|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| The '''mean effective pressure''' is a quantity relating to the operation of a [[reciprocating engine]] and is a valuable measure of an engine's capacity to do work that is independent of [[engine displacement]].<ref>Heywood (1988), page 50.</ref> When quoted as an ''indicated'' mean effective pressure or imep (defined below), it may be thought of as the average [[pressure]] over a cycle in the combustion chamber of the engine. | | The writer is called Wilber Pegues. Distributing manufacturing is how he makes a living. Mississippi is the only place I've been residing in but I will have to move in a year or two. To climb is some thing she would never give up.<br><br>my web-site; cheap psychic readings [[http://netwk.hannam.ac.kr/xe/data_2/85669 netwk.hannam.ac.kr]] |
| | |
| ==Derivation==
| |
| | |
| Let, <br>
| |
| :<math>W</math> = work per cycle in joule
| |
| :<math>P</math> = power output in watt
| |
| :<math>p_{me}</math> = mean effective pressure in pascal
| |
| :<math>V_d</math> = displacement volume in cubic metre
| |
| :<math>n_c</math> = number of revolutions per power stroke (for a 4-stroke engine <math>n_c = 2</math>)
| |
| :<math>N</math> = number of revolutions per second
| |
| :<math>T</math> = torque in newton-metre
| |
| | |
| The [[Power (physics)|power]] produced by the engine is equal to the work done per operating cycle times the number of operating cycles per second. If ''N'' is the number of revolutions per second, and <math>n_c</math> is the number of revolutions per cycle, the number of cycles per second is just their ratio. We can write
| |
| :<math>
| |
| W = {P n_c \over N}
| |
| </math>
| |
| By definition:
| |
| :<math>
| |
| W = p_{me} V_d
| |
| </math>
| |
| so that
| |
| :<math>
| |
| p_{me} = {P n_c \over V_d N}
| |
| </math>
| |
| | |
| Since the [[torque]] ''T'' is related to the angular speed (which is just ''N'' 2 ''π'') and power produced by
| |
| :<math>
| |
| P = T N {2 \pi}
| |
| </math>
| |
| | |
| Then the equation for mep in terms of torque becomes,
| |
| :<math>
| |
| p_{me} = {T n_c \over V_d} {2 \pi}
| |
| </math>
| |
| | |
| Notice that speed has dropped out of the equation and the only variables are the torque and displacement volume. Since the range of maximum brake mean effective pressures for good engine designs is well established, we now have an engine displacement independent measure of the torque producing capacity of an engine design (a specific torque of sorts). This is useful for comparing engines of different displacements. Mean effective pressure is also useful for initial design calculations; that is, given a torque, we can use standard mep values to estimate the required engine displacement. However, it is important to remember that mean effective pressure does not reflect the actual pressures inside an individual combustion chamber—although the two are certainly related—and serves only as a convenient measure of performance.
| |
| | |
| Brake Mean Effective Pressure or bmep is, as usual, calculated from measured [[dynamometer]] torque. Indicated mean effective pressure or imep is calculated using the [[horsepower|indicated power]]; i.e., the pressure volume integral in the work per cycle equation. Sometimes the term fmep (friction mean effective pressure) is used as an indicator of the mean effective pressure lost to friction (or friction torque) and is just the difference between imep and bmep.
| |
| | |
| ==Types of mean effective pressures==
| |
| | |
| Mean effective pressure (MEP) is defined by the location measurement and method of calculation, some commonly used MEPs are given here.
| |
| | |
| * Brake mean effective pressure (BMEP) - Mean effective pressure calculated from brake power
| |
| | |
| * Indicated mean effective pressure (IMEP) - Mean effective pressure calculated from in cylinder pressure, average in cylinder pressure over engine cycle (720° in a 4 stroke, 360° in a 2 stroke).
| |
| | |
| * Friction mean effective pressure (FMEP) - Theoretical mean effective pressure required to overcome engine friction, can be thought of as mean effective pressure lost due to friction. BMEP = IMEP - FMEP
| |
| | |
| ==BMEP typical values==
| |
| * Naturally aspirated spark-ignition engines : Maximum BMEP in the range 8.5 to 10.5 bar (850 to 1050 kPa; 125 to 150 lbf/in<sup>2</sup>), at the engine speed where maximum torque is obtained. At rated power, bmep values are typically 10 to 15% lower.<ref>Heywood, page 50.</ref> <!-- Using the formula provided on this page, I have found that many car engines have values well in excess of 10.5 bar. The Rover K-series engine ranges from 11.1 bar to 12.2 bar and the Honda K20A engine give a bmep of 13 bar -->
| |
| * Boosted spark ignition engines : Maximum BMEP in the 12.5 to 17 bar range (1.25 to 1.7 MPa; 180 to 250 lbf/in<sup>2</sup>).<ref>Heywood, page 50.</ref> <!-- F1 cars of the 1980s seem to have produced BMEP figures around 50psi, far in excess of the 17 bar maximum suggested here -->
| |
| * Naturally aspirated four-stroke diesels: Maximum BMEP in the 7 to 9 bar range (700 to 900 kPa; 100 to 130 lbf/in<sup>2</sup>).<ref>Heywood, page 50.</ref>
| |
| * Boosted automotive four-stroke diesels : Maximum BMEP in the 14 to 18 bar (1.4 to 1.8 MPa; 200 to 269 lbf/in<sup>2</sup>) range.
| |
| * Two-stroke diesels have comparable values, but very large low speed diesels like the [[Wärtsilä-Sulzer RTA96-C]] can run at BMEPs of up to 19 bar (1.9 MPa; 275 lbf/in<sup>2</sup>).
| |
| * Ultra boosted engines such as the engine used in the [[Koenigsegg Agera]] can run at BMEPs as high as 28 bar (32 bar for the Agera R)<ref>{{cite web|last=Drive|title=The 1140 hp Heart of a Hypercar - Inside Koenigsegg|url=http://www.youtube.com/watch?v=NzVc13UF2s8|accessdate=26 February 2013}}</ref>
| |
| * Top Fuel dragster engines: 80–100 bar (8.0-10 MPa)
| |
| | |
| For example, a four-stroke motor producing 160 N·m from 2 litres of displacement has a bmep of (4π)(160 N·m)/(0.002 m³) = 1,005,000 N/m<sup>2</sup> =1,005 kPa (10.05 [[Bar (unit)|bar]]). If the same engine produces 76 kW at 5400 rpm (90 Hz), its torque is 134 N·m and its bmep is 8.42 bar (842 kPa). As piston engines always have their maximum torque at a lower rotating speed than the maximum output, the BMEP is lower at full power.
| |
| | |
| ==See also==
| |
| * [[Compression ratio]]
| |
| | |
| == Notes and references==
| |
| {{Reflist}}
| |
| | |
| *Heywood, J. B., "Internal Combustion Engine Fundamentals", McGraw-Hill Inc., 1988
| |
| | |
| ==External links ==
| |
| *[http://www.factorypipe.com/t_brake.php Brake Mean Effective Pressure (bmep), Power and Torque], Factory Pipe
| |
| *[http://www.harleyc.com/prelude/articles/allaboutmeaneffectivepressure All About Mean Effective Pressure], Harleyc.com
| |
| | |
| <!--* The paragraph on typical values for bmep has been taken from Heywood and the Wikipedia article on the Wärtsilä-Sulzer RTA96-C. -->
| |
| *[http://www.flysteam.co.uk/recent.htm Tiddler steam engine]
| |
| | |
| {{Aircraft piston engine components}}
| |
| | |
| [[Category:Piston engines]]
| |
| [[Category:Engine technology]]
| |
The writer is called Wilber Pegues. Distributing manufacturing is how he makes a living. Mississippi is the only place I've been residing in but I will have to move in a year or two. To climb is some thing she would never give up.
my web-site; cheap psychic readings [netwk.hannam.ac.kr]