As holiday departures have begun and with sky-high 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 motor vehicles from 2035, the majority of passenger vehicles will be in circulation in France and on the planet with a thermal engine, that is, with an engine that uses petrol or diesel.
These engines have the function of converting the thermal energy coming from the combustion of fuel into mechanical energy, which will be used to start the vehicle. About 40 to 50% of the energy from the fuel is converted into mechanical energy, the rest is given off to heat. The mechanical energy is not completely 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?
How a heat engine works
A heat engine consists of a combustion chamber where the fuel is burned with air. This leads to an increase in the amount of gas in the combustion chamber, which will push a piston down. The latter is connected to a connecting rod, which in itself is connected to a crankshaft, which will convert the vertical movement of the piston into rotation. This rotation is transmitted via the mechanical transmission (especially the gearbox) to the wheels of the vehicle.
Valves will open and close to let air and fuel in and allow burned gases to escape through 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 the 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%.
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 you slide (or rub) two objects on each other, there will be a 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.
Tribology is the science that deals with contact and friction problems and how to control them. Recent tribological studies have made it possible to estimate friction losses in engine heaters and vehicle wheel transmissions. The figure above shows in yellow the contact areas where friction loss occurs in an engine. The largest 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 valve system. 10% of losses). The remaining 10% corresponds to losses in engine accessories.
The mechanical energy coming out of the engine is again reduced again reduced by the losses in the mechanical transmission, especially due to the 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 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. 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 carbon-based ones, which ensure 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 efficient lubrication.
Work 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.
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 combined with an improvement in the engines and, above all, a reduction in the size and mass of the vehicles and thus the width of the tires, a saving in fuel consumption of around 50% can be achieved. The growth of the SUV segment in the car market shows that it is unfortunately not a path that car manufacturers have taken in recent years.
What are the very short term 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 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. Carpooling allows, if the consumption is related to the number of passengers, to divide the 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, is the electric car, which is now favored by the EU and many manufacturers, a more efficient solution based on 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 still many obstacles to overcome 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.
This analysis is written by Noël Brunetière, research director at the University of Poitiers.
The original article was published on the site of The conversation.