Anyone who installs a fuel controller into a fuel-injected motorcycle may come across the terms "open loop" and "closed loop" mode.

Whenever you do a custom tune (again, on fuel-injected machines), you have to apply a custom map to the computer.

There are a number of ways of doing this — well, three main ways.

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.

Find a Dynojet Power Commander for your bike on Amazon

A third way is to install an O2 sensor modulator. These are an interface between the oxygen sensors in the exhaust and the ECU. They take the normal output and give the ECU an input that means "Yo, actually... richen it up a little." A couple of popular ones are the Booster Plug and the Rapid Bike Easy.

Regardless of how you reprogram your ECU, you're always altering the way it operates in two states: open loop and closed loop.

What do these mean? I'll try to explain it simply.

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In this guide...

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Closed Loop vs Open Loop in a nutshell

Closed and open loop conditions apply only to fuel-injected engines. Fuel-injected engines — basically, most motorcycles since the mid 2000s (and definitely any that I buy) that have sensors and fuel controllers.

In a nutshell, closed loop is where the engine operates with a feedback loop. A sensor closes the loop.

In open loop mode, the engine doesn't listen to nobody, it just runs off a set of instructions based on where the throttle is at, where the revs are, and so on.

The sensor in question is a lambda sensor, also known as an oxygen sensor or air/fuel ratio mixture sensor. In closed loop mode, the engine listens to the lambda sensor to adjust fuel/air mixture. In open loop mode, it doesn't!

See this diagram:

Diagram of an open-loop vs a closed-loop system
Open vs Closed Loop system

That diagram is a simplified representation of how closed and open loop systems work. In motorcycles (and cars), the "measuring element" is usually an oxygen sensor.

Below I'll explain this in more detail, including when a motorcycle is in closed loop vs open loop, and how it affects the way you ride.

Closed Loop: Targeting Efficient Mixture

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

The most important thing to understand about closed-loop combustion is that it's a compromise for fuel efficiency and environmental purposes.

If you were building an all-out gas guzzling race machine, you wouldn't need closed loop at all. But you'd use a lot more petrol and you'd never pass ever-tightening emissions standards.

Motorcycles switch to closed-loop mode when they're basically idling. This happens when:

  • They're idling (after they've warmed up)
  • You're at low-mid throttle (say under 40%) and constant speed

When you operate the motorcycle at a constant speed (constant load, constant throttle), the ECU switches to closed-loop mode.

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 the air/fuel ratio based on the actual result of the combustion. When the lambda sensor reports the mixture is rich, the ECU injects less fuel. And conversely, when the mixture is too lean, the ECU injects more fuel.

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

How lambda sensors work (and how fast)

Changes in fuel mixture happen many times a second. Not quite as often as how often fuel is injected and combusted, but quite a lot.

This is how often fuel is combusted in your motorcycle engine for an average given engine speed in RPM:

  • 3,000 rpm means 50 revolutions of the crankshaft per second,
  • 50 revolutions per second means 25 power strokes per second (assuming a four-stroke engine)
  • 25 power strokes per second means 25 combustions on a single, 50 combustions on a twin (what most people have), and 100 combustions on a four-cylinder engine... per second.

(...or some other number if you have a triple or a behemoth of a six-cylinder motorcycle. Do your own math!)

Even though fuel injection and combustion is happening hundreds of times a second, the lambda sensor is an analogue, organic device.

Lambda sensors are an electrochemical fuel cell with a zirconia element inside (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) that reacts to oxygen.

Lots of oxygen makes the lambda sensor's fuel cell put out low voltage (0.2V), and little to no oxygen makes it put out higher voltage (0.8V). Lots of oxygen means the fuel got fully burnt with oxygen to spare (too lean); no oxygen means that there wasn't enough oxygen to burn all the fuel (too rich).

To hammer the point home that a lambda sensor is an organic device, it's a non-linear scale, and remember they get dirty — and fail — because they're being hammered with hot exhaust gasses all freaking day for their entire lives.

Because the lambda sensor is an organic device, it can't give as fast feedback as a digital device would. So even with 100 firing events a second, you'd get far fewer feedback points a second — but still enough for your computer to operate as lean as possible.

Other inputs in closed-loop mode

When figuring out the mixture, the ECU doesn't just use the lambda sensor.

Just like in open-loop mode, the ECU uses a bunch of other inputs as well to determine optimal mixture.

The other inputs the ECU uses are:

  • 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 changes of throttle

In open loop mode, the ECU doesn't care what the lambda sensor says. Oh, the mixture's too rich? Too bad, sucker, here's your fuel!

In open loop mode, the injected fuel amount is preprogrammed by the information in the fuel map.

Just like in closed-loop mode, the ECU uses information from a whole bunch of sensors, including sensors reading

  • 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 actually set the motorcycle to always be in open loop mode... more on that in a second.

In open-loop mode, the engine relies mostly on a fuel map to understand how much fuel to add to the engine.

fuel map

The simplest and most common kind of fuel map relies on just two things — throttle position and engine RPM.

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

Different motorcycles obviously have different fuel maps. The difference isn't just in the values, either — they also have different levels of granularity. The tuning also targets different things. Some motorcycles might pursue a linear power curve, some might pursue outright power, and some might optimise for low-down torque.

Changing between open and closed-loop modes

In a standard configured motorcycle with an ECU, they switch from closed-loop to open-loop operation automatically.

For example if you're idling and the sensors are warm, you'll be in closed-loop mode. The engine is trying to use as little fuel as possible. If you then accelerate, the engine will be told "yo! the accelerator is on!" and switch to open-loop mode, relying on the fuelling map instead.

The switch from closed loop to open loop is effectively instant. And sometimes it can be jarring.

When conditions stabilise, for example if the throttle is returned to a low position and the RPM is constant, then the ECU will switch back to closed-loop mode. Again, this can sometimes be perceived as a jarring transition — it depends on how "smooth" the computer's tuning is. It can be a fraction of a second, or a few seconds.​

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

Sum-up

Hopefully this was a clear enough explanation of open loop vs closed-loop modes of fuel injection.

If there's one thing I want you to take away, it's this: closed loop is about saving fuel and being environmental, and open loop is about performance and responsiveness.

You can remove closed loop mode if you want. You'll go a bit faster and smoother, but you won't be saving the planet — and if you're in a tightly emissions controlled place, you might be breaking the law and unable to re-register your motorcycle.

But most importantly: stay safe, and stay alive.