09/2015 Exhaust gas emissions of modern diesel engines
These few lines hopefully will help you to better understand the present discussion about the exhaust gas emissions of some Volkswagen engines. We will give you some explanations about different emissions, tests and conclusions for our SVO technology.
The present controversial diesel emission data was actually discovered by the International Council on Clean Transportation (ICCT) already in 2013. The scandal came up when Volkswagen couldn’t give a plausible answer to the EPA on these emission result differences between test bench and real drive results. The most critical differences detected were nitrogen oxides (NOx). To give you a better understanding about these emissions we will first list all limited emissions for diesel engines, their effects, and explain methods to lower them. These methods are either focused on what is happening inside the combustion chamber of the engine itself – “Raw Emissions”, or rely on products and systems in the exhaust flow “Exhaust treatment equipment”
1. Hydrocarbon (HC) is an indicator for the quality of combustion; the more complete it is the less the hydrocarbon concentration in the exhaust gas. It is mostly proportional to the fuel consumption. Hydrocarbon (HC) emission increases cancer risk. Methods to lower this emission are improvement of the combustion process, i.e. better fuel mixture by increasing the spray pattern (raw emission) or installation of an oxidation catalytic converter (exhaust treatment equipment).
2. Carbon monoxide (CO) is an indicator for the quality of combustion, the more complete it is the less the carbon monoxide concentration in the exhaust gas. It is mostly proportional to the fuel consumption. Carbon monoxide (CO) is a respiratory poison. Methods to lower this emission are improvement of the combustion process, i.e. better fuel mixture by increasing the spray pattern, increasing the amount of oxygen/more air (raw emission) or installation of an oxidation catalytic converter (exhaust treatment equipment)
3. Particles are an indicator for the quality of combustion, the more complete it is the less the amount of particles in the exhaust gas. It is mostly proportional to the fuel consumption. Particles increase cancer risk. In the past a high amount of particles was simply visible as black smoke. With the newer injection systems this black smoke becomes invisible, because the particles became smaller and smaller. Methods to lower these emissions are improvement of the combustion process, i.e. better fuel mixture by increasing the spray pattern, increasing the amount of oxygen/more air (raw emission) or installation of an diesel particulate filter-DPF (exhaust treatment equipment)
4. Nitrogen oxide (NOx) is an indicator for the temperature level and unused oxygen during the combustion process. The higher the combustion temperature and the content of unused oxygen (O2) are, the higher is the Nitrogen oxide (NOx) content in the raw emission of the engine. More simply, the better your engine is optimized for low fuel consumption and high power, the higher the Nitrogen oxide (NOx) content in the raw emissions. By the way, we know this reaction very well from other processes in nature, i.e. thunderstorms. Lightening produce huge amount of Nitrogen oxides, because it heats up air, which is full of oxygen. There are some indicators about consequences of high nitrogen oxides, but the discussion is very confusing and disunited. If you read these days five different articles about diesel engines, you may get five different answers about bad consequences of high nitrogen oxides emissions. Methods to lower these emissions are lowering combustion temperature and proportion of unused oxygen, i.e. delaying injection temperature, exhaust gas recirculation (EGR), reducing intake air temperature (raw emission) or installation of a NOx-storage catalytic converter (NSC) or a selective catalytic reduction converter (SCR) - (exhaust treatment equipment).
From this short explanation you will understand easily that the first three mentioned emissions (HC, CO, particles) will simply work in the opposite way of the fourth emission (NOx). And this is more or less the bad news for the diesel engine engineers: the better your engine efficiency is, the higher the nitrogen oxide (NOx) emission - if we talk about the raw emissions.
The good news is that there is an existing technical solution for that used on the market for more than 5 years. After torturing the engines with high exhaust gas recirculation (EGR) rates over decades, there was introduced the selective catalytic reduction converter (SCR) technology, which may fix the nitrogen oxides emissions to very low levels below the limits. Diesel engines made a big step back to real low fuel consumption (decreasing by –minus 10-20%) by keeping the low nitrogen oxide (NOx) emission levels.
But why the Volkswagen engines didn’t show these good results at the International Council on Clean Transportation ICCT which conducted a real drive emission test (RDE). Technically it is definitely possible, but Volkswagen simply tried to reduce production cost of their units (roughly 150-300 USD/unit) by not using the SCR technology with an additional AdBlue (NH4 liquid) tank, which is necessary for a high level of nitrogen oxide reduction in the SCR units and is commonly found in many heavy vehicles, as well as other brands of passenger vehicles.
Instead of this they equipped some of their cars with the less efficient NOx-storage catalytic converter (NSC) and used a so called defeat device, which is software recognizing a set of criteria that indicates the vehicle is in a test procedure. This is illegal in the the US, most of West-European and some other countries. To be fair, there should be mentioned, Volkswagen was not the first using defeat devices, there is a longer history (https://en.wikipedia.org/wiki/Defeat_device).
The real damage of this scandal is being done to diesel technology, because these engines became really super clean during the last years – with the right equipment. The emissions in test and real world conditions are now on a level which is hard to measure; sometimes the exhaust is lower on limited emissions than the air going into the engine from the streetside!
Last but not least for people who want to burn SVO or WVO. All this development during the last years is really positive, because we like and need clean combustion and high combustion temperatures. So the introduction of the SCR systems was a great step for us to use alternative fuels even in newer engines!
For people who think the petrol engine is the better option and want to return to it now: The diesel engine still far in front efficiency wise and will definitely help to reduce greenhouse gas emission (Carbon dioxide - CO2). But it is very important to know if you talk about particles, that new diesel engines today are better than petrol engines, because petrol engines are not yet fitted with particulate filters. Ever since the petrol engines switched to direct injection a few years ago, they emit more particles, but nobody is talking about that – so far.