Good info from Madzng, he has covered most things,
The Carnot cycle, (Google it) is the most efficient thermodynamic cycle, and all other theoretical and practical cycles are compared to this.
The Carnot efficiency is based on the difference between the air temperature at the engine inlet and exhaust divided by the exhaust temperature. (T4 -T1)/T4.
The turbocharger and air cooler are all part of the engine cycle, so you should take the air temperature at the TC inlet not after the air cooler.
Therefore, Mathematically, to acheve 100% efficiency the air inlet temperature would have to zero absolute, (MINUS 273 Celsius)
So with a typical Engine Room air temperature of about 30 C and a maximum exhaust temperature of about 430C due to materials limitations, (say 300K & 700 K respectively)
The maximum theoretical efficiency works out about ((700-300)/700) or 57%
JK should be able to do a bit better with her ships in the frozen North. Though she may find that the high density of the air overloads the turbochargers. Obviously the rest of us can not cool the air inlet temperature, running a giant A/C plant to cool the air intake wont do anything for our overall efficiency! Although warships always have fresh air supplies ducted from outside the ER directly into boiler and engine intakes, as part of their NBCD measures, but the presure drop in the ducting probably balances out any gain from the reduced temperatures.
We will just have to wait for better materials that will allow higher exhaust gas temperatures, and hope to get some of the waste heat out of the exhaust gases after the turbocharger.
I have just sailed on a ship with a B & W slow speed engine, when I joined a service Engineer was on board to inspect the engine, he pronounced that all 6 pistons and liners would have to be replaced due to scuffing damage and liner wear. He put this down to running the engine at too low a charge air temperature. Investigation into the History of the ship showed that the sea water tubes of the charge air cooler had been heavily choked and the ship had been running with high charge air temperatures without any problems. In dry dock a new charge air cooler was fitted and afterwards the charge air temperature was run at 40C, running in the Gulf and South China sea, with high humidity, the condensation was washing off the cylinder lubrication, within a few months the Engineers started to notice problems with broken piston rings, and within a year the engine was wrecked. The owners were not amused.
Most Engine makers warn that charge air temperature should be maintained sufficiently high to avoid condensation in the charge air manifold, in Tropical conditions that can mean 50C or higher.
I agree with Mazdng that at full load the charge air temperature should be kept to the minimum permitted by the engine builder, but be aware that excessively cold air temperatures increase thermal stresses inside the engine and can cause cracking of cylinder heads, liners and pistons. Be aware of the dangers he and I have mentioned.
At part load, efficiency can be increased by raising the the charge air temperature. This will raise the exhaust temperature and therefore the ratio between the air inlet and exhaust temperatures. It can also help improve combustion and improve the efficiency of the conversion of the potential chemical energy in the fuel into heat energy. It will also make more energy available for recovery in the economiser, if fitted.
I am not sure about lowering the charge air temperature to reduce the amount of water going into the engine, water vapour in the air will not absorb any latent heat of evaporation because it is already a vapour. Cooling the charge air may cause some vapour in the air to condense into water droplets which will then absorb heat in the engine when they evaporate, as well as destroying the cylinder lubrication and causing cold corrosion. Unfortunatly, efficient co alescing filters to remove all the water droplets from the air would cause a pressure drop in the charge air and reduce the efficiency. The only solution to all three problems is to raise the charge air temperature.
Charge air temperature control is a big problem, on older ships with Sea Water cooled air coolers. Modern Medium speed Engines tend to have 2 stage coolers, using Jacket water to preheat the air at low load and as a first stage cooler at high load and LT water for second stage cooling at high load. I am not sure if the latest slow speed engines do this, but it is definitly the way to go, precise control of the optimum temperature throughout the power range. I suppose you could even integrate charge air humidity control, to keep the charge air above its Dew Point.
One of the problems with Sea Water cooling is that throttling the SW flow will raise the mean scavenge air temperature, but some of the air flow is cooled almost to the SW inlet temperature causing condensation even if the mean or average temperature is above the Dew Point. It is better to raise the mean scavenge temperature by recirculating the SW to raise its temperature rather than throttling the SW outlet valve or by passing the cooler.
It is always better to ask a stupid question than to do a stupid thing.