Start of Injection Skoda Fabia 1.9 PD 101hp

by **Sascha712** » Wed Jan 23, 2013 8:52 pm

Hello,

I have a question about the Start of injection map.

I am trying to get more out of my fuel. I also know that, if the SOI is earlier the engine produce more power....if the fuel is injected to late, it doesnt burn complete.

But how can i figure out. with which setting my car would get the best economy?
And where are the limits, which i shouldnt pass?
I have a friend, who knows many about these things. But he wont tell my everything.

Sry for my english. I live in germany, but i am from russia.

So, it would be nice, if you could help me ;)

by **Relic** » Thu Jan 24, 2013 11:33 am

You cannot set this properly without a dyno.
You advance SOI until power increase stops at same rpm and load.
Then retard to SOI a little to protect the engine and still make almost maximum power.

This will give you the most effiecient use of the fuel.....or you wouldnt have the most power.
Fuel efficiency is based on the BSFC or brake specific fuel consumption.
Most diesels are 0.38 lb hp hr.
This determines how many lbs of fuel you use per hp.
Optimising SOI will maximise the BSFC.

IF SOI is too late the piston is too far past TDC to make the most use of the rapid expansion so fuel is wasted.
If SOI is too early you gain no power....but start driving the crankshaft backward from a certain point of view...which will brake rods/cranks or seriously reduce bearing life.

by **Sascha712** » Fri Jan 25, 2013 4:29 am

At first, thank you for your answer ;)

So have to use a dyno?
I could use VAG COM to log speed, and than i could use a programm to look, how much power i have.

So, this could be usefull for adjusting SOI for high IQ (50mg/r for example)

But i think, that i dont have to change much at high IQ.

IMHO i have to make changes at the IQ i using to cruise.
So how should i start with that?

Lets use this for an example:

My engine ATD has a SOI at 4° BTDC (15mg/r @ 1500 rpm)
The Duration for this is 7,8°. So the fuel is injected completly at 3,8° ATDC
Or doesn´t it matter how long the duration is? Does the fuel start to burn before its injected completly?

If i understand all right, i have to Start the injection earlier? for exampe at 6° BTDC?

by **Relic** » Fri Jan 25, 2013 11:46 am

I have got a thread running on another forum concerning this tuning SOI vs tuning Injector opening.....

I have noted on a few forums where the posters have suggested using 0 BTDC as the end of injection.
Which basically got me thinking.

If the fuel is injected too early before TDC the engine will be driven backward.
If the fuel is injected too late after TDC you dont get the most power from the fuel and expansion.
Now with diesels its not a case of spark and instant bang.
The fuel is injected and burns/expands all the time it is being injected.
So where with petrol, the burn process is initiated at a single point in time.....with diesel its over a period of time.

With that in mind, ignition is not a set point when the spark occurs but an ongoing process while the injectors are open with diesels.
So thinking out of the box I wondered what would happen if you considered the mean point of fuel injection duration as the way to map diesel fueling.

To do this you need to know when the fuel is injected and how long for.... for each rpm and IQ.
I created a table .. SOI/Duration degrees (X:1) vs rpm and plotted 0-75mg at 5mg increments.
Looking at the OEM values there did seem to be a general pattern.
On average the ratio is 0:1 at 0 rpm and then rises roughly in proportion to rpm.
[NB a ratio of 1 would mean the EOI was 0 degrees or TDC and a ratio of 0 would mean SOI was at TDC]

Now this does make sense to me anyway.
The faster the engine spins the earlier you should be able to inject BTDC ...as the the expansion process will take the same time...but there will be less time as the rpm is higher. So you need to move the injection process earlier to make the most power from the expansion

In the attached pdf I created OEM and modified Tables based on using a uniform MBT ratio and length of injection per mg of fuel.
Is this a better way of tuning ??? I dont know. But things look a damn site prettier ....if a little unusual

Of special note is how the modified EOI looks.
Rather than the 0 degrees of many stated earlier.
It gets futher away from 0 depending on the IQ being injected for each rpm

Comments thoughts ideas opinions ?

Explanations of the tables and calculations can be found at the very back of the pdf.
Tables 1~6

by **KenTDI140** » Sun Feb 17, 2013 10:08 am

Hi I am curently working on something very similar with my golf GTTDI140. I have all the soi map locations and have already had it remapped and had the egr valve deleted.
From what I can see and find out all our moder diesel cars have quite severly retarded soi from where it needs to be top produce best power and economy so that they all meet the emissions laws we now have. so I am working on modding the soi maps above the 46c range to produce better economy. I have also fitted a aux heater from a peaugeot 306 hdi so that during warm up the engine water is heated alowing the engine to warm up faster which I hope would further improve the economy under general too and fro from work journies.

How ever the soi maps are probably going to take me weeks of experimentation as I do not now have acess to a roling road as i used to a few years ago.so I am hoping to alter the advance just a couple of degris at a time with in the area where the egr used to open and see what results i get over a week of ddriving the usual routs.

Ps I would like to read the PDF you mention but as yet dont have the required permissions to download it.

by **Tbracing** » Tue Mar 18, 2014 8:12 pm

how many degrees`?

by **zok** » Tue Mar 18, 2014 9:58 pm

Yes, SOI should be retarded in the EGR rpm area because the engine intakes more oxygen and combustion is faster - there is less combustion lag despite colder intake gasses.

I've read on some other forum that SOI on PD engines should be advanced for 1,5 to 2,5 degrees at whole rpm range for optimal economy. Didn't try it myself.