why does the injection time, rise with RPM??

by **settra** » Sun Jan 26, 2014 12:33 pm

Hello guys! i am trying really hard to understand something... when you are trying to achieve stoichometric, you an air-fuel ration of about 17:1 (or whatever is the correct).
i get that, to inject the correct amount of fuel, the injector has to stay open for certain time.
and that in higher RPM the injector , Opens, more frequently.
BUT, as i see in injection maps, the injection time, also rises with RPM. why is that?? shouldn't the fuel needed, be a variant , only of the air induced???
(sorry if it is an extremely noobic question :P )

by **biela** » Mon Jan 27, 2014 8:10 am

Which injection maps are you speaking about?
In which ECU?

by **settra** » Mon Jan 27, 2014 12:47 pm

i dont have something specific in my mind.... i study how ECU works, because i am in the processes of making one (microprocessors and everything) , and i think i have seen somewhere that injection time ,rises with RPM...

i think i might be starting to get it... engine load, is the Actuall, air , comming into the engine. BUT diferent sensors, mesure it, in diferent ways.
so if you use a MAP sensor, the engine, takes into acount RPM and MAP input, and assumes engine load.
BUT if you use ,MAF sensor, then you have actaull air flow readings, so you dont need the rpm, to determine the engine load...
am i correct??

by **biela** » Mon Jan 27, 2014 5:23 pm

May be you are speaking abouy "Injection duty cycle"?

by **settra** » Mon Jan 27, 2014 5:30 pm

nope. duty cycle , rises anyway, since they have to fire more often...
so, injection time, dosent have to rise, with rpm ??

by **xsashox** » Sun Feb 02, 2014 2:40 pm

I dont really understand what you are asking about.. As i understood from your question, you are looking at injection duration?

First off you have to clarify something; Injection duration is controlled by rail pressure and IQ (on a diesel, EDC15). Therefore, you have a rail map, which is used by the ECU to check what rail pressure is used at X rpm to inject X IQ.

Since the crankshaft is traveling faster at 3500RPM than at 2000RPM, injector has a limited amount of time to inject the same amount of fuel in the engine.

Example:
BMW E39 530d -

45mm3 at 2000RPM is 984bar
45mm3 at 3500RPM is 1320bar

Injection duration for
45mm3 at 984bar is 799usec
45mm3 at 1320bar is 671usec (about that, i have the 1000bar and 1350bar values)

Which means:
CADeg: 360°C (1 RPM)
799usec = is about 0,26RPM or 9,5° CADeg @2000RPM
671usec = is about 0,39RPM or 14°CADeg @3500RPM

So, injection duration only tells the ECU how long he has to keep the injector open to inject a proper value of diesel. Dont be confusing the AFR with duration, its a different thing. Smoke maps take care of AFR.

by **bastunas** » Wed Feb 12, 2014 2:10 am

As RMP rises, engine injects more fuel. To inject more fuel you need more time (injection starts earlier before Top Dead Center, or more praesure to inject more per same time.
Try read here, realy nice explanation
https://sites.google.com/site/vagecumap ... ne-control
in first topic 1. Diesel engine control

by **zok** » Thu Feb 20, 2014 9:52 am

Injection duration [in microseconds] doesn't rise (much) because fuel rail pressure rises, and fuel flows quicker - so more fuel is injected at a specific duration.

Injection duration [in degrees] rises because of increased angular velocity of the engine.

One minute doesn't extend either, but you can measure 800 revolutions in a minute at idle and 3000 revolutions in a minute when driving - that is a similar thing. :)