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Auto News: Mr. Green Car: Turbochargers and the downsizing of engines

October 17, 2012

Image courtesy of engineerography.com

By Allen Penticoff
Free-lance Writer

In my last “Mr. Green Car” column (Oct. 3-9 issue), I test drove a new Dodge Dart Rallye with a 1.4-liter turbocharged engine. This is part of an industry-wide trend in downsizing engines for increased fuel economy while still providing the power consumers expect in passing and acceleration through the use of turbochargers.

The root of this technology is the turbocharger (and, in some cases, the electric hybrid motor). Turbochargers have been around for a long time, but mostly seen in trucks, aircraft and ships. What a turbocharger does is use the wasted exhaust gas heat heading out the tailpipe to create compressed air. This compression of air is routed to the engine’s air intake, where the denser air can be used to create more power for a given displacement.

When the gasoline or diesel burns in your engine, relatively little of it goes to actually making your vehicle move. Most of it goes out the exhaust. The turbocharger recovers that energy with a small turbine wheel that is connected to a small centrifugal compressor by a solid shaft. It is a very simple device in itself, but it needs high-tolerance metal work and exotic materials to withstand the heat. A valve on the exhaust system that controls the flow of exhaust to the turbocharger versus going directly out the exhaust is called a “wastegate.” Some systems have no wastegate, but control the compressed air going to the intake. Either way, the result is the same.

The system works particularly well at steady high power settings — which is why they work well on aircraft and big trucks. In aircraft, they are used to make up for the lower density of air at higher altitudes — turbochargers will have the same benefit in vehicles as well. But in the city, where the need for power is low, they have less effect because there is not enough exhaust flow to make the turbocharger do its job. The manufacturer has to balance drive-ability with the extra power — computers do this now, but there is no getting around the physics of how the system works. A turbocharged car or truck will not be quick off the line (in normal situations), but will have much more power as the vehicle gains speed (and more engine rpm).

New cars, as in the case of the Dodge Dart Rallye, are after high fuel economy for city and highway driving. Moving slowly in normal city traffic, only a small amount of horsepower is needed to do the job — such that a small displacement engine will suffice. But customers still expect good highway acceleration — so at these speeds, a turbocharger can provide the needed boost in performance to yield the desired need to pass. After the acceleration (and its high exhaust pressure) is over, the turbo sort of coasts along, and the engine reverts to a more normally aspirated mode, where the highway fuel economy of a smaller engine can maintain a cruising speed.

If you choose to buy a turbocharged car or truck, you will need to be religious about oil changes. Turbochargers run very hot and are hard on oil. Ignore your oil changes, and the turbocharger will die from baking dirty oil on its bearings. Synthetic oils have relieved some of this concern, but I would not shirk the recommended changes of the manufacturer to save a few dollars. You will also need to adjust how you drive. The car/truck could be a bit sluggish in the first attempt to go — but will soon get much faster. You’ll need a bit more space to get into traffic if it is slow and tight — less so if it is moving fast — as with merging onto a freeway.

Not all turbocharged systems are created equal. You’ll need to drive it and see for yourself if the engine responds as you think it needs for your kind of driving. Don’t mistake a turbocharger for a supercharger — an entirely different system that does produce more power at lower speed (and favored by drag racers) but with a whole new set of complications — thus rarely seen on consumer street vehicles.

We’re heading in this direction. So, it is up to us to adapt to the new paradigm. It is good — don’t reject it for the old tried and true just because it is different.

From the Oct. 17-23, 2012,  issue

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