Mean Effective Pressure - BMEP Typical Values

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.
• 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).
• Naturally aspirated four-stroke diesels: Maximum BMEP in the 7 to 9 bar range (700 to 900 kPa; 100 to 130 lbf/in2).
• 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 just like 2-strokes, so the engine's BMEP can be calculated as just under 129 bar.