diesel or petrol engine

Both pollute in similar as well as different ways and amounts.

No.

Depends on what you consider friendly. Neither one is very clean but I would say a modern gasoline engine has a slight edge over a diesel.

This is not a comparison that can be made accurately and easily. Both create CO2 in the combustion process, which is a big potential issue.

Diesel is probably less likely to contribute to high nitrogen oxide contamination but may release more carbon particulates..

Both alter the atmospheric composition over time, and the differences may well be slight.

The British put out about a decade's worth of propaganda extolling the environmental benignity of the diesel engine. So much cleaner and greener, they said. Do be a love and try to get a diesel car rather than a petrol one, they said.

Oops, and then it turned out that the particulates diesel engines spew are highly carcinogenic, and that made it a lot harder to disregard the high levels of NOx they likewise spew. Diesels' "cleanliness" advantage is in reduced CO2 per mile (a good thing if you're in a country that has signed the Kyoto Accord) and in reduced HC and CO emissions compared to an

**UNCONTROLLED** gasoline engine.

Thing is, uncontrolled gasoline engines were last installed in new cars in the US in the mid-1960s.

The Brits are not the ones to listen to on auto exhaust emissions. They finally got round to deciding that perhaps there might be something to all this balderdash about unleaded petrol...a decade and a half after it was mandated for new cars in the US, and just 18 months before it was mandated in *Mexico*.

It is an almost unanwerable question. The Diesel has different exhaust emission products than the SI engine. Much less CO, but at certain power levels, more nitrous oxides. And more soot. Is soot worse for you than CO? I am not sure anyone knows.

Even the greenhouse gas thing is complicated. The Diesel gets a better milage in terms of pounds of fuel used, but the fuel likely has a different ratio of carbon atoms per pound than gasoline, so it is hard to say which puts out more CO2- actually I'd like to see figures if anyone is aware of any studies with conclusions on this one.

Part of the problem is that emission controls for diesels lag some years behind those of gassers, for various reasons: one might argue for factors such as lack of opinion and regulatory pressure, incomplete vision of the full scope of the problem, and still-evolving understanding of the health effects of the very small (2.5 micron on down, and especially the nanosized) particulate matter.

The array of technologies needed is at various stages from lab R&D to commercialization. The advent of low- and then ultralow-sulfur fuels has also been a while coming, and most of the pollution control technologies depend on such fuels (that older engines can live with!). Some improvements have been made, and are still being made, in the engines themselves.

Note also that the bulk of diesel-related pollution is from "off-highway" use -- trains, construction and mining equipment, ships, etc. -- although trucks and buses and cars also make up an important contribution and are particularly concentrated in the same places where people are. Many of those off-highway applications use diesels because they're by far the best suited technology, and in many cases we're talking about big engines with big price tags whose owners hope they have big lifetimes.

See for instance

links therein, such as particularly (no pun intended and not much of one achieved) for asurvey of the health issues,

Anyway, that's on the output side. I don't know if there is much difference on the input side in their environmental impact, at least if you assume that they're both going to run on a fairly sophisticated petroleum-derived fuel. I wonder if diesel of that sort is easier or nicer on the environment to make than is gasoline. Dunno.

Cheers,

--Joe

Thanks, guys

John

Do you have any references to the carcinogenicity of diesel particulates? I'm not doubting you, just wonder where you got this information.

I have seen some relatively recent work that indicates that fullerenes may have rather unsavory and unsuspected consequences to the human organism. I have not been able to find any good source to whether fullerenes are produced in diesel exhaust - even in minute quantities - but would not doubt that it could happen.

As a postscript, I have seen a number of reports on the carcinogenicity of diesel, and the authorities seem to waffle about how bad it really is. The particulates would be intuitively considered to be contributing to lung disease, including cancer. The nonparticulate emissions are also items which are not particularly healthy.

Of course, gasoline engines intrinsically put out some of the same things. Gasoline which was formulated with high levels of BTX (benzene-toluene-xylene reformates) was pretty ugly itself.

From what I have read, the diesel engine inherently puts out much less oxides of nitrogen than does a typical gasoline engine BUT the use of modern catalytic converter systems can reduce these levels to diesel values or even below.

One thing we chemists have learned slowly, and sometimes the hard way: meaningful data is difficult to collect and interpret. In the early stages where chemical toxicities were being evaluated, the pure compounds were often tested, giving some benchmark data at any rate. It was soon found that the interactions between apparently 'low level toxicity' by-products often produced systems which were much more toxic than the components. In some cases, chemical families were assumed to be similar in toxicity, while further work showed this was not always the case...For example, alkanes or paraffinics were thought to be universally rather benign. The unusual hexane toxicity taught us not to generalize.

Some time ago our company was developing some sensors for soot control. I talked quite often with a truck fleet operator in the area. He said t hat while the more modern of his trucks had controls that would prevent heavy soot formation, the engine controls were overridable. When overridden they provided a bit more power. Some drivers were able to learn how to override the controls, others had their mechanic buddies do it. :-(

I have seen trucks, locomotives, and construction equipment with engines obviously sadly in need of tuneups or overhauls, but the operators figure it is more expensive to fix them than to pay the very infrequent pollution control fines. Some of these vehicles are even operated by local and county governments :-(

From what I have read, the aldehydes are pretty much accepted as being carcinogenic, but the jury is still out on carbon particulates (soot).

snipped-for-privacy@nospam.nix wrote:

Formaldehyde is certainly carcinogenic, BUT we have been using it for decades with little problem. Other aldehydes may not be carcinogenic at all.

I agree the jury is still out on particulates. And it may depend heavily on the exact morphology of the particulate as to the level of risk.