Ignition and Injection Questions (ECU models)

Newbie Joined: 17 Nov 2004 Posts: 13

Hi!

I was reading some articles, and 2 questions poped:
1st. Regarding the ignition. It is stated in all books that the spark timing avance is controlled by the ECU (or, in the old carburator days by vaccum advance and rpm advance). OK. Clear. Now, it is the distributor that "passes" the voltge (spark) to the correct spark plug. OK. Clear.
Normally, the spark will take place when the rotor arm will pass near the mount corresponding to a plug in the distributor.

The question is: how can you advance the timing, when anyhow the current will flow only when the rotor reaches the spark plug mount in the distributor? The rotor arm play is fixed, and can not be "pushed" forward or reverse with 10 degrees. The distributor cap is also static.

I mean, the rotor will pass the mount as the camshaft moves, and it does not know anything about advance???

I was thinking that maybe, the rotor arm conductive surface is much wider, and the time duration the contact surface of the rotor when passing the Spark plug mount is much longer, and in that long period of time, the coil will trigger the "shot" at the correct time?

Is it like that?

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2. Second question is about fuel injectors. Ref MPI. I read in some articles that all injectors are triggered at the same time per revolution (or half a complete cycle). Or in some cases you can trigger them in pair of two.

Is it posible? Ok. for one injector is logic, but how do you inject fuel in all cilinders in the same time. It might be that one cilinder is in admision, but can be the rest? Can they be in compression or exaust stroke???

How can you inject fuel in compresion or exaust stroke?

What do you think?

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

question 1... correct, you have the idea. also dont forget the camshafts (and rotor arm) rotate at half crank speed so there is ample time as the rotor passes the relevant contact for a few degrees of advance or retard either way

as for injection, it is intuitive the think that fuel should be injected when the intake valve is actually open, but in practice its unimportant.

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

1) Spark is advanced by moving the the dizzy cap in relation to the rotor arm. So if you move the dizzy so that the contacts are "seen" earlier than usual by the rotor arm, the spark will occur sooner. On electronic managment it is controlled by the injection map, you tell it when to fire in relation to TDC.

2) If a system is semi-sequential/non-sequaential i.e. injection takes place for all for at the same time (usually mechanical) or in pairs, then the fuel that is injected to the cylinder which is not on induction simply stays behind the valve until it opens. It may sound lioke a silly thing to do, but it helps evaporation.

Newbie Joined: 17 Nov 2004 Posts: 13

@Neal I believe during the compression, blow and exaust the intake valve must be closed. Am I wrong? In that case, why MPI? when it is almost the same thing as SPI?

@stan: That is my point. I do not belive (but i may be awfully wrong) that the dizzy cap can advance with respect to the rotor arm. At least the dizzy cap is bolted, and the rotor arm is fixed to the camshaft. Unless the whole camshaft advances.

If the fuel stays behind the valve, why MPI? What is the real gain with respect to SPI? I thought the injector sprays the fuel into the intake manifold, near the intake port just when the valve opens (is opened).

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

With SPI, injecting into the plenum for example, the cylinders will never all get the same amount of fuel.

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

You can move the body of the distributer, which the cap is mouted to. On the F7P you could do it if u slotted the circular bolt hole (the other when is slotted).

In simple, A LOT more air is available to the intake system. This increases the V.E and so peak cylinder pressure. Also the mixture state.....droplet size/vapour level/"trickle" level. With MPI a more homogenous mixture with better composition (smaller droplet size, more vapour) is obtainable

edit: But like i said with electronic control you basically tell the ECU when to discharge a voltage to create a spark using the TDC as a reference

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

Also with SPI the inlet manifold has to flow two materials with totally different flow characteristics....fuel and air. with MPI the inlet only has to flow air, so less of a comprimise can be made.

Neil...
thats a lesser problem....cycle-by-cycle and cylinder-to-cylinder variations will occur no matter what the inlet sytem/fuelling design is.

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

Right you are all dancing around the right answer here but you really haven't hit it on the head.

First the ignition timing.

Moving the distributor on the 16V's or any other ignition module car will do sfa.

The tip of the rotor arm is rather wide. A common mod I/clued up peeps do when they tune a car or add forced induction the rotor arm is narrowed down to prevent cross firing within the cap.

The width of the rotor arm may not seem a lot but the width covers quite a bit of crankshaft rotation. Like Neal stated the cams turn at half engine speed where the dizzy is mounted (OHV like project Y cars are driven off the camshaft via a skew gear, some also turn the oil pump, and snap too which is fun).

To alter timign on cars like project Y you do as Stan says, rotate the distributor in relation to the rotor arm.

On ignition module set ups the only way to alter the timing is to alter the position of the crank trigger.

As for the injection, answers are right but seem to be getting muddled with each other.

You can have batch fire, semi sequential of fully sequential.

In the article I refer to batch and semi injection as after all its retro renault and I was covering cars up to the late 90's.

Anyway batch fire fires all the injectors at the same time, this does mean some fuel will puddle in theory, but due to the time scale in which the combustion process occurs thats not really a problem.

Like Neal stated with SPI cylinders will never get equal mixtures, also like I stated in the article thats a long way for the mixture to travel which is not conductive to efficiency.

But like Stan stated no 2 cylinders are the same so the fueling requirement will be different on a mass produced engine, indeed Bosch injectors have a tollerence range of +-3% so that means there can be an overall devience of 6% between injectors.

Like Stan stated all the MPI manifold has to worry about is flowing air. Less chance of bad back fires etc

Also theres the benefit of sharper throttle response with batch fire but this is never really realised unless its a proper race car, even then its usually a mapping fault that allows the difference to be felt.