10.3.2004
Opel Twin-Turbo revolutionizes diesel engine technology
In the latest issue of the US Consumer Reports magazine, published by
a highly respected consumer survey institute with a big impact in North
America, only eight European car models, all less than a year old, are
rated as a 'recommended buy' with regards to quality.
The Opel Vectra
OPC engineering study presented at the Essen show in November 2003 and
now at the Geneva Motor Show has a 1.9-liter CDTI twin-turbo
engine, which produces 156 kW (212 hp). The specific power output of
112 hp per liter displacement is a world record for turbo-diesel powered
passenger cars. The 1.9 CDTI twin-turbo unit starts to deliver its maximum
torque of 400 Nm at only 1400 rpm, complies with Euro 4 exhaust emission
standards, and uses only 6.0 liters of diesel fuel per 100 kilometers
in the European test cycle.
The heart of the twin-turbo technology is
forced aspiration of the diesel engine through two exhaust-driven turbochargers,
which unlike "biturbo" systems
operate in series rather than in parallel: A smaller, highly responsive
turbocharger for the low engine speed range, a larger turbocharger designed
for high output to take over as the revs build up. Thanks to its superior
efficiency, the twin-turbo system can achieve an extremely high specific
power output, without disadvantages in fuel consumption and exhaust emissions.
The
secret behind "twin-turbo" is the clever two-stage forced
aspiration principle. By using a small high-pressure turbocharger for
the first stage, the engine responds readily to the gas pedal at lower
speeds without suffering from "turbo lag". Up to 1800 rpm
this high-pressure turbocharger works alone and compresses the intake
air at up to 3.2 bar boost pressure. Between 1800 and 3000 rpm, a larger
low pressure turbocharger joins in – both turbines run together
in this engine speed range. Above 3000 rpm, only the larger turbocharger
continues to deliver charge air to the cylinders. The complex control
of both chargers is via a valve in the engine's exhaust system,
controlled by engine speed and load.