More power: Supercharger vs turbo

A turbocharged Subaru Impreza WRX. Photo by Abubaker Lubowa

Both turbocharger and supercharger are forced induction systems, which are designed to force air into the engine. The more air and fuel into the engine, the more power it will make.

Superchargers and turbochargers have been around since 1930s, but they were not as refined as those installed in today’s machines.
The way the supercharger works allowed for better performance than the initial turbochargers, which eventually faded out due to the invention of fuel injection. However, by the 1980s, the turbo made a comeback, and has been constantly refined to offer the amazing results we see today.

More air, more fuel
In this environmentally conscious era, car makers are able to reduce engine displacement by installing a turbocharger or supercharger for extra power. Consequently, cars with engines as small as 1,000 cc can be more powerful than your average Toyota Premio while maintaining good reliability.

At the top end of the market, technology has offered performance cars two or more turbochargers where you hear buzz words like twin-turbo or Kompressor (which is really Mercedes’ marketing to mean supercharger).

Ultimately, smaller engine displacement means less fuel consumption and—the less cared about in Uganda—better carbon emissions.

Both turbochargers and superchargers work on the principle of forcing more air and fuel into the engine for a bigger explosion.

Without digressing and going into detail, I will describe the engine as essentially a big air pump working at atmospheric pressure, with the pistons sucking in air into the cylinder when they go down. The air is then combined with fuel and compressed when the piston moves up inside the cylinder.

What is the difference?
Once compressed, it is ignited by the spark plug. The subsequent explosion forces the piston back, where the gases leave the cylinder via the exhaust system, and so the cycle continues. In the most basic form, a supercharger and turbocharger force air into a standard engine running at atmospheric pressure. What are the difference between the two?

The turbocharger is anchored to the exhaust manifold, where the exhaust gases from the cylinders make it spin. The more exhaust gases passing through the turbocharger, the faster it spins thus pumping more air into the cylinders.
Turbos have to spin at speeds of up to 150,000 RPM (Revolutions Per Minute). For contrast, the tachometer on your car increases in intervals of 1,000 RPM. Because of this most modern turbochargers, use a fluid bearing instead of standard bearings.

Using this type of bearing allows the shaft to be supported by a layer of oil, which cools the shaft and turbo components, plus it reduces friction.
Some turbochargers are designed to tap into the normal engine’s oil system for lubrication, an Achilles heel when you use poor quality oil or take long to do an oil change.
This fact is confirmed by Donald Lule, an instructor with Nakawa Vocational Institute’s Motor Vehicle Section.

He observes it is one of the main causes of turbocharger failures. He adds that “a choked air filter will cause oil to be drawn into the turbo due to the high sucking pressure of the turbo charger.”
The downside to turbochargers is they do not provide an immediate response when you hit the accelerator.

This is because it takes a while (depending on engine revs and how much exhaust is produced) for the turbine blades to get up to speed before it creates the right amount of pressure. It is called “turbo lag”—a feeling of slow acceleration followed by a boost. Through different engine designs, there has been considerable progress towards reducing turbo lag.

Dealing with turbo lag

One way is reducing inertia of any spinning components by reducing their weight using ceramic blades. A case in point is in the Vertex Edition of Toyota Aristo with a 3L VVTi Twin Turbo. This allows the turbine and compressor to operate faster, producing boost as early as 1,800 RPM. Then there is use of two turbos of different sizes to counter lag. The smaller one goes first as it gets to speed faster while the larger turbo kicks in at higher engine speeds to provide extra boost. A good example is the twin-turbo Subaru Legacy B4.

Most turbos use an intercooler to maintain air supply to the engine to aid performance. Cool air is denser and contains more air molecules than hot air.

The intercooler is like radiator, with air passing over it from the outside, as well as air passing through sealed passageways within. Superchargers work on the same principle as turbochargers, although they create air pressure in a different way. While turbochargers use exhaust gases to create air pressure, a supercharger uses a belt connected to the crankshaft. Simply put, the engine spins the compressor, more air is forced into the engine. Some rotors in the compressor come in various designs, but its job is to suck in air, pressurise it and discharge it into the engine.

There is no lag
Like the turbo, supercharger’s moving components create heat under exertion, so intercoolers are needed. As the supercharger works from instant acceleration and does not rely on build-up of pressure from exhaust gases, it does not suffer “lag”.

You get instant power from low RPM all the way up. Also, superchargers do not run on hot exhaust gasses and do not spin nearly as fast as a turbo making them less prone to failure.

Another advantage is a supercharger is less complex and requires less modification of the exhaust system. It is usually bolted to the top of the engine, thus making it less expensive to install and maintain. But the downside is that it uses part of the engine’s power to spin the compressor.