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Understanding Fuel Injection and Tuning: Open vs. Closed Loop

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If you are looking for high quality products for your car or motorbike, look no further. We are sure you will find the perfect product for you at Racext. Do not hesitate to contact us with any questions or requests. We are here to help you make your vehicle perfect.Understanding Fuel Injection and Tuning: Open vs. Closed Loop

Anyone installing a fuel controller in a fuel-injected motorcycle will come across the terms “open-loop” and “closed-loop.”

When performing a custom tune (again, on fuel-injected machines), a custom map must be applied to the computer.

But, before I get into all the different ways to apply fuel maps to your motorcycle (or car), I’d like to quickly go over what open-loop and closed-loop modes are, because understanding what went wrong and how to fix it is essential.

What do these terms mean? I’ll try to explain it as simply as possible.

 

In a nutshell, Open Loop vs Closed Loop -Understanding Fuel Injection and Tuning: Open vs. Closed Loop

 

Only fuel-injected engines have closed and open loop conditions. Fuel-injected engines — essentially, most motorcycles sold since the mid-2000s (and nearly all that I buy) that have sensors and fuel controllers.

In a nutshell, a closed loop engine is one that uses a feedback loop. The loop is closed by a sensor.

The engine does not listen to anyone in open loop mode; it simply runs a set of instructions based on where the throttle is, where the revs are, and so on.

A lambda sensor, also known as an oxygen sensor or air/fuel ratio mixture sensor, is the sensor in question. The engine adjusts the fuel/air mixture in closed loop mode by listening to the lambda sensor. It doesn’t in open loop mode!

Consider the following diagram:

That diagram depicts the operation of closed and open loop systems in a simplified manner. The “measuring element” in motorcycles (and cars) is typically an oxygen sensor.

I’ll go over this in more detail below, including when a motorcycle is in closed loop versus open loop and how it affects how you ride.

Closed Loop: Aiming for an Efficient Mixture -Understanding Fuel Injection and Tuning: Open vs. Closed Loop

 

The goal of closed-loop is to get as close to an efficient air/fuel mixture as possible.

The most important aspect of closed-loop combustion is that it is a compromise between fuel efficiency and environmental concerns.

Closed loop would be unnecessary if you were building an all-out gas-guzzling race machine. However, you’d use a lot more gasoline and would never meet ever-tightening emissions standards.

Motorcycles enter closed-loop mode when they are essentially idling. This occurs when:

• They’re idling (after warming up).
• You’re running at a low-to-medium throttle (say, less than 40%) and at a constant speed.

When you ride at a constant speed (constant load, constant throttle), the ECU enters closed-loop mode.

In closed loop mode, the ECU will use the lambda sensor in the exhaust to determine how much fuel to inject into the engine.

The ECU can then adjust the air/fuel ratio based on the actual combustion result. When the lambda sensor detects a rich mixture, the ECU injects less fuel. In the event that the mixture is too lean, the ECU will inject more fuel.

The ECU is attempting to lean out the mixture as much as possible without damaging the engine, using data from the lambda sensor — as well as other inputs — to determine how well the fuel combusted (see below).

How lambda sensors work (and how fast)

Fuel mixture changes occur many times per second. Not quite as frequently as fuel is injected and burned, but quite frequently.

For an average given engine speed in RPM, this is how frequently fuel is combusted in your motorcycle engine:

• 3,000 rpm equals 50 crankshaft revolutions per second, and 50 crankshaft revolutions per second equals 25 power strokes per second (assuming a four-stroke engine)
• 25 power strokes per second equates to 25 combustions on a single, 50 combustions on a twin (the most common configuration), and 100 combustions on a four-cylinder engine… per second.
• (…or another number if you have a triple or a six-cylinder behemoth.) Perform your own calculations!)

Despite the fact that fuel injection and combustion occur hundreds of times per second, the lambda sensor is an analogue, organic device.

Lambda sensors are electrochemical fuel cells with a zirconia element inside that reacts to oxygen (yes, the same zirconia, a.k.a. zirconium dioxide, used in that fake diamond you bought your ex, but in powder form in a battery… so romantic).

When there is a lot of oxygen, the lambda sensor’s fuel cell produces a low voltage (0.2V), and when there is little to no oxygen, it produces a higher voltage (0.8V). Lots of oxygen indicates that the fuel was completely burned with oxygen left over (too lean); no oxygen indicates that there was insufficient oxygen to completely burn the fuel (too rich).

To emphasize that a lambda sensor is an organic device, it has a non-linear scale, and remember that they get dirty — and fail — because they are bombarded with hot exhaust gasses all day, every day of their lives.

Because the lambda sensor is organic, it cannot provide as fast feedback as a digital device. Even at 100 firing events per second, you’d get far fewer feedback points per second — but enough for your computer to run as efficiently as possible.

In closed-loop mode, other inputs

 

The ECU does not simply use the lambda sensor to calculate the mixture.

The ECU, like in open-loop mode, uses a variety of other inputs to determine the optimal mixture.

The ECU also uses the following inputs:

 

• Air flow into the manifold
• Ambient air temperature
• Engine block temperature
• Air conditions: altitude and humidity
• Engine RPMs
• Throttle position

The inputs used vary from motorcycle to motorcycle.

Open Loop: Idling, low RPM, and throttle changes

The ECU doesn’t care what the lambda sensor says in open loop mode. Oh, the mixture is overly rich? Sorry, sucker, but here’s your fuel!

The amount of fuel injected in open loop mode is preprogrammed by the information in the fuel map.

The ECU, like in closed-loop mode, uses data from a variety of sensors, including sensor readings.

 

• Air flow into the manifold
• Ambient air temperature
• Engine block temperature
• Air conditions: altitude and humidity
• Engine RPMs
• Throttle position

— and possibly other inputs, too.

The ECU is operates in open loop mode in these situations:

• Start-up/warm-up. The engine isn’t trying to optimise for anything here other than warming up — much like using a choke on simpler engines or older motorcycles.
• Medium throttle/acceleration: The engine needs more fuel and isn’t optimising for efficiency any more.
• Deceleration (Engine braking): The engine RPM isn’t constant, even if the throttle is (it might even be held off).

You can set the motorcycle to always run in open loop mode… More on that in a minute.

The engine in open-loop mode relies heavily on a fuel map to determine how much fuel to add to the engine.

The most basic and widely used type of fuel map is based on only two variables: throttle position and engine RPM.

The people who tuned the engine figured out how much fuel it needs at different combinations of those variables to make the motorcycle respond well — usually the engineers who built it, but sometimes an aftermarket tuner.

Obviously, different motorcycles have different fuel maps. The distinction is not only in the values; they also have different levels of granularity. Different things are also targeted by the tuning. Some motorcycles may pursue a linear power curve, others may pursue outright power, and still others may optimize for low-down torque.

Changing between open and closed-loop modes

They automatically switch from closed-loop to open-loop operation in a standard-configured motorcycle with an ECU.

You’ll be in closed-loop mode, for example, if you’re idling and the sensors are warm. The engine is attempting to use the least amount of fuel possible. When you accelerate, the engine hears “yo! the accelerator is on!” and switch to open-loop mode, relying instead on the fuel map

The transition from closed to open loop is practically instant. And it can be startling at times.

When conditions stabilize, such as when the throttle is returned to a low position and the RPM remains constant, the ECU will return to closed-loop mode. Again, depending on how “smooth” the computer’s tuning is, this can sometimes be perceived as a jarring transition. It could be a few seconds or a fraction of a second. ​

The fuel map is how the computer thinks about this (see the picture above). If the computer observes the same conditions for a few seconds, it thinks to itself, “hey, things seem stable enough,” and switches from open-loop to closed-loop mode.

Problems with Switching Between Open Loop and Closed Loop

There are a number of common issues that can cause people to complain about “poor fueling” and long for the days before emissions controls and possibly before fuel injection.

One of the most common issues is abruptly switching to closed-loop mode.

Manufacturers do this because they must maintain high combustion efficiency in order to meet emissions regulations. This means you may encounter things like:

• You’re keeping constant speed on a flat surface and the throttle is nearly closed: The motorcycle lurches and jerks. This is the engine switching to closed-loop mode, or switching between odes because it’s confused.
• You have hesitation as you accelerate hard, usually in the low-mid RPM range: This is the computer being hesitant to enter open-loop mode.

These issues can be resolved by changing the fuel map and, in some cases, the sensors. Motorcycle owners will sometimes simply remove the sensors.

Other more conventional problems can be solved by remapping the fuel injection, such as providing an optimal fuel map to a high-flow exhaust and intake system (to improve low-end response, reduce exhaust pops and crackles under deceleration, and improve top-end power).

How to Modify your Fuel Map – Understanding Fuel Injection and Tuning: Open vs. Closed Loop

When you have fueling issues, your first thought is: How can I change the fuel map?

There are several methods for applying a custom map to an ECU — three in particular.

The first way is to reprogram the motorcycle’s ECU. Workshops can always do this, but sometimes users can do it too. For example, I tuned my Triumph Scrambler’s ECU using TuneECU (software you can buy for an Android phone or a Windows PC) and a programming cable I got on Amazon.

The second way is to install a full computer system. The most noted of these is the DynoJet Power Commander, but other notable brands are Bazzaz, Rexxer, and Rapid Bike.

Installing an O2 sensor modulator is a third option. These serve as a link between the exhaust oxygen sensors and the ECU. They take the standard output and send it to the ECU with the instruction, “Yo, actually… richen it up a little.” The Booster Plug and the Rapid Bike Easy are two popular ones.

The most aggressive approach is to completely remove the O2 sensors. You can’t just take them out (usually) because it will cause an ECU error. People frequently remove them and remap the ECU to prevent the error from appearing.

As a result of removing the O2 sensors, the motorcycle is always in open loop mode.

This will increase fuel consumption and emissions. However, with a good tune, that impact can be reduced as well.

Sum-up – Understanding Fuel Injection and Tuning: Open vs. Closed Loop

Hopefully, this clarified the difference between open loop and closed loop fuel injection modes.

If there’s one thing I want you to remember, it’s that closed loop is about saving fuel and being environmentally friendly, whereas open loop is about performance and responsiveness.

If you prefer, you can disable closed loop mode. You’ll go faster and smoother, but you won’t save the planet — and if you’re in an area with strict emissions regulations, you might be breaking the law and unable to re-register your motorcycle.

Most importantly, remain safe and alive.

 

Understanding Fuel Injection and Tuning: Open vs. Closed Loop

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Understanding Fuel Injection and Tuning: Open vs. Closed Loop

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