Internal combustion engine and hybrid powertrains

POLLUTANT FORMATION AND CONTROL IN CI ENGINES

Problem difficult and important:

Important: Diesel engines are today Major source of NOx and particulate; in fact we know that in SI engines, with the 3W catalyst, we can completely eliminate Nox emissions;

Difficult to control: Efficiency penalty and some discomfort of use;

In addition, let's take in account the main characteristics of the Diesel combustion, which are:

• High in-cylinder temperature and excess O2 give more Nox formation
• Lean combustion so we don't have significant CO (=very small quantity, so not an important problem)
• We don't have uniform mixture in Diesel engine (=very rich region and very lean region). In these conditions we have HC emissions.
• Because we have excess air (=lean combustion) we have both low exhaust temperature and excess of O2.
• In these conditions we cannot reduce Nox. We cannot use 3W catalyst in Diesel engines because we are not operating under stoichiometric conditions but in

lean ones. In addition, we have low exhaust temperature, so the efficiency of the catalytic convert is low too. Definitely, it's difficult to reduce Nox and we will see how to reduce it.

Pollutant formation in CI engines

CO : = 0 but not important (we will not talk about that)

HC: moderately important

NOx: very important. Nox in atmosphere gives smog.

Particulates (PM): very important (reduce visibility, health issue) also called soot (polveri sottili, fuliggine, particolato).

In addition, there's a trade-off (=compromesso) between Nox-particulates matters. In Diesel engine is actually difficult to control both Nox and particulates matters emissions. In fact we will see that if we reduce Nox we will increase particulate matter and if we do something in order to reduce particulate matter we will increase Nox. We have to find a trade-off between the two pollutants.

Now let's see what are the conditions of Diesel combustion:

We have said we have a premixed combustion phase, a

mixingcontrolled combustion phase and a later mixing controlled combustionphase.During this premixed combustion, the fuel is very rich. In the first rapidenergy release we have noise and we have soot formation. In addition,at the end of the mixing region, we have very lean region and in this wewill have HC production (because the mixture is too lean in order toburn efficiently).Then, when we go to the mixing controlled combustion, we will havehigh temperature reactions; in this diffusion flame where we obtainhigh temperature reactions, NO forms.In addition, if the spray touches the wall, we will have flame quenchingand again HC emissions (=where vaporization is more difficult).We will see that also at the end of injection, from the injector someliquid fuel comes out and this again gives HC emissions.Dicembre Pagina 237Now, as we have done with the SI engines, we will see in detail HC, Nox and then PM emissions.We will not talk about CO anymorePollutant formation in CI engines: HC WeHo visto che in un getto abbiamo una miscela molto molto ricca. Ciò significa Φ=10. Poi abbiamo una regione con Φ=2-4 (anch'essa molto ricca). In queste regioni si formano le emissioni di HC perché la miscela è molto ricca. Poi abbiamo una regione in cui raggiungiamo Φ=1, una miscela stechiometrica. Fuori dal getto avremo solo aria o una miscela molto molto povera (miscela poco miscelata). Qui avremo HC perché la miscela è troppo povera per bruciare efficientemente. Quindi avremo emissioni di HC perché la miscela è troppo ricca o perché è troppo povera. La figura b) rappresenta una spiegazione di ciò che accade durante la fase di combustione controllata dalla diffusione della miscela. Avremo una miscela molto ricca e poi una fiamma di diffusione a Φ 1 (=combustione efficiente). Lontano da questa fiamma di diffusione abbiamo una miscela molto povera. I bordi esterni del getto sono troppo poveri per bruciare. Un po' di carburante si deposita sulle pareti. Quindi quando il carburante entra in contatto con la parete, diventa difficile per esso evaporare.This causes unburned HC formation. Injector residual volume (SAC): at the end of injection we will have some residual fuel coming out from the injector and this are called SAC volumes. These SAC volumes are a source of HC emissions. Let's see in more detail the different sources of HC emissions. The "Sac" is a small within the flow volume fuel path of an electronic fuel injector. In this study, it is defined as the volume between the valve seat (fuel shut off point) and the entrance to the final metering orifice of the injector. We see that if mixture is too lean, combustion doesn't occur very efficiently. So when engine is at very low load (=idling or very light load operations) we have more mixture which is too lean to burn efficiently. So when we go in light spark load conditions or at ad idle, load diminishes and also Φ. So we will have more regions with too lean mixture and HC production will increase. However, we have

saidthat Diesel load stops around at Φ=0.75 or, if it is turbocharged, even a lower load (about Φ ≃0.62).

If we go close to the stoichiometric or slightly lean mixture, then we will have a very rapid increase in HC production. So, in this case, if the engine is over-fueled, HC emissions increase very substantially.

Dicembre Pagina 238

Pollutant formation in CI engines: HC overleaning

Then we have also the opposite problem: if the mixture is too lean, so there’s overleaning. So the problem is when the fuel has too longer time in order to mix with air. In this condition, it’s more probable that we obtain very lean mixture which are not able to burn efficiently.

So if ignition delay τid increases We will have a very lean mixture And so a major production of HC.

So if ignition delay becomes longer, the exhaust unburned HC increase.

Pollutant formation in CI engines: HC SAC volume

Again, the other problem is with the injector. When the plunger closes the hole, we still have a

little bit of fuel which remains in the SAC volumes and inside the small holes. This fuel will come out from the injector very slowly and with high-diameter droplets. If we make a graph with SAC volume on abscissa and exhaust HC in ordinate, we will see that the higher the SAC Volume, the higher the unburned HC emissions. In that graph we have marked the minimum SAC volume that can be used. In fact, it's not very convenient not using a SAC volume, because if we don't have any sac at all, this gives overpressure waves into the incoming fuel pipes. Definitely, if we have a big sac volume, We have higher exhaust HC. If we have a very small SAC volume, we will have lower exhaust HC. Then we have HC also if we make a null SAC volume because we still have the hole volume in which some fuel is present. Anche se annulliamo i volumi residui dell'iniettore, sono comunque presenti volumi interstiziali dei fori che non possiamo eliminare. Da questi volumetti, il combustibile, invece di

essere sparato nella camera, sgocciola e quindi da delle gocce di diametro elevato che hanno difficoltà a vaporizzare. Dicembre Pagina 239

Strategies for HC reduction in Diesel engines:

• Careful control of A/F.
• Pressure boost. One of the problem is that fuel can come in touch with walls. If this happens, vaporization is very difficult. So, in order to avoid this phenomenon, we increase air density. Aumentiamo la densità dell'aria in modo che i proiettili vengano rallentati e non sbattano contro la parete. Inoltre, diminuiamo le emissioni di idrocarburi incombusti poiché favoriamo l'evaporazione del combustibile.
• Injection time advance at light load.
• Exhaust oxidation catalyst for HC and (the small amount) CO (=after-threat HC)
• Pollutant formation in CI engines: NOx

We have a quiescent DI Diesel engines (no swirl). In point S+ we have a probe (=sonda) which takes a small amount of gas and analyzes the composition of this small amount of gas. Then we have a

graph of pressure and temperature against crank angle. We notice that when combustion starts we have a change in pressure derivative (=punto angoloso): the pressure increases much faster than before. Also temperature increases very rapidly. S+ we can measure equivalent ratio: at the very beginning (=when the injection is already started) Φ is higher than zero but very low. Then, as fuel travels and reaches point S+, the equivalence ratio increases up to values Of about 1.5 -2. Closer to fuel jet, we have an even higher equivalence ratio ( Φ ≃10) S+ So in this point we see that first we have very low oxygen concentration because

Φ is high. The graph represents mole fractionscurve. So we have, for example, quantity of oxygen O2 as compared With quantity of other chemical species.

So let’s see what happens to NO: it increases very rapidly as temperature increases and then decreases very slowly Whentemperature decreases. The problem is the chemical reaction rate.

On the left we have different values of different compositions of fuel: we can go from higher carbon number (C=6) to methane. Ofcourse we don’t have methane in the fuel: it is a first partial decomposition of the original fuel.

Quindi questi combustibili che troviamo nel grafico non sono contenuti nel gasolio, il quale ha numero di atomi di carboniosuperiore a 12. Questi composti sono primi prodotti parziali di combustione, che aumentano e poi spariscono mano mano chebruciano.

As this partial combustion products first increase and then they burn, we will see the increasing of CO2Then the mole fraction of CO2 decreases, Not the CO2 production!

That's because we have more oxygen, so the mole fraction of CO2 decreases. Avendo frazioni molari, se c'è più ossigeno, la componente CO2 conta di meno. Dicembre Pagina 240 So in Diesel engine we have largely NO and modest content of NO2. We have to remember that in SI engines we don't have any NO2. We have seen that the Nox forms in region where we have high temperature and equivalence ratio about 1. Then, when we have the mixing of NOx with air, the temperature decreases because of that mixing (=air has a lower temperature). The temperature starts decreasing and Nox reduction rate becomes very slow. So the No concentration freezes (=composizione congelata, nel senso che non è più la composizione relativa a quella di equilibrio, ma a una composizione di un equilibrio.