Thermodynamics and Heat Engine4 (3+1)

LESSON - 47 ENGINE EFFICIENCIES AND PERFORMANCE

47.1.2. Thermal Efficiency

It is defined as power produced by the engine per unit heat energy consumed by it per unit time.

Indicated Thermal Efficiency (η_ith)  =  ;     

                  where,  is mass flow rate of fuel  into the engine & CV is calorific value of fuel.

Brake thermal efficiency (η_bth) =

47.1.3. Volumetric efficiency

It applies to 4-stroke engines. It is defined as the ratio of volume (referred to NTP) of air/charge admitted to the engine cylinder during suction stroke to the displacement volume of cylinder. Charge efficiency is another parameter similar to volumetric efficiency. It is defined as the ratio of mass of air/charge admitted to the engine cylinder during suction stroke to the mass of air/charge corresponding to displacement volume of cylinder at NTP.

η_charge =    ;   ρ is density of air/charge at NTP ,  v_s  is stroke volume

47.1.4. Scavenge efficiency

It applies to 2-stroke engines. It is the measure of extent to which burnt gases are removed from the cylinder & cylinder is filled with fresh air/charge.

η_scavenge =

where, m_a is mass of air/charge retained in the cylinder for compression, v_c is clearance volume , v_s is swept volume;  ρ is density of air/charge at NTP or at scavenging temperature & pressure.

47.1.5. Relative efficiency

It is defined as ratio of actual thermal efficiency to ideal cycle efficiency for same compression ratio. Depending upon actual thermal efficiency taken, it is called indicated or brake relative efficiency.

η_{indicated relative efficiency} = 

η_{brake relative efficiency}      =  

47.1.6. Specific fuel consumption (sfc)

It is defined as mass of fuel consumed by engine per unit energy produced by it.  

isfc =  ;         bsfc = ; kg/kWh

where,   'isfc' is indicated  specific fuel consumption,    'bsfc' is brake specific fuel consumption and is mass flow rate of fuel into the engine.

47.2. PERFORMANCE OF ENGINES

Engine performance means how well an engine is putting out or doing its job with relation to the energy supplied in or how effectively it provides energy in relation to some comparable engines. 

IC engines are designed to produce their maximum efficiency at a certain speed, depending on the type of engine. Tests carried out on these engines show that, as the speed increases, the efficiency will increase until the speed for maximum efficiency is reached; if the speed is still further increased the efficiency will fall. The hourse-power also increases with the speed, and will continue to increase after the speed for maximum efficiency is passed, but there will be a more rapid increase in the fuel consumption beyond this speed. If the speed is still further increased the horse-power will reach its maximum, after which it decreases on any further increase of speed.

In Fig. 47.1 curves are plotted for a high speed spark ignition IC engine showing IP, BP, mechanical efficiency, and brake specific fuel consumption, on a base representing the engine speed in revs. per min. The brake specific fuel consumption is a measure of overall efficiency of the engine. The best speed of this engine is the speed of 1300 rpm at which the brake specific fuel consumption will be a minimum; at this speed the engine is producing power at the least cost. It will be noticed from these curves that although more power can be developed by speeding-up this engine above the speed for maximum efficiency, the cost per BP will be increased by so doing, as the brake specific fuel consumption increases at a great rate.

 Fig. 47.1.

The curves obtained from tests on a compression ignition IC engine having compression ratio of 16.5:1 are shown in Fig 47.2 and Fig. 47.3. In Fig. 47.2 the curves are plotted for brake specific fuel consumption, brake power, and brake mean effective pressure, on a base representing the engine speed in rpm. It is noticed that minimum specific fuel consumption occurs at a speed of 1370 rpm; this condition corresponds to the maximum efficiency and is the most economical speed for the engine when running under the conditions used in the test. If the speed is increased beyond this amount, the brake specific fuel consumption increases and efficiency is consequently reduced. The BP curve shows that the brake power increases with the speed throughout the whole range; hence, the maximum power does not occur at the condition for maximum efficiency. The brake mean effective pressure appears to have its maximum value at a speed slightly above the most economical speed.

A curve showing the variation of brake specific fuel consumption, with BP obtained during these tests, is shown in Fig. 47.3. It will be noticed from the curve that the most economical condition occurs when the engine is developing between 14.5 and 20 BP.

Fig. 47.2.

Fig. 47.3.

 Curves showing the variation of b.m.e.p. and specific fuel consumption with mixture strength for the engine are shown in Fig. 47.4; these were obtained during tests at constant speed.

Fig. 47.4.