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Citroën A-Type Tuning

Engine Management System

© Copyright: J.Cats


This page only provides information based on the personal opinion of the author. The author can't be held responsible for damage in any way that could result from modifications mentioned here.

What is an EMS?

EMS stands for Engine Management System. As the name implies, it's a system that contains a number of components to manage your engine. With managing an engine we really mean managing the timing of the sparks and the fuel distribution. Normaly the sparks are managed by the points box (distributor on other cars). And the fuel distribution is managed by the carburettor.

How does an EMS work?

As you might or might not know, all this managing is done by a computer which is called ECU (electronic control unit) here.

The ECU will tell the coil when to fire. For this it needs to know where the engine currently is. Just as with the points where you set the timing with the timing light, the ECU needs to know when the crankshaft / camshaft is at a certain point in it cycle. This is done by fitting a sensor which reads the position of either the camshaft or the crankshaft. On an A-type we can fit the sensor where the points box would originally be and look at the camshaft. But it's simpler to fit a sensor to the crankshaft just as Citroën did with the Visa 652cc engine. Here's an example of how to fit a crankshaft position sensor to the flywheel yourself.
Click for full screen Click for full screen
pictures by Aad.
The sensor used is a standard visa 652cc sensor, these are available new from 2cv specialists. The trigger on the crankshaft is mild steel simply bolted to the flywheel. With this sensor (usually called rpm sensor) the ECU knows how fast the engine is turning and when a spark is needed. The ECU knows the optimal coil soak time and with this it can calculate when to put power on the coil and when not.

The ECU already knows how fast the engine is spinning. All we need to know is the load the engine is under. In aftermarket systems the following systems of measuring load are very popular.

Speed density
Here a Manifold Absolute Pressure sensor (MAP) is used to see how much vacum there is between throttle plate and cylinderhead(s). A lot of vacum is presumed to be low load, little vacum such as under wide open throttle (WOT) is presumed to be high load. For each combination of pressure and rpm the amount of fuel needed has been programmed into the ECU. The result is a 3 dimensional map / chart. This works ok until you start using very wild cams, because of reverse flow. However on the Citroën engines discussed here it works very well. This is also the prefered choice if the engine is not normally aspirated.

Here the Throttle Position Sensor (TPS) is used. Slightly open throttle is presumed low load, WOT is presumed high load. This is how many aftermarket EFI systems work because it's so easy to measure. For each combination of throttle position and rpm the amount of fuel needed has been programmed into the ECU. The result is a 3 dimensional map / chart.

On production cars where performance is not the main issue, other systems are also used. This involves air flow meters, hot-wire sensors, or other devices. They are used widely on production cars because on productions cars emissions are set, and with a known amount of air, known efficiency of the engine, fuel requirement is easy to calculate. However, this method requires lots of R&D to set up and are not very flexible outside their designed package. Therefor there are rarely used on modified cars.

With engine rpm and load being monitored, the ECU controls the time the injectors are kept open, thereby controllign the amount of fuel squirted into the engine.

Why should an EMS be used?

The big question here is: Is electronically controlled ignition and fuelling better then mechanical?
Most of you will be aware of the problems with points ignition, being wear. Wear causes you to replace points and set them up. Worn points do not give the same ignition timing and dwell (coil soak time) as unworn points. Fuelling is a bit more difficult for most people. The carb seems to work ok most of the time, and it does. However, when you start modifying engines, the fuel requirements change and the carb needs to be rejetted. Jetting is not easy, it's more like an art. There is also a very limited amount of modifications you can make before the carb can not be modified enough to cope. So when very precise ignition timing is needed and / or there are highly modified fuel requirements, EMS can offer a solution. Always remember that the original Citroën setup works very well, is highly reliable (if you know how to maintain it), updated and proven for over 20 years and was develloped with a large corporate budget. And some amature (you and me) is not going to improve on this system that easily by just adding some electronic gizmos.

How do I fit an EMS to my car?

There is no simple answer to this one. Injectors will have to be fitted. Fuel rails and lines capable of withstanding much higher pressures then on carb cars will need to be made up. A high pressure fuel pump will be needed together with the correct plumbing to and from the fuel tank. Sensor will have to be fitted to the engine and made to work reliably. Lots of wiring to connect everything to the ECU which will need to be programmed. All in all, not a simple modification as you can see.

Where can I get an EMS?

Try the general tuning page for links. I'm working on getting the DIY-EFI Megasquirt to work on 602, 652 and G engines.

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