Low Speed Pre-Ignition
for Non-Technical People
Created for Non-Technical People
But I’m a Mathematician, so I this will have graphs
What is Low Speed Pre-Ignition?
• Vehicle manufacturers are constantly trying to decrease fuel consumption while increasing
speed-torque to the tires (or at least keep it the same). This is the definition of increasing fuel
• In it’s simplest form, you want to increase engine pressure at the optimal engine speed. The
engine is tuned to the ”power band”, where the fuel to (speed-torque) ratio is the lowest.
• For simplicity’s sake, the ”power band” exists when the engine speed is low and the engine
pressure is very high. (1500 – 2500 RPM)
• Because of an emphasis on fuel economy, the “power band’s” torque is being elevated which is
done by increasing pressure inside of the engine cylinder. We’re pushing the limit of what current
combustion engines can successfully accomplish.
• This increased pressure in the cylinder is causing a phenomenon called
low speed pre-ignition. This is where the fuel is combusting before the spark plug fires. As you
can imagine this is bad. Think about when this happens on the up-stroke. It’s like hitting the top of
the piston with a maul! This causes all kinds of damages. Even broken rods!
• No one really knows what causes this LSPI, but there are some theories and proven actions which
minimize the amount of LSPI
We have to learn some fundamentals first!
So put your thinking caps on!
This chart will be explained fully over the next few slides, but as a
general overview please note:
• The lower the number, the more fuel efficient the vehicle is
• The area above the red line is the area where LSPI is likely to
• The area below the blue line has higher, less fuel efficient
numbers, so are not optimal for the vehicle to be tuned to.
• This is a chart of a vehicle’s fuel to power ratio. It’s the whole
vehicle system fuel economy combined into one chart.
Image courtesy pceo.com
What the heck does this chart mean?!?!?!?
If you search for LSPI, you’ll run across this chart. It’s tough to
digest mainly because it looks like a 2d graph, but it’s actually 4
dimensional, so you are looking at 4 variables, 2 of which are
wrapped up in the color and map lines.
• BMEP is basically pressure inside the cylinder (bar)
• Engine speed (rpm)
Not inherently visible - BSFC
• Fuel consumption (g)
• Torque (Nm)
Notice the number in the middle of the chart (230 in light blue),
that’s the BSFC. The lower the number, the better the fuel
efficiency. This number is the amount of fuel versus the rpm-
torque output. For fuel economy reasons, we really want to
operate in the LSPI region “power band”
Image courtesy pceo.com
Pressure vs Engine Speed Chart
To start to understand LSPI we will simplify this
chart by using constant fuel input and power
output while assuming 100% energy
This chart represents a BMEP (pressure) vs.
engine speed using a constant rate of fuel
input. It’s important to note, that the power
output is constant too for this entire graph.
Power output is the speed / pressure (or
BMEP means ”Mean effective pressure” which
is an average of cylinder pressure
Introducing BSFC – Step 1
BSFC means brake specific fuel
consumption. Which is the correlation
between fuel consumption and actual
torque output. As said before, there are 4
variables at play here. Fuel, pressure,
RPM, and torque output. This is a realistic
view of engine efficiency. Still, we are
considering constant fuel input and
torque, but notice the speed is not
constant. The “power band” looks to be
between 1000 and 3000 rpm. The POWER
(which is rpm-torque) is at it’s highest
level in that range. So the fuel / power
ratio will be the highest in this range … for
this fuel input level.
Introducing BSFC – Step 2
BSFC is the ratio of fuel to power.
As we alter the fuel input, the BSFC graph will change. Creating a 3
BSFC – Step 3
So the rings that you see on this graph are
the fuel / power ratio. It’s a topographical
map (think upside mountains), the lower
the number, the more fuel efficient the
The light blue range (230) in the middle of
this chart just happens to operate in a bad
range which is near the LSPI area (above
the red line). LSPI is a type of knock that
causes more engine damage that you can’t
control by spark plug timing.
Where we operate our engines
So we want to operate near the LSPI range
because there’s more area and it’s easier to
keep an engine in that range under most
conditions. Using electronic control of the
transmission, we keep the torque load in the
area that keeps the engine near the LSPI
region, which maximizes fuel efficiency.
The last few slides have explained the LSPI
region which is really close to the optimal
fuel efficient region, and we sometimes drift
into. Now I will talk about combating LSPI.
Low Speed Pre-Ignition
• Happens at low speeds where the pressure is very
high inside the cylinder
• It’s very bad for the engine, much worse than the
• Combustion occurs spontaneously
• No one really knows why, but there are some
interesting claims which point to:
• Calcium detergents are becoming superheated and
autoignite the fuel
• Oil droplets are mixing with the fuel causing spontaneous
• As vehicles get more power dense and fuel efficient
LSPI will become a major issue because engines will
get tuned closer and into the dangerous LSPI
Image courtesy Afton Chemical
• IT’S VERY IMPORTANT
• More and more vehicles are operating at or near this BSFC range that harbors LSPI
• OEMs such as General Motors and bodies such as API and ILSAC see the need to combat
LSPI and will soon be requiring motor oils to pass LSPI tests to gain new service
• It looks as the best way is to use a lubricant that is designed to combat LSPI.
• Detergents such as calcium, which are prone to cause LSPI will be used less and less
• Better friction modifiers such as molybdenum and phosphorous will be used to decrease
engine temps and prevent deposit buildup
• Better base oils with higher lubricating boundaries will be used to help prevent LSPI
• Higher quality finished lubricants will have to be used!