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Performance Specialists Since 1963

Understanding Turbo Boost

Created on 2011-03-09 by IPD Staff, Last Updated on 2021-05-03

How do we turn explosions in to thrust?

The name of the internal combustion game is converting heat energy to movement. Inside an engine, we ignite a fuel like gasoline, and the hot, expanding fuel/air gases push down on pistons. The up-and-down pistons are connected to the rotating crankshaft in the bottom of the engine, turning this vertical motion in to reciprocating motion. Hook it all up to a transmission connected to the wheels, and off you go!

How do we make bigger explosions for more thrust?

If you've ever stoked a campfire to make s'mores, you probably remember the three parts of the "fire triangle": air, fuel, and ignition source. It's the same at your campfire as it is inside your engine, with air flowing in to the intake combining with fuel from your fuel injectors and getting ignition from your spark plugs.

If we want to increase power, we need to make sure we have enough of all three components of our fire triangle. In an engine, that means if we inject more fuel, we also have to make sure we're getting more air (and thus more oxygen) to burn all the fuel, otherwise you end up with the internal combustion equivalent of a smothered campfire. More power = more fuel and more air, with the ideal ratio of air to fuel being about 14.7 parts by mass air to one part gasoline.

Enter forced induction

"There's no replacement for displacement" is a phrase from the big block V8 days, where more power meant physically larger engines that gulped more air and fuel in to the cylinders; back in the day, the way you added more air/fuel was by increasing overall engine size.

More size means also more weight, so some engineers struck on the idea of getting more air in to the engine by pumping it in: instead of making the engine physically larger to suck in more air, push more air in to the same size engine. The pressurized air being pumped in to the engine is called boost, with the pressure increase compared to ambient air pressure measured in PSI or bar/kilopascals.

These first forced-induction air pumps were called superchargers, and they were powered by the engine itself via a belt attached to the engine's crankshaft. That 14.7:1 stoichiometric ratio means every little bit of extra fuel requires 14.7X as much air, so it's no surprise superchargers use a huge amount of energy (sometimes as much as 20% of total engine power!) to pump all that air around.

More oomph with fewer losses - the turbocharger

Size and weight are bad things in cars and worse things in planes, which is were a lot of early forced induction development was taking place. Not only does a heavy engine make for a heavy plane, but a physcally large engine also makes for a bulky, non-aerodynamic fuselage. In aircraft, there was added incentive to pressurize incoming air to compensate for air getting thinner at high elevations, keeping engine power from falling off at high altitude.

Swiss aeronautical enginer Alfred Büchi had a brainwave on improving supercharger efficiency: instead of powering the compressor directly from the engine via a belt or gears, power it from a turbine wheel spinning in the waste exhaust stream already flowing out of the engine. These early "turbine superchargers" or "turbo-superchargers" eventually went on to power many racing planes, bombers, and fighters in the 1930s and 1940s.

Turbochargers were considered advanced aerospace technology at the time, with parts rotating at hundreds of thousands of RPMs and turbine wheels exposed to exhaust gas temperatures as high as 1800°F/1000°C. Adopting such an expensive piece of equipment in cars was thus slow and experimental at first, with a handful of models like the Chevrolet Corvair appearing from the 1950s onwards with an optional turbo engine.

It was the energy crises of the 1970s that really pushed automakers to start seriously looking at turbochargers as a way to downsize engines (and improve emissions and fuel economy) without sacrificing power.

Power and control

The 1970s and 1980s also coincided with the computer revolution, and these advanced fuel and engine control technologies proved well suited to turbocharger performance and longevity. From the first analog sensors in the 1970s to multiple networked control units in the 2000s and beyond, systems advanced to keep up with the demand to squeeze as much energy as possible out of a drop of fuel:

  • Mass airflow sensors, to measure the amount of air going in to the engine
  • Electronic fuel injection, to meter out the right ratio of fuel to go with the known amount of air
  • Oxygen/Lambda sensors, measuring combustion byproducts in the exhaust to see how close to the ideal 14.7:1 air/fuel stoichiometric ratio the engine is running
  • Knock sensors to measure the health and timing of combustion events
  • Coil-on-plug direct ignition, to adjust spark plug timing to prevent knock
  • Digitial Engine Control Units (ECUs) to continually measure all these inputs and adjust the outputs
  • Torque Request engine management schemes, to figure out exactly how much power the driver's requesting with their right foot and "work backwards" to calculate the amount of open throttle, fuel, and boost needed to efficiently hit the driver's power target

Finely controlled engine load and temperatures, tighter machining tolerances and balance, and more advanced alloys all played their part in improving turbocharger reliability and performance. As the 80s and 90s progressed, turbocharging became more mainstream, with predictable power outputs and turbo time-between-overhauls now reaching 100,000 miles or more.

Turbo design was changing too, first with computer-controlled vacuum solenoids opening and closing the wastegate to control overall boost, and with fundamental changes to the turbo itself like twin-scroll and variable geometry turbine housings boosting turbo efficiency by extracting much energy as possible from the exhaust stream. 

As we keep marching in to the 21st century, turbochargers are key to squeezing maximum efficiency out of combustion engines before electric vehicles are ready to take over in mainstream cars. The turbo has been with us for almost as long as the car itself, but it still has some work to do.

For more detailed information on turbocharger components and system service, see our article on common turbo problems.

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