Growing concerns about the engine-destroying effects of low speed pre-ignition, likely caused by droplets of oil and fuel mixed together and auto-igniting in the combustion chamber, are pushing automakers and lubricant and fuel formulators to seek solutions. Jon Pilbeam, Afton Chemicals senior research and development engineering specialist, based in Bracknell, Berkshire, U.K., described the new challenge of low speed pre-ignition and the role lube formulations play at the ICIS World Base Oils & Lubricants Conference in London in February.
Low speed pre-ignition is spontaneous ignition that occurs at low engine speeds before normal spark ignition in downsized direct fuel injection turbocharged engines, Pilbeam explained, and it is a very serious matter that can destroy engines.
Downsized gasoline engines are a direct response to legislation mandating lower carbon dioxide emissions and improved fuel economy, and such legislation is becoming more prevalent in both mature and emerging markets. But unfortunately, the most fuel efficient [operating] area for a small engine is also the area where it is most susceptible to LSPI, Pilbeam said. As a result, car manufacturers calibrate their engines to avoid LSPI by injecting more fuel than necessary, with detrimental effects on fuel economy. LSPI also drives automakers to return to stronger engine components, resulting in higher cost and weight, degrading carbon dioxide.
What happens when LSPI occurs? The oil droplet theory is currently the prevailing theory to explain what is happening, Pilbeam continued. Researchers believe that droplets of lubricant and fuel mix together in the combustion chamber, and the inclusion of the lubricant lowers the auto-ignition temperature of the droplet.
Numerous factors influence the occurrence of LSPI. Engine architecture is likely most important, particularly the spray direction of the fuel into the combustion chamber. Side-mounted wall-wetting injectors are worst due to impingement of fuel onto the cylinder wall, said Pilbeam. Engine operation is also a critical factor. Boosts in pressure increase combustion chamber temperature, and lower engine speeds allow more time for a reactive fuel-oil mixture to auto-ignite.
Fuel composition has been shown to influence LSPI as well, Pilbeam noted. Less volatile and older fuel can increase LSPI as a larger fraction remains with the lubricant on the cylinder wall, and high aromatic content can increase LSPI.
Turning to the influence of lubricants, Pilbeam said that recent research by Takeuchi and others has shown that base oils can make a difference. The researchers found that higher API Group base oils correlated with lower LSPI frequency.
Lube additives were also fingered by the researchers. Higher calcium levels correlate with higher LSPI frequency, and similar LSPI frequencies are observed for different types of calcium. On the other hand, said Pilbeam, increasing phosphorous and molybdenum levels through ZDDP decreased LSPI frequency.
Of particular note, Takeuchi and his colleagues found a clear correlation between oxidative reactivity and frequency of LSPI, Pilbeam said, but there was no clear correlation between lubricant volatility and LSPI. Finally, the presence of wear metals in the lubricant increased the frequency of LSPI, and used oil may prompt higher LSPI frequency.
Tests are being developed by the industry to evaluate the propensity for LSPI, Pilbeam said, including one for the next passenger car specification GF-6 and another for General Motors next-generation Dexos1. Optimal [lubricant] formulation strategies will be used to reduce the risk of LSPI, Pilbeam concluded. We must balance this new demand against the existing plethora of requirements through the use of relevant industry tests.