Mean effective pressure
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. When quoted as an indicated mean effective pressure or IMEP (defined below), it may be thought of as the average pressure acting on a piston during the different portions of its cycle.
- = work per cycle in joule
- = power output in watt
- = mean effective pressure in pascal
- = displacement volume in cubic metre
- = number of revolutions per power stroke (for a 4-stroke engine )
- = number of revolutions per second
- = 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 is the number of revolutions per power stroke, the number of power strokes per second is just their ratio. We can write
Reordering to put work on the left:
Since the torque T is related to the angular speed (which is just N 2 π) and power produced by
Then the equation for mep in terms of torque becomes,
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 a 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, standard MEP values can be used 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 (BMEP) is calculated from measured dynamometer torque. Net indicated mean effective pressure (IMEPn) 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 IMEPn 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 measured brake torque.
- Gross indicated mean effective pressure (IMEPg) - Mean effective pressure calculated from in-cylinder pressure over compression and expansion portion of engine cycle (360° in a four-stroke, 180° in a two-stroke). Direct measurement requires cylinder pressure sensing equipment.
- Net indicated mean effective pressure (IMEPn) - Mean effective pressure calculated from in-cylinder pressure over the complete engine cycle (720° in a four-stroke, 360° in a two-stroke). Direct measurement requires cylinder pressure sensing equipment.
- Pumping mean effective pressure (PMEP) - Mean effective pressure from work moving air in and out of the cylinder, across the intake and exhaust valves. Calculated from in-cylinder pressure over intake and exhaust portions of engine cycle (360° in a four-stroke, 0° in a two-stroke). Direct measurement requires cylinder pressure sensing equipment. PMEP = IMEPg - IMEPn.
- 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. Friction mean effective pressure calculation requires accurate measurement of cylinder pressure and dynamometer brake torque. FMEP = IMEPn - BMEP.
BMEP typical values
Petrol Engines :
- Naturally aspirated spark-ignition engines : current VW petrol engines maximum MEP range (in bar and psi) from 11.6 to 13.3 bar (168 to 193 psi), 10.1 to 11.4 bar (146 to 165 psi) at maximum power.
- 28-cylinder Pratt & Whitney R-4360 Wasp Major with 115/145 Octane World War II avgas, 3,600 HP at 2,700 rpm and 17.2 bar (249.4psi) BMEP.
- Napier Sabre 7, at peak HP, 3055 HP at 3850rpm, 19.4 bar (281.3psi) BMEP
- Rolls-Royce Merlin 130/131, at peak HP, 2030 HP at 2900rpm, 23.1 bar (335psi) BMEP
- Boosted spark ignition engines : current VW petrol engines maximum MEP range from 16.4 to 23.1 bar (238 to 335 psi), 15.3 to 19.3 bar (222 to 280 psi) at maximum power.
- High boost engines such as the Koenigsegg Agera can run at BMEPs as high as 28 bar (32 bar for the Agera R)
- Formula One engines : in 1986 the 1.5L Williams-Honda FW11 Turbo produced 1,400 hp (1,044 kW) at 12000 rpm for 831 Nm of torque, a 69.6 bar (1,009 psi) BMEP. In 2006 the 2.4L Toyota TF106 produced 740 hp (552 kW) at 19,000rpm for 277 Nm of torque, a 14.5 bar (210 psi) MEP.
- Top Fuel dragster engines: 80–100 bar (8.0-10 MPa; 1160-1450 psi)
Diesel Engines :
- Naturally aspirated four-stroke diesels: current VW diesel engines have a 8.9 bar (129 psi) maximum MEP, 8 bar (120 psi) at max power.
- Boosted automotive four-stroke diesels : current VW diesel engines have a 17.7 to 31.9 bar (257 to 463 psi) maximum MEP, 14.5 to 26.8 bar (210 to 389 psi) at max power.
- 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).
- The Wärtsilä W31, the world's most efficient four-stroke diesel engine as of October 2015, has a mean effective pressure of 30.1 bar.
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 kPa (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 usually have their maximum torque at a lower rotating speed than the maximum power output, the BMEP is lower at full power (at higher rotating speed). The exception is with engines with sharp artificial torque limiter past certain RPM, as in 320 N·m, 90 kW RZ4E for trucks, with calculated BMEP of 21.16 bar from 1600 to 2800 rpm, peak power at 2800 rpm.
Notes and references
- Heywood, J. B., "Internal Combustion Engine Fundamentals", McGraw-Hill Inc., 1988
- Brake Mean Effective Pressure (bmep), Power and Torque, Factory Pipe
- All About Mean Effective Pressure, Harleyc.com