BMW and MINI (as well as most other manufacturers) are producing more and more engines that incorporate intricate engine management and control systems in order to meet emissions requirements, reduce vehicle weight (smaller engines) and increase power output. While it seems that the goals of reducing engine size to achieve higher fuel economy and lighter vehicle weight, while producing more power are at opposite ends of the field, the manufacturers have been achieving both of these goals through the use of Gasoline Direct Injection (GDI) and forced induction (turbo or super charging)*.
These small displacement, forced induction, GDI engines are running within a very tight optimum operational envelope in order to achieve the desired fuel economy and power output. The tightest area of the engine management mapping is what we might call “cruise”. This is when the vehicle is running at a more or less steady speed, in the top gear. During this operational parameter, the engine is being run as lean as possible (for best economy and emissions). Contrary to what we might think, the engine is under a fair amount of load (cylinder pressures and temperatures) when in this mode. Additionally, if the load is moderately increased (not enough to cause a gear downshift), by a slight uphill grade, minor acceleration, etc., the engines cylinder pressures and temperatures are significantly increased.
Due to all of the above points, these small displacement, forced induction, GDI engines can exhibit a phenomena known as LSPI (Low Speed Pre-Ignition). Similar to detonation, this condition creates extremely high cylinder pressures and can be highly destructive to the engine. We commonly know pre-ignition and detonation as “knock” or “ping”. These conditions occur when the charge in the combustion chamber ignites prematurely or burns too quickly. The expanding combustion charge either is trying to push the piston down while it is still moving upward or expands in volume faster than the piston can move downward. The knock or ping we detect is the piston rattling in the cylinder bore, the cylinder head rattling on the engine block, crankshaft bearings being hammered and general distress on the engine parts.
LSPI is thought to be generated in these small displacement engines due to combustion soot or other particles from “dirty” oil that are in the combustion chambers during the compression cycle. The heat in the combustion chambers heats the particles to a burning point and the particles act as a source of ignition to the compressed charge. As soon as the fuel begins to be injected, the charge is ignited … before the spark plug fires. This is the pre-ignition condition.
Is there anything we can do to help assure that our BMW or MINI engines don’t suffer from LSPI? Since it is believed that a potential source of the LSPI is particles in the combustion chamber, we would suggest the following:
1) Forced induction GDI engines tend to produce more soot in the combustion cycle. Therefore, use a high quality fuel system cleaner and a carbon cleaner as a periodical treatment to help prevent build-up of soot in the combustion chambers.
2) Dirty oil has a higher particulate content than fresh oil, even though the oil may still be fully within its operational envelope. This particulate content may contribute to the combustion chamber particles that are available for potential pre-ignition. Using a high quality synthetic motor oil and applying shorter change intervals will help to keep the particulate content low.
* What is GDI and Forced Induction?
GDI injects the fuel directly into the combustion chamber at the precise moment that is optimum for cylinder combustion at any given engine operational parameter. This allows a far more exact level of engine management. In order to increase power from these smaller engines, forced induction using turbocharging or supercharging may be employed. As the name implies, forced induction “forces” additional air into the cylinders during the intake cycle. Normally, atmospheric pressure pushes the air into the cylinder as the piston falls and creates the volume in the cylinder. Forced induction adds pressure over that of the atmospheric pressure, which forces additional air mass into the cylinder. This increased air mass can now have an additional amount of fuel injected (to keep the proper air/fuel ratio). The resulting combustion is of greater power as if a larger cylinder (larger engine size) were being used. Therefore, with GDI and forced induction, a smaller engine can be made to have the power of a larger engine while also achieving higher fuel economy, and lighter weight, compared to the larger engine.