|Antara GTC Twin-turbo
With much fanfare, GM Opel introduced a new 1.9-liter, twin-turbo diesel crossover concept, the Opel Antara GTC at the Frankfurt IAA. Notably absent from the hoopla, however, was the Opel Astra GTC diesel hybrid concept GM had unveiled in January in Detroit. More on this below.
As applied, the twin-turbo, based on the popular 1.9-liter diesel ECOTEC family, dramatically increases the power and torque range of that engine family while maintaining comparable levels of fuel consumption.
The 1.9-liter twin-turbo generates 156 kW (212 hp) of power and 400 Nm of torque from 1,400 rpm—more than twice the power output of the new 74-kW entry-level 1.9-liter CDTI.
GM simulations calculate that the engine, combined with the six-speed automatic transmission, will give the Antara GTC concept a top speed of more than 210 km/h and an acceleration from zero to 100 km/h in around 8 seconds.
GM has been working on the twin-turbo technology for several years, first applying it publicly in an earlier engineering study in an Opel Vectra presented at the Essen show in November 2003. The Vectra—a smaller car than the Antara—used the same size twin-turbo engine (1.9 liters) and produced the same power and torque output, with fuel consumption of 6.0 liters/100km (39 mpg US). GM gave no fuel consumption figures for the Antara GTC.
|Opel’s twin-turbo diesel technology. Click to enlarge.
The heart of the twin-turbo technology is forced aspiration of the diesel engine through two exhaust-driven, variable-blade geometry turbochargers, which, unlike bi-turbo systems, operate in series rather than in parallel.
A smaller, highly responsive turbocharger handles the low engine speed range, while a larger turbocharger designed for high output takes over as the revs build up.
This is also unlike Volkswagen’s new production dual-charged technology, in which a supercharger handles low revs and a turbocharger kicks in for higher speed boosting. (Earlier post.) By only using turbochargers, GM avoids introducing the parasitic load of a belt-driven supercharger for low-speed boosting.
The first-stage, small high-pressure turbocharger works alone up to 1,800 rpm and compresses the intake air at up to 3.2 bar boost pressure. Between 1,800 and 3,000 rpm, the larger low-pressure turbocharger kicks in—both chargers run in this speed range. Above 3,000 rpm, only the large 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.
Where traditional turbo-diesels have a mean effective pressure of 17 to 19 bar, the 1.9-litre twin-turbo reaches 26 bar. The mean effective pressure of an engine is average working pressure acting on the pistons during the combustion process—the higher the value, the higher the power output.
|Opel Mid-Range Diesel Engines
|Antara Concept 1.9-liter Twin-turbo
|Vectra Concept 1.9-liter Twin-turbo
Depending on the development objective, the twin-turbo can be designed either for high performance or for efficiency in its consumption of fuel. Compared with a naturally aspirated diesel engine, power outputs can be raised dramatically without increasing fuel consumption. (See table above.)
Alternatively, consumption can be reduced by as much as a quarter without loss of power—i.e., downsizing. Opel chose the first route for both its the 1.9 CDTI twin-turbo engine studies (2003 Vectra and 2005 Antara GTC).
Like all Opel’s 1.9 CDTI production cars, the Antara GTC concept is equipped with the maintenance-free diesel particulate filter system (DPF).
As noted above the Opel Astra GTC diesel hybrid concept GM introduced in January along with the Graphyte, GM’s large format SUV hybrid concept, was absent. (Earlier post.)
The Graphyte, by contrast, is prominent at the Frankfurt IAA, which is being heralded as its European debut.
The Astra Diesel hybrid, based on the GM/DaimlerChrysler two-mode hybrid powertrain, used a 1.7-liter turbo diesel engine delivering 125 hp (92 kW) of power and 206 lb-ft (280 Nm) torque with two electric motors, rated at 30 kw and 40 kw, respectively. The diesel hybrid prototype delivered zippy 0–100 km/h acceleration of less than 8 seconds—but with fuel consumption of less than 4-liters/100km (58.8 mpg)—25% more fuel-efficient than the conventional diesel model with the same displacement engine.
At the reveal event for the diesel hybrid, a GM executive made the following observation:
We decided to use a diesel-powered car as a starting point because in the mid-term, we don’t see a demand for gasoline hybrids in Europe. Our state-of-the-art CDTI engines already deliver impressive dynamics and low fuel consumption. The Astra concept demonstrates that fuel efficiency and vehicle dynamics can be significantly improved by hybrid technology.—Hans H. Demant, GM European engineering vice president and Opel managing director
But the absence of the Astra diesel hybrid from Frankfurt hype, combined with the reveal of the twin-turbo Antara concept implies that GM is focusing on increasing power and maintaining fuel consumption, rather than downsizing for fuel savings.
Although GM’s stated objective for the Antara concept was “to show just how dynamic and athletic an SUV (Sport Utility Vehicle) can now look, with this four-wheel drive Opel concept vehicle,” it could have achieved the goal while making a further statement about improving fuel efficiency. In other words, it could have downsized on the turbo, or continued to work with the hybrid powertrain concepts.
The really interesting concept would have been a downsized twin-turbo diesel hybrid.
Again by contrast, Graphyte was touted as a glimpse at future SUVs—a future that will first appear on the full-size Yukon and Tahoe in 2007.