"Real" diesels vs. "Kinda" diesels

[LowGear]

My local small diesel engine dealer tells me real diesels, those with more than 19:1 compression, get better fuel economy and are better machines.  Is he just full of it or has he got a valid case?

Casey

[compig]

Higher compression ratio = higher efficiency.

[mobile_bob]

not necessarily true

there certainly are real diesels that run at 11.6:1 (ntc400 cummins albeit turbo)

the changfa s195 idi is 20:1, but
the varient di engine is 15.5:1 and uses less fuel per hp or kwatt

efficiency in theory goes up with comp ratio, along with other things like friction
so you get to a point of diminished returns

generally good design dictates using no higher compression ratio than that needed to provide
for ease of starting, in the climate the engine is targeted for, burning the fuels that are commonly available.

there is a lot more to efficiency than compression ratio, so raising it chasing the last bit of efficiency
is likely the last path i would take.

millions of real diesels have been built that run in the 15:1 ratio range, with as many below as there is above
i would expect.

bob g

[compig]

I should have said generally speaking and assuming otherwise good design.

[PaulMoffenger]

Hello,
Typically the only diesels that are above 18:1 are Indirect injected or Naturally aspirated direct injected (not many either).

Paul

[lowspeedlife]

I'd say that's his opinion & you know what they say about opinions. everybody's got one & most of them stink!. some guys would say "real" diesels are water cooled, some would say they are multi cylinder & blown or turboed. Rudolph's first engine surely wasn't 19 to 1 compression, so i'd ignore him unless he's got sum'thin you want.
    Scott R.

[LowGear]

Sage advice Scott R.

[compig]

Just checked the CR of my Citroen common rail turbo diesel , 18:1. The NA version is 21.8:1.  It's all dependant on the overall design. The early diesels were probably unable to use high CR's due to the metallurgy of the time and the difficulty then in making high pressure injection pumps.

[Quinnf]

That's what Rudolf Diesel thought, too.  He experimented extensively with high compression ratios and found to his surprise that waht worked best for overall efficiency was to have the compression ratio only high enough to ensure reliable initiation of combustion.  It wasn't a case of metallurgy, but one of combustion engineering.

You can read all about it in detail in http://www.amazon.com/Diesels-Engine-Lyle-Jr-Cummins/dp/0917308034 written by Lyle Cummins.  It's a good read.

Quinn

[compig]

I suppose there is going to be an optimum CR for a given engine design. Thanks for the book tip , I'll buy a copy.