The main cause for concern over urban air quality is NO2, which is known to be harmful to health if inhaled in high concentrations. But are diesel engines to blame?
The demonisation of diesel has reached almost hysterical levels, but is it really justified and is it wise to consider banning all diesel cars from the roads altogether as some are suggesting?
NOx is the culprit causing the widespread alarm. NOx is not the chemical name of an individual gas but a generic term for the two most common oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO2). The main cause for concern over urban air quality is NO2, which is known to be harmful to health if inhaled in high concentrations.
NOx is not unique to diesel, says Jaguar Land Rover head of engine calibration Alan Jones. “The generation of NOx has nothing to do with the type of fuel,” he says, “NOx is created by high temperature combustion. If you have nitrogen and oxygen in the same high-temperature environment, the two will combine to form NOx.” NOx is created in both petrol and diesel engines and by other sources too, like central heating systems and atmospheric lightning.
Are diesels to blame for NOx or has the legislation been too lax?
Cycle cheating aside, the emissions performance of any car is only as good as the test cycle it is designed to meet. The current New European Driving Cycle (NEDC) was designed in the 1980s, introduced in 1996 and last updated way back in 1997. It is widely criticised from being unrepresentative of current real-world driving. The maximum speed driven in the test cycle is only 70mph for 10 seconds, the average speed over the test is just 21mph and tests are performed with all ancillaries such as air conditioning, heated seats and the infotainment system turned off.
European emissions regulations began with EU1 in 1992 when emissions limits were set sufficiently high that diesels didn’t need any exhaust after-treatment at all. Diesel oxidation catalysts (DOCs) were first used on large cars in 1996 and spread to all diesels in 2000 with tougher EU3 regulations. Along with the introduction of direct injection, DOCs controlled hydrocarbons (HC), carbon monoxide (CO) and, to some extent, particulates as well. The limit for NOx then was 0.5gm/km (half a gramme, or about the weight of a small pea). Older diesels do emit more HC, CO and NOx, but they do what legislation required of them at the time they were manufactured.
Diesel particulate filters were introduced in 2009. “DPF technology is so effective that NOx is now the only real consideration with diesel emissions,” says Jon Andersson, global technical expert on emissions at consultant engineering firm Ricardo. It wasn’t until 2014 and the introduction of the first stage of EU6 that any efficient NOx after-treatment control was required to meet the regulations. Now, the limit for NOx is set at 80mg/km (eighty one-thousandths of a gramme, more than six times lower than 2000 levels).
Is there a fix?
The NEDC will be complemented by the worldwide harmonised light vehicles test cycle (WLTC) and real driving emissions legislation (RDE) in September. WLTC testing is laboratory-based while RDE takes place on the road and the combination of the two is very effective. RDE testing is carried out using PEMS (portable emissions measuring system) attached to the car.
PEMS consists of sensitive laboratory equipment repurposed to be used on board a car and its use is not straightforward. It’s essential that all systems are carefully set to ensure accurate results, says Andersson. “When we do any RDE testing at Ricardo, the first thing we do is correlate the PEMS system against the analysers used in the chassis dynamometer [rolling road]. We also compare a bottle gas standard with the analyser. We find that using the bottled gas standard alone isn’t sufficient to ensure high enough confidence in the accuracy of the results.”
“The new test cycle is much more realistic and representative of the 50th percentile driving style," explains Jones. “By comparison, the NEDC test represents 5% of driving styles. This is why the new cycle is relevant and the old cycle is irrelevant.”
The new test cycles are much harder to pass than the NEDC. Like a driving test, the RDE test can be carried out by certification agencies on roads anywhere in Europe. Within certain maximum limits like gradients, manufacturers have no way of predicting the location of a test, ambient temperature, driving style, payload or use of ancillaries such as air conditioning. “We have to try to generate a worst case scenario by taking our own cars and doing extreme testing to make sure we can pass,” says Jones. “RDE is more dynamic and the cars are made to work much harder.”
EU6 came into force in September 2014 but is being rolled out in several increasingly tough stages ending with EU6d in 2020 for new types of cars. The latest generation of diesels are very clean, says Jones: “Engines that meet RDE absolutely walk the NEDC test.”
Can diesels ever better petrol engine NOx emissions of 60mg/km?
“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.”
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.
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.
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.”