is gasoline the best fuel for advanced diesel engines? – fuel effects
Post on 12-Feb-2022
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Is gasoline the best fuel for advanced
diesel engines? – Fuel effects in
“premixed-enough” compression
ignition engines.
Gautam T. Kalghatgi
Shell Global Solutions (UK)
Per Risberg, KTH Stockholm
Shell Global Solutions
Low-NOx, low-smoke combustion
• Promoting mixing of fuel and air before ignition to reduce smoke
• Low-temperature combustion to reduce NOx – lean mixtures +
EGR, heat release after TDC….
• HCCI is one end of this spectrum – no “in-cycle” control over
heat release. Engine control very difficult also only low load.
• Practical implementation in diesel engines – Toyota UNIBUS
system (early fuel injection/HCCI), Nissan MK (late injection +
EGR) “premixed enough” combustion.
• Combination of the two – multiple injections
• Enhancing mixing – high injection pressures, more swirl …
These systems are usually run on diesel fuels (CN > 30, RON <
60) which are very prone to auto-ignition.
Importance of high Engine Ignition Delay (EID)
• We can define EID = CA50 – SOI (Start of injection). CA50 is the
crank angle at which 50% of total heat release occurs
• EID can be taken as measure of “mixedness”.
• The larger the EID, the more premixed the fuel and air at the
time of heat release. This reduces smoke and also NOx if the
global mixture strength is sufficiently lean
• Lots of EGR has to be used to increase EID with conventional
diesel fuels.
• In general with diesel fuels to get low NOx and low smoke at
high IMEP is very difficult
Can we make use of the auto-ignition resistance of fuels to
increase EID and improve performance ?
Previous work with diesel fuels of different auto-
ignition quality
•Different diesel fuels tested with different
Injection strategies in SAE 2005-01-2127
•At fixed operating conditions in Nissan
MK combustion, injection timing sweeps
• Higher EID and lower NOx for lower
Cetane (more resistant to auto-ignition)
fuels at the same CA50
• But these tests were done at low Comp.
Ratio (CR) of 11.4 and low loads 3.1-4.2
bar IMEP.
0
5
10
15
20
25
30
-10 -5 0 5 10 15 20 25 30
CA50, CAD
En
gin
e Ig
nit
io D
ela
y, (C
A50-S
OI)
, C
AD
EU Diesel, 53.5 CN
Swedish MK1, 53 CN
n-heptane, 54.5 CN
Kero 3, 45.7 CN
90% Kero, 41.5 CN
US Diesel, 39.5 CN
85% Kero, 39.5 CN
Pin = 1.5 bar abs, 1200 RPM,
25% EGR, = 4, Tin = 40 C
Experiments
• 2L single cylinder engine. CR = 14
• 1200 RPM, Tin = 40º C, EGR using exhaust from stoichiometric SI
engine (3-way catalyst), characterised by CO2 content of intake air –
e.g 25 % EGR ~ 3.8 % CO2.
• 8 hole nozzle, 1300 bar injection pressure
• Measured CA50, NOx, Smoke, HC, CO etc
Three Diesel fuels
and one 95 RON
gasoline.
** Lower Heating Value
CN Density IBP T10 T50 T90 FBP Aromatics LHV**
Fuel g/cc ºC ºC ºC ºC ºC % vol MJ/kg
Swedish MK1 54 0.81 195 208 240 273 297 ~ 3 43.8
Diesel 1 39 0.81 167 179 196 220 246 34 43.5
Diesel 2 30 0.83 167 179 198 222 246 50 43.3
Gasoline ~15* 0.726 32 50 102 144 176 29 43.2
Experimental Detail
Phase I – Comparison between fuels. All four fuels tested
with Pin = 1.5 bar abs. at two conditions –a) 0.6 g/s fuel
without any EGR ( ~ 3.8) and b) 0.8 g/s with 10% EGR*
( ~ 2.6). Gasoline also tested at 0.8 g/s without EGR ( ~
3) and diesels at 0.8 g/s with 25% EGR** ( ~ 2).
Phase II – Further tests with gasoline with higher inlet
pressure, higher fuelling rates and higher EGR to see if
higher IMEPs could be reached with low NOx and low
smoke.
Phase III – Double Injection
* 21% ** 35%
Phase I results – Comparison between Fuels
• Engine will not run on gasoline
with very early SOI in HCCI mode
Expected results –
• CA50 decreases with CN
0.6 g/s, No EGR, Pin =1.5 bar abs, Tin = 40 C, 1200 RPM
-5
0
5
10
15
20
25
30
-40 -30 -20 -10 0 10
SOI, CAD
CA
50,
CA
D
54CN, 0.6 g/s,No EGR
39 CN,0.6 g/s, no EGR
30 CN, 0.6 g/s,No EGR
Gasoline,0.6 g/s, No EGR
Phase I results – Engine Ignition delay
• Significantly higher ignition
delay for gasoline
•Difference between 39 CN
and 54 CN less than when CR
was 11.4
0
10
20
30
40
50
60
-5 0 5 10 15 20 25 30
CA50, CAD
EID
, E
ng
ine Ig
nit
ion
Dela
y (
CA
50-S
OI)
, C
AD
54CN, 0.6 g/s,No EGR
39 CN,0.6 g/s, no EGR
30 CN, 0.6 g/s,No EGR
Gasoline,0.6 g/s, No EGR
0.6 g/s, No EGR, Pin =1.5 bar abs, Tin = 40 C, 1200 RPM
Comparison between gasoline and diesel
(Swedish MK1) heat release patterns
-50
0
50
100
150
200
250
300
350
-25 -5 15 35
Crank Angle, CAD
He
at
Re
lea
se
Ra
te,
J/d
eg
Gasoline HRR
Gasoline NL
Diesel HRR
Diesel NL
0.6 g/s, No EGR, Pin =1.5 bar abs, Tin = 40 C, 1200 RPM
Low smoke (< 1% AVL opacity) because of high oxygen for both fuels
concentration but higher NOx with diesel.
Phase I results - NOx
NOx decreases with ignition delay
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 60
EID, Engine Ignition Delay (CA50-SOI), CAD
NO
x, p
pm
54CN, 0.6 g/s,No EGR
39 CN,0.6 g/s, no EGR
30 CN, 0.6 g/s,No EGR
Gasoline,0.6 g/s, No EGR
0.6 g/s, No EGR, Pin =1.5 bar abs, Tin = 40 C, 1200 RPM
Phase I results – NOx and IMEP
Very much lower NOx for the same IMEP for gasoline because of higher EID.
Increasing injection rate will increase IMEP but also smoke for the diesel fuel
and NOx for all fuels. NOx can be reduced by EGR
0
200
400
600
800
1000
1200
1400
0 1 2 3 4 5 6
IMEP,bar
NO
x, p
pm
54CN, 0.6 g/s,No EGR
39 CN,0.6 g/s, no EGR
30 CN, 0.6 g/s,No EGR
Gasoline,0.6 g/s, No EGR
IN THE REST OF THE PRESENTATION SWEDISH MK1 DIESEL WILL BEIN THE REST OF THE PRESENTATION SWEDISH MK1 DIESEL WILL BE
COMPARED WITH GASOLINE with Pin = 2 bar abs, EGR ~ 25% COMPARED WITH GASOLINE with Pin = 2 bar abs, EGR ~ 25% stoichstoich
Smoke increases with injection rate
SOI at TDC, different inj. rates
0
2
4
6
8
10
12
14
16
18
20
7 9 11 13
IMEP, bar
AV
L s
mo
ke
op
ac
ity
%
-100
0
100
200
300
400
500
600
0 5 10 15 20 25 30
Crank Angle Degree, CAD
Heat
Rele
ase R
ate
, J/C
AD
7.35 bar 1%AVL smoke
7.35 bar NL
12.35 bar 17.4% AVLsmoke
12.35 bar NL
Swedish MK1 diesel fuel, Pin = 2 bar abs, EGR ~ 25% Swedish MK1 diesel fuel, Pin = 2 bar abs, EGR ~ 25% stoichstoich
Single Injection starting at TDCSingle Injection starting at TDC
Smoke Smoke vs vs IMEP, SOI at TDCIMEP, SOI at TDC Heat release rate and needle lift forHeat release rate and needle lift for
low and high smoke caseslow and high smoke cases
Injection timing and smoke
Swedish MK1 Diesel
0
5
10
15
20
25
-4 -2 0 2 4 6 8
Start of Injection, SOI, CAD
AV
L S
mo
ke
Op
ac
ity
% a
nd
CA
50, C
AD
11
11.5
12
12.5
13
Me
an
IM
EP
, b
ar
Smoke
CA50
Mean IMEP
Gasoline
0
5
10
15
20
25
-20 -15 -10 -5
Start of Injection, SOI, CAD
AV
L S
mo
ke
Op
ac
ity
% a
nd
CA
50, C
AD
12
12.5
13
13.5
14
Me
an
IM
EP
, b
ar
Smoke
CA50
Mean IMEP
Pin = 2 bar abs, 1.2 g/s fuel, EGR ~ 25% Pin = 2 bar abs, 1.2 g/s fuel, EGR ~ 25% stoich stoich (38%)(38%)
Single InjectionSingle Injection
Smoke decreases as injection is retarded for diesel but very low for gasolineSmoke decreases as injection is retarded for diesel but very low for gasoline
Low smoke for gasoline because of higher EID
0
5
10
15
20
25
0 5 10 15 20 25 30
Engine Ignition Delay, EID = CA50 - SOI, CAD
AV
L S
mo
ke
,%
Gasoline
Diesel
Pin = 2 bar abs, 1.2 g/s fuel, EGR ~ 25% Pin = 2 bar abs, 1.2 g/s fuel, EGR ~ 25% stoichstoich (38%) (38%)
-200
0
200
400
600
800
1000
-15 -5 5 15 25 35
Crank Angle, CAD
He
at
Re
lea
se
Ra
te,
J/C
AD
HRR, diesel, 11.5 bar, 1.6% smokeNL, diesel 11.5 bar, 1.6% smokeHRR, diesel, 12.5 bar, 20.7% smokeNL, diesel 12.5 bar, 20.7% smokeHRR, gas, 12.9bar, 0.11%smokeNL, gas, 12.9 bar, 0.11%smoke
Smoke Smoke vs vs ignition delayignition delay HRR and needle lift for three casesHRR and needle lift for three cases
Gasoline fuel rate can be increased up to a point
0
2
4
6
8
10
12
5 10 15 20
IMEP, bar
ISN
Ox, g
/kw
h
No EGR, 2 bar Pi, 0.8 g/s fuel
10% EGR, 2 bar Pi, 0.8g/s fuel
25% EGR, 2 bar Pi, 0.91 g/s fuel
25%EGR, 2 Pi, diff mfuel
Excessive Smoke
150
170
190
210
230
250
270
0 5 10 15 20
IMEP, barIS
FC
, g
/kw
h
No EGR, 2 bar Pi, 0.8 g/s fuel
10% EGR, 2 bar Pi, 0.8g/s fuel
25% EGR, 2 bar Pi, 0.91 g/s fuel
25%EGR, 2 Pi, diff mfuel
HCCI from Ref.17
Excessive Smoke
Pin = 2 bar abs, different injection rates and SOI and EGRPin = 2 bar abs, different injection rates and SOI and EGR
NOx NOx increases with IMEP (fuel rate) but can be controlled with EGR. ISFCincreases with IMEP (fuel rate) but can be controlled with EGR. ISFC
decreases with IMEPdecreases with IMEP
HC and CO decrease as fuel rate is increased
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20
IMEP, bar
ISH
C, g
/kw
h
No EGR, 2 bar Pi, 0.8 g/s fuel
10% EGR, 2 bar Pi, 0.8g/s fue
25% EGR, 2 bar Pi, 0.91 g/s f
25%EGR, 2 Pi, diff mfuel
Excessive Smoke
0
10
20
30
40
50
60
0 5 10 15 20
IMEP, bar
ISC
O,
g/k
wh
No EGR, 2 bar Pi, 0.8 g/s fuel
10% EGR, 2 bar Pi, 0.8g/s fuel
25% EGR, 2 bar Pi, 0.91 g/s fuel
25%EGR, 2 Pi, diff mfuel
Excessive Smoke
Pin = 2 bar abs, different injection rates and SOI and EGRPin = 2 bar abs, different injection rates and SOI and EGR
GasolineGasoline
High IMEP point with gasoline, single injection
-200
0
200
400
600
800
1000
1200
1400
-30 -10 10 30
Crank Angle Degree, CAD
He
at
Re
lea
se
Ra
te,
J/d
eg
0
20
40
60
80
100
120
140
160
Pre
ss
ure
, b
ar
Heat Release Rate
Needle lift
Pressure
Pressure, heat release rate and needle lift curves for gasoline with 14.86 bar IMEP (0.1
bar std), 1.8% smoke and ISNOx , ISFC, ISCO and ISHC of 1.21 g/kWh, 178 g/kWh, 3
g/ kWh and 3.6 g/kWh respectively. Needle lift, arbitrary scale.
-200
0
200
400
600
800
1000
1200
1400
1600
-17 -7 3 13
Crank Angle Degree, CAD
He
at
Re
lea
se
Ra
te,
J/d
eg
HRR, low smokeNeedle lift, low smoke HRR, high smokeNeedle lift, high smoke
More overlap between heat release and injection event for high smoke caseMore overlap between heat release and injection event for high smoke case
Double Injection Strategies Used
• Total injection of 1.2 g/s. Pilot injection at fixed injection
rate and SOI of 150 CAD before TDC (when the valves
close), sweep of SOI of main injection near TDC of fixed
injection rate - at two different fractions of the total fuel
mass in the pilot for gasoline
• For the diesel fuel, for a total injection rate of 1.2 g/s, the
main injection was fixed at 0.84 g/s at TDC and the SOI of
the pilot was varied
• Main injection was fixed at 1.19 g/s with SOI at 11 CAD
before TDC in most cases, and the pilot injection quantity,
with SOI fixed at 150 CAD before TDC, was varied for
gasoline.
The two limits are too early combustion and high smoke
Comparison between diesel and gasoline
-50
0
50
100
150
200
250
300
-150 -100 -50 0 50
Crank Angle, CAD
Heat
Rele
ase R
ate
, J/d
eg
Heat Release Rate
Needle Lift
-100
0
100
200
300
400
500
600
700
-150 -100 -50 0 50
Crank Angle, CADH
ea
t R
ele
as
e R
ate
, J
/de
g
Heat Release Rate
Needle Lift
Swedish MK1 diesel. Main injection SOI
at – 6 CAD. Mean IMEP 11.84 (std 0.115 bar) bar.
AVL smoke opacity 8.7%. In g/kWh, ISFC= 183,
ISNOx = 0.3, ISHC = 11.3, ISCO = 10
Gasoline. Main injection SOI at – 9 CAD.
Mean IMEP 12.86 (std 0.108 bar) bar.
AVL smoke opacity 0.9%. In g/kWh, ISFC= 174,
ISNOx = 0.39, ISHC = 6.8, ISCO = 9
Total fuel rate 1.2 g/s, 70% in main injection. Lowest possible smoke with diesel was 7.8Total fuel rate 1.2 g/s, 70% in main injection. Lowest possible smoke with diesel was 7.8
Comparison between diesel and gasoline -
pressure
Total fuel rate 1.2 g/s, 70% in main injection. Lowest possible smoke with diesel was 7.8Total fuel rate 1.2 g/s, 70% in main injection. Lowest possible smoke with diesel was 7.8%
0
20
40
60
80
100
120
-150 -100 -50 0 50
Crank Angle, CAD
He
at
Re
lea
se
Ra
te,
J/d
eg
Diesel
Gasoline
Smoke – total fuel rate 1.2 g/s different injection
strategies
Lowest smoke possible is lower with single injection for dieselLowest smoke possible is lower with single injection for diesel
This is because early injection causes heat release duringThis is because early injection causes heat release during
compression stroke compression stroke –– reduces ignition delay for main reduces ignition delay for main injinj..
••Smoke level much lower for gasoline at high IMEP Smoke level much lower for gasoline at high IMEP
0
5
10
15
20
25
8 9 10 11 12 13 14
IMEP, bar
AV
L S
mo
ke
Op
ac
ity
%
Gasoline, single
Diesel, single
Gasoline, 32% pilot, main sw eep
Gasoline, 19% pilot, main sw eep
Diesel, 30%, sw eep for main
Diesel 30% pilot sw eep, main at TDC
Fuel consumption – total fuel rate 1.2 g/s
different injection strategies
•• With diesel fuel, double injection increases ISFC compared to single injectionWith diesel fuel, double injection increases ISFC compared to single injection
•• With gasoline, double injection does not increase ISFC With gasoline, double injection does not increase ISFC
•• ISFC decreases as IMEP increases ISFC decreases as IMEP increases
150
160
170
180
190
200
210
220
-20 -10 0 10 20 30
CA50, CAD
ISF
C, g
/kW
h
Gasoline, single
Diesel, single
Gasoline, 32% pilot, main sweep
Gasoline, 19% pilot, main sweep
Diesel, 30% SOI sweep for main
Diesel 30% pilot sweep, main at T
150
160
170
180
190
200
210
220
8 9 10 11 12 13 14
Mean IMEP, bar
ISF
C, g
/kW
h
Gasoline, single
Diesel, single
Gasoline, 32% pilot, main sweep
Gasoline, 19% pilot, main sweep
Diesel, 30% SOI sweep for main
Diesel 30% pilot sweep, main at T
Smoke at high IMEP with gasoline
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5 10 15 20
Mean IMEP, bar
AV
L S
mo
ke
Op
ac
ity
%
No Pilot, 1.2g/s
No Pilot, 0.8 g/s
No Pilot, 1 g/s
Pilot, 1.2 g/s total
Double,1.2 g/s late, increasing pilot
Single Inj, increasing fuel
•• With gasoline, double injection helps reduce smoke at high load With gasoline, double injection helps reduce smoke at high load
•• Even with double injection smoke increases eventually at high load Even with double injection smoke increases eventually at high load
Gasoline – single vs double injection
0
20
40
60
80
100
120
140
160
-40 -20 0 20 40
Crank Angle Degree, CAD
Pre
ss
ure
, b
ar
14.86 bar IMEP, single inj
15.95 bar IMEP. Double inj
15.07 bar IMEP, double inj.
-200
0
200
400
600
800
1000
1200
1400
-20 -10 0 10 20 30 40
Crank Angle Degree, CAD
He
at
Re
lea
se
Ra
te,
J/d
eg
14.86 bar IMEP, single inj
15.95 bar IMEP double inj
15.07 bar IMEP, double inj
Injection Fuel Rate CO2 IMEP* IMEP* AVL ISNOx ISFC ISHC ISCO MHRR* Angle of
Mean Intake Mean std smoke MHRR*
g/s % bar bar % opacity g/kWh g/kWh g/kWh g/kWh J/deg CAD
Single** 1.436 4.05 14.86 0.115 1.81 1.21 178 3.6 3.4 1446 10.8
Double*** 1.46 4.14 15.07 0.138 0.28 0.59 179 3.0 5.8 817 18.2
Double*** 1.549 4.16 15.95 0.112 0.33 0.58 179 2.9 6.8 1393 14.1
* Mean from 100 cycles
** SOI @ -16 CAD from SAE 2006-01-3385
*** 1.19 g/s @ -11 CAD and rest at -150 CAD
Double injection
allows MHRR to be
reduced and
delayed without
increasing cyclic
variation and at
lower emissions.
All experiments at
bar abs. inlet
pressure, 40 C inle
temp. ~35% EGR
based on actual
exhaust.
Conclusions (1)
• The engine can be run on gasoline in partially pre-mixed mode even
when it cannot be run in HCCI mode.
• Much higher ignition delay for gasoline compared to diesel at a given
set of operating conditions and hence lower smoke (and lower NOx).
• Double injection helps reduce maximum heat release rate while
maintaining IMEP,low emissions and fuel consumption for gasoline
• Much higher IMEP possible with gasoline compared to diesel for low
smoke and NOx
• IMEP = 15.95 bar, smoke < 0.07 FSN, ISNOx, ISCO, ISHC and ISFC
of ~ 0.6, 6.8, 2.9, 179 g/kWh. This was with 23% pilot, 2 bar abs. Pin
and 35% EGR (actual).
• Highest IMEP possible with diesel fuel for this low smoke < 6.5 bar
• Further improvements should be possible with optimisation of
injection and mixture preparation (multiple injections, more injector
holes….)
Conclusions (2)
• Further work is needed to understand the effect of higher speeds and
the lowest loads that can be run in partially premixed mode on gasoline
• In general, if smoke and NOx are to be reduced by promoting premixed
combustion, the fuel needs to be as much like gasoline as possible
• In practice, the extent to which this is possible depends on other critical
requirements – low noise, cold starting, low load operation … - being
met
• What currently matters is the diesel fuel quality required by future
diesel engines.
• If the strategy to reduce smoke and NOx in such engines is to promote
premixed combustion, increasing fuel cetane number will not help – it
might actually make control of smoke more difficult. Higher volatility for
the fuel might be beneficial.
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