You may have caught an earlier piece we did introducing the concept of forced induction on an internal combustion engine, so let's dive a little deeper into turbochargers.
To quickly catch you up, turbochargers are a device used on some vehicle engines to pump more air (and fuel) into each induction cycle (thereby forcing the induction) so as to get more power from that engine.
Turbochargers are driven by harnessing some of the exhaust energy on its way out to the atmosphere, and so if there's insufficient exhaust being produced, the turbocharger won't be spinning fast enough to produce the boost pressure that we're requesting of it the moment we go to accelerate.
The operating range of a typical engine turbocharger will be somewhere above 80,000rpm and below 120,000rpm, and when that turbo has been allowed to slow down it needs a moment to get back up to speed. We call this event spooling up, and the time it takes is perceived as a delay so it's called lag. It's kind of like you needing to breathe in before you can start inflating a party balloon.
There are many different ways that engineers and manufacturers have set about reducing the lag to an acceptable minimum, or even eliminating it altogether in applications where the costs can be justified (and/or where the racing rules allow it).
Turbochargers can be configured in multiple ways. For simplicity, and to stay within our word count for this week, we'll stick with single-turbo applications.
The simplest solution to lag is to mount the turbo very close to the engine. That way, the maximum amount of exhaust energy is available, which is good, but that also means more heat management is needed to cool the intake charge (which already heats up a lot, as any gas does when it gets compressed).
Size helps. Smaller turbochargers need less exhaust to get up to speed, but they also produce less peak power than a larger one, so a compromise needs to be made.
A solution on diesels that started slowly entering the market more than 30 years ago on some vehicles is the variable-geometry turbo. These have movable flaps inside the exhaust housing of the turbo to effectively make that housing act like it's smaller or bigger. Porsche used this solution on 911s for a short period from 2006, but because petrol burns much hotter than diesel, the cost of the fancy metals needed was just silly.
Lighter moving parts, along with better housing and turbine blade designs, also help a turbo spool up more swiftly, but other solutions have been used in various applications.
The simplest was back in the '80s when turbochargers on race cars got ludicrously big in the pursuit of power, and drivers learned to left-foot brake so they could modulate the throttle with their right in order to keep (or get) the turbo spooled up ready for the corner exit. They didn't necessarily share this trick with one another though. Many had to figure it out for themselves.
The invention of the blow-off valve helped. This vents intake charge when you close the throttle so that the turbine wheels don't slow down as soon.
In the '90s rally teams added an anti-lag device. This solution deliberately puts unburned fuel, and likely another spark source, into the exhaust manifold to power the turbo. However, they gulped a mind-bending 100L/100km and their use reduces the operational lifespan of the turbo dramatically, so it's a pretty terrible solution overall.
The current solution to this issue of lag is electricity. Since 2014 Formula 1 cars have used a single-turbo engine with a hybrid motor/generator directly assisting the engine (or harvesting energy as needed), as well as one connected to the mainshaft of the turbocharger. The motor/generator on the turbo also harvests energy when the turbo is spinning fast enough already, and it is used to spin the turbo up to operating speed so there's genuinely no lag as well. In fact there's so much grunt available immediately it took some drivers a few races to get used to it.
A few turbocharger manufacturers are also trying to get electrically-assisted turbos to catch on in road vehicles, whether it becomes original equipment or as a retrofit. As far as power delivery goes, it's about as perfect as forced induction can get.
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