Development ofHigh Efficiency Gas Engine forGreen House Gas Reduction
TAKESHI ARAI* 1
TAKAYUKI YAMAMOTO* 2
JUNNOSUKE ANDO* 2
HIROYOKI ISHIDA* 3

Most of the methane gas generated in a coalmine at the time of coal digging is unused and discharged into
atmosphere, which becomes one of the causes of global warming. Because of the low methane concentration andlarge variation in the concentration, the coalmine methane gas has so far been hard to put into effective use. The
new technical development is based on the lean-burn gas engine using the micro-pilot ignition system in order toreduce the emission of green house gasses, and incorporates development of a gas engine capable of high-efficiency
power generation while using the coalmine methane gas as fuel. Mitsubishi Heavy Industries, Ltd. (MHI) has so
far learned combustion characteristics of coalmine methane gas and studied on the gas supply system suitable forthe coal mine methane gas.
methane concentration is confined to a mere 30-50%
because of the inclusion of air, etc. during the processof recovery.
Further, because of a large variation of methane concentration in the gas, methane gas is considered hard touse as a stable energy source, so that only a part of it iscurrently being used as a source of heat, etc.
Since methane gas is evaluated to have the global
warming potential 21 times higher than CO2, it could bean effective means of reducing the emissions of green
house gasses only if the methane gas currently discharged to the atmosphere were burned and put toeffective use.

1. Introduction
With the global call for preventing emissions of green
house gasses increasing, the reduction in emission of
methane gas from a coal mine at the time of coal digging
and the effective use of the gas as a source of energy aredrawing attention.
Approximately 90% of the total volume of coalmine
methane gas is currently being discharged into atmosphere unused. Methane gas, with a large green houseeffect, amounts approximately to 500 million tons interms of the CO 2 emission per year in the world, whichcomes to be almost 40% of the annual emission of CO2 inwhole Japan.
Originally, methane gas is a clean energy source.
However, because of the low concentration and large
variation of the concentration, the coalmine methanegas has so far not been used as a fuel for boilers or gasturbines.
In order to solve such problems MHI is developing ahigh-efficiency, high-power gas engine capable of using
the coalmine methane gas as fuel on the basis of the leanburn gas engine "MACH-30G" using micro-pilot ignition
system.

Discharge to atmosphere
Methane concentration level: 30- 50%(Intrusion of air during the process ofgas drainage)

Ventilation fanVentilation pit

Coal bedVent hole

2. Outline of coalmine methane gas
Fig. 1 shows the outline (1) of coalmine methane gas.
The methane gas in coal bed has a high degree of concentration amounting normally to 90%. However, it isrecovered by pump or is leaned with air and discharged
into atmosphere in order to prevent explosion in themine cavity. The concentration of the recovered gas
differs according to the coal mine and the recovery system, but with the present technology, the restored*1 Yokohama Dockyard & Machinery Works*2 Yokohama Research & Development Center, Technical Headquarters*3 Nagasaki Research & Development Center, Technical Headquarters

Discharge ofmethane gas

Fig. 1 Outline of coalmine methane gas
The intrusion of air during the process of gas recovery causes thegas concentration to deteriorate, with a large amount discharged
into the atmosphere.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

1

World-wide emission of coalmine methane gas: 22.4 million tons/year
If 35% of the discharged coalmine
methane gas is recovered andused for power generation

The quantity of methane used comes to be 8 million tons/year.
Discharge afterpower generation

Methane has greenhouseeffect 21 times as high as CO2.
170 million tons

-0.2

0.4

Energy-saving
effect (Effective utilization of natural resources)

Effect of direct reductionof greenhouse gasses

Approximately 200 million tons/year of CO2 can be reduced.

Further, if it could be used as a fuel for power generation, the consumption of other fossil fuels could be
cut down, contributing to the reduction in CO 2 emisFig.2
sion (Fig.2
Fig.2). Accordingly, power generation using
coalmine methane gas is drawing attention of the worldin recent years.

4. Technical Problems and Countermeasures
4.1 Ignition system
The pilot ignition system(2) used in MACH-30G gas engine was adopted as the ignition system for the gas engine
corresponding to coalmine methane gas.
In the case of a conventional spark ignition gas engine, the air-fuel mixture with the air-fuel ratio close to
the theoretical air-fuel ratio level is supplied to the precombustion chamber. The air-fuel ratio varies due to thechange in fuel gas concentration, resulting in unstableignition. The output was also limited in order to ensure
longer ignition-plug life.
In the case of micro-pilot ignition system, however, a small
quantity of liquid fuel is injected into the pre-combustion
chamber to allow self-ignition. Since the ignition energy inthis system is large, stable ignition for lean air-fuel mixture
can be ensured. In other words, there is no need to supply
air-fuel mixture with the air-fuel ratio close to the theoretical level, ensuring stable ignition even for the coalmine
methane gas with variable fuel gas concentration.
The pilot ignition system is compared with the sparkignition system in Fig. 33.

3. Objective
The brake mean effective pressure (BMEP) of a conventional spark ignition gas engine is approximately 0.6-0.8 MPa when coalmine methane gas is used. The output is lower than a diesel engine or a natural gas engineof the same class. Further, the ignition was unstable
when the methane concentration level in the coalmine
methane gas became lower, causing a problem for continuous operation of the engine.
This research, therefore, aims at developing acoalmine methane gas engine to correspond to the highpower, high-efficiency and variation in gas concentration,
based on the MACH-30G gas engine with highest levelof efficiency in the world. This paper introduces, as thefirst step, the fundamental combustion test and verification test for practical performance in order to develop
the required fundamental technologies. The tests arecarried out by using a small-size gas engine testing equipment. The objectives of development using the small-size
testing equipment are given in Table 11.
Characteristics of spark ignition
Because of the small ignition energy(approximately 0.1 J) , stable ignition cannot
be expected except at limited pre-combustion
chamber air-fuel concentration.

Fig. 2 Effect of coalmine
methane gas power
generation on
reduction of greenhouse gas emissions
Not only the direct reduction,
but also the reduction in emissions by reducing the consumption of other fossil fuels.

Table 1 Performance target levels
Output5 000 kW class

Efficiency at generator terminal

38 % or above

Variation in methane concentration

Independent supplyof gas to the precombustion chamber

35~50 %

Lean air-fuelmixture

Ignition plug

When coalmine methane gas is used
The ignition gets unstable becauseof the variation in fuel gas concentration.
Characteristics of pilot ignition
Large ignition energy (approximately 800 J) ensures
stable ignition at a wide range of air-fuel concentration.

Spark ignition
Pilot oilInjection
valve

Lean air-fuelmixtureWhen coalmine methane gas is used
Stable ignition can be obtained evenwhen the fuel gas goes through variation.

Fig. 3 Comparison of ignition
systems
Adoption of pilot ignition systemcorresponds to the variation ingas concentration

Pilot ignitionMitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)2

4.2 Gas supply system
The coalmine methane gas has about 1/2-1/3 the calorific value of natural gas, so that the methane gas 2-3
times the quantity of natural gas must be supplied to agas engine.
In the case of MACH-30G gas engine, the gas is supplied at the position immediately before each cylinder.
The fuel gas feed pressure must be higher than the inlet
manifold pressure. The engine efficiency is down whencoalmine methane gas with high flow rate is used, because the power for raising fuel gas pressure increases.
In the case of medium- and small-size gas engine, thepower for raising fuel gas pressure is not necessary because the air-fuel mixture is formed before the turbocharger. However, approximate measures have to betaken against the back fire inside the inlet manifold andthe blow by of air-fuel mixture.

Fig. 4 shows comparison between the system supplying gas before cylinder and the system pre-mixing gas andair before turbocharger. As for the testing equipment, thepre-mixing system was adopted in the simulated gas test
to verify the applicability to the MACH-30G gas engine.
4.3 Control system
MACH-30G gas engine uses a unique combustion control technology called Mitsubishi Real-time Intelligent
Control System (M-RICS) (3) . In this system real-time
diagnosis of the combustion condition is carried out basedon the combustion pressure data in each cylinder, andensures high-efficiency operation while averting the abFig. 5
normal combustion (Fig.
5).
The utilization of M-RICS ensures instantaneous correspondence to the variation in fuel gas concentrationeven when coalmine methane gas is used as fuel, enablinghigh-efficiency and stable operation (Fig. 6)
6).
Gas supply
electromagnetic valve

Fuel gas
Turbocharger

Electronically controlled
pilot injection system(common rail)

Turbocharger

Turbocharger

Combustion
pressure sensor
Air

Fuel gas

Generator

Air

Electronic governor

Combustion
chamber
Pre-mixing before
turbocharger

MACH-30G

Pilot controller

M-RICS
combustion diagnosis unit
Optimum combustion control ofeach cylinder; automatic evasive
control at abnormal combustion

Combustion
chamber

Air-fuel ratio control

Conventional MACH-30G
gas engine

Main controller, DIASYS

Fig. 4 Comparison of gas supply systems
The system conventionally adopted in MACH-30G
compared with the mixing system before turbocharger

Charged airtemperature control

Fig. 5 Outline of M-RICS
The system detects the combustion pressure to control each cylinder,
ensuring both high efficiency and stability.

M-RICS abnormal combustion control
with knocking automatically averted

M-RICS abnormal combustion control
with the concerned cylinder cut off toallow continuous operation

M-RICS optimum combustion control
with the combustion constantly
adjusted to appropriate stateKnockingCombustion
pressure: HighAppropriateCombustion
pressure: Low

Knocking zone
Combustion changed according to thevariation in methane concentration level
RichMethane concentration level in the gas(fuel-air mixture concentration)

Accidental
fireUnstablecombustion zone
Lean

Fig. 6 Combustion control using M-RICS
The M-RICS ensures control corresponding to the variation in methane gas concentration.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

3

25

50.0
Air-fuel ratio(Air flow rate/fuel gas flow rate)

Thermal efficiency

Indicated mean effective
pressure (IMEP)

52.048.046.0: Natural gas: Simulated coalmine methane gas

44.0
1.70
1.60
1.50
1.40

20151051.30
1.20020406080
CH4 content in fuel gas (%)

100

020406080
100
CH4 content in fuel gas (%)

5. Fundamental combustion test

Fig. 7 Results of elementary
combustion test
The results indicate methane
concentration during combustion vs. thermal efficiency andair-fuel ratio.
Even the coalmine methanegas can give the combustion
performance equivalent to thatof natural gas through optimization of air-fuel ratio.

Table 2 Specifications of testing equipment

5.1 Testing equipment
A S6R engine manufactured by General Machinery &Special Vehicle Headquarters of MHI was remodeled intoa pilot ignition type gas engine. The remodeled engine
was used as the testing equipment for coalmine methane gas fundamental combustion test. The test wascarried out using simulated gas. The specifications ofthe testing equipment are given in Table 22.
5.2 Combustion test results
7 The reThe main test results are shown in Fig. 7.
sults show that, with the adoption of micro-pilot ignition
technology, stable operation can be obtained even by using coalmine methane gas with the output and efficiency
as high as that of natural gas. Further, the optimum airfuel ratio per gas concentration rate was learned.

Specification

Item
Rated output

400 kW

Rotational frequency

1 500 rpm

Number of cylinders

6

Bore X Stroke

170 mm X 180 mm

Compression ratio

10.0

Combustion system

Pre-combustion chamber type

FuelGas, light oil

adoption of pilot ignition system ensures the power generation efficiency level equivalent to that of natural gas.
MHI is determined to make further verification of thesystem in order to put the MACH-30G engine using
coalmine methane gas into practical use.
The research has been a part of the "Joint Research
Program of Technological Development in the Private
Sectors" for fiscal year from 2002 to 2004 of the ResearchInstitute of Innovative Technology for the Earth (RITE).
Corporation to this regard by a large number of concerned
people is highly appreciated.

6. Application to actual machine
Based on the study and combustion test results so far
obtained, the MACH-30G using coalmine gas as fuel ison the way to commercialization.
At present, verification using simulated gas is underway to verify the control system when coalmine methane gas as used for MACH-30G engine as fuel.

References(1) Hirasawa, H., Coal Mine Methane Recovery and Utilization
Project in China, Douryoku Vol.259 (2002) p.52
(2) Nakano et al., Development of High Power KU30GA Gas Engine, Mitsubishi Heavy Industries Technical Review Vol.38
No.3 (2001)(3) Nakano et al., Development of Advanced MACH-30G Gas
Engine, Mitsubishi Juko Giho Vol.41 No.1 (2004)

7. Conclusion
A gas engine capable of high-efficiency power generation is under way by using the coalmine methane gas sofar discharged in great amount into air as fuel in orderto reduce emissions of greenhouse gasses. The combustions tests and studies so far made indicate that theTakeshi Arai

Takayuki Yamamoto

Junnosuke Ando

Hiroyoki Ishida

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

4