Turbochargers. How They Work

Everybody’ heard of a turbo. It’s become a byword for enhanced or supreme performance for something spinning faster and, in an engine, that’s exactly what they do; squeeze more performance out of a standard engine.

They manage this by packing more air into the cylinder, allowing far more fuel to be burned at every ignition, thus supplying more power, more thrust from the engine.

Superchargers perform the same job, ramming air into the combustion chamber, but do so through being powered off the crankshaft, robbing the engine of power that might otherwise go to the driveshaft. They also require a beefing up of the engine to cope with the added strain. Turbochargers however, don’t rob the engine of any power, operating as they do off the exhaust.

3D Supercharger animation

In 1885, Gottlieb Daimler first patented the technique of pumping air into an internal combustion engine, but it wasn’t until 1905 that Alfred Buchi patented the use of exhaust gases in creating this extra compression.

Their early development was through the aero engine industry, competing as they were with superchargers in enhancing engine performance, but they were also used extensively in locomotives and shipping. In the Second World War however, they were employed to great effect in the B-24 Liberator, the B-17 Flying Fortress and the P-38 Lightning. The technique of compressing and pushing more air into the combustion chamber coming into its own in the rarified atmospheres above 18,000ft where air is at a premium.

How they work

Unlike superchargers, turbochargers are powered by the superheated gases being pushed out of the engine, which are employed to power a pump that compresses more fresh air, for the air inlet.

However, it’s actually quite a complicated undertaking, as the air going in needs to be cool to prevent premature ignition, so there’s quite a lot of technology employed in separating the power that the exhaust provides from any adverse effects due to the intense heat involved.

But it’s this compression of air that creates extra power from an engine. The turbocharger is an impeller, which pushes a much higher volume of air into the air inlet than atmospheric pressure usually would. This in turn allows for a much greater quantity of fuel to be burned at each combustion, pushing the pistons down with far greater force.

Components

A turbo essentially consists of three main components: a radial inflow turbine, a centrifugal compressor and a centre housing.

·     The turbine harnesses the exhaust gases and can rotate at speeds of 250,000rpm.

·     The compressor is a mechanism employing an impeller and a diffuser.

·     The hub rotating assembly or ‘centre housing’ is the complex part, translating the rotation of the turbine to the compressor, at very high speeds and with extremely low friction. This can be achieved with either ball or fluid bearings and can be both oil and watercooled.

However, they have continued to get more and more complex, utilising auxiliary technologies to help improve their performance, notably with wastegates, intercoolers and blow off valves, but also more recently with e-boosting. 

Wastegates and blow off valves are concerned with the problems of too much pressure on the air inlet, which can produce knocking or premature ignition and also help reduce lag.

Intercoolers regulate and reduce the increase in temperature that compressing air creates, ensuring that the air going in is cool enough to prevent knocking.

One of the major problems encountered with turbochargers is ‘lag’. This is the time it takes for the engine to respond to a foot on the accelerator. With superchargers, the response is immediate, as it’s driven from the engine directly, but as the turbo operates from the last point in the combustion process, the exhaust system, there will always be a delay in the engine responding.

Straight turbochargers only start producing boost when a high amount of energy is being provided by the exhaust; anyone familiar with driving a turbocharged engine will be familiar with the distinct kick as the revs go up and the turbo kicks in.

There are several methods of changing an engine’s design to help compensate for lag, such as altering the turbine’s aspect ratio, increasing the compressor discharge and wastegate response, use variable nozzle or twin-scroll turbos, use an ‘antilag’ system or even use multiple turbos either sequentially, or in parallel.

The use of twin turbos, one small one large, is comes under this last prescribed method, as the turbos cut in at very different engine speeds, providing a smoother transfer of power to the wheels. This can be hugely effective.

Twin turbo outlaw race car testing

One of the most recent leaps in turbo evolution however, is the use of E-boosting. This is the use of electrical boosting, particularly in hybrid engines, which focuses on the improvement of the compressor speed, but also crucially, makes it independent from the turbine speed. The electric engine can rotate at speeds of up to 120,000rpm and, by divorcing itself from the exhaust turbine, also does away with the dreaded lag.

You can begin to see the advantages of that…

Mercedes hybrid turbo