Turbocharger TechPutting wasted energy to good use
It's amazing just how long turbos have been in existence. The first design drawings for a diesel engine turbocharger were patented in 1905 by Buchi, a Swiss engineer and inventor. He knew he could increase the output or torque of the engine dramatically simply by increasing the airflow and fuel into the cylinders.
Today, that same principle still applies to both gasoline and diesel engines, and turbocharging is now a routine way to produce more power. In fact, Gale Banks, a noted turbocharger expert, believes that a diesel engine isn't complete until it's been turbocharged.
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.
Buchi didn't live long enough to see his turbo design catch on, which really became popular in the 1960s, especially in Formula 1 engines. Despite having only a puny 1.8 liters of displacement, they could pump out more than 1,000 hp! Today, turbos are found on everything from dragstrip Mustangs to lumbering RVs.
Mix 'n Match
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.
What about twin turbo set ups? As Duttweiler points out, "A single large turbo can make as much horsepower as two small ones, but the advantage of a dual system is that you can get them to respond earlier and make more power down low in the powerband. Sometimes maximum horsepower is not as important as a broad powerband."
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