Newsgroups: sci.energy
Subject: Re: Internal Combustion Engines
From: srgxbhh@grace.dsir.govt.nz
Date: 17 Apr 92 07:12:44 +1200
Organization: DSIR, Gracefield, New Zealand

In article <1992Apr15.170303.10462@fmsrl7.srl.ford.com>, wreck@fmsrl7.srl.ford.com (R. Cage) writes:
> In <1992Apr15.223236.16722@grace.dsir.govt.nz> srgxbhh@grace.dsir.govt.nz says:
>>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.
> 
> It has every relevance.  Octane rating is a measure of resistance
> to pre-ignition.  The ease of ignition of hydrogen translates directly
> into a very low octane rating (and maximum compression ratio).  If
> memory serves, hydrogen ignites in air at about 400 degrees F (477 K),
> which is about equal with (or even lower than) heptane.  

I obviously didn't make myself clear. Octane ratings are actual measurements
of liquid hydrocarbon fuels in variable compression engines using standard
settings - however the measurement is alway a comparison between the sample
and blends of iso-octane ( ON=100 ) and n-heptane ( ON=0 ), or if the ON
is greater than 100 then alkyl lead compounds or toluene are added.
Octane numbers for gaseous fuels can not be generated under the same
conditions and the actual conditions are not usually reported. 
Knock occurs in the spark-ignition piston engine when the severity of the
operating conditions is sufficient to permit the end-gas mixture to ignite
spontaneously before being consumed by the advancing spark-ignited flame.
The tendency towards fuel knock is is increased by any factor which causes:
 (a) A decrease in the main flame velocity in the combustion chamber
    [ see my earlier post for ** indications ** of relative flame speeds ]
 (b) A shorter period for ignition of the unburnt gases beyond the flame
     front.

In practice many such engine factors exist and the requirements for avoidance
of knock are vested in the fuel alone. I assume vehicle manufacturers in the
US still perform Road Octane Ratings - but they could have sufficient data
that these are no longer required. The tendency to knock can vary with :-
compression ratio, degree of supercharge, shape of combustion chamber,
position of valve ports and spark plugs, oil consumption and general 
cleanliness of the engine, temperature and pressure of the charge, fuel
distribution in the chamber,mixture strength, turbulence within the chamber,
ignition timing and type of spark plug. In the early 70s Jaguar had an
engine ( with a head designed by Hay? ) that provided high compression and
still operated on fuels of medium Research and Motor ON. 

I'm not aware of a significant correlation between autoignition 
temperatures and Octane Number for different classes of fuels. There may
be one within chemical families. I'm not suggesting hydrogen is a suitable
fuel for automobiles, but I believe alternative fuels should not be assessed
historical hydrocarbon fuel quality criteria such as Octane Number. 

It's interesting that the SAE assessment of Hydrogen in SAE J1297 JUN90
"Alternative Automotive Fuels - SAE Information Report" states
"The use of hydrogen would require revolutionary changes in fuel manufacturing,
distribution and handling systems. At present, application of hydrogen to
vehicles as a motor fuel is in the research stage, and problems in its 
use are not clelyar defined. Safety and methods for storage of hydrogen
fuel will require additional investigation.
[ some text deleted ]
High cost storage problems and safety considerations represent substantial
deterrents to the use of hydrogen as a vehicle fuel."  
 

> Summary:  Hydrogen makes a lousy fuel for an internal combustion engine.
> -- 
> Russ Cage              wreck@fmsrl7.srl.ford.com  russ%rsi@sharkey.cc.umich.edu
>   *  When Ford pays me for my opinions, THEN they can call them theirs. *

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