“The best diesel will never match the best petrol engine for NOx,” says Andersson, “but the best diesel can get way below the 80mg/km limit and also way below the 60mg/km limit for petrol cars. At Ricardo, we are benchmarking diesel vehicles already on sale and finding they are substantially below EU6d and look to be capable of meeting the regulations for 2020 and beyond.”
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Why do diesel engines generate more NOx than petrol engines?
In a modern petrol engine, just enough air is mixed with the fuel to consume both air and fuel completely and the exhaust after-treatment consists of the well-established ‘three-way’ catalytic converter. Diesel engines are ‘lean-burn’ with excess air left in the cylinder after all the fuel is burned and the high temperature burns nitrogen in the air to generate NOx. Three-way catalysts do not work with any lean-burn engines, whether petrol or diesel. Lean-burn petrol engines have been tried in the past, but they produce just as much NOx as a diesel and need the same type of exhaust after-treatment.
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Getting rid of NOx
The first step is to reduce the amount of NOx leaving the engine using minute control of high-pressure direct fuel injection, plus exhaust gas recirculation to cool combustion and reduce the amount of oxygen present in the cylinder. Then to meet the latest EU6 emissions, diesels need either a lean NOx trap (LNT) in the exhaust system or more complex and bulky selective catalytic reduction catalyst (SCR). With SCR, urea fluid held in a separate tank is injected into a catalytic converter to neutralise the higher levels of NOx.
Jones says LNTs are less efficient and can only scavenge about 60% of NOx from the exhaust. SCR is much more effective when fully warmed up with a percentage clean-up rate “in the high 90s”, he adds. For small cars using LNTs, a switch to SCR to satisfy RDE may pose a problem because the SCR system is more bulky and expensive. That may force manufacturers of A and B segment cars to stick with downsized petrol engines, but dropping small diesel engines will make it harder to meet their fleet CO2 average.
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What if diesels are banned completely?
“Diesel CO2 emissions are still 20% lower than petrol,” says Jones. “If we prematurely abandon diesels, Europe will not meet its 95gm/km CO2 target by end of the decade.” Andersson agrees, adding: “Another thing that doesn’t seem to be discussed enough is personal mobility. If there is a drive to replace older diesels with hybrids or EVs, even with a scrappage scheme, how many people with a 15-year-old diesel can afford to take the £2000 and go and buy a new plug-in hybrid?”
Chairman of the mobility solutions sector of technology giant Bosch, Rolf Bulander, said that while clean air must be a priority, banning diesels would be a mistake. Bosch is one of the world's leading developers of EV powertrain technology, as well as diesel and petrol.
He said: “In our view, this is ecologically misguided, or at best environmental protection from a blinkered perspective. Blinkered because driving bans ignore diesel’s outstanding efficiency, which is still needed to limit global warming. It also underestimates the potential still latent in this technology, since emissions from diesel and gasoline engines can be cut even further, further than any current legislation requires. Politicians should not restrict our engineers creativity by favouring or disadvantaging a certain technology.”