Turbocharger (FAQ)
From Nissan 350Z & 370Z Wiki
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What Is a Turbocharger, And What Does It Do?
A turbocharger is an exhaust gas driven compressor used in internal-combustion engines to increase the power output of the engine by increasing the mass of oxygen entering the engine. The compressor increases the pressure of the air entering the engine, so a greater mass of oxygen enters the combustion chamber in the same time interval This greatly improves the volumetric efficiency of the engine, and thereby creates more power.
A turbocharger increases the pressure in the system, and the increase in pressure is called "boost". Boost is usually measured in pascals, bars or lbf/in², p.s.i, or mmHG. The energy from the extra air mixed with the right amount of fuel leads to more overall engine power. For example, at 100% efficiency a turbocharger providing 14.7 PSI of boost would effectively double the amount of air entering the engine because the total pressure is twice atmospheric pressure. However, there are some parasitic losses due to heat and exhaust backpressure from the turbine, so turbochargers are generally only about 70-80% efficient, at peak efficiency, because it takes some work for the engine to push those gases through the turbocharger turbine (which is acting as a restriction in the exhaust) and the now-compressed intake air has been heated, reducing its density.
For automobile use, typical boost pressure is in the general area of 8 - 14.7 p.s.i., but it can be much more depending on the vehicle and manufacturer. Because it is a centrifugal pump, a typical turbocharger, depending on design, will only start to deliver boost from a certain rpm where the engine starts producing enough exhaust gas to spin the turbocharger fast enough to make pressure. This engine rpm is referred to as the boost threshold.
Parts of a Turbocharged System
Most turbos use intercoolers, wastegates and compressor bypass valves to help provide more useable power. An intercooler is a heat exchanger that is used to cool the air heated by the turbocharger's compressor. Therefore, the intercooler can significantly boost power because the cooler air it provides is more dense than the hot air straight out of the turbo. An intercooler also takes thermal load off of the engine's cooling system by cooling the intake air. A wastegate is designed to regulate boost pressure, and is simply a valve in the exhaust system that allows some of the exhaust to bypass the the turbo when it is not needed. The bypass valve is a valve between the throttle and the turbo that vents extra boost pressure. When this valve vents to the outside air, it is called a blow off valve, and when it vents back into the inlet of the turbo, it is called a bypass valve.
A turbocharged engine's compression ratio must be lowered by using a lower compression piston, since an excessive amount of pressure will wear on the piston, connecting rods, and crankshaft, and destroy the engine. All of these parts then, as well as the transmission, must be strengthened on a turbocharged engine or it will be torn apart by the increased horsepower.
What Are The Downsides?
A disadvantage in gasoline engines is that the compression ratio should be lowered (so as not to exceed maximum compression pressure and to prevent engine knocking) which reduces engine efficiency when operating at low power. This disadvantage does not apply to specifically designed turbocharged diesel engines. However, for operation at altitude, the power recovery of a turbocharger makes a big difference to total power output of both engine types. This last factor makes turbocharging aircraft engines considerably advantageous, and was the original reason for development of the device.
Another problem turbocharged engines most overcome is that pressurizing air increases its temperature. This increase in charge temperature is a limiting factor for petrol engines that can only tolerate a limited increase in charge temperature before detonation occurs. The higher temperature is a volumetric efficiency downgrade for both types of engine. The pumping-effect heating can be alleviated by aftercooling (sometimes called intercooling) or water/methanol injection.
Also, some turbos often require cool down periods after use (meaning you have to let the car idle for a few minutes) everytime you want to shut the engine off. This is done to prevent the oil inside the turbocharger's bearings from "coking" or turning into sludge. Todays stock and aftermarket turbochargers are likely to be oil and water cooled meaning there is little or no time needed for cooldown after a drive.
General
Why is a Turbocharger Different Than a Supercharger?
The primary difference between centrifugal superchargers and turbochargers is the method in which they receive power. The term supercharger is very often used when referring to a mechanically driven turbocharger, which is most often driven from the engine's crankshaft by means of a belt (otherwise, and in many aircraft engines, by a geartrain), whereas a turbocharger is exhaust-driven, the name turbocharger being a contraction of the earlier "turbosupercharger". Because the turbine of a turbocharger is in-itself a heat engine, a turbocharger equipped engine will normally compress the intake air more efficiently than a mechanical supercharger. But because of "turbo lag" (see below), engines with mechanical superchargers are typically more responsive.
Can I install a turbocharger on the 350Z by myself?
Turbo installations can be much more involved than supercharger installations. A single turbo kit can be installed in a day or more on the 350Z, but a twin turbo kit can take 60hrs or longer to do right. Feel free to look over the installation manuals to see if this is something you can tackle, of if it is something better left to a professional.
Which one is the best?
This question is not answered easily. Please research each kit and decide for yourself what best fits your goals and budget.
Common Turbocharger myths
- Turbochargers all have lag and superchargers are way better!
First: All turbochargers have lag, whether they be run of the mill types used on a tractor trailer or of the semi exotic variable geometry ceramic vane variety. But in the shocker of the century all superchargers have lag as well. Lag is a function of the laws of physics and no matter what variety of forced induction you go with it will be there. On the other hand modern turbo technologies, specially those dealing with material science and aerodynamics, have advanced to the point where a well designed street setup will have lag that is order of magnitudes smaller than previous generations of turbocharged vehicles.
Second: Lag is the time delay between the time you stab the gas and the time in which the turbocharger spools to its limited maximum boost. Most people, when they speak of lag, are referring to boost threshold. Boost threshold is the time it takes for the turbo to produce any tangible boost. As with lag, boost threshold has been improved over the years and a modern, well designed system makes it much less of an annoyance than it once was.
Parts of a Turbo
