Engine Management
This page explains a little of engine management system and what's in it that we should know when we are out shopping for that 2000 dollars system.
The Electronic Control Unit (ECU) that we discuss here is the “brain” of your engine. It is a computer inside a protective metal box under the carpet in the passenger foot well. Taking information from many sensors, it coordinates the fuel injectors, spark plugs and, where appropriate, turbo boost. We will discuss each of these briefly so that you have an idea what's the minimum a EM system should have.
Fuelling
Closed loop – this method uses the lambda sensor to set the air/fuel mixture to 14.7:1 which is the point for complete combustion of petrol. This gives good emissions and economy at idle and cruise. It is not useful for high power.
Open loop – this method mainly uses inputs from the mass airflow (MAF) sensor and engine speed to control the amount of fuel required at higher engine loads – typically when on boost when richer mixtures are required for power and detonation control. When the MAF reads and translate a known amount of air going into the engine a desired air:fuel ratio can be achieved. A failed MAF sensor leads to poor running, and usually a check engine light and “limp home” mode where engine speed is limited to typically 3000 RPM and boost to 8 PSI. When the MAF fails, the consequence of this is that the ECU runs too lean and too much ignition advance. This can pose a serious threat to the engine from detonation. With a working knock sensor the ECU will sense this detonation and try to control it by adding fuel and retarding the ignition timing.
Ignition timing
This is determined mainly from the MAF sensor and the engine speed. There are maps for high octane and low octane fuels and the ECU selects between them depending on knock sensor activity. The ECU also learns ignition timing from the knock sensor and effectively sculptures these maps over time.
The Subaru ECU runs highly active timing where it relies on the knock sensor to advance or retard the time the plug sparks. On boosted engines, very often the limiting factor on maximum timing one can run is the pressure within the combustion chamber. Boost is one that raise this pressure significantly. Minimum best torque (MBT) timing is the point to aim for on all engines – practically on a turbocharged engine detonation limits ignition advance before the torque starts to drop off from advancing the timing further.
