Audi offers its solution to the decades-old problem of turbo lag with a new engine that features electrically-driven turbocharging. It’s based on the 3.0 litre BiTDI V6 (yes, the diesel that can sing), which has two turbos; only here, one of them’s a bigger, conventional exhaust-driven unit while the other is a smaller, electric motor-driven compressor.
Turbo lag refers to the time lapse between the press of the accelerator pedal and the desired power actually arriving – because turbochargers are driven by exhaust from the engine, a lag occurs when exhaust pressure is not high enough to force the turbine blades to spin and push more air into the engine. Generally, the bigger the turbo, the higher the revs and exhaust pressure required to make it work, and therefore, the bigger the time delay.
Sequential twin-turbo systems help this phenomenon somewhat – a smaller turbo works at low revs, switching to a bigger one at higher revs – but however minimal, there’s still a lag. In this new engine, the electric motor can “accelerate its turbine to very high speeds in an extremely short time.” Audi says the energy used to drive the compressor is mostly replaced by battery regeneration during coasting, so overall efficiency is preserved.
The electrically-driven compressor is placed after the main turbo in the charge air path – most of the time, the charge air is bypassed around the former, but when the main turbo’s energy output drops, a flap in the bypass shuts and the air goes into the electric compressor to be compressed a second time.
Tests have been conducted on prototype units, and the company found that the new system can yield a staggering two car-length lead over a conventional twin-turbodiesel in the first three seconds of acceleration, when fitted to the same car. So boost arrives noticeably earlier.
We don’t know when the engine will go into production, but there is speculation that the next-gen Audi A4 may be a recipient.