Engine Downsizing Comes With Downside

BIRMINGHAM, MI – Downsized turbocharged engines continue to grow in popularity because they offer impressive fuel efficiency and squeeze out the power. But there’s a downside.

Their high-compression nature in certain circumstances can damage engine parts, sometimes catastrophically, says Joachim Wagenblast, director of R&D-engine systems and components for engine supplier Mahle USA.

He cites broken piston rings, cracked off tops of pistons and bent connecting rods.

The culprit is called low-speed pre-ignition, or LSPI. It is an abnormal combustion phenomenon that occurs before the normal spark-plug ignition and is not widely known.

To facilitate downspeeding, which is changing gear ratios to lower engine speed in order to achieve better fuel economy, the smaller engines must produce high torque at low speeds.

The pressure required to achieve high levels of low-speed torque and quick throttle responsiveness can cause LSPI, which generates extremely high combustion-chamber pressures, Wagenblast says here at a technology presentation entitled “Problems Ahead for New Fuel-Efficient Engines.”

LSPI is unpredictable, but aggressive driving and towing are contributing factors, he says.   

Mahle says the need to address the issue is dire, especially because of the increasing popularity of downsized turbocharged engines that can be 30% smaller than their naturally aspirated equivalents.

As the auto industry strives to meet high government-mandated fuel-economy standards, downsized engines are expected to power more than 50% of all vehicles sold in the U.S. by 2025, the EPA says.

Solutions for LSPI range from stronger parts to more durable coatings. Mahle is working on both. For example, incorporating a high-strength ring carrier in a piston’s first ring grove helps prevent LSPI-related part failures, Wagenblast says.  

“Our development teams are focused on creating engine components and systems designed to withstand low-speed, pre-ignition events, as well as assist in preventing their occurrence,” he says. 

The company also is developing advanced test protocols aimed specifically at LSPI.

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