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Clearing the turbocharged air
Consumer Reports makes vague claims about the latest crop of turbocharged cars, forgets to include 'context.'
Posted February 19, 2013
My husband remembers how, when he was a teenager driving his dad's old Saab 900 Turbo around, he'd routinely be challenged to drag races. Other teenage boys (apparently sent down from central casting) would become enraptured by the word "Turbo" emblazoned on the car. To them it seemed to mean "afterburner" or "rocket boosters" or "way faster than you, little man," and they couldn't help it. They just had to see if they could beat it.

Some of this confusion over what "turbo" means in an automotive context still surfaces today. You can even see some evidence of it in the recent Consumer Reports critique, which charges that "small turbo engines don't deliver on fuel economy claims." CR takes a thorough, important look at the actual performance of some turbocharged engines and shows some challenging results, but they miss the boat in a few areas.

First of all, they start by saying that the cars don't meet the EPA fuel-economy estimates. Well, duh. Hardly any cars do that - it's well documented that the EPA's laboratory tests bear little resemblance to real-world road conditions, and that road performance should be expected to yield considerably lower MPG than the number the EPA puts on the window sticker. As the Dothraki say, this is known. So this is not a very substantive critique.

More significantly, CR doesn't give enough context for turbochargers' claims to provide efficiency benefits. No one claims that turbochargers will somehow reduce the amount of gas your engine burns. The claim rests on the idea that you want or need a certain amount of power from your car, power you'd usually have to get by having a big engine (higher number cylinders, larger cylinders, or both). The turbocharger says (figuratively speaking) that it can provide this same power with a smaller engine, by using a turbine to force more air and fuel into the cylinders every time they fire. So you can go down in size of car and of engine without making any sacrifices in power.

Did you spot the flaw, tucked away there, in assuming that something about this process will produce fuel economy? If you said "Wait - more fuel?" then you got it right! Turbocharger cylinders can be thirsty little buggers. They squeeze more air and more fuel into the cylinders, causing them to produce more power than they otherwise would - keywords more fuel.

How then does a turbocharger promote fuel economy, if it does? First of all, with a turbo charger on board, the engine and car can be lighter, which improves MPG. So, given a certain desired level of horsepower, it can be achieved with a smaller, lighter car. It also does a lot to boost the engine's internal efficiency - meaning that more of the energy created by the engine and captured and used, and less of it is lost and wasted.

For example, turbo is a direct injection fuel technology, which is exactly what it sounds like: gasoline is injected directly into the car's combustion chamber (mixing with air flowing in through a valve). This means that more of the gasoline is directly captured by the engine - so less of it is needed to push the pistons. Also, because the gas has less distance to travel to reach the combustion chamber, it has less time to heat up, meaning it's cooler when it gets there. That makes it possible to compress the gas more and pack it more tightly into the cylinder, thus creating more power.

The turbocharger also yields greater low-end torque than a conventional engine. That means you can get more power at lower RPMs, so you can stay in a lower gear during regular (not high-speed) in-town driving.

Finally, turbo delivers power on-demand. If you're driving an 8-cylinder 5-liter truck because you need the power sometimes, it's nevertheless using all eight cylinders all the time and consuming all the gas necessary to fuel all eight whether you particularly need them or not. (That said, there is recent technology that allows 8-cylinder engines to shut down some cylinders when not needed, but that's an optional exception, not the norm.) But a turbocharged engine goes along much of time not even using the turbocharger, kicking in only when you need to merge onto a highway or pass a truck (or street race like a whippersnapper, I suppose).

Two things (at minimum) should be clear by now about the turbocharger's so-called claims to improved fuel economy. First, it has to be understood as relative to the engine's horsepower. The claim is not that it's fuel-efficient in the way a hybrid is, using really small amounts of gas. It's that you can get the power of a larger car/engine in a smaller vehicle and smaller engine.

Second, it really matters how you drive it. The importance of this cannot be overstated. Think about it: turbo only kicks in when you really need power, but while it's delivering the power, it's guzzling gasoline like Gatorade. So, what happens if you just happened to like the feeling of that power kicking in? If you love peeling away from stoplights, if you have to pass every other car on principle, your turbocharger will not deliver any fuel economy benefits. Just the opposite, in fact. That's true of most cars, to some extent - lead-foot equals lousy gas mileage - but it's really true of turbocharged cars. To make it really deliver on the fuel economy promise, you have to drive like you're nearing the third level of transcendental meditation.

The CR report makes no reference to the strong influence of driving behavior - though to be fair, CR performs its fuel economy tests under controlled conditions and drive all the cars exactly the same way. So if a turbocharged car emerges from that test with the same mileage as the corresponding non-turbo version, that's important - but without considering how a turbocharged car would deliver on its fuel economy potential, CR doesn't have as much support for its conclusions.

Neither does it address the need to put the cars' MPG in the context of horsepower. The report
Blithely compares the mileage of the turbocharged cars first with this car and then with that, sometimes mentioning the horsepower and sometimes not, never expressing the mileage and horsepower as a ratio or even in regard to one another. If you're trying to test the claim that you can have a smaller engine without sacrificing power, it seems like power should be a prominent and consistent part of the analysis.

When CR does discuss power, it's in a way that's completely out of whack with how the turbocharge works. In Europe, automakers have been using turbochargers for much longer, and they use it quite differently from North American automakers. Europeans tend to produce way-underpowered cars (say, 1.5 liters, 2 cylinders) and then use a small turbocharger to keep you from getting crushed by a semi when you try to merge onto the highway. Europeans pay little heed to North American's all-hallowed zero-to-sixty benchmark - that's what they call a "sprint."

Instead, Europeans are more interested in "overtake" as a benchmark - the car's ability to steadily and consistently accelerate on demand while already at moderate to high speeds. That's totally practical, if you think about it, because that necessity comes up a lot more than the need to get to sixty MPH as fast as possible. Measuring acceleration only in terms of time from zero to sixty pretty much ensures that you wouldn't capture the power benefits of the turbocharge.

In the end, turbochargers in fuel efficient little cars aren't there to add fuel efficiency - indeed, they'll reduce it if you drive like a whippersnapper. To have a car that's good on gas when you drive like you care with the added bonus of being able to overtake a tank truck before the passing lane ends, that's the bonus. And that's what it's for.
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Newsroom Notes
Clearing the turbocharged air

File Under:
Engineering, Green, Automotive, Consumer Reports
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