Reducing emissions is now a major imperative for most countries, whether it's point source pollution in large cities or carbon dioxide (CO2) which contributes to global warming.
Industrial activities also have an impact, of course, but here the focus is on road mobility.
Emissions related to road mobility are not limited to those produced by vehicles, so-called tank-to-wheel emissions. In fact, extracting and transporting petroleum then refining and transporting fuel all use energy that releases so-called well-to-tank emissions. The same is true for biofuel between the production site and the tank.
That's not the case for electric vehicles since no tank-to-wheel emissions are produced. Well-to-wheel emissions are therefore those that are produced all the way from the primary energy source to the battery or fuel cell. These emissions depend on the country's energy mix. Therefore an electric vehicle can indirectly release polluting emissions and carbon dioxide if the country's electricity is produced in thermal power stations, especially coal-fired plants...
Taking on the problem of emissions as a whole requires taking into consideration each country's specific circumstances. Here we focus specifically on the tank-to-wheel aspects: the fuel, the vehicle's fuel consumption, and exhaust gas after-treatment.
Reducing air pollution emissions
There has been a significant decrease in urban pollution despite an increase in traffic, in no small part because internal combustion vehicles are regularly checked and constantly improved, namely with increasingly strict electronic combustion control systems. Consequently, 80 % of the remaining pollution comes from 20 % of the oldest vehicles.
We must go further, and manufacturers have produced prototypes that are even more efficient, particularly for spark-ignition engines.
Unfortunately, traffic jams remain a problem since the engines are at their least efficient (5 %). Of course, stop and start systems and regenerative braking limit emissions when the engine is restarted, but they can't make up for the energy used to propel the vehicle forward! Reducing emissions is therefore another reason to enhance connectivity in order to improve traffic flow.
Possible illustration: diesel fuel desulfurization in the USA and Europe. Book Andre Douaud p.106
How can we reduce CO2 emissions?
First of all, by improving fuel efficiency. Car manufacturers play a primary role, but drivers can also do their part by using a vehicle that is fuel-efficient, well-maintained, and equipped with energy-efficient tires. They can also adopt energy-efficient driving techniques and avoid traffic jams.
Additionally, drivers can also choose alternative fuels such as natural gases with the best hydrogen/carbon ratios, or biofuels. The advantage of biofuels is that they recycle atmospheric CO2 used in plant growth. Priority is given to non-food biomass production.
Finally, the most promising option is undoubtedly hybrid vehicles as a first step, and then electric vehicles. CO2 emissions will then be limited to electric energy production sites which can capture and sequester CO2 (CCS for carbon capture and sequestration). They will depend only on the primary energy source used in energy production.
Reducing and treating polluting emissions from spark-ignition engines
Noticeable improvements have been made in terms of consumption and pollution thanks to the development of so-called "clean" fuels, which are unleaded and contain minimum amounts of sulfur and benzene; the electronic regulation of fuel injection and combustion; hybridization, which provides optimal conditions for internal combustion engines; and aftertreatment of exhaust gas.
Catalytic converters are used to reduce pollution from spark-ignition engines. This technology is called a three-way catalyst since it simultaneously eliminates the three pollutants (CO, HC, and NOx) and can be used with spark-ignition engines whether they run on gasoline, biofuel, or natural gas.
Three chemical reactions take place simultaneously: while carbon monoxide CO is oxidized into carbon dioxide CO2 and unburned hydrocarbons HC are oxidized into water H20 and carbon dioxide CO2, nitrogen oxides NOx are reduced to nitrogen N2. The kinetics of these three reactions would be much too slow without the presence of a catalyst at exhaust gas temperature.
Catalytic treatment is very effective, operating at close to 90 % efficiency, and very robust. But several requirements must be met:
- The air-fuel ratio must be as close as possible (better than 1 %) to stoichiometry, that is 14.5 grams of air per gram of fuel, even though fuel efficiency is not optimal at that rate. Only electronically controlled injection can guarantee such precision.
- The use of catalytic converters required the elimination of sulfur and lead from gasoline, since they contaminated the catalysis.
- The catalysts are precious metals such as platinum, palladium, and rhodium. One or two grams are used per converter, but they are usually recovered when it is recycled.
- A minimum gas temperature is required for the catalytic reactions to take place. Improvements to catalytic converters mean that they can now function as soon the engine starts.
Catalytic converters have therefore established themselves as the most efficient and affordable solution for eliminating pollution from spark-ignition engines.
Reducing and treating polluting emissions from diesel engines
Diesel engines emit particulate matter (PM), nitrogen oxides NOx, carbon monoxide CO, and carbon dioxide CO2.
The air-fuel ratio is not homogeneous in a diesel engine, on principle. Tiny amounts of excess fuel can appear and cause incomplete combustion due to lack of oxygen. This results in the formation of small molecules of solid carbon. These particles clump together and form very small clumps of several nanometers (1 nm = 10-9 m), or several thousandths of a micron. Given their very large specific surface, the carbon particles capture hydrocarbons and other substances produced by the fuel and lubricants such as sulfates.
Particle clumping continues up to the exhaust phase. We can then find particulate in sizes varying from 0.01 to 10 microns (1 µm = 10-6 m). A direct injection diesel emits particulates ranging from 0.1 to 0.5 microns.
Air quality analyses in cities generally measure two average particulate sizes: PM 2.5 and PM 10, which correspond to 2.5 and 10 microns, respectively.
The relationship between the particulate mass that is emitted and the mass of fuel that is consumed is very weak today, but regulations in different countries are becoming more and more strict. For example, the Euro 5 standard which has applied to all new European vehicles since 2011 limits maximum emissions to 5 mg/km or 1/10,000 of the fuel mass for a vehicle which consumes around 6 l/100 km.
It would be impossible to envisage such low levels without effective filtering of exhaust gas.
Diesel Particulate Filter (DPF)
The first treatment of exhaust generally starts with the catalytic oxidation of unburned HC gases, carbon monoxide CO, and part of the volatile soot compounds.
The second step involves the particulate filter itself, a honeycomb structure generally made of silicon carbide that is resistant to the high temperatures required for soot combustion. The filter has alternate channel plugs to force the exhaust gases through a controlled porosity wall.
Several independent studies have shown that the effectiveness of these filters is very high since the total of all sizes of particulate mass emissions is reduced by 90 %. The total amount of particulate matter that is emitted is therefore divided by 10,000!
The sticking point remains the elimination of the accumulated particulate remaining in the filter. Indeed, the soot must be regularly burned in order to prevent accumulation, which would then necessitate very high temperatures. A second catalytic oxidation is used to improve combustion at a moderate temperature.
Two catalytic processes can be used:
1) The catalyst is added to the fuel
An additive containing the catalyst is placed in a tank and automatically added to the fuel. The catalyst, composed of cerium oxide and iron, is activated by combustion and binds closely to the particulates. Combustion in the filter is thus more effective at a relatively low temperature close to 450°C.
The filter regeneration cycle, or the burning off of soot, is activated by monitors that measure the differences in pressure between entry and exit into the filter. It is important not to stop the engine during this regeneration phase.
2) The catalyst is added to the filter
This method is easier but to avoid the accumulation of soot that will be out of the catalyst's reach, frequent or even continuous regeneration cycles are necessary (CRT for Continuous Regenerative Trap).
How is regeneration activated? There are three possible methods:
- Post-injection using the engine's own injectors, but late enough in the cycle for the exhaust gas temperature to be high enough to burn off the soot.
- Placing a 5th injector at the filter opening in the exhaust pipe.
- Using the oxidizing function of the oxidation catalyst that is placed in front of the filter to transform NO into NO2, a very powerful oxidant used to initiate the combustion of soot accumulated on the filter. This process allows for continuous regeneration without big temperature increases. However there is a risk of increasing final NO2 emissions.
3) Retrofitted filters
Vehicle emissions regulations generally only apply to new vehicles, even though 80 % of the pollution is caused by 20 % of the oldest vehicles. It would therefore be a good idea to equip older diesel vehicles with a minimum of pollution reduction mechanisms. Filters such as the ones mentioned above would quickly be rendered inoperable by a too-heavy flow of particulates. Different types of special filters have thus been developed which divert the gas stream and cause solid particulates to impact with and stick to the coated sides of the catalyst. Regeneration is automatic and continuous. They are less effective than the previously mentioned filters, particularly for very fine particulates, but they are cheaper. And they make a very clear and considerable difference when retrofitted to older vehicles.
Aftertreatment of NOx in diesel engines
Since diesel engines run with excess air, nitrogen monoxide NO produced by combustion finds itself amongst the exhaust gases in an oxidizing environment (reduction is the opposite of oxidation). It is therefore extremely difficult to reduce it to N2 + 02 as spark-ignition engines do.
Three main principles are used to eliminate nitrogen oxides from diesel engines:
1) Selective Catalytic Reduction (SCR)
First used in large industrial facilities, the chemical reduction of NOx in smoke when oxygen is present has been increasingly used in heavy trucks since the end of the 2000s. It is also being progressively added to certain high-end cars.
SCR technology injects urea into the gas to be treated, which is then hydrolyzed into ammonia before reaching the catalyst. The SCR catalyst then treats the combustion gases which contain nitrogen oxides and ammonia. An additional treatment eliminates all trace of ammonia by transforming it into nitrogen.
The SCR system is complex but has two main advantages: since aftertreatment is separate from combustion, combustion can be optimized to increase fuel-efficiency. The presence of sulfur in the fuel is less of a problem than with other systems.
2) The NOx Trap
When an engine is running normally, the exhaust gas contains oxygen and nitrogen monoxide is easily oxidized into NO2 in the presence of a pre-catalyst. This nitrogen dioxide is then stored as nitrate in the catalytic trap. When the trap is full, the engine control unit injects excess fuel into the combustion chamber, which temporarily turns the exhaust gases into reducers thanks to the presence of carbon monoxide CO and unburned HC as well as the lack of oxygen. The nitrates leave the trap and are reduced to nitrogen in this reducing environment.
Though it is simpler than SCR because no reductant is required, the NOx trap does have several disadvantages:
- The injection of excess fuel leads to over-consumption (several %).
- Its effectiveness can reach 80 % but is limited by the presence of even small amounts of sulfur in the fuel (sulphate formation inhibits nitrate formation).
- The management of the rich exhaust phase is quite complex, even with NO probes in the exhaust line.
- Is the elimination of nitrates from the trap reversible in the long-term?
3) Exhaust Gas Recirculation (EGR)
This is the oldest and most efficient way to reduce NOx emissions: the exhaust gas is recirculated back to the engine cylinders. Thanks to its dilutive effect, the exhaust gas lowers the combustion temperature and therefore reduces the quantity of NO that is produced. Cooling the exhaust gas before it is recirculated further improves the effectiveness of this method.
Direct injection used in diesel engines can handle very high rates of recirculation before efficiency is affected. Furthermore, the recirculation of exhaust gas plays an important role in new homogeneous combustion processes (HCCI, etc.).
Total pollutant elimination from diesel exhaust: the "4-way catalyst"
This process eliminates carbon monoxide, unburned hydrocarbons HC, and solid PM through oxidation, as well as nitrogen oxides NO and NO2 through reduction. Equipping small vehicles with each of the 4 systems mentioned above is not a viable option. So a so-called "four-way" system combines these four pollutant elimination techniques into one single mechanism.
The disadvantage is that it's not effective for very long, but its adaptability to small cars and homogeneous combustion engines make it a promising solution (example of the Toyota DPNR).
Illustration such as sketch André Douaud "Cars and energies" p. 100
Drastic reductions in polluting emissions are now technically and financially feasible, and standards in different countries are responding accordingly.
One difficult issue remains: retrofitting older vehicles. Although there are fewer of them every day, the black clouds that escape from the exhaust pipes of these cars are still a reality...