Fo­cus on

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.

Leisure Wheels (South Africa) - - CONTENTS - Text: GG van Rooyen

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.

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