Diesel engine pollute more only according to NOx and particulates emissions. They actually have much lower emission of CO. The high rate of NOx comes from the high temperature in the chamber which favors the oxidation of N. Particulates comes from the diffusion flame that diesel engine uses. They are created as droplets of fuel do not have enough time to evaporate while travelling toward the front flame and then pass through it.

If you want, you could find more details in a reference I gave you in an other topic a few days ago.

I think that the major problem in engine pollution for health are particulates. Indeed, even if the biggest are stopped by our "natural filter", the smallest one go directly in our lungs and can stay there for a long time... It's particulary harmful for kids, old people, people suffer from asthma and can even created cancer... This problem was only a diesel problem until direct injection for gasoline engine appeared.

It's a big problem for Europe as governments oriented a big part of cars population to diesel and gasoline direct injection has been spread some years ago, whereas North America mostly kept gasoline engine with port fuel injection, direct injection only appearing now.

Diesel engines work in leaner ranges of fuel, so the emissions of CO and HC are supposedly lesser than in SI. But when we go for NOx, as the compression ratios are high, the temperatures and pressures are high in CI, when compared to SI engines.Also the soot or particulates are present in CI emissions because of non homogeneous mixing.

When we have non homogeneous mixing, we can have a fuel drop sprayed into the combustion chamber which is already at high temperature (high temperature can be because of the rapid initial combustion), and the fuel drop might get cracked because of high temperature, even before it finds the air it needs.

And yes, particulates are more hazardous to human body as explained earlier by Jonathan.

Thanks for these very much clear responses..

In the U.S. there is practically no difference between emissions from diesel and gasoline (petrol) automobiles, trucks and buses. Both diesel and gasoline vehicles and trucks are regulated to the exact same emissions standards for PM, NOx and NMHC or NMOG and must comply with the exact in-use emissions compliance provisions. Diesels using CDPFs for PM control have PM emissions that are at, or below, those of the best PFI/SI gasoline vehicles and trucks. Heavy-duty diesel trucks using copper-zeolite urea-SCR have comparable NOx emissions to heavy-duty SI gasoline trucks using PGM-based 3-way catalysts. Similar systems used in light-duty vehicle applications are capable of achieving complying with stringent Tier 2 Bin 5 emissions standards (0.07 g/mi NOx at 120,000 miles). In general, the exhaust catalyst systems and high-pressure common rail injection systems necessary for diesels to ahieve parity with stoichiometric SI gasoline engine emissions are more expensive, but for some applications the improved low-rpm torque characteristics and much improved part-load efficiency improvements more than offset the increased up-front costs, particularly for heavy applictions such as Class 6-8 trucks and buses. For lighter applications, improvements in the torque characteristics and efficiency of SI gasoline engines, particularly some of the newer TGDI designs, tilt the balance in their favor once the cost of the emissions control and fuel systems are factored in.

With respect to pre-catalyst NOx and HC emissions, in my experience these are much higher for stoichiometric homegenous SI combustion than for lean, diffusion-controlled diesel combustion - the difference in tailpipe emissions is due to the relative ease with which PGM 3-way catalysts can reduce NOx and HC emissions. Typical pre-catalyst NOx emissions concentrations over the light-duty FTP cycle are often 2-4 times higher for stoichiometric SI gasoline comubstion relative to comparable light-duty diesels and HC emissions are often 2 orders of magnitude higher or more for pre-catalyst SI engnes. With modern 3-way catalyst systems, NOx and HC emissions drop to levels that approach zero emissions. Modern urea-SCR systems or combinations of SCR and NOx adsporpton catalysts (NAC) together with combustion controls (cooled EGR, HPCR) provide comparable levels of emissons control for diesels.

The fuel is injected over the compressed air which makes it completely heterogeneous mixture leading to incomplete combustion. This attributes to the formation of particulate matter and soot.  But in case of gasoline engine the mixture is either partially or completely homogeneous and the combustion propagates in the forma of flames from the tip of the spark plug to entire combustion chamber volume so no soot occurs in case of gasoline engine.

The combustion mechanism is mainly responsible for the difference in emissions from diesels and gasoline engines.

The attached figure will clearly explains you about the combustion process.... please find the attachment..

 Dear Sharna, how much are approximately formed particle matter  when combust 1 litre diesel ? What kind of particle matters emerge ...heavy metals or VOC? And, can u say that What is difference particle matter as g/km between Euro 5 and Euro 6 when combust ?