Contributed by Bob Chabot
Resolving Low-Speed Pre-Ignition (LSPI)
Unless resolved, LSPI is a barrier to better engines
Market demand and legislation are driving automakers to find ways to improve fuel economy and reduce CO2 emissions across their vehicle fleets. U.S. regulations will see passenger car fuel economy standards jump from an average of 35.5 mpg in 2016 to 54.5 mpg in 2025, while ever-lowering tolerances for greenhouse gas emissions. Even more aggressive standards have been proposed in Europe and Asia.
Unexpected Low Speed Consequences for More Power, Torque and Pressure
To achieve these goals, automakers initially focused on downsizing gasoline engines. For example, Ford Motor Co. and General Motors Corp. have launched 3-cylinder, 1.0-L boosted engines (EcoBoost and Ecotec, respectively) that deliver the same output as their 1.6-L four-cylinder engines, but also provide an approximately 20 percent improvement in fuel economy and lower CO2 emissions. These reduced engine displacements delivered improved engine efficiencies that included lower pumping mechanical friction losses; down-speeding of the engine by using higher transmission gear ratios; higher engine torque at lower engine speeds; and lower gases-to-wall heat transfer.
The number of turbocharged gasoline direct injection engines has begun to dominate global vehicle production. Correspondingly, concern for LSPI has risen over the timeframe in the graph above. (Image — Lubrizol)
Both Ford and GM say they have plans to increase production of these smaller engines to meet growing consumer demand in markets worldwide. Ford already produces 100,000 EcoBoost engines per month, and aims to offer the technology in approximately 80 percent of its vehicles by 2016. GM says it expects to produce 2.5 million units annually at five global plants by 2017.
But early downsized engines also sacrificed engine performance. To offset lower power output, automakers began adding turbochargers to boost engine operating pressure. In addition, the rapid proliferation of turbocharged, higher pressure gasoline direct injection (TGDI) engine technology in the past five years has been stunning.
What is Low Speed Pre-Ignition (LSPI)?
LSPI is an unexpected consequence of downsizing and boosting engines. Also known as stochastic pre-ignition (SPI), megaknock, superknock or deto-knock. LSPI most commonly occurs at low speeds during a period of rapid acceleration. LSPI is believed to be caused by droplets or particles in the combustion chamber — combinations of fuel and oil — that ignite prior to spark, resulting in uncontrolled, abnormal combustion. This creates spikes in engine pressure, ultimately causing the damage. In some cases, researchers reported that just a single LSPI event was sufficient to cause severe engine damage.
In the early days of downsizing and boosting engines, it was not clear how much impact LSPI would have. It soon became apparent that LSPI events were more widespread and that lubricant, fuel and engine design research was needed to determine an optimal path forward. The first step required industry consortia to develop more general knowledge about LSPI.
Two examples of piston damage due to LSPI observed during testing. (Image — Southwest Research Institute)
Initially it was thought that pre-ignition sources were located at hot spots in the cylinder, or were from soot accumulation. However, further optical investigation revealed that pre-ignition actually occurred randomly throughout the combustion chamber, which means surface ignition is not the only source of LSPI.
More current research suggests that the auto-ignition of oil droplets or deposit particles is probably the major cause of LSPI. In addition, it soon became apparent that LSPI events are more widespread than previously thought and that they represent a barrier to automakers safely maximizing performance and fuel efficiency simultaneously, let alone meeting more stringent regulatory standards.
LSPI is Poised to Become an Increasing Service/Repair Problem
Just five years from now, it's estimated that a quarter of all cars on the road in North America and 39 percent of global production will utilize this combination of engine hardware. However, despite the benefits of downsized, boosted gasoline engines that operate at low speeds and high torque, an unintended, but serious consequence has emerged that the industry must resolve — LSPI.
The challenge in a nutshell? As a larger percentage of today's engines are downsized and turbocharged over time, more vehicles in operation will suffer low-speed pre-ignition and potentially experience engine damage and/or failure. In addition, as more stringent fuel economy standards take effect, vehicles will increasingly be operating in regimes where LSPI events are more likely to occur.
SWRI lays out where and when LSPI occurs. (Image — Southwest Research Institute)
Addressing LSPI sooner than later is top of mind for manufacturers. At the most recent JSAE/SAE Powertrains, Fuels and Lubricants International Meeting held in September 2015 in Kyoto, Japan, LSPI was one of the primary agenda topics. Attendees recognized that advanced downsized and boosted engines now spend much more time in a low-speed high-torque regime that easily leads to LSPI.
At the meeting, industry experts reached consensus on several points:
- LSPI arises from interactions between lubricants, fuels and engine design/operation. Resolving LSPI will likely require a holistic approach that addresses all three areas.
- LSPI occurs very close to the engine's optimum operating area where fuel economy, performance, and drivability are balanced under constant load (e.g. highway cruising).
- Downsized, boosted engines can easily slip into LSPI in that zone, which can cause potential engine damage (e.g. broken pistons, bent connecting rods or severe engine failure).
- Until LSPI is rectified, automakers may be restricted in their ability to maximize the performance and fuel efficiency of their advanced engine designs, creating a barrier to meet future demanding fuel-performance and emissions requirements.
No Simple Fix
"LSPI is not a simple problem," explained Thomas Briggs, Jr., manager of engine systems research and development at Southwest Research Institute (SWRI) in San Antonio, TX. Briggs led SWRI's Pre-ignition Prevention Program team that include industry partners (GM, Ford, Honda, Infineum International, Afton Chemical and others) that began investigating LSPI in 2011.
"When the fuel is injected directly into the combustion chamber, it dilutes the oil film lining the cylinder," Briggs shared. "This fuel dilution reduces the surface tension and viscosity of the oil, causing an oil-fuel mixture to accumulate in the upper reaches of the piston top land crevice. The mechanical energy of the upstroke during compression pushes droplets into the combustion chamber, where they vaporize and can auto-ignite prior to spark ignition and subsequent engine damage."
"The team used high-speed video, crevice sampling and other specialized tools to better determine the source of the problem. Captured video showed droplets of material were coming out of the piston crevice in the engine. Sometimes those droplets burned, leading to LSPI and strong engine knock. The video also showed that the material was a complex cocktail of fluids — fuel, lubricant, soot and other material. It's clear to us now that resolving LSPI issues will require addressing lubricants, fuels, engine design and more."
Southwest Research Institute and its partners are now involved in a follow-up consortium which started in 2014 and is scheduled to wrap up in 2018. Currently, ways are being investigated to replicate what happens in an engine on a test bench, and develop standardized tests for LSPI. This will give researchers greater access and more tools to understand what is going on. The goal is to determine the chemical steps that lead to LSPI, which will help in figuring out what should be done to address it.
The program wrapped up in 2014, having produced information that gave additive companies a rough idea of what was needed to solve the problem. "We already know that LSPI arises when engines are in the 1500-2000 rpm range, a relatively slow speed, while still exerting plenty of torque," Briggs said. "That's also when a large supply of fuel is present. Yesterday's engines spent little time in these operating conditions, but it's very possible they too would experience LSPI if they had."
According to Afton Chemical's Ian Bell, market-based solutions to LSPI issues need to meet five key pillars of a holistic solution. (Image — Afton Chemicals)
Complexity Requires a Holistic Systems Approach
Industry researchers and manufacturers increasingly realize that vehicles are complex systems that require a holistic approach to resolving problems. Certainly an advance in one area may help but doing so without considering other areas has already led to unintended consequences. The serious issues caused by LSPI, after downsizing then boosting engines, have amply demonstrated that.
"An area that SWRI is working on now is LSPI testing," Briggs shared. "While LSPI tests already exist that can readily discriminate between the LSPI impact of various lubricant formulations, they are being further refined into upcoming ILSAC GF-6 and GM dexos 1. But we also need to standardize testing across the industry and incorporate new understanding on a dynamic basis into LSPI testing so it remains relevant over time. For instance, continued research on the mechanisms behind and possible fixes for LSPI in areas outside of lubricant formulation need to be considered as LSPI testing evolves."
"We know LSPI occurs in the combustion chamber, and the most practical solution is a holistic solution that takes into account engine design, engine oil formulation and fuel quality," advised Ian Bell, Research and Development Director for Afton Chemical in Richmond, VA, a company in the lubricant formulation and lubricant manufacturing business. For any manufacturer in one of those areas, solutions must integrate a multi-perspective approach. In addition, positive innovation in one area must not severely compromise overall system performance. Whether engine design, lubricant formulation, fuel quality or backwards compatibility, R & D in one area must also include consideration of the others."
For example, Bell noted that when Afton recently presented a survey of LSPI performance for a broad range of commercially-available GF-5 lubricant products, all failed proposed LSPI requirements for the new GF-6 standard about to be introduced. "Lubricant formulations will have to change. But the lubricant manufacturers' holistic solution will need to reduce or eliminate LSPI while still providing improved performance in other areas it hasn't been concerned with before."
Specifically, Bell cited:
- Engine Durability — To preserve the chief function of engine oil, a formulation must be able to cool, lubricate and clean engines to reduce wear and maximize service life.
- Oil Formulation — Oil additives, such as viscosity modifiers, ensure oil performs in both hot and cold weather, while performance additives ensure oil remains clean, durable and effective. There is a clear impact from all organic materials, and base oils can also be significant, so lubricant manufacturers must bear this in mind going forward.
- Fuel Quality — Today more than ever, oils depend on friction modifiers in fuels to help maximize the amount of usable energy, while maintaining stable viscosity to reduce engine drag.
- Backwards Compatibility — The average age of a light vehicle in the U.S. is more than 11 years old, and it's growing. Tomorrow's lubricants must address LSPI in the vehicles of today, yet remain fully compatible and effective in the vehicles of yesterday. The new ILSAC GF-6 and next-generation GM dexos 1 (2015) standards do that.
Lubricant formulation is just one of the elements to resolve the LSPI challenge. Manufacturers in other areas must take a similar approach and collaborate with each other. It's about raising the competency bar across the industry landscape for us all — from those who build vehicles to those who service them.
[Editor's note: Read MOTOR Magazine's September 2016 issue for the latest diagnostic and automotive service insights.]