Think you know what turbo lag is? The real definition might surprise you.
More and more vehicles are being equipped with turbochargers. Turbodiesels have been popular in bakkies and off-roaders for a while now, but now even smaller, affordable vehicles are being fitted with turbochargers. A good example is the new Toyota C-HR. A few years ago, this little compact crossover would probably have been fitted with a 1.6-litre or even 2.0-litre petrol powerplant. Instead, it is fitted with a 1.2-litre turbo petrol engine. Why? Efficiency. Both petrol and diesel engines benefit greatly from the addition of a turbocharger. Instead of fitting a vehicle with a bigger and thirstier naturally aspirated engine, you can give it a turbocharger, which improves performance without increasing fuel consumption too much. So, if this is the case, why aren’t all vehicles fitted with a turbocharger. Well, a turbo engine is more complex and therefore more expensive than a naturally aspirated one. Also, building a great turbocharged engine isn’t easy. Bad turbo engines can feel underpowered at times and suffer from lag, which results in an unpleasant driving experience. As with just about everything in life, a turbocharged engine requires some compromise.
How does a turbocharger work?
Let’s start by quickly looking at how a turbo actually works. Power is generated in an engine’s cylinders when air mixes with fuel to combust and drive a piston. The quicker the engine is able to burn fuel, the more power it can create. One way to increase the burn rate is to add cylinders to the engine, another is to add a turbocharger. A turbocharger is powered by exhaust gasses, and is mounted inside a vehicle’s air intake. The gases spin a turbine wheel, which is in turn connected to a compressor wheel. This set-up forces compressed air into an engine, which increases the rate of combustion, and therefore generates more power.
What turbo lag isn’t
When you put your foot down in a naturally aspirated vehicle, the power delivery is linear. As you continue to press down on the accelerator pedal, the power and torque increases along a steady curve, the vehicle picks up speed in a predictable way, there are no sudden jolts. When you throw a turbo into the mix, things change. Press down hard on the accelerator and, well, little happens. At first, the engine struggles to produce much power on torque, then, when the turbocharger kicks in, you get a sudden boost of power. This period – before you get that boost of power – is not actually turbo lag, although it is caused by the turbo and can be very annoying. Although technology has advanced a lot, and this phenomenon is almost non-existent in some vehicles, it is particularly hard to get rid of completely, since it is an inevitable by-product of how turbos function.
Like any part of an engine, the turbo must be matched to a specific rev range. You can’t get a turbo to perform at its maximum across the range. Compromise is always needed. For instance, if you fitted a turbo that could provide a lot of boost at low revolutions, it would spin too fast and tear itself apart at greater speeds. Instead, manufacturers fit turbos that perform optimally in the range where vehicles do most of their work. That’s about 1 500–4 000r/min for oilburners and 3 000–6 000r/min for petrol engines. The range where the turbo kicks in is called the boost threshold. When the turbo isn’t functioning properly at 1 000r/min, you’re not dealing with turbo lag – it’s just that the engine isn’t within the turbo’s boost threshold yet.
What is turbo lag?
Even when an engine is operating above the boost threshold of the turbocharger, you’ll often find that there’s a hesitation between when you put your foot down, and when the engine responds. That is turbo lag. Why does it happen? When it’s not under full load, pressure is released from the turbo to protect it and increase its longevity, so when you decide to accelerate, full pressure must be reintroduced before you can get that turbo boost. As mentioned earlier, the great advantage of a naturally aspirated engine is that its power delivery is so linear. You don’t have to wait for the engine to reach the turbo’s boost threshold, and there’s no lag between you signalling your intention to accelerate and the engine providing more oomph.
That’s why a lot of people believe that naturally aspirated vehicles are the better drivers’ cars. They respond instantly and predictably. Put your foot down in a naturally aspirated V8, and the needle will just keep climbing steadily until you reach the redline. A turbocharged engine, meanwhile, requires you to drive a bit differently. You simply have to wait for the engine to reach the boost threshold, and you need to keep in mind that there will be a pause between your throttle adjustment and the boost from the turbo.
The modern turbo
The good news, though, is that things are changing. Turbocharged engines are behaving more like naturally aspirated ones, delivering (almost) immediate and linear power right from idle. BMW – whose performance vehicles were once all naturally aspirated but are now supercharged – makes use of twin-scroll turbochargers, while many other companies now use variable geometry turbochargers (VGT) that have small adjustable vanes that direct gases onto the turbine blades. Then, of course, we also now find that manufacturers like BMW and VW are fitting multiple turbochargers to their engines. Next month, we’ll look at some of the most interesting approaches to modern turbocharging.
Text: GG van Rooyen