Brake Horsepower
Content
Brake Horsepower
Not all the power (I. Hp) developed by combustion of fuel is available as useful power to do
outside work. Part of it is absorbed by the engine itself and referred to as mechanical losses or fractional
horsepower (F. Hp).
The mechanical losses may be grouped as under,
A) Friction – between the cylinder surface and the piston rings in bearing, gears valve
mechanisms
B) The work changing – absorbed during the exhaust and suction strokes
C) Resistance of air to flywheel rotation
D) Power required driving the auxiliaries fuel pumps governor, lubricating oil and water
circulating pump.
Thus, the power available is about 15% to 30% less than the indicated power.
The output horsepower is called the shaft horsepower or brake horsepower (B. Hp).
Note:
It may be noted that friction loss is really not a loss of energy, since energy cannot be lost or
destroyed but is only a transformation of work into heat.
This heat is dissipated to the atmosphere and not available for purpose for which the engine is
created.
Thus, B Hp = I Hp – F Hp
Specific Fuel Consumption
Specific fuel consumption is defined by
SFC =
Where:
,
= mass rate of fuel flow into engine
⁄
W = power KW or Hp
Brake power gives brake specific fuel consumption.
=
=
Indicated power gives indicated specific fuel consumption.
=
Not all the power (I. Hp) developed by combustion of fuel is available as useful power to do
outside work. Part of it is absorbed by the engine itself and referred to as mechanical losses or fractional
horsepower (F. Hp).
The mechanical losses may be grouped as under,
A) Friction – between the cylinder surface and the piston rings in bearing, gears valve
mechanisms
B) The work changing – absorbed during the exhaust and suction strokes
C) Resistance of air to flywheel rotation
D) Power required driving the auxiliaries fuel pumps governor, lubricating oil and water
circulating pump.
Thus, the power available is about 15% to 30% less than the indicated power.
The output horsepower is called the shaft horsepower or brake horsepower (B. Hp).
Note:
It may be noted that friction loss is really not a loss of energy, since energy cannot be lost or
destroyed but is only a transformation of work into heat.
This heat is dissipated to the atmosphere and not available for purpose for which the engine is
created.
Thus, B Hp = I Hp – F Hp
Specific Fuel Consumption
Specific fuel consumption is defined by
SFC =
Where:
,
= mass rate of fuel flow into engine
⁄
W = power KW or Hp
Brake power gives brake specific fuel consumption.
=
=
Indicated power gives indicated specific fuel consumption.
=
