Mean effective pressure

Mean effective pressure

The mean effective pressure is a quantity related to the operation of an reciprocating engine and is a valuable measure of an engine's capacity to do work that is independent of engine displacement[1]. 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.




W = work per cycle in joule
P = power output in watt
pmep = mean effective pressure in pascal
Vd = displacement volume in cubic metre
nc = number of revolutions per cycle (for a 4-stroke engine nc = 2)
N = number of revolutions per second
T = torque in newton-metre

The 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 nc is the number of revolutions per cycle, the number of cycles per second is just their ratio. We can write

W = {P n_c \over N}

By definition:

W = pmep * Vd

so that

p_{mep} = {P n_c \over V_d N}

Since the torque T is related to the angular speed (which is just N 2 π) and power produced by

P = TN

Then the equation for mep in terms of torque becomes,

p_{mep} = {T n_c \over V_d} {2 \pi}

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 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 torque
  • Indicated mean effective pressure (IMEP) - Mean effective pressure calculated from in cylinder pressure, average in cylinder pressure over engine cycle, 720°.
  • Friction mean effective pressure (FMEP) - Theoretical mean effective pressure required over come 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/in2), at the engine speed where maximum torque is obtained. At rated power, bmep values are typically 10 to 15% lower. [2]
  • Boosted spark ignition engines : Maximum BMEP in the 12.5 to 17 bar range (1.25 to 1.7 MPa; 180 to 250 lbf/in2). [3]
  • Naturally aspirated four-stroke diesels: Maximum BMEP in the 7 to 9 bar range (700 to 900 kPa; 100 to 130 lbf/in2).[4]
  • Boosted automotive four-stroke diesels : Maximum BMEP in the 14 to 18 bar (1.4 to 1.8 MPa; 200 to 269 lbf/in2) 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/in2).
  • Ultra boosted engines such as the engine used in the Koenigsegg Agera can run at BMEPs as high as 28 bar
  • 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/m2 =1,005 MPa (10.05 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.

It may be of interest to consider the, 0.13 cc displacement, high speed uniflow steam engine "Tiddler", built by model engineer Geoff Wolfe. It appears to manage a full load output of 20w@5,300 rpm . Reciprocating steam engines have nc = 1 just like 2-strokes, so the engine's BMEP can be calculated as just under 129 bar.

See also

Notes and references

  1. ^ Heywood (1988), page 50.
  2. ^ Heywood, page 50.
  3. ^ Heywood, page 50.
  4. ^ Heywood, page 50.
  • Heywood, J. B., "Internal Combustion Engine Fundamentals", McGraw-Hill Inc., 1988

External links

Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • mean effective pressure — vidutinis slėgis statusas T sritis Standartizacija ir metrologija apibrėžtis Lygiavertis vidutinis slėgis, kurį kuria oro kompresoriaus ar kitokio įrenginio stūmoklis vienos jo eigos metu. atitikmenys: angl. mean effective pressure; mean pressure …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • mean effective pressure: (MEP) — The average pressure of the burning fuel on the power stroke subtracted by the average pressure on the other three strokes. Pressure is in pounds per square inch. Average pressure on a surface when a changing pressure condition exists. Also see… …   Dictionary of automotive terms

  • brake mean effective pressure — A measure of mean effective pressure in a cylinder of a reciprocating engine. It is the average in excess of ambient pressure inside the cylinder during the power stroke. It is measured in pounds per square inch (psi), kilopascals (kPa), or bars… …   Aviation dictionary

  • friction mean effective pressure — That part of the indicated mean effective pressure (IMEP) used to overcome internal friction in a combustion chamber of a reciprocating engine. The difference between the IMEP and the brake mean effective pressure (BMEP) is the FMEP. The FMEP and …   Aviation dictionary

  • indicated mean effective pressure — The average pressure produced in the combustion chamber during the operating cycle. It is an expression of the theoretical, frictionless power known as indicated horsepower. In addition to completely disregarding power lost to friction, indicated …   Aviation dictionary

  • effective pressure — See brake mean effective pressure indicated mean effective pressure mean effective pressure …   Dictionary of automotive terms

  • Mean arterial pressure — The mean arterial pressure (MAP) is a term used in medicine to describe an average blood pressure in an individual.[1] It is defined as the average arterial pressure during a single cardiac cycle. Contents 1 Calculation 2 Estimation …   Wikipedia

  • brake mean effective pressure — noun the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output. Syn: BMEP …   Wiktionary

  • brake mean effective pressure — (BMEP) The average pressure in the cylinders of an engine divided by its mechanical efficiency, i.e., the ratio of the power actually delivered at an output shaft to the power developed in the cylinders. It is used as an indication of torque …   Dictionary of automotive terms

  • indicated mean effective pressure — (IMEP) The average pressure within an engine cylinder during a working cycle, calculated from an indicator diagram …   Dictionary of automotive terms

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”