A turbocharger in theory gives free additional power. This means that the turbo charger itself does not use any energy that could have been used elsewhere. The reason for this is that the turbo is driven by the exhaust gases as they leave the engine.
A turbo charger consists of two joined rotors, one in the exhaust flow, and one in the inlet flow. The exhaust gases pass through a housing that contains the exhaust rotor causing it to spin; this in turn spins up the one in the inlet. This rotation pressurises the air and allows a much denser charge of air than would otherwise have been possible, in effect it gives a small engine the displacement of a much larger unit. That is basically it and everything else in the system is simply ways of enhancing and controlling the operation.
The waste gate controls boost pressure by bleeding off excessive pressure, there are a few variations with the more modern ones being computer controlled.
Primarily for tuned and race cars the dump valve removes all of the pressure in the inlet manifold when changing gear. The reason for this is that the pressure in the system can dramatically slow the turbo causing lag. (Delay in power take-up)
The majority of current vehicles with turbos now have intercoolers fitted; this is because the air compressed by the turbo charger can be very hot due to the proximity of the exhaust gases and the act of compressing the air. Hot air entering the engine causes a couple of problems the first being an unwelcome increase in cylinder temperatures and secondly reduced engine efficiency. The reason for this is that hot air takes up a greater volume than cold and therefore the cylinder cannot fill to its capacity. Putting the air through an intercooler reduces the temperature but not the pressure as the turbo will have enough spare capacity to cope.
Turbo charged engines produce much more power than its normally aspirated cousin. It will also give off more heat, and have higher stresses placed on it. Typically a turbo charged engine will have lower compression*, better cooling and beefier engine internals, simply bolting a turbo charger onto a normal engine is not an option.
A turbo spins at insanely high speeds, (100,000 rpm is not unheard of) because of this good lubrication is essential or a seizure is always possible. For this reason turbo charged vehicles will often have a shorter interval between oil changes and specify fully synthetic oil. When an engine is stopped oil stops pumping, but a turbo charger can take a few minutes to spool down for this reason it is good practice to let the vehicle tick over for a few minutes before switching off, this is especially important after being driven hard.
Turbo lag has been mentioned before, it is simply the time it takes for the turbo to spool up when power is applied, basically the bigger the turbo the greater the lag but the greater the final amount of power. Some manufacturers solve this by fitting two smaller units rather than one monster, that being said all turbo charged vehicles have lag to some degree.
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