By Jim Kerr
Turbocharging is becoming popular again. BMW, GM, Mazda and Acura have already introduced, or soon will be introducing new turbocharged models. Sure, there have always been vehicles around with turbos such as Audi, Saab or VW, but why the resurgence in turbos now? It’s because of two things – performance and horsepower.
With fuel prices where they are, it is hard to believe the manufacturers are in a horsepower race, but they are and consumers are eager to buy them. Yes, there are a few big horsepower cars without turbochargers that are putting out more than 500 ponies, but even small displacement engines are putting out more horsepower per litre displacement than specifically designed race cars did just a couple decades ago. Turbocharging is one way of increasing that output per litre, yet keeping the engine small so fuel economy is good too.
Turbochargers, in the simplest terms are nothing more than air pumps. Pump more air into an engine and you can add more fuel to produce more power. That makes it sound like a turbocharger would decrease fuel economy, but a turbocharger only produces “boost” or higher air pressure when the driver demands more performance. Most of the time, it is sitting there, idling along.
A turbo is driven by exhaust gases – energy normally wasted in non-turbocharged engines. Internal combustion engines typically only use about 30% of the fuel’s heat energy, so harnessing more of it with a turbocharger makes the engine more efficient. There are three main parts to a turbocharger. The centre housing contains the bearings, shaft and seals. On each end of the shaft, there is a finned wheel inside a scroll housing (think of a snail shell – the housing gets progressively smaller as it forms a coil). One housing is connected into the exhaust system and a finned wheel, called the turbine, spins as exhaust flows past it. This turns the shaft that spins the second wheel, called the compressor wheel, that compresses air into the intake manifold.
Turbochargers spin very fast. Cruising down the highway, the turbo wheels may be “idling” at 30,000 rpm. Step in the gas pedal and the wheels accelerate perhaps from 80,000 up to 100,000 rpm as more hot exhaust gases are forced past the turbine wheel. It takes a little time
for the turbocharger wheels to accelerate after a driver steps on the gas pedal and this delay in boost and performance is known as turbo lag. Large displacement engines may have enough torque that turbo lag isn’t noticeable, but it was on smaller engines. However, vehicle manufacturers have largely overcome turbo lag using newer materials and methods.
BMW 3.0-litre, twin-turbo inline six cylinder engine. Click image to enlarge
One of the simplest ways is to install two small turbos instead of one big one. “Bi-turbo” systems have smaller diameter turbine and compressor wheels, so they can accelerate more quickly yet still deliver the same volume of air as a larger turbo. Lag is not perceptible because of the small turbo’s quick acceleration. Adding a second turbo may sound complicated but Bi-turbo systems can actually be easier to install on “V” design engines such as V6’s or V8’s. Exhaust system routing is often simpler on “V” engines, although BMW is using Bi-turbo on an inline six-cylinder engine too for more responsive performance.
Another method of reducing turbo lag is to use variable vane turbochargers. These units have moveable blades inside the exhaust scroll that change the direction of exhaust gases at the turbine wheel. Controlled by the engine computer, the blades open to allow gases past
the turbine wheel at cruising speeds but change the exhaust flow to more directly at the turbine wheel during acceleration. Variable vane or variable geometry turbochargers offer the performance of a small turbo with the capacity of a larger one.
Intercooling goes hand in hand with turbocharging. Any time air is compressed, it generates a lot of heat. Hot air expands and contains less oxygen per litre, so engine performance decreases. Intercooling is a method of cooling the compressed air before it goes into the engine so performance remains high. Most intercoolers are air-to-air systems. The compressed air is forced through a radiator-like heat exchanger and cooler outside air flows over the fins to cool the compressed air.
Liquid-to-air intercoolers have been used too, where coolant is pumped through part of the heat exchanger housing to cool the air inside. Liquid-to air systems provide more consistent performance as they don’t have to depend on outside air temperature variations, but they add complexity to the system, so most manufacturers use air-to-air intercoolers.
Another benefit of turbocharging is the high swirl produced by forcing the air into the cylinders. This swirling air mixes well with the fuel and promotes complete combustion. Direct fuel injected engines often use turbocharging to improve the combustion process in the cylinder.
Because a turbocharged engine produces lots of power per litre, engine parts have to be stronger to withstand the additional stresses. This is no different than building any hi-performance engine, but does add to the cost of production.
Finally, lubrication is critical for turbocharger survival. The turbocharger shaft is spinning as soon as we start the engine, and letting the engine oil warm up before demanding performance from the engine will help the bearings (bushings actually) survive. Also, after driving at higher speeds, the turbo is extremely hot – as hot as the exhaust manifolds. Letting the engine idle for a minute or two so that lubricating oil can help cool the turbo bearings and housing will extend turbocharger life.