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I have an overall question that's been bothering me for years. Which is more dangerous for our health, a diesel car or a gasoline car?

This is quite a cliche question and I can find a lot of newspapers reporting that diesel is way more dangerous, but I would like to hear a more engineering/scientific/economic approach to this. Can anyone help me with this? I have a series of other questions that would help me to answer this overall question. Here is goes...

In this article, they write:

Diesel fumes are significantly more damaging to health than those from petrol engines, according to research which shows that related air pollution contributes to lung disease, heart attacks, asthma and other respiratory problems.

And they are referencing a nice report written by Department for Energy and Climate Change.

So it is agreed by many that Diesel is more dangerous than gasoline. But at the same time, the diesel engine has a higher efficiency.

Q1: Is the efficiency also reflected in the amount of gases getting out? In other words, an engine with a higher efficiency releases a lower amount of gases?

Then there is a second question, which might sound a bit stupid but,

Q2: Why are people so much for buying Diesel cars (especially in Europe)? They are way more expensive, and yet so popular in Europe.

Q3: Is the lower price of Diesel fuel really making a difference? Or is it the power and reliability?

At the same time, the regulations in US are more strict to Diesel cars.

Q4: Is it because of the emissions or is there another reason for this?

Q5: Is really NOx worse for our health than COx?

Q6: What are the engineering limitations in reducing the NOx's?

I know that some question are not strictly related to engineering, but I want to avoid posting the same questions in other places. However, I would appreciate if you could answer to any of the questions. I would highly appreciate any help in understanding this.

I think this is a very important issue, because most of people (like me) are unaware of these issues.

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  • $\begingroup$ From what I'm aware of, gasoline fumes are more dangerous to human health than diesel fumes, it's why gasoline engines are not allowed in underground environments such as tunnelling projects or mines. $\endgroup$ – Fred Nov 25 '16 at 11:06
  • $\begingroup$ Interesting. So you are saying the contrary. But if you do a simple Google search you would actually see that Diesel is the bad one. Apparently it is the NOx which makes it dangerous and Diesel releases more. Especially the <4 Euro standards. Can you find a reference or some numbers for you statement? Thank you! $\endgroup$ – Physther Nov 25 '16 at 15:25
  • $\begingroup$ What's the problem you're trying to solve? The question sounds a bit like asking whether I'd prefer to be stabbed with a bread knife or a kitchen knife - the obvious answer is that both present avoidable harm. $\endgroup$ – EnergyNumbers Nov 25 '16 at 18:03
  • $\begingroup$ Hehe, that's good analogy. Which one is the kitchen knife and which one is the bread knife? But Chris wrote a nice answer. $\endgroup$ – Physther Nov 26 '16 at 8:44
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1) In an engineering context efficiency has a very specific meaning ie it is the ratio between the energy you put into a system and the useful work that you get out. In itself this doesn't tell you very much about the amount of pollution produced in the process.

Fuel economy is a bit different as it considers the overall energy(fuel) use of the car as a whole system and while engine efficiency is part of this the mass and drag of the vehicle are equally important.

Pollution emissions is more closely related to fuel economy than efficiency but even then they are not identical. Clearly all else being equal if you burn more fuel you produce more emissions and equally an efficient engine is not necessarily clean, indeed there can be situations where emissions and efficiency are in conflict (especially in the case of NOx emissions).

2 & 3) Fuel economy is the main selling point for diesel engines. Even if the fuel is not cheaper per litre (in fact it has been substantially more expensive than gasoline in the recent past) the overall savings in fuel uses can be significant, especially for high mileage users. In very general terms diesel engines provide more low rev torque and are more robust and durable

4) Diesel engines tend to produce more emissions which are immediately dangerous to humans, although this tends to be a local problem first and foremost, particularity in urban areas.

5) It depends, carbon dioxide is an asphyxiatiant in large enough concentrations but not actually toxic per-se, carbon monoxide is (sort of) toxic in that it binds to haemoglobin and can cause asphyxiation although this is only a real problem in moderate concentrations in enclosed spaces rather than being a long term cumulative toxin. Actually one of the biggest concerns with diesel is particulates especially nano-scale carbon particles (soot) which can accumulate in the lungs with the problems associated with any fine particle inhalation and may also be small enough to cross biological membranes.

6) In some cases reducing NOx emissions can be in direct conflict with efficiency as they are produced more readily in lean-burning, high compression conditions which are preferred for efficiency. There is also the fact that catalytic converters (which are intended, among other things, to Reduce NOx emissions) require fairly specific exhaust conditions to work properly and can be damaged and clogged by the particulates in diesel emissions. There are also diesel particulate filters but these are themselves fraught with engineering challenges and, in particular, tend not to work well in the low mileage, low speed conditions encountered in urban driving where particulate emissions are the biggest problem.

Having said all of this there is a difference between the health effects of local pollution and the global effects of more persistent emissions. CO2 and its effect as a greenhouse gas is the obvious one but there are also more general effects from petroleum extraction, handling and processing to consider.

Equally you can also look at the dangers inherent in the fuel itself. Gasoline is certainly more dangerous to store and handle and being more volatile can contribute to atmospheric pollution just by evaporating.

Yet another complication is that many fuels now have at least some bio-fuel component ethanol for gasoline and vegetable oils for diesel.

Also in this context technology is changing rapidly in response to consumer demand and government regulation so there is always a danger of comparing apples and oranges eg a 2 to 5 litre truck engine is a very different thing from a 1.2 litre hatchback regardless of petrol vs diesel.

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    $\begingroup$ Diesel is often cheaper than gasoline in many European countries. This is due to lower taxation and because diesel is easier to make than gasoline. I pay about 20% less for diesel than gasoline where I live. $\endgroup$ – MikaS Nov 25 '16 at 17:45
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    $\begingroup$ Indeed, it is hard to generalise about fuel costs especially when they are substantially taxed but it is certainly fair to say that a diesel engine will use substantially less fuel than a gasoline equivalent. In the UK retail diesel prices have fluctuated a lot in the last 10 years or so. $\endgroup$ – Chris Johns Nov 25 '16 at 17:51
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My understanding:

Petrol engines produce far more carbon monoxide. Carbon monoxide is a colourless, odourless and highly toxic gas. This makes running petrol engines indoors far more dangerous than running diesel engines indoors.

Outdoors however carbon monoxide is not so bad. It's lighter than air so it won't tend to persist near ground level.

Diesel engines produce more particulates and NOx, these things aren't going to kill you immediately but chronic exposure is not healthy and since they are heavy they are more likely to remain near ground level where people can breathe them.

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