How Turbochargers and Superchargers work and why modern cars are using them again.
Forcing life back into smaller engines
Ever-tightening vehicle emissions regulations have led to manufacturers having to come up with a more complex way of reducing the amount of pollution coming from our car exhausts. One approach taken is what is often referred to as ‘engine downsizing’; engineering smaller capacity engines and using forced induction via turbochargers and superchargers to boost power output, thus enabling smaller engines to be used in larger cars. Both turbocharging and supercharging provide noticeable increases in performance, although there is no definitive answer as to which provides the best solution.
Turbochargers work by having the exhaust gas from the engine’s combustion cylinders directed into a turbine that spins at a very high speed. On the same shaft as the turbine is a compressor wheel that is in the inlet of the engine. This forces more air into the cylinder for the combustion process.
This method has two main benefits. The first is to help improve fuel economy while allowing the smaller engine to provide similar levels of power typically seen in larger capacity engines. When less power is required from the engine, usually when engine speeds are lower, the turbocharger is only partially or not in use. Where this often occurs is when driving in urban situations where the speed is lower, therefore increasing the levels of fuel economy, as it is effectively operating as a smaller engine.
Audi R18 e-tron quattro: VTG turbocharger ? the variable turbine geometry (VTG) improves responsiveness.
A familiar trait of older, less advanced turbocharged engines, was an occurrence referred to as turbo lag. This related to the delay in power delivery between the throttle being pressed by the driver and the turbocharger spooling up and delivering the greater power.
Turbocharging has enabled manufacturers to develop engines as small as 1.0-litre in capacity with as much as 140hp. The technology also features in almost all diesel engines now. Usually, these have a higher engine capacity, between 1.5- and 2.0-litre, although multiple turbochargers are often now used in larger diesel engines to deliver impressive performance levels with zero lag. When two turbochargers are used, it's quite common that one will be smaller to spin up quickly and one larger that takes over as engine speed increases.
3.0 TDI with electric biturbo
Advanced systems now even use smaller, electrically powered equivalents, to eliminate any noticeable effects of lag. The result is an increase in the levels of power that are available virtually from idle. Anti-lag systems in various guises have been in use in Motorsport for decades so it's not new technology it has just become more refined. As you can see from the two anti-lag solutions in the videos below, neither are really suitable for road use!
One of the downsides to modern turbocharging is that is does drive up the cost of producing such engines. The car maker carries some of this cost and absorbs it into the total car development program, but overall the prices of new cars with more advanced smaller turbocharged engines are increasing. They're also more sophisticated in terms of electronics so there's more to potentially to go wrong later.
An alternative to turbocharging used by some car manufacturers is the supercharger. The overall principle is the same, but the two work in different ways. Unlike turbos, superchargers are mechanically driven, usually powered via a belt that is driven by the engine’s crankshaft. This method means that the additional power created by the supercharger rises in line with the engine speed, resulting in a more linear power delivery.
Typically superchargers are more commonly seen on larger capacity engines. The way in which a supercharger works can put more of a load on the engine at lower speeds, so they suit bigger and more torquey engines. The upside is that superchargers can be more compact compared to turbochargers, which require additional piping and cooling systems. Another reason for a supercharger to be chosen is that it can be easier to package into an engine bay that may already be filled with a bigger motor. Fitting or replacing them can also be a less labour-intensive job.
Volkswagen is one of the few current car makers to use both turbocharging and supercharging. The concept of 'twin charging' was first employed by Lancia in its iconic Delta S4 Group B rally car in 1984, before making its way into the road going Delta S4 Stradale. This set-up eliminates the lag caused by a turbocharger and adds more power at the top end of the rev range, but naturally increases cost and complexity.
Of the two main principles of increasing performance, it is turbocharging that has been the more popular choice amongst mainstream manufacturers and this trend looks set to continue for the foreseeable future. Challenges that pose include engine, engine bay and inlet air cooling and long-term reliability, but the benefits certainly seem to outweigh such concerns for now.