Last month, we discussed the technical topics of boost threshold and turbo lag. Here we look at the different turbos on offer, and how manufacturers are dealing with the issues mentioned above.
The latest turbo technology
When it comes to building an engine, turbochargers can offer an excellent return on investment. A good turbo can provide extra oomph, without the need to increase engine capacity, which means less fuel and fewer emissions. But, as with anything in life, there are compromises. Firstly, a turbocharger kicks in properly only once it reaches its boost threshold. As long as engine revs stay below this point, engine delivery feels flat and uninspiring.
Secondly, once you’ve crossed the threshold, there’s still the issue of turbo lag to contend with. You’ll often find that there’s a hesitation between when you put your foot down, and when the engine responds. This happens because pressure is released from a turbo when not under full load so as to protect it and increase its longevity. So, when you put your foot down again, full pressure needs to first be introduced, which is where the lag comes from.
Love or hate the turbocharger, it’s here to stay. The days of the naturally aspirated V8 are over. Not even a vehicle like the F-150 Raptor boasts a big V8 these days. So, manufacturers are forced to come up with innovative ways to reduce lag and widen the effective rev range. Here are six common turbocharger configurations.
1. Single turbo
The simplest, and still most common, type of turbo is the single turbocharger set-up. Because of its relative simplicity (compared to the other options on this list), it is the cheapest, but arguably not the best, turbo option available. A single turbocharger can’t be effective across the whole rev range. Generally speaking, you have to choose either a smaller turbo that kicks in lower down on the rev counter and offers good low-down torque, or a larger one that will provide more topend power. In either case, the boost range will be fairly narrow, and lag will be an issue.
2. Sequential turbos
Having to choose between a small low-end turbo or a big high-end one presents a rather intuitive solution: fit two, one small and one large. That way you have a small turbo that kicks in early and provides good torque, and a bigger one that offers top-end grunt, resulting in a wide and flat torque curve. On the downside, quite obviously, you’re left with an engine set-up that is expensive, heavy and complex.
3. Twin-scroll turbo
A turbo is powered by exhaust gases that are redirected to spin turbine blades and force air into the engine. Now, an engine’s cylinders fire in sequence, meaning that exhaust gases enter the turbo in pulses. As you can probably imagine, these pulses can easily overlap and interfere with one another when powering the turbo, and a twin-scroll turbocharger solves this issue by using a dividedinlet turbine housing and a specific exhaust manifold that pairs the right cylinders to each scroll. In a four-cylinder vehicle, you can then have the first and fourth cylinders powering one scroll, and two and three powering another. This means that there’s less pulse overlap and less lag.
4. VGT turbo
A variable geometry turbo (VGT) is an expensive and complex power solution that’s especially prevalent in diesel engines. A VGT has a ring of aerodynamically-shaped vanes in the turbine housing that can alter their area-toradius ratio to match the revolutions of the engine. At low revs, area-to-radius ratio creates more pressure and velocity to spool up the turbo more effectively. At higher revolutions, the ratio increases to let in more air. The result is a wider boost range and less lag.
5. Variable twinscroll turbo
A variable twin-scroll turbo combines a VGT with a twin-scroll setup, so at low revolutions, one of the scrolls is closed completely, forcing all the air into the other. This results in good turbo response and low-end power. As you speed up, a valve opens to allow air into the other scroll (this is a completely variable process, meaning the valve opens in small increments), you get good high-end performance. You get the sort of performance from a single turbo that you’d normally only be able to get from a twin-turbo set-up.
6. Electric turbo
A very recent development is the introduction of turbos with electric compressors. An example is BorgWarner’s eBooster, which is an electrically powered compressor. The compressor provides instant boost to the engine, until the turbocharger has spooled up enough. A similar version of this can be found in Audi’s SQ7. With instant boost, lag becomes a thing of the past, but again, the system is expensive and complex. A compressor needs a motor, which in turn needs to be powered, so this is not a simple system to implement.
Left Page: The Audi SQ7 TDI has a compressor that instantly provides air to a turbo, thereby reducing lag greatly. Below: The BorgWarner eBooster is one example of this sort of turbo compressor. Bottom and inset: The latest BMW 750d has no less than four turbos.