Years ago, automakers began phasing out V6 engines and replacing them with small turbocharged four-cylinder units. The promise was the power of a V6 with the fuel economy of a naturally aspirated inline-four mill. But, as we know today, posted fuel economy ratings often don’t match what a driver sees.
And it comes down to how small turbocharged engines operate. Jason Fenske of Engineering Explained produced a handy video to show why said engines normally end up overpromising fuel economy targets.
It begins with how a turbocharger operates. Turbos are spooled up via exhaust gases and shove more air into the engine. More oxygen means more power. But, aside from weight benefits and the simplicity of smaller engines, automakers have to watch out for knock. Knock can occur when a piston compresses the air and fuel mixture in the cylinder. When the spark plug is busy igniting the mixture, the temperature and pressure in a turbo engine can be so great that pockets ignite themselves and colliding flames become a problem that can cause serious engine damage.
That’s where turbo engines become inefficient. To stave off knock, automakers will lower engine compression ratios and a tweaked air-fuel mixture at high loads.
Say the driver floors the throttle. The engine knows it needs to stay away from high-temperature scenarios that can cause knock. So, the engine sends an extremely rich air-fuel mixture to the cylinder. Injecting more fuel actually brings the temperature down to avoid knock. Thus, fuel is used only to keep the cylinder temperature down and create the power needed under full acceleration.
A larger engine, say a V6, has its benefits and weaknesses. But, each cylinder performs less work and the air-fuel mixture is also less, which naturally keeps knock at bay. See the complete explanation in the video right up above.