1. Stoichiometry. I liken it to New York City which is overflowing with single women all hunting for the very limited and selective pool of Mr. Rights, introducing even MORE single women will not make it MORE likely that they will all get married to their trophy husband. Every time the piston is in place for its combustion cycle to start, the air in the combustion chamber needs to have enough, or more, air molecules to support "marriage" for each of the hydrocarbon molecules.
2. Heat. Cold mixtures/suspensions/solutions do not react as well or predictably as they could if they were at a temperature that meets or exceeds whatever minimum they may require for complete and consistent combustion, and chains of combustion.
3. Quench. The air near the internal surfaces of the combustion chamber will often be cooler than it is at the flame front. Somewhere near that surface, temperatures can be too cool to support flame travel.
4. Detonation. Even on a diesel engine, detonation is not an attractive proposition, but gasoline engines tend to be designed without enough strength to overcome this problem. Diesel combustion tends to be deflagration, not flame front propagation. The combustion event rides the front of a shock wave, so it is more like a detonation than a combustion. Uncontrolled detonation, which has combustion events happening where and when they should not, has a violent jackhammer effect on the interior surfaces of the combustion chamber. Flame front combustion is like being hit with a big, heavy pillow, and deflagration is like being hit by a big rubber mallet.
How do we address these issues? With what do we combat the factors that contribute to the present paradigm, which is incomplete combustion?