There have been numerous studies published during the last decade reporting on the high levels of pollution generated by diesel engines and their possible consequences.
The public scandal and the controversy however, are a matter of the past few months. Clearly encouraged by the irresponsibility and misconduct demonstrated by the VW Group in approving some of its diesel engines with specialist software for anti-pollution testing, much of the specialist communications media investigated the topic and the results were not slow in coming.
Toxic emissions from diesel vehicles in real operating conditions exceed and multiply several times over the “expected” quantities and maximum approved values.
To this day, it might be said that the problem is widespread and includes practically all manufacturers. In the majority of cases they have taken advantage of small legal loopholes in the approval tests in order to pass the emission tests and obtain the consumption data/official emissions which, moreover, are usually those that are publicised.
Small tricks such as increasing the tyre pressures, switching off all the electrical consumers including the power steering, using specific lubricants, among others, The test cycle is carried out on rollers which has little to do with actual driving conditions, either on the highway or in the city.
In this way a reality has been hidden which the residents of Europe’s leading capital cities have been suffering from for some years now. Photochemical smog, that brownish grey blanket which we can see above cities, is only the visible part of an invisible but very real danger, the high levels of nitrous oxides (NOx) produced by diesel engines.
In recent times the replacement of the vehicle fleet by diesel vehicles has been promoted by authorities in several countries in the form of incentives and purchasing aids. The lower amount of CO2/km emitted was a determining factor, and the fight against global warming the objective. The diesel vehicle has been “sold” as being more environmentally friendly than petroleum, which is only a half-truth, and not the whole truth.
The reality of the diesel revolution is much more dangerous than expected and we need to know why.
The nature of fuel and the combustion processes in diesel or petrol engines determine the different composition and proportion of their respective exhaust gases.
In spite of the fact petrol engines at first produce a higher quantity of polluting substances than diesel engines, they are easily transformed into innocuous substances by means of a three-way catalytic converter, managing to reduce their quantity by 95% and more.
The petrol/air mixture inside the cylinders is always maintained at a ratio that is close to ideal (stoichiometric ratio). The fuel is vaporised and easily mixed with the air thanks to its low density, forming a very homogeneous mixture. The small size of the molecules of hydrocarbons that make up petrol favour its full oxidation in a short period of time so that combustion is practically simultaneous in the whole combustion chamber, and very rapid. The reaction is complete in approximately 1.5 - 2 milliseconds, varying little depending on the design of the combustion chamber, the compression ratio, the engine temperature and the air/fuel ratio. The amount of mixture contained inside the combustion chamber has minimum influence on the duration of the combustion reaction.
In this way, the resulting gases are mostly N2 and CO2 along with 1% of CO and 1% O2 as a result of incomplete combustion and a minimum amount of NOx and HCs, which the catalytic converter subsequently transforms almost entirely into water vapour, CO2 and N2.
The process of combustion in diesel cycle engines is somewhat more complex due to the nature of the fuel itself and its self-igniting operating cycle.
The hydrocarbons that form diesel are very different in nature, and are basically molecules of a much greater size and more complex atomic structure than those of petrol. Consequently, their full oxidation is slower and releases more energy than in the case of petrol. Hence diesel fuel is denser and less volatile which means it is more difficult to mix well with air or vaporise.
Just look at the different speed with which both fuels evaporate at ambient temperature to get an idea of their chemical “predisposition” to be mixed with air.
The diesel fuel is injected into the combustion chamber at high pressure. Nowadays, to achieve a good spray of fuel we use multiple-hole injectors (5 to 7 generally in passenger cars), injection systems of more than 2000 bar pressure and high turbulence intake pipes. Despite this, the homogenisation of air/diesel is not complete.
The question is somewhat less complicated. The "mix" should be made in a minuscule period of time which decreases as the engine speed/revs increase and, paradoxically, the amount of fuel that must be injected also increases. As a result of this, the only way to ensure that the highest amount of injected fuel is utilised and oxidised is always to introduce a quantity of O2 into the combustion chamber that is much higher than chemically necessary. To prevent the formation of soot particles (partially burnt particles) a minimum excess of O2 of 9-12 % must be maintained. It is for this reason that, historically, the majority of diesel engines have worked without a choke on the air intake, so that the work obtained from the engine is only regulated through the amount of injected fuel.
In parallel, the duration of the combustion and the temperature of the process depend substantially on the quantity of participant fuel. For faster engine speeds, more fuel is needed, which delays the burning time and achieves a higher temperature.
The high injection pressure required to get a good spray and penetration/homogenisation of the fuel in the combustion chamber causes the diesel to cool instantly when it passes from the liquid state inside the injector to a gaseous state in the combustion chamber. While cooling means a delay in the ignition of the fuel and a real headache to synchronize the start of combustion with the top dead centre of the pistons, remember that it self-ignites due to the elevated temperature of the compressed air, we must consider that it has a positive part, and that this allows a “minimum” time for the atomised fuel to mix with the air. Thus, combustion is not initiated right at the output of the injector holes but rather focussed circularly inside the combustion chamber.
The high concentration of "cold" fuel in the centre of the combustion chamber contrasts with its scarcity in the outermost area of the chamber, so that the combustion, once initiated, takes place differently depending on the air/fuel mixture in each area of the combustion chamber. If we establish the middle area as “ideal”, it is clear that the central area will have an oxygen deficit during practically the whole injection process and the area furthest away a notable excess of oxygen. And this is where the problems begin...
During the combustion process in unfavourable conditions, different pollutant substances are generated.
As a consequence of the excessive concentration of diesel in the central area, the air is displaced and with it the required oxygen, giving rise to the formation of solid soot particles, partially oxidised hydrocarbons and even the presence of unburnt hydrocarbons in the exhaust gases, especially at high engine speeds and high loads.
The excess of oxygen in the outermost area of the combustion chamber is still more harmful. The combustion process at the front of the flame with excess oxygen and high temperature causes a parasitic chemical reaction and the creation of nitrous oxides of a different structure. The nitrogen (N2) present in the air in a proportion of 78% reacts with the oxygen (O2) at temperatures above 1500°C and especially above 1800°C.
The combination of accidental or undesirable N2 and O2 poses a double problem. In addition to the creation of different nitrous oxides, the amount of oxygen available for combustion is also reduced, so, depending on the proportion this can result in partial combustions which increase the amount of CO and solid particles in the exhaust gases.
Just as in petrol engines, the hydrocarbons and CO resulting from the combustion can be easily removed using an oxidation catalytic converter. The solid particles and/or soot are retained and subsequently removed using a porous particulate filter. The NOx however are even more difficult and costly to remove.
At this point it is easy to determine because modern diesel engines produce more NOx than even a relatively old engine.
Increased thermodynamic and mechanical performance of the engine require higher pressure in the fuel chamber and to achieve this even higher combustion temperatures are needed which encourage the formation of NOx. To this end, more fuel must be burned in less time, at the precise time during a limited period of time.
Burning more fuel requires more oxygen (O2) and accordingly more air (78% N2), a task which is undertaken by the modern variable-geometry turbochargers. Burning it in less time requires a higher quantity to be injected in less time, which is achieved with higher injection pressures and more holes in the nozzles/injectors. More injection holes entails a higher number of flame fronts which is precisely where the NOx originate.
In this way it is easy to understand the evolution of diesel engines once their weak points have been dug up. Old and slow diesel engines did not have the benefits of those of the present day, but nor did they have the amount of NOx of the latest ones. With a single injector hole in the majority of cases, and naturally aspirated intakes or low blow turbos, the high combustion temperatures that facilitate the formation of such an “irritant” substance are not reached.
The evolution of vehicles towards the diesel has become a reality in recent times... along with the increase in the amounts and concentrations of NOx, especially in big cities.
Levels of NOx expected in the large European cities have been clearly exceeded and most probably due to the differences between theoretical quantities and actual quantities produced by many relatively new vehicles.
Now that we know how they are formed, in the next entries in our blog we will talk more about the NOx, its hazardous nature, how to prevent it forming and how to eliminate it or transform it into a non-toxic substance...