TECHNICAL CENTRE

ENGINE ELECTRONICS

Why does my Subaru engine have so many electrical components compared to much older cars?

Back in the days, there were no computers or sensors to help run the show. Engines operated 100% mechanically with carburettors and distributors. Engines were raw and either ran like a dream or ran like a nightmare. They needed a lot of attention to keep them running 100%. However, that all changed with the introduction of Electronic Fuel Injection (EFI). This effectively opened to gateway to an entirely new automotive industry and changed the way that engines were built and tuned forever.


Modern day engines are much more advanced, more refined, more efficient and more powerful thanks to all this electrical magic. Although they have a lot of electrical components to help run the entire show, it does offer much more control and refinement of an engine and the way it performs. Some typical components that you will find in modern day engines include an Engine Control Unit (ECU), fuel injectors, air-fuel sensors, oxygen sensors, air flow meters, MAP sensors, crank angle sensors, cam angle sensors, knock sensors, coolant temperature sensors, the list goes on and on!

So what do all the typical electrical components in an engine do exactly?

Here is a list of the main electrical components that you will typically find in most Subaru engines today.

Engine Control Unit (ECU)

Located in the passenger footwell under a kick panel, this unit controls the entire show by taking readings from all the sensors connected and makes any changes to its settings if necessary to suit the running conditions. It also stores and displays any information regarding problems or faults that have been detected via the information display in the dash or when a computer is connected to it.

Fuel Injectors

This component is connected to fuel rails and feeds fuel into an engine's combustion chamber (cylinder) at a rate determined the ECU based on other parameters such as throttle position, airflow, air-fuel mixture and crankshaft position. Each cylinder in an engine typically has 1 injector.

Air-fuel Sensor (AF sensor)

Depending on the model of your Subaru, this sensor is located in the exhaust manifold before the up-pipe and determines the amount of oxygen is present in gasses during the combustion process. This sensor works in conjunction with the oxygen sensor to determine how much fuel is needed at any given time to target a set air-fuel ratio.

Oxygen Sensor (O2 sensor)

Located in the exhaust system after the main catalytic converter, this sensor determines how much oxygen is present after gasses have passed through the catalytic converter. The ECU takes readings from both the AF sensor and O2 sensor and then determines how much fuel is needed at any given time to target a set air-fuel ratio. This sensor is not to be confused with the AF sensor as they are completely different, even though they have similar roles.

Air Flow Meter (AFM)

This unit is located in the engine's intake system, usually on the air filter box. This sensor allows an ECU to measure in voltages how much airflow the engine is taking in at any given time. Some air flow meters also allow the ECU to monitor intake air temperatures. The ECU takes these parameters along with other parameters from other sensors to determine the optimum fuelling and ignition timing requirements.

Crank Angle Sensor

This sensor is located just under the alternator and is mounted directly above the harmonic balancer (crank pulley). It determines the engine speed and position of the crankshaft within a few degrees. The ECU uses this information to determine ignition timing and injector timing. This sensor is the most important sensor in modern day engines. Without it, the engine will not start or run.

Cam Angle Sensor

This unit is located just under the oil filler neck and is mounted directly above the camshaft. This sensor, along with the Crank Angle Sensor determines the engine speed and position of the camshaft within a few degrees. The ECU also uses this information to determine ignition timing and injector timing. Engines equipped with variable valve timing which is known as Active Valve Control System (AVCS) use this sensor to control a solenoid that advances or retards the camshaft rotation by up to 35 degrees by calculating engine speed and engine load.

Knock Sensor

This unit is located just under the the throttle body and is mounted into the engine block. This sensor detects any engine knock which is caused by detonation. Detonation is when a cylinder fires before it is meant to which is very dangerous for an engine. Detonation can occur as a result of poor quality fuel or extremely high pressure and temperatures in combustion chambers. When knock occurs, the ECU will retard ignition timing and add more fuel in order to protect the engine. Depending on how severe the knock is, the ECU may throw a warning.

Coolant Temperature Sensor

This sensor is located in the coolant crosspipe under the inlet manifold and determines the water temperature. The ECU uses this information to determine and adjust idle, fuelling and ignition timing parameters accordingly. A faulty coolant temperature sensor can cause rough idling during cold starts and over-fuelling during normal operating temperature.