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Catastrophic damage to a piston and rings due to low speed pre ignition (LSPI) events. Imagine what the cylinder bore looks like. (Mahle image)

Mahle tests for 'mega knock' in downsized boosted engines

Low Speed Pre Ignition (LSPI) is an abnormal combustion event with the potential to cause catastrophic damage to pistons and related components in downsized engines.

Combining gasoline turbocharged direct injection (GTDI) with downsized cylinder displacements has enabled automakers to reduce vehicle emissions and increase fuel efficiency. But when such engines operate in a low rpm speed/high torque condition, the likelihood of an LSPI event puts pistons and other components at high risk.

“It’s very important for the future of GTDI that we keep LSPI under control, either by reducing the occurrence of LSPI or by designing more robust engine components,” explained Dr. Joachim Wagenblast, Director of Product Development North America for Engine Systems and Components at Mahle USA.

The LSPI phenomenon, based on current knowledge, can occur when the air/fuel mixture ignites before normal ignition timing. A single LSPI event can produce cylinder pressures higher than 300 bar (4351 psi), resulting in a ‘mega’ combustion knock.

To better understand the causes and effects of an LSPI event, Mahle researchers in Michigan and Germany have devised new testing procedures to better understand the causes and effects. One testing protocol involves intentionally inducing LSPI events through spark advance. In this testing scenario, engine parts are ranked for robustness based on the number of LSPI events that occur before component failure.

Another method, referred to as a natural LSPI test cycle, bypasses the need for intentionally inducing a spark advance.

“We’re putting in place special conditions that will lead to a more natural LSPI event. For instance, we’re using an oil that we know will increase the propensity of an LSPI event,” Dr. Wagenblast told Automotive Engineering. He pointed out that the oil used for testing typically isn’t used in field applications.

Researchers want to address LSPI triggers as a way to reduce or eliminate these unwanted, uncontrolled events. “If oil droplets are pushed into the combustion chamber, an LSPI event is likely. It’s also likely to occur in the hot spots,” Dr. Wagenblast said, referencing specific combustion chamber areas, such as where the fuel injector’s spray bloom hits the piston bowl and the high-temperature surfaces of exhaust valves.

Certain driving scenarios can be LSPI event triggers. “It depends very much on how the vehicle is driven,” he said. “For example, if you’re driving a pickup truck or an SUV with a heavy tow-load and you stomp on the accelerator, an LSPI event will probably happen.”

LSPI test cycle component failures include damage to the coatings, skirts and ring lands of pistons. Mahle engineers are evaluating several potential solutions for increasing component robustness, such as incorporating a high-strength ring carrier into a piston’s first ring land.

With the use of downsized/boosted engines in larger vehicles growing, solving LSPI issues is becoming more critical, noted Dr. Wagenblast.

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