How to identify different pipes / hoses on aircraft engines?
You can guess what most of the tubing goes to if you know a few of the parts under the cowl and have a general understanding of how engines work. You can also get a good idea of what each part is by studying the parts catalog for the aircraft. You can view the Parts Manual for my Cherokee and an Arrow that I fly. 

Piper Arrow Six Cylinder Continental



Let’s start with easy ones to identify. You should be able to find the tailpipe rather easily.  Notice in the picture of the Arrow that there are a lot of big orange tubes. That’s called scat tubing and it is used to move air around the engine compartment. In most singles, there is a shroud around the muffler that has orange hoses coming out of it. The ones going into the firewall provide cabin heat and defrost. The ones going to the carburetor provide carb heat to prevent icing.

Notice the rusty pipes coming out of the bottom of the engine. Those are the exhaust pipes and it’s hard to see in this picture, but they connect to the muffler. The big black pipe at the top of the engine is connected to the air filter and is the air intake. The clean pipes along the top of the engine are the air intake tubes. Most fuel-injected engines have the air intakes on the top of the engine.

Fuel injected engines also have small lines that put fuel directly into each cylinder. The line entering the "spider" at the top is a fuel line and you can trace it back to the fuel pump. 



The brown hose in the picture below going to the spider is a fuel line.



Cherokee Lycoming Four Cylinder Engine



Compare this to the carbureted engine in the Cherokee. The more rusty pipes are the exhaust and the cleaner blueish pipes come from the carburetor with air and fuel.

If you can find the vacuum pump (the dark item to the right of the oil filter with the red bands) you can identify the hose going to the attitude indicator and heading indicator. There’s usually a filter attached to the hoses and a line leading to the vacuum gauge—although sometimes that is inside the cabin. The big open-ended hose is the exhaust for the pump.



Here’s another four cylinder where you can see the intake and exhaust. Notice the silver lines with the blue fitting. Those are primer lines. At the bottom left is an oil cooler. There is scat tubing bringing air to the cooler and two lines going in and out.  



Oil coolers are often in the front of the cowl like this one. If there are lines attached to something that looks like this, they are oil lines.



This is a different method of plumbing carb heat. Rather than coming off the muffler hot air comes directly from the engine.



Here’s a picture with lots of hoses.



The two small hoses at the bottom of the picture are attached to the oil cooler. If you follow them back you’ll find the oil pump.

You can see the carburetor with a scat tube coming from the air filter and one on the back side coming from the muffler shroud. The two small hoses going into the carb are fuel lines.

The vertical silver tube running from the top of the picture, down past the muffler, and out the bottom is the crankcase breather line.

The box in the bottom right-hand corner has the electric fuel pump (boost pump). The line coming out of it is a fuel line. The silver tube in the box goes to the gascolator.

You should also see an oil line going through the firewall that measures oil pressure and one for fuel pressure but I don’t have pictures that show them.