Closed loop system. What does it means?

In several entries, the term “closed loop system” is mentioned. Now it’s time to explain bit more about this notion.

What do we see in this picture? The man goes along the rope, trying to maintain the equilibrium, using a pole. If he starts to deviate to the left side, he will move the pole to the right side and vice versa, till the equilibrium is restored.


What this image has to do with “closed loop system”? This is mechanical “closed loop system”!

We can “carry” this mechanical system to the car industry, for example, fuel supply system: the measuring of the equilibrium will be fuel pressure sensor;

the pole will be PWM – electrical power, supplied to pump;

(men’s) existing position – real pressure in the system in an exact moment.

The fuel pressure closed loop system tries to maintain exact fuel pressure. For this reason, the pressure is measured with a pressure sensor. As soon as the system detects differences between desired and ideal values, it performs immediate correction (in other “direction”), to reach the perfect pressure. Turns out, that the performance of the closed-loop system is very simple!

It should be noted, that main parameter, which can be used to evaluate, how the system “succeeds” with maintaining the required parameter is -“pole” (in case of Acrobat) or PWM in case of the fuel system. If the “pole” parameter is average (between left/right side, between min/max values) – the system is in middle point: it’s good news, which means, that the system is ready to correct the situation to both sides – increase/decrease the performance of the pump (in case of fuel pressure monitoring system). If the system is close to any of side values, it means, that it has problems maintaining required parameter. In case of an acrobat – strong side wind is blowing, in case of fuel pressure system – for example, there is fuel leakage, and the pump has to work with increased power.

If the conditions, when the system is not able to maintain the “equilibrium” anymore, appear – the acrobat falls, but MSD (and also all other management/electrical systems) are recording corresponding error message.

For example, MSD has recorder the error message – insufficient pressure, 4700 hPa (instead of necessary 5000 hPa). It is important to understand – although the measured value is only 300 hPa lower than perfect value, actually, the problem is serious! MSD has tried to reach 5000 hPa, increasing PWM from “middle point” or around 50% to 100% (it means, full power supply); electrical power, supplied to pump, was increased for around 3 times, but even with 3x increased power it was not able to reach “equilibrium” or required pressure. The situation is analogous if the acrobat would have moved to the pole to right side till the end, but even that would not be enough in this situation!


Such closed-loop systems are used very often, for example:

  • throttle (a system consists of throttle, it’s position sensors and the motor, that drives it);
  • VANOS (valve, camshaft sensor and the mechanism, which turns camshaft);
  • fuel pressure LPFP (pump, it’s power, which can be changed with PWM, pressure sensor);
  • fuel pressure HPFP (pump, it’s valve, which is directed with PWM, mechanism, which pumps the fuel, pressure sensor);
  • oil pressure system (oil pump, pressure sensor, valve, directed with PWM);
  • water pump (pump, a current sensor, PWM, which is used to change the flow-rate of the pump);
  • the thermostat in Mapped mode (heating element, temperature sensor, PWM management for heating element) etc.


Even fuel mixture (Lambda) maintenance is closed loop system!

This system consists of Lambda probe, DME, which changes the opening time for injectors, and integrators or STFT, which show “balance” situation. If the existing system is close to the perfect middle point, integrators are close to 0 – required correction is small, if the disbalance is more significant, also required correction is higher – the values of integrators will be significantly different from 0.


Even more – the example with acrobat explains very well, how the corrections are different from integrators. The acrobat has chosen the pole heavy and long not without a reason – in this way he reaches additional stability. He cannot accidentally swiftly drag the pole, the more heavy pole is a support point by itself. Exactly this principle is used in some closed loop systems – the system’s reaction time to reach the required correction is limited, to avoid swiftly changing and uncontrollable situations.

Correction – theoretically required correction in the exact moment, to reach the ideal situation;

Integrator – correction, “endowed” with variance, limited in time or the “heavy pole” from analogy with acrobat. Fuel mixture control systems use this principle of “heavy pole” to avoid too swift changes in fuel proportion. Too swift changes can cause uneven performance of the engine.


When we open ../F5/F6 for the running engine, scroll down the menu, we can see:

If we check the marked areas, we see, that the corrections are changing very swiftly, but the integrator – slower, more “solid”, without unnecessary twitching. This principle solves several tasks: if very rapid correction is required – it allows to avoid uncontrollable situations, in turn, parameters, necessary in long-term (in this situation – Lambda), are maintained very correctly.