Knock in Gasoline Engines - the Effects of Mixture Composition on Knock Onset and Heat Transfer

Sammanfattning: This thesis is based on experimental investigations into engine knock in, (a) a multi-cylinder turbocharged engine, and (b) a single-cylinder engine featuring optical access. The optical access facilitated the measurement of temperature in the end-gas using coherent anti-Stokes Raman spectroscopy (CARS).

The multi-cylinder engine tests showed that cooled EGR (exhaust gas re-circulation) can be used as a replacement for fuel-enrichment at high loads. Consequently a stoichiometric mixture can be used at all loads. Lean mixtures can also be used to suppress knock, as peak pressures are kept low in comparison to a stoichiometric mixture.

Measurements of the end-gas temperature showed the presence of cool-flame chemistry in mixtures with various air-to-fuel ratios and in a stoichiometric mixture with EGR. First-stage ignition appeared at temperatures ranging from 750-850 K, and final ignition occurred at temperatures above 1,050 K for all mixtures. A comparison between the isentropic and measured end-gas temperatures revealed that fuel-rich mixtures had the highest heat-release from low-temperature chemical processes. Despite this, the leanest mixture showed the highest knock-intensity at constant load and with the same position of 50% burnt charge as the other mixtures. The stoichiometric mixture with EGR showed a very low tendency to knock, even though the flame speed and cylinder-pressure development were very similar to that seen with the lean mixture.

This thesis presents the effect of knock on heat transfer. Statistical evaluation of the heat transfer shows that heat flux was affected when knock intensity was above 200 kPa for lean mixtures, and above 100 kPa for stoichiometric and rich mixtures. The increased heat transfer is most likely brought about by an increased charge motion associated with the autoignition event.

Measurements of the thermal-boundary layer are also presented in this thesis. It was found that the thermal-boundary layer has a thickness of around 0.5 mm prior to flame arrival. The boundary layer and the temperature profile were heavily perturbed at temperatures above 850 K due to heat released from low-temperature chemical processes.