after coolers

An aftercooler is like a radiator that cools the intake air after it is forced into the system by the turbo. Cooler air burns better, and the aftercooler "cools" or makes the air not as hot as it was when compressed by the turbo (when you compress air it heats up). Hope that helps.

I will try and keep it simple but there is more to the story than just the cooler itself.

A diesel produces power by burning fuel in air. The more air you can put in the cylinders in a given time, the more fuel you can burn and the more power the engine will produce.

The "after" in aftercooler means it is in the air path "after" the turbocharger. The turbocharger is there to recover energy from the exhaust that would otherwise be wasted. The turbocharger does this by using the energy in the exhaust stream to spin a centrifugal compressor on the air side of the unit. This takes in air at atmospheric pressure and compresses it to a higher pressure.

Compressing air raises its temperature, air leaving the turbocharger is very hot. Hot air, even though it is compressed is less dense (wieghs less) than cool air of the same volume. This is where the aftercooler comes into play. Air passes through tubes in the cooler and,in the case of the 3126, heat is removed by cool seawater on the other side of the tubes. Much cooler and denser air comes out downstream for use in the cylinders.

If you look at some engines that are available in different versions, you will see that a normally aspirated or non-turbo version might make say 150 hp, a turbocharged version might produce up to 250 hp, a turbocharged and aftercooled version might make 300 hp or more. The reason this can happen is that more fuel can be burned in a given time because more weight of air is available in the cylinders, thanks to the turbocharger and the aftercooler.

l hope that wasn't more than you wanted or not enough. It was a good question.

In most engines the turbocharger is always in the exhaust gas path. I say most because there are exceptions and some engines with multiple turbos have a control system to put more online to suit the engine load requirements. But for the most part all the exhaust gas goes through the turbo.

At low loads (not just rpm - a diesel can run at full speed but still have no load on it - generators are an example of this) the weight and volume of the exhaust gas is comparitively low and the gases are relatively cool so little work is being done by the turbocharger turbine, there is not much air being compressed. As engine load increases more fuel is injected and the weight and temperature of the exhaust gas increases. This puts more energy into the turbine and the compressor spins faster, compressing more air for use in the cylinders.

There are some engines which turn the compressor through gearing at low loads because the engine needs air to clear the cylinders of exhaust gases and when the load increases to the point the exhaust contains enough energy the turbine takes over and spins the compressor instead of the gears. That engine is a 2-stroke though and it is becoming less common nowadays because of emissions rules and economy.

The discolaration you see is on the air side of the system. The air coming out of the compressor can be around 200 degrees C at high loads ... the air out of the afterooler seldom exceeds around 45 degrees C (except in some small engines with jacket water cooled aftercoolers) there is always an exception so it is difficult to give specific figures.

"Is this normal?"

Sorry, after that long winded explanation I see I didn't answer your question ... yes, it is normal. It is hard to find a white paint that will withstand high temperatures for long periods without some discoloration.

Hi,

Marmot has written an excellent and well detailed explanation as to the workings of aftercoolers and turbos.

A way to beat the yellowing of your hot bits is to leave the engines the original CAT Yellow colour. This paint does not show the discolouration as quickly as White engines. I have been lucky enough to sail on two yachts with the engines still painted yellow. One I inherited and the other was my choice with the Owners agreement( My Avtar is the front of the Port Engine of this boat)- I know of two other yachts where the engines are being left yellow for the same reasons. It is thinner and not such a battle to get the wrench on nuts and bolts and when work/maintenance has been done it can be repaired with aerosol cans of touch up paint and it looks perfect again.

I have found with White engines that have been in service that when any paint repairs are done the are nearly always plainly visible as repairs and it detracts from the overall appearance of the engines.

Hi,

I have not worked on this model engine so can't tell you what cools the After Cooler.

Take a look at your sea Water Pump pipework. Where does the outlet go to? If it goes straight to the Air Cooler then you can be pretty sure it's Sea Water Cooled. If it doesn't then it's probably jacket water cooled although it could be cooled by jacket water that does not cool the engine but is a separate circuit.

How many header/expansion tanks does the coolant system have for this engine?

Posted at 0839 today: "Air passes through tubes in the cooler and, in the case of the 3126, heat is removed by cool seawater on the other side of the tubes."

There is one raw water pump. Is there more to your question than just an academic interest in how aftercoolers work?