BMEP of Wankel Rotary Engine?

[ Follow Ups ] [ [ Home ][ Forum Archive ] [ Active Engineering Forum ] [ Design & Engineering Data ]

Posted by Carmon Colvin (12.166.72.98) on November 18, 2002 at 17:12:57:

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: /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

< "> Subject: Re: BMEP of Wankel Rotary Engine?