Distinguishing between pre-ignition and super-knock

A study led by a team from Tsinghua University elucidates the relationship between pre-ignition and super-knock.

Pre-ignition/super-knock tend to occur at low-speed, high-load conditions and are an obstacle to using a higher compression ratio to improve power density and fuel economy.

In their work, reported in the International Journal of Engine Research, they used experimentation and numerical simulation to distinguish the relationship between super-knock and pre-ignition.

The experiment was conducted on a turbocharged gasoline direct injection engine with compression ratio of 10, operating at 1750 rpm and BMEP of 2.0 MPa under stoichiometric conditions.

Super-knock is the severe engine knock triggered by pre-ignition. Pre-ignition may lead to super-knock, heavy-knock, slight-knock, and non-knock. Significantly advancing spark timing can only simulate pre-ignition, not super-knock. Although knock intensity tends to increase with earlier pre-ignition timing, higher unburned mixture fraction at start of knock, and higher temperature and pressure of the unburned mixture at start of knock, knock intensity cannot be simply correlated to any of the parameters above.

A one-dimensional model is set up to numerically simulate the possible combustion process of the end-gas after pre-ignition. Two distinct end-gas combustion modes are identified depending on the pressure and temperature of the mixture: deflagration and detonation. Hot-spot in the mixture at typical near top dead center pressure and temperature condition can only induce deflagration. Hot-spot in the unburned end-gas mixture at temperature and pressure conditions above “deto-curve” may induce detonation. The mechanism of deto-knock may be described as hot-spot-triggered pre-ignition followed by hotspot-induced deflagration to detonation.