The global drive to lower emissions is increasing demand for gasoline direct injection (GDI) technology. When coupled with a turbocharger (TGDI), this can increase power, notably improve efficiency, and in turn reduce the level of greenhouse gas emissions.
One of the challenges for GDI and TGDI engines is the increase in fine exhaust particles (PM2.5). The World Health Organization has stated that there is a strengthened link between cardiovascular and respiratory ill health and PM2.5. The response from legislators worldwide has been a tightening of tailpipe emissions limits, with a focus on both particulate matter (PM) and particulate number (PN) for gasoline engine systems.
Such a focus on global emissions is driving the use of gasoline particulate filters (GPFs); an after-treatment device whose role is to remove small but harmful particulate matter from the vehicle’s exhaust gases.
During the life of a vehicle, the GPF collects a sizable portion of these particulates, but also captures additional combustion by-products such as ash, which can lead to GPF blockage. One source of ash is the engine’s lubricant. Whilst the oil is performing its primary role of engine lubrication, residual amounts of lubricant are inadvertently burnt during combustion creating lubricant ash. Unlike particulates, ash cannot be removed from the GPF whilst fitted to the vehicle and therefore continues to accumulate over time. The more ash present in the lubricant, the faster this process occurs, eventually blocking the filter.
By incorporating a lower ash level from the outset, a correctly formulated lubricant can be designed to minimize GPF blockage, so enabling it to continue working effectively for the entire life of the vehicle.
While particulate filters have been used in diesel cars for many years now, there are notable differences between diesel particulate filters (DPFs) and GPFs, which have a direct impact on the engine’s lubricant. Materials used in GPF construction are typically different to those used in DPFs, and the associated porosity of both filter variants can also differ. Additionally, the exhaust temperature passing through a GPF is notably higher than that found in a DPF. Ultimately, these differences offer new challenges with respect to formulating high performance lubricants that satisfy the individual needs of these alternative technologies.
The lubricant used in vehicles fitted with GPFs is vitally important. In ordinary vehicle usage, a small amount of lubricant finds its way above the top piston ring and into the combustion chamber and is subsequently burnt. The byproducts of burning the lubricant vary depending on the lubricant’s specific chemistry; those by-products can have a negative impact on exhaust after-treatment systems such as GPFs and DPFs.
If the chemistry within a lubricant contains sulphated ash, phosphorus and sulphur (SAPS) above OEM-specified limits, then the levels of ash created during combustion can begin to fill up the channels within a GPF. This will eventually block the filtration channels completely, causing large expense to the vehicle owner.
SAPS are important components in the formulation of a lubricant, providing wear protection and cleanliness performance amongst many other benefits. Creating a lubricant with sufficiently low levels of these components, whilst ensuring an OEM’s precise performance requirements are met, requires a different formulating approach and bespoke additive chemistry.