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Re: [T3] Dead Injectors Puzzler

On 10 Jan 2001, at 8:49, Brian Schlepp wrote:

> Is this FI system a batch fire or sequential?  This may be a stupid
> question, but I was under the impression that electrically fired
> injectors would be sequentially fired.  I know the early VW/Audi
> mechanical systems only had a fuel line running to the injector and was
> under the impression that these systems were batch fire.

The rabbits used a system dubbed CIS, for Continuous Injection 
System, which was not timed, but continuous.

The D-Jetronic system used in type 3s is a timed injection system, 
but in the complete engine cycle there are only 2 times when 
injectors are fired, thus injectors are fired in pairs. This was done in 
the name of simplicity and economy in designing the system.

It may be worth noting that Mercedes sold a few D-jetronic cars in 
the early 70s which were all V-8s. These cars had distributors with 
4 trigger points and fired the cylinders pairwise, but at 4 times per 
engine cycle. To my knowledge all the other D-Jetronic cars were 4 
cylinder, so you can imagine how much more expensive the MB 
parts were! ;-)

Since the gas is injected against the backside of the intake valve 
(also called port injection), the injection timing is not particularly 
critical unlike direct fuel injection, as used in Diesels, where the 
fuel must be injected directly into the combusion chamber at 
exactly the right moment.

> If the type3 FI is sequential then cross-wiring injectors would mess
> with the engine running as the FI would no longer be following the
> firing order.

In our case the firing order is 1 4 3 2, and the 1 & 4 injectors fire at 
the same time, as do the 3 & 2 injectors. The 1 and 3 injectors fire 
as their intake valve is opening while the 4 and 2 injectors fire a half 
cycle ahead of theirs. This means that this fuel has to wait around 
for a moment before the intake valve opens.

This may make a TINY difference at idle, but at any higher speed 
the wait is too small to matter, and the fuel always gets where it's 
going anyway.

Jim Adney
Madison, WI 53711-3054

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