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Direct Injection Turbo

Understanding AFR

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Discussion and questions related to the course Understanding AFR

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Recently did a dyno baseline 2 VW cars with 2.0 direct injection turbo engine and it seems that these cars runs very lean out from factory at about 14:1 - 14.2:1 on the exhaust tail of the dynojet wideband. So if we were to tune these VW ecu flash to up boost alittle, is it still save to run factory A/F or it is generally advisable to richen up a little?

Hello Chris,

i have watched few cars that are Direct injections,yes most of them runs lean,i have seen leaner than what you mentioned in mercedes turbo cars.

for VW with 95 ron octane i run around 0.82-0.83 lambda at peak torque to 0.85 by redline

but every car is different...new bmw turbo cars(N55) i faced miss firing issue's anything below 12 afr and 13.5 made best power all over tapered to 12.8-12.9 by redline..you have to monitor EGT and knock many VW cars have EGT channel you can log that with VCDS

i have seen mercedes run 15 afr with piggy back modules so it can make more power but i could hear it knocking with low grade octane

It's not uncommon, particularly in the more modern DI turbo engines for the wide open throttle AFR to be quite a lot leaner than a conventional port injection engine, although this is often more based around efficiency rather than power. The engines will still respond to changes in AFR in the same way as a conventional port injection engine so what we discuss in the course is still totally applicable. You can also check the sort of AFR used in stock trim as a guideline - As usual if you are increasing the engine's power output then a richer AFR would be sensible to cope with the additional combustion temperature being generated.

It would be useful if you want to tune using what would normally be considered a lean AFR to also monitor EGT pre turbo to watch how the AFR effects exhaust temperature.

This is interesting and I want to do a case study on this. EGT would be great data on these.

Let's remember one other fundamental difference about GDI cars too, the fuel is directly injected into the cylinder at super high pressures. This has a remarkable cooling effect on combustion regarding performance. My guess is this is why they get away with running stoich mixture on 600hp GDI M5. In fact on those, the only time you see the mixture go rich is when torque limiting functions are seemingly active on the top end.

One day... I will get EGT data on these.

The advantage with DI is that you can have a directed spray of fuel into the combustion chamber at the precise time that it is needed, and in the location that it would be most effective. What is generally happening with these engines is that there is an injection of fuel into the combustion chamber aimed at the spark plug, and it is timed to arrive in it's greatest (richest) concentration just as the spark fires. This means that the actual combustion event is rich, but the overall cylinder mixture is lean. This allows for a much cleaner burn, with lower emissions at the tail pipe. It also means that there can be some tricks with tuning the engine regarding when the fuel is injected, and the spark is fired, that if are not understood, can result very quickly in a failed engine.

We have a DI specific course penned for sometime early next year which will help shed a lot more light on DI tuning. BlackRex it sounds like you're describing a stratified charge configuration? My own research into current GDI engines has indicated that the majority of manufacturers have moved back to a homogenous charge design. I'm interested to hear your own experience as ours is very firmly centred right now around the FA20 which in many ways is great but in other ways is not always representative of the mainstream DI engines on the market.

Hi Andre,

The advantage with a DI engine is that you can run in a number of different modes, dependant on the load on the engine. for light loads, you can inject the fuel later in the cycle (closer to the spark event), this retains the fuel in a tighter kernel around the spark plug, promoting a stratified burn for fuel consumption and emissions requirements. When the load comes up on the engine, you can start the injection earlier in the cycle, this promotes mixing of the air fuel mixture in the cylinder, making for a more homogeneous mix through the combustion chamber.

"We have a DI specific course penned for sometime early next year "

Did this ever come to fruition? I've done a good bit of searching and have not found anything.

There was a DI injection timing webinar not too long ago.

That is somewhat helpful, but I was really hoping for that "DI specific course".

I can't make a course for you but I could help fill in some knowledge gaps. Do you have any areas of interest or specific questions related to DI?

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