Closed Loop Fueling & Trims (Wideband/Narrowband O2 Sensors)
Concepts
Air-Fuel Ratio/Lambda
Combustion and Air-Fuel Ratio
Gasoline engines operate on the principle of fuel combustion. By approximating the mass (not volume) of air entering a cylinder from the intake manifold prior to fuel injection, a calculation can be made to determine the necessary mass of fuel based on the desired target air-fuel ratio, or the ratio of units air mass to units fuel mass, for the intended combustion.
Depending on the conditions of the engine, such as load (grams of air mass per crank revolution) and engine rotation speed (RPM), varying air-fuel ratios will be required. A carefully selected air-fuel ratio for the engine's operating conditions will include the following positive benefits:
- The resistance to, or avoidance of detonation (or "knock") occurring inside a cylinder during or prior to intended, controlled combustion (which starts with the spark).
- Ensuring complete combustion, where the injected fuel is ideally mixed and combusted, which among several things, results in greater efficiency and more effective extraction of possible force on the cylinder's piston.
- Avoidance of excessive combustion temperatures that can cause damage to any metal surfaces inside the cylinder (melting) or pressures that are outside of the design limitations (relating back to knock).
How Air-Fuel Ratio is Realized
Using the fuel mass derived from the air-fuel ratio and other factors, a calculation can be made to approximate the necessary duration of duty time ("pulse width") that the to-be-fired cylinder's corresponding injector should be electrically signaled to open, permitting fuel to spray into the cylinder. The longer the injector stays open, the more fuel mass is injected directly into the cylinder, and the shorter the time the injector is open, the less fuel is injected. This injection time is called the injector pulse width, or "IPW" for short, where width refers to the duration of time the injector should remain open, typically represented in milliseconds (ms).
Other physical effects are factored during this calculation of IPW, such as the fuel's delivery pressure behind the injector (fuel rail pressure), intake air density, coolant temperature, and so on, all in order to estimate the precise amount of fuel being delivered as accurately as possible. Any modifications that affect the IPW calculation are considered compensations (for physical effects) or trims (for controlling a target air-fuel ratio).
Open vs. Closed Loop Fueling (Abstract)
Open Loop and Positives/Negatives
Although it is possible to estimate the ideal injector pulse width (IPW) to within a close margin, error accumulating from all sensor readings (of which there are many involved), calibration imprecision, and even rounding issues within the ECU's binary logic will effectively cause the calculated pulse width to the injectors to never be perfect. This means that instead of injecting an ideal mass of fuel, the ECU will always be injecting some amount more (richer) or less (leaner) than the actual, intended mass as it relates to the physical target air-fuel ratio by some unpredictable margin.
The process of injecting fuel into a cylinder without any sensor feedback on the resulting air-fuel ratio (in this case, the setpoint) is called Open Loop.
Open Loop presents a problem when tight control over the real, physical air-fuel ratio is necessary: in a modern vehicle with lower margins of safety and greater incentive to optimize combustion efficiency, where the limitations to fueling are approached more aggressively, these errors can have a more significant impact on causing engine knock. Additionally, with an OEM assumption of the fuel's energy density, which is typically only ever assumed to be that of gasoline unless modified (i.e. flex fuel), combustion will be negatively impacted when fuels with unanticipated energy densities are used or mixed (i.e. E85). Due to these restrictions, often a tuner will enrichen (add fuel) the Open Loop air-fuel ratio target(s) in order to add a margin of safety to combustion, but this does not cover the bases for all scenarios where Open Loop has challenges.
On the other hand, Open Loop is fast. Where Closed Loop control requires a fixed amount of time to sense the combustion products and derive an approximate result of air-fuel ratio, Open Loop quickly commands an estimated value. Depending on the strategy taken by the tuner, Open Loop can be an effective way to control effective combustion in an engine in scenarios where delayed feedback (trim) to air-fuel ratio is not an option. Additionally, Open Loop is ideal in scenarios such as malfunctioning air-fuel sensors for Closed Loop, or simply not having them physically present/available. This can sometimes be the case on extremely high flow/horsepower engines, older vehicles without Closed Loop-specific feedback sensors, or even scenarios where the fuel type being used is trusted to be highly consistent.
Closed Loop and Positives/Negatives
Due to the challenges presented by Open Loop control, most modern ECUs control the injector pulse width based on the feedback signaled from physical sensors in the exhaust path. This feedback, in the form of the physical air-fuel ratio (often expressed as lambda), is used to adjust (trim) the injector pulse width to account for the error observed between the sensed air-fuel ratio and the target air-fuel ratio (also known as the setpoint). This process of relying on feedback to adjust the commanded value is referred to as Closed Loop control.
Closed Loop control solves several issues with Open Loop control:
- Open Loop is not sensitive to any changes in a fuel's energy density (i.e. blending ethanol), while Closed Loop control can adjust for these changes.
- Open Loop is not sensitive to long-term changes in combustion, such as those created by carbon build-up on intake values disrupting airflow, while Closed Loop control can learn these deltas over time.
- Open Loop is not sensitive to errors in the air-fuel calculation caused by an improper calibration of the Mass Airflow (MAF) sensor, for example when replacing an intake with an aftermarket intake, while Closed Loop control can learn from these discrepancies. This is a critically important advantage for calibrating aftermarket intakes.
Closed Loop control has drawbacks, however. In Closed Loop, an ECU will make adjustments based on the downstream products of combustion after it has happened. Along with time-consuming signal processing such as smoothing, the sensed lambda value will be electrically delayed but some deterministic amount of time after the commanded pulse width value had been electrically commanded, and combustion had physically occurred. Due to this discrepancy, errors in the approximated pulse width are never perfectly removed, but instead are corrected for with calculated trims. Also, if the feedback delay is out of a preset tolerable value (limit), then Closed Loop control cannot take place.
Closed Loop relies on sensors to determine error, which unfortunately means that if sensors relied upon for reporting accurate estimations of air-fuel ratios are not functioning properly, the corresponding calculated error (and therefore the trims) will be inaccurate as well.