Automobile Tech 101 – What is a Turbo? Supercharged?

Hybrid-electric cars have obviously received a lot of attention as a way to reduce fuel consumption. Higher fuel prices have also increased interest in an older technology like Diesels and turbos (a.k.a. turbochargers). Ford, in particular, is branding a new line of turbocharged engines tuned for fuel-efficiency as “EcoBoost.”

Some readers may remember turbocharged vehicles from the last fuel crisis as they were quite common in the late 1970s and 1980s. Since then, however, few manufacturers have been pushing turbocharged engines. Two obvious exceptions include Saab which has been a big proponent of the technology for economy and Porsche, which has offered crazy-fast turbo 911s on and off over the years. It’s also hard to leave out Subaru and Mitsubishi who make performance versions of the Impreza and Lancer with turbochargers… like the Porsche, these aren’t primarily intended to save fuel. Diesel fans also know that turbos and Diesel engines go together like red and Ferrari.

I imagine that there are a lot of car buyers who really don’t know what a turbocharger is, how it works, and how it helps to improve fuel efficiency. I’ll try to answer these questions. Additionally, I want to clarify the difference between a turbocharger and another common device the supercharger.

Basic Mechanics
To explain how turbos work we need to start with some basics of the internal combustion engine. Very basically (and overly simplified for this purpose), car engines function as air pumps… a certain volume of air enters the cylinders and is mixed with fuel. This fuel then ignites, expands, and pushes the piston, thus rotating the crankshaft allowing for propulsion of the vehicle. Again, very generally, the bigger the volume of the engine the more power it creates.

Superchargers and turbochargers both aim to increase the ratio between the volume of the engine (aka “displacement”) and the power it can create. They do this by compressing the air that enters the combustion chamber, thus pushing more air into the same space which can be mixed with more fuel making more power with each ignition cycle. So an engine of a certain size can produce more power by forcing more air and fuel into each cylinder than the same engine without a turbo.

Another way to understand this is to think about airplanes. Early in the 20th century, piston-powered airplanes were limited in the altitude at which they could fly because the thin air at altitude made for less power the higher one flew. Prior to WWII, aircraft engines started to employ turbochargers to compress the air entering the engines, thus making up for some of the power losses due to the altitude. Similarly, turbocharged cars tend to suffer less than normally aspirated vehicles at higher altitudes.

Turbo vs. Super
The manner in which this compression is created is the difference between a turbocharger and a supercharger. Superchargers are driven mechanically by the engine… commonly by a belt driven off of the crankshaft. This then turns the supercharger’s compressor which speeds the air entering the cylinders.

A turbocharger, on the other hand, makes use of otherwise wasted exhaust pressure to turn a compressor. The exhaust gases are routed through a turbine which in-turn is connected to the blade that speeds up the fresh air entering the engine. Because of its use of waste energy a turbo is ultimately more efficient than a supercharger. Here’s a good diagram of the internals of a turbo.

Efficiency
But wait, you ask, if you are simply pushing more air and more fuel into an engine, how does this save gas? Well, the answer is that the turbocharger isn’t running all the time. At wide open throttle (WOT) or when you need extra acceleration like entering a freeway, the turbo can run. At other times, say cruising at low speed or on a long stretch of freeway, you don’t need the turbo. The turbo basically allows you to use a smaller engine than you would normally need while allowing for performance equal to that of a larger engine. And smaller engines, all else equal, are more efficient when you are cruising. This is similar in thought to cylinder deactivation schemes employed by some manufacturers which run an 8-cylinder engine on only 4-cylinders when conditions allow.

Of course, there are other efficiency benefits to a small turbo charged engine over a cylinder deactivation system. One is that smaller engines can also weigh less than a bigger engine. A V8 running on 4 cylinders uses less fuel but you still have to carry around the other 4 cylinders all of the time. Engines with fewer cylinders also have fewer moving parts and thus lose less energy to friction.

Downsides to turbocharged engines
If turbos are so terrific, then why aren’t they used more often? There are downsides. Turbochargers themselves are expensive. They get extremely hot and turn at high speeds so they need to be made to very tight tolerances to survive.

Additionally, all of that extra air pressure puts more stress on the rest of the engine parts. Early turbocharged engines kept self-destruction to a minimum by reducing the compression of the engine from its normally aspirated form. However, this led to reduced power when not under boost (while the turbo was compressing the intake air).

The term “Turbo Lag” that you might see in enthusiast magazines refers to the delay between pushing on the throttle pedal and the engine providing full acceleration. Traditionally, turbocharged engines suffered from this lag which made driving around town or on a race track somewhat tricky. The reduced off-boost compression in turbocharged engines made this weak off-idle acceleration even more pronounced that in non-turbocharged engines of the same size. Further exacerbating turbo lag is that the turbocharger needs a flow of exhaust gas to turn quickly. Acceleration from idle or slow speeds caused a delay while the turbine in the turbocharger could receive this flow and start to rotate. This is one reason that superchargers are popular in some applications (like drag racing, for example). Because they create boost through a mechanical link with the engine they don’t need to “spool up” with exhaust gasses, they can help create more power nearly instantaneously.

These days, manufacturers claim to have largely solved many of these downsides. Some newer cars with turbos use two smaller turbochargers in place of one large unit. The theory is that smaller lighter turbines can accelerate more quickly than a large one. They can also be used in sequence, thus allowing for tuning at different levels of rpm. Ford’s aforementioned EcoBoost V6 uses twin superchargers, as an example.

Modern engine control electronics also allow for longer life without the need to reduce engine compression as much as in the old days. Direct injection (spraying the fuel directly into the combustion chambers rather than further upstream into an intake manifold) makes for increased efficiency and also allows for a cooler fuel spray into the engine, keeping that heat down.

Long-term reliability will be seen, but the basic idea of a modern turbocharged engine seems like a great way to increase fuel economy without reducing performance or increasing weight and complexity as with a hybrid-electric car. Surely this is another example of an old technology coming back to help with a modern problem.

Steve Haas

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