Time was when midsize sedans and SUVs offered a choice of 4-cylinder or V6 power. Today, many vehicles have replaced the V6 option with a turbocharged 4-cylinder that promises great gas mileage — and yet time and again we’ve found that real-world gas mileage is nowhere near the EPA fuel economy estimates. Why is that?
For those who are unfamiliar, let’s start with a quick explanation of how engines work. Gasoline engines develop power by burning a mixture of fuel and air. The more fuel and air an engine can burn, the more power that engine can make. Larger engines develop more power because they can burn more fuel and air.
A turbocharger is a device that makes a small engine work like a large one — temporarily, that is. Like people, engines breathe: The movement of the pistons sucks in air, which is mixed with fuel on its way into the engine. (The ratio of fuel to air in a gasoline engine is fairly constant, so the more air an engine sucks in, the more fuel can be sprayed in to go with it.) The turbocharger is basically an air pump that is driven by the exhaust. When active, it pumps in more air than the engine could breathe in on its own, so the small engine can develop as much power as a bigger one — the caveat being that it is also using more fuel.
So why bother with a turbocharger? Why not just make the engine bigger? The key is that the turbocharger is driven by the exhaust, so it only provides a power boost when the engine is working hard. A turbocharged engine only delivers big-engine power (and uses big-engine fuel) when the driver gives it the beans. Under low-power situations — say, cruising along on the freeway — the turbocharger is dormant, and the car gets small-engine fuel economy.
Sounds good, right? So how come it doesn’t always work in the real world?
The problem is that the EPA fuel economy tests don’t exactly duplicate real-world driving. A turbocharged engine that delivers good numbers in its EPA tests may not do so in the real world, because a real-world driver may be putting higher demands on the engine. Automakers know the EPA test process, but they don’t know for certain what every real-world driver can do — so they tune their engines for good EPA test results, but not necessarily for good real-world gas mileage.
The real-world fuel economy of a turbocharged engine is largely dependent on how often the turbocharger is called into action. Urban and suburban driving demands lots of power, as the engine must work harder to get the vehicle moving, and a smaller engine will require liberal help from the turbocharger. Same for high-speed driving: Gentle cruising at 55 to 65 mph is a fairly low demand, but higher speed limits — or drivers who go faster — require more power from the engine. The more that engine relies on its turbocharger, the worse its fuel economy is likely to be. One of our correspondents tested a car that offered a choice of 1.4- and 1.8-liter engines, both turbocharged, and found that despite its higher EPA estimates, the smaller engine delivered worse real-world gas mileage than the larger one.
So how can you tell if your turbocharged car will get the fuel economy promised? Sadly, there is no easy way. The EPA’s website, www.fueleconomy.gov, lists real-world fuel economy from owners; unfortunately, not all cars have enough participants to give meaningful answers. We recommend using the car’s built-in trip computer during your test drive; be sure to follow a route that mimics the type of driving you do on a day-to-day basis.
Meanwhile, if you own a turbocharged car, there are a few things you can do to maximize your fuel economy: Accelerate gently, but not too slowly; you want to build speed quickly enough to let the transmission shift to higher gears. Keep your highway speeds moderate and use cruise control when possible. In traffic, brake and accelerate moderately, and look far ahead so you can anticipate changes in speeds. When climbing steep hills, keep right and slow down if traffic conditions allow you to do so safely.