[FRIAM] Diesel OK?

[Steven A Smith]

On 3/29/18 9:45 PM, Eric Smith wrote:
> Steve, hi,
>
> Two quick questions since this brings back things I was curious about
> as a child:
>>
>> 1.
>>
>>
>>     Higher MPG
>>      1. higher energy density per liter of fuel
>>      2. higher compression engine (more efficient)
>>      3. leaner combustion at idle or under low load
>>
> I seem to remember this as being associated with higher nitrogen oxide
> emissions than richer-burning.  Has that long since been fixed?
I believe that NO emissions are associated with the lean burn of
Petrol... I'm not sure why Diesel is less apt to that... perhaps the
longer chain hydrocarbons, the higher compressions (a different pointin
the pressure/volume/temp space?) or the more "natural" or complete
combustion conditions without a spark? 
>>
>>      4. better emmisions *except* particulates
>>      5. Usually are coupled with a turbo (they benefit more than petrol?)
>>
> I had assumed that this was because petrol engines continue to be
> spark ignited, not only for manifold injection but even for direct
> cylinder injection.  The flash point of a petrol-air mixture limited
> the compression ratio you could boost to somewhere below 8:1 (mist of
> memory), whereas since Diesels (I thought) were timed by when the
> injection is done, you could compress them as high as you had
> mechanical tolerance for, continuing to gain efficiency as burn
> temperatures were increased.
Turbo (or Super) charging increases *manifold* pressures, therefore the
compression *ratio* between intake/exhaust doesn't increase, just the
density of air/fuel charge.   I seem to remember Petrol engines in the
10:1 ratio, but those definitely required high octane fuel to avoid
pre-ignition.
>
> Are those ways of thinking even applicable to the engineering
> standards today?   It’s now been (?) 40 years since these cryptic
> memories were formed.
I think it all applies, but surely there are myriad episystems designed
(not unlike bruce's urea "scrubber" in the exhaust) which may move these
around a little.   Marcos Lopez (only on the WedTech list?) seems to be
much more up on modern auto engine design/processes.

- Steve

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