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Automotive Engineer

Diesel: Compression test

Experts from FEV, Ford and Renault detail the improvements in fuel injection, valvetrains and boosting that will keep diesel competitive

Simon Bickerstaffe in Focus.
  • Published in Focus.

Gaspar Gascon

Renault’s vice-president of projects and partnerships

Marc Bodin

Renault’s vice-president of powertrain strategy

The big challenge is aftertreatment. In terms of consumption diesel engines are achieving results very close to hybrids. But cost will increase a lot with the new emissions regulations. The regulators say in 2015 there will be Euro 6, but then in two months they’ll change the regulations a bit, saying that there’ll be two steps with different OBD requirements and so on. 

It complicates development because it’s quite difficult to stabilise on what’s the best technology to apply.

Injection pressures will increase, but not so much. Most of our engines are 1,600bar, including our latest 1.6, but we’re preparing that for an additional 200bar. Pressure will increase in the bigger, more powerful engines too –  we’re thinking about 2,000 or even 2,200bar systems. We’re preparing a new generation of 2-litre engines going up to that.


Gaspar Gascon at Renault

We’re getting more benefit by working on the number and timing of injections. This repays much more than just increasing the pressure. Reducing engine-out emissions minimises the cost of the aftertreatment and this is achieved by working on the injection pattern.

We’re also working on NOx traps and SCR. The latter is the ideal solution but it’s expensive and more of a packaging constraint. The issue is to create cheap and compact SCR systems. One solution could be a close-coupled SCR module at the back of the engine.

You can get additional benefits from VVA but it depends a lot on the base engine, the application, and the combination of aftertreatment devices. You can still gain 3-4% fuel economy but the problem is cost. We need a high value:cost ratio. This criterion allows us to decide if technologies are useful or not. We have a chart which says how much money we are ready to spend according to the CO2 benefit. You can keep on adding additional technologies but this increases cost.

Compression ratios were going down, but now they’re more or less stable. Around 14.5:1 is a good value but going lower can be tricky. There are some effects on engine starting, and this is also a very critical phase of the emissions cycle. You need good quality combustion at the very beginning if you want to reduce the precious metal in the catalyst coating.

Marc Bodin at Renault

There are two ways to reduce CO2 and have a good NOx compromise. One is low-pressure EGR; that works quite well in light applications. The other is twin-turbos; that works a bit better in the heavier applications. But three turbos? No. I cannot imagine that.

Having two is expensive and takes a lot of packaging, especially for carmakers like us who don’t have much room in the engine bay – it would just be a nightmare to put in three.

And when you add a twin-turbo some parts of the system move by 15mm between hot and cold. That’s enormous; you have to add lots of clearance. But it can take our 1.6 much higher than today’s 130hp.

There is another challenge for diesel, and that’s staying competitive throughout the electrification of small cars. Because of emissions regulations the cost of diesel will become an issue. Small electric cars are definitely a good solution for the near future. So the impact of electrification on diesel could depend on the segment.