POWER DEVELOPED AND EFFICIENCIES

1 Indicated Power

The power developed by the steam engine based on the indicator diagram is the indicated power.

Work done by the steam engine per cycle is given by,

w = Force × distance travelled by the piston

Work done per second is the indicated power

where  on cover end side.

 on crank end side.

d = diameter of piston rod.

If area of piston rod is neglected, then

Let A_i = area of indicator diagram

L_i = length of indicator diagram

The mean height of indicator diagram, 

Mean effective pressure, P_m = h × s

where s = spring scale or spring number, (N/cm²)/cm.

2 Brake Power

A part of the power developed in the engine cylinder is lost in overcoming friction at different parts of the engine. Therefore the power available at the engine crankshaft is less than the indicated power. The power available at the crankshaft is known as the brake power of the engine because it is generally measured by some type of brake.

A rope is wound around the brake drum whose one end is connected to the spring balance suspended from a support and the other end carries the load W, as shown in Fig. 5.13:

Let W = weight on rope, N

S = spring pull, N

D = outer diameter of brake drum

d_r = diameter of rope

The torque applied on brake drum,

Figure 5.13 Rope brake dynamometer

 

3 Efficiencies of Steam Engine

1. Mechanical efficiency, Frictional power, FP = IP − BP
2. Thermal efficiency = Heat supplied to engine per kg of steam = h₁ − h₂Where h₁ = heat per kg of steam entering the engineh_f 2 = heat per kg of condensate coming out of condenserm_s = mass of steam supplied per hourHeat supplied per hour = m_s (h₁ − h_f 2)Thermal efficiency of engine on IP basis, i.e., indicated thermal efficiencywhere m_si = specific consumption of steam (kg/kWh) on IP basiswhere m_sb = specific consumption of steam (kg/kWh) on BP basis
3. where m_f = mass of fuel burnt per second in the boiler    CV = calorific value of fuel in kJ/kg