Low Speed Pre-ignition (LSPI), Oxidation and Wear

Published September 2, 2013

Challenges for Gasoline Direct Injection Turbo-charged (GDI-T) Engine Technology

Applying GDI plus turbo-supercharger (turbocharger) intake air boosting technology to the typical light duty passenger vehicle is being adopted by light-vehicle engine builders globally. GDI and turbocharging are enabling technologies that allow engines to operate at low speed and high loads for optimal fuel efficiency. The direct spraying of fuel into the cylinder improves cooling from combustion allowing higher compression ratio and in engine torque as delivered by the turbo charger. The higher torque from the turbo charger allows smaller engines (3 and 4 cylinders to generate equivalent torque, power and performance of previous larger engines (6 and 8 cylinders). The greater power is also delivered with improved efficiency since the smaller engine has less friction due to fewer pistons, bearing and less weight of older larger engines. However, often with new technology comes unintended consequences that need to be addressed. GDI engines with turbocharging (GDI-T) increase soot-like particles in the oil that can increase engine wear. Higher thermal loads increase engine oil stress from oxidation and nitration. Pre-ignition at low engine speeds (LSPI) can lead to mechanical damage to engine pistons and connecting rods. Vehicles with high technology GDI-T engines need a matching high performance engine lubricant for maximum protection against premature failure.

Watch: Low Speed, Pri-Ignition
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Recent research illustrates how important the GDI plus turbo practice is becoming in Europe, with implications for other world regions. GDI penetration will reach 65% to 67% of new European gasoline-engine light vehicles by 2018. The larger-vehicle segments (D, E, and F) will have GDI as standard equipment by 2018. The smaller-vehicle segments (B and C), which are more cost-sensitive, will experience a more gradual penetration. The vast majority of Europe’s GDI cars will also be turbocharged (an estimated penetration of 90% to 92% by 2018). GDI system annual unit volume in Europe will triple between 2010 and 2018–welcome news for suppliers operating in this space.

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The GDI/turbo combination is already present in the North American and Japanese markets. Forecasts for new car sales in North America show GDI technology will reach 60% to 65% market penetration by 2018 similar to the levels in Europe.

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GDI technology will penetrate all other global markets.
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Watch: What is LSPI?
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The cost is substantially higher than a naturally aspirated engine with port injection. European CO2 emissions regulations have seen OEMs adopt GDI-t engines across all major manufacturers from Peugeot to Porsche. Japanese buyers like high-tech engines and pay high gasoline prices. Several of Japan’s OEMs have begun including GDI engines in their product line along with their HEV powertrains to improve their fleet fuel economy. Conversely, North American buyers are more value-driven and enjoy lower gasoline prices, so their adoption of costly higher-tech/fuel-saving powertrain features tend to be slower. However, GM and Ford have aggressively implemented GDI engines across their product line.

Original equipment manufacturers (OEMs) are trying to solve the unique technical challenges of GDI. For example, the kinetics and gas flow dynamics in the combustion chamber yield rich spots of incomplete combustion so that carbon particulate formation and engine-out particulate emissions are higher than for port-injected gasoline engines. As regulated particulate emissions limits tighten toward zero, the excess particulate problem must be addressed. Regeneratingsoot traps in the exhaust path–standard practice with highway diesel engines in key industrial countries–is one possibility.

Another challenge related to GDI soot formation is that carbon particulates can hide in crevices and get washed down cylinder walls or enter the crankcase via blow-by, causing problems with lube oil.

The active surfaces of these carbon particles readily absorb key additives in lube oil that minimize oxidation and wear, compromising lube performance. These carbonaceous particles also tend to be abrasive increasing wear on critical engine parts such as cams and valve lifters. Such suspended carbon particles can reach 1% to 3% of the total lube oil mass. Lube oil blenders and lube additive suppliers, such as Lubrizol, are developing measures to offset these GDI side effects.

Some GDI engines also have an unwelcome tendency toward pre-ignition, or spontaneous combustion before normal spark ignition, especially at lower speeds coupled with higher loads and high cylinder pressures. This unwanted phenomenon is called low speed pre-ignition (LSPI). Combustion events occurring before they are wanted or expected can hammer the mechanical internals of gasoline engines (evident with severe knocking), because pistons and connecting rods are rising when this downward combustion “bang” occurs. In the worst case, ongoing pre-ignition can lead to broken piston rings, damaged ring lands and/or bent connecting rods. The mechanism of GDI pre-ignition is not yet fully understood. Multiple parallel strategies to mitigate GDI pre-ignition involve lube oil/lube additives, engine mapping (spark ignition and fuel injection event timing) and mechanical changes.

Low speed pre-ignition (LSPI) can in severe cases lead to engine damage such as a broken piston as shown above.

The engine operating area where LSPI occurs is very close to where fuel economy, performance and drivability are optimally balanced. Operating at relatively low RPM and constant load, such as highway cruising, the engine can slip into the LSPI zone where potential engine damage may occur.

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Low speed pre-ignition occurs very close to the engine’s optimum operating area where fuel economy, performance and drivability are balanced.

The thermal stresses in GDI/turbo engines with high specific power ratings (hp/liter) are formidable, so dedicated external lube oil coolers are recommended. Also, many OEMs arrange a jet of lube oil to continuously hit and cool the underside of each piston crown. This heat stress could lead to thermal breakdown and in extreme cases, coking of lube oils if not properly managed. Lubrizol additives can help lube oil better cope with the thermal demands of high-output GDI/turbo engines.

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