As vacation departures approach and with soaring 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 combustion engine vehicles from 2035, the majority of passenger vehicles in circulation in France and on the planet operate with this type of engine, i.e. using gasoline or diesel fuel.
These engines have the function of transforming thermal energy, resulting from the combustion of fuel, into mechanical energy which will be used to set the vehicle in motion. About 40% to 50% of the energy supplied by the fuel is transformed into mechanical energy, the rest being dissipated into heat. The mechanical energy is not entirely returned to the wheels of the vehicle and nearly 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?
Half of thermal energy becomes mechanical energy
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 linked to a connecting rod, itself connected to a crankshaft which will transform the vertical movement of the piston into rotation. This rotation is transmitted by the mechanical transmission (in particular the gearbox) to the wheels of the vehicle.
Valves will open and close to let in air and fuel and allow burnt gases to exit through the exhaust pipe. Only part (40% to 50%) of the thermal energy of combustion is transformed 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 transformed 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 appears in the contact zones between them will oppose the sliding of one 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, such as when the ground is icy, the sliding will be facilitated between our shoes and the ground and it becomes very difficult to move while walking. On the other hand, one can then opt for skates, which will use the low friction with the ground to allow movement by sliding.
When we slide (or rub) two objects on each other, there will therefore be a resistance due to friction. This leads to a loss of energy in the form of heat, which is noticeable when rubbing hands. This is exactly what will happen between the moving parts in the engine and in the mechanical transmission and the consequences of which we will assess.
Zephyris via Wikimedia Commons. |
Tribology is the science concerned with problems of contact and friction and how to control them. Recent tribology studies have made it possible to estimate friction losses in heat engines and transmissions to vehicle wheels. The figure above shows in yellow the contact areas where friction losses occur in an engine.
The greatest losses occur around the piston (about 45%), in the connections between the connecting rod, the crankshaft and the engine block (about 30%) and around the valves and their actuation system (about 10% ). The remaining 15% correspond to losses in engine accessories.
The mechanical energy that leaves the engine is again reduced by the losses in the mechanical transmission, in particular because of the friction in the gears of the gearbox. The mechanical energy supplied by the combustion within the internal combustion engine is ultimately reduced by approximately 30%, under the average conditions of use of the vehicle, due to all of these losses.
Since 30% of the fuel is used to overcome the friction between the moving mechanical parts, a reduction in these losses would therefore suggest a substantial gain in consumption. But we must now focus on the elements in friction to discuss 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 the wear of these surfaces. To further reduce friction losses, tribology research focuses on two axes.
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. For this, the development of new additives, added to the lubricant and allowing the creation of protective layers with low friction on the surfaces, is also a subject of research.
The second part concerns the improvement of the surfaces themselves, in particular thanks to the production of carbon-based coatings which ensure the 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 effective lubrication.
more efficient engines,
less massive cars
Work that we have recently carried out at the Pprime Institute in Poitiers (National Center for Scientific Research, University of Poitiers, National School of Mechanics and Aerotechnics) has shown that it is possible to reduce friction by 50% in certain types of contact, thanks to surface texturing.
In the case of combustion engine vehicles, various studies confirm that these new technologies can make it possible, in the medium term, to reduce friction losses by 50% to 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 especially with a reduction in the size and mass of the vehicles and consequently in the width of the tyres, savings in fuel consumption of the order of 50% are attainable. The growth of the SUV segment in the automotive market shows that this is unfortunately not a path that has been chosen by manufacturers in recent years.
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 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 an individual, because comparative studies are, for the time being, only available in the scientific and technical literature and therefore reserved for an informed public.
On the other hand, let’s not forget that vehicles are designed to carry several passengers. Carpooling authorizes, if the consumption is related to the number of passengers, to divide the consumption by two, three, four or more. A rational use of vehicles remains the most effective and simplest solution to reduce the energy bill.
In the longer term, is the electric car, which is now favored by the European Union and many manufacturers, a more efficient solution from the point of view of friction losses? The answer is yes. The number of mechanical parts in friction being very limited, these losses are evaluated 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 necessary for their manufacture and their recycling.
This article is republished from The Conversation under a Creative Commons license. Read the original article.