Plain and simple, turbochargers can make a hell of a lot of power. Forcing compressed air into the cylinders of an engine is a surefire way of punching out fat torque and kilowatt numbers. Sounds like perfection doesn’t it. Well as we have all heard before from our mother, boss or significant other, we don’t live in a perfect world, and this unfortunately applies to the world of turbos. It is inevitable that all there will be some negatives to the positives when considering forced induction. In this article we will take a look at the positive and negative aspects of turbocharging which should shed some light onto the processes involved in setting up a good turbo system.
Negatives:
Heat is one issue that turbochargers have to content with in large doses. The problem arises due to the exhaust turbine being so close to the intake. The heated exhaust gases spin the exhaust turbine, but it is difficult to prevent heat transfer from occurring. Heating the intake air charge is the last thing anyone with a turbocharger wants, and is the reason intercooling is so important. When the intake charge is heated because of heat transfer, the adiabatic efficiency of the turbo is adversely affected.
Another problem which stems from the need for a turbo to have exhaust gases and intake charge running very near each other is plumbing and location in an engine bay. Nowadays most engine bays are jam packed with all sorts of items, mainly due to the increased use of electronics. This leaves minimal room to fit turbo chargers and the necessary piping. Engines such as inline 4 and inline 6 cylinders are easier to fit because of their size and narrow lay out as opposed to a V configured engine with two separate cylinder banks. Piping is still difficult though. A turbocharger is inevitably going to be equipped with an intercooler and this means extensive piping allowing more heat to enter. Pressure loss is also a side effect and more potential for laggy response because the air has to travel further.
The peaky delivery of turbo boost is also not a desired characteristic. This power delivery stems from the fact that boost is produced in relation to the exhaust gases which are spinning the impellers. As opposed to say a centrifugal supercharger, which spins in direct relation to the speed of the engine, the turbo charger produces boost as the exhaust gases flow faster, creating a more peaky effect in power delivery. This is why it is so crucial to size a turbo relative to the size of the engine. This involves matching the turbine size and design to the impeller size and design, in order to produce a smooth, strong power delivery.
Lastly, a turbo charger is technically a restriction on the exhaust system. This is why turbocharged cars are generally quieter than naturally aspirated or supercharged cars. The turbo acts as a muffler to a degree. A supercharger needs horse power to actually be turned which is obviously an instant loss, but exhaust flow will inevitably be affected in a turbocharged set up .
Positive:
As explained above, a turbo is powered by the exhaust air flow. When the exhaust valve is open, the pressure which is in the cylinder is still significant. The cam opens the valve which releases this pressure. The pressure forces the exhaust gas out of the cylinder at an incredibly rapid velocity. The turbocharger harnesses this wasted energy from the fast flowing exhaust pressure and uses it to spin the exhaust turbine, which in turn spins the compressor to compress or pressurise the intake charge effectively
Another thing to note is that because the turbo is a restriction to the exhaust system, it actually raises the pressure in the exhaust system. If this was a naturally aspirated system, a restriction in the exhaust system would inevitably result in a loss of power. This is correct, but because the turbo captures this energy that would otherwise be sent on its way into the atmosphere and wasted, the turbo is capable of moving a sufficient amount of mass with less effort and fuel than a supercharged system. A simple way of looking at it is the turbo pressurises the intake by pressurising the exhaust.
While a turbocharged set up is often more complex to build and design, and there are quite a few factors to pay attention to, there is no getting around the facts. A well designed and properly tuned and set up turbo system will allow an engine to develop more horsepower for a given amount of fuel. The brutal fact is that turbochargers are much more efficient than a supercharger set up when tuned properly.
Apart from the efficiency of a turbocharger, they can also be tuned to adjust power out-put at a wide variety of loads and engine speeds. This can alter the characteristics of the car and its performance, as well as the delivery of the power and the peak output. Electronic boost controllers also give turbocharged vehicles the opportunity to change their boost pressures on the fly, just incase a little bit more psi is necessary.
The widespread usage of electronic fuel injection in passenger cars also paved the way for turbocharging. Accurate tuning was possible and nowadays turbochargers are being used to produce power in a number of cars. Almost all diesel engines are powered by turbochargers and manufacturers are turning to turbos as the way of the future. They can produce nice power and yet do it efficiently and cleanly by still staying under the stringent pollution laws.
In the older days, when tuning and setting up a solid reliable turbo system was quite a difficult task, supercharging was the ideal choice. Turbos gained a bad reputation and were still seen as a bit of an enigma. While people have been experimenting with them for a long time, it was the turbo F1 cars that really demonstrated the potential of turbocharging. With the BMW 1.5 litre turbo churning out an estimated 1500 hp in qualifying tune, it was only a matter of time before turbo charged hit the streets. |
