B001

BUY: Engine - Reciprocation - 2-stroke

Brake Mean Effective Pressure:

Definition:

Brake mean effective pressure, or BMEP, is the measure of efficiency that express engine output relative to engine displacement. Simply put, BMEP is the average effective pressure of the cylinders as they progress through intake, compression, ignition and exhaust strokes.

The work accomplished during one engine cycle divided by the engine swept volume. It is essentially the engine torque normalized by the engine displacement. The word "brake" denotes the actual torque/power available at the engine flywheel as measured on a dynamometer. Thus, BMEP is a measure of the useful power output of the engine.

BMEP ≈ torque / capacity. A high BMEP is indicative of higher performance. A low BMEP tends to be indicative of higher reliability.

The highest BMEP [BMEP_MAX] will occur at the point of maximum torque, which also happens to be where the peak volumetric efficiency occurs.

2-cycles require only one half of 4-cycle BMEP to produce the same power at the same rpm.

Two cycle:... BMEP = ( HP * 6500 ) / ( L * RPM )

Four cycle:... BMEP = ( HP * 13000 ) / ( L * RPM )

BMEP	Brake Mean Effective Pressure
L	Engine capacity (volume)	[liters]
Q	Torque	[foot pounds]

Power = work (Q * rpm) / time

hp =( Q * rpm) / 5252

BMEP = (hp * 6500) / (L * rpm)

BMEP = (Q * 6500) / (L * 5252)

Stock motorcycle engines will have a BMEP of 80 - 90 psi. Road racers can get up to 125 -130 psi.

Notes from BJ Schramm's talk at Arlington:

BMEP = ((900 * hp * 14.7)/ L) / rpm This is the equation that Schramm gave. It appears to be for 4-stroke engines, not 2-stroke. The additional column [Schramm's PSI / 2] has been added to his table below, to divide his PSI by 2. This gives values that are more realistic.

Helicopter	Engine	Schramm's PSI	Schramm's PSI / 2	Life:
Mini 500	Rotax 582	234	117	35 hours
	Rotax 633	215	108
Helicycle 700	Rotax 700cc	202	101
Helicycle 800	Rotax 800cc	177	89
Ultralight aircraft	Rotax 582	170	85	500 hours

Mini 500 calculations using Schramm's BMEP figure above divided by 2 (I.e. 117 psi)

BMEP_MAX = (hp * 6500) / (L * rpm)

hp = (L * rpm * BMEP_MAX) / 6500

hp = (.582 * 6000 * 117) / 6500 = 63 hp at maximum torque

The database's helicopter calculations show that the Mini-500 requires 56 hp to hover out of ground effect at the engine's maximum torque rpm of 6000.

________________________

The database's helicopter calculations show that the Helicycle requires 51 hp to hover out of ground effect at the engine's maximum torque rpm of 6000.

BMEP Considerations re the UniCopter:

Engine:		BMEP
2-cycle:	BMEP = (Q_MAX 6500) / (L * 5252)*
Hirth F-30 85 hp	(78 * 6500) / (1.042 * 5252) =	92.6
Hirth F-30 ES 110 hp	(89 * 6500) / (1.042 * 5252) =	105.7
4-cycle:	BMEP = (Q_MAX 13000) / (L * 5252)*
Rotax 912	(75.9 * 13000) / (1.211 * 5252) =	155.1 / 2 = 77.6
	BMEP = (hp 13000) / (L * rpm)*
Lycoming O-360 (Robinson R-22)	(131 * 13000) / (5.24386 * 2652) =	122.45 / 2 = 61.1
Lycoming O-235	(100 * 13000) / (3.85096 * 2400) =	140.65 / 2 = 70.3

Oil for 2-stroke:

Leon, the manufacture of Zanzottera, said that they had tested a lot of oils and recommend Castrol TTS. It is expensive and hard to come by.

Time Between Overhauls ~ Hirth:

Date: 5-Oct-01 13:55
Author: R. K.
Subject: beware of TBO times

I asked the Hirth factory rep if they had an engine that actually has gone 1000hrs. The answer was no. I then asked him if they had ever run one to failure. The answer was no. The 1000 hr TBO is not from testing or usage. It just sounds nice.

Last Revised: October 8, 2001