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Low Speed Pre-Ignition

This is a short description and explanation of what LSPI is and why low speed pre-ignition is important. It discusses graphs and explains fuel economy and how to prevent LSPI.

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Low Speed Pre-Ignition

  1. 1. Low Speed Pre-Ignition for Non-Technical People Created for Non-Technical People But I’m a Mathematician, so I this will have graphs
  2. 2. 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 economy. • 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
  3. 3. Get ready! We have to learn some fundamentals first! So put your thinking caps on!
  4. 4. A CHART 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 occur. • 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
  5. 5. 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. Charted • 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
  6. 6. 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 conversion. 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 torque) BMEP means ”Mean effective pressure” which is an average of cylinder pressure
  7. 7. 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.
  8. 8. Introducing BSFC – Step 2 BSFC is the ratio of fuel to power. 𝑓𝑢𝑒𝑙 𝑝𝑜𝑤𝑒𝑟 = BSFC As we alter the fuel input, the BSFC graph will change. Creating a 3 dimensional graph.
  9. 9. 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 engine is. 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.
  10. 10. 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.
  11. 11. 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 typical knock! • 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 combustion • 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 region. Image courtesy Afton Chemical
  12. 12. Combating LSPI • 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 classifications • 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 occurrences. • Higher quality finished lubricants will have to be used!