Newsgroups: sci.energy
Subject: Hydrogen in Internal Combustion Engines
From: srgxbhh@grace.dsir.govt.nz
Date: 17 Apr 92 20:00:00 +1200
Organization: DSIR, Gracefield, New Zealand

 
Well to add a bit more information to my previous posts, I located my
photocopy of a chapter from " Alternative Fuels - Chemical Energy Resources"
by E.M.Goodger. This confirmed my disquiet about some of the numbers
being posted on hydrogen as an internal combustion fuel. I could not
understand why the SAE would ignore apparent fundamental problems such
as the quoted low maximum value for theoretical compression ratio.
 
I highly recommend this book for technical information about alternative
fuels. Unfortunately it has disappeared from our library, so I can't quote
publication date or ISBN.
 
The Spontaneous Ignition Temperature is defined as the minimum temperature
to which the fuel may be heated in the presence of air to promote ignition,
in the absence of any localised source within the mixture.
                                SIT ( degrees C )
  Hydrogen  ( gaseous )         570
  Methanol                      470
  Motor Gasoline                270 - 410
Consequently the theoretical maximum compression ratio may be significantly
higher than previously suggested in this newsgroup.  But read on...
 
" 7.1.3 Spark Ignition Performance of Hydrogen and other fuels.
 
  [ selected abstracts from a three page section. [...] indicates
  intervening text was omitted ]
 
[... discussion on the history of hydrogen as an IC fuel omitted.
     A discussion of the combustion problems was initiated ]
 
This readiness for abnormal combustion in its various forms is no doubt due
to the very low requirement of ignition energy, the high flame speed, and
the very small quenching distance which allows flame to pass through the
narrow openings of inlet valves causing backfire. In connection with the
first-named property, the work of King showed carbon deposits in the
combustion chamber to be a major cause of pre-ignition, which in turn
promotes knock. By adopting a "clean engine" technique and lowering the
jacket coolant temperature from 100C to 60C, knock-free operation at 900
rev/min was possible up to a compression ratio of 10:1.
 
[....]
 
In one sense, a high flame speed is helpful in reducing knock since the
end gases are consumed by the advancing flame before they can complete
their spontaneous ignition pre-reactions and delay period. Nevertheless,
the compression heating effect on the end gases during combustion of
hydrogen, coupled with the very low minimium energy serves to augment the
tendency to knock. Flame speeds of about 48.3 m/s were reported for the
hydrogen engine at a compression ratio of 12:1 and an engine speed 1500
rev/min, whereas the corresponding value for gasoline would be about
16.5 m/s only.
 
[... note - CFR engine = Single cylinder, variable compression ratio ]
 
By furthur adapting a CFR engine with a sodium cooled exhaust valve, a
cool variety of sparking plug, and eliminating the introduction into the
combustion chamber of lubricating oil from the top ring groove, King
found normal combustion possible up to 14:1 compression ratio, and also
at 16:1 compression ratio with engine sppeds restricted to 1100 rev/min.
The maximun imep reached was 986 kPa ( 143 lbf/in.^2 ), and maximum
indicated thermal efficiency was over 51%.
In more recent work using the Oklahoma State University ( OSU ) engine
reported by Murray, close attention was paid to emissions.
 
[.... discussion on hydrogen/air and gasoline/air predictions omitted ]
 
However, Murray's results show a marked reduction in NOx with hydrogen
fuel in comparison with gasoline. This was no doubt due to the fact that
the hydrogen was injected directly into the cylinder, coupled with a
wide open throttle, the power output being controlled by the fuel
injection period alone, as in diesel and stratified charge practice.
However as shown by Varde, when compared with the conventional gasoline
engine, the stratified charge gasoline engine shows an even greater
reduction in NOx than does the hydrogen engine.
 
[... ]
 
Later work by Billings has confirmed the very low concentrations of NOx
at weak mixtures, and the sharp increases with equivalence ratio to peak
at about ER=0.8. Water injection reduces the concentration of NOx, but
also the power output and efficiency.
 
[...]
 
In summary, the hydrogen spark-ignition piston engine, when compared with
its conventional gasoline counterpart, appears to offer the following.
 
 1. Wider range of operating mixture strength.
 2. High thermal efficiency but low power at weak mixtures.
 3. Comparable power but high fuel consumption at stoichiometric and rich
    mixtures.
 4. Theoretically more NOx emissions, but in practice much less if timed
    fuel injection and unthrottled air flow are adopted, as in some
    stratified charge engines.
 5. Strong tendencies to pre-ignition and back-firing unless timed fuel
    injection after inlet valve closure is adopted, together with
    elimination of hot spots and carbon particles in the cylinder, and
    ignition timing retarded to near tdc.
 6. Fairly strong tendencies to knock unless weak mixtures and cool
    operation adopted, with pre-ignition eliminated, in which case high
    compression ratios are possible."
 
 
Bruce Hamilton         SRGXBHH@grv.dsir.govt.nz