[Gasification] Prime Mover Knock

[AJH]

On Wed, 17 May 2006 01:23:09 -0400, Jeff Davis wrote:

>Dear AJH,
>
>Interesting misinterpretation of my e-mail.

Did I miss something in an earlier thread that lead to this?

I hoped I was explaining the drawbacks of Diesel's early ideas. Gasman
has posted in this thread some of the reasons for Diesel choosing this
method.

>
>It's a shame that so many people had vicious criticism for his prime mover. 

I'm certainly not criticising the diesel cycle as a prime mover. The
thread has thrown up salient points from Kevin and Gasman as well.

One point being that the original diesel was a slow speed device and
the combustion took place from the top of the stroke to a small way
down it and then continued expanding. This is analogous to the steam
cycle when steam is injected for, say 10%, of the stroke and then
expanded, doing work against the piston.

The Otto cycle completes it's burning much nearer TDC, largely before
the piston has started moving down. This actually gives it an
efficiency advantage as all the charge expands from one high pressure
to the exhaust pressure. 

Now what Diesel designed is a long way from what we see as modern
diesels, they are now much nearer to Otto cycle, as Gasman pointed
out, and no longer have constant charge of air, because they nearly
all have turbochargers.

The advantages of the modern high speed diesel are now mostly to do
with their better volumetric efficiency, as Kevin pointed out they can
suck in a full charge of air whilst only heating it with a small
amount of diesel, the spark ignition engine needs both air and fuel be
supplied at a near constant ratio.

Also as ever note most fuel is sold by volume, gasoline has 10% less
energy per litre than diesel.

These two factors lead people to expect diesels to be more economical
but at constant load and given the same compression ratio the Otto
cycle is more efficient.

AJH