Turbos and octane

Date:3 August 2013 Tags:, , ,

QI’ve read that turbochargers on petrol cars are designed for high-octane fuel but can use regular, and the engine adjusts to the octane level. I would think regular fuel in such an engine would use more fuel and negate the advantage of the turbocharger. What’s the deal?

A: High-octane fuel is used in many turbocharged engines to reduce knock. A turbocharger’s job is to stuff more air into the combustion chamber, and as the piston moves up, the air is squeezed, and this drives pressures and temperatures up. If the temperature gets too high, the fuel in the chamber will spontaneously ignite. This is called knock, or premature detonation, and it can cause serious damage. As a result, all engines are fitted with a knock sensor that warns the computer to change the injection and spark timing to protect the engine. This does reduce power output and fuel economy, but the engine doesn’t turn into a paperweight.

Modern turbo engines are different, though. The widespread use of computer-controlled turbocharger pressure-relief valves (wastegates), individual ignition coils for each cylinder, and high-pressure direct-fuel injection allows engineers to control ignition much more precisely. Fuel can be injected exactly when it’s needed in the engine and even in several waves during combustion for maximum output and efficiency, all while the liquid petrol cools the intake air charge. These advances have led some manufacturers to allow fuel as low as 87 octane for their turbo engines – even though the engines usually get more power with high-octane. So to answer your question: it depends. If a manufacturer says you can run regular fuel, the engine will be safe, but you might still find gains from running high-octane.

By Ben Wodjdyla

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