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An experimental investigation of hydrogen-enriched gasoline in a Wankel rotary engine

F. Amrouche a,*, P. Erickson b, J. Park b, S. Varnhagen b

aRenewable Energy Development Center, Hydrogen e Renewable Energy, B.P. 62, Observatory Street, Bouzareah,

Algiers 16340, AlgeriabMechanical and Aerospace Engineering Department, UC Davis, CA 95616, USA

a r t i c l e i n f o

Article history:

Received 29 January 2014

Received in revised form

13 March 2014

Accepted 22 March 2014

Available online 19 April 2014

Keywords:

Hydrogen

Gasoline

Wankel rotary engine

Combustion

Emissions

* Corresponding author. Tel.: 213 771125072E-mail address: [email protected]

http://dx.doi.org/10.1016/j.ijhydene.2014.03.10360-3199/Copyright 2014, Hydrogen Ener

a b s t r a c t

The Wankel rotary engine is a potential alternative to the reciprocating engine in hybrid

applications because of its favorable energy to weight ratio. In this study, a Wankel rotary

engine was modified to run on a hydrogenegasoline blend. Hydrogen enrichment

improved the performance of a lean-burn spark-ignition rotary engine operating at high

speed and wide open throttle conditions with the original ignition timing, using 0%, %2, 4%,

5%, 7%, and 10% hydrogen energy fractions at the intake. The experimental results showed

that adding hydrogen to gasoline in the engine improved the thermal efficiency and the

power output. Hydrocarbon and carbon monoxide emissions were reduced while nitrogen

oxide emissions increased with the increase of hydrogen fraction.

Copyright 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.

Introduction

Pollution and the fossil fuel depletion are among the most

important concerns in the last decade. The transportation

sector that largely depends on fossil fuels and is responsible

for a large portion of pollution emissions has received

particular attention from scientific and political communities.

Hydrogen is considered an alternative for conventional fuels

because it can be produced from renewable energy sources

and also, when burned in air produces only NOx as harmful

emissions [1]. However, when hydrogen is used in the spark

ignition engine, it can lead to combustion problems such as

back firing, auto-ignition and pre-ignition. Also, the low

volumetric energy density of gaseous hydrogenmakes storage

.om (F. Amrouche).72gy Publications, LLC. Publ

onboard the vehicle a great technical challenge for the

transportation sector. Therefore, blends of hydrogen with a

hydrocarbon fuel can be an alternative that offers the possi-

bility of enhancing the mixture by taking advantage of prop-

erties from both fuels. Indeed, the use of hydrogen as

enrichment technique is shown to improve the engine emis-

sions and performance for many fuels such as landfill gas [2],

natural gas [3] andmethane [4], gasoline [5e8], ethanol [9], and

methanol [10]. Hydrogen blending can also partially resolve

the difficulties of onboard storage of hydrogen by requiring a

much smaller amount of hydrogen fuel to be stored and

transported. Furthermore, hydrogen blending could poten-

tially serve as the first step toward widespread use of

hydrogen fuel as applied to the transportation sector.

ished by Elsevier Ltd. All rights reserved.
mailto:[email protected]://crossmark.crossref.org/dialog/?doi=10.1016/j.ijhydene.2014.03.172&domain=pdfwww.sciencedirect.com/science/journal/03603199www.elsevier.com/locate/hehttp://dx.doi.org/10.1016/j.ijhydene.2014.03.172http://dx.doi.org/10.1016/j.ijhydene.2014.03.172http://dx.doi.org/10.1016/j.ijhydene.2014.03.172

Fig. 1 e Schematic of Wankel rotary engine.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 8 5 2 5e8 5 3 48526

Hydrogen enrichment has been shown to extend the lean

operating limit of many fuel-air flames [11]. Lean burn has

been proven to increase thermal efficiency, and decrease

brake specific fuel consumption but can potentially decrease

engine flame stability [11]. The lean operating limit in an en-

gine depends on combustion parameters such the flame speed

and the fuel flammability limit. A slow flame speed typically

will result in combustion instability, decreased heat release,

and increased HC emissions [12]. The lean flammability limit

of a fuel/air mixture also depends on the thermal diffusivity,

reaction rate, and specific heat of the reactants. Both the

thermal diffusivity and reaction rate are increased with the

addition of hydrogen. Moreover, hydrogen has a diffusion

coefficient of 0.61 cm3/s which is larger than most fuels. This

improved diffusion improves homogeneity of themixture [13].

In the literature, reciprocating spark ignition engines

fueled by gasoline and hydrogen have been reported [5e8].

However, there are no reports of hydrogen enrichment with

the Wankel rotary engine which is an alternative to the

reciprocating engine [14,15], especially for hybrid vehicle ap-

plications [16].

The Wankel engine, by using an eccentric rotary design

instead of reciprocating pistons to produce motion is less

complicated and has an advantage of high power to weight

ratio as compared to similar reciprocating engines. The

Wankel is generally more compact, has less vibration, and

lower noise and power losses than a similar reciprocating

engine. The Wankel also generally has flat torque character-

istics. In the Wankel engine three combustion events occur

with each full rotation of the rotor yielding high power espe-

cially at the higher end of the operating speed domain. This

results in higher performance at high speeds with a smooth

operation of the engine [14,15].

However, the Wankel engine also has some distinct dis-

advantages such as: Sealing problems [14] and low power

output at low speeds. Furthermore, Wankel engines typically

have lower thermodynamic efficiency because of the long

combustion-chamber shape. High emissions of NOx, CO and

especially HC are another problem in the rotary engine. These

drawbacks are mainly caused by the rotarys long geometry

and relatively larger quenching area. Additionally, unburned

gas leakage past seals to the exhaust chamber, creates further

emissions [15].

The experience gained through the years in operating the

reciprocating engine with hydrogen has led to the promise of

theWankel engine being particularly well suited for hydrogen

enrichment. Theoretical reasons for this are as follows. The

presence of hydrogen can make a significant difference in

terms of emissions and performance by extending the lean

operating conditions because of the wide flammability limits

of hydrogen. Moreover, the absence of an exhaust valve which

is a common hot spot in reciprocating engines causing pre-

ignition and the physical separation of combustion and

exhaust chambers can prevent pre-ignition and/or backfire.

Also, the higher turbulence within the rotary combustion

chamber could allow a leaner operating limit to be reached.

Furthermore, the high hydrogen flame speed and relatively

small quenching distance potentially eliminate difficulties

with the rotary engine flame quenching. Brown et al. [1]

experimentally investigated the performance of a single

rotorWankel engine fueled on pure hydrogen. The enginewas

operated at a part and open throttle at relative equivalence

ratios varying from 1 to 4. They found out that the maximum

power was decreased but the brake thermal efficiency in-

creases at part load as compared to gasoline while the NOx

emissions change was found to be negligible. Salanki [17]

carried out an experimental study of the hydrogen fueled ro-

tary engine for hybrid vehicle applications. They used a 2.2 kW

Wankel rotary engine at WOT and lean conditions. The pre-

liminary results showed an absence of combustion difficulties

and NOx emissions are on the order of 5 ppm for power out-

puts up to 70% of the maximum attainable on hydrogen fuel.

Mazda has been developing hydrogen vehicles with

hydrogen Rotary Engines (RE) from the early 1990s. Recently,

the company has delivered the RX-8 Hydrogen RE powered. To

achieve high power density, injectors are installed on the top

of the housing for direct injection, and other injectors are

placed on air intake ports. The combination of the direct and

port injection has brought simultaneous achievement of high

power, NOx reduction and better thermal efficiency. For

further improvement in fuel efficiency, lean burn operation is

used [18].

Because all published studies found in literature are

focused on the performance of pure hydrogen in the Wankel

engine, we have a strong motivation to investigate the effect

of hydrogen addition to a gasoline fueled single rotor Wankel

engine. This study provides experimental data on the engine

performance and emissions at or near the Lower Operating

Limit as found with gasoline only operation. Because the best

operating of the Wankel engine is reached at high speed, a

speed of 3000 rpm and a wide open throttle was used during

these tests.

Experimental procedure

Experimental setup

The engine used in this test bench is a 0.530 L single rotor, air

cooled Wankel engine, using a single spark plug. The
http://dx.doi.org/10.1016/j.ijhydene.2014.03.172http://dx.doi.org/10.1016/j.ijhydene.2014.03.172

Table 1 e OMC Wankel engine technical specifications.

Type of engine Rotary Wankel engine

Number of rotors Single rotor

Cooling Air cooled

Engine manufacturer Outboard Marine Corps, USA

Ignition source Spark plug

Fuel supply system Fuel injection

Displacement (cc) 530 cm3

Compression ratio 8.75e1

Power output 26 kW @ 5500 rpm

Weight 27 kg

Fuel 50 parts gasoline, 1 part oil

Engine geometry

Eccentricity 3 generating radius 3 width of rotor housing (mm) 13.97 mm 92.202 mm 77.7748 mm

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 8 5 2 5e8 5 3 4 8527

schematic of theWankel engine used in this study is shown in

Fig. 1. This engine is manufactured by Outboard Marine Cor-

poration, USA. The engine Technical Specifications are listed

in Table 1.

This OMC Wankel engine was initially built with a carbu-

retor to prepare the intake air/fuel mixture. To achieve real

time control over the air/fuel mixture preparation as well as

hydrogen addition, two fuel injection systems were added,

one for gasoline and the other for hydrogen, the two injectors

were mounted into an aluminum block in the fueling system,

this block became a mixing chamber that allows the gasoline

vapor and fumigated hydrogen tomix in the intake air stream.

The aluminum injector block was mounted between the en-

gine and the carburetor. The carburetor is used only as a

throttle body for the engine. The gasoline fuel flow was

metered by a calibrated Micromotion CMF010M Coriolis flow

meter with an RFT9739 transmitter with an accuracy of 0.10%

of rate for flow rates of 0e23 g/s. The hydrogen used in this

study was bottled industrial hydrogen (99.95% purity) that is

regulated down to 35 PSIG (2.41 bar) after two steps of pressure

reduction, and is regulated and metered via a calibrated Aal-

borg differential pressure mass flow controller, model GFC 47,

with an accuracy of 3% (0e20% full scale), and 1.5%(20e100% full scale). The air mass flow was metered via a

specifically designed orifice plate, and a large air chamber was

added past the intake to damp cyclic variations in air flow

caused by the engine operation. The inlet temperature of air

was also measured. The schematic of the experimental set up

is shown in Fig. 2.

The spark timing, injection timing and pulse durations of

hydrogen and gasoline were controlled via a hybrid electronic

control unit (HECU) developed in the laboratory. That HECU

uses the original electromotive ECU, two different softwares

(Wintech Electromotive and National Instruments (Austin TX)

LabView version 7 software) and computers for the control of

the injection timing and pulse durations of gasoline and

hydrogen according to the desired air-to-fuel ratio and the

specified hydrogen energy fractions in the intake.

The exhaust emissions were filtered, and water vapor was

removed. The sample was then split between three calibrated

emissions measurement systems. These were:

1) a Horiba MEXA-574GE emissions analyzer, which analyzes

the HC, CO, CO2 and O2. The O2 and HC emissions were

determined by a hydrogen flame ionization detection

method, and the CO and CO2 were measured by a non-

dispersive infrared method. The Horiba has a time

response less than 10 s, a repeatability of 0.04%, 20 ppm,

0.1%, and 0.4%, respectively, for CO, HC, CO2 and O2.

2) a California Analytical model 400 Heated Chem-

iluminescence Photodiode Detector (HCLD) for measuring

NO and NOx. The resolution of this equipment is 0.1 ppm,

repeatability of 0.5% of full-scale, and sensitivity up to

5 ppm.

3) a California Analytical model 300 Non dispersive Infrared

(NDIR) Analyzer for measuring CO, and CO2, it has a line-

arity and repeatability of better than 1%, and a 90%

response time of in 2 s.

During the experiment, the measured emissions showed a

good consistency between the different systems.

The air to fuel ratio was determined in three ways. There

were:

1) Monitoring by the HoribaMEXA-574GE emissions analyzer;

2) Direct calculation of the ratio from the measured mass

flow rate of air, gasoline and hydrogen;

3) A Bosch Universal Exhaust Gas Oxygen (UEGO) sensor

installed in the exhaust stream. This sensor was controlled

by an Innovative Motorsports LC-1 wideband controller

which determined the real time oxygen content in the

exhaust stream and output a predicted air-to-fuel ratio.

This air-to-fuel ratio was used as a feedback signal ratio, in

a closed loop control system, by the ECU to achieve more

precise control of the air-to-fuel ratio.

The calculated andmeasured air to fuel ratios has shown a

good agreement between the three methods during all the

experiments.

The specific equivalence ratio of the gaso-

lineehydrogeneair mixture was calculated by Eq. (1) [5]:

l _mair.

_mg$AFst;g _mH2$AFst;H2

(1)

where _mair, _mg, _mH2are respectively the measured air, gasoline

and hydrogen mass flow rates (kg/h). AFst,g and AFst;H2 are the

stoichiometric air-to-fuel ratios of gasoline and hydrogen,

such as AFst,g 14.6 and AFst;H2 34:3.A calibrated Kistler 6051B high temperature piezoelectric

pressure transducer was used to capture the working

Fig. 2 e Schematic of the experimental set up.

Table 2 e Engine operating conditions during theexperiments.

Hydrogen energy fraction 0%, %2, 4%, 5%, 7%, 10%

Ignition timing

(degree Crank Angle BTDC)

15

Engine speed (rpm) 3000

Equivalence ratio f LOL of gasoline

Throttle position Wide open


chamber pressure data. This transducer is mounted in the

engine housing that was machined and tapped for this pur-

pose. The signal was amplified using a Kistler 5010B charge

amplifier. A 58-tooth (60-2) trigger wheel and a Hall effect

magnetic sensor were mounted on the engines output shaft

to obtain crank angle position data, the crank angle signal was

synchronized with the pressure trace.

The pressure and crank angle were recorded using Na-

tional Instruments (Austin TX) LabView version 7 software.

Chamber pressure and crank angle signals for over 100

consecutive cycles were sampled and treated post process via

Matlab to obtain chamber pressure against crank angle

profiles.

A Telma CC100 eddy current dynamometer was coupled to

the engine to control and measure the engine speed and tor-

que output.

The control of the dynamometer as well as the majority of

data acquisition in the engine test cell was accomplished

using components from the Superflow 742 product line. The

brain of the system was the Superflow ProSensor box, which

communicated with the Superflow ProConsole and Superflow

WinDyn software for real time data display.

Tests conditions

The tests were performed at an engine speed of 3000 rpm and

at wide open throttle. After a warm-up period of the engine on

gasoline, the appropriate air to fuel ratio and hydrogen energy

fraction was set. The engine was again allowed to come to

steady state at each operating condition and the data were

collected. Table 2 summarizes the operating conditions used

in the experiments.

There are different methods of defining engine lean oper-

ating limit (LOL). As defined by Badr [19] the lean misfire limit

or LOL is defined as the equivalence ratio at which first misfire

occurs. Also it is defined by Shiomoto [20] as the equivalence

ratio at which CO and HC emissions starts to increase. Ma [12]

defined the LOL as the excess air ratio (defined as the recip-

rocal of equivalence ratio) at which coefficient of variations

(COV) in IMEP reaches 10%. This is because it is generally

accepted that a COV above 10%will be perceived by a driver as

a poor running condition. However, in this study, LOL was

based the equivalence ratio at which first misfire occurs when

the engine was running on pure gasoline.

For this work, the hydrogen input was adjusted according

to the intake energy of the gasoline fuel. For each energy

fraction of hydrogen added to the fuel mixture, gasoline flow

rate was reduced through decreasing gasoline flow rate by the

ECU to keep the global excess air ratio of the hydrogen gas-

olineeair mixture at an approximately fixed air-to-fuel

equivalence ratio for gasoline.

The hydrogen energy fraction in the total fuel energy varies

from 0% to 10%. The lean operating limit of gasoline which is

equal to an equivalence ratio of 0.77 was used during these

tests. Since we wanted to investigate the effect of different

amount of hydrogen addition on engine performance and

emissions with all engine operation parameters unchanged,

(a)

(b)

0 1 2 3 4 5 6 7 8 9 10 11 1210

12

14

16

18

20

22

24

26

Hydrogen Energy Fraction (%)

)raB(

erusserPrebmah

Cgnikro

Wmu

mi xaM

22.5

25

27.5

30)C

DTA

D(erusser


so the spark timing was kept at 15 CA BTDC of the originalengine. Such a fixed ignition timing control strategy was also

adopted by other researchers for exploring the performance of

hydrogen-enriched engines [5,21e25].

The energy fraction of hydrogen in the total intake gas is

defined as energy fraction, calculated as follow [26]:

%H2 "

_mH2 LHVH2

_mg LHVg _mH2 LHVH2

# 100 (2)

where _mH2 , _mg are respectively themass flow rate of hydrogen

and gasoline (g/s).

LHVH2 , LHVg are the lower heating value, respectively, of

hydrogen and gasoline (MJ/kg), such as LHVH2 120:1,LHVg 43.5.

The maximum working chamber temperature was calcu-

lated from the pressure in this paper using the ideal gas law

[27]:

Tmax Pmax Vmmix RMix

(3)

where Rmix,mmix, Pmax,V are respectively the gas constant, the

mass of the mixture, the maximum working chamber pres-

sure and working chamber volume.

The error bars shown represent the standard deviation.

2.5

5

7.5

10

12.5

15

17.5

20

Prebmah

Cgnikro

Wmu

mixaMrof

elgnA

kn

Results and discussion

Working chamber pressure trace

Fig. 3 shows the variations of the working chamber pressure

according to the crank angle with hydrogen enrichment levels

of 0%, 2%, 4%, 5%, 7%, and 10% energy fraction, at a constant

TDC BDC -90+902

4

6

8

10

12

14

16

18

20

22

Crank Angle(Degree)

Pres

sure

(bar

)

0% H22% H24% H25% H27% H210% H2

-70

Fig. 3 eWorking chamber pressure trace for 0%, 2%, 4%, 5%,

7% and 10% hydrogen energy fractions.

0 1 2 3 4 5 6 7 8 9 10 11 120

Hydrogen Energ Fraction(%)

arC

Fig. 4 e a) Maximum working chamber pressure at

different hydrogen energy fraction levels; b) the relevant

Crank angle of maximum working chamber pressure at

different hydrogen energy fractions.

spark timing of 15 DBTDC and at LOL. According to the pres-

sure trace data, the working chamber pressure increases as

the hydrogen fraction increases. Because of high flame speed

(six times faster than that of gasoline), a larger heating value

and diffusivity of hydrogen, with hydrogen addition, the peak

working chamber pressure is raised earlier and is much closer

to TDC than the gasoline fueled engine pressure trace, but

after reaching its peak value, pressure drops more quickly

than the pure gasoline fueled engine, which means the com-

bustion duration and post-ignition temperature is reduced

with hydrogen addition. This yields a more isochoric heat

release theoretically yielding better efficiency in the energy

conversion process. The sudden jump in pressure around

90 (Fig. 3) is due to the sensor coming into contact with thefollowing working chamber.

0 1 2 3 4 5 6 7 8 9 10 11 125

6

7

8

9

10

11

12

13

14

15


)w

K(rewoP

ekarB

detcerroC

EAS

Fig. 6 e SAE corrected indicated brake power at different

hydrogen energy fractions.


At lean conditions, the fuel is harder to ignite than at

stoichiometric condition which is why the spark timing has to

be advanced at LOL [5]. However, the spark timing in this

experiment is fixed at 15 DBTDC, which makes the combus-

tion for pure gasoline suffer. The high diffusivity, the low

flammability limit and the low ignition energy of the hydrogen

improve the combustion of the gasolineehydrogen fuel

mixture. This hydrogen addition makes the air/fuel mixture

more homogenous and increases laminar burning velocity

that results in increased maximum pressure. This increases

the maximum pressure in the working chamber as shown in

Fig. 4a. For pure gasoline, at fixed spark timing, the combus-

tion duration is longer when running close to LOL and the

gasoline in the working chamber is incompletely burnt

resulting in a reduced working chamber combustion temper-

ature and pressure. However, when hydrogen is added the

combustion is shorter and that phenomenon continues as the

hydrogen fraction increases [6].

As is shown in Fig. 4b, when hydrogen is added at fixed

spark timing, it advances the relevant crank angle for the peak

working chamber pressure. Indeed, since the same ignition

timing has been adopted, the increase of peak pressure value

and the shift of its position toward TDC for the blends prove

that combustion speed increases as the hydrogen content in

the blend increases. Furthermore, it was demonstrated by

Kahraman et al., [28] andMa et al., [22] the engine performance

peaks generally when the crank angle for the maximum cyl-

inder pressure appears around 10e15 DCA ATDC. Hydrogen

appears to shift the maximum peak pressure around this

location in DCA when the hydrogen energy fraction is over 4%.

This is likely due to the low ignition energy and fast flame

speed of hydrogen that improves the combustion by short-

ening the flame development and propagation durations,

which offer the same effect as increasing spark advance [5].

Brake thermal efficiency

The thermal efficiency of an engine is an important factor in

determining the performance of an automobile [15] and its

economics [5].

0 1 2 3 4 5 6 7 8 9 10 11 1210

12

14

16

18

20

22

24

26

28

30


Bra

ke T

herm

al E

ffici

ency

(%)

Fig. 5 e Brake thermal efficiency at different hydrogen

energy fractions.

Fig. 5 shows the change in brake thermal efficiency with

variation of the hydrogen energy fraction added to the gaso-

line at 3000 rpm, at the LOL. As it can be seen in Fig. 5, for pure

gasoline, the brake thermal efficiency of this engine is rela-

tively low. However, when hydrogen is added the brake

thermal efficiency increaseswith the increase of the hydrogen

fraction to about 28.8% with 10% hydrogen energy fraction.

When pure gasoline is used in the engine, the highest thermal

efficiency is found close stoichiometric conditions near 22%,

and with the increase of excess air found at the LOL, the

combustion becomes incomplete, resulting in a lower power

output and brake thermal efficiency [5].

Therefore, because of hydrogens high diffusion, heating

value (hydrogen is 120.97 MJ/kg, while gasoline is 44.4 MJ/kg),

flame speed (six times higher than that of gasoline), and the

wider flammability range of hydrogen, higher charge homo-

geneity and shortened burn duration and a more complete

and faster combustion of hydrogen enriched gasolineeair

mixtures at LOL is observed. This contributes to decreased

cooling and exhausts losses, leading to a higher brake thermal

efficiency than the baseline. Indeed, adding hydrogen to the

air/fuel mixture has a significant effect on reducing the flame

quenching distance, and also reduces the combustion dura-

tion during expansion, reducing losses during the combustion

by more closely approximating an ideal constant volume heat

release, which leads to an increase in thermal efficiency.

As it was noted by Salanki et al. [17], who ran a rotary en-

gine at 3300 rpmwith pure hydrogen, use of hydrogen leads to

an increase in thermal efficiency as compared to gasoline

especially in a lean condition where the equivalence ratio is

ranging between 0.3 and 1.

Compared to the reciprocating engine, the Wankel rotary

engine is known to have higher cooling losses because of a

larger surface to volume ratio of the working chamber and a

longer time required for one-cycle (1. 5 times longer than that

of the equivalent reciprocating engine). Hence, for higher

thermal efficiency it is effective to raise the burning speed

which allows a more isochoric combustion. The hydrogen

(a)

(b)

0 1 2 3 4 5 6 7 8 9 10 11 1230

40

50

60

70

80

90

100


Volu

met

ric E

ffici

ency

(%)

70 75 80 85 90 95 100 10550

55

60

65

70

75

80

85

90

95

100

Rotor Temparature(Degree Celcius)

)%(

ycneiciffEcirte

muloV

Fig. 7 e a) Volumetric efficiency at different hydrogen

energy fractions; b) volumetric efficiency dependence on

rotor temperature.

(a)

(b)

0 1 2 3 4 5 6 7 8 9 10 11 12200

225

250

275

300

325

350

375

400

425

450

475

500

525

550

575

600


)h-wk/g(noitp

musnoCleuF

cificepSekar

B

300 325 350 375 400 425 450 475 500 525 55015

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

Brake Specific Fuel Consumption(g/kw-h)

Bra

ke T

herm

al E

ffici

ency

(%)

Fig. 8 e a) Brake specific fuel consumption at different

hydrogen energy fractions; b) thermal efficiency

dependence on brake specific fuel consumption.

820

840

)suicl

1700

1800

(Deg

ree

Cel

cius

)Exhaust Temperature

Maximum Working Chamber Temperature


addition contributes to raising the burn velocity, which is a

solution to obtaining stronger squish flow and better output

performance which becomes especially true at high speed.

This confirms the fact that even a small amount of hydrogen

addition effectively improves the engine lean burn capability

in the Wankel engine as shown by the data collected.

0 2 4 6 8 10 12760

780

800


eC

eergeD(

erutarepmeTtsuahxE

-2 0 2 4 6 8 10 121400

1500

1600

Max

imum

Wor

king

Cha

mbe

r Tem

pera

ture

Fig. 9 e Exhaust gas temperature and the maximum

working chamber temperature for different hydrogen

energy fractions.

SAE corrected indicated brake power

To insure that repeatable data could be collected on a daywith

different ambient conditions, the power output was corrected

using SAE standard J1349. Fig. 6 shows the SAE corrected brake

power. As expected, running the engine on gasoline at the

LOL, reduces the power produced by the engine because less

fuel is burned completely. Therefore, when hydrogen is added

there is a slight improvement at these operating conditions.

The brake power of a Wankel rotary engine fueled on

hydrogen enriched gasoline showed an improvement in per-

formance as compared to pure gasoline. Compared to recip-

rocating engines, at high speed the Wankel rotary engine


produces more overall power due to less parasitic power los-

ses moving air in and out of the engine and smaller frictional

losses because of the absence of a reciprocating mass. Further

theWankel has a high charging efficiency due to longer intake

and exhaust strokes, and the absence of intake and exhaust

valves causing gas flow resistance. Also, since hydrogen has

fast burning characteristics, it is expected that better torque

results at high speed operation because in this condition the

engine needs a fuel which burns more quickly. Furthermore,

the engine brake torque increases with the increase of the

hydrogen energy fraction. Contributing to this torque increase

is the complete burn up of fuel as enabled by the smaller

quenching distance of the hydrogen fuel. The Wankel engine

has historically suffered from a large relative area and related

quenching by its rotor housing interface resulting in higher

squish flow. Decreasing the quenching distance by adding

hydrogen enables previously unburnt fuel to attain combus-

tion adding to the torque and the related efficiency.

Volumetric efficiency

Volumetric efficiency is a parameter related to the load and

temperature on the Wankel. An increase in the temperature

was indicative of a decrease in the density of themixture inlet

into the combustion chamber, reducing volumetric efficiency.

Since this engine is air-cooled, the operating temperature in-

side the housing is dependent on load rather than being

thermostatically controlled to a set value as in liquid-cooled

engines. As the operating temperature increases in the

housing there is less liquid fuel vaporized in the working

chamber, and the charge cooling decreases, thus reducing the

volumetric efficiency. As can be seen in Fig. 7a, as the

hydrogen energy fraction increases, there is less liquid fuel

vaporized inside the working chamber, which leads to less

charge cooling, resulting in lower volumetric efficiency.

Additionally, adding hydrogen increases the combustion

temperature, resulting in higher temperature of the rotor and

working chamber causing a lower volumetric efficiency

compared to the same equivalence ratio as shown in Fig. 7b.

Previous studies made on pure hydrogen fueled rotary engine

air cooled, have shown the same behavior of the engine

regarding the effect of the volumetric efficiency and the rotor

temperature [17]. It should also be noted that as a low-density

gaseous fuel, hydrogen displaces a significant amount of air

further lowering the volumetric efficiency.

Brake specific fuel consumption (BSFC)

Fig. 8a shows the brake specific fuel consumption for the

Wankel engine running on pure gasoline and hydrogen

enriched 0%, 2%, 4%, 5%, 7%, and 10% energy fraction of gas-

oline at LOL and 3000 rpm. According to Fig. 8a, the BFSC de-

creases from 506 g/kWh for pure gasoline to 312 g/kWh with

an addition of 10% by energy fraction of hydrogen which

represents a drop of about 38%, and this is attributedmainly to

an increased thermal efficiency (see Fig. 8b) and the higher

specific energy value of hydrogen. The higher the mean

effective pressure is, the better the specific fuel consumption

will be. The main reason for this is that a higher mean effec-

tive pressure relatively reduces the effect of more constant

frictional losses. Also, the increase of combustion duration

which occurs at low equivalence ratios increases the BSFC.

Exhaust temperature and the working chamber maximumtemperature

As can be seen in Fig. 9, at the LOL, when hydrogen enrich-

ment increases, the exhaust temperature dropped and is

proportional with the increase of hydrogen energy fraction. At

the LOL, because of the increased efficiency due to shorter

burn duration, the additions of hydrogen lead to a fast and

complete combustion of the fuel/air mixtures, less fuel is

burnt during the expansion stroke which decreases the post-

ignition combustion duration and decreases the exhaust los-

ses as manifested by exhaust temperature. This phenomenon

was also noted by other researchers [5]. This proves that

hydrogen is effective for reducing the exhaust losses, espe-

cially at lean conditions [29].

The calculated maximum working chamber temperature

increases as the hydrogen energy fraction increases as shown

in Fig. 9 and the maximum working chamber temperature

affects the NOx emissions. These results were also seen by Ji

et al. [29], where the in-cylinder temperature increased with

hydrogen addition at the same manifold pressure as

compared to pure gasoline engine operation.

Emissions

Fig. 10a illustrates the brake specific emissions of NOx at the

LOL, 3000 rpm and WOT. As is shown, the brake specific NOx

emissions increase with hydrogen blending levels. Moreover,

the brake specific NOx emissions rise from 0.29 g/kWh for

pure gasoline to 0.69 g/kWh for a hydrogen energy fraction of

10%, which represents an increase of over 100% of brake

specific emissions of NOx. As it was seen previously, this is

due to the increase of the working chamber pressure and

temperature that is activated by the fast burning velocity and

high flame temperature of hydrogen which tends to stimulate

the formation of thermal NOx.

However, as it can be seen in Fig. 10b, the HC emissions

decrease from 0.0309 g/kWh for pure gasoline to 0.0046 g/kWh

for hydrogen enriched at 10% hydrogen energy. This is a drop

of 85% in HC brake specific emissions. Indeed, when the pure

gasoline is burnt, as the fuel/air mixture approaches the LOL

there is less fuel burnt and more air which leads to an

incomplete combustion and some partial misfiring which in-

creases the production of a hydrocarbon emissions for pure

gasoline. On the other hand, because hydrogen has a high

flame speed, low ignition energy, wide flammability and

diffusivity and small quenching distance, HC emissions

caused by incomplete combustion decrease. As explained

previously flame quenching occurs when a flame propagates

close to the chamberwall or a crevice in the engine. The cooler

wall acts as a heat sink, preventing complete combustion in

the region close to the wall or within the crevice, increasing

the emissions of HC and CO. This phenomenon is more pro-

nounced in the case of the Wankel engine because of its ge-

ometry. Adding hydrogen has a significant effect on reducing

the flame quenching distance of the fuel mixture and the

flame can propagate much closer to the chamber wall than


that of gasoline, which reduces HC emissions caused by the

crevice effect [7].

As presented in Fig. 10c, the CO brake specific emissions

decrease with the increase of hydrogen energy fraction. It can

be seen in that Fig. 10c that the CO brake specific emissions

drop from 2.409 g/kWh for pure gasoline to 0.86 g/kWh for 10%

hydrogen which represents a drop of about 64% in CO brake

specific emissions. That is explained by the fact that because

the maximum working chamber temperature increased with

hydrogen addition the oxidation reaction of CO into CO2 is

stimulated. Moreover, because the gasoline is partly replaced

by hydrogen that does not contain a carbon atom, the

hydrogen enriched gasoline exhausts less carbonaceous

emissions than the original gasoline engine [7,5].

Fig. 10d shows the variation of the CO2 brake specific

emissions against the hydrogen energy fraction of 0%, 2%, 4%,

5%, 7%, and 10% at WOT, 3000 rpm and LOL. It can be found

from Fig. 10d that the CO2 brake specific emissions is reduced

from 116 g/kWh for pure gasoline to 74 g/kWh with the 10%

energy fraction of hydrogen added fuel mixture, which rep-

resents a drop of about 36%. This is due to both the fact that

hydrogen is carbon free and the fact that efficiency increases.

(a)

(b)

-2 0 2 4 6 8 10 120

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


)h-wk/g(x

ON

snoissimE

cificep Sekar

B

0 1 2 3 4 5 6 7 8 9 10 11 120

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04


)h-wk/g()viuqE

naxeh-n(C

Hsnoissi

mEcificepS

ekarB

(c

(

Fig. 10 e Brake specific emissions at different hydrog

Conclusion

The Wankel rotary engine is a promising alternative to the

reciprocating engine, especially for hybrid applications.

Because of the geometry of the Wankel engine, the concept of

hydrogen enrichment is well suited for the improvement of its

performance. It has been conclusively proven that a small

amount of hydrogen by energy fraction improves the perfor-

mance and emissions with exception of NOx for a Gasoline

Wankel Rotary Engine at LOL, 3000 rpm, fixed spark timing

andWOT. Themain conclusions resulting from this paper are

summarized as follows:

1) At fixed spark timing and LOL, the pressure trace indicated

an increase in the peak working chamber pressure with

advancing the relevant crank angle for peak pressure of a

gasoline Wankel engine with the increase of hydrogen

energy fraction added to the fuel mixture, which increases

the maximum working chamber temperature.

2) The brake thermal efficiency of the original gasoline

Wankel rotary engine was enhanced by about 28% over the

)

d)

0 1 2 3 4 5 6 7 8 9 10 11 120

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

2.75

3


)hWk/g(

OC

s noissimE

cificepSekar

B

0 1 2 3 4 5 6 7 8 9 10 11 1250

60

70

80

90

100

110

120

130


)hWk/g(

2O

Csnoissi

mEcificepS

ekarB

en energy fractions: a) NOx; b) HC; c) CO; d) CO2.


baseline by adding 10% energy fraction of hydrogen in the

gasoline fuel mixture, decreasing the brake specific fuel

consumption and the exhaust temperature of the engine.

3) Because of the air cooling system of the engine, the volu-

metric efficiency is proved to be highly sensitive to the

operating temperature of the engine (the rotor) and volu-

metric efficiency decreased with hydrogen enrichment.

This is also due to the low volumetric energy density of

hydrogen displacing intake air.

4) The performance of the original gasolineWankel engine as

indicated by the Bmep, Torque and Power are improved as

the energy fraction of hydrogen increases in the fuel

mixture at the conditions tested.

5) Because the increase of the peakworking chamber pressure

and temperature, the NOx brake specific emissions are

raised by 137% as the energy fraction of hydrogen increased

from 0% to 10%. However, reductions in the brake specific

emissions of HC by 85%, COby 64% and CO2 by 36% occurred

with increasing hydrogen fraction due to better combustion

of the air/fuel mixture at the conditions tested.

Acknowledgments

The authors thank Moller International for their donation of

the Wankel research engines. The University of California,

Green Transportation Laboratory and the Hydrogen Produc-

tion and Utilization Laboratory and all their associated

members made this work possible.

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experimental investigation of hydrogen-enriched gasoline in a Wankel rotary engineIntroductionExperimental procedureExperimental setupTests conditionsResults and discussionWorking chamber pressure traceBrake thermal efficiencySAE corrected indicated brake powerVolumetric efficiencyBrake specific fuel consumption (BSFC)Exhaust temperature and the working chamber maximum temperatureEmissionsConclusionAcknowledgmentsReferences

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