Math Check

[chucktoo]

I am trying to size the jets for my 2D systems.

I have a 2.5l Subaru [ ultimately going to 2.8l ] with 4 850cc/mim injectors . I will not come close to maxing the injectors till many more mods are done but I assume yuo should size the Aqua-Mist for the injector max ?

Do you use 4 x 850 c for 3200 cc / min or 2 times since fuel is not going in durring exhaust stroke ?

Fro 4 times at 20% water/methanol that is 640 cc/min which sounds like a lot to me. At 2 times tha is 320 cc/mim or a 1mm jet that feels right ?

In the jet table they list 5 bar and race pump - I assume the race pump pressure is ~130 psi ? This is from [285/215] squared times 5 bar times 14.7 psi ?

Thanks Charlie

[chucktoo]

Since it a 4 stroke engine each injector is really only open ~25% of the time - actaully a little longer since you can open the injector before the valve opens. So total fuel flow rate is injector cc x .27 x number injectors or in my case 850cc/mim x .27 x4= ~900 cc/mim .

Since the HSV follows the injector timing it too is only open ~25% of the time but has to match the flow of all the injectors at the desired ratio of water/methanol to fuel.

900 x .2 / .27 = 660 cc/mim or 2 of the 1.0 mm jets which sounds like a lot to me.

Too bad the FiA2 can't monitor 2 fuel injectors at same time or just double the pulse lenght to the HSV so it could go up to ~50% - this would give more uniform flow over time ?

Richard have I missed something here ?

Thanks Charlie

[hotrod]

Injector flow is rated at "static flow" ie running wide open. On OEM applications they size the injectors so they do not exceed 80% of max flow at max engine rpm.

The key here is the injector duty cycle depends on the engine rpm.

At 6000 rpm, a single engine revolution only takes 10 milliseconds ( 0.01 seconds). Based on that at 6000 rpm there is only a 20 millisecond time period for the injector to do its work. Bottom line, at high rpm the injector is open for a very large fraction of 2 engine revolutions. At 80% duty cycle in an engine at 6000 rpm, the injector would be open ( .8 x .020 ) = .016 seconds and closed for only 4 milliseconds. On the Subaru Injectors they have a dead time of about 1.3 milliseconds, so the injector basically would go static at around 18.7 milliseconds open time or 93.5% duty cycle.

One of the advantages of larger injectors is they don't need as much time to flow the same amount of fuel, so they can get the job done at higher rpms when the injection window is much smaller.

If you size your system so it is capable of matching between 80% - 90% of the maximum continous flow of all four injectors you should be pretty well covered.

In your case of 4 (850 cc/min) injectors you would have:

(4 x 850) = 3400 x .90 = 3060 cc/min max safe fuel flow.

As a double check, you can use other measures to verify your capacity numbers.

On a turbo charged engine, it typically takes about .6 lb/hr of fuel flow to produce 1 hp ( at typical rich AFRs like 10:1). So you can take your expected peak hp and multiply by .6 and that will give you an estimate of the amount of fuel you will need to produce that power. Gasoline typically weighs about 6.5 lb/gal.


You need to make these calculations based on the engines gross hp output at the flywheel.

If your putting down 450 hp (at the flywheel) the engine would require about (0.6 x 450 ) = 270 lb/hr of fuel or 270/6.5 gal/hr = 41.5 gal /hr of fuel. If you mulitiply that by 63 you will get the approx max fuel flow in cc/min. or about 2617 cc/min of fuel flow to hit 450 hp.

With your max flow capability, your injectors would be running at 76.9% duty cycle at 450 hp, a very safe setup.

At 2617 cc/min, you could then size your WI based on that to be 20% of fuel flow, you would have to provide for around 525 cc/min through the WI system.

Hope that helps?

You might find this work sheet on injectors helpful.

http://www.rceng.com/technical.htm#WORKSHEET

Larry

[chucktoo]

Thank you -I now understand that you can be injection fuel even when the valves are not open at high speeds.

So for my 4 850 cc/min injectors I will need 2 1mm jets and 2 HSVs to provide the 20% water/methanol to fuel rato I am looking for. Can you run two HSVs off one FiA2 ?

I ended up buying 2 2D systems since they are on sale to get all the parts.

Thanks Charlie