Turbocharging and direct injection have ushered in new levels of power and fuel economy from smaller engines, and cars equipped with that technology have begun to appear in NASA HPDE and racing classes.
However, that technology also has presented new challenges. A condition unique to turbocharged direct-injection engines, known as low-speed pre-ignition, can cause catastrophic engine failure.
Normal pre-ignition results from low octane or a faulty air/fuel ratio, excess engine temperature or advanced ignition timing. Regular pre-ignition is predictable and can be detected with knock sensors and dealt with through engine management systems.
Low-speed preignition, or LSPI, differs from regular pre-ignition knock in that it occurs at low temperatures, low speeds and typically under high-torque loads, such as when accelerating from a stoplight. According to Matt Erickson, technical manager with Amsoil in Superior, Wisc., LSPI earned the nickname “super knock,” because the event is so destructive.
“You won’t just hear knocking going. You won’t hear anything going on. It almost instantly shatters pistons, and you can break connecting rods,” Erickson said. “That’s the big difference. You get a really high-pressure spike on these, and the big issue, why you get the bigger pressure spike, is because it’s at such low engine rpm, so that’s the low-speed part of it.”
How and Why LSPI Happens
If you’re towing your racecar to the track with an EcoBoost-powered Ford truck, or your track car has direct injection and a turbo, you need to be aware of LSPI, how it happens — and how to prevent it. How it happens is really fascinating if you’re an engine geek.
When OEMs started downsizing engines and using turbocharging and direct injection in the quest for greater fuel economy, they began to see random, yet catastrophic engine failures in the field. General Motors became aware of the problem as early as 2007 in Europe, and the automotive, fuel and oil industries have only now gotten a handle on how and why it happens.
Amsoil has posted a video on YouTube explaining how low-speed pre-ignition takes place in a direct-injected, turbocharged engine.
According to Lake Speed Jr. a certified lubrication specialist formerly with Driven Racing Oil, now with Total Seal piston rings, no one has a chemical equation that shows the root cause of LSPI. Solutions have been elusive because the events are so random. Two theories exist as to how and why LSPI occurs.
One idea is that a carbon deposit flakes off and enters the combustion chamber and doesn’t exit after one combustion cycle. If that deposit remains for a second combustion cycle, it’s already heated up a bit, creates a hot spot, and once fuel is injected into a cylinder, it lights off early.
That’s why it is isolated to direct injection, because DI can deliver fuel to a cylinder at any moment of the piston stroke, fully independent of the valve train. What’s more, because there is no port injection to “wash” the back of the valves, deposits can be more prevalent.
The second, more common theory involves droplets of oil that sneak past the rings and mix with fuel. The two fluids mix into a droplet, create a hot spot, which causes the fuel mixture to ignite before the spark plug fires, while the piston is still rising in the cylinder. These droplets are still measured in microns, but they are larger than what you find in highly atomized fuel.
“There are certain known variables that make it worse, like low engine speeds and low engine temperatures make it worse, so it’s more likely to happen under those conditions,” Speed said. “I’m not saying it never happens in other conditions, but all the testing and data shows that this is where it’s most likely to occur. You would get more events if you ran low-speed, high-load settings in the engine. If you ran, say, a million cycles at 30 degrees Celsius water temperature, you would have more events than if you ran the exact same speed and load setting at 100 degrees Celsius. That we know.”
Another cause for LSPI involves the fuel mixture itself. With direct injection, engineers can achieve extra lean burn mixtures, as lean as 40:1 air/fuel ratio. Adding high cylinder pressures associated with turbocharging compounds the problem.
“Controlling the timing of when they’re injecting and how much fuel they’re injecting and what kind of pressures they’re running, spark timing, that’s really the key,” Erickson said. “Early on and even today, they’re not going to the real danger area in the fuel map, so to speak. Through software, they’re controlling the engines to operate in a safer zone to try to prevent LSPI as best they can.”
Addressing the Problem
Due to the destructive power of LSPI, it was in everyone’s best interest to find a solution to prevent it. OEMs have been researching the problem for at least 10 years. Some of things they have been doing to prevent damage include changing piston designs, strengthening ring lands, experimenting with fuel tumble within the cylinder and, of course, tuning through software changes.
Those software changes keep the engines out of dangerous lean-burn conditions, but engineers would like to be able to use more lean burn to increase fuel economy, gains that can be as high as 10 percent.
Southwest Research has been a leader in trying to find the root cause of LSPI, Speed said, which would help engineers achieve greater power and fuel economy without the catastrophic events that can result.
“They’re trying to capture oil from the ring lands of a running engine. It’s quite complex,” Speed said. “They’ve got some tubes and things built in so they can run this engine at a low speed at a high load and try to pull small samples of the oil from that area on a periodic basis to try to quantify the chemical makeup of that oil. So that’s some work that’s being done to try to better understand the chemical pathways that lead to LSPI. At this point, it’s still unknown. There are various theories, but it’s still unknown.”
Driven Racing Oil began its research by working with Oak Ridge National Laboratory, and the company realized fairly early on that if you took its base oil and mixed it with fuel, it had little impact on octane and it produced no LSPI events.
“So, we thought, ‘Hmmm, it’s probably not the base oil. It might have something to do with the additives.’ So then, you have to ask, well which additive is it? It could be 14 different additives in there so you better go check them all.”
OEMs are fighting it from a lot of different angles, but it turns out oil formulation is key preventing LSPI.
Solutions in Oil Formulation
Researchers found that some of the additives used for detergents were causing problems, specifically calcium and sodium. As a result, sodium is pretty much gone as a detergent and calcium levels have dropped. Research has basically shown that below 1,500 parts per million, calcium doesn’t really cause a problem.
“The trick has been that traditional oils have run somewhere between 2,000 and maybe 3,000 ppm calcium, and really you’ve got to be below 1,500 for it to be OK,” Speed said. “So that forced reformulations on pretty much everybody industry-wide.”
Oil manufacturers also found that ZDDP, or zinc, phosphorus, magnesium and molybdenum detergent additives helped suppress LSPI events, so they began to rebalance their additive packages accordingly. We’re talking about changes of a few hundred parts per million to prevent random, catastrophic engine failure. It’s mind-boggling how miniscule the changes are compared with how severe the damage can be.
It also has resulted in a new specification from the American Petroleum Institute. SN Plus is the new spec announced in 2018, that designates whether an oil is safe for use in a turbocharged direct injection engine. SN Plus oils have passed tests using Ford EcoBoost and GM EcoTec engines. The engines are run under conditions prone to LSPI, and they check to see how many events occur for LSPI.
“The problem is LSPI will end up ruining most engines without the right oil,” Erickson said. “So that’s why the oil has to be in place before they can make that switch in the fuel map, so that’s why the industry is changing, and it’s why GM came out with its Dexos I Gen II specification, and the American Petroleum Institute came out with its SN Plus specification. If an oil meets either one or both of those specs, each of those has an engine test that checks for LSPI and an oil’s ability to prevent it. You have to pass that test to get that spec on your label.”
Speed pointed out that even something as simple as adding larger-tube headers to a direct-injected engine can change the conditions that might foster LSPI. For example, at low engine speeds, a big-tube header has less velocity, which results in less cylinder scavenging and more trapped residuals, unburnt combustion gases, which can trigger LSPI.
Also, something as simple as warming up the car to operating temperature can keep LSPI from occurring while you’re at the track. For HPDE drivers, that could mean the difference between driving home and hitching a ride.
“For the guy who is tracking his car, now he needs to be considering the modifications that are making his car more appropriate for the track,” Speed said. “He really needs to be thinking about what he’s doing with oil choice. Just because you may be buying a racing oil, does not mean that oil won’t have a high calcium content or a high sodium content. He needs to know how much sodium and calcium is in his engine. Those are important factors when it comes to using a DI engine in a modified, highly tuned environment.”