WHITEPAPER
Next Generation Gas Engine Oils for
Improved Sustainability in the Power
Generation Market.
Kathy TELLIER, ExxonMobil Research & Engineering Co., Paulsboro, NJ (USA)
Gilles DELAFARGUE, ESSO S.A.F (France)
Thomas DIETZ, ExxonMobil Lubricants and Petroleum Specialities Company, Fairfax, VA (USA)
Kevin HARRINGTON, ExxonMobil Lubricants and Petroleum Specialities Company, Fairfax, VA
(USA)
ABSTRACT
Engine designs, operating conditions, customers’ needs and environmental factors continue to
place high demands on lubricants for natural gas engines. Projections for future energy supply
and demand indicate that oil, coal and natural gas will continue to be the predominant energy
sources through 2030.1 As energy prices fluctuate through market cycles, owners and operators
of natural gas engines will seek ways to reduce their energy costs. This will be especially true in
power generation markets where fuel costs directly affect profitability. In addition, the power
generation industry is seeking options to improve their productivity and support sustainability. In
many cases, a well designed lubricant can provide benefits that contribute to sustainability such
as extended oil life, reduced oil for disposition, energy efficiency and extended equipment life for
power generation applications, including cogeneration and landfill applications.
This paper will focus on next generation natural gas engine oil product developments utilising
leading edge product technology that provides extended oil life, excellent piston deposit control
and increased engine efficiency potentially reducing greenhouse gas emissions. Comprehensive
research test programs which have evaluated oxidation stability, high temperature thermal
stability and frictional characteristics of promising candidates will be discussed. The paper will
also provide highlights of the extensive engine durability test programs which evaluate the oil life,
piston cleanliness and wear performance of gas engine oils in shop and field applications.
Consequently the development of these next generation natural gas engine oils could
significantly enhance the sustainability benefits of natural gas fuelled power generation engines.
INTRODUCTION
Innovative lubricants can help deliver tangible performance and sustainability-related benefits as
well as material economic advantages to industry and consumers. Advanced lubricating oils can
help increase equipment operating efficiency and engine fuel economy, and help contribute to
reduced energy and resource use, lower emissions and cost savings for gas engines.
Innovations in synthetic-based product formulations help deliver longer lubricant performance
cycles, and help reduce lubricant consumption, used oil volumes and operating expenses.
Additional sustainability benefits can also be obtained by utilizing landfill or biogas as a fuel,
thereby reducing impact of the greenhouse gases that are generated. Gas engine oils can be
specifically formulated to address the requirements of landfill or biogas contaminants.
Recent trends in engine design include increasing power output and efficiency through higher
compression ratios and higher turbocharger pressures. At the same time, with increasing focus
on emissions, the amount of oil available in the engine to lubricate piston rings and cylinder liners
has been reduced.
In addition, with pressure on operators to reduce lubricant-related
maintenance costs, there is even greater need to maximise the effective life of the lubricant, as
well as enabling reduced engine downtime and labour cost for oil changes, and reduced volume
of waste oil produced to minimize disposal costs. Finally, with significant increasing cost of
energy, owners and operators of natural gas engines welcome any opportunity to improve energy
efficiency.
As with any new product development, ExxonMobil Research & Engineering engineers and
chemists first identify the key lubricant attributes required for an application, and then confirm
those parameters that need to be improved. For this development, the focus was to develop a top
performing synthetic gas engine lubricant with outstanding oxidation resistance with low oil
consumption, hence enabling oil drain interval extensions and excellent piston cleanliness control.
Friction reduction would also enable increased engine efficiency, resulting in reduced emissions.
The product profiling was done with equipment builders' advice, in association with the users and
maintainers of such equipment. A thorough understanding of how products are used, and the
factors that limit their performance, is essential if meaningful and beneficial improvements are to
be made.
Many manufacturers of finished lubricants utilise "off the shelf" additive packages to formulate a
lubricant for a particular application, which does not always lead to the best lubrication solution.
For high performance products, ExxonMobil prefers to utilise individual additive components in
combination with the highest quality base oil components. Synthetic basestocks (such as
polyalphaolefins) are utilised to achieve optimum performance in key areas.
Global product quality and consistency is also an important aspect of ExxonMobil's product
development strategy. In the long term, this offers the best value to the customer, not only in
terms of the cost and performance, but also in the protection afforded to the engine/equipment in
which they are used, wherever that may be.
PRODUCT DEVELOPMENT PROCESS OVERVIEW
There are a number of parameters essential to the successful development of new, high quality
synthetic gas engine oil.
At the heart of any lubricant is the base oil and this is especially important for a high performance
gas engine oil. Hence carefully selected synthetic base stocks, based on years of experience in
engine applications, ensure consistent, high level of performance of the finished synthetic product.
Selection of the correct balance of additives is also critical to finished product performance.
An important requirement for a premium lubricant is to maximise its effective in-service life. These
life extensions must coincide with other component service intervals to be beneficial to the end
user. Often, this may require double (or higher multiples) of the existing oil drain interval. Oil
drain interval extension is achieved by ensuring the factors that limit oil life, such as oxidation,
nitration, viscosity increase, TBN level, etc; all remain within acceptable limits for the intended life
of the lubricant. Engine deposits and wear levels must be maintained at satisfactory levels as
well.
The wear protection of the moving components within the engine is vital.
The oil's wear
protection additives ensure that expensive or heavily loaded components (such as pistons,
crankshafts, and bearings) remain within serviceable tolerance limits during their normally
expected life.
Cleanliness is also crucial for long term reliability of the engine, especially in the piston ring
grooves, and on the cylinder liner surfaces where lacquers or varnishes could interfere with
effective lubrication and control of lube oil consumption.
Minimal lubricant consumption in use is essential to controlling total cost of ownership and is
essential to achieving lower cost of engine operations.
High oil consumption rates lead to
increased lubricant purchases for the operator and could partially negate the advantages of
longer-life engine oil.
A reduction in friction would positively influence efficiency of the engine, and in turn generate
energy savings under mixed and boundary lubrication regimes.
Commensurate with improvements in fuel efficiency, CO2 emissions will also be reduced.
Conformance to equipment builder specifications and requirements is essential to ensure the
acceptance and commercial feasibility for the product.
These are just a few of the parameters necessary to ensure that any new premium synthetic gas
engine oil product provides a compelling value proposition for the customer.
ExxonMobil
formulators strive to attain "balanced performance" formulations, where optimised all-round
performance is achieved.
BENCH TEST PROGRAM
The use of effective lab screening tests is essential to enable rapid evaluation of experimental
lubricants under controlled conditions, with each test simulating a condition that the oil would
experience in service.
A comprehensive bench test program ensures that the best overall
candidates are selected for subsequent evaluation in real-life testing. Specific attributes can be
evaluated in laboratory tests, as outlined in the following.
In House Thin Film Oxidation Test (Hot Tube)
This test evaluates the ability of oil to prevent the formation of high temperature thin film deposits
in the hot areas of the engine (e.g. piston ring grooves).
Photo 1 - Reference ratings
A small volume of oil flows in heated glass tubes for a fixed period of time. The tubes are then
rated for deposits on a demerit scale, shown in Photo 1 (1= clean; 10= heavy). Typical results
are shown in Figure 1, comparing the new lubricant vs. two competitive natural gas engine oils.
Over time, this test has proven to be an excellent indicator of deposit formation on engine
components subjected to high temperatures.
Demerits,
1=clean,
10=heavy
deposits
Lower Number better
Performance
10
9
8
7
6
5
4
3
2
1
0
Candidate
Competitor A
Competitor B
Figure 1 – Typical hot tube test results
In House Oxidation / Nitration Bench Tests
Lubricant exposed to oxygen and/ or nitrogen oxide will degrade over time, forming oxidation
products that increase viscosity, and reduce lubricant life.
There are a number of different
methods to assess the potential performance of a candidate, one of which is described in the
following test(s).
Bulk Oxidation Test
This test evaluates the ability of oil to resist bulk oil oxidation and has been shown to be a good
indicator of its extended oil drain capabilities. The oil is heated at elevated temperatures for a
fixed period of time while air is bubbled through the sample in the presence of catalyst. The
viscosity increase is measured at the conclusion of the test. Typical results are shown in Figure 2,
comparing the new lubricant to the same competitive oils. Those oils with the lowest viscosity
increases in this test tend to demonstrate the extended oil life capabilities in the field. This test
has also been enhanced to allow evaluation of oil’s nitration resistance.
250
Lower Number better
Performance
200
% Viscosity 150
Increase
100
50
0
Candidate
Competitor A
Competitor B
Figure 2 - Typical bulk oxidation test results
University Testing
A program, sponsored by the U.S. Department of Energy, including representatives from
prominent U.S. universities and a key gas engine equipment builder, evaluated various piston
hardware options and lubricant technologies to successfully reduce friction in the piston ring
pack.2 University modelling reaffirmed that additional friction reductions could be achieved by
changes in lubricant characteristics.
Two phases of testing were conducted in a commercial scale gas engine. The objective of the
first phase was to evaluate the impact of viscosity on engine friction, engine efficiency and fuel
consumption. The second phase evaluated the impact of base oil type (at constant viscosity) on
the same parameters.
The test engine was a Waukesha VGF F18, in-line 6 cylinder natural gas engine used for
stationary power generation. Friction Mean Effective Pressure, mechanical efficiency and fuel
consumption were measured at two load points (70 and 100%) for each test oil.
The lubricant changing procedures were designed to replace as much oil as possible without
requiring a full engine rebuild. To start, oil was drained from the sump and oil consumption meter.
The oil filters were replaced. The engine was then filled with new oil and run for nearly an hour.
The engine was then shutdown and the oil pan and oil consumption meter were drained again.
The oil filters were also replaced again. The engine was then filled for a final time.
The brake specific fuel consumption (BSFC) results are shown in Figure 3.
The reference oil was tested at the beginning and end of each test phase. The two sets of test
results were compared to each other and did not show any significant variation between the two,
demonstrating excellent test repeatability. C
10.8
10.6
BSFC (MJ/bkW-hr)
10.4
Reference
10.2
SAE 30 - Phase 1
SAE 20 - Phase 1
10
SAE 30 - Phase 2
SAE 20 - Phase 2
9.8
9.6
9.4
9.2
70% Load
100% Load
Figure 3 - BSFC results - Phase 1 and 2
Based on the results shown in Figure 3, the fuel efficiency relative to a SAE 40 reference oil was
1.5 to 3% for the Phase 2 SAE 30 candidate and 3 to 4% for the Phase 2 SAE 20 candidate
based on 95% confidence interval.
As lubricant viscosity decreases, the impact on engine durability must be considered.
The
majority of natural gas engine oil applications now utilise SAE 40 oils. SAE 30 oils are the most
reasonable next step for commercial fuel efficient gas engine oils. SAE 20 oils will provide the
greatest fuel economy benefit but could have greater potential impact on engine durability.
The next sections of the paper will discuss how engine durability impacts were evaluated.
High Severity Engine Test
Once a candidate has been evaluated successfully in all relevant lab screening tests, appraisal in
a more realistic application is required. However, speed to market is essential, so ExxonMobil
uses a modified full scale gas engine, adapted to put the oil under maximum stress.
Photo 2 - High severity engine test
Modifications to the engine include:
Reduced sump volume
Elevated operating temperatures
Air-fuel ratio set for maximum oxidizing and nitrating conditions
Under such extreme operating conditions, any weaknesses in the lubricant's performance are
quickly revealed. Successful completion provides assurance that the product will be more than
capable of surviving in real life applications. Used oil analysis and piston demerit ratings are
conducted during each test run.
To evaluate the wear impact of lower viscosity candidates, the engine test protocol included
detailed metrology of key engine components (pistons, piston rings, liners, valves, valve guides,
bearings, etc) and oil condition monitoring. Photos 3 and 4 show a piston and set of valves at the
conclusion of a test run with SAE 30 candidate. End-of-test used oil wear metal levels are also
shown in Figure 4. Engine durability testing of lower viscosity candidates in the high severity
engine test showed equivalent wear performance to the SAE 40 reference. Figure 5 shows one
example of the metrology results, a comparison of liner wear steps measured with the SAE 40
reference oil and the SAE 30 candidate. A liner wear step (in units of microns) is measured by a
profilometer in the area of top ring reversal. A total of 12 measurements are taken around the
perimeter of each liner. Both the average and standard deviation was calculated for each liner.
There was no statistically significant difference between the two wear measurements.
Photo 3- Piston cleanliness at End-of Test
Photo 4 - Valves at End-of- Test
Wear Metals
50
Wear metals (ppm)
40
30
20
10
0
Reference
Candidate
Al
Cr
Fe
Pb
Figure 4 - Used oil wear metals at End-of-Test
Liner Wear Step
10
Liner Wear Step (um)
9
8
7
6
Reference
5
Candidate
4
3
2
1
0
0
1
2
3
4
5
6
7
Liner
Figure 5 - Liner wear step comparisons
FIELD DEMONSTRATIONS
Once the final commercial candidate is selected based on the bench screening and high severity
test engine results, field tests are conducted, including a wide variety of gas engine makes and
models, representative of the key main equipment builders in the market. The configuration of
each test used to evaluate lubricant candidates is shown in Appendix A.
Field Demonstrations
In addition to the initial fuel efficiency testing at the university and the in-house engine durability
testing, field testing was conducted to evaluate three aspects of candidate performance under
real life conditions:
1. engine durability impact of an SAE 30 lubricant over an extended period of time
2. extended oil drain capability
3. fuel efficiency improvement.
Engine Durability Testing
Engine durability testing was conducted in two (2), new 16 cylinder, 170 mm bore gas engines
operating on clean, natural gas in gas compression service at 95-100% load (Units 242 and 254).
The objective of this test was to evaluate the impact of lower viscosity lubricants on the durability
of key engine components, i.e. liners, valves and valve guides, piston rings, bearings, etc. The
test was conducted for over 7000 hours at full load. An intermediate boroscopic inspection was
conducted on both units at 4000 hours. Photos 5 and 6 show the excellent liner and cylinder
head condition (typical) at the intermediate inspection. A final inspection was conducted at over
8000 hours and two (2) power cylinder assemblies were removed for further inspection and
photographs. Photos 7 and 8 show the excellent piston and connecting rod bearing condition at
the end of the test period.
Oil condition monitoring was conducted throughout the test period with used oil sample collection
at 250 hour intervals. Samples were analyzed for key parameters, including kinematic viscosity
at 100°C, oxidation and nitration, and wear metals. Figures 6 and 7 provide trends of kinematic
viscosity and oxidation. Each chart also shows a commercial reference (in light blue) and the
OEM condemning limit (in red). All used oil parameters were satisfactory throughout the test and
well below the condemning limits, with the exception of lead which was determined to be related
to a lube oil cooler metallurgy issue. The used oil analysis results confirmed the excellent wear
performance of the lubricant and its extended oil life relative to the SAE 40 commercial reference.
Photo 5 - Liner crosshatching at 4000 hr intermediate inspection
Photo 6 - Cylinder head and valve condition at 4000 hr boroscopic inspection
Kinematic Viscosity @ 100C
Unit 242
Unit 254
Limit
SAE 40 Reference
cSt
16
14
12
10
0
2000
4000
6000
8000
Hours
Figure 6 - Used oil analysis - Kinematic viscosity
Oxidation
absorbance/cm
Unit 242
Unit 254
Limit
SAE 40 Reference
30
25
20
15
10
5
0
0
2000
4000
6000
8000
Hours
Figure 7 - Used oil analysis - Oxidation (as measured by FTIR in units of absorbance/cm)
Photo 7 - Piston at 8331 hours
Photo 8 - Connecting rod bearings at 8331 hours
Extended Oil Drain Interval Testing
The extended oil drain capability was evaluated in both units. Used oil analysis and monitoring
continues.
Unit 254 has reached 13,000+ hours without an oil drain while still maintaining
excellent viscosity control, oxidation and nitration control and good wear performance. The oil
was drained on Unit 242 at ~7000 hours due to elevated lead (caused by a lube oil cooler
metallurgy issue) as mentioned in above.
Figures 8 through 11 below show key used oil
parameters measured during the test period. The lube oil cooler on Unit 254 also contributed to
elevated lead levels in the used oil as shown in Figure 11.
Kinematic Viscosity @ 100C
cSt
Unit 242
Unit 254
Limit
16
15
14
13
12
11
10
0
2000
4000
6000
8000
10000
12000
14000
Hours
Figure 8 - Used oil analysis - Kinematic viscosity
Oxidation
Unit 254
Limit
30
20
10
0
0
2000
4000
6000
8000
10000 12000 14000
Hours
Figure 9 - Used oil analysis - Oxidation
Nitration
Unit 242
absorbance/cm
absorbance/cm
Unit 242
Unit 254
Limit
30
20
10
0
0
2000
4000
6000
8000
10000
12000
Hours
Figure 10 - Used oil analysis - Nitration
14000
Unit 254 Metals
ppm
Iron
Aluminum
Copper
Lead
70
60
50
40
30
20
10
0
0
2000
4000
6000
8000
Hours
10000
12000
14000
Figure 11 -Used oil analysis - Wear metals
Fuel Efficiency Confirmation Testing
Fuel efficiency confirmation testing was conducted on two (2), new 16 cylinder, 170 mm bore gas
engines operating with clean, natural gas in gas compression service at 95-100% load (Units 275
and 276). Each test engine was equipped with a temperature and pressure compensated fuel
consumption meter (Roots rotary meter 5M175). An additional oil storage tank and associated
piping was installed to facilitate oil switching between test cycles.
The experimental design
included four cycles conducted in an A-B-A-B sequence, alternating between candidate and
reference oils in Units 275 and 276 as shown in Figure 12. This enabled a rigorous statistical
analysis of the data.
Each test cycle was approximately 500 hours in duration.
Pressure and temperature
compensated fuel consumption readings and engine/compressor operational data (e.g. speed,
compressor discharge pressures and temperatures, air manifold pressure, etc) were recorded in
a data acquisition system at 5 minute intervals throughout the test.
The fuel meter and
engine/compressor operational data were merged together, resulting in 40,000 data points. Two
test oils were evaluated, a commercial SAE 40 reference and the SAE 30 candidate.
Test Cycle
Unit 275
Unit 276
A1
SAE 30
SAE 40 Ref.
B1
SAE 40 Ref.
SAE 30
A2
SAE 30
SAE 40 Ref.
B2
SAE 40 Ref.
SAE 30
Figure 12 - Table of Test Cycles
A statistical analysis (mean and 95% confidence interval) was conducted on the merged data set.
Figure 13 provides an overall comparison of fuel consumption in Units 275 and 276 when
operating with the SAE 40 reference (in black) vs. the SAE 30 candidate (in blue). The SAE 30
candidate consistently showed a statistically significant fuel efficiency improvement vs. the
commercial reference. The average efficiency benefit of the SAE 30 oil versus the conventional
SAE 40 gas engine was calculated to be 1.5%.
Fuel Consumption - Units 275 and 276
Reference vs. Candidate
300
280
M3/hr
260
240
220
200
Unit 275 Reference
Unit 275 Candidate
Unit 276 Reference
Unit 276 Candidate
Figure 13 - Comparison of fuel consumption
Several variables were considered during the statistical analysis including engine load, ambient
conditions, and humidity impact. Each variable was analyzed statistically to determine the
magnitude of its impact on flow measurements. Each will be discussed in more detail in the
subsequent paragraphs.
As indicated in Appendix A, fuel efficiency confirmation testing was conducted at a site in gas
compression service. Engine load was calculated based on compressor inlet and discharge
pressures using original equipment manufacturer compressor curves.
Ambient pressure and temperature are needed to convert actual fuel flow measurements to
standard conditions (15.6C and atmospheric pressure). Fuel consumption meters were equipped
with thermocouple and pressure transmitter inputs to provide a compensated flow signal for
analysis.
Absolute air humidity has an influence on the combustion since an increase in humidity can
decrease the combustion speed and reduce maximum combustion temperature. Therefore, as
humidity increases, engine efficiency decreases.3
CONCLUSIONS
In depth understanding of gas engine lubrication issues reported from in service applications, in
liaison with equipment builders enabled the targeting of a new synthetic lube oil product
performance profile. It is aimed at solving the increasingly severe lubrication needs of engines
operating on natural gas. A structured development utilizing effective and proprietary screening
programs, and extensive field-test monitoring, is beneficial for the commercial launch of such
products. Delivering proven outstanding performance including excellent wear protection,
extended oil drain intervals, reduced oil consumption, and energy savings up to 1.5% versus
conventional natural gas engine lubricants, Mobil SHC Pegasus provides advantages to gas
engine operators by improving the overall productivity and contributing to the sustainability of
their operations.
A corresponding reduction in emissions in natural gas engines globally is
equivalent to the annual CO2 emissions output of a 500 MW coal burning power plant.
REFERENCES
1. Hard Truths - Facing the Hard Truths about Energy, National Petroleum Council, July 2007
2. "Friction Reduction Due to Lubrication Oil Changes in a Lean-Burn 4-stroke Natural Gas
Engine: Experimental Results", Kris Quillen, Rudolf Stanglmaier, Victor Wong, Ed Reinbold, Rick
Donahue, Kathleen Tellier, Vincent Carey, Copyright 2007 ASME
3. "About the Influence of Ambient Conditions on Performance of Gas Engines", Position Paper
by the CIMAC working Group "Gas Engines", March 2009
Appendix A
Objective ---->
Demonstrate Engine
Durability
Engine
Duration
Application
Load
Test Oil
Cat G3516
7000 hours
Gas compression
100%
SAE 30 candidate
Equipment
Demonstrate
Extended Oil Drain
Interval
Cat G3516
12,000+ hours
Gas compression
100%
SAE 30 candidate
Confirm Fuel
Efficiency
Cat G3516
4000+ hours
Gas compression
100%
SAE 30 candidate
SAE 40 reference
Fuel consumption
meters; additional oil
storage tank to
facilitate oil switching