Four technologies to save the diesel engine

Published: 25 August 2016

► Four technologies to save the diesel
► Don't be put off by VW's dieselgate
► Cylinder deactivation just one answer 

Should the diesel engine be tossed onto the scrapheap? Headline writers shouldn’t forget that all combustion engines produce nitrogen oxides. While diesel NOx is higher than petrol (Euro6 NOx limits allow 80mg/l for diesel and 60mg/l for gasoline), the rollout of new NOx after-treatment systems will allow diesel to match gasoline. 

Chris Brace, automotive propulsion professor at Bath university, believes that ‘today’s after-treatments will fix emissions and air quality issues. When real driving emission tests are enforcing Euro 6.2 in 2017, the air quality issue will be largely beaten. Then it’s back to CO2 business as usual.’ 

Of course, that’s where denser diesel fuel has the edge, containing about 15% more energy than petrol, plus diesel engines are about 20% more efficient. The downside is that diesel produces 2.65kg of CO2 per kilo burnt, compared to 2.3kg for petrol. However, since diesel consumes 25% less fuel than a gasoline engine, it emits 15% less CO2. And that’ll be vital to help car makers avoid a €95 fine for every gram emitted over 95g/km – PER car sold – from 2021. Reason enough for the bean-counters to sanction these NOx-beating technologies… 

1) Exhaust after-treatments

Depending on the engine and vehicle size, different exhaust gas treatment (EGT) systems will be employed. Bosch has developed modular systems that incorporate a NOx Storage Convertor ahead of a double selective catalyst reduction (SCR).

The NOx trap appears to be tailor-made for many urban traffic situations due to the lower temperature limit of its operating range. The first SCR, with AdBlue urea injection, combined with a diesel particulate filter (DPF) reduces low temperature urban exhaust emissions; a second, optional, SCR further cleans the high temperature exhaust gases, if necessary.

2) Variable compression ratio 

Because compression ratio (CR) is fixed across all operating boundaries it is a compromise. A high CR is used for cold ambient start and engine warm-up – improving starting, hydrocarbon/carbon monoxide (HC/CO) reduction and increased thermal efficiency. With increased in-cylinder temperature at the start of injection the rate of ignition is improved and stabilised, resulting in more complete combustion and reduced HC/CO emissions.

It is also used for part load in warm engine conditions to increase the combustion’s thermal efficiency and decrease CO2 emissions and fuel consumption. A reduction in CR is needed, as the engine load increases, to protect components. For higher loads the amount of fuel and cylinder charge is increased along with cylinder peak pressure. 

3) Cylinder deactivation

Today’s Golf already has cylinder deactivation – but on its 1.4 petrol. But for diesel? Tula Technology’s Dynamic Skip Fire (DSF) uses audio electronics-derived digital signal processing to continuously vary the number of cylinders firing at any one time, controlling diesel exhaust temperature independent of engine load. Diesel after-treatment devices have a relatively narrow window of temperatures in which they are effective.

A common problem with NOx traps and SCR catalysts is that, at low loads, the exhaust is actually too cold for the after-treatment devices to work at peak effectiveness. Running DSF allows control of the exhaust temperature so the systems operate more effectively. As a separate but related advantage, with DSF it is easier to achieve the temperatures and equivalence ratios required to regenerate a particulate filter.

4) Electric turbochargers

The latest electric AirCharger (eAC) from Pierburg uses 12v or 48v to spin the turbo to maximum boost within 220-230 m/secs, bridging the gap normally associated with turbo lag caused by low engine load and speed.

This improves engine efficiency at low engine speeds, assisting in bringing the selective catalyst reduction (SCR) to operating temperature quicker, reducing emissions and boosting fuel economy by 3-5%. First application on a small capacity diesel, in 2017-2018.

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By Ian Adcock

CAR's engineering whizz, making sense of oily bits and megabytes