Newsgroups: sci.energy
Subject: Re: Internal Combustion Engines
From: srgxbhh@grace.dsir.govt.nz
Date: 15 Apr 92 22:32:36 +1200
 <1992Apr11.141805.16715@grace.dsir.govt.nz> <1992Apr12.183213.13212@eskimo.celestial.com>
Organization: DSIR, Gracefield, New Zealand

In article <1992Apr12.183213.13212@eskimo.celestial.com>, nanook@eskimo.celestial.com (Robert Dinse) writes:
> [ my earlier post deleted ]
> 
>      You raise a mixture of nitrogen and oxygen (air) to a high temperature
> under high pressure and some of it reacts and forms NOx..  Where is the
> mystery in this?

No mystery - I didn't intend "apparently" to mean "mysteriously". 
 
>      I can't see of any way to make an internal combustion engine that uses
> air as the oxidizer not have this problem.  You can reduce the problem by
> using lower temperatures and pressures, but you get lower thermo-dynamic
> effeciency.  This is one of the reason gas milage went to hell around '74
> when compression ratios were lowered in the name of meeting NOx emission
> requirements.

One of the reasons?. I suspect that the reduced CR wasn't only for meeting NOx
emission requirements, as there were techniques available to reduce tailpipe
NOx emissions. I can't comment on the US requirements at that time, nor the
reasons why the CR was lowered rather than improving combustion etc..  

> 
>      Hydrogen presents another problem in internal combustion engines, it has
> a very low octane rating, which makes it difficult to use without knocking.

I wonder if Octane Ratings actually have much relevance to gaseous fuels,
especially hydrogen with its strong tendency to pre-ignite and backfire
unless a " clean " engine with timed fuel injection is adopted. The only 
( admittedly old ) reference I have about Hydrogen indicates that the
stratified charge engine has much lower NOx than direct injection engines.
The need for weak mixtures and cool operation if knock is to be minimised
when using hydrogen fuels is not the only major problem. All the blowby gases
have to be purged to prevent explosions ( hydrogen has both a very wide
flammable range, and a small molecular size that enables it to leak 
through most seals). Also the fuel storage system has to be considered.
For a Total Energy Content of 1590MJ the following data was reported for
automotive fuel storage systems.
                             Fuel                  Fuel + Tank
                          kg        l              kg         l
Motor Gasoline            37       50              46        57
Liquid H2 ( Cryogenic )   13      187             105       276
Metallic Hydrides        183      208             214       234
Gaseous H2 at 13.8MPa     13     1140             710      1700    
 

>      One thing that reduces knocking, reduces NOx emissions and improves
> effeciency is water injection.

Interesting.... My notes from reading about hydrogen include the following
"The power output is comparable, however water injection ( to reduce NOx )
 lowers power and efficiency. All NOx would be eliminated by the use of O2"
Unfortunately I don't have the citation, but if water is so wonderful, why
not forget the hydrogen completely and run the engine on water. :-)

Bruce Hamilton            SRGXBHH@grv.dsir.govt.nz
Newsgroups: sci.energy