Injector Duty Cycle Questions and Interpretation

[Agent_S13]

After reading the thread about the "rom tune" regarding better gas mileage for our cars, I was looking through some of the more detailed answers/responses and it got me thinking.

GunluvS14 made a comment that I was hoping to get someone to chime in an explain in simpler terms -- mostly because I have no/very little automotive knowledge.

Here's what is said:

if u let off the gas with engine braking, no throttle, and STAYS IN GEAR, there's no enrichment there. I have Techtom to monitor the injector duty, if you just cruise in neutral, no load, no engine braking, then IACV comes into play and there's your enrichment. If you keep it in gear, let the rpm drops, the injector duty will drops WAY LOW

this is my case, 45mph slowing down in 2nd gear, injector duty 0.5~0.8%
45mph slowing down in NEUTRAL, injector duty 1.8~2.3%

The original post is here - http://zilvia.net/f/showpost.php?p=2121822&postcount=92

Just so you know I'm not editing any of the relevant information.

My question -- How do I interpret what my injectors are doing based upon the 2nd part of the quote, specifically:

"45mph slowing down in 2nd gear, injector duty 0.5~0.8%"

Can someone comment regarding the application of the %? Is that the load of the injectors? How about the application regarding time? Milliseconds? I too have a techtom and under that category its listing the time, not a percentage. The manual just notes the topic without further explanation.

I did some searched but didn't find anything that would be put into a simpler term.

TLDR Version - Injectors show 0.7 milliseconds while slowing down in gear; shows 3.4 milliseconds while coasting/neutral.

Is more gas coming out of the injectors during the slowing down in gear or while coasting in neutral?

I'd like to make sure I'm interpreting the information correctly so I can see if I should "adjust" my driving style to see if there's any gas economy benefits -- aka more mpg.

Sorry for the long question, but hopefully someone knowledgeable on the subject can school me.

Thanks in advance!

-Aaron
'91 240sx coupe

[WhatsADSM]

Injector duty is calculated based on the time that the injector was open and the total time that the injector COULD have been open (i.e. 2 revolutions, which is dependent on RPM).

So to calculate the duty cycle you would need to know the injector pulse time, along with the RPM. Here is a great page explaining more:
http://www.stealth316.com/2-calc-idc.htm

As for the other question... about fuel economy.

You will be using less fuel if you slow down with the vehicle still in gear and use engine braking. Many ECUs will actually be tuned to completely turn off the injectors when you do this for fuel economy and emissions. However the flip side is you will slow down much quicker if you use engine drag versus coasting.

[xsparc]

^what he said

[!Zar!]

Wtf.

Engine braking is due to compression, not duty cycle.

[WhatsADSM]

Originally Posted by !Zar!

Wtf.

Engine braking is due to compression, not duty cycle.

It sure is.

However when you engine brake (typically above a specific RPM) the ECU will completely shut off the injectors (i.e. reduce the duty cycle to nothing), again for emissions and fuel economy reasons. This is because the engine is mechanically being idled by the inertia of the vehicle and the road.

However if you stick the car in neutral and let it coast the engine is no longer effectively coupled to the road, and the ECU will have to idle the engine or else it will die. Hence in this case the ECU will have to open the IAC and inject fuel (again, higher duty cycle) to keep the engine from dying.... Same thing it has to do when you are stopped at a traffic light and you have the car in neutral.

So... yes, as mentioned before you use less fuel to slow down by engine braking instead of putting it in neutral and hitting the brakes.

[s13derfter]

Originally Posted by WhatsADSM

It sure is.

However when you engine brake (typically above a specific RPM) the ECU will completely shut off the injectors (i.e. reduce the duty cycle to nothing), again for emissions and fuel economy reasons. This is because the engine is mechanically being idled by the inertia of the vehicle and the road.

However if you stick the car in neutral and let it coast the engine is no longer effectively coupled to the road, and the ECU will have to idle the engine or else it will die. Hence in this case the ECU will have to open the IAC and inject fuel (again, higher duty cycle) to keep the engine from dying.... Same thing it has to do when you are stopped at a traffic light and you have the car in neutral.

So... yes, as mentioned before you use less fuel to slow down by engine braking instead of putting it in neutral and hitting the brakes.

ok so neutral your basically just using as much fuel as your engine needs to idle. but if your rev matching wont it use a bit of fuel?

[renegade_ewok]

if your rev matching, your hitting the gas which = using fuel

[Agent_S13]

Thanks for the responses and additional information.

Thanks, Zar for commenting since you too also posted in that other thread I swiped the post from.

I understand that engine breaking is more beneficial than coasting due to the way the ECU reads the situation.

I was hoping to get a bit more information specifically on the idea of the 2 situations:

Engine Braking with .07 milliseconds
vs.
Coasting with 3.4 milliseconds

This is what my Techtom is showing me as I slow down in each situations.

I'm trying to figure out what's being done in those milliseconds? Or what is happening to those injectors that it needs to be displayed in milliseconds?

Maybe I'm asking the wrong question? Instead what is the measurement in milliseconds compared to the use of a percentage (%)?

Are the injectors cycling through each other each .07 milliseconds while engine braking (which could be interpreted by more gas since it only takes Injector 1 .07 milliseconds before it fires again ~ I know this incorrect) vs the 3.4 milliseconds it takes while coasting in neutral?

I see this quote

However when you engine brake (typically above a specific RPM) the ECU will completely shut off the injectors (i.e. reduce the duty cycle to nothing), again for emissions and fuel economy reasons. This is because the engine is mechanically being idled by the inertia of the vehicle and the road.

and it makes sense. When the engine shuts off there is no need for the injectors to work (distribute fuel). Using a percentage here like 0% or 1% (of the injector's load) would make sense to me.

My concern is the application of the milliseconds measurement instead of the percentage?

I know its extremely basic stuff but I thought I'd ask you all first to get a better idea of the actual measurements the injectors are doing.

I guess my request was for a simplistic answer before I start to wrap my head around the next part of engine braking > coasting in neutral.

If there are other additional factors or issues that also should be taken into consideration, please post too.

Sorry for the confusion but there's a lot more technical breakdowns compared to the other forums. Hopefully my question isn't too simplistic!

Thanks!

-Aaron
'91 240sx coupe

[WhatsADSM]

Originally Posted by Agent_S13

Thanks for the responses and additional information.

Thanks, Zar for commenting since you too also posted in that other thread I swiped the post from.

I understand that engine breaking is more beneficial than coasting due to the way the ECU reads the situation.

I was hoping to get a bit more information specifically on the idea of the 2 situations:

Engine Braking with .07 milliseconds
vs.
Coasting with 3.4 milliseconds

This is what my Techtom is showing me as I slow down in each situations.

I'm trying to figure out what's being done in those milliseconds? Or what is happening to those injectors that it needs to be displayed in milliseconds?

Maybe I'm asking the wrong question? Instead what is the measurement in milliseconds compared to the use of a percentage (%)?

Are the injectors cycling through each other each .07 milliseconds while engine braking (which could be interpreted by more gas since it only takes Injector 1 .07 milliseconds before it fires again ~ I know this incorrect) vs the 3.4 milliseconds it takes while coasting in neutral?

I see this quote



and it makes sense. When the engine shuts off there is no need for the injectors to work (distribute fuel). Using a percentage here like 0% or 1% (of the injector's load) would make sense to me.

My concern is the application of the milliseconds measurement instead of the percentage?

I know its extremely basic stuff but I thought I'd ask you all first to get a better idea of the actual measurements the injectors are doing.

I guess my request was for a simplistic answer before I start to wrap my head around the next part of engine braking > coasting in neutral.

If there are other additional factors or issues that also should be taken into consideration, please post too.

Sorry for the confusion but there's a lot more technical breakdowns compared to the other forums. Hopefully my question isn't too simplistic!

Thanks!

-Aaron
'91 240sx coupe

The injector time you are seeing is the injector pulse width time. This is how long FOR EACH ENGINE REV (well actually 2 revs) the injectors are staying open.

Honestly the injector duty cycle (%) is just another way to look at this same data you already have however it also takes into consideration the RPM you were at when you took your time measurements.

Please see my first post with the link as it will explain it and even has a calculator on it.

[Agent_S13]

[QUOTE=WhatsADSM;2181411]The injector time you are seeing is the injector pulse width time. This is how long FOR EACH ENGINE REV (well actually 2 revs) the injectors are staying open.[QUOTE]

Actually I think you answered it right there.

The milliseconds are showing how long the injector is open.... when its engine braking they're not open very long because gas isn't needed.

Perfect, now I have a better understanding about the whole thing.

Thanks to all that answered!!

-Aaron
'91 240sx coupe

[!Zar!]

Originally Posted by WhatsADSM

It sure is.

However when you engine brake (typically above a specific RPM) the ECU will completely shut off the injectors (i.e. reduce the duty cycle to nothing), again for emissions and fuel economy reasons. This is because the engine is mechanically being idled by the inertia of the vehicle and the road.

However if you stick the car in neutral and let it coast the engine is no longer effectively coupled to the road, and the ECU will have to idle the engine or else it will die. Hence in this case the ECU will have to open the IAC and inject fuel (again, higher duty cycle) to keep the engine from dying.... Same thing it has to do when you are stopped at a traffic light and you have the car in neutral.

So... yes, as mentioned before you use less fuel to slow down by engine braking instead of putting it in neutral and hitting the brakes.

Ahh.

Much better.