Our cars “waste” two-thirds of their fuel

On the eve of holiday departures and with rising fuel prices, it is legitimate to ask the question: are our cars consuming too much? Even if Europe has decided to ban the production of vehicles thermal engine from 2035, the majority of passenger cars in circulation in France and around the world will drive with a thermal enginethat is, with an engine using a gasoline or diesel fuel.

These motors have the function of converting the resulting thermal energy combustion fuel into mechanical energy that is used movement the vehicle. Around 40 to 50% energy The energy provided by the fuel is converted into mechanical energy, the rest is dissipated warmth. The mechanical energy is not completely and almost not returned to the wheels of the vehicle 30% would be lost through friction. In the end, the energy used to actually move the vehicle is only about 30% of the total energy provided by the fuel. Where are these losses coming from? Can we reduce them? What profit can we expect in terms of vehicle consumption?

operation of a heat engine

A heat engine consists of a combustion chamber in which fuel is burnedAir. This leads to an increase in volume from gas in the combustion chamber pushing a piston down. The latter is connected to a connecting rod, which in turn is connected to a crankshaft whereby the vertical movement of the piston is converted into rotation. This rotation is transmitted above the mechanical power transmission (particularly the gearbox speed) on the wheels of the vehicle.

The valves open and close to let in air and fuel and let out burned gases above the exhaust pipe. Only part (40 to 50%) of the thermal combustion energy is converted into mechanical energy. The rest of this energy is lost and dissipated through the hot gases exiting the exhaust and through the cooler what cools the engine. Improved combustion combined with energy recovery systems can increase the percentage of energy converted and reduce fuel consumption by almost 30%.

friction losses

It is now useful to define what is meant by friction. When two objects are brought into contact, the friction that occurs in the zones of contact between these two objects opposes the sliding of one relative to the other. For example, the friction between our shoes and the ground allows us to move without slipping. When the friction is too low, for example when the ground is icy, sliding between our shoes and the ground becomes easier and it becomes very difficult to move while walking. On the other hand, one can then opt for pads that use the low friction with the ground to allow movement by sliding. Therefore, when we slide (or rub) two objects over each other, we get a Resistance due to friction. This leads to a loss of energy in the form of heat, which can be felt when rubbing your hands, for example. This is exactly what will happen between the moving parts in the engine and the mechanical transmission and we will evaluate its effects.

That tribology is the science that deals with contact and friction problems and how to control them. Current Studies of tribology made it possible to estimate the friction losses in the internal combustion engines and the transmissions to the wheels of the vehicle. The figure above shows in yellow the contact areas where friction losses occur in an engine. The largest losses occur around the piston (approx. 45% of the losses), in the connections between the connecting rod, crankshaft and engine block (approx. 30% of the losses) and around the valves and their actuation system (approx. 10% of the losses). of losses). The remaining 10% are losses in engine accessories.

The mechanical energy coming out of the engine is reduced again due to losses in the mechanical transmission, in particular due to friction in the gears of the transmission. All of these losses ultimately reduce the mechanical energy supplied by the combustion within the internal combustion engine by around 30% under the average operating conditions of the vehicle.

Can we reduce consumption by limiting friction losses?

Around 30% of the fuel is therefore used to overcome the friction between moving mechanical parts. A reduction in these losses indicates a significant gain in consumption. It is now necessary to focus on the friction elements to discuss the possible improvements. The engine and transmission parts are lubricated by an oil that is introduced between the surfaces and makes it possible to limit the friction and wear of these surfaces.

In order to further reduce friction losses, research in tribology deals with two areas. The first is the improvement of lubricant. This work aims to better control the variation of the lubricant’s properties, such as viscosity with temperature. In fact, friction is generally reduced when the viscosity is lower, but the oil film can become too thin, leading to contact with surface roughness and faster wear. In addition, the development of new additives added to the lubricant, which enable the build-up of low-friction protective layers on the surfaces, is also the subject of research.

The second part concerns the improvement of the finishes themselves thanks to the manufacture of coatings, in particular based on carbon which ensure the protection of the contact surfaces and less friction. Another way of limiting friction consists in using surfaces structured by a network of cavities whose dimensions are optimized to allow more effective lubrication.

Is working that we recently carried out at the Pprime Institute of Poitiers (CNRS, University of Poitiers, ISAE Ensma) showed that it is possible to reduce friction by 50% in certain types of contact thanks to surface texturing.

In the case of vehicles with internal combustion engines, various studies confirm that these new technologies can reduce friction losses by 50 to 60% in the medium term, with a consumption advantage of around 15%. This gain may seem small, but when combined with an improvement in engines and, above all, a reduction in size and Dimensions Vehicles and thus the width of the tiresfuel savings of in the order of 50% are achievable. The increase in the segment of SUV on the market automobileshows that this is unfortunately not the path that car manufacturers have taken in recent years.

What short-term solutions are there to reduce the bill? Excluded is the purchase of a new vehicle, the use of high performance lubricants can reduce consumption by a few percent, which remains small and does not compensate for the increase in fuel prices at the pump. In addition, the choice of a new lubricant remains complicated for the individual, since comparative studies are only available in the scientific and technical literature and are therefore reserved for an informed public.

On the other hand, we must not forget that vehicles are designed to carry multiple passengers. the carpooling entitled, if the consumption is based on the number of passengers, to divide the consumption by 2, 3, 4 or more. Rational use of vehicles remains the most effective and simple solution to reduce energy bills.

Longer term is the electric car, which is now being praised by the European Union and many manufacturers, is it a more effective solution from the point of view of friction losses? The answer is yes. Since the number of mechanical parts subjected to friction is very limited, these losses are rated at less than 5%. However, many remain Castles 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.