How
low-speed pre-ignition can damage your direct-injection turbo
engine
Bozi
Tatarevic
04 June 2020Manufacturers
are always chasing better efficiency goals. One of the more recent
trends is to downsize engines and add turbocharging, as we see many
six-cylinder engines being replaced by turbocharged four-cylinders
in a variety of applications. The phenomenon has resulted in
automakers’ mass adoption of supporting technologies, such as
direct injection. Direct injection systems run under much higher
pressures than traditional port injection systems, and
turbocharging stresses the engine even further, so these engines
have to be designed with safety in mind.
Even though modern engine computer programming mitigates the chance
of overexerting the engine and causing damage, there are some
unintended phenomena that can still pop up, albeit rarely. One is
the irregular detonation effect, called low-speed pre-ignition
(LSPI). LSPI can result in catastrophic engine damage under the
right conditions. Conditions I witnessed first-hand when my
brother’s Focus ST, which we had been doing some maintenance on,
suddenly lost power and started burning oil. We checked the car out
and found that one of the cylinders had lost almost all
compression. We decided to pull the engine and found that the
ringland of one of the pistons had cracked, leading us to believe
that there was some sort of knock in the engine that caused the
failure. Explaining how that happened proved challenging,
considering that the car was filled with 93-octane fuel (as Ford
recommends) and previous ECU logs from showed no knock at all.
After doing some digging, we came across the concept of LSPI, which
seemed to account for our unfortunate situation. Not knowing much
about LSPI, I decided reach out to an expert for more information
on how my brother and I got so lucky.
LSPI and direct-injected, turbo engines
LSPI appears to be most common in small displacement turbocharged
engines with direct-injection. The effect is classified as an
abnormal combustion event and happens in a similar manner to
traditional engine knock or detonation, in which a mixture is unintentionally
ignited, and often not by a spark plug.
Next, We spoke to Michael Warholic, light duty lubricant Technology
Manager at Valvoline, to find out more about the phenomenon and how
worried vehicle owners should be. LSPI can be mitigated to some
extent with the right engine oil formula. Warholic is a scientist
who previously formulated such oils, and he has been involved with
LSPI research and mitigation for almost a decade.
The issue of LSPI, Warholic notes, emerged as technologies like
direct injection and turbocharging started to enter wide use in
production vehicles. The industry responded in 2011, when
manufacturers and suppliers came together to create a consortium to
research the issue at Southwest Research Institute (SwRI) in Texas.
SwRI set up engines for monitoring and found that this type of
knock is most prominent in situations with low speed and high load.
The observation indicated high-pressure spikes and hot spots in the
cylinders in such situations, and SwRI’s first hunch was that there
were oil or fuel deposits on the cylinder walls that might be
auto-igniting. After additional research, researchers found that
there are spots in random areas of the cylinders that were getting
hot enough to auto-ignite under the right combustion conditions,
before the spark plug fired, causing this knock or detonation.
Warholic described these spots of auto-ignition as “fireflies”
because they would light up in the cylinder in various areas
without a clear pattern. Even though these events might be happening
in an engine, Warholic says, they may not necessarily be
catastrophic enough to cause engine damage—the catastrophic version
of these events only happens under perfect conditions for LSPI. In
that scenario, fuel or oil droplets are ignited, creating a high-pressure
spike that can result in broken connecting rods, broken rings or
ringlands, or even a cracked piston.
The secret is in the oil
Once researchers started digging into why these events were
happening, they found that a detergent used commonly in engine oil,
calcium sulfonate, was reactive to the conditions of LSPI. Testing
reduced calcium sulfonate in engine, they noticed that LSPI events
were significantly curbed. After these findings were verified, most
oil companies and marketers decided to rebalance the detergents in
their oils to reduce calcium sulfonate and replace it with
magnesium sulfonate. This resulted in a new specification, called
API SN Plus, that in 2018 required oil companies to reformulate
their oils to be LSPI-friendly. The specification was based on a
test created by Ford to measure LSPI events in EcoBoost engines.
There are other methods to reduce LSPI, such as increasing zinc or
molybdenum content, but these are often found in racing oils.
Street cars required measured usage of such ingredients, as zinc
can poison catalytic converters and molybdenum can be corrosive.
Manufacturers are testing various combinations of detergents and
additives, but their conclusion has been that the oil chemistry can
greatly impact LSPI events. All of the previous tests and
research have been completed with fresh oil, and manufacturers are
now developing tests to examine aged or used oil. Many believe that
the effect can become worse as oil becomes older. Traditional
engine knock was pretty common in older engines, and it still
happens in modern engines when combustion events happen early. The
biggest difference with LSPI is the huge pressure spike.
Direct-injection and turbocharging are the main culprits of
increased pressure when LSPI-type knock happens. The engines used
in testing and research have transducers mounted in the cylinders
to monitor pressures, recording in-cylinder pressures that reach
1000 psi or more during LSPI events. Normal operation may show
these pressures at half of that amount. Luckily, the events aren’t
frequent, at least in testing as there may be 5 of these spikes
over 100,000 engine cycles.
In addition to the changes in oil formulation, manufacturers can
also make changes to their direct-injection systems to reduce
chances of LSPI. Reducing pressures for direct-injection systems or
richening up the fuel mixture is one approach, but OEMs try to
avoid those changes given that doing so consume more fuel, thereby
defeating the purpose of these systems in the first place.
Should you be worried about LSPI?
We asked Warholic if he would buy a small turbocharged and
direct-injected engine, considering what he knows about the risks
of LSPI. He said that he certainly would, but only while using
the correct oil. (Fuel is not a risk factor for those in the United
States, as fuel here is generally of high quality.) Warholic
recommends a careful reading of the owner’s manual to find what API
rating has been specified for the engine, and to make sure to only
use that specification of oil to reduce the chances of LSPI damage.
He believes that these new engines are incredibly efficient—he
would not hesitate to buy one.
The specifications that owners of these engines might find in their
manuals are SN Plus, SP, or GF6. SN Plus was the first interim
solution to LSPI and an improvement over the SN standard. The
detergents and additives in that specification are based on the
earliest LSPI tests completed against a Ford EcoBoost engine. GF6,
an improvement on SN Plus, is the latest and greatest specification
from ILSAC when it comes to LSPI prevention. It includes all of the
previous LSPI testing as well as a new chain wear test from Ford,
better deposit requirements, and better fuel economy requirements.
The GF6 specification was only released in the last month, so it
may not be on the shelves yet, but Warholic expects that GF60-rated
oils should be on the shelf sometime this summer. SP is the latest
specification from API and mostly mirrors the GF6 specification.
Any of these three specifications should help to prevent LSPI, but
SP and GF6 are the latest versions owners should look for when
making a purchase to ensure that they are getting the best for
their engine. According to Warholic, Valovline will spread the GF6
and SP specification to essentially its entire line. GF6 will be
used for oils up to a 10W30 weight, as that is the maximum ILSAC
grade, but API SP will be in use for their heavier oils. In short
order, the rest of the industry is likely to follow the same path
for oil formulation.
Small-displacement turbocharged engines are not going away, and
direct-injection systems will likely become more advanced, so it’s
important to prevent these issues before they, say, break your
piston.
https://www.hagerty.com/media/maintenance-and-tech/how-low-speed-pre-ignition-can-damage-your-direct-injection-turbo-engine/
My 2009 R56 JCW during happier times.
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