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Diesel engines are diferent from gas engines in that only air is entering the cylinder when the intake valves or ports are open. Diesel engines draw much more air than is actually necessary for combustion most of the time, although this is changing with new engines that recirculate exhaust gases. The gasoline engine works on a constant volume cycle. When the valves have closed, all air and fuel that will enter the cylinder have done so. Thus, volume remains constant.. As combustion begines, pressure rises. As the piston falls, pressure drops because all fuel has burned and the original volume was a combination of fuel and air. The Diesel cycle is called a constant pressure cycle. Only air enteres the cylinder. When compression is complete, injection begines. pressure rises with combustion. As the piston falls, pressure remains constant, as injection continues to "feed the fire". Diesel fuel has a higher heat value than gasoline and needs more air to burn completely, this with the fact that diesel burns slower, greater combustion time per stroke is why Diesel engines produce more than twice the torque as horspower with a higher fuel efficiency. locomotive engines do have intercoolers. Supercharging, whether by blower or turbo is when intake manifold pressure is higher than atmospheric pressure. The blower on a two stroke diesel is not for supercharging. The pressure provided by a blower for a two stroke Diesel is only a few inches of water. This is to help exhaust scavenging. A guage measures the difference in pressure between a given source and the atmosphere. Such a measurement in an engineering environment is denoted PSIG for measurements in pounds per square inch. A measurement of absolute pressure in pounds per square inch is denoted PSIA. 25 pounds of boost at sea level is not the same as 25 pounds of boost at high altitude. Surging happens in turbine engines when the compressor stage produces more air than can pass through the turbine and "burps" or "farts". Newer Diesel engines have this problem because manufacturers have gone to using smaller turbos that can spool up quicker, and improvements in design have made them more efficient.
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