Our cars “waste” two-thirds of their fuel

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On the eve of the holidays and the increase in fuel prices, it is legitimate to ask: are our cars consuming too much? Even if Europe decided to ban the production of cars in thermal motor from 2035, the majority of passenger vehicles in circulation in France and on the planet will operate on a thermal motormeaning an engine that uses gasoline or diesel fuel.

These machines have the function of converting thermal energy resulting from combustion fuel into mechanical energy to be used in mobility the car. About 40 to 50% energy provided by the fuel will be converted into mechanical energy, the rest will be lost heat. The mechanical energy is not fully returned to the wheels of the car and almost 30% is lost due to friction. Finally, the energy used in the actual movement of 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 benefits can we expect from car consumption?

The operation of a hot engine

A heat engine consists of a combustion chamber where the fuel is burned inWIND. This leads to an increase in quantity on gas in the combustion chamber which pushes a piston down. The latter is linked to a connecting rod, itself connected to a crankshaft which will convert the vertical motion of the piston into rotation. This rotation is transmitted through the mechanical transmission (especially the gearbox rapidly) to the wheels of the car.

The valves open and close to let air and fuel in and burn gases through the exhaust pipe. Only part (40 to 50%) of the heat energy of combustion is converted into mechanical energy. The rest of this energy is lost and displaced by the hot gases that exit the exhaust and the radiator which cools the engine. Improved combustion combined with energy recovery systems can increase the percentage of energy conversion and reduce fuel consumption by around 30%.

Friction losses

It is now useful to explain what is meant by friction. When two objects are brought into contact, the friction that appears in the contact zones between these two objects opposes the sliding of one relative to another. 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 frozen, sliding can be accelerated between our shoes and the ground and it becomes very difficult to move while walking. Alternatively, one can choose pads that use low friction on the ground to allow movement by sliding. If we slide (or rub) two objects on top of each other, we have a OPPOSITION due to friction. This leads to the loss of energy in the form of heat that appears when one rubs a hand for example. This is exactly what happens between the moving parts of the engine and the mechanical transmission and whose effect we will examine.

the tribology is the science concerned with contact and friction problems and how to control them. New studies in tribology, makes it possible to estimate losses through friction in internal combustion engines and transmissions to car wheels. The figure above shows the yellow contact areas where friction loss occurs in an engine. The most important losses occur around the piston (about 45% of the losses), at the connections between the connecting rod, the crankshaft and the engine block (about 30% of the losses) and around the valves and their movement system (for about 10% of losses). The remaining 10% corresponds to the loss of engine accessories.

The mechanical energy coming out of the motor is reduced again through mechanical transmission losses, mainly due to the friction of the gearbox gears. The mechanical energy provided by the combustion within the internal combustion engine is ultimately reduced, under normal conditions of use of the vehicle, by approximately 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 the moving mechanical parts. The reduction of these losses suggests a large gain in consumption. It is now necessary to focus on the elements of friction to discuss possible developments. The engine and parts of the transmission are lubricated with an oil that is inserted between the surfaces and makes it possible to limit the friction and the wear of these surfaces.

To further reduce friction loss, tribology research is concerned with two areas. The first is the development of lubricants. This work aims at a better control of the variation of lubricant properties such as viscosity with temperature. In fact, friction is generally reduced when the viscosity is lower, but the oil film can be very thin and lead to contact surface adhesion and faster wear. For this, the development of new additives added to the lubricant that allows the creation of protective layers with less 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, mainly based on carbon which ensures the protection of contact surfaces and a low friction. Another way to limit friction involves the use of surfaces textured with a network of holes whose dimensions are optimized to make lubrication more effective.

Works which 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 some types of contact thanks to surface texturing.

In the case of combustion engine vehicles, various studies have confirmed that these new technologies can in the medium term reduce friction losses by 50 to 60% for a gain in fuel consumption of approx. 15% This profit may seem small, but if it is combined with an improvement in machines and above all a reduction in size and mass cars and therefore the width of surroundingfuel consumption savings on the order of 50% is achieved. The increase in share of SUV in the market vehiclesshows that this is unfortunately not a path that motor vehicle manufacturers have taken in recent years.

In the short term, what are the solutions to reduce the bill? Besides buying a new car, the use of higher performance lubricants can reduce consumption by a few percent, which remains low and does not compensate for the increase in the price of gasoline at the pump. In addition, the choice of a new lubricant remains complicated for an individual, because comparative studies, at the moment, are only available in the scientific and technical literature and are therefore reserved for an informed which is public.

On the other hand, we must not forget that the vehicles are designed to carry many passengers. the carpooling allows, if the consumption is related to the number of passengers, to divide the consumption by 2, 3, 4 or more. The rational use of vehicles remains the most effective and simplest solution to reduce energy bills.

In the longer term, the electric car, which is now recognized 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. The number of mechanical friction parts is very limited, these losses are estimated at less than 5%. There remain, however, many locks to increase the production of this ideal solution: the weight and the price of the batteries, the acquisition of the materials necessary for their production and their recycling.The Conversation

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