Every human behind the wheel of a vehicle propelled by an internal combustion engine has experienced some form of engine lugging at one time or another. Although this condition can happen in any internal combustion engine, the damaging effects associated with it are most commonly seen in small displacement turbocharged engines and has a high probability of causing severe engine damage.

Engine lugging occurs when the driver smashes the throttle to the floorboard, usually while overtaking on the highway or climbing a hill in too high of a gear and causing the engine to experience high load at too low of an RPM.

For Engines In General
When you attempt to accelerate at too low of an engine speed and in too high of a gear, crank driven oil pumps won’t be able supply a large enough volume of oil to support the load being put on the internal components and leading to oil starvation related failures.

Another common problem when lugging is that the engine has less available torque and has to work much harder to propel the car forward than if the transmission were in a shorter gear. This reduces the engines already poor efficiency, and when an engine becomes less efficient, cylinder temperature rises, and you start pushing the boundaries of the knock resistance of the fuel and will begin to see very high knock counts (detonation). Heat related lugging issues are of even greater concern for air-cooled engines, where the fan speed is controlled directly by engine RPM.

Knock is often considered to be the equivalent of hitting the top of your piston with an explosive hammer. Knock can cause severe damage to an engine’s cylinder walls, pistons, connecting rods, spark plugs, bearings, and hardware, presenting you with an engine that burns more oil and makes less power, if it still runs at all.

A diagram comparing a normal combustion event (left) vs a knock event (right).

Although knock related damage is less of a concern on most modern EFI engines because they have a failsafe that pulls timing and or adds fuel to counter high knock counts.
For a more in-depth look at detonation and its associated effects, be sure to check out our article on the topic here.

Small Displacement Turbo Engines
Low speed pre-ignition (LSPI) is considered a serious problem for turbocharged small displacement direct injection engines. It is becoming more common as manufacturers continue to increase the stress being put on these engines by reducing displacement and increasing pressure and ignition timing to meet the ever more demanding fuel economy and emissions standards.

Graph showing the average brake mean effective pressure (BMEP) of an LSPI event vs a normal knock event. With brake specific fuel consumption islands. (Image courtesy of PCEO)

LSPI occurs before the spark plug even fires, and while engineers have yet to identify all of the variables that may cause LSPI, they have been able to pinpoint several secondary ignition sources such as cylinder temperature/pressure, hot engine deposits free-floating in the combustion chamber, and large oil droplets after mixing with the fuel injected on the intake stroke.

In addition to avoiding lugging, a few studies presented at the JSAE/SAE Powertrains, Fuels and Lubricants International Meeting in 2015 showed that even reducing the calcium carbonate (CaCO3) levels being used in engine oil as a detergent greatly reduced the occurrence of LSPI during testing. A catch can system will also dramatically reduce the amount of blow-by that is able to enter the combustion chamber and reduce the occurrence of oil deposit build up and foreign materials in the combustion chamber.  To learn more about about reducing blow-by, check out our guide on catch can systems.

Low speed pre-ignition has the ability to cause severe engine damage, because the flame front propagates and builds cylinder pressure faster than the piston can travel at that low of an RPM and creates extreme pressure spikes, or it may even ignite on the compression stroke and build against the upward motion of the piston in rare situations.

Graph showing the crank angle and cylinder pressure of a normal combustion event vs an LSPI event.

Pre-ignition cannot be prevented by pulling ignition timing or adjusting the spark plug heat range, so auto manufacturers have worked very hard to prevent LSPI during the design phase of the engine and by adding as much fuel in these load cells as possible without noticeably affecting efficiency.

The Takeaway
While LSPI may not be completely understood at this point and large displacement engines are less susceptible to knock and pre-ignition at low RPM and high load, we do know that it can happen to just about any internal combustion engine when the conditions are right, especially turbocharged small displacement direct injection engines and will often lead to piston damage or a snapped rod putting a nice new window in your block skirt.