Why do our cars “waste” two-thirds of their fuel?

At a time when millions of vehicles are going to cross paths on the road to summer vacation, the increase in fuel is in everyone's discussion. But do we know how a heat engine works and especially how our cars “waste” two-thirds of their fuel? Explanations. You will also be interested [IN VIDEO] Electric car versus thermal car: the point in 5 received ideas From 2035, the sale of diesel or gasoline vehicles will be prohibited in the European Union. Autonomy, cost, ecology… Are electric cars really competitive? On the eve of vacation departures and with soaring fuel prices, it is legitimate to ask the question: do our cars consume too much? Even if Europe has decided to ban the production of thermal engine vehicles from 2035, the majority of passenger vehicles in circulation in France and on the planet operate with a thermal engine, that is to say with an engine using gasoline or diesel type fuel. These engines have the function of transforming the thermal energy resulting from the combustion of fuel into mechanical energy which will be used to set the vehicle in motion. About 40 to 50% of the energy supplied by the fuel is transformed into mechanical energy, the rest being dissipated into heat. The mechanical energy is not entirely returned to the wheels of the vehicle and nearly 30% would be lost by friction. In the end, the energy used to actually move the vehicle represents only about 30% of the total energy provided by the fuel. Where do these losses come from? Can we reduce them? What gain can we expect on vehicle consumption? Operation of a heat engineA heat engine consists of a combustion chamber in which the fuel is burned with air. This leads to an increase in the volume of gas in the combustion chamber which will push a piston down. The latter is linked to a connecting rod, itself connected to a crankshaft which will transform the vertical movement of the piston into rotation. This rotation is transmitted via the mechanical transmission (in particular the gearbox) to the wheels of the vehicle. Valves will open and close to let in air and fuel and allow burnt gases to exit through the exhaust pipe. Only part (40 to 50%) of the thermal energy of combustion is transformed into mechanical energy. The rest of this energy is lost and evacuated by the hot gases coming out of the exhaust and by the radiator which cools the engine. Improving combustion combined with energy recovery systems can increase the percentage of energy transformed and reduce fuel consumption by almost 30%. Losses by friction It is now useful to define what is meant by friction. When two objects are brought into contact, the friction which appears in the contact zones between these two objects will oppose the sliding of one relative to the other. For example, the friction between our shoes and the ground allows us to move without slipping. If the friction is too low, for example when the ground is icy, it will be easier to slip between our shoes and the ground and it becomes very difficult to move around while walking. On the other hand, one can then opt for pads which will use the low friction with the ground to allow movement by sliding. When you slide (or rub) two objects on each other, there will therefore be a resistance due to friction. This leads to a loss of energy in the form of heat which is perceptible when one rubs one's hands for example. This is exactly what will happen between the moving parts in the engine and in the mechanical transmission and whose impact we will assess. Tribology is the science concerned with contact and friction problems and how to control them. Recent tribology studies have made it possible to estimate friction losses in heat engines and transmissions to vehicle wheels. The figure above shows in yellow the contact areas where friction losses occur in an engine. The most significant losses occur around the piston (about 45% of losses), in the connections between the connecting rod, the crankshaft and the engine block (about 30% of losses) and around the valves and their actuation system (for about 10% of losses). The remaining 10% correspond to losses in engine accessories. The mechanical energy that comes out of the engine is again reduced by the losses in the mechanical transmission, in particular because of the friction in the gears of the gearbox. The mechanical energy supplied by the combustion within the internal combustion engine is ultimately reduced, under the average conditions of use of the vehicle, by approximately 30% due to all of these losses. Can fuel consumption be reduced by limiting friction losses? Approximately 30% of fuel is therefore used to overcome friction between moving mechanical parts. A reduction in these losses suggests a substantial gain in consumption. It is now necessary to focus on the elements in friction to discuss the possible improvements. The engine and transmission parts are lubricated by an oil which is inserted between the surfaces and makes it possible to limit the friction and the wear of these surfaces. To further reduce friction losses, research in tribology concerns two areas. The first is improved lubricants. This work aims at a better control of the variation of the properties of the lubricant such as the viscosity with the temperature. In fact, friction is generally reduced when the viscosity is lower, but the oil film can become too thin and lead to contact with surface roughness and faster wear. For this, the development of new additives added to the lubricant which allow the creation of protective layers with low friction on the surfaces is also a subject of research. The second part concerns the improvement of the surfaces themselves thanks to the production of coatings, in particular carbon-based, which ensure the protection of the surfaces in contact and lower friction. Another way of limiting friction involves the use of surfaces textured by a network of cavities whose dimensions are optimized to allow more effective lubrication. Work that we recently carried out at the Pprime Institute in Poitiers (CNRS, University of Poitiers, ISAE Ensma) has shown that it is possible to reduce friction by 50% in certain types of contact thanks to surface texturing. In the case of combustion engine vehicles, various studies confirm that these new technologies can in the medium term reduce friction losses by 50 to 60% for a gain in fuel consumption of around 15%. This gain may seem small, but if it is combined with an improvement in the engines and above all a reduction in the size and mass of the vehicles and consequently the width of the tyres, savings in fuel consumption of around 50 % are achievable. The growth of the SUV segment in the automotive market shows that this is unfortunately not a path that has been taken by automotive manufacturers in recent years. In the very short term, what are the solutions to reduce the bill? If we exclude the purchase of a new vehicle, the use of more efficient lubricants can reduce consumption by a few percent, which remains low and does not compensate for the increase in fuel prices at the pump. In addition, the choice of a new lubricant remains complicated for an individual, because comparative studies are, for the time being, only available in the scientific and technical literature and therefore reserved for an informed public. On the other hand, let's not forget that vehicles are designed to carry several passengers. Carpooling authorizes, if the consumption is related to the number of passengers, to divide the consumption by 2, 3, 4 or more. A rational use of vehicles remains the most effective and simplest solution to reduce the energy bill. In the longer term, is the electric car, which is now favored by the European Union and many manufacturers, a more efficient solution from the point of view of friction losses? The answer is yes. The number of mechanical parts in friction being very limited, these losses are evaluated at less than 5%. However, there are still many obstacles to overcome to make it the ideal solution: the weight and price of the batteries, the extraction of the materials necessary for their manufacture and their recycling. Interested in what you just read?