Originally posted by Kurfürst
I often seen the Merlins "aftercooler" referenced as an "intercooler". And while you are correct with your points, and certainly understand the subject far better than me, let me note that, as I heard, the British and the Americans use different terms to describe the same thing, the former call it an IC, the latter know it as an AC.
An aftercooler is
after the supercharger. An
intercooler is between the the 2 stages of the supercharger.
justin, the P&W H-3730, the Chrysler IV--2220 and the Allison V-1710-119 had aftercoolers. Pics show the cooler between the supercharger and the induction manifold.
description of the V-1650
The supercharger is a two-stage two speed gear driven unit with intercooling and aftercooling. Two stages are used to obtain relatively high pressure ratio's efficiently, two-speed operation allows improved performance at high altitude without the loss of power at sea-level which is inherent at high blower speeds, and
intercooling and aftercooling significantly reduce charge temperature, allowing higher boost without detonation.
For low speed operation, the supercharger turns at 6.391 times crankshaft speed, for high speed the blower spins at 8.095 times crankshaft speed. The supercharger is driven through a gear train coupled to the spring drive through the supercharger driving gear in the wheelcase. Three independent planetary gear trains are arranged at 120 degree intervals around the driving gear, the driving gear thus driving three planetary pinions. The ring gears for the three planetary drives have internal and external teeth, the external teeth are coupled to the supercharger pinion which directly drives the supercharger, the internal teeth are coupled to planetary gears coupled to planetary pinion gears. For low speed, the planetary pinions are directly locked to their respective ring gears through planetary gears and clutches, so the ring gear is turning at the speed on the input to the planetary geartrain. For high speed, the planetary gears are allowed to rotate about the sun gear, increasing the speed of the ring gear with respect to the input shaft. A hydraulic clutch (three really, one for each planetary gear train) combined with over-running "sprag" clutches is used to effect speed changes. The hydraulic clutch uses moderate engine oil pressure controlled with 24V electrically actuated solenoid -- the solenoid is actuated to apply oil pressure for high speed, and pressure is released for low speed operation.
The supercharger itself consists of two impellers on the same shaft, both turning the same speed. The first stage uses a 12.0" diameter impeller, while the second stage uses a 10.1" impeller. The intercooler is an integral part of the intermediate volute case, located between the first and second stage impellers. The compressed air from the first stage passes through the cooled volute and passage to the second stage. The aftercooler is located between the exit of the second supercharger and the intake plenum, and is a conventional air/water heat exchanger (liquid/air radiator). A separate cooling system with its own pump was provided (permitting cooler water than is possible by using the engine coolant) was provided for aircraft use -- a 40% reduction in intake temperature was reported by Rolls-Royce at maximum speed and power with a coolant flow of about 30 gallons per minute. For boat racing, the aftercooler is usually replaced with simple plenum tube (called a tube or ADI tube) due to disruptions in airflow and mixture which occur in the aftercooler matrix at very high power levels -- at least some of which is though to result from air-fuel-ADI separation.
An automatic boost regulator is standard equipment for aviation use. The boost regulator automatically retards the throttle as full boost is reached, eliminating manual control of this critical function. War emergency boost is often provided whereby the pilot can over-ride the regulator when necessary, an indication such as a broken seal is usually provided to alert maintenance that the engine has been overstressed. For planes with ADI, there is usually an interlock provided that restricts maximum boost to a lower setting when ADI is not functional.
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