Is gasoline the best fuel for advanced diesel engines? â€“ Fuel effects

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






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


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






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





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





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


10% EGR, 2 bar Pi, 0.8g/s fue

25% EGR, 2 bar Pi, 0.91 g/s f


Excessive Smoke

0

10

20

30

40

50

60

0 5 10 15 20

IMEP, bar

ISC

O,

g/k

wh





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


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


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


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



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


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


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.

Is gasoline the best fuel for advanced diesel engines? â€“ Fuel effects

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