[sustran] low compression vs high compression

[Eric Bruun]

On 14 Apr 1998, Roberto Verzola wrote:

>  >The old reasons against high compression are not necessarily
>  >true any longer, as the combustion can now be regulated to 
>  >vary in temperature and to have more idealized "swirl".  
> 
> Barry Commoner was quite definite that high-compression engines
> produced worse pollution than low-compression ones. Can you please
> substantiate your statement, which seems to contradict Commoner's
> conclusions? (Not that I am looking for a debate; I was simply
> convinced by Commoner's very strong arguments, and I haven't seen
> convincing counter-arguments or substantiation yet from postings
> here.)

  You are not going to get the kind of definitive argument that
  you demand in short answers to postings.  The point that Barry
  Commoner made is true, but in a misleading sense. If we had
  all low compression engines there would be less NOx, for example,
  but we would be consuming far more total fuel because of the
  low efficiency of low compression -- it is a thermodynamic
  fact of life that higher temperatures (which go with the higher
  pressures) give higher efficiency. Thus bigger engines
  would offset many of the gains from lower compression. That
  is why I mentioned compromise solutions where combustion chambers
  are designed carefully to burn part of the charge at lower and
  part at higher temperature. This is how the Honda CVCC engine
  complied with pollution regulations far ahead of deadlines.

> 
>  >Most two stroke motorcycles have relatively low compression
>  >ratios.  They can be boosted, but at great expense in reliability,
> 
> This confirms what I suspected: that motorcycles in general are
> low-compression engines. According to Commoner again, the main
> products of combustion in a low-compression engine are water, carbon
> dioxide and unburnt fuel. In high-compression (and therefore
> high-temperature too) engines, chemistry kicks in and produces a
> different set of combustion by-products, which are worse pollutants.
> 
>  >are polluters more because of their primitive design of 
>  >carburetion and lack of valves as much as anything else. 
> 
> This seems to imply that a more primitive design means more pollution,
> which is not necessarily so. In fact, Commoner's message, in his
> chapter "The Technological Flaw" was that often more modern designs
> are more counter-ecological (and he cited quite a number of examples,
> not only automobiles), because design is guided less by ecological
> than by profit considerations.
> 
> Obet Verzola
> 
  When I say "primitive" I mean that they use non-carefully matched
  and tuned components. These motorcycles do not have the best
  carburation, internal cylinder porting , and timing for optimum
  pollution performance.  If they used the same equipment (at 
  higher cost, of course) that was sold in Europe or the United
  States, just this change alone could already decrease pollution.

  At any given power output level and displacement, modern 
  engines can always be made to pollute less than older designs. 
  I believe it is not just profit 
  considerations (this is more relevant to selling in rich 
  countries) it is cost considerations that cause the sale 
  of obsolete designs in the poorer countries. 
> 
>