The trouble with turbos: Why fuel economy can be worse, not better

BY PETER CHENEY

If you’ve ever noticed that your car burns more gas than its official rating suggests, you’re not alone, especially if you drive a car with a turbocharger.

Turbochargers are the new darlings of the automotive engineering world. Pushed to achieve government-mandated fuel efficiency gains, car designers increasingly turn to small-displacement, turbocharged engines. Almost everyone’s doing it. Ford offers its F-150 pickup with a turbocharged V-6 instead of the traditional V-8. Porsche just downsized the motor in its iconic 911 sports car from 3.8 litres to 3.0, but made it more powerful by adding a pair of turbochargers.

In theory, these new power plants offer the best of both worlds, offering at least as much power as a larger engine, but with lower fuel consumption. At least that’s the theory. The real world is another matter. Let’s have a look at a recent example.

When I picked up the Ford Focus One-Litre Ecoboost for an extended test drive, I was looking forward to some spectacular fuel economy numbers. On paper, the little Focus looks like a genuine efficiency contender: its has a six-speed transmission, start-stop technology, and a one-litre, three-cylinder engine with direct fuel injection, a variable-pressure oil pump, and a turbocharger.

The EPA rates the One-Liter Ecoboost Focus at 30 mpg U.S. city (7.8 L/100 km) and 42 highway (5.6 L/100 km). Those are very good numbers. But my results didn’t come close.

Around town, the One-Liter Ecoboost routinely burned 11 to 12.5 L/100 km – not much better than a 2.5 litre, all-wheel drive Subaru Outback I tested last winter. On the highway, the Ecoboost’s consumption fell to 8 L/100 km or so, which wasn’t terrible, but nothing to write home about, either. For a three-cylinder, one-litre car purposely designed to maximize fuel-efficiency, these were dismal results.

I wasn’t actually surprised. As I’ve learned through years of testing, turbocharged cars often return much worse fuel economy than their official ratings would lead you to expect. My findings parallel those of other testers.

Consumer Reports, for example, concluded that downsized, turbocharged engines typically achieve worse mileage than larger engines without turbochargers. In their tests, Ecoboost Ford Fusions using turbocharged, four-cylinder engines burned more fuel than their larger, naturally-aspirated counterparts.

When CR tested a turbocharged, 2.0-litre Ecoboost Ford Escape against a Toyota RAV4 with a 3.5-litre, non-turbo V-6, both vehicles got 22 mpg U.S. But in an acceleration test from zero to 100 km/h, the Ford was 1.5 seconds slower.

Green Car Reports also ran a series of tests to determine whether smaller, turbocharged engines were really the best way to get good gas mileage. Their conclusion: “… with the latest generation of small turbocharged engines replacing larger variants, the real-world gas mileage improvement is nowhere near that of the on-paper advantage in official economy.”

The real-world results that drivers are getting with turbocharged cars seem to defy engineering theory. The concept behind turbocharging is a sound one: by using the exhaust gas to spin a turbine that compresses the fresh air going into the engine, you recapture thermal energy that would otherwise be lost out the exhaust pipe.

Driven carefully, turbochargers do offer efficiency gains. But that efficiency can quickly disappear if you don’t drive with discipline. A turbocharged engine turns into a fuel-hog under hard acceleration, because the large volume of air being pumped into the cylinders must be matched by a larger volume of fuel.

Explaining the abnormally high consumption of a turbocharged engine under high load takes us into some interesting areas of engineering. To perform properly (and not destroy itself) an engine must mix air and fuel in a precise ratio. The perfect air/fuel ratio is about 14.7 parts of air to one part of fuel. This is known as a “stoichiometric” ratio, which ensures a chemically complete combustion event. If you introduce more fuel than necessary, you create a “rich” mixture, and part of the fuel passes through the engine unburnt, wasting gas and creating extra pollution. A lean mixture, on the other hand, saves fuel, but makes the engine run hotter.

A turbocharger changes the picture. Because it compresses the incoming air, the fuel injection system automatically adds more fuel to keep the mixture at the correct ratio. And this is where the problems begin. As the pressure in the combustion chambers rises, you run the risk of pre-ignition (commonly known as “knock”) – this is caused when raw ignites before the spark plug fires.

Knock is destructive (imagine millions of tiny grenades exploding inside your engine), but is easily prevented by computerized engine control systems that monitor fuel flow and cylinder in real time. If your engine is on the verge of knocking, the computers have an instant fix: they shoot extra fuel into the cylinders to cool things down. As you can imagine, this hurts fuel economy. But it does help your engine last.

With a turbocharger, hard acceleration results in extremely high cylinder pressures. In response, the fuel system shoots in plenty of extra fuel – and there go your fuel savings.

Avoiding fuel consumption spikes in a turbocharged car calls for common-sense techniques that work in any vehicle, but matter more than ever with a turbo. The key is to avoid large throttle openings. Accelerate gently, and don’t cruise at high speed. Keep your car as light as possible by taking out unneeded luggage, and cut down aerodynamic drag as much as you can – keep the windows closed, and don’t use a roof rack unless you need it.

The turbocharger is a smart piece of engineering. It can make a small engine feel like a larger one, and it can turn big engines into performance monsters. Just remember: your mileage may vary.

Peter Cheney is a former columnist, correspondent and feature writer for The Globe and Mail