously lean.
• With the Closed Loop operation in place on the modern fuel injection system, the ECU will automatically adjust the AFR back to factory level in Closed Loop mode. And as high power conditions are almost exclusively Closed Loop territory, the engine will not suffer.
• Acceleration is obviously an Open Loop thing and there will be no AFR compensation here, but as you are only accelerating for a limited time, there is no risk of damaging the engine during acceleration.
• Idle and low speed running are also Open Loop mode, but at low RPM, the air flow in the aftermarket filters and exhaust is only a little higher than in the standard parts. And as this is a low power situation, your engine will survive without problems.
• The aftermarket exhaust and filters means that you WILL be running the engine slightly leaner in Open Loop mode, and even though this will not harm your engine, the rideability issues mentioned in an earlier chapter, will be even more pronounced. But the closed Loop mode and the Lambda sensor can obviously not be blamed for this disadvantage :-)
All in all, the Closed loop mode and the Lambda sensor gives the bike owner more advantages than problems, and you should think carefully before tampering with the Lambda sensor or bypassing it !
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