Calculate AI flow based on %Fuel volume flow (cc/min) or mass flow (lb/hr)?
 

I posted this question elsewere but got ignored so here it is again.

I've seen people calculate AI flow based on target HP, BSFC and either cc/min or lb/hr fuel flow, which way is the right one?

gasoline, water and alcohol all have different densities so mass lb/hr of each is not the same when converting to cc/min.

Using these values:
gasoline = 6.073 lb/USgal
water = 8.32 lb/USgal
USgal = 3785 cc

Example for a 450 flywheel hp rotary using 15% water only as AI.

bsfc = 0.64 (some people use 0.62)

450 x 0.64 = 288 lb/hr gasoline

mass flow calculation: 15% of 288 = 43.2 lb/hr water needed
43.2 lb/hr water = 327.5cc/min

volume flow calculation: 288 lb/hr gasoline = 2991.6 cc/min gasoline
15% of 2991.6 = 448.7 cc/min water

It makes a significant difference how we calculate it!

SO which one is it?

I'm not claiming to be an expert but I don't think there is necessarily a "right" way. Optimal AI flow is an empirically-derived number, 15% is only a rule of thumb. I think a lot of people are just experimenting to see how much they can put in and still make their power goals. What you might be looking for is what that 15% rule of thumb refers to, either a mass ratio or volume ratio. I couldn't tell you that. I think most people will tell you to start somewhere around those numbers, vary it, and see what your results are. Otherwise you can look at studies or other peoples' experiences and determine your flow based on their observations.

I would like to hear what other input there is on your question (and my answer hehe).

yeah Im not worried about the 15% as some people experiment with different ratios and with alcohol they go even higher. My question is, as you mentioned, % of what? mass or volume?

I think you need to multiply the mass ratio of ..... when you represent mass flow by volume: water/gasoline = 8.073/6.073

if I do that with the mass calculation result of 327 I end up with the volume result....

327 x 8.32 / 6.073 = 448 ???????????????????

these guys just use mass calculation:

http://www.rbracing-rsr.com/waterinjection.html

i completely agree w post 2.

take your pick. get tuning and see where it leads. most scientists/engineers use mass. i use CCs as we all think in CCs when we consider our base fuel injectors.

as far as how much of course it depends on your objectives. i run alcohol which is superior to gasoline so the more alcohol, for me, the better. i run 1400 CC.

the Langley Air Force 1942 water report showed them running a ton of water at high boost and hp levels. they spoke in terms of mass.

hc

I calculated how much methanol to inject based on mass flow and my fuel injector pulse width (my fuel map), fuel pressure, injector size and the percentage of methanol I wanted to inject. This got me close to my solenoid duty cycles I needed when setting up my methanol injection map.

Forget using the BSFC, use your actual fuel injector pulse widths, fuel pressure, fuel injector size (measured flow if you have it), etc. Your BSFC is just a guideline and unless you actually KNOW the value, it is just a gross approximation.

If you are calculating the flow, you use mass and not volume.