In modern-day natural gas engines the importance of using a state-of-the-art lubricant should not be under-estimated, especially when it comes to improved efficiency and extended equipment life

 

With modern gas engines placing greater and greater demands on the oil that keeps them running, PEi explores the development process and presents the potential benefits of using bespoke synthetic engine lubricants. 

Ask any natural gas engine operator to list their biggest concerns, and likely as not fuel consumption, maintenance costs, durability and emissions will all be at the top of that list. Of all the factors that can impact these one of the most influential is engine oil performance.

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 global power generation industry is looking for options to improve productivity and sustainability. In many cases, a well designed lubricant can provide benefits that contribute to extended oil life, reduced oil for disposition, increased energy efficiency and extended
equipment life.

With this in mind, we explore the development process and potential benefits of bespoke engine oils in such applications.

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 operating efficiency and fuel economy, and help contribute to reduced energy and resource use, lower emissions and longer lubricant performance cycles.

Recent trends in engine design include increasing power output and efficiency through higher compression ratios and higher turbocharger pressures. At the same time, with the increasing focus on emissions, the amount of oil available in the engine to lubricate piston rings and cylinder liners has been reduced.

In addition, increasing and unstable energy prices, combined with greater regulation and associated costs in disposing of waste oil, has placed increased pressure on operators to reduce lubricant-related maintenance costs. There is now a greater need than ever to maximize the effective life of the lubricant, as well as reduce engine downtime, labour costs for oil changes and waste oil disposal costs.

We are now in a situation where there is less oil inside the engine, it is working harder to cope with increased compression ratios, higher turbocharger pressures and increased power outputs, yet there is increasing demand from operators for greater durability and fuel efficiency, and fewer oil changes. To solve this dichotomy ExxonMobil research engineers and chemists set about creating an improved synthetic lubricant for gas engines. 

PRODUCT DEVELOPMENT process 

As with any new product development, ExxonMobil research team first identified the key lubricant attributes required for the application, and then confirmed those parameters that need to be improved.

For this development, the focus was to create a top performing synthetic gas engine lubricant with outstanding oxidation resistance and 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 and fuel consumption.

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 are essential if meaningful and beneficial improvements are to be made.

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, combined with the correct selection and balance of additives are used to ensure a consistently high level of performance for the finished product.

An important requirement for a premium lubricant is to maximize its effective in-service life; this however must coincide with the service intervals of other components 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, 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 throughout their expected service life. However, conformance to equipment builder specifications and requirements is essential to ensure the acceptance and commercial feasibility for the product. 

BENCH TESTING 

The use of effective laboratory 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 programme ensures that the best overall candidates are selected for subsequent evaluation in real-life testing. ExxonMobil utilizes a range of tests to evaluate specific attributes.

One such test is the hot tube test, used to assess the thin film oxidation properties of a candidate lubricant. 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).

In Figure 1, the lubricant candidate demonstrated better protection against deposit formation than 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.

Figure 1: Typical hot tube test results, demonstrating that the candidate lubricant exhibited good protection against deposit formation

Another important parameter measured by ExxonMobil research is the oxidation/nitration properties of the candidate lubricant.

Lubricant exposed to oxygen and/or nitrogen oxide will degrade over time, forming oxidation products that increase viscosity, and reduce lubricant life. One of the methods used to assess the potential performance of a candidate is the 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 a catalyst. The viscosity increase is then measured.

Typical results are shown in Figure 2. Those oils with the lowest viscosity increase 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.

Figure 2: Lubricants with the lowest viscosity increase in the bulk oxidation test tend to demonstrate extended oil drain capabilities in the field
 

University Testing programme 

Further to these in-house tests ExxonMobil also entered in a programme sponsored by the US Department of Energy, including representatives from prominent universities in the USA and a key gas engine equipment builder. The aim was to evaluate various piston hardware options and lubricant technologies to successfully reduce friction in the piston ring pack.1

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 brake specific fuel consumption (BSFC) were measured at two load points – 70 per cent and 100 per cent for each test oil. The BSFC results are shown in Figure 3.

Figure 3: BSFC performance of a Waukesha VGF F18 test engine at two load points – 70 per cent and 100 per cent

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. 

High Severity Engine Stress Test 

Fuel consumption however is not the only consideration in evaluating the effectiveness of an engine lubricant. Engine durability is perhaps the primary consideration in selecting the optimum oil for any engine.

As lubricant viscosity decreases, the impact on engine durability must be considered. The majority of natural gas engine oil applications now utilize SAE 40 oils. SAE 30 oils are the most reasonable next step for fuel efficient commercial gas engine oils. SAE 20 oils will provide the greatest fuel economy benefit but could have greater potential impact on
engine durability.

Once a candidate oil has been successfully evaluated in all relevant laboratory 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, with a reduced sump volume, elevated operating temperatures, and an air to fuel ratio set for maximum oxidizing and nitrating conditions to put the oil under maximum stress.

Under such extreme operating conditions, any weaknesses in the lubricants 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. End-of-test used oil wear metal levels are also measured. Engine durability testing of a lower viscosity candidate in the high severity engine test showed equivalent wear performance to the SAE 40 reference (Figure 4).

Figure 4: Liner wear step comparisons showed a similar wear performance between candidate and reference lubricants

 

Real world durability testing 

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:

  • Engine durability impact of an SAE 30 lubricant over an extended period of time
  • Extended oil drain capability
  • Fuel efficiency improvement 

Engine durability testing was conducted in two new 16 cylinder, 170 mm bore gas engines operating on clean, natural gas in gas compression service at 95-100 per cent 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 and bearings.

The test was conducted over 7000 hours at full load. An intermediate boroscopic inspection was conducted on both units at 4000 hours and a final inspection was conducted at over 8000 hours and two power cylinder assemblies were removed for further inspection.

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 5 & 6).

Figure 5: Kinematic viscosity results from used oil analysis in test engines
Figure 6: Oxidation of used oil in test engines (as measured by FTIR in units of absorbance/cm)

All used oil parameters were satisfactory throughout the test and well below the OEM 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 SAE 30 lubricant candidate and its extended oil life relative to the SAE 40 commercial reference.

The extended oil drain capability was also evaluated in both units. Used oil analysis and monitoring continues, so Unit 254 has surpassed 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 around 7000 hours due to elevated lead caused by the lube oil cooler metallurgy problem. Figures 7-9 show key used oil parameters measured during the test period.

Figure 7: Kinematic viscosity results from used oil analysis in test engines
Figure 8: Oxidation results from used oil analysis in test engines
Figure 9: Nitration results from used oil analysis in test engines

It was also important to conduct fuel efficiency confirmation testing in the field. This was conducted on two new, 16 cylinder, 170 mm bore gas engines operating with clean, natural gas in gas compression service at 95-100 per cent load (Units 275 and 276). Each test engine was equipped with a temperature and pressure compensated fuel
consumption meter.

An additional oil storage tank and associated piping was installed to facilitate oil switching between test cycles. The experimental design included four cycles, alternating between candidate and reference oils in Units 275 and 276. This enabled a rigorous statistical analysis of
the data.

Each test cycle was approximately 500 hours. Pressure and temperature compensated fuel consumption readings, and engine/compressor operational data (e.g. speed, compressor discharge pressures and temperatures, and air manifold pressure) were recorded in a data acquisition system at five minute intervals throughout the test. Two test oils were evaluated, a commercial SAE 40 reference and the
SAE 30 candidate.

Figure 10 provides an overall comparison of fuel consumption in Units 275 and 276 when operating with the SAE 40 reference and the SAE 30 candidate oils. 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 per cent.

Figure 10: Comparison of fuel consumption between candidate and reference lubricants

Several variables were taken into consideration during the statistical analysis, including engine load, ambient conditions and humidity. Each variable was analyzed statistically to determine the magnitude of its impact on the flow measurements.

Ambient pressure and temperature are needed to convert actual fuel flow measurements to standard conditions (15.6 °C 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.2 

New lubricant, greater engine productivity 

An in depth understanding of gas engine lubrication issues as reported from in service applications, in liaison with equipment builders, has enabled ExxonMobil to successfully target a new synthetic lube oil product performance profile aimed at satisfying the increasingly severe lubrication needs of modern gas engines.

A structured development utilizing effective and proprietary screening programmes, 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 per cent versus conventional natural gas engine lubricants, the new Mobil SHC Pegasus (SAE 30 candidate) can benefit gas engine operators by improving the overall productivity and contributing to the sustainability of their operations.

1 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

2 About the Influence of Ambient Conditions on Performance of Gas Engines’, Position Paper by the CIMAC working Group “Gas Engines”, March 2009

The article is based on a paper ‘Next generation gas engine oils for improved sustainability in the power generation market’, which was presented at POWER-GEN Europe, 8-10 June 2010, Amsterdam, the Netherlands.

Appreciation goes to the authors of the original paper Kathleen H. Tellier, ExxonMobil Research & Engineering Company, USA, Gilles Delafargue, ESSO SAF, France and Thomas Dietz and Kevin Harrington, ExxonMobil Lubricants and Petroleum Specialities Company, USA. 

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