Drawthrough & Meth/Water Injection
12-22-10, 05:31 PM
#52
Well, I mean, yeah, if the compressor is out of its efficiency range that will rob power through higher IATs, or if the hotside is too restrictive that will rob power through increased exhaust backpressure. But by going with a bigger turbo it may (or may not) give you a less desirable powerband. If you can find a compressor map for your turbo that will help give you an idea.
My target is 10psi... thus a pressure ration of 1.68
12-22-10, 06:05 PM
#53
My target is 10psi... thus a pressure ratio of 1.68
My engine's max air flow = Air flow equals: (Cid X RPM X 0.5 X Ev) / 1728 = (70cu X 8000rpm x 0.5 x 0.80) / 1728 = ~130 CFM
(or 162 CFM with 100% efficiency)
1 cfm = 0.028 m3/min
Thus 130 cfm = 3.68 m3/min
(or 162 cfm @ 100% efficiency = 4.59 m3/min)
So.... plugging these numbers into my turbos flow map looks like this.... is that good?
My engine's max air flow = Air flow equals: (Cid X RPM X 0.5 X Ev) / 1728 = (70cu X 8000rpm x 0.5 x 0.80) / 1728 = ~130 CFM
(or 162 CFM with 100% efficiency)
1 cfm = 0.028 m3/min
Thus 130 cfm = 3.68 m3/min
(or 162 cfm @ 100% efficiency = 4.59 m3/min)
So.... plugging these numbers into my turbos flow map looks like this.... is that good?
12-22-10, 06:23 PM
#54
I think your max air flow value (CFM) is incorrect. That formula works fine for a 4 stroke reciprocating engine, but a rotary does not follow the same line of thought. Consider the fact that intake, combustion and exhaust are all happening at the same time in the rotor housing at all times essentially. So the engine more closely resembles a 2 stroke in it's method of power delivery. Using a formula for a 2 stroke may prove more viable. Sterling, as in Sterling Carbs posted some snippets about determining VE and flow for a rotary engine as well. But if memory serves me it's closer to the 350-400 cfm range at 10k rpm and 100% VE (he used those numbers for easy math) thusly he determined that his carb (which is capable of 450 or so CFM is plenty big enough for a stock to streetport 12a to quiet the neh sayers saying to use a 600 Holley.
A little off topic, I know. But the idea here is that your figures are off, and that's also not considering that VE increases exponentially on a rotary when you start adding boost. Keep in mind, there are no valves to get in the way of boost, so as soon as the port opens it's a hailmary into the rotor housing.
I say you should triple that CFM value and you'll be a little closer. But those compressor maps do not take into account the fuel that your are compressing through the impeller housing, which will definitely impact efficiency.
A little off topic, I know. But the idea here is that your figures are off, and that's also not considering that VE increases exponentially on a rotary when you start adding boost. Keep in mind, there are no valves to get in the way of boost, so as soon as the port opens it's a hailmary into the rotor housing.
I say you should triple that CFM value and you'll be a little closer. But those compressor maps do not take into account the fuel that your are compressing through the impeller housing, which will definitely impact efficiency.
12-22-10, 06:57 PM
#55
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Not a terribly useful map, unfortunately. There's only one efficiency line and it's not labelled. Also, it states airflow by volume and not mass, which is annoying. But let's just take it at face value.
At 10 PSI it looks like the efficiency line is around 11 m^3/min, or about 388 CFM. At 7500 RPM and 90% VE and 10 PSI boost a 12A will be sucking in about (1.146*2*90*1.68)/5660=459.2 CFM. Oops! You're beyond that efficiency line, whatever it is.
Too bad he never posted his reasoning for wanting to use such a big pressure drop for his carb sizing calculations.
12-22-10, 10:58 PM
#56
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It has a much higher latent heat of vaporization than gasoline, and you're putting a lot more of it into the motor to achieve the same lambda. Result is ridiculously low IATs. Combine that with its higher octane rating, and you see why you'll typically run out of fuel system flow long before you see any knock readings.
12-23-10, 12:23 AM
#57
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Latent heat of vaporization is exactly that: the heat required to vaporize the fuel. Has nothing to do with combustion. Doesn't matter (much) where you inject it, as it's going to get vaporized basically as it's injected. That lowers the temperature of the air going into the motor.
Not sure what you mean by "pass by the IAT", unless you're talking about an IAT sensor, which isn't used in a carb setup.
Correcting my typo in case someone was confused by my math.
Not sure what you mean by "pass by the IAT", unless you're talking about an IAT sensor, which isn't used in a carb setup.
Correcting my typo in case someone was confused by my math.
12-23-10, 01:20 AM
#58
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Latent heat of vaporization is exactly that: the heat required to vaporize the fuel. Has nothing to do with combustion. Doesn't matter (much) where you inject it, as it's going to get vaporized basically as it's injected. That lowers the temperature of the air going into the motor.
Not sure what you mean by "pass by the IAT", unless you're talking about an IAT sensor, which isn't used in a carb setup.
Correcting my typo in case someone was confused by my math.
Not sure what you mean by "pass by the IAT", unless you're talking about an IAT sensor, which isn't used in a carb setup.
Correcting my typo in case someone was confused by my math.
And obviously carbs don't have IAT's sensors, but people that have done this before DO, and you wouldn't be able to say anything for certain were it not for IAT sensors.
^speaking of which, water injection is documented to do little to the IAT's, but does SIGNIFICANTLY increase knock resistance. So I'm not understanding why the high latent heat of vap. decreases IAT's for fuels (meth/eth etc) but not water.
12-23-10, 02:42 AM
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The latent heat of water is also much higher than gasoline, but that doesn't matter till it hits the combustion chamber. How can we apply different logic to e85? i'm asking.
And obviously carbs don't have IAT's sensors, but people that have done this before DO, and you wouldn't be able to say anything for certain were it not for IAT sensors.
^speaking of which, water injection is documented to do little to the IAT's, but does SIGNIFICANTLY increase knock resistance. So I'm not understanding why the high latent heat of vap. decreases IAT's for fuels (meth/eth etc) but not water.
And obviously carbs don't have IAT's sensors, but people that have done this before DO, and you wouldn't be able to say anything for certain were it not for IAT sensors.
^speaking of which, water injection is documented to do little to the IAT's, but does SIGNIFICANTLY increase knock resistance. So I'm not understanding why the high latent heat of vap. decreases IAT's for fuels (meth/eth etc) but not water.
Search Howard Coleman's name. He's got some great posts on here comparing water and alcohol as auxiliary injectants.
12-23-10, 03:05 AM
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You're correct that water injection won't significantly lower IATs. This is because it doesn't actually vaporize in the intake tract. Typical intake temps in boost are well below 100 degrees Celsius. Water works its magic as a knock suppressant inside the chamber itself as the charge is compressed and then ignited. Until then, it's mostly just suspended in a fine mist. Alcohol, on the other hand, does vaporize as it's injected, thanks to its low boiling point. As it does, IATs drop drastically.
Search Howard Coleman's name. He's got some great posts on here comparing water and alcohol as auxiliary injectants.
Search Howard Coleman's name. He's got some great posts on here comparing water and alcohol as auxiliary injectants.
I'm well aware of what DOES happen. We're more discussing the why.
12-23-10, 11:06 AM
#61
Don't plan on trying to use e85 and couldn't source it locally if I wanted to. I appreciate you'all participating in this thread, but, it's not about e85.
81WideMariah & PercentSevenC, thanks for your input on my turbo sizing question. I really haven't decided what route i'm going yet. Ideally a t04 or similar sized turbo, but, the prices are scaring me away.. lol
Hell, even sourcing a s5 turbo is getting to be an expensive endeavor
81WideMariah & PercentSevenC, thanks for your input on my turbo sizing question. I really haven't decided what route i'm going yet. Ideally a t04 or similar sized turbo, but, the prices are scaring me away.. lol
Hell, even sourcing a s5 turbo is getting to be an expensive endeavor
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