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How To: Turbocharger Basics

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Quite simply, a turbo is merely an exhaust-driven compressor. Imagine a small shaft about the size and length of a new pencil. Now rigidly attach a pinwheel to each end of the pencil. One pinwheel (called the turbine) is placed in the path of the exhaust gases which are exiting the engine. These gasses are caught in the turbine, causing it to spin. This in turn spins the whole shaft, along with the pinwheel on the other end (called the compressor). The compressor is placed in the intake air’s path; once it begins spinning, it actually compresses the air on its way into the engine.

Why is this beneficial? Well, normally aspirated engines have to work to draw in their intake air (this is where a Ram Air system can benefit normally-aspirated engines)! In other words, as the intake valves open, the piston’s downward movement creates a vacuum which ‘sucks in’ some air through the intake system. Ideally, the piston’s movement would suck in 100% of the air that could fill the combustion chamber. In the real world this is not the case; the typical engine will draw in only about 80% of the total volume of the combustion chamber. There are many reasons for this: intake restrictions, valve timing, camshaft design, and much more.

Now imagine that the engine mentioned above has a turbocharger. When the turbo compresses the air it builds up pressure in the intake manifold. Now when the intake valves open, air is actually forced into the combustion chamber. (This is one reason why turbocharged engines are sometimes referred to as ‘forced-induction’ engines.) As you might imagine, this allows more air to fill the chamber.

The beauty of turbochargers is that they make use of wasted energy thrown away by the engine. Unlike superchargers that require a mechanical drive off the engine’s crankshaft (and thus rob power), a turbo is driven by the pulses and pressures of the exhaust gasses. This exhaust flow spins a turbine wheel that shares an axle shaft with an air compressor, so they turn in tandem at the same rate. Once their rotation comes up to a certain speed, pressure or boost builds up in the manifold. So instead of the engine having to suck in the air/fuel mixture by the vacuum draw of the pistons, this pressurized intake charge rushes into the cylinders and “overfills” them, producing more torque and horsepower.

Although the basic principles behind a turbo are fairly simple, the components themselves are complex, especially in the mechanical tolerances required to rotate parts in excess of 100,000 rpm. If the balance of the shaft and wheel assembly is not absolutely precise, they will self-destruct.

Not only that, a turbo has to be matched to a specific application. Getting a dragster off the line requires a completely different approach than getting a Winnebago to climb up a hill. Here the design of the compressor’s wheel and housing comes into play, because that determines the maximum flow (measured in pounds per minute) of air at a given amount of manifold pressure (boost). For example, as renowned engine builder Ken Duttweiler points out, “If you want to make 500 hp from a single turbo, you need a compressor that flows at least 50 lbs/min of air. You can figure this by dividing the maximum horsepower by 10 to get the airflow required.”

The turbine wheel spun by hot exhaust gases has a comparatively easier job. Even though the compressor is the most important thing to pick first, the wrong turbine wheel combination can ruin a good compressor match. The turbine wheel’s diameter and exducer (exit point) size are important for controlling speed and response time. For instance, a larger outlet will increase the amount of flow, but can slow turbo response. Also, the turbine must be lightweight, yet heat-resistant at the same time, so sometimes they’re made of exotic material such as inconel or Mar-M (a steel-nickel alloy developed in the aerospace industry), particularly where long-term, hard use is expected. Otherwise, a steel alloy called GMR is more commonly used.
Due to the high temperatures and rotation speeds, oil lubrication is the lifeblood of a turbo system. The only thing supporting the shaft and wheel assembly is a micro-thin film of oil, usually supplied by the engine. Note, however, that oil can break down or cook from high temperatures and cause what is known as coking, leaving deposits on precisely machined components. To prevent this from happening, before shutdown, a turbocharged engine should be run at a low engine speed to reduce heat and shaft speed. This procedure will also prevent a loss of oil pressure to the wheel and shaft assembly, which may still continue to spin after shut down and require lubrication. Some companies offer a turbo timer that maintains oil pressure for a set period of time after the engine shuts off.

Due to the critical importance of oil lubrication, the turbo housing must be positioned with the oil inlet on top and the oil return facing down. Make sure the oil return line is as large as possible to reduce to possibility of flow restriction.

When starting up an engine with a new turbo, allow it to reach operating temperature before running hard. Never rev a turbo engine right after startup, because there is insufficient oil pressure and film on the bearings. Not following this procedure can knock out a set of bearings and gall up the shaft.
 
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