 When my daughter dreams of transportation of future, she imagines vehicle power by alternative energy. The autonomy of electrical cars is already 150 km, but she prefers the car carrying her to vacation 1000 km from home. This car is powered by an internal combustion engine, a technology invented in the 80s and 60s. This engine is widely used today and certainly for still long. Its greatest quality is to enable driving 1000 km, with a 50-liter fuel tank. A recent International Energy Hoclux planned a 40% increase of the world demand for transportation fuel in the next 30 years. This is mainly due to the expected increase of the car market in India and China. We hunter the transition period awaiting the time when alternative technologies will be dominant. During this period, internal combustion engine has to be made the cleanest as possible. Increasing engine efficiency will favor climate change mitigation. Combustion does not only emit greenhouse gases, but also many traces pollutants which are often neglected. Many of them are known to be toxic for men or harmful for environment. The full range of these pollutants has to be investigated. I realize the importance of traces pollutants in Grenoble. Scientists there analyzed the air bubble in high score taken from Halpin Glacier. This bubble contains many chemicals which can be attributed to transport emissions. Combustion chemistry can be investigated in sample laboratory devices. A quartz vessel of a size of a small orange is used to study the chemistry in diesel engines. Founded by ERC, my team with Chinese colleagues has coupled this reactor to a special type of mass spectrometer. This apparatus uses the radiation of a synchrotron. This particular radiation has a low enough energy for not breaking the chemical during ionization. This new apparatus has enabled the detection of many chemicals produced from combustion. Some of them were thin for the first time under condition close to those in engines. The newly detected chemicals are important for both increasing engine efficiency and better understanding environmental emission. Concerning engine efficiency, we have seen ketoadroparoxide where their formation has been accepted for more than 50 years as responsible for cognition. This chemical was first experimentally characterized during a war. A better understanding of auto ignition chemistry will favor the development of advanced technology engines. Current types of engines are either poly-efficient gasoline engines or dirty diesel engines. Advanced technology engines are designed as hybrid between both types. With them, again, 40% in fuel economy can be targeted with very low emission. Concerning environmental emission, we have discovered large amount of carboxylic acid. Carboxylic acid are particularly responsible for the occurrence of acid rains. Since our study, this chemical are newly considered in engine chemistry study. In future, scientists should be able to probe still more chemical, even the most elusive one. This should be performed under actual fuel condition for the largest range of fuel, including those produced from biomass. I hope I convince you of the importance of studying trace pollutant. This does not only concern nitrogen oxide and soot particulate. The teeny one whose adverse effect will only be discovered in future years should not be forgotten. Studying trace chemical is also a key toward clean and efficient engine. Developing such engine will help to best manage the maybe long transition period between fossil fuel and alternative energy. This is urgent to preserve our planet and its nicest landscapes.