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Re: BMEP of Wankel Rotary Engine?
Re: BMEP of Wankel Rotary Engine? -- Carmon Colvin	Post Reply	Top of thread	Forum

Posted by: Niko 04/10/2003, 13:49:43 Edit

: 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 First of all please forgive me for my little English i'm an italian wankelieer web surfer so.. Following your reasoning i believe the formula is: 4-stroke BMEP = (HP * (6500 / .5)) / (L * RPM) 2-stroke BMEP = (HP * (6500 / 1.0)) / (L * RPM) Wankel Rotary BMEP (HP * (6500 / 3)) / (L * RPM) cause 4-stroke have 1 combustion every 2 revolution 720? 2-stroke have 1 combustion every 1 revolution 360? Wankel have 3 combustion every 1 revolution 360? alias 4-stroke have 0.5 combustion every 1 revol. 360? 2-stroke have 1 combustion every 1 revolution 360? Wankel have 3 combustion every 1 revolution 360? because 4-strokes have 1 combustion every 2 revolution wankel 6 combustion every 2 revolution and supposing same hp form 4t and Wk (at same conditions, rpm, ...), Wk will have 1/6 BPEM of 4t alias 4t=0.5 Wk=3, these two values in the formula works reducing BPEM as i wrote. only an example for your car: BMEP = (168 * (6500 / 3)) / (1.308 * 7000) BMEP = (168 * 2166) / 9156 BMEP = 363888 / 9156 BMEP = 39.7 i do not add more, because i believe this task is too old, if not reply and i'll continue...... bye wankel friends.

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Re: Re: BMEP of Wankel Rotary Engine?
Re: BMEP of Wankel Rotary Engine? -- Carmon Colvin	Post Reply	Top of thread	Forum

Posted by: sarvesh 12/01/2002, 00:49:24 Edit

: How to plot/ calculate the pressure - crankangle diagram for engine ?

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Re: BMEP of Wankel Rotary Engine?
Re: BMEP of Wankel Rotary Engine? -- Carmon Colvin	Post Reply	Top of thread	Forum

Posted by: sarv 11/23/2002, 10:51:10 Edit

How to plot/ calculate the pressure - crankangle diagram for engine ?

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Re: Re: Re: BMEP of Wankel Rotary Engine?
Re: Re: BMEP of Wankel Rotary Engine? -- sarv	Post Reply	Top of thread	Forum

Posted by: Sander 02/26/2003, 05:20:38 Edit

: : How to plot/ calculate the pressure - crankangle diagram for engine ? Well, I'm not an engineer too, but do know somethings about rotary engines... Here's a formula how to caculate the volume of such engine... It follows : Excentricity X Radius X Width X 5.196. Excentricity about how much the excentricity is on the central shaft.. Lets say 15mm... Radius is the distance from the centre of the rotor. lets that one is 105mm . Width is the width of a rotor.That's let say 80mm.. (actual these are from the mazda 787B engine ) Calculate all measurements in cm! 1.5cm X 10.5cm X 8cm X 5.196 = 654.7ccm... Per Rotor chamber that is.. Since the displacement formula equals to the formula how to calculate the power of that engine, that is how it follow for calculating the power of an rotary engine... Although, it might only be an estimate of how much it produces. You still have to concider the compression-ratio and intake/exhaust size (and timing for creating the prevered power-curve..).. Displacement X presure per square cm. X central shaft Rotation per Second! The out come must be devided by ten, or alse you have an ten time more powerfull engine out come in theory. This a formula I picked up in a book from my school... Lets say : 8.7kg/cm (for a compr.rat. of about 9 to one)X 654,7 X116.6rotations per second / 10 = 66343Watts!!! Is about 66,3kW@7000revs/minute(90HP). Note : per Rotor... If you have a two-rotor engine you have, obviosly, multiply by the number of rotors the engine has.. Only thing that puzzles me is that the 787B engine has presure calculated at a 11.8kg/cm.. with a compression of 10 to 1!!! I geuss it's deu to the peripheral port (ports directly mounded in the trochoids..) Ans second of all : It's a racing engine... That figures... Hope you you vcan use these formula's, but you only have to recalculate it into inches of needed... Greetig from Holland

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