The high load pressures of modern gas engine operation lead to high oil stress, which can quickly impede operational efficiency. Thijs Schasfoort, a product application specialist at Shell Lubricants Companies, explains how long life, high performance lubricants can optimize engine efficiency and translate into maintenance savings.
Although gas engines can enable power operators to realise high conversion efficiency during power generation, increasing demand for high availability and reliability of power at lower costs per unit, places a great deal of pressure on engine performance. Operators need to be able to meet these challenges or else risk facing costly accelerated component wear and unscheduled stoppages.
Modern gas engines in power generation applications can often convert more than 40 per cent of the fuel energy into electricity. In addition, they can convert approximately 50 per cent of the fuel energy into valuable heat, providing operators with an extremely high conversion rate. However, high load pressure can lead to high oil stress, which can quickly impede engine and operational efficiency. Therefore, engines must be correctly maintained and lubricated in order to meet oil stress levels and ensure efficient and uninterrupted performance.
Although gas engine lubrication can be a complex subject, with the right technical support and expertise, gas engine operators can effectively manage lubrication to improve performance. Starting with an understanding of lubrication essentials. The role of the lubricant in an engine is to provide more than just lubrication. Lubricants must also cool the pistons and bearings, provide protection against corrosion, minimize running-in and start up wear, and help prevent the formation of deposits on the engine’s internal workings and components.
In a gas engine, there are a number of particular challenges that the lubricant must meet; both technical and operational. Firstly, the ignition and combustion process within a gas engine needs to be controlled. If the ignition of the air fuel mixture is uncontrolled, then shock waves (often referred to as detonation or knock) with high heat release can occur. These can cause mechanical and thermal overloading of combustion chamber components and will eventually result in serious damage to the engine.
Figure 1. A long life, high performance lubricant plays a key role in generating long term maintenance savings for operators of gas engine-based power plants
Knock can be caused by a number of factors, one of them being the presence of local hot spots in the combustion chamber. Carbon deposits resulting from oxidized lubricants are potential hot spots and can be found on the piston crown and top land, the upper part of the liner and the exhaust valves. As a result, the lubricant must be formulated with a very high resistance to oxidation and the high temperatures of the combustion chamber components, thus helping to maintain a safe margin against knock.
In electronically controlled engines, a reduced margin against knock will be detected. The engine control system will then try to restore the margin against knock by adjusting ignition timing or air fuel ratio (with a negative effect on engine efficiency), or by limiting the output level (thereby affecting the production of saleable electricity and heat). The increased fuel bill and reduced production have a direct impact on the operator’s bottom line.
Good resistance to oxidation will also prevent build up of deposits at critical hot areas outside the combustion chamber. Deposits on the underside of the piston crown would hamper the heat transfer from piston crown to the lubricant, increasing the temperature of the piston crown. Not only does this reduce piston crown lifetime, it also reduces the margin against knocking. Deposits behind piston rings will have an immediate effect on the wear of rings and liner, reducing overhaul intervals and limiting component lifetime.
A high performance gas engine lubricant such as Shell Mysella XL, a new long life oil designed specifically for high output four-stroke gas engines, can assist operators in overcoming these challenges, safeguarding fuel efficiency and energy production, while helping to reduce maintenance costs.
The oil drain interval of Mysella XL is more than twice as long as the oil drain interval of industry standard life oil products. This is especially beneficial for operators whose engines are running on unmanned sites. Simply put, previously, a service engineer visiting the installation for maintenance purposes would also carry out an oil drain. Now that engine manufacturers have succeeded in increasing spark plug lifetime significantly, the frequency of maintenance visits has been reduced and oil drain intervals of standard life oils are no longer synchronised with the scheduled visits. This means that the service engineer would have to make special visits purely to carry out an oil change. In many cases, the cost of man-hours attributed to the oil change could exceed the cost of the oil volume being changed. With Mysella XL, the oil drain interval is long enough so that it can be synchronised again with the engine maintenance intervals.
The amount of energy to which one gram of lubricant is being exposed to over its lifetime is the so-called ‘oil stress’. In modern gas engines the oil is severely stressed. Higher specific outputs and relatively smaller sump sizes mean that a given amount of oil will be exposed to more energy before it is replaced.
Figure 2. Rolls Royce G4 engine: clean piston after 3000 hours of operation on Shell’s advanced lubricant, Mysella XL, designed especially for high-output four-stroke gas engines
Engine manufacturers have been very successful in reducing lube oil consumption, which can be as low as 0.1 g/kWh. This is essential for the lifetime of the exhaust gas catalyst, as well as for cleanliness of the exhaust gas boiler. Low oil consumption implies, however, a low refreshment rate of the lubricant, which means that the amount of energy a lubricant molecule will see during its lifetime is further increased.
Finally, the drive for maximizing the overall efficiency of combined heat and power installations means that the temperatures of cooling water, exhaust gas and lube oil are increased, further increasing the amount of energy that a lubricant is being exposed to before being replaced.
The high oil stress in modern gas engines requires a high performance lubricant. It is the combination of base oil and additives that determines the performance of the lubricant. One aspect of gas engine oil is the sulphated ash content, which generally represents the amount of additives in the lubricant.
Low ash oils
Figure 3. Deutz engine: a deposit-free cylinder head after 11 000 hours of operation on Mysella XL. Advanced lubricants eneble the operator to synchronize oil drains with engine maintenance intervals
All major engine manufacturers now prefer low ash oils for their engine portfolio, i.e. a sulphated ash content of less than 0.5 per cent. The amount of sulphated ash producing additives in the oil must be enough to protect valves and valve seats, provide acid neutralisation and provide wear protection of gears, bearings, liners and piston rings. However, it must also be sufficiently low to prevent knock, liner polishing and scuffing (as a result of ash deposits on the piston land) and fouling of spark plugs, turbochargers, catalysts and exhaust gas boilers.
Finding the correct balance of lube oil ash is crucial to the successful operation of a gas engine. The composition of the additives is also important – the ash should be both soft as well as easily removable.
Shell Mysella XL has been formulated with these requirements in mind. Extensive field trials and around 100 field references have confirmed that Mysella XL’s new technology resists oil stress better than previous products. Its characteristics are beneficial for prolonging filter lifetime and reducing deposits in intercoolers, exhaust gas boilers and catalysts. It provides the operator with extended oil and engine component life, even in very demanding engines with high piston temperatures. It allows synchronization of oil drain intervals with engine maintenance intervals.
Ultimately, a long life, high performance lubricant plays a key role in optimizing engine efficiency and can translate into long term maintenance savings.