How do you calculate the airflow rate (in cfm) for a rotary engine? I know how to do it for a piston engine, but...

The end result of all this is I am trying to figure out how to size a turbo/supercharger for our engine. Also, does anyone know the Volumetric efficiency for a 13B of the top of their head? Thanks much.

 

On a 2 rotor you base it on a 2.6 liter engine. Since the rotary makes about 10 hp per pound of air and a piston engine makes about 7 hp per pound of air used, you need to compensate for this. This means you need to factor in about 1.43 times 2.6. This equals a 3.71 liter engine in terms of airflow useage on the intake side. It's pretty close.

 

How or where do you get these figures. Is this backwards, this doesn't explain why a rotary engine needs such a huge turbo to make power.

 

I'm pretty sure the 10hp per lb of air thing is fuel dependant. It's a theoretical energy content and stoichiometric ratio thing. In an ideal world ~10 hp/lb air is what you can expect from any engine running gasoline with %100 Volumetric Efficiency. If it make less than this it can be attributed to the engine running less than %100 VE.

 

well old world used Cubic feet per minute or CFM , but the more realistic way is in pounds per minute, or LBS./HR. but us regular guys just try things, a rotary N/A can handle 650CFM 4 barrel, a turbo 13B can handle 1000 or more. dont forget there aint no valves in the way to restrict air flow, valves being the most restricting part of a piston unit. anyway things today are starting to use weights for both air and fuel, GM uses for fuel GRAMS per second or minute and it probably is more accurate, and GM is winning, with CADILLAC, CORVETTE, and that pesky upstart 4 cyl. ECOTEC GEN ll engine. Ron

 

if you go by the old world teachings! I say a rotarys VE is 200%

 

1000 CFM looks doable.

 

Oops. Good spot. Swap that. The rotary makes about 7hp per pound of air and a typical piston engine makes about 10 hp per pound of air used. I was tired. These are based on average street engines. This can drastically change with more aggresive timing and other things. They are approximations.

 

Well, this needs some clarification:

For example, referring to the chart posted above, the 324CFM figure at 7000 is correct for what the engine can move assuming 100% volumetric efficiency. The engine is nothing more than a constant volume air pump. VE's greater than 100% really do not exist, so the max CFM through the engine at any given RPM and 100% VE is going to be the 0 PSI figure in the chart. I know this seems counterintuitive, but remember you're dealing with a fixed space in the engine. It doesn't matter if you have 60 PSI of boost or -20hg of vacuum, you are only going to get 324 cfm through the engine (at 7000 RPM) period.

What does change is the density. A cubic foot is a cubic foot regardless of whether it's at -20Hg or +50 PSI. This is why we calculate air (and fuel) based on mass (e.g., pounds per hour, etc.). It has always been done this way at the engineering level. The 1000 CFM figure cited above is, at best, a rough figure at the compressor inlet. Problem is, depending on a number of variables including ambient conditions, compressor efficiency, intercooler efficiency, volumetric efficiency, etc., that RPM vs. CFM number can vary widely. Calculating mass flow allows us to correct for varying conditions and standardize the results into something meaning for comparison.

I ran a few calculations and determined that it would take about 4140#/hr or about 920 inlet CFM (at standard conditions) to produce 575 BHP, which should be at or over 500 WHP. This assumes a 0.6#/hr/hp BSFC, a typical figure. Note that this is about 7.2# of air per HP (not the other way around). The amount of power that an engine can extract from one pound of air is really a question of mechanical and thermal efficiencies at a given operating point. Note that seems to make the engine that needs 10# of air per HP less efficient (assuming the same AFR). Also note that a higher charge density entering the combustion chamber of the engine will result in increased CFM flow at the compressor inlets, all else being equal. This is an important point--please give it some thought.

So, yea, the 1000 standard CFM (~4500#/hr) is doable. It equates to about 31 PSI of boost at 7000 RPM assuming 100% density recovery, 100% VE and no plumbing losses. Factor in estimated losses and that figure is going to be higher. More like 37+ PSI boost.

Hope this helps.

 

How the hell am I getting my numbers so mixed up? I need to go find my airflow testing notes. Now I'm a bit perplexed.

 

I went back and looked at the airflow notes from engine dyno testing during my friend's Renesis supercharger development. The numbers show that the test engines make anywhere between 7.5- 8.5 hp per pound of air used depending on what engine was tested. The Renesis uses around 8.5 hp per pound of air. Admittedly these were not corrected numbers but they won't be that far off.

 

OK I've pulled up some more numbers. It takes 14 cubic feet of air (1728 cu in) to equal 1 pound of air at sea level pressures on a 60*F dry day. Standard correction temperature. If our engine can flow 324 cfm at 100% efficiency at 7000 rpm (or any rpm for that matter), that means it is flowing 324/14= 23.14 pounds of air per minute. If the engine makes, lets just say 200 hp, that equals 8.64 hp per pound of air. Those are just random numbers to show the math. The actual numbers I have were from a mass air flow sensor on an engine on an engine dyno and they worked out to a stock Renesis making about 8.5 hp per pound of air max.

 

Bring it back.

Anyone have any new info over the past 6 years?

 

umm

13b NA s4 AFM maxes at 600 cube metres per hour ( 36.6 cu-in/hr )

and 13b turbo s4 AFM maxes at 720 cube metres per hour ( 43.9 cu-in/ hr )

cause someone bothered to share the N318 training manual.....

 

Thanks, That was nice because I just picked up an S4 turbo!



Ben