Injector placement for starting

[AxelRipper]

Our engine guy is looking for opinions about the placement of the injector on our car.

What our thinking is, since we've been having troubles with the hot start of the engine (stroked CRF450x), if we were to move the injector closer to the intake port, then it would start easier. I know that having the injector farther away helps with fuel atomization and will give more power, but you can't use that power if you can't start the engine.

Does anyone have any ideas on this? we've searched around and can't find anything regarding this.

[AxelRipper]

Our engine guy is looking for opinions about the placement of the injector on our car.

What our thinking is, since we've been having troubles with the hot start of the engine (stroked CRF450x), if we were to move the injector closer to the intake port, then it would start easier. I know that having the injector farther away helps with fuel atomization and will give more power, but you can't use that power if you can't start the engine.

Does anyone have any ideas on this? we've searched around and can't find anything regarding this.

[B Lewis @ PE Engine Management]

AxelRipper,

I don't think that the injector placement will necessarily improve you starting. Make sure that you still have some starting compensation active even with the engine is hot. Most engines still need additional fuel to get started. This is especially true if the fuel temperature has increased. The density of fuel changes with temperature and can effect the starting of a heat soaked engine. We have seen many occasions where hot-starting issues are cured by adding some additional starting compensation when hot.

Good luck.

[Kirk Feldkamp]

Can you post a picture of where you currently have the injector?

Cal also had *significant* issues with starting their CRF450X over the last few years. If you want to see what Honda came up with for injector positioning, take a look at where they put them on the 2009+ CRF450R's. I think it's pretty close to the valves, and essentially shooting directly at the back of the valves.

-Kirk

[Wesley]

The issue with your placement is that, at low rpm, your flow velocity is so slow your fuel falls out of suspension before it gets to the cylinder. It helps at high rpm, but your low-enine speed response and atomization will be poor and you end up with fuel puddling. Closer is generally better for starting.

[Hector]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by AxelRipper:
I know that having the injector farther away helps with fuel atomization and will give more power, but you can't use that power if you can't start the engine. </div></BLOCKQUOTE>

If that were true, wouldn't we still be running throttle-body injection instead of multi-port or direct injection? As I understand it, when the fuel sprays, it actually forms droplets downstream and "de-atomizes", if you will. There a whole class of wet flow benches that are clear and inject fuel into the plenum so you can see just how bad it is. And let me tell you - it's bad. The walls get smeared with fuel.

A couple years ago an engine judge at design recommended getting our injectors as close to the valve as possible, and injecting fuel early onto the intake valve to :
A)Cool the valve
B)Evaporate and atomize the fuel

Though I've heard some crazy stuff, even from design judges http://fsae.com/groupee_common/emoticons/icon_wink.gif

I'll second enriching your starting fuel - even when hot.

But I'm a lowly drivetrain guy, don't take it from me...

[AxelRipper]

here is our current intake setup. sorry for the crappy pictures that I snapped with my phone with me holding the intake in the car since the dyno engine is torn apart

http://sphotos.ak.fbcdn.net/hphotos-ak-ash1/hs493.ash1/26935_417137771759_648196759_5706531_6782331_n.jpg

http://sphotos.ak.fbcdn.net/hphotos-ak-ash1/hs493.ash1/26935_417137786759_648196759_5706533_6295703_n.jpg

Thanks for your input everyone. we've been getting conflicting inputs from different sources regarding tuning information, which I suppose you get when you ask people who tune dirtbikes and the like just from experience.

[AxelRipper]

pictoral post processing

[Wesley]

Well you've got two things fighting you in that case - it's a bit far to the cylinder and gravity is pulling your fuel droplets down to the bottom of the runner.

Perhaps if the runner stuck straight up like most of the four-cylinder guys it wouldn't be as much of a problem, but it seems with that horizontal setup you might try pushing the injectors a bit closer to improve atomization.

[oz_olly]

Our injectors are probably atleast halfway between where your injector is and your intake port. We haven't had too many starting issues. We went with the idea mentioned above in that we were trying to get the injector pointing at the back of the valve. I was speaking to some guys running the Aprillia v-twin engine at FSAE-Aus last year and they said their starting was really sensitive to injector placement, pretty much anywhere other than where Aprillia has it didn't work. In the end they came up with a pretty cool intake using standard plastic plumbing parts (they painted it black so it took a keen eye to recognise the plumbing).

Cheers

Olly

[SamB]

They're way too far back. In terms of atomisation, don't forget the different spray patterns available. Last year we used a set from magnetti morelli which output two cones to help increase atomisation. You can probably angle them to the flow also, again a lot of injectors have a flow angled to their central axis. Check out the mangetti morelli website for full specs but I'm pretty sure you can solve this problem through changing your injectors.

Sam

[RenM]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Hector:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by AxelRipper:
I know that having the injector farther away helps with fuel atomization and will give more power, but you can't use that power if you can't start the engine. </div></BLOCKQUOTE>

If that were true, wouldn't we still be running throttle-body injection instead of multi-port or direct injection? As I understand it, when the fuel sprays, it actually forms droplets downstream and "de-atomizes", if you will. There a whole class of wet flow benches that are clear and inject fuel into the plenum so you can see just how bad it is. And let me tell you - it's bad. The walls get smeared with fuel.
</div></BLOCKQUOTE>

Be Carefull, not to mix up the reasons.
The reason no one is using a throttle body injection anymore is because you will get a huge wall layer of unevaporated fuel in the airbox, that can be very bad for your fuel consumption as your overrun fuel cut off isnt going to work very well. Plus You will get uneven fuel distribution to the cylinders.

The reason direct injection is used is for several reasons:
First you can use stratified charge injection, that allows you to run a lot leaner.
Second your overflow losses wont have an effect on fuel consumption (very important for charged engines)
Third your evaporation enthalpy will be fully used to cool down your charge in the cylinder, allowing you to run a higher compression ratio .

Moving your injectors as far away from your intake valve as possible is good to increase your mixture preperation and to increase your charge mass, as you can use the evaportion enthalpy to cooldown the mixture. Your throttle response is going to get bad though.
Thats the reason why some engines have dual staged injection. You can have all the advantadges of an injector that is far away from the intake valve without the disadvantadges.

[Hector]

Good to know. Like I said, I'm just a drivetrain guy, and that makes a lot of sense.