It's good to see a sticky on this. It was also good to see a cutaway from a Mack truck engine turbo. I once worked in a shop (or three) that repaired trucks. Déjà vu.
The reason we use turbos is twofold.
Here at sea level, we use turbos to increase the "volumetric efficiency" of an engine. In other words, by forcing more air into an engine we can also add more fuel, and so we can get more power from an engine by making its combustion "volume" more "efficient." We can moderate the boost by controlling the amount of exhaust gas volume entering the turbine wheel by means of a "waste gate." The exhaust we don't need can be "wasted" by this controllable valve, sending it around to bypass the turbo and on into the exhaust system.
The other reason we use turbos is to maintain power. In many reciprocating aircraft engines, turbos are used to provide enough boost to maintain the power output of an engine running at sea level as the aircraft climbs in the atmosphere, where the air becomes progressively thinner. Aviation calls this “Turbonormalizing.”
Things to know: turbos spin incredibly fast, on the order of 20,000 rpm and more. They need air that is very clean, because particles of dirt can wear the compressor fins, and you don't want to be packing dirty air into an engine.
Turbos also need oil, and lots of it. Many turbo applications use a shutdown oil pump, designed to maintain oil pressure and oil flow to the turbo's bearings to carry away heat after shutdown. This really extends the life of a turbo, and as some of you know, they are not cheap to replace.
The same pump is often used as a "pre-luber," designed to supply oil pressure to every bearing in the engine and the turbo BEFORE it is faced with the loads of starting and initial running. This practice of pre- and post- lubricating extends the life of both the turbo and the engine it serves. If you want more miles out of a turbocharged engine, or if you want to extend the life of a supercharged engine, a lube pump is a great way to go.
One factor that shortens the life of a turbo is the level of maintenance the engine receives. I have seen blocked oil passages due to infrequent oil changes, leading to a total failure of the "center section" of a turbo. The center section is the part with the shaft, the wheels, and the bearings. The center section lives in the compressor and turbine shells or "snails," as described above.
Now for the downside. We pay a price in engine life, even with extra lubrication, when we increase the volumetric efficiency of many engines. Most of this is due to the increased power being produced by an engine that was not originally designed to be turbocharged, and the other factor is heat. The engine bay of a Mack or Freightliner is huge, and there is a massive, I mean REALLY massive radiator and intercooler there to keep heat at bay. In a Camry or other auto engine, we have a cramped space with few choices for dealing with heat. Why is that bad?
First, heat is created whenever we compress a gas, and in terms of physics, atmospheric air is a combination of gasses, mostly nitrogen. Engines don’t like hot air, which is why you see “cold air intake” kits for sale. Cooler air is more dense than warm air, and therefore has more oxygen per unit volume to support combustion. So, once we compress the air, we need to get rid of that heat so the air entering the engine will be more dense. An intercooler, either air-to-air or air-to-water is how we do that. Engines run better on cool, dense air. Drag racers want a low “density altitude” for their runs, a factor that encompasses both air temperature and pressure. So, getting the compressed air coming from the turbo to get “cool” is a priority for the best efficiency and the highest performance, here on earth or in the sky.
Second, if the engine was not specifically designed to be turbocharged, that engine will be challenged to deal with the additional stress and higher temperatures not faced by its normally aspirated cousin. In fact, even if it was designed to receive boost, the engine will not live as long without a rebuild. For example, a typical small aircraft engine will last around 2,000 hours of running time. When it is turbocharged, the same engine will be expected to be serviceable for several hundred hours less.
Clean air in
Lots of oil
Cool the air
Watch engine heat
Consider pre- and post- lubrication.
Last edited by timebuilder; 01-19-2007 at 10:08 PM.