Torque Dip

[sierra]

There was no response to this post in the Perrin thread and I think it's too important to let slip by unnoticed?
I wondered at what revs the port and direct injection come into play and this chart shows exactly that. Interestingly the direct inject is a constant, at least from 2,300rpm when this chart starts but the port injection shuts down from 3,000 - 5,000rpm.

Perhaps this was done for emissions reasons in that rpm range?

The question is, does the direct injection struggle to supply enough fuel on it's own and is that the cause of the torque dip?

If that's the case then why can't the port injection be turned on for the entire rpm range to fill the dip?

I realise that this is far too simple a solution and would have been implemented if was that easy but just interested now that i've seen this chart?

[ahausheer]

I think the general consensus is that the torque dip is not due to injectors. I think the going idea is that it is caused by poor cylinder filing in that rpm range due to intake manifold harmonics. However, as far as I know there is no conclusive proof of this. Many engines have variable length/volume intake manifold designs (TVIS) in an attempt to develop a flatter torque curve. To my knowledge this engine does not have such a feature but could benefit from it.

[sierra]

Originally Posted by ahausheer

I think the general consensus is that the torque dip is not due to injectors. I think the going idea is that it is caused by poor cylinder filing in that rpm range due to intake manifold harmonics. However, as far as I know there is no conclusive proof of this. Many engines have variable length/volume intake manifold designs (TVIS) in an attempt to develop a flatter torque curve. To my knowledge this engine does not have such a feature but could benefit from it.

So why do you think they shut down the port injection at 3,000rpm and increase the direct injection to compensate?
Notice that the opening of the port injection again at 5,000rpm also results in a net gain which is exactly what I feel at a constant throttle when it hits 5,000rpm.

[ziggz501]

has nobody messed with the ECU to keep the port injectors on? it would be interesting to see what the torque curve would look like if they were just kept on and tuned to keep proper air:fuel.

but yeah, a major objective of the direct/port injection system is for emissions.

[ahausheer]

Port and direct injection is so complicated that Subaru tried to implement it before this joint venture with Toyota, but literally gave up. It is far more complicated than it seems. One thing that is interesting to note is that the injection volume reaches somewhat of a low point at 4K rpm. I think this may relate to the fact that there is less air in the cylinders due to poor filing.

Edit: The reason as to the dip has been disused to great length on this and every other FR-S forum, FYI.

[ziggz501]

here's an in depth look at the evolution of the 4-DS system if you want to go read the intricacies of the setup and how it works. i set it aside for future reading. it's a lot...

http://www.ft86club.com/forums/showthread.php?t=3172

[lazyluka]

I was thinking the same thing as the OP when I first saw this, but after reading the comments above, I think it's more complicated.

Im a noob on this topic, so I could be way off, but if you look at the AFR ratio, it's fairly steady and doesn't show a dip, so regardless of where you inject the fuel from, it's not really going to make too much difference to that dip. Or will it?

[sierra]

Originally Posted by lazyluka

I was thinking the same thing as the OP when I first saw this, but after reading the comments above, I think it's more complicated.

Im a noob on this topic, so I could be way off, but if you look at the AFR ratio, it's fairly steady and doesn't show a dip, so regardless of where you inject the fuel from, it's not really going to make too much difference to that dip. Or will it?

Who's to say the throttle isn't limited in the 3-5,000rpm range to match the limited fuel available? It's electronic so we wouldn't have a clue.

It seems a big coincidence that the torque dip is where the port injection is turned off and the direct injection total time open is no match for the combined port and direct.

[ziggz501]

the torque curve dips around 4000~rpms if i recall correctly, but the port injectors open up at 5000, which almost doubles the amount of fuel being dumped into the engine. my guess is that the engine is using the DI mainly for the purpose of emissions in the early rpms.

[sierra]

Originally Posted by ziggz501

the torque curve dips around 4000~rpms if i recall correctly, but the port injectors open up at 5000, which almost doubles the amount of fuel being dumped into the engine. my guess is that the engine is using the DI mainly for the purpose of emissions in the early rpms.

It pulls well until 3,000rpm when the port injection shuts down and it's like a switch being thrown at 5,000rpm when it comes back on again in an instant.
So if this period from 3-5,000rpm is for emissions using the more efficient direct injection, why can't ecutek or one of the others turn the port injection back on?

[ziggz501]

it seems like it would be a simple process of turning it back on and correcting the air:fuel, but then again, i'm no programmer... or engineer, lol.

[b.e]

I think some of you are missing the importance of the green (INJECTION VOLUME) curve. The fuel rate isn't dropping when the type of injection changes.

[sierra]

Originally Posted by ziggz501

it seems like it would be a simple process of turning it back on and correcting the air:fuel, but then again, i'm no programmer... or engineer, lol.

I have an engineering background but the ECU side of engines is new ground and I can only apply old fashioned logic to this.
In all the talk about the torque dip in the past I don't recollect any mention of the port injection just happening to be turned off for the exact range of that dip? Lots of 'is this the point where the port injection stops and the direct injection takes over?'

[ziggz501]

Originally Posted by b.e

I think some of you are missing the importance of the green (INJECTION VOLUME) curve. The fuel rate isn't dropping when the type of injection changes.

Truf! but most DI engines use forced induction as well. i think the method and spread of the fuel within the cylinder chamber has an effect on the torque that the engine can produce.