#cars #waste #twothirds #fuel
On the eve of going on vacation and with fuel prices on the rise, it is legitimate to ask: Do our cars consume too much? Although Europe has decided to ban the production of vehicles with a thermal engine from 2035, the majority of passenger cars on the road in France and on the planet are powered by a thermal engine, that is, with an engine that uses gasoline or diesel fuel.
These engines have the function of transforming the thermal energy resulting from the combustion of the fuel into mechanical energy that will be used to start the vehicle. About 40 to 50% of the energy supplied by the fuel is transformed into mechanical energy, the rest is dissipated into heat. Mechanical energy is not fully returned to the vehicle’s wheels and nearly 30% would be lost through friction.
In the end, the energy used to move the vehicle represents only around 30% of the total energy provided by the fuel. Where do these losses come from? Can we reduce them? What gain can we expect in vehicle consumption?
Operation of a heat engine.
A 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 attached to a connecting rod, in turn connected to a crankshaft that will transform the vertical movement of the piston into rotation. This rotation is transmitted through the mechanical transmission (particularly the gearbox) to the wheels of the vehicle.
The valves will open and close to let in air and fuel and allow burned gases to exit through the exhaust pipe. Only a part (40 to 50%) of the thermal energy of combustion is transformed into mechanical energy. The rest of this energy is lost and is evacuated by the hot gases that come out of the exhaust and by the radiator that cools the engine. Combustion improvement combined with energy recovery systems can increase the percentage of energy transformed and reduce fuel consumption by almost 30%.
Now it is useful to define what is meant by friction. When two objects are put in contact, the friction that appears in the contact zones between these two objects will oppose the sliding of one with respect 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 frozen, it will be easier to slide between our shoes and the ground and it becomes very difficult to move when walking. On the other hand, you can opt for pads that will take advantage of the low friction with the ground to allow movement by sliding. When you slide (or rub) two objects against each other, there will be a resistance due to friction. This leads to a loss of energy in the form of heat that is noticeable when you rub your hands together, for example. This is exactly what will happen between the moving parts of the engine and the mechanical transmission and whose impact we will evaluate.
Tribology is the science that deals with contact and friction problems and how to control them. Recent tribology studies have made it possible to estimate friction losses in thermal engines and vehicle wheel transmissions. The greatest losses occur around the piston (about 45% of losses), at the connections between the connecting rod, crankshaft and cylinder block (about 30% of losses) and around the valves and their actuation system (for approximately 10% of losses). The remaining 10% corresponds to losses in engine accessories.
The mechanical energy leaving the motor is again reduced by losses in the mechanical transmission, in particular by friction in the gears of the gearbox. The mechanical energy provided by combustion inside the internal combustion engine is finally reduced, in the average conditions of use of the vehicle, by approximately 30% due to all these losses.
Can we reduce consumption by limiting friction losses?
Therefore, about 30% of the fuel is used to overcome friction between moving mechanical parts. A reduction in these losses suggests a substantial gain in consumption. Now it is necessary to focus on the friction elements to discuss possible improvements. The engine and transmission parts are lubricated by an oil that is inserted between the surfaces and makes it possible to limit the friction and wear of these surfaces.
To further reduce friction losses, tribology research focuses on two areas. The first is improved lubricants. This work aims to better control 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 become too thin and cause rough surface contact and faster wear. For this, the development of new additives added to the lubricant that allow the creation of low-friction protective layers on the surfaces is also being investigated.
The second part refers to the improvement of the surfaces themselves thanks to the production of coatings, in particular carbon-based, which guarantee the protection of the surfaces in contact and less friction. Another way to limit friction is by using surfaces textured by a network of cavities whose dimensions are optimized to allow more effective lubrication.
A piece of work that we have recently carried out at the Pprime de Poitiers Institute (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 vehicles with a combustion engine, various studies confirm that these new technologies can reduce friction losses in the medium term by between 50 and 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 tires, savings in consumption can be achieved. about 50% fuel. The growth of the SUV segment in the automotive market shows that unfortunately this is not a path that car manufacturers have taken in recent years.
What solutions to reduce the bill?
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 small percentage, which is still 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, at the moment, only available in the scientific and technical literature and are therefore reserved for an informed public.
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On the other hand, let’s not forget that vehicles are designed to carry multiple passengers. Carpooling authorizes, if the consumption is related to the number of passengers, dividing the consumption by 2, 3, 4 or more. A rational use of vehicles continues to be the most effective and simple solution to reduce the energy bill.
In the longer term, is the electric car, now favored by the European Union and many manufacturers, a more efficient solution from a friction loss point of view? The answer is yes. Being very limited the number of mechanical parts in friction, these losses are evaluated in less than 5%. However, there are still many obstacles to overcome for it to be the ideal solution: the weight and price of the batteries, the extraction of the materials necessary for their manufacture and their recycling.