Extending the lifetime of components, as well as that of oil charges, has taken on the sound of a mantra! Modern lubricants should be designed increasingly with the purpose of remaining usable for longer. The mineral oil industry is looking increasingly to better, often synthetic base oils as the simplest way of achieving this aim. In contrast to pure mineral oil refining, the molecular structures of the base oils are changed during the production process in such a way as to create positive characteristics, like an increased ageing stability or improved viscosity-temperature behaviour. Modern high-potency oxidation inhibitors based on salicylates, phenols or amines also help to retard the ageing process of the lubricants during use. Nowadays it is possible even for some high-performance industrial gear and hydraulic oils, as well as turbine oils, to remain in use for over 50,000 operating hours, or around 10 years. Nevertheless, even the best long-term lubricant has its limits.

In contrast to the aim of achieving longer lifetimes, many lubricants are still being changed even when their potential is far from being fully exploited. This means that enormous potential is being destroyed every year. If the approximately 1 million tonnes of lubricant used in Germany alone was only replaced when it was technically necessary, around 30% of that, or 300,000 tonnes/350 million litres, could be saved every year. Although more and more companies are managing their oil change intervals using trend analyses, only a fraction of this enormous savings potential is being realised. However, rather than changing oils at fixed intervals, OEMs are now increasingly recommending that oils be changed according to their condition. For this purpose, many machine manufacturers offer OELCHECK analysis kits, often as part of their spare parts ranges. The analyses not only allow oil changes to be made on a condition-dependent basis, they also allow damage to be detected at an early stage, thereby ensuring fail-safe operation of machines and systems. The monitoring of oils and motors in biogas plants, for example, which are exposed to particular risk on account of the often variable, sometimes aggressive gas compositions, prove how well this can work. Lubricant analyses are also the indispensable method of choice when it comes to using new types of lubricant for which there is no empirical evidence to indicate a possible application period.

For private individuals, good thermal insulation reduces household heating costs, while service technicians are promised that the energy consumption of their vehicles and machines can be reduced by using selected lubricants. However, given the vague nature of these promises, they are not worth very much. Ultimately the much-touted lubricants tend to be considerably more expensive; buying them should be economically profitable and result in a marked reduction in energy costs. Before there is a wholesale change in the type of lubricant used, ideally a single system should be changed to begin with, whereby the energy consumption of the system is ascertained under the same operating conditions and the values from before and after the oil change are compared.

However, comparing the energy consumption is not always possible, or may only be possible at considerable expense. It becomes very tricky when, for example, lubricant manufacturers claim that the efficiency of the main gears in a wind turbine, for instance, will be increased to such an extent that more electricity will be fed into the grid at the same wind speed as a result of reduced friction loss in the gears. The electricity feed-in from an individual wind turbine is dependent on several factors and it is difficult to verify that the lubricant has had the expected effect. The prospects of increasing the energy efficiency of engines, hydraulic systems and industrial gears with lubricants are extremely variable. The extent of the lubricants‘ influence diverges significantly. Moreover, every application must be considered individually.

To ensure the safe operation of innovative engines and fail-safe use of their exhaust gas treatment systems, appropriately designed engine oils are absolutely essential. At the same time, they should also help to reduce fuel consumption. The requirements for engine oils will be much higher in future. The latest ACEA and API specifications even include engine tests which can detect an energy saving. In practice, when using appropriately designed engines and under optimal conditions, fuel savings of between 1.8 % and 5.5 % can be achieved compared to a reference oil – usually an SAE 15W40 or 20W50 – by switching to another type of oil with a different viscosity.

In principal, an engine runs more smoothly with a low-viscosity oil thanks to reduced pump and splash losses, and therefore also consumes less fuel. As a result, the trend is toward thinner and thinner engine oils. For example, an SAE 0W16-class engine oil has already been designed for one type of engine, and it is likely that engine oils in SAE classes 0W12 and 0W8 will soon be available on the market. However, there are also limits to this development.

On the one hand, the low-viscosity engine oil must be able to form a resilient fi lm in order to reliably protect the moving parts against friction and wear. On the other hand, there can be problems with evaporation loss (to find out more see OELCHECKER Spring 2015), which usually increases as the viscosity of the base oils decreases. Some of the engine oil which has evaporated as a result of increased engine oil temperatures in the oil sump is led through the crankcase ventilation system into the air-fuel mixture and burned with it. The combustion residues can then impair the effect of catalytic converters or soot filters. It is also true that the lower the loss of oil due to evaporation, the lower the oil consumption is and the more stable its viscosity characteristics are. However, low-viscosity engine oils tend to display greater evaporation loss, which can lead to increased viscosity during operation. The originally much-praised low-friction characteristics of the oil therefore decline as a result, while the fuel and oil consumption increases. The relatively low-viscosity engine oils in viscosity class SAE 0W-30 that are currently available on the market can only be achieved using thinner synthetic base oils.

If the viscosity is to be reduced even further in future, this will present a particular challenge for lubricant manufacturers as they will have to endow these engine oils with characteristics that will ensure low evaporation loss, as well as all of the other characteristics that are important for the safe operation of the engines. However, products designed in this way will certainly come at a price. And even the perfect engine oil will only be able to lower the fuel and oil consumption significantly if the driving is suitably prudent. It should also be taken into account that the newly developed oil types can no longer be used wholesale for older engine types, as their components are not designed for such low-viscosity oils.

The performance of hydraulic systems is increasing, and the systems themselves are getting smaller. Reduced gap tolerances in valves, pumps and engines and better surface qualities allow higher operating pressures and enable system components to work more precisely. At the same time, ever increasing system availability, even under extreme operating conditions, is required. However, smaller oil charges, higher pressures and increased operating temperatures create intensified working conditions which a conventional mineral-oil-based hydraulic oil in class HLP or HLPD can barely withstand. All lubricants and power transmission media also age during their lifetime. This ageing process is caused in part by oxidation. High temperatures, long service lives, high pressures and in some cases even contaminants and wear particles all accelerate the ageing process. In order to delay ageing for as long as possible, the fluids contain antioxidants. However, these can also decompose during oil use, just like extreme pressure agents or anti-wear agents. With the addition of extreme operating conditions, temperatures increase as a result of power dissipation, which further accelerates oil ageing.

And throughout this process, the viscosity of the oil also decreases, which is a key factor affecting the efficiency of the hydraulic system. Extensive tests with vane pumps, gear pumps and piston pumps have demonstrated that the viscosity of the hydraulic fluid has a considerable effect on the efficiency of the pump. Its hydraulic efficiency is dependent on the oil viscosity at the pump inlet, as well as on the pump speed and pressure. Therefore, the oil viscosity not only affects the efficiency of the pump as a whole but the energy consumption as well. For this reason, the viscosity of the hydraulic fluid should remain as constant as possible for the entire period of operation, from start-up right up to high-load operation. To ensure this, increasingly hydraulic oils with multi-grade characteristics of type HVLP or HVLPD are being used, which, in contrast to HLP oils, have a very high viscosity index of almost 200. In principle, these types of oils have a positive effect on energy consumption because even at temperatures of over 80 °C the minimum viscosity is not exceeded. However, the relatively high proportion of viscosity index improvers often present in the multi-grade oils can impair the air separation ability to such an extent that cavitation damage starts to appear. To avoid this as much as possible, hydraulics manufacturers allow oils that are thinner, with the result that the maximum permissible temperature for use of these types of oils must be reduced. This is the only way to be on the safe side in case of any shearing of the viscosity index improvers. In practice, the use of these multi-grade hydraulic oils at low start-up temperatures is ideal, but at very high operating temperatures they reach their limits. If the fluid becomes too „thin“, the hydraulics in construction machinery, for example, become more difficult to control or start to work imprecisely.

In an attempt to resolve this dilemma, one of the leading international additive manufacturers has designed a special package of hydraulic oil active agents that is used by a number of lubricant manufacturers. The ready-made products, which are usually based on base oils in Groups I or II, meet the requirements of DIN 51524/3 for HVLP hydraulic oils. The new technology allows low-viscosity hydraulic oils to be used over a wider operating temperature range. This means that the efficiency of the hydraulic systems is increasing, while the energy consumption of the systems is decreasing. Compared to a conventional HLP hydraulic oil, the efficiency of the machine overall can only be increased by changing the type of oil. In practice, this means higher hydraulic pressures under full load, more precise reaction of the system and, above all, reduced energy consumption, meaning a reduction in the resultant oil temperature and energy-intensive cooling output. The manufacturer of the additive package expects a potential efficiency increase of around 5% and has demonstrated this in mobile hydraulic systems with extensive field tests. Whether using one of the somewhat expensive fluids included in the innovative additive package pays off simply on account of the reduced energy consumption must be considered on a case-by-case basis.

Specially designed gear oils are playing an increasingly important role in increasing the efficiency of manual transmission, automatic and hypoid vehicle gears, as well as industrial gears. However, this cannot be achieved, as is customary with engine oils and hydraulic oils, by using a thinner oil in conjunction with structural changes. To improve the energy efficiency of industrial oils, ingenious mixtures of different types of base oils and synergetic additive combinations are used. The gears should be able to transmit power as efficiently as possible. The level of efficiency is equal to the ratio of output power to drive power, while the output power equals the difference between drive power and power dissipation. If at a certain output power the efficiency level increases and power dissipation decreases, then less energy is required to drive the gears. This can be checked, for example, by measuring the power consumption. Minimising power dissipation, which is usually reflected by a lower temperature in a gearbox without additional cooling, also serves as a guide to increasing energy efficiency. A considerable reduction in power consumption and operating temperature is therefore indicative of reduced friction, lower power dissipation and higher gear effi ciency. At the same time, this type of lubricant oxidises considerably less, not least because of the lower operating temperatures, and can therefore be used over a longer period.

The effect of a gear oil on gear efficiency can be examined in practice using, among other things, the FZG‘s (Gear Research Centre at the Technische Universität München) tension test. Moreover, the programme WTplus, developed at the FZG as part of research projects commissioned by the FVA, is available for calculating the losses and thermal behaviour of complete gear systems. However, the extent to which the respective gear oil can positively affect the efficiency, and therefore efficiency rating, of a gear type in practice can only be determined by practical application, taking into account the individual operating conditions. It is beyond question that gear oil can affect efficiency, and therefore energy efficiency. However, it is important to bear in mind the relationships in individual cases. The percentages that are often used in lubricant sales to demonstrate savings potential usually only refer to the efficiency or power dissipation in the gearing. Here is a simplified example: if the drive power is 100 kW and the output power is 97 kW, then the power dissipation is 3 kW or 3% of the drive power. If a very high reduction in power dissipation of 25% is achieved, then this 25% does not relate to the 100 kW, but just to the 3 kW of power dissipation. This means that the gears need 0.75 kW or 750 W less power. It is doubtful in this case that this 750 W reduction in friction loss would be noticeable in the form of a lower gear oil temperature. Even here, the individual conditions must again be considered carefully and all factors taken into account.

Almost all lubricants can remain in use for longer, provided their use is accompanied by lubricant analyses and the length of intervals between changes are condition-dependent. Compared to conventional oils, specially designed lubricants have the ability to positively affect the energy efficiency of systems. However, every application must be considered individually and the cost-benefit ratio carefully calculated. As these decision-making processes can often be very complex, the OELCHECK advisory service is on hand to help.