Kathy Tellier, ExxonMobil Research & Engineering, USA
As energy prices fluctuate through market cycles, owners and operators of natural gas engines will seek ways to reduce their energy costs. This is especially true in power generation markets where fuel costs directly affect profitability.
In addition, the power generation industry is looking for options to improve their productivity and sustainability. In many cases, innovative lubricants with synthetic-based formulations can offer both performance and economic advantages through extending lubricant performance cycles, while simultaneously cutting lubricant consumption, used oil volumes and operating expenses – all contributing to the promotion of greater sustainability.
Engine designs, operating conditions, customer’s needs and environmental factors, however, continue to place high demands on lubricants for natural gas engines.
Recent trends in engine design have raised compression ratios and turbocharger pressures to increase power output and efficiency. Meanwhile, with increasing concern for emissions, the amount of oil available in the engine to lubricate piston rings and cylinder liners has been reduced. Pressure on operators to reduce lubricant-related maintenance costs has further raised the need to maximize the effective life of the lubricant and thereby to cut engine downtime and oil change labour, costs along with waste oil volumes and disposal costs. Finally, the rising cost of energy makes owners and operators of natural gas engines welcome any opportunity to improve energy efficiency.
Last year, in association with PennEnergy, ExxonMobil published a research report on gas engine efficiency and productivity measures in the energy sector1. Energy industry experts indicated that efficient lubrication is a factor in maximizing productivity and profitability. Over half of those polled directly associated maintenance relating to gas engine lubrication with their company’s bottom line, and two-thirds of survey respondents indicated that they felt that the energy efficiency of their industrial lubricant impacts the running costs of their company’s gas engines to a “moderate/great” extent.
In an article, published in the June 2010 issue of Power Engineering International, laboratory bench testing, in-house engine testing and preliminary field test results, which are all part of a robust product development cycle, were presented2. Here, we now focus on the continuing field test programmes that further validates the performance of the Mobil SHC Pegasus product line.
PROOF OF CONCEPT TESTING
ExxonMobil Research and Engineering Company participated in a joint research programme sponsored by the US Department of Energy entitled ‘ARES’ (Advanced Reciprocating Engine Systems), which also included Waukesha, a major gas engine OEM, Colorado State University and MIT.
The project investigated the impact of lubricants on gas engine friction loss and energy efficiency3. A Waukesha VGF F18GL was instrumented to measure the effect of lubricant on friction, brake thermal efficiency and brake specific fuel consumption. Figure 1 shows a comparison of brake specific fuel consumption (BSFC) at 70 per cent and 100 per cent load for a conventional SAE 40 grade gas engine oil (reference) vs two experimental gas engine oil candidates (SAE 30 and SAE 20).
|Figure 1: Brake specific fuel consumption results for SAE 30 vs reference|
The objective of Phase 1 was to evaluate the impact of lubricant viscosity on fuel consumption, while Phase 2 aimed to evaluate the effect of viscosity and base stock combinations on fuel consumption. Figure 2 shows a comparison of both SAE 30 and SAE 20 candidates in comparison to the SAE 40 reference oil.
|Figure 2: Brake specific fuel consumption results for SAE 30 and 20 vs reference|
The following performance objectives were established during the product development process for the next-generation gas engine oil:
- To improve energy efficiency
- To deliver excellent balanced performance through:
- – wear control – piston rings and cylinder liners; bearings; and valve seats and valve stems
- – deposit control – clean pistons; clean exhaust heat exchangers; and low impact on catalyst life
- – extended oil drain intervals vs conventional oils – viscosity control; oxidation and nitration control; and additive retention
BENCH TEST PROGRAMME
The use of effective laboratory screening tests is essential for the 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. Specific attributes can be evaluated in laboratory tests, as outlined in the following paragraphs.
Next-generation gas engine lubrication is a key component for reducing friction, thereby increasing efficiency, in several areas of the engine. The traction coefficient was measured employing the mini traction machine (MTM) traction rig – a fully automated traction measurement device. The test specimens and apparatus configuration are designed so realistic pressures, temperatures and speeds can be attained without requiring very large loads, motors or structures. The Stribeck curve generated on the MTM rig represents the lubrication regimes in an engine (boundary, mixed/EHL, hydrodynamic). The lower friction/traction coefficient of the next generation oil (SAE 30) compared to the reference oil clealry demonstrated the potential to improves energy efficiency.
Low temperature properties for a lubricant can be quantified by two key physical properties. The first test is known as the ‘cold cranking simulator’ (CCS), which measures the low-temperature, high-shear viscosity used to measure the ability of a lubricant to allow the engine crankshaft to spin during start-up. The lower a lubricant’s cold crank viscosity, the easier an engine will turn over in cold temperatures. The second key test for low temperature properties is the mini-rotary viscometer (MRV), which is used to measure the ability of a lubricant to flow toward the suction of an engine oil pump. It represents the low-temperature pumpability performance of an engine oil and can be used to predict and prevent oil starvation during engine start-up.
Both the CCS and the MRV viscosity for the Mobil SHC Pegasus 30 was found to be significantly lower than the reference oil
High temperature high shear (HTHS) viscosity is a measurement that reflects the operating viscosity temperature and shear circumstances typical of an engine operating under severe conditions. It simulates the shear rates found in piston cylinders and cam regions. The Mobil SHC Pegasus 30 demonstrated that it provided equivalent HTHS at elevated temperaturs, such as would occur in the piston ring pack area, to the reference oil.
A high temperature thermal stability test evaluates the ability of oil to prevent the formation of high temperature thin film deposits in the hot areas of the engine such as piston ring grooves. 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 (1= clean; 10= heavy).
In the test, the Mobil SHC Pegasus exhibited a much lower level of deposit formation than the reference oil. Over time, this test has proven to be an excellent indicator of deposit formation on engine components subjected to high temperatures.
Lubricant exposed to oxygen and/or nitrogen oxide will degrade over time, forming oxidation products that increase viscosity and reduce lubricant life. Several methods can be used to assess the potential performance of a candidate oil, one of which involves heating oil 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. Oils with the lowest viscosity increases in this test, such as seen with the Mobil SHC Pegasus, tend to demonstrate extended oil life capabilities in the field. This test has also been enhanced to allow evaluation of oil’s nitration resistance.
In addition to the initial fuel efficiency testing at Colorado State University and the ExxonMobil Research and Engineering 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. Energy efficiency improvement
Table 1 provides a summary of the five field demonstrations. The field demonstrations represent a wide cross section of engine makes and models and applications. Each field test was designed to evaluate specific performance attributes of Mobil SHC Pegasus, such as engine durability, energy efficiency and extended oil drain interval (ODI).
|Table 1: A summary of the five field demonstrations|
Field Test Site A
This field test was designed to evaluate the durability performance of Mobil SHC Pegasus on a Jenbacher J412 engine (see Figure 3). Used oil analysis will be conducted at 250 hour intervals and detailed engine inspections will be conducted to evaluate wear of key engine components (piston rings, liners, bearings). Used oil analysis results through 2606 oil hours is provided in Table 2.
|Figure 3: The Jenbacher J142 engine|
|Table 2: Used oil analysis for durability with a Jenbacher J412 engine|
Field Test Site B
This field test was designed to evaluate the energy efficiency performance of Mobil SHC Pegasus. Used oil analysis was conducted at 150 hour intervals. Energy efficiency was measured by dividing the power generation output in kW by the fuel flow rate multiplied by fuel heating value. Efficiency was calculated for both the conventional SAE 40 gas engine oil reference (Pegasus 805) and Mobil SHC Pegasus. The efficiency gain with Mobil SHC Pegasus over a two-month period was reported by Dalkia as 1.67 per cent.
Field Test Site C
This field test was designed to evaluate the durability and extended ODI performance of Mobil SHC Pegasus on a Waukesha P9390GL engine. Used oil analysis will be conducted at 250 hour intervals and detailed engine inspections will be conducted to evaluate wear of key engine components (piston rings, liners, bearings). Used oil analysis results following approximately 1500 oil hours is shown in Figure 4.
|Figure 4: Used oil analysis results from a Waukesha P9390GL engine|
Field Test Site D
This field test was conducted to evaluate the extended ODI and energy efficiency performance of Mobil SHC Pegasus on a Caterpillar G3516 engine. Used oil analysis was conducted at 250 hour intervals. Intermediate borescopic and end-of-test engine inspections were conducted to evaluate the impact of Mobil SHC Pegasus on liner, piston ring and connecting rod bearing wear, as well as engine cleanliness and deposit control. Fuel consumption was measured via a statistically designed set of experiments (A-B-B-A) and results demonstrated an average 1.5 per cent improvement over the conventional SAE 40 gas engine oil reference (see Figure 5).
|Table 5: The efficiency gain in a Caterpillar G3516 engine (red = Mobil SHC Pegasus, blue = reference oil)|
Field Test Site E
This field test was initiated to evaluate the engine durability and energy efficiency performance of the SAE 20 candidate. Used oil analysis was conducted at 250 hour intervals. Following approximatley 1000 hours an energy efficiency improvement of 1.13 per cent was calculated by the same method as noted for Field Test B.
The development of next-generation gas engine oils is based on several factors, the most important being understanding the market needs for the product. Based on a clearly identified customer need for lubricants that can contribute to gas engines’ energy efficiency, ExxonMobil Research and Engineering embarked upon a structured product development process. It focused on measuring key performance parameters, such as low and high-temperature properties, friction coefficient, high-temperature thermal stability and oxidation control.
It also demonstrates the important role that field testing plays in developing a next-generation gas engine lubricant. In this case, a comprehensive field test programme designed to validate the performance of the Mobil SHC Pegasus 30, which was launched to the market themiddle of last year, and a prototype SAE 20 grade in terms of energy efficiency improvements, engine durability, extended oil drain and filter change intervals.
1. Engine Efficiency, Preventative Maintenance and the Bottom Line; PennEnergy, May 2010.
2. What does it take to make a modern engine oil? Power Engineering Internatrional, Vol. 18: No.6 (June 2010).
3. Friction Reduction Due to Lubrication Changes in a Lean-burn 4-stroke Natural Gas Engine: Experimental Results; K. Quillen, R. Stanglmaier, V. Wong, E. Reinbold, R. Donahue, K. Tellier & V. Carey.
The article was co-authored by Gilles Delafargue, ESSO SAF, based in France; and Kevin Harrington and Reda Kamal, ExxonMobil Lubricants and Petroleum Specialities Company, based in the USA.
The article is based on a paper, which was presented at POWER-GEN Europe 2011, which took place in the Fiera Milano City, Milan, Italy, between 7–9 June.