Study of Combustion and Emission Characteristics of

Gasoline Engine with Miller Cycle

Jian Wu1,a, Wei Fan1, Yang Hua1, Yunlong Li2, Shaozhe Zhang2, Yiqiang pei2 1College of Vehicle and Traffic Engineering, Henan University of Science and Technology, Luoyang,

471003, China

2 The State Key Laboratory for The Internal Combustion Engine Combustion Learn, Tianjin University, Tianjin, 300072, China

awujian8402@163.com

Keywords: Variable valve timing, Miller cycle, Combustion characteristics, Emission characteristics

Abstract. On the basis of original engine, high compression ratio miller cycle can be realized,

through perfecting the inlet cam profile, using higher geometry compression ratio, combining VVT

control technology. The results indicate that the miller cycle achieved by VVT control technology

can reduce pumping loss, and improve the effect utilization of energy. The combustion heat release

rate is lower than the original engine, and combustion heat release are mainly concentrated on TDC

later, lower the burning temperature. Compared with the original engine, NOX emissions decrease

significantly, but CO and HC emissions increase somewhat.

Introduction

Traditional otto cycle engine can't use the high compression ratio because of limitation of

deflagration. Miller cycle can reduce the effective compression ratio by closing the inlet and

exhaust valve behindhand, and improve thermal efficiency by high expansion ratios. Miller cycle

engine can reduce pumping losses, fuel consumption rate and NOX emissions [2-3]. Low effective

compression ratio can prevent the deflagration and increase power, and also can reduce the

institutions of engine and heat load, equivalent to improve the durability [3].

Through a comparative study of the otto cycle and miller cycle, it can be found that the purpose

of low fuel consumption can be achieved by improving variable intake timing VVT and variable

chamber volume [4-5]. The simple way of achieving miller cycle is to increase the compression

ratio and inlet valve closed late [6]. This article realized miller cycle by controlling VVT on a motor

gasoline engine, and also studied and analyzed the combustion and emission characteristics of

engine.

Experimental facilities and research method

The experiment was conducted on a port fuel injection vehicle gasoline engine, with its

compression ratio is 10.4. The specific parameters are shown in Table 1, and the bench experiment

system is shown in Fig.1. The main equipments used in the experiment have YiKe hydraulic

dynamometer, AVL7351CME type fuel consumption instrument, AVLZI31-Y5S type sensor,

ETAS LA4 air-fuel ratio instrument. VVT is controlled by the Delphi MT22.1 open ECU, and

emission characteristics is measured by HORIBA MEXA-7100DEGR type emission instrument.

On the basis of original engine, this paper makes the intake duration increase by replacing the

inlet cam profile, increases geometry compression ratio by replacing piston, and achieves miller

cycle of high compression ratio by combining with VVT control technology. Combined with the

common working conditions of passenger vehicles, conduct a load characteristics experiment of

2000r/min on the original engine (otto cycle) and miller cycle. In the test, adjusting the inlet and

exhaust VVT and ignition advance angle make specific fuel consumption and cycle fluctuation

minimum, which keep excess air coefficient 1 and water temperature between 85oC to 90oC. In

heavy load test, adjusting the air-fuel ratio makes the exhaust temperature below 850oC to protect

Advanced Materials Research Vols. 960-961 (2014) pp 1411-1415 Online: 2014-06-18© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.960-961.1411

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three-way catalytic converter and both fuel consumption. If there is deflagration, the ignition angle

delay 1 to 2oCA. Because intake duration of miller cycle is longer than the original engine, inlet

angle close later than the original engine, thus increase the throttle opening to maintain air input.

Experimental results and analysis

Variable valve timing (VVT). In the experiment, intake duration of the original engine (otto cycle

engine) and the miller cycle engine are 265oCA and 285

oCA, through the adoption of different inlet

cam profile. The increase of intake duration of miller cycle is conductive to provide plenty of fresh

impulse to cylinders, which can meet the needs of the combustion process in cylinder. At the same

time, the original engine and miller cycle engine use different structure of pistons, and geometry

compression ratio are 10.4 and 12.6. The adjusting of intake late closing angle and exhaust advance

angle is combined with the variable timing VVT, the effective compression ratio and expansion

ratio are shown in Table 2. Under the common effect of variable timing VVT and geometry

compression ratio, the effective compression ratio of miller cycle reduce at low and middle loads,

but the effective expansion ratio has an obvious increase in the entire loads. Effective compression

ratio and effective expansion ratio will affect the cycle thermal efficiency and fuel economy

collectively.

Pumping loss is the main causes of poor fuel economy of otto cycle engine. In the experiment, in

order to ensure adequate air inflow, according to different conditions to optimize the throttle

opening. After optimization, the throttle opening of miller cycle is average 13.3% larger than that of

original engine. The increase of throttle opening is helpful to decrease the throttling losses near

throttle, and also can improve the intake pressure. Figure 2 shows that the intake pressure of miller

cycle is about 12.7% larger than that of original engine. Under middle and low loads, the intake

pressure increase quickly with the increase of loads. While in high loads, intake pressure increase

gently. And the technology of late inlet valve close (LIVC) could part replace throttle to adjust the

load. As shown in Figure 3, compared with the original engine, the pumping loss of miller cycle

reduce obviously. Because the intake pressure increase, and the increase of expansion ratio also will

lead to exhaust pressure reduce. Pumping loss of miller cycle engine is average 53.3% lower than

that of original engine at 2000r/min.

Table 1 Main technical parameters of

gasoline engine

Model Inline, DVVT

Bore / mm 78

Stroke / mm 83.6

Rod length / mm 134.7

Displacement / L 1.6

Compression ratio 10.4

Maximum torque / Nm 150

Power / kW 82/6000r/min

Fig.1. Schematic diagram of the test rig.

Table. 2 Comparison of effective compression ratio and effective expansion ratio

BMEP

/100kPa

Effective Compression Ratio Effective Expansion Ratio

Otto Miller Otto Miller

2 5.95 5.52 8.77 11.37

4 6.45 5.05 9.45 11.4

6 7.24 6.05 9.45 11.4

8 8.14 8.61 8.6 9.75

9.5 8.95 9.52 8.35 9.33

1412 Thermal, Power and Electrical Engineering III

Thus it can be seen that, miller cycle engine can reduce pumping loss significantly, improve the

effective utilization of energy, and reduce the fuel consumption through using variable valve timing

technology VVT.

Combustion process analysis. Figure 4 shows that the combustion heat release rate changes with

the crank angle under different loads of 2000r/min. Figure 5 shows the contrast of ignition advance

angle under different loads of 2000r/min. As is shown in Figure 5, compared with original engine,

the ignition advance angle reduce to avoid detonation combustion. And because the effective

compression stroke of miller cycle is small, the air motion inside the cylinder is relatively weak, the

burning speed slow, these lead to the combustion process of miller cycle focus after the top dead

center.

As is shown in Figure 4, the heat release rate of miller cycle is slower under low loads. This is

because that compared with the original engine, the effective compression ratio of miller cycle is

low, and the throttle opening is bigger, thus the air motion inside the cylinder is relatively weak.

These lead to that the burning speed slow and the peak heat release rate is lower. Under high loads,

because of the increase in effective compression ratio, the air motion produced in compression

process enhance, burning speed is accelerated, and the peak heat release rate increase.

Table 3 shows the contrast of maximum combustion temperature and exhaust temperature under

different loads of 2000r/min. Because the combustion heat release rate of miller cycle is slower than

original engine, and combustion focus on later top dead center, the combustion temperature

Fig. 2.Comparison of intake pressure Fig. 3. Comparison of pumping loss

0

20

40

60

80

100

120

0 2 4 6 8 10

BMEP/100kPa

I

n

t

a

k

e

P

r

e

s

s

u

r

e

/

k

P

a

Otto

Miller

-90

-70

-50

-30

-10

10

0 2 4 6 8 10

BMEP/100kPa

P

u

m

p

i

n

g

L

o

s

s

/

1

0

0

k

P

a

Otto

Miller

-5

0

5

10

15

20

25

30

-40 -20 0 20 40 60

Crank Angle/°CA

d

Q

/

d

θ

/

J

/

°

C

A

otto

miller

-10

0

10

20

30

40

50

-40 -20 0 20 40 60

Crank Angle/°CA

d

Q

/

d

θ

/

J

/

°

C

A

otto

miller

-10

0

10

20

30

40

50

60

-40 -20 0 20 40 60

Crank Angle/°CA

d

Q

/

d

θ

/

J

/

°

C

A

otto

miller

Fig. 4. Comparison of combustion heat release rate (dQ/dθ)

0

10

20

30

40

50

2.0 6.0 8.0

BMEP/100kPa

S

p

a

r

k

A

n

g

l

e

/

°

C

A

Otto

Miller

Fig. 5 Comparison of spark angle

Table. 3. Comparison of combustion

temperature and exhaust temperature

BME

P/kPa

combustion

temperature/K

Exhaust

temperature/K

Otto Miller Otto Miller

200 2543 2398 699 666

600 2550 2418 735 719

800 2450 2380 759 732

Advanced Materials Research Vols. 960-961 1413

decrease somewhat. Because of the increase of expansion ratio, the exhaust temperature of miller

cycle decrease somewhat, compared with original engine.

Emission analysis. The main source of NOx emissions is the nitrogen in the air which

participated in combustion. The generation mechanism of NOx is Zeldvich mechanism[7]. NOx

formation is mainly related to temperature and excess air coefficient. Excess air coefficient in this

experiment keep 1, through regulating. So the combustion temperature has a great influence on

NOx emissions. Figure 6 shows the contrast of NOx emissions. As is shown in the figure, compared

with original engine, NOx emissions of miller cycle have obvious lower in the whole loads. NOx

emissions of miller cycle reduce 50% at low load, and 30% in middle and high loads. This is

because combustion temperature in cylinder of miller cycle reduce.

CO emissions is a result of incomplete combustion of fuel in cylinders, which is intermediate

products caused by the lack of oxygen[8]. Figure 7 shows the contrast of CO emissions. As is

shown in the figure, compared with original engine (otto cycle), CO emissions of miller cycle

increase somewhat, especially in middle and low loads. Although the excess air coefficient is kept 1,

there will be the phenomena of part hypoxia influenced by burned gases. And the combustion

process of miller cycle mainly focus on the top dead center later, the combustion temperature is

reduced, then the incomplete combustion increases, this lead to CO emissions increase.

HC emissions of gasoline engine is mainly the product of incomplete combustion, and the

quality of mixed gas is one of the important factors that affect the combustion. Figure 8 shows the

contrast of HC emissions. As is shown in the figure, compared with original engine (otto cycle), HC

emissions of miller cycle increase somewhat in the whole loads. This is mainly caused by residual

gas in the cylinder. The inlet and exhaust overlap angle of miller cycle engine is increased (Figure

9), through the regulation of variable timing VVT. This lead to residual gas in the cylinder increase,

which would hinder the formation of flame center and flame spread, even interrupt the flame spread.

This lead to the increase of HC emissions because of incomplete combustion.

Conclusions

(a) By perfecting the inlet cam profile, using higher geometry compression ratio, and combining

VVT control technology, the engine can achieves miller cycle of high compression ratio. The

pumping loss can be reduced and the effective utilization of energy can be improved.

0

500

1000

1500

2000

2500

3000

2.0 4.0 6.0 8.0

BMEP/100kPa

N

O

x

/

p

p

m

c

Otto

Miller

Fig. 6. Comparison of NOx emissions Fig. 7. Comparison of CO emissions

0

2000

4000

6000

8000

2.0 4.0 6.0 8.0

BMEP/100kPa

C

O

/

p

p

m

Otto

Miller

0

500

1000

1500

2000

2500

3000

2.0 4.0 6.0 8.0

BMEP/100kPa

H

C

/

p

p

m

c

Otto

Miller

Fig. 8. Comparison of HC emissions Fig. 9. Comparison of valve overlap

0

10

20

30

40

50

60

1 2 3 4

BMEP/100kPa

V

a

l

v

e

O

v

e

r

l

a

p

/

°

C

A

Otto

Miller

1414 Thermal, Power and Electrical Engineering III

(b) The combustion heat release rate of miller cycle achieved by VVT control technology is

slower than that of the original engine, and combustion heat release are mainly concentrated on

TDC later. The combustion process can be improved through inlet pressurization and combustion

system optimization.

(c) The miller cycle achieved by VVT control technology can reduces the combustion

temperature, which can reduce NOX emissions effectively, but CO and HC emissions increase

somewhat.

References

[1] Tuttle J H. Controlling engine load by means of late intake-valve closing[J]. SAE Paper 800794,

1980.

[2] Wang Y, Lin L, Roskilly A P, et al. An analytic study of applying Miller cycle to reduce NOx

emission from petrol engine[J]. Applied thermal engineering, 2007, 27(11): 1779-1789.

[3] Xue Jun. Miller cycle gas engine[J]. Design and Manufacture of Diesel Engine, 1999(2):

28-33(in Chinese).

[4] Luria D, Taitel Y, and Stotter A. The Otto-Atkinson Engine- a New Concept in Automotive

Economy[J]. SAE Paper 820352, 1982.

[5] Boggs D L, Hilbert H S, Schechter M M. The Otto-Atkinson cycle engine: fuel economy and

emissions results and hardware design[J], SAE Paper 950089,1995.

[6] Zhao Jinxing, Xu Min, Li Mian, et al. Design and optimization of an Atkinson cycle engine with

the Artificial Neural Network Method[J], Applied Energy, 2012, 92: 492-502(in Chinese).

[7] Georg Tinschmann, Marco Taschek, Mr. Heiner Haberland, et al. Combustion System

Development for IMO Tier 2[C], CIMAC Congress 2007, Vienna, 2007, 148.

[8] Gong Jinke. Automobile emission and control technology[M]. Beijing: People's

Communications Press, 2007:10-11(in Chinese).

Advanced Materials Research Vols. 960-961 1415

Thermal, Power and Electrical Engineering III 10.4028/www.scientific.net/AMR.960-961 Study of Combustion and Emission Characteristics of Gasoline Engine with Miller Cycle 10.4028/www.scientific.net/AMR.960-961.1411

DOI References

[2] Wang Y, Lin L, Roskilly A P, et al. An analytic study of applying Miller cycle to reduce NOx emission

from petrol engine[J]. Applied thermal engineering, 2007, 27(11): 1779-1789.

http://dx.doi.org/10.1016/j.applthermaleng.2007.01.013 [5] Boggs D L, Hilbert H S, Schechter M M. The Otto-Atkinson cycle engine: fuel economy and emissions

results and hardware design[J], SAE Paper 950089, (1995).

http://dx.doi.org/10.4271/950089