BMEP of Wankel Rotary Engine?

[CarmonColvin]

First of all I am NOT an engineer so please forgive me if I use the wrong terminology or similar laymen mistake.

I am trying to calculate the BMEP (Brake Mean Effective Pressure) of a Wankel rotary engine. Searching the net I have found one formula that is slightly different for a 4-stroke and 2-stroke type of engine.

2-stroke BMEP = (HP * 6500) / (L * RPM)
4-stroke BMEP = (HP * 13000) / (L * RPM)
source:

http://engineersedge.com/engine_formula_automotive.htm

The only difference between the two formulas are the numbers 6500 and 13000. The only correlation with a 2x difference between the two types of motors are the 2-stroke motor uses 100% of is displacement for combustion during one revolution where a 4-stroke motor uses 50% of its displacement for one revolution.

The only way I can make this work between those formulas is to look at it this way.


BMEP = (HP * (6500 / DU%) ) / (L * RPM)
Using the Displacement % used as DU%

OR

2-stroke BMEP = (HP * (6500 / 1.0)) / (L * RPM)
4-stroke BMEP = (HP * (6500 / .5)) / (L * RPM)

A Wankel Rotary engine uses 33% of its displacement for combustion for one revolution output from the motor.

Wankel Rotary BMEP (HP * (6500 / .333)) / (L * RPM)

IS THIS A CORRECT ASSUMPTION?

Data from an actual dyno run (measured at the wheels) of a slightly modified Mazda 13b (1308cc) non turbo motor put out 168 HP at 7000 RPM and 147 ft lbs of torque @ 4000 RPM.

BMEP = (168 * (6500 / .333)) / (1.308 * 7000)
BMEP = (168 * 19500) / 9156
BMEP = 3276000 / 9156
BMEP = 357.8
This number seems really high compared to other BMEP readings I have seen for 4 and 2 stroke engines.
And am I correct in assuming this is in pounds per liter?

If this is completely wrong please help me with a formula that will accurately give me BMEP of a Rotary Engine.

Thanks.

Carmon Colvin

http://www.ThePartsTrader.com

 

[EngJW]

Carmon,

This forumla is good for any 4-stroke engine (which a rotary engine is):

bmep = 792000 P/(VN)

where P is power, V is total engine cubic inches, and N is rpm. Bmep is psi.

Displacement for a rotary engine is the displacement of one chamber (654cc for a Mazda) times the number of rotors, times 2 to make rotary displacement equivalent to reciprocating displacement. Don't forget to convert to cubic inches.

You should get in the area of 80-120 psi for a non turbocharged engine. This is the range for reciprocating engines, but I think a rotary engine will fall toward the low side because of port limitations and less efficient cooling.

John Woodward

 

[RennaissanceBoy]

I'm a 16 year old aspiring auto engineer. I have been absolutely fascinated with rotary engines ever since my cousin took me for a ride in his 310 hp RX-7, which you can imagine what that was like.
Yes we of course also must take in to consideration the rotary's poor cooling. I have done some testing with the torque of a free flight model airplane engine and noticed you guys might be able to help me. Like I said I am riveted with the calculus of a rotary and need to know more about it. I know the essentials but am thirsty for some more details. I really appreciate it, thanx. -Jack

 

[EngJW]

Jack-

Contact me at jlwoodward@worldnet.att.net

I have lots of notes on rotary engine design and you are welcome to what I have. I also know of several books and many SAE papers.

A few years ago I worked for Teledyne Continental Motors and started off in their rotary engine program. It never went anywhere because it was horribly mismanaged. We also noticed the cooling difficulties you mentioned. What model airplane engine do you have? I have the rotary engine on my bookshelf. I never tried to run it, though.

John Woodward

 

[Tacitus]

Dear Staff:
Have wankel engines been stacked in a system similar to that used by rotary piston engines on WWII bomber
engines?

 

[evelrod]

Strictly a pedestrian point of view but, yes. I have seen several Mazda test and production, multi chambered, rotary engines being tested. A couple were being setup for aircraft application.

Rod

 

[EngJW]

Two chambers are typical (RX7) and Mazda has used 4 for a racing engine. With more than 2 it becomes increasingly difficult to provide a bearing and support the crankshaft in the intermediate housings.

John Woodward