How Does a Software Update Fix Diesel Emissions?
After 3 years the after-effects of “Dieselgate” are still rumbling on, with the recent arrest of Rupert Stadler the CEO of Audi and the German authorities forcing Daimler to recall hundreds of thousands of vehicles for software updates, I think we have not seen or heard the last of the troubles from Dieselgate. But, how is it that a simple software update is able to fix these problems, and if it was as easy as that, why did the manufacturers cheat in the first place? To understand this we have to look at the performance and emissions of a diesel engine. Diesel engines work by compression ignition of their fuel, unlike a conventional gasoline engine which use spark plugs to ignite the air-fuel mixture. In a modern diesel engine, diesel is injected under very high pressure into the combustion chamber with ultra-precision as the engine has pressurised and heated the intake air. The fuel is ignited in the cylinder and due to the high pressure and temperature at the time of combustion it burns very effectively and the engine can capture more of the energy generated. This high temperature and pressure combustion are what gives diesel engines their high thermal efficiency which in turn means better miles per gallon (l/100km) fuel consumption and lower CO2 emissions. Achieving very complete combustion of the fuel. So far so good, but it is also this high temperature and pressure combustion that results in the formation of NOx and fine particulates in the exhaust from the engine. So the diesel’s strength from an efficiency point of view is also its weakness from a pollution point of view, and the better we have become at creating very efficient high temperature and pressure combustion through common rail high-pressure diesel fuel injection systems and high-pressure turbocharging has led to a problem with NOx and PM production. (Note: this is also an issue with high-efficiency gasoline direct ignition engines GDI, HCCI systems) In order to control the formation of harmful NOx and PM various techniques are employed. Techniques that reduce the combustion temperature and pressure control the production inside the engine but have a negative consequence of reducing the fundamental combustion performance that gives the diesel engine its efficiency. This includes the use of Exhaust Gas Recirculation, EGR and changing the timing of the fuel injection into the engine relative to the compression. EGR works by taking off gas from the exhaust stream cooling it and putting it back into the inlet of the engine. What this is actually doing is reducing the oxygen content in the inlet air charge of the engine and reducing the combustion temperature. Whilst this helps to reduce NOx it has a negative impact on fuel consumption. So the use of EGR needs to be carefully managed in order to achieve a good fuel efficiency of the engine. The amount of EGR is controlled by a valve that is operated by the engine management system and controlled by the management software depending on the operating conditions of the engine. Adjusting the timing of the fuel injection into the engine can also reduce NOx and PM formation but again this has a negative impact on fuel consumption so there is a balance to try and maintain the best fuel efficiency but minimise the formation of PM and NOx. The injection system is operated by the engine management system controlling the point and duration of the fuel injected into the engine under all different engine operating conditions. Finally, there is after treatment for the diesel exhaust. After treatment systems for diesel engines take the exhaust gas and treat it to remove harmful NOx and PM. A typical exhaust after-treatment system will consist of a particulate filter to remove most of the particulates and then either a Selective Catalytic Reactor (SCR) or a Lean NOx trap (LNT) to remove NOx. Selective Catalytic Reaction or SCR involves spraying a fine mist of ammonia solution (marketed as AdBlue) into the exhaust system. The ammonia reacts with the exhaust gas to reduce the NOx content in the exhaust. SCR can be very effective but requires the right conditions to work, principally the exhaust gas needs to be hot enough for the reaction to take place. NOx traps (LNT) were favoured by some vehicle manufacturers over SCR because the SCR systems were relatively more expensive and require an additional fluid (AdBlue) to be stored on the vehicle and consumed in use. In an LNT the NOx is stored in what is basically a chemical filter in the exhaust system and then reduced during a regeneration cycle where the NOx is converted to less harmful compounds. Once the LNT is “full” it stops absorbing NOx so it must then be regenerated. This regeneration process uses fuel and has a negative impact on fuel efficiency. It was this LNT regeneration that has been the main cause of trouble in the industry. OEM’s were allowing the LNT to remain full and therefore inoperative in order to minimise fuel consumption. So many of the software updates done are to simply increase the frequency of the LNT regeneration, and also increase the use of EGR and injection timing controls to ensure NOx stays within limits. This has the impact of increasing fuel consumption and also reducing the performance of the vehicle. When the engine is up to its operating temperature and the exhaust gas temperature is within the right range SCR systems are very effective with a high conversion efficiency. When the engine is not in the correct operating temperature range for the SCR to work then the engine management system will utilise EGR and injection timing to reduce NOx production in combustion. But this has the negative consequence of reducing fuel efficiency. Again in some circumstances even on vehicles with SCR manufacturers were found to be reducing the function of EGR and injection timing controls when the vehicle was not on the emissions test, meaning that emissions would increase particularly when the engine was cold. Because of the cost of the SCR systems themselves and also the operating cost from the use of AdBlue, OEM’s have been incentivised to minimise the size of the SCR systems and their use to the emissions testing driving cycles which are very lightly loaded when compared to real-world driving, this has resulted in SCR systems that are undersized and result in a test pass but are significantly undersized to deliver real-world results. The old testing regimes for passenger vehicles were also conducted against a very controlled driving cycle (NEDC etc.), basically, no one actually drives like this cycle in real life, someone once commented to me that you would probably fail your driving test for driving so gently on the road! Unlike heavy-duty engines where a wide range of operating points including very heavily loaded portions of the engines operating cycle were covered in the emissions testing cycles. Using these lightly loaded driving cycles it was possible to highly optimise the performance of the vehicle in this very narrow operating band meaning that whilst the vehicle passed the tests it never came close to achieving the standards under real-world driving, note that whilst this is clearly not in the spirit of the emissions legislation this is considered to not be illegal and vehicles with such highly optimised emissions and wide variance between as tested and real world are not subject to the various recalls. The term “Defeat device” is being used to describe software control that reduces LNT regeneration frequency and reduces EGR and injection timing intervention when the vehicle is not specifically on the emissions test cycle there is no physical piece of hardware in the system. The “Defeat device” software in the engine control unit recognised when the vehicle was following the emissions driving cycle test pattern and allowed higher LNT regeneration, EGR operation and injection timing controls ensuring the emissions test was passed but then reverting back to a lower level of performance when the vehicle was in normal use. All diesel cars currently on the market do not achieve Euro 6 RDE and there are significant challenges to make this happen whilst still maintaining acceptable fuel consumption and CO2 emissions. This is resulting in the deployment of a number of technologies including:
- The use of larger SCR systems with wider temperature operating bands
- The redevelopment/deployment of EGR systems where some manufacturers had deleted these and were relying solely on the after-treatment system.
- The implementation of more advanced engine thermal management strategies to reduce engine warm-up and maintain more consistent operating temperatures such as
- Split cylinder block/head cooling loops to allow separate control of coolant flow in the engine block and cylinder head of the engine
- Electronically actuated and controlled thermostat valves
- Electrically operated variable speed engine coolant pumps
- The use of Mild and Full hybrid systems to reduce engine loads and provide torque infill during zones of engine operation and also to improve driving performance to offset the use of the various emissions control strategies.