The 4th AUN/SEED-Net Regional Conference on New and Renewable Energy
Simulation study on potential addition of HHO gas
in a motorcycle engine using AVL Boost
Le Anh Tuan1, Nguyen Duc Khanh1, Cao Van Tai2
1)
School of Transportation Engineering, Hanoi University of Science and Technology
2)
Nha Trang Vocational Colleges
tuan.leanh05@gmail.com
Abstract increased under lean conditions [1]. The burn duration
This paper describes a simulation study on the impacts and cycle-to-cycle variation decreased [2]. Hydrogen
of hydrogen-water gases mixture (HHO gas) addition has a number of properties that make it an attractive
into intake manifold on motorcycle engine’s fuel as an additive or on its own. These properties are
characteristics with the support of AVL Boost software. listed in Table 1 along with those of gasoline for
The simulation results at 3000rpm, wide open throttle comparison.
of the engine indicated that the thermal efficiency, the
engine power and the brake specific fuel consumption Table 1 Properties of gasoline and hydrogen
(BSFC) are improved with the grow of added HHO Properties Gasoline Hydrogen
content. NOx and CO emissions are increased while Chemical formula C8H18 H2
HC emission is deteriorated. At lean conditions, the Molecular weight (g/mol) 107-114 2,02
CO emission tends to reduce but NOx emission raises Density (kg/m3) 721-785 0,0838
strongly. A/F ratio 14,6 34,3
If engine power is kept constant, and at 3000rpm Minimum ignition energy (mJ) 0,24 0,02
engine speed, during HHO gas is added, the BSFC is Auto-ignition temperature (K) 533-733 858
improved 13%, 16% and 19% when 2 liters/min, 4 Flame speed (cm/s) 41,5 237
liters/min and 6 liters/min HHO gas are added into Lower Heating Value (MJ/kg) 44 120
intake manifold, respectively. The NOx emission Research Octane Number 92-98 130
surged dramatically, whereas CO and HC emissions Quenching gap (cm) 0,2 0,064
significant reduced due to lean condition.
HHO gas is the mixture of H2 and O2 in a ratio 2:1 by
Keywords volume – products of water electrolysis, which is
HHO gas (Hydroxyl gas), engine performance, exhaust invented since March, 1978 by Yull Brown [3]. Hence,
emissions, AVL Boost simulation electrolytic gas often called as “Brown’s gas” or
Hydrogen Rich Gas (HRG). In recent years, there are
some investigations on the effects of HHO gas addition
1. INTRODUCTION on performance of spark ignition (SI) and compression
ignition (CI) engine. These studies indicated that the
The consumption of fossil fuel in internal combustion addition of HHO gas seemed to affect engine
(IC) engines and the associated environmental impacts performance in the same way as if the hydrogen had
are now the worldwide concerns. Hence, studies on been used on its now: fuel consumption reduced [4],
alternative fuels for IC engines are regarded as one of the torque and indicate mean effective pressure (IMEP)
the major research areas for the age. Many countries surged, the combustion duration and cycle-to-cycle
over the world have been trying to use the alternative variation also declined. NO emissions increased as well
fuels with IC engines to reduce the pressure of emptied [5]. And the effect of HHO gas addition is most
oil on state security and economy. Hydrogen is an apparent at light load with leaned mixtures [6].
example of these alternative fuels.
It is always beneficial to predict the effects of HHO gas
Hydrogen is a kind of green renewable energy with the addition on engine performance, emission and
high heating value, whose combustion products are combustion characteristics by advanced simulation
mainly water. Using hydrogen as the alternative fuel software like AVL Boost, without performing actual
can solve a great deal of problems. Firstly, petroleum engine tests. It provides a higher flexibility in changing
fuel will be exhausted in the next few decades; parameters, savings in time and money.
secondly, the combustion of fossil fuels has resulted in
the severe air pollution. With the addition of hydrogen, 2. SIMULATION STUDY
engine power and brake specific fuel consumption
(BSFC) both witness an improvement, but each 2.1 Simulation model
indicate specific benefit. The concentration of carbon
monoxide (CO) and hydrocarbon (HC) emissions The simulation tool used here is AVL Boost v.2009.1,
reduced whereas nitrogen oxide (NOx) emission which is one-dimensional unstable flow simulation
HCMUT, Ho Chi Minh, Vietnam 50 Oct 12 – 13, 2011
The 4th AUN/SEED-Net Regional Conference on New and Renewable Energy
software. As the model described in the Fig.1, there are reactions are represented together in Table 3 [7]. The
two injection elements located in the intake manifold to calculation of NOx formation is begun at the
simulate the gasoline injection and the HHO nozzle of combustion start. The concentration of N2O is obtained
the real model. Other elements are based on the actual by the following relation:
configurations and parameters of the engine which are N 2O 18,71 (1)
1,1802.10 6 T10 ,6125 exp
shortened in Table 2. N2 O2 RT
Table 2 Specification of motorcycle engine The NO formation rate is calculated by Eq. 2:
Engine type S.I. engine d NO R1e R4 e p
Number of cylinder 1
2 1 2
(2)
dt 1 K 2 1 K 4 RT
Bore x Stroke (mm) 50 x 49,5
Displacement (cm3) 97
2.3. CO formation model
Compression ratio 9:1
Ignition timing (0CA, before 12
The CO value can be computed by solving a
top dead center - BTDC)
differential equation based on the following reactions
[8]:
CO + OH ↔ CO2 + H (3)
CO2 + O ↔ CO + O2 (4)
and expressing the resulting CO reaction rate as:
d CO CO (5)
R1 R2 1
dt CO e
where [CO]e is the predicted equilibrium concentration
of CO, and the rates R1, R2 are given by:
R1 k1 COe OH e (6)
R2 k 2 CO e O2 e (7)
2.4 HC formation model
Fig. 1 One-dimension model of the motorcycle engine In S.I. engine the unburned hydrocarbons have
1: Air Cleaner, 2: Restriction, 3: HHO nozzle, 4: Fuel
Injection, 5: Cylinder, 6: Catalyst, 7: Plenum
different sources. The following major sources of
unburned hydrocarbons can be identified [8]:
2.2 NOx formation model
1. A fraction of the charge enters the crevice volumes
For the calculation of NOx formation in IC engines, a and is not burned since the flame quenches at
computational program, which is based on a reaction- entrance.
kinetic model was developed by Pattas and Häfner [7]. 2. Fuel vapor is absorbed into the oil layer and
It calculates the NOx formation with the input of deposits on the cylinder wall during intake and
engine speed and fuel data, as well as the following: compression.
zero dimensionally determined pressure, temperature, 3. Quench layers on the combustion chamber wall
equivalence ratio, volume and mass, depending on time which are left as the flame extinguishes prior to
in the burned zone. The latter is necessary because the reaching the walls.
theory for NOx formation is based on the dissociation 4. Occasional partial burning or complete misfire
of N2 and O2 molecules following the high temperature occurring when combustion quality is poor.
of the gas in the front of the flame.
The HC prediction model evaluates their oxidation
The reactions utilized for the calculation of NOx process during the exhaust stroke; an Arrhenius
formation and the rate constants used for these equation takes into account the slow HC post-oxidation
Table 3 Reactions for NOx formation. Rate coefficients in the form k = ATB exp(-E/T)
No Reaction Forward Reaction Backward Reaction
A (cm3/mol s) B (–) E (kcal/mol K) A (cm3/mol s) B (–) E (kcal/mol K)
1 N2 + O ↔ NO + N 4.93 x 1013 0.0472 - 75.59 1.6 x 1013 0 0
2 O2 + N ↔ NO + O 1.48 x 108 1.5 - 5.68 1.25 x 107 1612 - 37.69
3 OH + N ↔ NO + H 4.22 x 1013 0 0 6.76 x 1014 - 0.212 - 49.34
4 N2O + O ↔ NO + NO 4.58 x 1013 0 - 24.1 7.39 x 108 0.89 - 58.93
5 O2 + N2 ↔ N2O + O 2.25 x 1010 0.825 - 102.5 3.82 x 1013 0 - 24.1
6 OH + N2 ↔ N2O + H 9.14 x 107 1.148 - 71.9 2.95 x 1013 0 - 10.8
HCMUT, Ho Chi Minh, Vietnam 51 Oct 12 – 13, 2011
The 4th AUN/SEED-Net Regional Conference on New and Renewable Energy
[9]: efficiency versus lambda is shown in Fig. 1. As it is
d HC E / RT
(8) seen in Fig. 1, engine thermal efficiencies of the HHO
c HC AHC e HC gw HC O2
a b
dt gas enriched engines are higher than those of original
one especially at lean conditions.
where AHC and EHC are constants equal to 7,7x109
((m3/mole)a+b-1/s) and to 156222 (J/mole), respectively. Since the flame speed of hydrogen is five times as
R is the universal gas constant equal to 8314 large as that of gasoline and the flammability range of
(J/mole/K), Tgw is the average gas temperature in the hydrogen is much wider than gasoline, the HHO gas-
thermal boundary layer, assumed equal to gasoline mixture will have a faster burning velocity
(Tgas+Tcyl.wall)/2, with Tgas = cylinder bulk gas and an extended flame limit than gasoline, which can
temperature and Tcyl.wall = cylinder wall temperature at achieve a shorter burning duration and a more
that location, [HC] and [O2] are the concentration of complete burning. Therefore, the faster flame speed of
HC and O2 (mole/m3) in the gas, a and b are the gasoline-HHO gas mixture leads to a higher degree of
exponents generally to 1,0 and cHC is a calibration constant volume combustion, meaning that the engine
constant used to adjust the reaction rate to a specific operates much closer to the ideal cycle, and gains a
case. higher thermal efficiency than gasoline at the same
lambda. In addition, due to the wider flammable range,
2.5 Heat transfer model the engine using hydrogen as a fuel has high efficiency
in a wide range of lambda beyond the stoichiometric.
The heat transfer to the walls of the combustion Hence, with the increase of lambda, a HHO gas
chamber, i.e. the cylinder head, the piston, and the enriched gasoline engine moves into a relatively more
cylinder liner, is calculated from [10]: complete combustion, although the engine power
Qwi Ai w Tc Twi (9) decreases by reason of the gradually decreased fuel
flow.
where Qwi is wall heat flow, Ai is surface area, αw is
heat transfer coefficient. Tc and Twi are gas temperature
in the cylinder and wall temperature, respectively.
3. SIMULATING PROCEDURE
The simulation was carried out at a motorcycle engine
with engine speed of 3000 rpm, wide open throttle
(WOT) and the flows of the added HHO gas were 2
liters/min (Gasoline + 2 HHO), 4 liters/min (Gasoline
+ 4 HHO) and 6 liters/min (Gasoline + 6 HHO). The
lambda (λ) of gasoline-HHO gas mixture can be
calculated by Eq. 10: Fig. 1 Effect of addition HHO gas flow rate on thermal
mair (10) efficiency against lambda (λ) at 3000 rpm, WOT
m g . A / F g m HHO . A / F HHO
4.2 Combustion characteristics
In Eq. (10), mair, mg and mHHO in gram per second (g/s)
represent the measured air, gasoline and HHO gas Fig. 2 shows the profiles of in-cylinder pressure and
mass flow rates, respectively. (A/F)g and (A/F)HHO are pressure rise rate against crank angle (CA) at 3000 rpm,
the stoichiometric air-to-fuel ratios of gasoline and WOT and λ = 1.4. Due to the fast burn characteristic of
HHO {(A/F)g = 14.6; (A/F)HHO = 0}, respectively. hydrogen, the pressure curves of Gasoline + 2 HHO,
Gasoline + 4 HHO and Gasoline + 6 HHO cases rise
For each given HHO gas fraction, gasoline flow rate earlier than the original engine pressure curve.
was gradually reduced to keep the lambda constant. Meanwhile, because of the high flame temperature and
Because the total intake flow rate is kept roughly high flame speed of hydrogen, the peak in-cylinder
constant at the same engine speed and ambient pressure is increased after HHO gas addition. As a
condition; the ambient air is gradually reduced with the result, a little fuel is burnt during the expansion stroke,
increase of added HHO gas fraction in the total intake the post-combustion is reduced and the in-cylinder
gas. While the gasoline mass flow rate is controlled by pressure after reaching its peak value drops more
the flow rate of ambient air. quickly than original engine with the HHO gas
enriched. As a consequence of the variations of
4. RESULTS AND DISSCUSSIONS pressure rise rate when adding HHO gas are larger than
engine using pure gasoline, as shown in Fig. 2.
4.1 Engine thermal efficiency
Engine thermal efficiency is crucial to evaluating the
engine economic and overall performance. The thermal
HCMUT, Ho Chi Minh, Vietnam 52 Oct 12 – 13, 2011
The 4th AUN/SEED-Net Regional Conference on New and Renewable Energy
Fig. 2 Effect of added HHO gas flow rate on in- Fig. 4 Effect of addition HHO gas flow rate on engine
cylinder pressure and pressure rise rate at 3000 rpm, power against lambda at 3000 rpm, WOT
WOT and λ = 1.4
As a results of the climb of engine power and the drop
The results suggest that, the combustion durations are of supplying fuel due to the air flow reduced, BSFC of
reduced with the addition of HHO gas. The combustion HHO gas enriched engine are less than engine using
duration reduces about 18.31%, 29.17% and 29.29% gasoline (Fig. 5).
when adding 2, 4 and 6 liters/min HHO, respectively.
Fig. 3 shows the profiles of heat release rate (HRR)
and in-cylinder temperature with CA at 3000 rpm,
WOT and λ = 1.4. From Fig. 3, it is easy to find that
the peaks of temperature and those of HRR increase
and closer to top dead centre with the increase of HHO
gas addition fraction either at rich or lean conditions,
due to the high adiabatic flame temperature and high
flame speed of hydrogen.
Fig. 5 Effect of addition HHO gas flow rate on BSFC
against lambda at 3000 rpm, WOT
4.4 Exhaust emissions
Fig. 6 indicates the variations of NOx emission versus
lambda at four HHO gas blending levels. It can be
found that NOx emission gradually increase with the
climb of lambda, especially at lean conditions. NOx
emission depends on the combustion temperature and
Fig. 3 Effect of addition HHO gas flow rate on HRR oxygen concentration in the cylinder. Due to the high
and in-cylinder temperature at 3000 rpm, WOT and λ temperature (as shown in Fig. 3) and the oxygen
equal 1.4 concentration of HHO gas, the NOx emission increases
dramatically with the surge in HHO addition. Average
4.3 Engine power and BSFC over all range of lambda from rich to lean conditions,
the NOx emission increases by 57.9%, 133.3% and
Fig. 4 displays the variations of engine power versus 207.3% with 2 liters/min, 4 liters/min and 6 liters/min
lambda when adding 2, 4 and 6 liters/min HHO at 3000 added HHO gas flow rates (relative to original engine),
rpm and WOT. It can be seen from Fig. 4 that engine respectively.
power raises with the increase of HHO gas addition
fraction over the period from rich to lean conditions. The effect of HHO gas addition on CO emission shows
The increase of engine thermal efficiency and the peak a contrast between rich and lean conditions, as it is
pressure, incorporate with the diminishing of post- displayed in Fig. 7. CO emission tends to increase with
combustion, are reasons for the engine power the HHO gas addition fraction when being operated at
improvement. rich conditions. This is a result of the shorter
combustion duration after HHO addition, which limits
the necessary time for CO oxidation reaction, causing
slow reaction kinetics of CO oxidation into CO2.
However, when the engine runs under lean conditions,
CO emission is improved by enhancing HHO addition
fraction. The availability of excess oxygen helps
HCMUT, Ho Chi Minh, Vietnam 53 Oct 12 – 13, 2011
The 4th AUN/SEED-Net Regional Conference on New and Renewable Energy
convert CO into CO2 at lean conditions. And the flame of gasoline-HHO gas mixture propagate much
increased in-cylinder temperature after HHO addition closer to the cylinder wall and crevice than that of
also contributes to stimulating the oxidation reaction of gasoline. Hence, the average of HC emission collapses
CO into CO2. With 2 liters/min, 4 liters/min and 6 around 0.51%, 3.49% and 10.06% in all operation
liters/min HHO gas addition cases, average over all conditions; 0.84%, 8.25% and 17.68% under lean
range of lambda, the CO emission increases about conditions with the HHO gas flow rates are in turn 2
4.6%, 11.0% and 23.5% in rich conditions; and under liters/min, 4 liters/min and 6 liters/min.
lean conditions (λ ≥ 1.2), the CO emission deteriorates
around 1.74%, 1.97% and 0.93%, respectively. In case the engine power is fixed during HHO gas is
added into the intake manifold (the engine power of
original engine is fixed at 1.97 kW) at the working
condition of 3000 rpm of the engine, the flow rate of
main supplying fuel (gasoline) has to be adjusted. The
simulation result shows that, the brake specific fuel
consumption will be saved about 13%, 16% and 19%
when adding 2 liters/min, 4 liters/min and 6 liters/min
HHO gas flow rates, respectively. The changes of
BSFC, pollution emissions and thermal efficiency are
shown in Table 4.
Table 4 The variations of BSFC, emissions and thermal
Fig. 6 Effect of addition HHO gas flow rate on NOx efficiency at 3000 rpm of the engine when engine
emission against equivalence ratio λ at 3000 rpm power is kept constant (1.97kW), and different HHO
flow rates (↑: increase, ↓: decrease)
HHO
flow NOx CO HC BSFC Eff.
rates
2 l/min ↑ 247% ↓ 95% ↓ 43% ↓ 13% ↑ 14%
4 l/min ↑ 322% ↓ 99% ↓ 49% ↓ 16% ↑ 18%
6 l/min ↑ 372% ↓ 99% ↓ 57% ↓ 19% ↑ 23%
5. CONCLUSIONS
A simulation study based on the AVL Boost software
aiming at investigating the performance of the HHO
Fig. 7 Effect of addition HHO gas flow rates on CO
gas enriched gasoline engine is introduced in this paper.
emission against lambda at 3000 rpm, WOT
The main conclusions are listed below:
The addition of HHO gas (at 3000 rpm, wide open
throttle of the engine) provides:
Positive effects on combustion process. The peak
in-cylinder pressure is improved; the combustion
duration and post-combustion are shortened with
the increase of HHO gas addition.
Both thermal efficiency and engine power are
surged; and the BSFC is improved with the addition
of HHO gas.
Averaged NOx and CO emissions over the lambda
Fig. 8 Effect of addition HHO gas flow rates on HC ranging from 0.8 to 1.4 are increased while HC
emission against lambda at 3000 rpm, WOT emission is reduced. It is also observed that, at lean
The variations of HC emission versus lambda at 3000 conditions, CO deteriorates slightly.
rpm and WOT are shown in Fig. 8. It can be found that The effect of HHO gas addition was explained in
HC emission drop with the increase of lambda, terms of well known influence of hydrogen, the
especially under lean conditions. Gasoline-HHO gas main component of HHO gas. That effect is most
mixtures can be more fully burnt and emit less HC obvious at lean conditions.
emission than gasoline due to the improved chain
reaction. The quenching distance of hydrogen shorter If engine power is kept constant and at 3000 rpm
than that of gasoline is another possible reason for the engine speed, during HHO gas is added, the brake
decline HC emission. The quenching gap of hydrogen specific fuel consumption and the engine thermal
is one-third as long as that of gasoline, which help the efficiency are improved. The NOx emission increases
HCMUT, Ho Chi Minh, Vietnam 54 Oct 12 – 13, 2011
The 4th AUN/SEED-Net Regional Conference on New and Renewable Energy
strongly, while CO and HC emissions significantly
deteriorate.
In the future, the ignition timing and the coherent fuel
consumption with the addition HHO gas flow rates
must be considered to obtain highest efficiency.
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HCMUT, Ho Chi Minh, Vietnam 55 Oct 12 – 13, 2011
