[Diy_efi] sequential injection timing

[Mark Claywell]

Hello,
 
 First, end of injection (EOI) is usually considered best for control of
injection timing. 

Most cars inject most if not all of the fuel on a closed valve for econonmy
(somewhat) and emissions concerns. This also cools the intake valve.
Mixture preperation (ie - coverting fuel to vapor phase and mixing) is key
to performance, economy, and emissions. Of course injecting at any point on
a closed valve, means that some of the fuel has a chance to escape during
overlap, reducing economy. 

Injecting on an open valve can often provide a power increase over
injecting completely during the valve being closed. However, to what degree
is engine dependant. 

Increasing fuel flow rates for the same injector with the resulting lower
injection pulse width (via higher pressure), for the same rpm/load point,
can offer you the ability to move the injection pulse around (timing wise)
with more freedom. If you are injecting during a large portion of an engine
cycle, timing will become a lesser issue. Obviously at 100% duty it would
be no issue. Increasing pressure can also result in lower SMD values and
being able to squeeze most, if not all, of the fuel injection pulse, in
after overlap. 

Targeting and port velocity have an influence on MBT (max brake torque) EOI
timing. How close you can get to EOI being to IVC is dependant on targeting
and port velocity, among other things. 

I've seen EOI for MBT go to 140 CAD BTDC(compression) on some high overlap
bikes. MBT EOI all depends on the engine and injector configuration. It is
safe to say though, that just about any engine with good targeting will
like injection on a open valve and will result in higher brake torque.
Occasionaly throttle response with poorer fuels can suffer noticeably, due
to poor mixture prep. 

If you really must have a starting point (which I am leary of), is I
wouldn't let EOI go past the crank angle at which the intake valve is 1-2mm
from closing. That's for a high performance engine with good targeting.
Start from there and go back (ie - ending inj. with more valve lift). 

You should be able to tune EOI as a function of rpm only. Using load and
rpm would be better. If the port temperatures are changing vastly for
whatever reason (ie- seasons; no cooling control) you can compensate EOI
with water/coolant temp, although it is rarely needed. 

Hope that helps. 

mark


On 27 Jul 2002, Kent Martin wrote:
> Does any one know at what angle you should start fuel injection
> in a sequential injection arrangement. I would have assumed 
> that you would start injection just before or when the intake begins 
> opening. I had a look on my car and found that they started injection 
> somewhere around 70 or 80 degrees before TDC. The valve is not
> open at this time, why do they do it then?
> This was measured at idle, and they might have been timing it so at
> maximum load the injection finished when the valve was closing at the
> expense of spraying on a closed valve at lower loads. Does anyone
> have an explanation for this? What are the advantages and disadvantages 
> of different injection start times?
> Kent.
> 
> 
> 
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