If you've ever owned a car with a small turbocharged engine, you may have noticed a difference in fuel economy between the advertised numbers and your real-world numbers. When I owned a Fiesta ST, I could never quite achieve the highway rating, despite my conservative driving. Well, turns out there's a reason for that.
Jason Fenske of put together a short video showing us why exactly small turbocharged engines might not always achieve those EPA rated fuel efficiency numbers. You see, it all has to do with protecting the engine from knock. Compared to a naturally aspirated engine, there is significantly more pressure in the combustion chamber for a turbocharged engine. With enough of this pressure, the air-fuel mixture could ignite on its own without the help of a spark plug, causing knock, which could destroy the engine.
Turbocharged cars are programmed to prevent knock from happening by injecting more fuel into the cylinder, which lowers the temperature inside the chamber. While the engine makes more power this way, it also uses a lot more fuel, which brings down efficiency. Fenske explains the process in detail, in the video below.
If you can't see the YouTube video above, .
Fenske points out that while most new small turbo engines operate this way, some manufacturers, like Mazda, have figured out a way to find power without sacrificing fuel economy by dumping more fuel into the engine.