21 Ind Thermal Eff = ƞ ti = wf= fuel supplied per hour in m 3 for gaseous fuel and kg for liquid fuel HHV = high heating volume of fuel KJ/kg or KJ/m 3 The brake thermal efficiency flows what fraction of heat supplied was actually delivered by the engine shaft Brake Thermal Eff = ƞ tb = Conversion: 575 Example 2 -2 The engine in problem 2-1 is connected to a dynamometer which give a brake output torque reading of 205 N-m at 3600 RPM. At this speed, air enters the cylinder at 85 kpa and 600C and the Mech Eff of the engine is 85%. Calculate : a) Brake power (KW) e) fmep b) Indicated Power (kW) f) Power lost to friction c) Bmep g) brake work per unit mass d) Imep Solution :
22 a.) N m = ; = = = 90.9 kw b.) B mep = = B mep = 859 000 = 859 Kpa c.) I mep = = = 1010 Kpa d.) f mep = I mep - B mep = 1010 - 859 = 151 kpa e.) Power lost = f = i - b = 90.9 – 77.3 = 13.6 kw f.) b = where m a is mass of air Brake work for one cylinder B mep = Wb = B mep x Vd = 859 x 0.0005m 3 = 0.43 kj Since gas is air : m a = where V is volume at BDC
3 c t =
= = =
25 Standard values of surrounding air pressure and temperature can be used to find density Po (std) = 101.3 kpa or 14.7 psia To (std) = 25 o c 0r 298 k P o = pressure of surrounding air T o = temperature of surrounding air R = gas constant of air – 0.287 At standard conductions: ρ air = 1.181 ; ρ a = Typical values of Nv for an engine at WOT are in a range of 75% to 90% going down to much lower value as the throttle is closed.
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- Summer '19
- Combustion, Internal combustion engine