the positive-displacement (Roots or screw) type due to its ability to produce boost at relatively low engine speeds. Otherwise, low-speed power will suffer. Alternatively, a turbocharger can be used for greater efficiency, if low-speed operation is not required, or supplemented with electric motors. In the Miller-cycle engine, the piston begins to compress the fuel-air mixture only after the intake valve closes; and the intake valve closes after the piston has traveled a certain distance above its bottom-most position: around 20 to 30% of the total piston travel of this upward stroke. So in the Miller cycle engine, the piston actually compresses the fuel-air mixture only during the latter 70% to 80% of the compression stroke. During the initial part of the compression stroke, the piston pushes part of the fuel-air mixture through the still-open intake valve, and back into the intake manifold. Charge temperature The charge air is compressed using a supercharger (and cooled by an intercooler) to a pressure higher than that needed for the engine cycle, but filling of the cylinders is reduced by suitable timing of the inlet valve. Thus the expansion of the air and the consequent cooling take place in the cylinders and partially in the inlet. Reducing the temperature of the air/fuel charge allows the power of a given engine to be increased without making any major changes such as increasing the cylinder/piston compression relationship. When the temperature is lower at the beginning of the cycle, the air density is increased without a change in pressure (the mechanical limit of the engine is shifted to a higher power). At the same time, the thermal load limit shifts due to the lower mean temperatures of the cycle. This allows ignition timing to be advanced beyond what is normally allowed before the onset of detonation, thus increasing the overall efficiency still further. An additional advantage of the lower final charge temperature is that the emission of NOx in diesel engines is decreased, which is an important design parameter in large diesel engines on board ships and power plants. Compression ratio Efficiency is increased by having the same effective compression ratio and a larger expansion ratio. This allows more work to be extracted from the expanding gases as they are expanded almost to atmospheric pressure. In an ordinary spark ignition engine at the end of the expansion stroke of a wide open throttle cycle, the gases are