‘Dieselgate’ plunged the compression-ignition engine’s future into a state of uncertainty, but new technology is allowing oil burners to regain ground…
Internal-combustion engines (ICE), especially the diesel variety, are under threat. France has announced that by 2040 no new vehicles fitted with an ICE can be sold. The electric revolution has started and an all-electric powertrain future is fast becoming a reality. Should we get out the violins for the last diesel engine to be used in a passenger vehicle? Not according to Siegfried Pint from the Audi drivetrain development team, with whom I spoke at the recent Audi Summit in Barcelona. The Volkswagen Group, which was in the media for all the wrong reasons, is now leading the diesel fight-back charge.
The case for oil-burners
Fuel economy: There are many reasons diesel-engined vehicles achieve better fuel economy than their petrol equivalents. The fact that diesel fuel is more energy-dense per litre is a good start, but the main reasons are that diesel has a higher compression ratio, which is thermally more efficient; and, unlike petrol engines that need to control the intake air volume (throttle) to achieve the stoichiometric (ideal) air-fuel ratio of 14,7 to 1, diesel engines run “un-throttled” and that means there is less pumping loss during part-load conditions to get fresh air inside the combustion chambers. Another advantage of high fuel efficiency is that less CO2 is produced. If manufacturers plan to meet the low fleet-average targets in Europe of 95 g of CO2 per kilometre by 2020, but still use ICE, diesel will have to be part of the plan.
Diesel fuel: Refining crude oil (the most common method of diesel production) creates a multitude of products, including fuels such as diesel and petrol. If diesel engines disappear altogether, there will be little demand for diesel fuel, which will mean an imbalance in the current refinery process. It therefore makes sense to use diesel as a fuel as long as petrol is produced. Per litre, diesel fuel is more energy dense (its chemical potential energy) than petrol (around 36 MJ/L versus 34 MJ/L) and is a lower fire risk because it has a higher flashpoint than petrol (above 52 °C versus minus-43 °C).
Driveability: Because of high in-cylinder pressures during combustion, diesel engines (especially turbodiesels) are known for high torque outputs across the engine-speed range. This results in excellent in-gear performance and enhances driveability. Compared with a petrol unit, a diesel’s limited engine-speed range is less of an issue with today’s proficient automatic transmissions.
The reason that diesels fell out of favour was poor real-world emissions results. It therefore makes sense that engineers target the main concern head-on: proving diesels can be environmentally friendly, but can also comply with the strict Euro 7 emission standards that regulators are planning for 2023. According to Siegfried Pint, with the introduction of Euro 7, there will be no difference between the emissions requirements for diesels and petrols, placing the two ICE types on an equal footing.
48 V mild-hybrid systems:
This technology consists mainly of an integrated starter motor (ISG) and a small lithium-ion battery pack (round 0,5 kWh) that runs a 48 V system. The ISG is an electric motor connected to the engine via the auxiliary belt and can recuperate vehicle kinetic energy during braking or downhill situations by generating electricity when the vehicle’s brakes are applied. In the new Audi A8, the system generates up to 12 kW of electric power. Using this stored electric energy, it can also perform a boosting function by adding to the engine output. This system lowers emissions in both diesel and petrol powertrains, but in diesels, known for high levels of noise, vibration and harshness, it offers a much smoother stop/start function than achieved with the conventional starter motor.
Electrically heated catalysts:
With regard to catalytic converters, a downside of a diesel engine is that its exhaust gas is much cooler than the equivalent petrol version’s. This means long heat-up times for catalysts that function efficiently only at operating temperature. However, a new type of catalyst, showcased by Continental at the 2017 TechShow, has an electric heater element that improves heat-up times considerably and keeps the catalyst at optimum temperature. This helps further lower emissions during, for example, low loads or when the engine stops during stop-start system operation. This tech, together with a 48 V mild-hybrid system, was demonstrated on a Volkswagen Golf GTD, the result of which was 60% less CO2 emissions compared with the standard car. Selective catalyst-reduction systems and diesel-particulate filters (DPF) are mandatory after-treatment systems to meet strict emissions regulations.
Double-decker intake manifolds:
To achieve efficient and complete diesel combustion and minimise emissions, it is important that all the diesel that’s injected mixes with the air before combustion takes place. To achieve this, two techniques are employed: swirl, where the air circles the combustion chamber; or tumble, where the air tumbles like a wave into the combustion chamber during the intake (and compression) stroke. For the first time, Audi is employing a double-decker intake manifold design in its new 3,0-litre TDI engine in which the air can either flow through the top or bottom sections, resulting in either swirl or tumble. This allows engineers to choose the most efficient method of mixing depending on the engine’s speed and load condition. High-pressure injection systems (2 500 bar) and multiple injections per cycle help further.
As we know, eliminating turbo lag improves driveability in turbocharged engines. Having an electric turbocharger in parallel with the conventional turbo achieves zero lag because the electric unit can reach peak boost in less than a quarter of a second. Thereafter, the conventional turbo takes over. The turbo is powered by the 48 V mild-hybrid system we mentioned earlier and this technology is used on diesel and petrol engines. The Audi SQ7 TDI is the first production model featuring this tech.
Low-pressure EGR system:
During part-load conditions in a diesel, some of the burnt exhaust gasses are recycled back into the intake to lower the oxygen content in the combustion chamber via the exhaust gas-recirculation (EGR) valve. The effect is reduced combustion temperatures during the part-load condition and fewer NOx emissions. The traditional, high-pressure EGR system routes hot exhaust gases before the turbo back to the intake manifold after the intercooler. The problem is that the pressure differential is not enough to allow adequate EGR flow rates. A new system is now used which takes colder exhaust gas after the DPF and pipes it to the intake system before it gets to the turbo’s compressor. Adding low pressure EGR flow to the high EGR flow further reduces emissions.
Author: Nicol Louw