Some of the drawbacks to turbochargers have been discussed in our supercharger section. Here we will talk about the benefits to turbochargers and whether or not they have any effect on fuel economy. A turbocharger, (they used to be referred to as turbosuperchargers), is attached to the exhaust system of a vehicle. Some are found directly after the exhaust manifold or much further down the line in the exhaust stream. Turbos use the high temperature exhaust gas to spin a turbine which in turn rotates a compressor. The compressor takes ambient air like a supercharger and further compresses it to be reintroduced into the motor for additional power. The principles of operation are similar to superchargers in that they both compress air, however the main difference is that a turbo uses spent exhaust gases to spin the device while a supercharger uses the power of the crankshaft to turn a belt.
Turbochargers can offer massive gains in vehicle horsepower, from increases on the order of tens to even hundreds of horsepower. This all depends on the size of the motor being modified, how much boost is run on the turbo, the fuel system, etc. Boost is a term used in forced induction to denote the pressure above atmospheric pressure that the turbo is putting into the system. The higher the boost pressure the more horsepower and torque the vehicle will make. However there is a limit to how high boost pressure can reach before damaging vital engine parts. This differs between vehicles. This safety limit is usually controlled via use of a waste gate within the turbo system. A waste gate will open when a turbo is spinning at too high of an rpm such that no additional boost is created that would damage the motor.
Intercoolers are sometimes used in turbo systems to further cool the compressed, heated air before it enters the intake manifold. As a gas is compressed its temperature increases. So naturally as the turbo compresses air it increases its temperature. This has the negative side effect of decreasing the density of the air intake charge which decreases its horsepower potential. So by compressing the air and then cooling it even further, more power can be created than before. Intercoolers are quite similar to radiators however most use an air to air exchange system. This means that the air flowing into the intercooler as you drive cools the fins which cool the heated compressed air running through the system. There is no liquid involved as in a regular radiator that uses antifreeze.
Fuel Economy Drawbacks
In some cases increased horsepower comes at a cost: fuel economy. Just like with supercharger systems, turbochargers require additional fuel added to the system to combust with the extra oxygen now produced by the turbocharger. The extra fuel consumption decreases fuel economy when the turbo is engaged. However if you drive the vehicle around normally at a low enough rpm most of the time such that you do not significantly engage the turbo then you may not lose too much gas mileage. The heavy loss occurs at higher rpms.
Additional Pros and Cons
Turbocharged systems are more efficient power producers than superchargers because they utilize already existing exhaust gas to provide the power necessary to spin their turbines. Unlike superchargers they do not rob extra horsepower from the motor to operate. This means there is the possibility for more power available in a turbocharged system, all other variables being considered equal of course. In supercharged systems sometimes the power robbed from the engine to drive the supercharger is up to 60% of the power gained in the end! This means that if you gained 50 horsepower from a supercharger you may actually have gained 125 horsepower but it took 75 horsepower from your car to spin the charger so your end result net gain was only 50 horsepower.