Abstract Lean fuel/air mixtures with various fuel/fuel ratios betwee

M Furutani

Y Ohta

Y Yamaya

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FORMALDEHYDE 
FOR  THE  PREMIXED  COMPRESSION-IGNITION  ENGINE 
- AN  ADDITIVE  FOR  IGNITION-TIMING  CONTROL - 
 
Y. Yamaya, M.  Furutani  and  Y. Ohta 
 
Nagoya Institute of Technology 
Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan 
+81-52-735-5329, +81-52-735-5566: Fax 
ohta@megw.mech.nitech.ac.jp 
 
Abstract 
Lean fuel/air mixtures with various fuel/fuel ratios between methane and n-butane were supplied to a premixed 
compression-ignition engine (i.e. homogeneous charge compression ignition engine, HCCI) with or without 
supplementary gaseous formaldehyde induction as an ignition controlling additive. In the no additive case the 
methane and butane function as the two fuels in the high/low-octane two-fuel premixed compression-ignition 
operation we proposed previously as another ignition control procedure. The formaldehyde addition to the methane/ 
butane/air mixtures has given the engine desired and stable ignition timings controllable by the amount of 
formaldehyde to be added, almost independent on the fuel/fuel ratios between methane and butane. The efficacy of 
formaldehyde has been confirmed as an ignition controlling medium for the piston-compression ignition of 
hydrocarbon/air mixtures.  
Key Words: Internal Combustion Engine, Ignition, Low-Temperature Flame, Preflame  
Reaction, Ignition Control, HCCI 
 
1. Introduction 
 
The present obstacle to realize the premixed compression-ignition engine (HCCI) is the lack of 
effective ignition control procedure. We have proposed and confirmed the formaldehyde as an 
efficacious additive into the mixture to realize wide-range premixed compression-ignition 
operation of the natural gas engine (1). The main constituent of natural gas is methane, and the 
remainder are ethane, propane, butane and propylene. It is well known that the methane would not 
generate cool flame during its preflame period in the autoignition processes; quite different from 
the higher hydrocarbon fuels including propane, butane, gasoline and diesel oils (2). The most 
important intermediate during the preflame period up to the hot-flame establishment of the 
hydrocarbon fuels is the formaldehyde, which is closely related to the cool-flame appear- ance. It 
can be easily expected that the formaldehyde is effective in promoting ignition of the 
non-cool-flame generating fuel such as methane; the other way around about the cool-flame 
generating fuels.  
 
The amount and combination of each component species in natural gas fuel, especially the ratio 
between propane and butane, change day by day, which results in the ignition-characteristics 
fluctuation of the engine fuel gases. The increase of butane content compared to propane would 
bring the engine to earlier ignition timings. Measures should be found out to eliminate the 
fluctuation of ignition. We propose herewith an ignition-control concept utilizing the gaseous 
formaldehyde as an additive to realize premixed compression-ignition engines and to maintain 
stabilized operation of spark-ignited natural gas engines.  
 
 291
 
Journal of KONES Internal Combustion Engines 2002 No. 1‐2 ISSN 1231 ‐ 4005 
 
                                                                                    
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                                                                           
2. Experimentals 
 
A single cylinder diesel, Yanmar L60ADDP, four-stroke, air-cooled, 273 cm3-displacement 
volume engine, was used to confirm the proposing concept. The combustion chamber was 
simplified to a pancake type. The compression ratio ε was 18 (real compression ratio: 14.8). 
A pressure transducer (Kyowa Electronic Instruments: PE-200K WS) and a needle-type 
high-speed electromagnetic sampling valve (Tsukasa: NW42) were installed on the cylinder head. 
 
The engine was connected to an AC dynamometer to absorb the engine power output as well as to 
enable motoring operations without a break. The engine was operated under the WOT condition; 
the intake system had no throttle valve. The engine speed was 600 rev/min (10/s) during whole 
experiments. Cylinder head temperature was fixed at 80 °C. 
 
Lean fuel/air mixtures with various fuel/fuel ratios between methane (octane rating: 120) and 
n-butane (octane rating: 94) were supplied to a single cylinder, premixed compression-ignition 
engine with or without supplementary gaseous formaldehyde induction as an ignition controlling 
additive. The n-butane represented the cool-flame generating fuels contained in natural gases. Fuel 
gases and formaldehyde were introduced continuously from the uppermost stream part of intake 
pipe as the gaseous state to obtain homogeneous mixtures. The formaldehyde adulterated into the 
intake air was a standard gas balanced or diluted by nitrogen or our own-make pure and 
nitrogen-diluted gases.  
 
The in-cylinder gases were sampled once every ten-cycle interval with a valve opening period of 
1.5 ms (equivalent to 5.4° CA at engine speed of 600 rpm). Container was connected via the valve 
and kept warm at 385 K. The sampled gasses were analyzed by gaschromatographs having TCD 
and FID detectors (GL Science: GC-390 DDTF, -390DDT) with a Porapak Q-S and Molecular 
Sieve 13X columns. The target species were CO, CO2, HCHO, H2O, H2, O2, 1-C4H8, C2H4, CH4 
and n-C4H10. The CO, CO2 and HCHO gasses were deoxidized to methane by a catalytic 
converter and then detected by a FID. 
 
 
3. Results and Discussion 
 
3.1  Ignition-timing depending on high/low octane-rating two-fuel characteristics 
 
Premixed piston-compression ignition operations using n-butane/methane/air mixtures are shown 
as pressure histories in Fig. 1, in which n-butane ratio to the total fuels is 0.359, 0.420 and 0.454 by 
volume under a fixed equivalence ratio φ of 0.52. The compression ratio ε was fixed to be 18.0. 
Total chemical energy retained in the mixtures are almost constant. Timings the hot-flame ignition 
can be achieved were 10.8, 2.0 and 0.8 degrees after top dead center (° aTDC) respectively. An 
increase of n-butane ratio to the total fuels brings about an advanced timing of hot-flame ignition. 
As a matter of course, the highest output torque was obtained in the 0.8° aTDC case. The nature of 
"high/low octane-rating two-fuel ignition-timing control" (3) for the premixed 
compression-ignition engine operations was revealed here again, though the difference of octane 
rating between methane and n-butane was not large.  
The butane content lower than 0.35 could not lead to a reliable hot-flame appearance, associated 
with excessively retarded and unstable ignition timings. An 0.27 butane-content case is shown 
together in the figure. A wider range ignition timing and/or output torque control by the high/low 
octane-rating two-fuel system could be accomplished by using two fuels with a larger 
octane-rating difference between the two (4). 
 292
3.2  Formaldehyde addition to the mixtures with various fuel/fuel ratios 
 
A few amount of formaldehyde was introduced to the above-mentioned n-butane/methane/air 
mixtures at the intake pipe portion. The formaldehyde content added to the whole fuel/air mixtures 
was 1 350 or 2 100 ppm. A relation between ignition timing and n-butane content in the total fuel 
is shown in Fig. 2. The pressure traces in these two cases are shown in Fig. 3 together with the 
traces of no addition cases. The timings hot-flame ignition would occur were around 5° and 
3° aTDC independent of fuel/fuel ratio. The ignition timing is determined through the amount of 
formaldehyde added to the fuel/air mixture.  
 
For the non-cool-flame generating methane n-butane mixed with it as a supplementary fuel acts as 
an intense ignition promoter, even though its octane rating is almost as high as the commercial 
gasoline. The ignition promoting effect of n-butane to the methane, however, would saturate at a 
certain extent. When the formaldehyde is introduced into the mixture the amount of formaldehyde 
added to the fuel/air mixture becomes a sole controlling factor to the ignition timing. It would be 
deserving special mention that the effect of formaldehyde addition is inhibitive for the mixtures 
with advanced hot-flame ignition tendency as contrasted with a promoting effect to the mixtures 
with retarded ignition tendency. It is also demonstrated that the operating range of premixed 
compression-ignition engine expands to both methane-abundant and butane-abundant directions. 
 
3.3  Leaner mixture cases 
 
In the 0.52 equivalence-ratio case the n-butane content, or fuel/fuel ratio, has poor controlling 
linearity. Another lean condition, a leaner equivalence ratio case with φ = 0.44 was examined. No 
change was given on the compression ratio. The n-butane content was varied from 0.37 to 0.50. As 
a whole, leaner mixture supply has led retarded ignition timings beyond the top dead center.  
 
Pressure traces in the formaldehyde adulterated case demonstrated the formaldehyde efficacy, in 
which the formaldehyde content added to the whole fuel/air mixtures was the same as the φ = 0.44 
case; 1 350 or 2 100 ppm. The ignition timings become controllable and are settled to a certain 
crank angle according to the amount of formaldehyde introduced into the mixture, even for the 
0.31 butane-content one. 
 
For the leaner mixture, ignition-timing controllability by the fuel/fuel ratio is improved having 
a higher linearity. Engine operating range is also expanded. 
 
3.4  Chemical species histories to the hot-flame ignition 
 
In-cylinder gas sampled consecutively up to the hot-flame occurrences under the compression 
ratio 18 were analyzed concerning both of a simple methane/butane/air mixture case and 
a formaldehyde adulterated case. The equivalence ratio of the mixture was fixed to be 0.52. 
The initial formaldehyde concentration introduced in the mixture was set to be 0 and 1 300 ppm. 
In-cylinder pressure and gas constituent histories of engine operation under the condition the 
butane content was 0.45 are shown in Fig. 4 as a typical example. The abscissa is the crank angle. 
Every concentration was plotted at the halfway timing of the whole valve opening period. 
 
The starting of distinct heat release due to hot-flame occurrence could be recognized when 
compared to the pressure trace of air-only supplied trace. It is clearly shown that in the case with 1 
300-ppm initial formaldehyde concentration the hot flame began at +6° CA and -1° CA near the 
TDC in the simple mixture case without formaldehyde addition. According to the raised 
 293
formaldehyde concentration the hot-flame ignition timing is retarded due to the high butane 
content with an advanced ignition tendency. 
 
The beginning of formaldehyde consumption was at +4° CA in the case with 1 300-ppm initial 
formaldehyde concentration and at the TDC in the simple mixture case. The earlier hot-flame 
occurrence is accompanied with the earlier start of formaldehyde consumption. 
 
The carbon-monoxide concentration was always higher than that of carbon-dioxide before 
the ignition. The formaldehyde generated from the simple mixture grew monotonously up to the 
hot flame occurrence. The timing of maximum carbon-monoxide concentration was not far away 
from the timing the carbon dioxide generated most rapidly. The carbon-monoxide concentration 
seems to be closely related to the heat release rate of hot-flame explosion. 
 
A piston compression of a simple charge of methane/butane and air contained no intake 
formaldehyde addition showed a gradual formaldehyde generation during the preflame period and 
slight consumption at the final stage before the hot-flame occurrence. In the formaldehyde 
adulterated case the formaldehyde concentration shows a slight rise followed by prompt decrease 
at the final stage of the ignition delay period; so-called blue-flame period, but seemingly stable 
during almost the whole induction period up to the ignition. 
 
Methane consumption is very low during the whole induction period. Concerning the n-butane 
consumption, degenerate nature can be recognized only in the no addition case. 
 
The effect of formaldehyde added into the methane/butane/air mixture leading to the ignition 
would be an inhibitive event for the cool-flame generating constituent and a promoting event of 
the preflame reaction of the non-cool-flame generating constituent in the fuel. 
 
 
4. Concluding Remarks 
 
Methane is the main constituent of natural gas, and the remainder is ethane, propane, butane and 
others. The former is a non-cool-flame generating fuel as contrasted with cool-flame generating 
remainders.  
 
•  Cool-flame generating constituent of natural gas will act as an ignition promoter for the main 
constituent, methane. The nature of high/low octane-rating two-fuel ignition-timing control for 
the premixed compression-ignition engine operation is revealed for the fluctuation of natural 
gas constituents. An increase of cool-flame generating remainder ratio brings about an 
advanced timing of hot-flame ignition. The ignition promoting effect to the methane, however, 
would saturate at a certain extent.  
 
A small amount of formaldehyde added into the methane/ butane/air mixtures has given the engine 
desired and stable ignition timings according to the amount of formaldehyde added, almost 
independent on the fuel/fuel ratios between methane and butane.  
 
•  Amount of formaldehyde added to the fuel/air mixture becomes a sole controlling factor to the 
ignition timing. The effect of formaldehyde addition is inhibitive for the mixtures with 
advanced hot-flame ignition timings as contrasted with a promoting effect to the mixtures with 
retarded ignition timings.  
 294
•  Troubles due to the day-by-day ignition-characteristic fluctuation of the engine fuel gases can 
be eliminated by using a small amount of formaldehyde for the premixed compression-ignition 
and spark-ignited natural gas engines.  
•  Operating range of premixed compression-ignition engine expands to both methane-abundant 
and butane-abundant directions. 
 
Formaldehyde is efficacious as an ignition controlling medium for the hydrocarbon/air mixtures in 
engine cylinders. The effect of formaldehyde leading to the ignition would be an inhibitive event 
for the cool-flame generating constituent and a promoting event of the preflame reaction of the 
non-cool-flame generating constituent in the fuel gasses. 
 
 
References 
 
(1)  Ohta, Y., Furutani, M., Kojima, M. and Kono, M., Premixed-Compression-Ignition Natural-Gas 
Engine, JSAE Proceedings #73-99, (1999), Paper #37, 28th FISITA Congress, F2000A083, (2000), 
and Ignition-Timing Controlled Premixed Compression-Ignition Natural-Gas Engines, 2nd Dessau 
Gas Engine Conference, (2001), WTZ Rosslau, Germany. 
(2)  Levedahl, W. J. and Yokley, C. R., New Data on Automotive Combustion: Research on the Nature of 
Engine Knock, Technical News Bulletin, 37-8, (1953), 113, National Bureau of Standards. 
(3)  Furutani, M., Kawashima, K. and Ohta, Y., An Ultra-Lean Premixed Compression-Ignition Engine 
Concept, 12th Internal Combustion Engine Symp., 259~264, (1995), Trans. JSME, 62B-595, (1996), 
1240~1246. 
(4) Furutani, M., Ohta, Y. Kono, M. and Hasegawa, M., An Ultra-Lean Premixed Compression-Ignition 
Engine and its Characteristics, 4th COMODIA, (1998) 173~177. 
 
 
 
 
 
8
6
4
2
0
Pr
es
su
re
  [
M
Pa
]
= 0.45n- C4H10
Total Fuel
n- C4H10
Total Fuel
n- C4H10
Total Fuel
Air only
Crank Angle  [deg. aTDC]
TDC 10 20 30-10-20-30
= 0.36
n- C4H10
Total Fuel
= 0.42
= 0.27
Compression Ratio ε = 18.0
Equivalence Ratio φ = 0.52
CH4/n-C4H10/Air Mixture
 
 
Fig. 1.  Pressure traces of methane/butane/air ignition at an equivalence ratio φ = 0.52 with various fuel/fuel ratios. 
Compression ratio ε = 18. 
 295
n-C4H10
Total Fuel [%]
Ig
ni
tio
n 
Ti
m
in
g 
[C
ra
nk
 a
ng
le
 d
eg
. a
TD
C
]
12
10
8
6
4
2
TDC
50403020
Compression Ratioε =18
Equivalence Ratio φ = 0.52
HCHO added:
2 100 ppm
HCHO added:
1 350 ppm
 No HCHO Addition
 
Fig. 2. Effect of HCHO addition on the ignition timing stabilization and fuel constituent fluctuation. Compression 
Ratio ε = 18. 
 
P
re
ss
ur
e 
 [M
P
a]
HCHO added: 
 1 350 ppm
8
6
4
2
0
Crank Angle  [deg. aTDC]
TDC 10 20 30-10-20-30
P
re
ss
ur
e 
 [M
P
a]
HCHO added: 
2 100 ppm
Air only
8
6
4
2
0
Air only
Compression Ratio ε = 18.0  
Equivalence Ratio φ = 0.52 
CH4/n-C4H10/HCHO/Air Mixture
Compression Ratio ε = 18.0  
Equivalence Ratio φ = 0.52 
CH4/n-C4H10/HCHO/Air Mixture
 
Fig. 3.  Pressure traces of methane/butane/air ignition when HCHO is added into the mixture having various 
fuel/fuel ratios and a fixed equivalence ratio φ = 0.52. Compression ratio ε = 18. 
 296
10
8
6
4
2
0
P
re
ss
ur
e 
 [M
P
a]  No HCHO Addition
0
1.2
n-
C
4H
10
 [%
] 1.0
0.8
0.6
0.4
0.2
0.0
HCHO added:
1 350 ppm
Compression Ratio ε  = 18.0
Equivalence Ratio φ  = 0.52
n-C4H10/CH4/HCHO/Air Mixture
  n-C4H10
Total Fuel
 = 0.45 HCHO added:
1 350 ppm
4 000
3 000
2 000
1 000n
-C
4H
10
C
on
su
m
pt
io
n 
[p
pm
]
HCHO added:
1 350 ppm
Crank Angle  [deg. aTDC]
TDC 10 20 30-10-20-30
H
C
H
O
 [p
pm
]
2 000
1 500
1 000
500
0
HCHO added:
1 350 ppm
5
4
3
2
1
0
C
O
2 
[%
]
C
O
 [p
pm
]
4 000
3 000
2 000
1 000
0
C
H
4 
[%
]
1.2
0.8
0.4
0.0
HCHO added:
1 350 ppm
HCHO added:
1 350 ppm
HCHO added:
1 350 ppm
 No HCHO Addition
 No HCHO Addition
 No HCHO Addition
 No HCHO Addition
 No HCHO Addition
No HCHO Addition
 
Fig. 4.  Species concentration histories in 0.45-butane-content methane/butane/HCHO/air ignition at an 
equivalence ratio φ = 0.52 when the intake HCHO concentration 1350 ppm, associated with pressure traces. 
Compression ratio ε = 18.  
 297


FORMALDEHYDE FOR THE PREMIXED COMPRESSION-IGNITION ENGINE -AN ADDITIVE FOR IGNITION-TIMING CONTROL

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FORMALDEHYDE FOR THE PREMIXED COMPRESSION-IGNITION ENGINE -AN ADDITIVE FOR IGNITION-TIMING CONTROL

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