Thermal efficiency, engine

Fig. 5. NO formation in a hydrogen engine having spark at 17° before top-dead center (BTC) rpm, 2900 and compression ratio, 5.5 1, where A is nitric oxide B, backfire C, power and D, brake thermal efficiency, (a) Effect of equivalence ratio, ( ) and (b), effect of water induction at 0 = 0.625.

Many preliminary analyses of gas turbines are based on the assumption of a closed air standard cyclic plant, and for such analyses the use of tj as a thermal efficiency is entirely correct (as discussed in the early part of Chapter 3 of this book). But most practical gas turbines are of the open type and the rational efficiency should strictly be used, or at least its approximate form, the arbitrary overall efficiency tjq. We have followed this practice in the latter part of Chapter 3 and subsequent chapters even though some engineers consider this differentiation to be a somewhat pedantic point and many authors refer to tjo as a thermal efficiency (or sometimes the lower heating value thermal efficiency ). 

The second law of thermodynamics may be used to show that a cyclic heat power plant (or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle called the Carnot cycle for a given (maximum) temperature of supply (T ax) and given (minimum) temperature of heat rejection (T jn). Such a Carnot power plant receives all its heat (Qq) at the maximum temperature (i.e. Tq = and rejects all its heat (Q ) at the minimum temperature (i.e. 7 = 7, in) the other processes are reversible and adiabatic and therefore isentropic (see the temperature-entropy diagram of Fig. 1.8). Its thermal efficiency is... 

Vehicle fuel economy is normally measured in miles per gallon. At any given instant, it depends on the energy content of a gallon of fuel (Qf), the vehicle velocity (V.,) and power required (P,-5q), the thermal efficiency with which the engine converts fuel energy into useful output work (rj,.), and the mechanical efficiency with which the driveline delivers that work to the vehicle wheels (r j). Specifically,... 

Rudolph Diesel s original intent was to produce a low-heat-rejection internal-combustion engine without the need for a cooling system. He believed that this would provide less heat losses from the combustion process and provide him with a superior heat, or thermally efficient (TE), design concept. To his chagrin. 

Superior fuel economy (more thermally efficient) particularly at low speeds due to lack of restriction of the air flow air flow restriction occurs in a gasoline engine through the throttling action of the gas pedal. 

Wood was the easiest fuel to use in early steam locomotives, but it was soon realized that the logistics of wood fuel were limiting. Steam engines were developed that could burn coal, peat, or (later) oil where those fuels were more abundant. For intercity railroads (especially in the Americas, Asia, Australia, and Africa), coal remained the fuel of choice for one hundred years. Despite impressive technology development, steam locomotives never could achieve thermal efficiencies greater than about 6 to 8 percent. 

A diesel-electric locomotive uses as its prime mover a large, self-igniting, internal combustion engine of the type invented by RudolfDiesel and first successfully demonstrated in 1897. Thermal efficiency of these engines exceeded 30 percent, compared... 

Valenti, M. (1995). Breaking the Thermal Efficiency Barrier. Mechanical Engineering 117(7) 86—89. 

The overall efficiency, rjb of a reciprocating engine is the product of the thermal efficiency, rj and its mechanical efficiency rjni- Thus ... 

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