Part 2: Turbochargers: One man’s trash is another man’s treasure
In Part 1 of this series, we covered the basics of supercharging. If you didn’t have a chance to read it, we recommend that you do so as we’ll be alluding to some of the concepts presented in that piece as we embark on the second half of our journey to understand forced induction.
Turbochargers have the same goal as superchargers: increase the density of the air that enters the cylinder, allowing for more fuel to be combusted and more power to be created. They’re also very similar in design to the centrifugal superchargers described in Part 1. Turbochargers use an impeller to draw air into the unit, which is then compressed via travel through a diffuser and a volute before entering the engine. However, unlike the centrifugal supercharger, which is mechanically-driven by the engine, turbochargers scavenge energy from expanding exhaust gases to generate boost. The exhaust drives a turbine that causes the impeller to rotate. The more exhaust gas that influences the turbine, the faster the impeller spins. This means the higher the engine speed, the higher the boost.
But increasing pressure of a gas also leads to an increase in temperature, which isn’t ideal for your engine. If the air/fuel mixture heats up too much, it can detonate, causing knocking in the engine. And when an engine knocks, it means that the fuel has ignited before the piston has neared the end of the compression stroke. The expanding gases make it hard for the piston to finish the stroke, and there is no power to drive it back down after it passes Top Dead Center (TDC). Luckily there are quite a few options to reduce the likelihood of knocking due to forced induction, but the most common are:
- Increase the octane of the fuel
- Decrease the compression ratio of the engine
- Cool the air after it exits the compressor, but before it enters the cylinder
That last option is usually accomplished by use of an intercooler. In the automotive world, an intercooler is a device that cools the heated air leaving a compressor (either a supercharger or turbocharger) before it enters the engine. Most automotive intercoolers are air-to-air heat exchangers. The hot air is piped through a radiator-like housing (visible on the front of both cars above) that is exposed to ambient air. The excess heat is rejected into the atmosphere, and the cooled charge proceeds to the engine.
So far, the sound of using exhaust to make power probably sounds like automotive alchemy. But there are a few drawbacks. While turbochargers increase exhaust back pressure, they reduce the pumping efficiency of the engine, which leads to their biggest flaw: turbo lag. The centrifugal supercharger described in Part 1 is susceptible to lag too, but it’s reduced due to the mechanical linkage between the engine and the supercharger. With a turbo, however, the connection is based on a compressible gas. A certain amount of pressure needs to build up before the turbo will start to spin and generate boost, which means with many turbocharged cars, you could be left waiting for that power boost for several awkward seconds after you stomp the gas. Luckily, there are a few creative ways to mitigate this nuisance.
The easiest way to reduce turbo lag is to make the turbo smaller, either physically or by utilizing electrical boosting or variable geometry turbos. Doing so means the turbo will reach its boost threshold RPM (the minimum engine speed required to begin compressing incoming air) faster, improving responsiveness. But this limits the maximum boost that can be created by the turbo, and can put the engine at risk due to exceeding its maximum boost pressure. This is mitigated by a wastegate, which allows exhaust gases to bypass the turbine when maximum boost is reached. Still, an alternative to the wastegate setup, and a better solution to limited boost, is a twin-turbo setup.
This system is one in which there are two turbos on the same engine, and it falls into one of two different layouts: parallel or sequential. Parallel involves two identical turbos, which are often smaller in size, splitting the work of providing boost for the engine. Sequential turbos is a setup in which a smaller turbo provides boost at low RPM and then a larger turbo provides boost at high RPM. This effectively reduces turbo lag and provides top end power.
One of the most recognizable things about a turbocharged engine is the sound it makes when you lift off the throttle. This sound is created by air flowing backwards against the impeller and then being released by a blow-off valve. Unlike the positive displacement superchargers we discussed in Part 1, centrifugal superchargers and turbochargers spin at extremely high RPM, as high as 300,000 RPM for turbos, and they’re spinning at their fastest speed right at the shift point. When the throttle plate slams shut, there’s still momentum in the compressors and no outlet for the air. The remaining air can go backward against the rotation of the impeller, which could cause significant damage to the compressor. Enter the blow-off valve: It allows the high pressure air at the outlet of the compressor to be vented to the atmosphere, which saves the compressor and gives off that characteristic “pssst” sound we all know and love.
So there you have it, folks — the fundamentals of forced induction. This is an extremely complex topic, but we’ve done our best to reduce into two bite-size chunks. There are a lot of resources out there on the web (including Autos Cheat Sheet) for further study, so if you think a turbocharged car is in your future, I encourage you to read up, save up, and boost up.
Like classics? It’s always Throwback Thursday somewhere.