32 Gas generator and compound schemes compared with the turbocharged engine

Figures 3.1a, 3.1c, and 3. Id represent, respectively, the simplest forms of turbocharged engine, gas generator and compound engine. Since in examining the potential of alternative arrangements the design point rating is of crucial importance, it is usual to base such calculations on boost pressure in conjunction with an assumed air-fuel ratio. Using a very simple diesel engine cycle model (see section 1.3, Chapter 1), with appropriate assumptions for compression ratio and maximum cylinder pressure, as well as compressor and turbine efficiency, it is possible to construct curves of specific power W/m (where W = power and m = rate of air mass flow) for engine, compressor and turbine as a function of boost pressure.

Figure 3.2 represents such a simplified specific power relationship over a wide range of boost ratios for a four-stroke engine with Pmax = 138 bar, and compression ratio reduced progressively from 15:1 at low boost in order to maintain the set limit of Pmax. The assumed air-fuel ratio is 25:1. The heat loss to coolant is adjusted so as to give a constant turbine inlet temperature of 600°C. Under these somewhat artificial assumptions engine power We exceeds compressor power Wc over the whole of the boost pressure ratio range up to 7:1. Thus power balance between engine and compressor could be achieved only at pressure ratios of 7:1 or above, a highly unrealistic state of affairs. In practice the gas generator concept has been applied only to two-stroke engines in which the specific airflow and the resultant overall air-fuel ratio are considerably higher than for four-stroke engines. Such gas generators have operated at pressure ratios of the order of 4:1. It will also be observed that turbine power consistently exceeds compressor power so that theoretically, since Wc = Wc, the useful shaft power output of such a unit, namely that of the turbine, Wt, should exceed engine power, implying an output shaft efficiency greater than that of the engine. This train of reasoning constitutes the theoretical justification for the gas generator cycle. In practice, part load efficiency is strongly dependent on boost, both diminishing rapidly with decreasing load. This inherent disadvantage, coupled

76 Diesel Engine Reference Book 500 -

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