Performance and Tuning Discussion forum for various performance tuning techniques and questions

some help guys... some maths dilema

Thread Tools
 
Search this Thread
 
Old 06-13-2002, 09:07 PM
  #1  
New Member
Thread Starter
 
Agent911's Avatar
 
Join Date: May 2001
Posts: 424
Likes: 0
Received 0 Likes on 0 Posts
Default 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>
Old 06-14-2002, 04:51 AM
  #2  
New Member
 
Electron Man's Avatar
 
Join Date: May 2000
Posts: 184
Likes: 0
Received 0 Likes on 0 Posts
Default 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.
Old 06-14-2002, 09:02 AM
  #3  
Member
 
dingster's Avatar
 
Join Date: Mar 2000
Posts: 3,348
Likes: 0
Received 0 Likes on 0 Posts
Default 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.
Old 06-14-2002, 10:55 AM
  #4  
Member
 
smallTTs's Avatar
 
Join Date: Apr 2000
Posts: 4,008
Likes: 0
Received 0 Likes on 0 Posts
Default 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.
Old 06-14-2002, 12:11 PM
  #5  
New Member
Thread Starter
 
Agent911's Avatar
 
Join Date: May 2001
Posts: 424
Likes: 0
Received 0 Likes on 0 Posts
Default 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???
Old 06-14-2002, 12:13 PM
  #6  
New Member
Thread Starter
 
Agent911's Avatar
 
Join Date: May 2001
Posts: 424
Likes: 0
Received 0 Likes on 0 Posts
Default

ya so.. injector stays open regardless of the valve? i c...
Old 06-14-2002, 12:15 PM
  #7  
Member
 
dingster's Avatar
 
Join Date: Mar 2000
Posts: 3,348
Likes: 0
Received 0 Likes on 0 Posts
Default

Yes, injector can stay open when valves are closed
Old 06-14-2002, 01:09 PM
  #8  
New Member
 
Electron Man's Avatar
 
Join Date: May 2000
Posts: 184
Likes: 0
Received 0 Likes on 0 Posts
Default

Thanks, hadn't had my coffee yet.
Old 06-14-2002, 01:43 PM
  #9  
Member
 
smallTTs's Avatar
 
Join Date: Apr 2000
Posts: 4,008
Likes: 0
Received 0 Likes on 0 Posts
Default 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>
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
skiwi
A8 / S8 (D2 Platform) Discussion
4
05-01-2007 12:08 AM
kazino21
Audi A3 / S3 / RS 3
1
04-11-2004 01:09 AM
HarnessedLunacy
S4 (B6 & B7 Platforms) Discussion
5
03-25-2004 08:43 AM
Agent911
Performance and Tuning
2
06-17-2002 10:23 PM
zafrir
Performance and Tuning
13
08-02-2001 04:22 AM



Quick Reply: some help guys... some maths dilema



All times are GMT -8. The time now is 11:28 AM.