Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
To enable these smaller boosted engines to be used as part of OEMs’ fuel economy and CO2
emission reduction strategies it is
clearly essential to fully understand the mechanisms and causes of LSPI. And, because these combustion events are widespread and lubricant design can have an impact on LSPI, Infineum began its own research in this area.
The causes of LSPI Initially it was thought that pre-ignition sources were located at hot spots in the cylinder, or were from soot accumulation. However, further optical investigation revealed that pre-ignition occurred randomly throughout the combustion chamber, which means surface ignition is not the only source of LSPI. Our current thinking is that the auto-ignition of oil droplets or deposit particles is probably the major cause of LSPI.
When the fuel is injected directly into the combustion chamber, it dilutes the oil film lining the cylinder. This fuel dilution reduces the surface tension and viscosity of the oil, causing an oil-fuel mixture to accumulate in the upper reaches of the piston top land crevice. The mechanical energy of the upstroke during compression pushes droplets into the combustion chamber, where they vaporise and can auto-ignite prior to spark ignition.
Lubricant formulation
In addition to investigating the causes of LSPI, Infineum is studying the effects of lubricant composition to better understand how it might contribute to the suppression of LSPI events. Work has already been undertaken to investigate the effects of using different types and levels of base stocks and additives.
The initial focus of the research has been to evaluate the effect of using different detergent chemistries, Zinc dialkyl dithiophosphate (ZDDP) types as well as varying the levels of other additives. Some chemistries have been found to have major credits while others have significant debits on LSPI events.
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This research has been very challenging. The destructive nature and random occurrence of LSPI has necessitated the development of sophisticated testing and simulation methods in an attempt to fully investigate the phenomenon. Despite the challenge, the initial findings from this research have identified a number of lubricant and fuel chemistries and hardware and operational conditions that can suppress or increase LSPI activity.
Test developments Work to date has stimulated discussions about the possibility of new chemical limits for future OEM and industry lubricant specifications. Infineum’s position here is clear: achieving performance in an engine test that correlates to the field is the preferred method because arbitrary chemical limits do not always result in relevance in the field. Introducing a new test rather than new chemical limits will allow formulators to use their full expertise to find the best solutions to overcome the LSPI challenge. For example, the deleterious effects of one component may be mitigated by other additive chemistries and combinations, which can be optimised to deliver the required engine protection. Tightening chemical limits has the potential to stifle creativity and technical innovation.
Controlling LSPI will have a very significant impact on the composition of next generation lubricants, required to meet OEM and industry specifications. Our collaborative efforts with other stakeholders to develop a meaningful engine test to measure the effects of lubricant composition on the occurrence of LSPI will continue.
Low-viscosity formulations A second area of focus for Infineum research and development is on the formulation of lubricants that help to meet the latest fuel economy legislation without compromising engine durability.
Fuel economy derived from advanced lubricant technology is an attractive option for OEMs because it comes at a smaller cost than redesigning hardware. Reducing viscosity, which in turn reduces engine friction, is clearly an effective way for lubricants to contribute to vehicle fuel economy performance. We already see SAE 0W-16 in the market, and some OEMs are beginning to look at viscosities as low as SAE 0W-8, or even 0W-4. In this low viscosity era it is becoming increasingly important to understand the implications this has on the formulation envelope and on hardware protection.
Moving to ultra-low viscosity Lowering a lubricant’s high temperature high shear (HTHS) viscosity might result in fairly small improvements in fuel consumption. But, the potentially significant fines for non- compliance to emissions and fuel economy legislation mean OEMs value every contribution to help them meet their fleet- wide efficiency targets. This means the trend to ultra low viscosities is extremely likely to continue.
Infineum found that ZDDP type can impact LSPI events. Source: Infineum International Limited
26 LUBE MAGAZINE NO.128 AUGUST 2015
However, because vehicle population age and OEM market share both impact viscosity grade trends, it will take some years before these new viscosity grades make up a significant portion of the market. Take North America as an example, here SAE 5W-30 is currently the most common viscosity grade, but it took
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