How multiple turbochargers maintain performance while cutting emissions

BTN Turbo's Phil Wilson explains how increasingly complex turbos are essential for cutting diesel emissions

How multiple turbochargers maintain performance while cutting emissions

One of the routes to greater efficiency is to combine two, three or even four turbos, which use low pressure and high pressure forced induction to precisely match engine speeds and loads.

These units were previously only found on performance cars but are now being used on standard road cars.

Two-stage turbocharging

BorgWarner’s two-stage R2S turbochargers are used on Volkswagen’s 2.0 litre turbo diesels, connecting two turbos in series.

At lower engine speeds, exhaust gases drive a small high-pressure variable turbine geometry (HP VTG) turbo.

As engine revs build to medium speeds, this turbo’s wastegate opens to supply a larger, water-cooled turbo optimised for low-pressure exhaust gas recirculation (LP EGR).

The exhaust gases are enriched with fresh air, feeding the LP turbo’s compressor.

As engine speeds increase further, a bypass valve cuts the supply to the smaller HP turbo and the larger LP turbo takes over.

Although some exhaust gases keep the small turbo spinning, the large turbo now provides all the charged air pressure.

This system improves response, with almost instant acceleration at lower engine speeds and more consistent delivery at higher speeds, and more efficient, cleaner combustion throughout.

Triple turbo

For its R3S turbochargers (used in BMW’s M-Performance 3.0 litre diesel engines), BorgWarner combines two electrically-controlled VTG HP turbos with one wastegate-controlled LP turbo.

In this triple turbo, the individual units come into play sequentially.

At lower revs just the first HP turbo operates, providing immediate charge air pressure for optimum dynamic response.

As engine speed and loads increase, intake air enters the large LP turbo and gradually this turbo’s wastegate opens, allowing exhaust gases to drive the second HP turbo, reaching maximum charge air pressure.

The benefits of the extra turbo are dramatic.

When fitted to the same 3.0 litre diesel engine, the R3S triple turbo produces 25 per cent more performance and eight per cent lower fuel consumption compared to the R2S two-stage turbo.

Quad turbo

BorgWarner’s quad turbo is fitted to the BMW six-cylinder diesels used in their 750d XDrive, 5 Series, X5, X6 and X7 models.

Based on the same principle as the R2S, it has two HP turbos plus two smaller LP turbos.

The first HP turbo quickly generates charged air pressure without turbo lag.

As engine speeds increase, the LP turbos come on stream, both smaller units working together.

At higher speeds, the final HP turbo is introduced, for maximum output.

This approach produces an even faster response from the turbos, providing extra power and smooth torque curve without increasing the system’s weight.

Keeping diesel relevant

These are complex systems, but with their technology means more becomes less.

Impressive gains in efficiency are balanced with lower emissions, helping to keep diesel engines competitive at a time when many purchasers are moving away to petrol, hybrid or electric power.

The complicated nature of these turbos, particularly the precise operation of their VTG, wastegate and bypass valves, mean they require specialist knowledge from turbocharger experts.

For further information about BTN Turbo select ‘more details’ below.

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