Tuesday, December 1, 2015

What's the deal with 'turbocharged'?

A turbocharged engine.
A turbocharged engine. iStock

(MCT) -- QUESTION: I've noticed a lot of new cars are now "turbocharged", Can you explain how this works and if it's a good idea to buy one?

ANSWER: You're correct that turbochargers are becoming very popular, to improve fuel economy and increase performance. Once found mostly on performance vehicles and diesel trucks, and some four cylinder engines in the 1980-90s, they're now mounted in about 1 in 5 vehicles in the United States. I should add that turbos have been very popular in Europe, mostly on diesel engines, for a long time.

Turbochargers boost engine power by around 25 percent on a gasoline engine and about 40 percent on a diesel, allowing a smaller, more efficient engine to be used in place of a larger one. Instead of power being largely developed only at high engine speeds, a turbo flattens the torque curve, improving driveability and allowing more efficient low-speed engine operation. Tiny three cylinder turbocharged engines are helping cars reach more than 40 mpg and performance oriented turbos allow four- and six-cylinder engines to develop as much as 400 horsepower.

Turbochargers pump air into an engine at a higher rate than would occur if the engine were naturally aspirated. Driven by exhaust gases, a turbine wheel rotates at as much as 200,000 rpm. At the opposite end of a connecting shaft, a compressor wheel draws in air and pumps it to the engine's intake manifold. A computer-managed wastegate controls boost pressure to safe levels, maximizing performance while preventing engine damaging detonation (an explosion, rather than a burn of combustion gases). An attractive feature of turbochargers is that they use wasted energy, rather than consuming it like a belt-driven supercharger.

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  • A slippery modern car requires only about 20 horsepower to move down a level road at highway speed. Additional power is required for acceleration and hill climbing, which happens infrequently. A turbo matches this behavior well as it loafs along until spooled up by higher throttle opening and increased exhaust flow. This slight delay in operation can be mitigated by the use of a small, low mass turbo, a combination of two turbos, or a variable geometry turbo, which can adjust to conditions.

    The downside of a turbocharged engine is additional initial cost, a somewhat more congested engine compartment, and the chance the turbo may not last the life of the engine. Good maintenance is essential, as the turbo's very precise bearings are lubricated with engine oil, which is subjected to greater stress due to high temperature. Cooling system maintenance, always important, will help ensure proper cooling of the turbo. Premium fuel is often required to prevent detonation.

    I'd have no concerns buying a modern turbocharged vehicle. Coupled with direct fuel injection, performance and efficiency could be almost double that of engines made perhaps 10 years ago. Intake valve carbon deposits have been a problem for some direct injection engines, but design and operating improvements seem to be turning the tide.

    McClatchy-Tribune News Service
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