Understanding Turbo Boost

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. This suck-squeeze-bang-blow 4-stroke cycle moves pistons up and down in their cylinders, and via connecting rods hooked up to a crankshaft, this reciprocating motion is turned in to rotary motion. Bolt a transmission to the end of the crankshaft, 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, as the extra fuel won't burn unless it has more oxygen with which to combine and combust.

Chemistry has a word for the ideal amount of reactants in an equation to balance out with no leftovers: stoichiometry. In a gasoline engine, the stoichiometric ratio of oxygen-containing air to fuel is 14.7 parts (by mass) air to 1 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. Bigger cylinders could suck in not just more fuel but more air to combust completely with the fuel, and this gave you more power.

Unfortunately, more size also means more weight, so some engineers instead 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 their bladed compressor wheels were powered by the engine itself via belts or gears driven off the engine's crankshaft. Stoichiometry 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 less waste - 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.

With power and weight at such a premium in aircraft, Swiss aeronautical enginer Alfred Büchi had a brainwave on getting rid of that 20% supercharger power loss: instead of using engine power via a belt/gears to spin the compressor, connect the compressor wheel to a matching turbine wheel in exhaust system, capturing energy from the otherwise-wasted exhaust flow like a windmill captures energy from the breeze.

These early "turbine superchargers" or "turbo-superchargers" eventually went on to power many racing planes, bombers, and fighters in the 1930s and 1940s, and 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.

Turbocharger development marched hand-in-hand with gas turbine (jet engine) development throughout the 1950s and 1960s. In addition to better materials able to withstand the high temperatures and pressures in the hot side of the turbo, the general layout of the turbocharger eventually standardized:

• Cold side housing, which routes inlet air in to the turbo
  • Compressor wheel, which pressurizes the air
  • Compressor bypass, which opens when you lift off the gas to keep boost air from building up behind the closed throttle plate and causing compressor stall
• CHRA (center housing rotating assembly, also sometimes called the "cartridge")
  • Shaft on which the compressor and turbine wheels are both attached
  • Shaft bearings to allow the shaft to spin freely
  • Oiling and cooling
• Hot side housing, which routes air from the exhaust manifold in to the turbo
  • Turbine wheel, which captures energy from the exhaust
  • Wastegate, which opens when the turbo reaches target boost and sends extra exhaust past the turbine so it doesn't spin faster

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 temperature and flow rate 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:

• The Lambda Sond (oxygen sensor) system, with Volvo being the first automaker to use this combination of sensors to meter 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 any leftover fuel or oxygen in the exhaust to see how close to 14.7:1 stoichiometric 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
  • A: figure out exactly how much power the driver (via the driver's right foot on the gas pedal) is asking for
  • B: "work backwards", calculating the smallest amount of open throttle, fuel, and boost needed to 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.

Design of the turbo itself was also changing rapidly, with computer fluid dynamics modeling making efficiency improvements to the basic design, and direct computer control of the turbo itself now starting to appear:

• Turbo Control Valves (TCVs) give the ECU control of the turbo: the turbo wastegate still opens at a set boost level, bypassing any additional exhaust past the turbine to keep it from spinning any faster and building extra boost...but the ECU-controlled TCV now sits between the wastegate line and compressor housing line. The TCV mixes boost air with intake vacuum before it reaches the wastegate actuator, the actuator "sees" less boost and delays opening the wastegate, and the car keeps building boost until the ECU deems it safe.
• Variable geometry turbine housings add movable stator vanes outside the spinning turbine wheel, allowing the turbo's Aspect Ratio (A/R) to be adjusted on-the-fly between low volume/high velocity (ideal for low speed and quick spool) to high volume (ideal for high speed and turbo efficiency).
• Twin-scroll turbine housings split the exhaust flow from different cylinders, reducing backpressure and improving scavenging by minimizing exhaust pulse overlap.

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.