Sizing Injectors for high reving motors

[whittlebeast]

Here is a calculator I put together for calcing the max allowable Duty Cycle of the injectors. The typical calculator that you find on the net does not account for high reving motors.

http://www.nbs-stl.com/MLVDemo/Injector%20Max%20DC.xls As an example 8000 RPM gives 80% 9000 RPM gives 77.5%

This sheet works by allowing the injector enough time to reset after the fueling event. This is the DC that you feed into a calculator that you use in the RC Engineering calculator.

http://www.rceng.com/technical.aspx

Estimate your crank HP.

Number of injectors is obvious.

Brake specific fuel consumption of .6

Use the max Duty Cycle from the spread sheet above for the Max RPM you plan to run

43.5 PSI Fuel Pressure as this is really the difference between your absolute pressure in the intake compared to the absolute pressure in the fuel rail.

Keep in mind, your tune must be set up for whatever injectors you choose. This is not just a size thing but injector size is the largest part of this setup in the ECU.

If you don't have Excel at home, here is a screen shot of the Sheet

Maximum acceptable duty cycle to allow reliable fueling

Injector Reset Time (ms)
________3__________

________RPM____Max Allowable Duty Cycle
________6000_______85.0%
________6200_______84.5%
________6400_______84.0%
________6600_______83.5%
________6800_______83.0%
________7000_______82.5%
________7200_______82.0%
________7400_______81.5%
________7600_______81.0%
________7800_______80.5%
________8000_______80.0%
________8200_______79.5%
________8400_______79.0%
________8600_______78.5%
________8800_______78.0%
________9000_______77.5%
________9200_______77.0%
________9400_______76.5%
________9600_______76.0%
________9800_______75.5%
_______10000_______75.0%

In MLV I use the formula Max Allowable DC = 100*((120000/[RPM])-3)/(120000/[RPM])

Andy

[dontz125]

Reset time? How is this different from dead time, and where would you get injectors with a 3ms dead time?!

Barring F1, high-revving engines tend to be smaller (I think the highest-reading tach I've seen on a production engine was 22,000 RPM on a CBR250), so smaller or mid-sized injectors with sub-1.0 ms injectors should be readily available.

[whittlebeast]

Dead time is the opening time - closing time

Lets say the opening time is 2 ms and the closing time is 1 ms, the calculations think your motor wants 10 ms of spray time.

at time 0 the injector goes to ground
at time 2 ms the injector starts to actually spray
at 11 ms the injector looses power and the field starts to discharge.
at 12 ms the injector stops spraying fuel for 10 ms of effective spray time.

The ECU commanded time is 11 ms but the spray time is actually 10 ms. You lost 1 ms of spray time that we arbitrarily call "dead time" for lack of a better name.

Now lets assume the rpm is 10000 rpm. The entire 720 degrees of crank rotation takes 120000/10000 = 12 ms We need to start the entire sequence again in 2 ms and we are getting close to running out of total cycle time.

I made up the name "reset time". Whats a better name? We already use the name "dead time"

It appears that about 3 ms of "reset time" is what the industry standard is. That works out to 85% DC at 6000 rpm or 3 ms of "reset time".

It is similar to shooting guns fast. The cycle time is tied up in moving your finger, not the time of sending the bullet down the barrel.

Andy

[whittlebeast]

Here is a page out of the Manual

http://www.msextra.com/doc/pdf/html/Meg ... .4-38.html

I contend that the Max duty cycle is driven by the time till this sequence starts over.

[ewflyer]

Do you think it would be accurate to say that today's Super Sport street motorcycles represent state of the art development for road-going high revving engines?

If it can be agreed that these engines do set the standard, then 2 injectors per cylinder must be the correct solution to the question of correct fuel injector sizing for high revving engines. Each cylinder gets one injector downstream from the throttle body butterfly and a second one in the airbox, aimed into the cylinder's intake stack.

[whittlebeast]

Staged injection opens a new bag of worms.... Still, you can not run too high of a duty cycle before you start running into trouble. My formula assumes 1 injector per cylinder and one squirt per engine cycle.

Andy

[whittlebeast]

Here is what it looks like in MLV. This motor is right on the edge at WOT.

I use the formula 100*((120000/[RPM])-3)/(120000/[RPM])

http://www.nbs-stl.com/MLVDemo/975%20Du ... eserve.png

Andy

[elaw]

I can think of another solution... it would require a firmware change but probably a pretty simple one: alternating injectors. Two injectors on each port, not staged but alternating meaning on one cycle one fires, then on the next cycle the other fires, and so on. Tuning could be tough... since they'd be in different locations you'd probably get a somewhat different AFR from each injector.

[prof315]

Good point Andy....Over the summer one of my customers switched from motorcycle ITBs to Jenvey DCOE replacement throttle bodies on his racecar engine that turns 9000 rpms. The engine builder and I worked with the folks at Marren Fuel Injection to select appropriate injectors and rpm was indeed one of the factors we used in choosing what we wanted.

[jsmcortina]

I think Andy's really onto something here.

James

[muythaibxr]

IMHO if you're going to be running a high-rev engine, staged injection (table-based) is the best option.

Configured properly with the injectors in the right places, and used with table-based staging, you can get the low-rev low-load drivability of small injectors with the ability to scale up to high rpm that larger secondaries give you without any weird stutter issues when the secondaries engage.

Most of the higher-revving car and bike engines out there use this method from the factory.

The Mazda RX7 and RX8 series of engines do this, and most high-revving/high power bike engines... (Ducati 1199 and 1299 Panigale to name a couple).