Compression-ignition (‘diesel’) engines for large-scale CHP are predominantly four-stroke, direct-injection machines fitted with turbochargers and intercoolers. Diesel engines will accept gas-oil and can also be designed to operate on heavy residual fuel oils and natural gas. Operation on natural gas is, in reality, a dual-fuel mode, as a small quantity of oil (about 5% of the total heat input) has to be injected with the gas to ensure ignition. As this engine can also run at full output on oil as an alternative fuel, it is suited to the interruptible gas tariff. Shaft efficiencies are 35-45%, and the output range is from 1 MW up to 15 MW. Cooling systems are more complex than on spark-ignition engines, and temperatures are lower (typically 85°C maximum), thereby limiting the scope for heat recovery. Exhaust excess air levels are high and supplementary firing is practicable.
Compression-ignition engines run at speeds up to 1,500 rev/min. In general, engines up to about 2 MW CHP engine and its energy balance (GCV) are derivatives of the original automotive diesels, operate on gas-oil and run at the upper end of the speed range. Above 2 MW, they evolved from marine diesels and are dual-fuel or residual oil machines running at medium to low speed.
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