Turbocharged gasoline direction injection (TGDI) engines are supporting the delivery of fuel economy targets in today’s passenger vehicles. However low-speed pre-ignition (LSPI) can occur, potentially causing significant engine damage. Here we discuss how advanced lubricants can lessen the chances of LSPI occurring while continuing to deliver increased fuel economy.
Over recent years, increased fuel economy, along with reduced greenhouse gas emissions, has been the biggest driver in the automotive industry. This has forced Original Equipment Manufacturers (OEMs) to develop innovative hardware solutions, typically by producing more power and torque from smaller engines.
Together with turbocharging and gasoline direct injection, TGDI engine configurations allow for some extremely power-dense engines—offering a great balance of power and fuel economy. Today, TGDI engines are a major means by which OEMs are delivering on efficiency and fuel economy targets, and advanced lubricants can help ensure that success.
GDI engines offer higher power density in comparison to traditional port fuel injected engines. This means GDI engines can be made smaller and can operate at higher loads and lower speeds in order to achieve maximum fuel efficiency. OEMs have further incorporated turbocharger technology in order to recoup energy that would otherwise be lost with engine exhaust. GDI and turbochargers complement each other well, however these downsized TGDI engines operate at such extreme conditions—higher temperatures and higher pressures—that they have been known to experience a phenomenon called LSPI.
LSPI is an uncontrolled combustion event that takes place in the combustion chamber prior to spark in GDI engines. It is believed that LSPI is caused by droplets or particles in the combustion chamber—combinations of fuel and oil—that ignite prior to spark, resulting in uncontrolled combustion. It can lead to extremely high pressures in the combustion chamber, high pressure rise rates and pressure wave reversals that are violent enough to result in catastrophic engine damage. Most commonly, damage includes broken pistons and rings. When this occurs, the engine must be replaced.
Many of the hardware design and calibration strategies to mitigate LSPI have an adverse impact on fuel economy. For example, it has been shown that increasing piston ring tension can reduce LSPI, but it increases engine friction, which in turn reduces fuel economy. Overfuelling, or using an air-fuel ratio richer than the stoichiometric ratio also has been shown to mitigate LSPI, but also reduces fuel economy.
Our View
One solution to LSPI which does not reduce fuel economy is the use of engine oils that have been specifically-designed for TGDI engines. It's important that additive companies, automotive manufacturers, third party laboratories and the wider industry work closely together to understand and develop specific formulations that effectively mitigate LSPI. Differing additive technology can lessen the chances of LSPI occurring, highlighting the importance of additives that can help stop the phenomenon. It is critical to remember that a given lubricant does not cause LSPI; however a lubricant can effectively reduce the likelihood that LSPI will occur.The continued focus on LSPI prevention chemistry is a key element, highlighting the importance for performance additives, performance polymers and base oils being carefully formulated to work in harmony with advanced engine hardware designs, delivering increased fuel economy and reduced emissions alongside reliable and robust protection.
To learn more about LSPI and the importance of the lubricant, contact your Lubrizol representative.
