Diesel: Small improvements
Downsizing is a growing trend for gasoline engines but OEMs want efficiency gains from diesels too. AVL' three cylinder concept explores the limits
- Published in Focus.
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1.05 litre triple uses a supercharger and a turbo in series to match four cylinders for power
Diesel engines are efficient, but expensive. That’s why so much powertrain R&D spend is going on gasoline engines right now. Cost-efficient improvements here will help OEMs cut fleet CO2 emissions averages.
Compression ignition engines are already highly evolved, especially compared to their spark-ignition counterparts, but further gains can come from heavy downsizing and reducing the number of cylinders.
Small displacement triples are available in the B-segment, such as Volkswagen’s 1.2-litre TDI in the Polo Bluemotion, but these offer reduced output.
What’s also needed are three cylinder diesel engines capable of replacing the inline fours used in C- and, increasingly, D-segment vehicles, with no loss of performance or driveability. Development firm AVL, working with Renault, has been studying the limits of downsizing the OEM’s 1.5-litre diesel.
“The main aim was aggressive downsizing to see what could be physically achieved,” says Dr Helfried Sorger, AVL’s head of design for powertrain systems, engineering and technology. “Renault came to us and asked if we could help – it wanted some innovative ideas.”
Renault’s 1.5 dCi engine is used in vehicles, from the A-segment Twingo to the D-segment Laguna. The 1,461cc unit features a cast iron cylinder block, an 8V SOHC head and a solenoid injector commonrail system.
The target was to maintain the 80kW/ 240Nm rating but improve fuel efficiency on the NEDC cycle through reduced friction and thermal losses and achieve Euro 6 compliance.
The concept AVL developed is a 1,055cc three cylinder. Its specific output of more than 75kW/litre exceeds the production benchmark set by BMW’s six-cylinder engines with dual-stage turbocharging.
“We also investigated dual-stage turbos but with this arrangement the compressor wheel of the wastegate turbo was too small,” says Sorger. “Therefore low-end torque couldn’t be achieved by exhaust gas turbocharging alone. So we went for a combination of a turbo and a supercharger.
“What’s interesting is that, unlike some gasoline engines where the supercharger is used for low-end torque, it’s the other way around in our diesel.”
The boosting system uses an Eaton supercharger in series with a turbo. This arrangement gives a maximum pressure ratio of 3.6, well above that possible from a single stage system.
