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Effect of hydroxy (HHO) gas addition on performance and
exhaust emissions in compression ignition engines
Ali Can Yilmaz, Erinc¸ Uludamar, Kadir Aydin*
Department of Mechanical Engineering, C¸ ukurova University, 01330 Adana, Turkey
article info
Article history:
Received 8 May 2010
Received in revised form
5 July 2010
Accepted 5 July 2010
Available online 11 August 2010
Keywords:
Hydrogen
Hydroxy
Enrichment
Combustion
Performance
Emissions
abstract
In this study, hydroxy gas (HHO) was produced by the electrolysis process of different
electrolytes (KOH
(aq)
, NaOH
(aq)
, NaCl
(aq)
) with various electrode designs in a leak proof
plexiglass reactor (hydrogen generator). Hydroxy gas was used as a supplementary fuel in
a four cylinder, four stroke, compression ignition (CI) engine without any modification and
without need for storage tanks. Its effects on exhaust emissions and engine performance
characteristics were investigated. Experiments showed that constant HHO flow rate at low
engine speeds (under the critical speed of 1750 rpm for this experimental study), turned
advantages of HHO system into disadvantages for engine torque, carbon monoxide (CO),
hydrocarbon (HC) emissions and specific fuel consumption (SFC). Investigations demon-
strated that HHO flow rate had to be diminished in relation to engine speed below 1750 rpm
due to the long opening time of intake manifolds at low speeds. This caused excessive
volume occupation of hydroxy in cylinders which prevented correct air to be taken into the
combustion chambers and consequently, decreased volumetric efficiency was inevitable.
Decreased volumetric efficiency influenced combustion efficiency which had negative
effects on engine torque and exhaust emissions. Therefore, a hydroxy electronic control unit
(HECU) was designed and manufactured to decrease HHO flow rate by decreasing voltage and
current automatically by programming the data logger to compensate disadvantages of HHO
gas on SFC, engine torque and exhaust emissions under engine speed of 1750 rpm. The flow
rate of HHO gas was measured by using various amounts of KOH, NaOH, NaCl (catalysts).
These catalysts were added into the water to diminish hydrogen and oxygen bonds and
NaOH was specified as the most appropriate catalyst. It was observed that if the molality of
NaOH in solution exceeded 1% by mass, electrical current supplied from the battery
increased dramatically due to the too much reduction of electrical resistance. HHO system
addition to the engine without any modification resulted in increasing engine torque output
by an average of 19.1%, reducing CO emissions by an average of 13.5%, HC emissions by an
average of 5% and SFC by an average of 14%.
ª2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1. Introduction
Faced with the ever increasing cost of conventional fossil
fuels, researches worldwide are working overtime to cost-
effectively improve internal combustion engine (ICE) fuel
economy and emission characteristics. In recent years, many
researchers have focused on the study of alternative fuels
which benefit enhancing the engine economic and emissions
*Corresponding author. Tel.: þ90 5335107585; fax: þ90 3223386126.
E-mail address: kdraydin@cu.cdu.tr (K. Aydin).
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy 35 (2010) 11366e11372
0360-3199/$ esee front matter ª2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2010.07.040
characteristics. The main pollutants from the conventional
hydrocarbon fuels are unburned/partially burned hydro-
carbon (UBHC), CO, oxides of nitrogen (NO
x
), smoke and
particulate matter. It is very important to reduce exhaust
emissions and to improve thermal efficiency. The higher
thermal efficiency of diesel engines certainly has advantages
for conserving energy and also solving the greenhouse
problem. Among all fuels, hydrogen is a long term renewable,
recyclable and non-polluting fuel. Hydrogen has some pecu-
liar features compared to hydrocarbon fuels, the most signif-
icant being the absence of carbon. Very high burning velocity
yields very rapid combustion and the wide flammability limit
of hydrogen varies from an equivalence ratio (f) of 0.1e7.1,
hence the engine can be operated with a wide range of air/fuel
ratio. The properties of hydrogen are given in Table 1 [1]. Due
to the low ignition energy and wide flammable range of
hydrogen, hydrogen engines are quite suitable to run at lean
conditions which are helpful for the enhanced engine
economic and emissions performance [2,3]. All regulated
pollutant emissions, except nitrogen oxides, can be simply
reduced by using a carbon-free fuel. This is true whatever the
alternative fuel source if the production of this carbon-free
fuel in large plants is more efficient and therefore produces
less CO
2
than the direct conversion of the fuel source into
mechanical power in the internal combustion engine. The
combination of its molecular composition and some of its
peculiar properties (high laminar flame speed, wide flamma-
bility range, etc.) reveals hydrogen as an attractive fuel for
ICEs [4]. Besides, compared with traditional fossil fuels,
hydrogen is a carbonless fuel whose combustion doesn’t
generate emissions such as HC, CO and CO
2
[5].
The concept of using hydrogen as an alternative fuel for
diesel engines is recent. The self ignition temperature of
hydrogen is 858 K, so hydrogen cannot be used directly in a CI
engine without a spark plug or glow plug. This makes
hydrogen unsuitable as a sole fuel for diesel engines [1]. There
are several reasons for applying hydrogen as an additional
fuel to accompany diesel fuel in CI engine. Firstly, it increases
the H/C ratio of the entire fuel. Secondly, injecting small
amounts of hydrogen to a diesel engine could decrease
heterogeneity of a diesel fuel spray due to the high diffusivity
of hydrogen which makes the combustible mixture better
premixed with air and more uniform. It could also reduce the
combustion duration due to hydrogen’s high speed of flame
propagation in relation to other fuels [6].
Throughout history, there have been many studies
regarding hydrogen as a fuel in ICEs. First, Reverend Cecil in
England planned to use hydrogen as fuel in 1820. Bursanti and
Matteucci in Italy improved the hydrogen engine with a free
piston in 1854. Rudolf Erren conducted studies with the
hydrogen engine in Germany in 1920. Ricardo achieved high
efficiency when working with hydrogen in an engine in 1924
[7]. In 1992, as a result of the Second World Renewable Energy
Congress held in Reading, the world renewable energy
network (WREN) has been formed. The first author of this
paper is the founder member of WREN. This network is
dedicated to promoting renewable energy throughout the
world [8]. Also, there have been many investigations on
hydrogen-enriched fuel operation in ICEs. Saravanan and
Nagarajan [9] experimentally investigated the hydrogen-
enriched air induction in a diesel engine system. The test
results showed that an efficiency of 27.9% was achieved
without knocking over the entire load range with 30%
hydrogen enrichment. Also, they observed that specific fuel
consumption decreased with increase in hydrogen percentage
over the entire range of operation. Saravanan et al. [10] did an
experimental investigation on hydrogen as a dual fuel for
diesel engine system with exhaust gas recirculation (EGR)
technique. The test results demonstrated that the SFC
decreased without EGR with 20 L/min of hydrogen flow and
they concluded that the reason for reduction in SFC is due to
the operation of hydrogen fueled engine under lean burn
conditions. Masood et al. [11] studied on experimental verifi-
cation of computational combustion and emission analysis of
hydrogenediesel blends and the test results showed that the
hydrogenediesel co-fueling solved the drawback of lean
operation of hydrocarbon fuels such as diesel, which were
hard to ignite and resulted in reduced power output, by
reducing misfires, improving emissions, performance and fuel
economy. Saravanan and Nagarajan [12] studied on an
experimental investigation on optimized manifold injection
in a direct-injection diesel engine with various hydrogen
flowrates. The test results showed that in the manifold
injection technique, the optimized condition was the start of
injection at gas exchange top dead center (TDC) with injection
duration of 30crank angle (CA) with a hydrogen flow rate of
Table 1 eThe properties of hydrogen.
Properties Diesel Unleaded gasoline Hydrogen
Autoignition temperature (K) 530 533e733 858
Minimum ignition energy (mJ) e0.24 0.02
Flammability limits (volume % in air) 0.7e5 1.4e7.6 4e75
Stoichiometric air-fuel ratio on mass basis 14.5 14.6 34.3
Limits of flammability (equivalence ratio) e0.7e3.8 0.1e7.1
Density at 16 C and 1.01 bar (kg/m
3
) 833e881 721e785 0.0838
Net heating value (MJ/kg) 42.5 43.9 119.93
Flame velocity (cm/s) 30 37e43 265e325
Quenching gap in NTP air (cm) e0.2 0.064
Diffusivity in air (cm
2
/s) e0.08 0.63
Research octane number 30 92e98 130
Motor octane number e80e90 e
international journal of hydrogen energy 35 (2010) 11366e11372 11367
7.5 L/min. The brake thermal efficiency was increased by 9%
compared to pure diesel fuel operation. CO emissions varied
from 0.03 to 0.12 vol% compared to 0.08e0.14 vol% in a diesel
fuel investigation. Naber and Siebers [13] successfully inves-
tigated the hydrogen autoignition process under diesel
conditions. The autoignition of hydrogen was investigated in
a constant-volume combustion vessel. The varied parameters
were as follows: the injection pressure and temperature, the
orifice diameter, and the ambient gas pressure, temperature
and composition. They obtained a strong Arrhenius correla-
tion between ignition delay and temperature. Senthil et al. [14]
conducted research on applying hydrogen to improve
combustion of vegetable oil in a diesel engine. In their work,
experiments were conducted to evaluate the engine perfor-
mance while using small quantities of hydrogen in
a compression ignition engine primarily fueled with a vege-
table oil, namely Jatropha oil. Results indicated an increase in
the brake thermal efficiency from 27.3% to a maximum of
29.3% at 7% of hydrogen mass share at the maximum power
output. They also noticed significant smoke reduction by 20%.
There was also a reduction in HC and CO emissions from 130
to 100 ppm and 0.26e0.17% (by volume), respectively, at
maximum power output.
The ability for H
2
ICEs to burn cleanly and operate efficiently
is owed to the unique combustion characteristics of hydrogen
that allow ultra-lean combustion with dramatically reduced
NO
x
production and efficient low-engine load operation. In
contrast, the same combustion characteristics impose tech-
nical challenges at high engine-loads due to an increased
propensity to preignite the hydrogeneair mixture [15].Atlow
loads, the load can be controlled by the equivalence ratio
(qualitative approach), as combustion temperatures then stay
below the NO
x
formation temperature. The engine is then run
under wide openthrottle conditions, so that pumpinglosses are
negligible which benefits the brake thermal efficiency.
However, hydrogen may cause some problems at high engine-
loads [16]. Hydrogen has high autoignition temperature
compared to diesel and this causes some challenges on oper-
ating a diesel engine just by increasing compression ratio.
Therefore,a glow plug or a spark plug(external ignition sources)
should be oftenused. Also hydrogen usage as a sole fuel in spark
ignition engine brings some disadvantages to be overcome like
backfire, pre-ignition and knock. Therefore, hydrogen control
in engine should be managed by an electronic system. Since
hydrogen has the smallest molecular size and is the lightest
element in nature,its storage becomes a crucial problem. While
electrochemically reacting hydrogen in fuel cells is considered
to be the cleanest and most efficient means of using hydrogen,
it is believed by many to be a technology of the distant future.
Currently, fuel cell technology is expensive and bulky. In the
near term, the use of hydrogen in an ICE may be feasible as
a low-cost technologyto reduce emissions of criteria pollutants
and global warming via carbon dioxide (CO
2
)[17].
The aim of this experimental investigation was, to make
a spectacular combination of anodes and cathodes in a simply
adaptable ambient within the fuel system and to obtain an
enhancement in combustion and reduction in exhaust emis-
sions with electrolysis reaction without the need for storage
tanks. In this experimental study, instead of pure hydrogen
addition to diesel fuel, produced hydrogen gas along with
oxygen (hydroxy gas, HHO, Brown’s gas) was fed to the intake
manifold of a direct-injection CI engine by a hydroxy system
and a hydroxy electronic control unit (HECU) under various
loads, which caused engine speed to decrease from 2800 to
1200 rpm. Hydroxy gas is in brown color and the form of
unseparated hydrogen and oxygen generated by the electrol-
ysis process of water (NaOH, KOH or NaCl additives for more
HHO production and optimum molality to keep electrical
resistance-conductivity balance) by a unique electrode design.
Hydrogen and oxygen did not form into O
2
and H
2
molecules.
They were in their monoatomic state (a single atom per
molecule). Water was split by electricity to form its various
elements, oxygen and hydrogen. When HHO mixture was
ignited, both explosion and implosion occured to form water,
releasing the energy that was found in the bonds of the two
elements in the form of heat. In the monoatomic portion,
there weren’t any atomic bonds needed to be broken (the
bonds of the H
2
and O
2
respectively) before turning back into
water. The key difference of HHO gas was the fact that some of
the hydrogen and oxygen never go into a diatomic state.
Hence, HHO gas had more energy because these bonds were
never made. In this state, which was an unstable state of H
2
O
vapor, more energy was achieved compared to hydrogen
burning with oxygen. Pulverized water clashed the fuel and
they united. Water became the core and the fuel tended to be
the water shell (due to density difference). During compres-
sion stroke, pressure and heat increased, the water exploded
to steam and consequently, the fuel got atomized. After igni-
tion, in-cylinder temperature increased rapidly which resul-
ted water to be splitted into hydrogen and oxygen and
reigniton occured which yielded increased combustion effi-
ciency. Due to the oxgen atoms coming out with hydrogen
(monoatomic structure), autoignition temperature of hydroxy
was not as high as hydrogen (diatomic structure). Thus,
hydroxy gas did not need an external ignition source like
spark or glow plug and due to the simultaneous production
and consumption of hydrogen; no storage was necessary,
which resulted in safe operation [18]. Hydroxy gas was
generated and used as a sole fuel in diesel engine to benefit
from peculiar features and minimize disadvantages of
hydrogen. It was observed that hydroxy system provided
advantages in engine performance, emissions and specific
fuel consumption at high engine speeds under lean condi-
tions. At mid and low speeds, these specifications turned into
disadvantages, due to minimum ignition energy of hydroxy
which is a strongly decreasing function of equivalence ratio,
pre-ignition and knock occured. Also, low lean-flammability
limit of hydroxy resulted advantages only under dilute (lean)
conditions unless HECU was added to the HHO system.
Experiments without HECU demonstrated that, compared to
pure diesel fuel operation, engine torque was increased by an
average of 27.1% above the engine speed of 1750 rpm and
decreased by an average of 46.9% under 1750 rpm. An average
reduction of 23.8% (>1750 rpm) and increment of 22.7%
(<1750 rpm) in HC emissions were observed. An average of
2.1% (>1750 rpm) reduction and 4.6% (<1750 rpm) increment
were observed for CO emissions. The average values for SFC
were 13% reduction above 1750 rpm and 15.8% increment
below 1750 rpm. Average values, obtained from experiments
with HECU addition to the hydroxy system, were 19.1%
international journal of hydrogen energy 35 (2010) 11366e1137211368
increment for engine torque, 13.5% reduction for CO emis-
sions, 5% reduction for HC emissions and 14% reduction for
SFC at all engine speeds.
2. Experimental set-up and procedure
The hydroxy system was added to the engine without any
modification. HHO gas was generated in reactor container
(plexiglass) by various types of electrodes (reactors) in various
molality aqueous solutions of catalysts. The positive current
positively charged the anodes which yielded the electrolysis
reaction of the electrolytic solution and eventually released
gaseous oxygen and hydrogen were generated which, in turn,
surfaced at the top portion of reactor container. Electrical
power that fed the electrodes was measured and it was
observed that reaction field was the major factor that influ-
enced the amount of hydroxy gas generated. Experiments on
aqueous solutions of catalysts demonstrated that HHO gas
flowrate increased in relation to mass fraction of catalyst in
water. However, if the molality of NaOH in solution exceeded
1% by mass, current supplied from battery increased
dramatically due to the too much reduction of total electrical
resistance. The plate electrode and NaOH
(aq)
were found the
most efficient reactors and catalysts in relation to electrical
power consumed. Technical specifications of the engine used
in this experimental study are shown in Table 2.
An electronic control unit was designed and manufactured
to decrease HHO flow rate by decreasing voltage and current.
Experiments depicted that voltage around 7.3 V and current
around 5.9 A were suitable values for the engine speed below
1750 rpm and data logger was programmed according to these
values. HECU was designed according to working principles of
a Pulse Width Modulation (PWM) Circuit based on the 555
Timer which is the process of switching the power to a device
on and off at a given frequency, with varying on and off times
with aid of a Metal Oxide Semiconductor Field Effect Transistor
(MOSFET). IRFZ46N MOSFET was used due to its high electrical
current endurance (50 A) and high triggering capacity. Sche-
matic diagram of the circuit and pin descriptions for the 555
Timer are shown in Fig. 1 and Table 3 respectively.
Electrodes were made of 316L stainless steel due to its high
corrosion resistance. Every test was repeated three times and
averages were taken as results. A multimeter was used to
measure output voltage and current, a flowmeter was used to
measure the flow rate of hydroxy gas and a gas analyzer was
used to observe the exhaust emissions. Technical specifica-
tions of the HHO system are given in Table 4.
A float system was assembled into the reactor container to
prevent short circuits through hydroxy gas expanded in
reaction pot. Dynamometer which has a torque range of
0e1700 Nm, speed range of 0e7500 rpm was connected with
the engine to control the speed by automatically adjusting the
Table 2 eTechnical specifications of the engine.
Configuration In-line 4
Type Direct-injection diesel
with glow plug
Swept volume 3567 cm
3
Bore 104 mm
Stroke 105 mm
Oil cooler Water-cooled
Maximum torque 255 Nm at 1800 rpm
Maximum brake power 80 kW at 3500 rpm
Recomended
maximum speed
3600 rpm
Fig. 1 eSchematic diagram of the hydroxy electronic
control unit (HECU).
Table 3 ePin descriptions for the 555 Timer.
Pin Description Purpose
1 Ground DC Ground
2 Trigger The trigger pin triggers the beginning of
the timing sequence. When it goes LOW (0),
it causes the output pin to go HIGH (1).
The trigger is activated when the voltage
falls below 1/3 of þV on pin 8.
3 Output The output pin is used to drive external
circuitry. It has a “totem pole” configuration,
which means that it can source or sink
current. The HIGH (1) output is usually
about 1.7 V lower than þV when sourcing
current. The output pin can sink up
to 200 mA of current. The output pin is
driven HIGH (1) when the trigger pin is taken
LOW (0). The output pin is driven LOW (0)
when the threshold pin is taken HIGH (1),
or the reset pin is taken LOW (0).
4 Reset The reset pin is used to drive the output
LOW, regardless of the state of the circuit.
5 Control Voltage The control voltage pin allows the input of
external voltages to affect the timing of
the 555 chip. When not used, it should be
bypassed to ground
through a 0.01 mF capacitor.
6 Threshold The threshold pin causes the output to be
driven LOW when its voltage rises
above 2/3 of þV.
7 Discharge The discharge pin shorts to ground
when the output pin goes HIGH. This is
normally used to discharge the timing
capacitor during oscillation
8þV DC Power/þ3toþ18VDC
international journal of hydrogen energy 35 (2010) 11366e11372 11369
load. The engine speed, power output, SFC, HC and CO emis-
sions were measured by the computer via a data logging
software. Hydroxy gas was firstly sent to a water safety system
to prevent backflash using a 1/3 water-filled pot before being
sent to the intake manifold. Sensors were located on the
container to observe excess growth of water temperature and
gas pressure. A return-safety valve was used to prevent rising
of gas pressure over 1 bar in the container. General view of
experimental set-up is shown in Fig. 2.
3. Results and discussion
3.1. Engine torque
Fig. 3 shows the variation of engine torque with engine speed.
An average of 19.1% increment in engine torque is obtained
with using HHO system compared to pure diesel operation.
The increase in power is due to oxygen concentration of
hydroxy gas and better mixing of hydroxy with air and fuel
that yield enhanced combustion. High laminar flame velocity
of hydroxy yields decreased ignition delay and shorter
combustion period that provides lower heat losses, much
closer to ideal constant-volume combustion which results
increased compression ratio and thermal efficiency. High
burning velocity of hydroxy provides faster increment in
pressure and temperature which may minimize the knocking
especially at idle conditions (low or no load). Also, ignition
delay period reduction yields diminished engine noise. The
results show that the addition of hydroxy can significantly
enlarge the flammable region and extend the flammability
limit to lower equivalence ratios. At high speeds (1750 rpm)
the weakened in-cylinder charge flow and increased residual
gas fraction are formed, which block the fuel to be fast and
completely burnt. Since hydroxy has a low ignition energy and
fast flame speed, the hydroxyediesel mixture can be more
easily ignited and quickly combusted than the pure diesel fuel.
Thus, improved torques at high speeds can be obtained. Low
lean-flammability limit of hydroxy gas allows stable
combustion at highly dilute (lean) circumstances. However, it
is observed that hydroxy gas cannot have a positive effect on
power output at around stoichiometric (richer) conditions.
Since the energy density of hydrogen on volume basis is much
lower than that of diesel fuel, the reduced fuel energy flow rate
is attained and finally results in the dropped engine torque at
low speeds. The impairments of HHO at low speeds can be
turned into advantages with the aid of HECU.
3.2. SFC
The variation SFC with engine speed is shown in Fig. 4.An
average gain of 14% is achieved on SFC by using hydroxy
Table 4 eTechnical specifications of the HHO system.
Maximum gas supply 5 L/min
Electrodes (anodeecathode) 316L stainless
steel plates
Maximum electrolysis voltage
and current (1750 rpm)
12 Ve10 A
Electrolysis voltage and
current (1750 rpm with HECU)
7.3 Ve5.9 A
Electrolyte (1% by mass) NaOH aqueous solution
Reactor container volume 8.5 L
Water level control Float system
Water temperature 40e45 C
Dimensions 170 400 mm
(diameter height)
Weight 3.5 kg
Water
Safe
Float
Water
Tank
Water level system
Direct
Current
Device
+_
Engine
Dynamometer
Gas
Analyzer
HHO Flow
Flowmeter
Exhaust
Computer Data Logger
Electronic
Circuit
Fig. 2 eGeneral view of the experimental set-up.
Fig. 3 eVariation of engine torque with engine speed.
Fig. 4 eVariation of specific fuel consumption (SFC) with
engine speed.
international journal of hydrogen energy 35 (2010) 11366e1137211370
system. Brake thermal efficiency is usually used to symbolize
the engine economic performance. The improvement in
engine brake thermal efficiency for the hydroxy enriched CI
engine is more evidently seen at high speed conditions. The
reduction in SFC is due to uniform mixing of hydroxy with air
(high diffusivity of hydroxy) as well as oxygen index of hydroxy
gas which assists gasoline during combustion process and
yields better combustion. This can be attributed to that, at high
speeds, the diesel fuel is hard to be completely burnt at lean
conditions due to the increased residual gas fraction and poor
mixing. Since HHO gains a high flame speed and wide flam-
mability, the addition of hydrogen would help the fuel to be
burned faster and more complete at high speed conditions.
Also, low ignition energy of hydroxyeair mixture derives diesel
fuel even to be burned safely under leaner conditions.
However, at low speeds (1750 rpm), low lean-flammability
limit prevents hydroxy to have positive influence on combus-
tion efficiency due to mixture requirement around stoichio-
metric conditions. Increased CR may cause pre-ignition and
high volume occupation of hydroxy causes reduced volumetric
efficiency unless HECU is included to the system.
3.3. HC emissions
The variation of HC emission with engine speed is depicted in
Fig. 5. An average reduction of 5% at HC emission is achieved
above the engine speed of 1750 rpm. At high speed conditions,
short opening time of manifolds prevents adequate air to be
taken into the cylinder and diesel fuel cannot be burned suffi-
ciently. Short quenching distance and wide flammability range
of hydrogen yield engine to expel less HC emissions especially
under high speed conditions and low speed conditions with the
aid of HECU. Besides, oxygen index of hydroxy yields better
combustion which diminishes HC emission. At low engine
speeds, due to high volume occupation of HHO gas, correct air
cannot be taken into the cylinders which prevents fuel to be
combusted completely. Besides, higher in-cylinder pressure,
temperature and high volume occupation of hydroxy espe-
cially at low engine speeds may increase soot formation if HHO
flow rate is not diminished at about 1.6 L/min.
3.4. CO emissions
Fig. 6 shows the variation of CO emissions with engine speed.
An average reduction of 13.5% is gained at CO emissions at
mid and higher engine speeds (1750 rpm). Absence of carbon
in hydroxy gas is a major reason for CO reduction. Wide
flammability range and high flame speed of hydroxy ensure
engine to be operated at low loads. The HHOediesel fuel
mixture burns faster and more completely than the pure
diesel fuel operation. Thus, CO emission at high speed and
lean conditions is effectively reduced after HHO addition.
Since HHO gas contains oxygen, higher combustion efficiency
is obtained and increment for CO emission is slower unless
HHO flow rate is diminished to appropriate flow rate values
while approaching low speeds.
4. Conclusions
At mid and higher engine speeds; the HHO system with diesel
fuel yields higher engine torque output compared to pure
diesel fueled engine operation unless HECU is added to the
system. High burning velocity and low ignition energy of
hydroxyeair mixture minimize the effect of the weakened in-
cylinder charge flow and increased residual gas fraction which
block the fuel to be fast and completely burnt at high speeds.
However, increased CR may cause pre-ignition and this can be
minimized by direct HHO injection into the cylinder. At low
engine speeds, low lean-flammability limits of hydroxy causes
challenges at higher equivalence ratios. Due to the long
opening time of intake manifold at low speeds, high volume
occupation (reduced volumetric efficiency) of HHO becomes
inevitable. Since minimum ignition energy of hydroxyeair
mixture is a decreasing function of equivalence ratio till
stoichiometric (richer) conditions, torque is reduced after
HHO gas addition. A control unit has to be used to obtain
appropriate electrolysis voltage and current (gas flow rate) to
terminate the impairments of hydroxy gas at low speeds.
Uniform and improved mixing of hydroxyeair and oxygen
content of HHO stimulate combustion which has a major
effect on SFC by using an adequate capacity system. Wide
flammability range, high flame speed and short quenching
distance of hydroxy yield diesel fuel to be combusted
completely under high speed conditions without HECU and
low speed conditions with HECU.
Fig. 5 eVariation of hydrocarbon (HC) emissions with
engine speed.
Fig. 6 eVariation of carbon monoxide (CO) emissions with
engine speed.
international journal of hydrogen energy 35 (2010) 11366e11372 11371
High burning velocity, wide flammability range, oxygen
content and absence of carbon make HHO gas an appro-
priate fuel addition to obtain adequate combustion which
yield reputable reduction of HC and CO emissions when
a sufficient hydroxy system is used at mid and higher
speeds of engine without HECU and low speed conditions
with HECU.
A control unit, which decreases electrolysis voltage and
current automatically when the engine speed decreases
under 1750 rpm (critical speed for this experiment), has to be
designed and manufactured to eliminate the impairments
of hydroxy enriched diesel fuel combustion at low speeds
and to provide energy economy.
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