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Published: May 11, 2012

These days almost all cars are fitted with an ECU (Electronic Control Unit) which is basically a computer programmed with all the information (known as parameters) to make the engine run.

Inside View of a typical ECU.

The Electronic Control Unit is basically a computer programmed with all the information (known as parameters) to make the engine run, controling such things as the ignition & fueling of an engine.

The ignition can be controlled by the ECU in the form of a ‘spark advance map’ or by a distributor and contacts.
If controlled by the ECU it will need at least 1 input (2D system) but preferably 2(3D system) or more- These inputs will usually be:
• Crank Position sensor: This uses a ‘toothed’ wheel (usually 36-1) with one tooth missing- the missing tooth is mounted next to a magnetic pick off sensor at 90degrees BTDC. The ECU uses this sensor to measure the engine RPM and also the position of no1 piston in relation to TDC (Top-Dead-Centre)
• A Throttle Position sensor, The ECU uses this to measure the throttle position & thus calculates the engine load.
Within the ECU there is a table/graph of Engine RPM * Throttle position. The spark advance will be pre programmed for each ‘load site’ to tell the engine how much spark advance is needed (see article on engine combustion for spark advance reasons).

Older cars use ‘distributors’ and use the inlet manifold vacuum to adjust for advance- these, although crude work pretty well for road & fast road applications.

Open & closed loop control explained.

The fuelling also has a map similar to the ignition, but this can run using ‘open or closed loop- with feedback’.
Open loop uses a minimum of 2 inputs to calculate the injector pulse; they are: Throttle position & Engine RPM (other inputs may include: Inlet manifold pressure, ambient air temperature, coolant temp, humidity & many more!). The fuelling control for this type of system has no ‘feedback’ i.e. it has no way of knowing if it actually IS giving the engine the correct supply of fuel other than what it was originally programmed for- This creates a problem of over/under fuelling in different driving conditions and although this is not noticed by most drivers it doesn’t help fuel economy or emissions.
With the introduction of emission laws, engines builders were forced to find a way of reducing fuel consumption & lowering carbon emissions, this was done by introducing a ‘catalytic converter’(CAT) which reduces the carbon monoxide. The only problem was that CAT’s need very constant exhaust co2 content & this cannot be measured in a typical ‘open loop’ system.

Closed loop systems use ‘feedback’ from a lambda sensor.
This is a sensor mounted in the exhaust system to measure the amount of un-burnt oxygen in the exhaust gas. It uses this feedback to correct the fuelling to what its ideal content should be for the CAT & thus can reduce emissions.

Performance Chips!

An increasing trend in the search for ‘cheap & easy’ power gains is 'ECU chipping'. I am not talking about the type where you get a rolling road to ‘tune you car’ to its peak performance- But many people buy ‘pre programmed chips’ and simply plug these in their ECU & claim that they have a 10% gain in power!.
Ok so this is one area of performance increases that the vehicle manufactures missed, they spent thousands on other parts of the engine development but completely forgot to try to tune the ECU program! – hmm Sounds untrue? It is, but yet so many people fall for this con.
Although there is room for improvement in the standard ECU this can only be done using a proper rolling road & even then only noticeable gains will be seen if other modifications have been done (up rated exhaust-induction kit etc) not much power increase will be seen, and even if it is you will no doubt loose some in other conditions.
The vehicle manufactures although tuning for fuel economy, also have to put in large safety margins for varying fuel qualities, Air density (different at altitudes etc) & Maintenance of the engine (air filters-spark plugs etc). These safety margins can be exploited only by way of ‘proper’ rolling road tuning.

This Article is written completely by XLR8. It is our property & is protected by copyright 2005. Any unauthorized copying is strictly prohibited.