some help guys... some maths dilema
06-13-2002, 09:07 PM
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some help guys... some maths dilema
i was just doing some maths...
it has occured to me:
at <b>5000 rev/min = 83.33333 rev/sec</b> (5000/60sec)
this means <b>0.012 sec</b> is needed for <b>1 rev</b>. (1/83.3333)
since there are 4 cycles to 1 complete engine rev, i understood it that you can only insert air and fuel when the piston is "moving down" and "not moving up". which is 1/2 time of 1 rev.
so if you split that time <b>0.012 sec</b> by 1/2, you get 0.006sec or <b>6ms</b> for which the injector can open at 5000rpm.
completley different from the link below, can someone clear this one up?
thx.<ul><li><a href="https://forums.audiworld.com/performance/msgs/42816.phtml">https://forums.audiworld.com/performance/msgs/42816.phtml</a</li></ul>
it has occured to me:
at <b>5000 rev/min = 83.33333 rev/sec</b> (5000/60sec)
this means <b>0.012 sec</b> is needed for <b>1 rev</b>. (1/83.3333)
since there are 4 cycles to 1 complete engine rev, i understood it that you can only insert air and fuel when the piston is "moving down" and "not moving up". which is 1/2 time of 1 rev.
so if you split that time <b>0.012 sec</b> by 1/2, you get 0.006sec or <b>6ms</b> for which the injector can open at 5000rpm.
completley different from the link below, can someone clear this one up?
thx.<ul><li><a href="https://forums.audiworld.com/performance/msgs/42816.phtml">https://forums.audiworld.com/performance/msgs/42816.phtml</a</li></ul>
06-14-2002, 04:51 AM
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Your math appears correct. ECU pulses the injectors regardless of whether the intake valves...
...are open or closed. A small amount of fuel *may* accumulate on top of the intake valve prior to opening, but the airflow is high enough at that rpm to negate most of the downside.
06-14-2002, 09:02 AM
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Math is not correct. There are only 2 cycles to 1 crank rev in a 4 cycle engine
He needs to multiply 12ms by 2 which is 24ms. That's the maximum possible time the injectors can stay open at 5000rpm.
06-14-2002, 10:55 AM
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Your are quite correct. 12 ms per rev or 24 ms per 2 rev cycle.
Inlet valves are off their seats more than 180 crankshaft degrees. (typically about 250-300 degrees total, but effectively more like 200-230 degrees). Nothing says the injector can only fire into the inlet manifold during the time when the valve is open. Perhaps that's what confused 911.
FWIW: Constant flow injection spits fuel all the time.
As dingster said, the limit is 100% firing, or 24 ms @ 5000 (or 16 ms @ 7500). Obviously you'd need an injector capable of spitting out enough fuel in something less than 24 ms for the engine to produce max power at or above 5000. That's generally what's done.
Injector size (capacity) is a compromise of max power requirements and low flow (idle, etc) requirements. When you modify the engine enough to max out the stock injectors, you need to put in "bigger" ones. Too big and you get into idle and part throttle problems.
FWIW: Constant flow injection spits fuel all the time.
As dingster said, the limit is 100% firing, or 24 ms @ 5000 (or 16 ms @ 7500). Obviously you'd need an injector capable of spitting out enough fuel in something less than 24 ms for the engine to produce max power at or above 5000. That's generally what's done.
Injector size (capacity) is a compromise of max power requirements and low flow (idle, etc) requirements. When you modify the engine enough to max out the stock injectors, you need to put in "bigger" ones. Too big and you get into idle and part throttle problems.
06-14-2002, 12:11 PM
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ya i made a mistake there, 1 rev = 2 cycle, 2 rev = 4 cycle...
the cycles as i understand..
1) inlet valve opens - piston moves down.
2) inlet valve close - piston moves up to compress
*spark
3) piston moves down due to explosion
4) outlet valve open - miston moves up to eject exhaust
there is 2 rev to this one complete cycle. time taken to do 2 rev at 5000rpm = 24ms. only 1/4 of that time the inlet valve is open to let air and fuel which is <b>6ms</b>
so why is it that the link above says 24ms for the fuel injector pulse? (i assume thats the time the injector stays open.
is it that the injector stays open even if there is no airflow? i.e the valves are close???
1) inlet valve opens - piston moves down.
2) inlet valve close - piston moves up to compress
*spark
3) piston moves down due to explosion
4) outlet valve open - miston moves up to eject exhaust
there is 2 rev to this one complete cycle. time taken to do 2 rev at 5000rpm = 24ms. only 1/4 of that time the inlet valve is open to let air and fuel which is <b>6ms</b>
so why is it that the link above says 24ms for the fuel injector pulse? (i assume thats the time the injector stays open.
is it that the injector stays open even if there is no airflow? i.e the valves are close???
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06-14-2002, 01:43 PM
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You might try this link. It starts of with the basics but then gets..
more detailed.
When you get to "How Fuel Injection Systems Work", click on that and follow along. It's quite well done, IMO.<ul><li><a href="http://www.howstuffworks.com/engine.htm">http://www.howstuffworks.com/engine.htm</a</li></ul>
When you get to "How Fuel Injection Systems Work", click on that and follow along. It's quite well done, IMO.<ul><li><a href="http://www.howstuffworks.com/engine.htm">http://www.howstuffworks.com/engine.htm</a</li></ul>
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