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

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On the eve of holiday departures and with sky-high fuel prices, it is legitimate to ask the question: do our cars consume too much? Even though Europe has decided to ban the production of vehicles at thermal motor from 2035, the majority of passenger vehicles in circulation in France and on the planet will operate with a thermal motorthat is, with an engine that uses a gasoline or diesel type fuel.

These motors have the function of converting the thermal energy that results from combustion fuel for mechanical energy, which will be used for movement the vehicle. About 40 to 50% energy supplied by the fuel is converted into mechanical energy, the rest is dispersed heat. The mechanical energy is not fully returned to the wheels of the vehicle and almost 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 engine

A heat engine consists of one combustion chamber in which fuel is burned withair. This leads to an increase in bind of gas in the combustion chamber, which will push a piston down. The latter is connected to a connecting rod, even connected to one crankshaft which will convert the vertical movement of the piston into rotation. This rotation is transmitted via the mechanical transmission (especially the gearbox velocity) to the wheels of the vehicle.

Valves will open and close to let air and fuel in and burn out gases via the exhaust pipe. Only a part (40 to 50%) of the thermal energy by combustion is converted into mechanical energy. The rest of this energy is lost and evacuated by the hot gases coming out of the exhaust and by radiator which cools the engine. Improving combustion combined with energy recovery systems can increase the percentage of energy converted and reduce fuel consumption by almost 30%.

Friction loss

It is now useful to define what is meant by friction. When two objects are brought into contact, the friction that occurs in the contact zones between these two objects will prevent one from sliding 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, the slippage between our shoes and the ground will be facilitated and it will be very difficult to move while walking. On the other hand, one can then choose cushions that will use the low friction with the ground to allow movement by sliding. Therefore, when we slide (or rub) two objects on top of each other, there will be one resistance due to friction. This leads to a loss of energy in the form of heat, which can be felt when e.g. rubs his hands. This is exactly what will happen between the moving parts in the engine and in the mechanical transmission, and whose impact we will assess.

That tribology is science dealing with problems of contact and friction and how to control them. Recent studies of tribology, made it possible to estimate the losses due to friction in the internal combustion engines and the transmissions to the vehicle wheels. The figure above shows in yellow the contact areas where friction loss occurs in an engine. The most significant losses occur around the piston (approx. 45% of the losses), in the connections between the connecting rod, the crankshaft and the engine block (approx. 30% of the losses) and around the valves and their actuation system (for approx. 10% of losses). The remaining 10% corresponds to losses in engine accessories.

The mechanical energy that comes out of the engine is reduced again in case of loss in the mechanical transmission, especially due to friction in the gearbox gear. The mechanical energy provided by the combustion in the internal combustion engine is ultimately reduced, under the average operating conditions of the vehicle, by approx. 30% due to all these losses.

Can we reduce consumption by limiting friction losses?

About 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 elements of 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 wear on these surfaces.

To further reduce friction losses, research in tribology relates to two areas. The first is the improvement of lubricants. This work aims at a better control of the variation of the lubricant 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 and lead to contact with surface roughness and faster wear. To this end, the development of new additives added to the lubricant, which allows the creation of protective layers with low friction on the surfaces, is also a subject of research.

The second part deals with the improvement of the surfaces themselves thanks to the production of coatings, especially based on carbon which ensures protection of the surfaces in contact and a lower friction. Another way to limit friction is through the use of surfaces textured by a network of cavities whose dimensions are optimized to allow more efficient lubrication.

Working as we recently performed 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.

For vehicles with internal combustion engines, various studies confirm that these new technologies can reduce the friction loss by 50 to 60% in the medium term for a gain in fuel consumption of around 15%. This gain may seem small, but if it is combined with an improvement of the engines and above all a reduction of the size and lot vehicles and therefore the width of tirefuel consumption savings on on the order of 50% are achievable. The increase in the segment of SUV on the market the automotive industryshows that unfortunately it is not a road that has gone by the motor vehicle manufacturers in recent years.

What are the very short term solutions to reduce the bill? Apart from buying a new vehicle, using higher performance 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 the individual because comparative studies are so far only available in the scientific and technical literature and therefore reserved for an informed public.

On the other hand, let us not forget that vehicles are designed to carry more passengers. that carpooling authorizes, if consumption is related to the number of passengers, to divide consumption by 2, 3, 4 or more. Rational use of vehicles remains the most efficient and simplest solution to reduce energy bills.

In the long run will electric car, now recognized by the EU and many manufacturers, is it a more effective solution in terms of friction loss? The answer is yes. As the number of mechanical parts in friction is very limited, these losses are estimated at less than 5%. However, there are many left lock to raise to make it the ideal solution: the weight and price of the batteries, the extraction of the materials needed for their manufacture and their recycling.The conversation

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