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

On the eve of vacation departures and with soaring fuel prices, it is legitimate to ask the question: do our cars consume too much? Even if Europe has decided to prohibit 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 to say with an engine using a fuel of the gasoline or diesel type.

These engines have the function of transforming the thermal energy resulting from the combustion fuel into mechanical energy which will be used to movement the vehicle. About 40 to 50% energy provided by the fuel is transformed into mechanical energy, the rest being dissipated in heat. The mechanical energy is not fully 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?

Operation of a heat engine

A heat engine consists of a combustion chamber in which fuel is burned withair. 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 via the mechanical transmission (in particular the gearbox speed) to the wheels of the vehicle.

Valves will open and close to let air and fuel in and burnt gases out via 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%.

Friction losses

It is now useful to define what is meant by friction. When two objects are brought into contact, the friction which appears in the contact zones between these two objects 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, for example 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 pads which will use the low friction with the ground to allow movement by sliding. When we slide (or rub) two objects on top of 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 perceptible when one rubs one’s hands for example. This is exactly what will happen between the moving parts in the engine and in the mechanical transmission and whose impact we will assess.

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

The mechanical energy that comes out of the motor is reduced again by losses in the mechanical transmission, in particular due to friction in the gears of the gearbox. The mechanical energy provided by the combustion within the internal combustion engine is ultimately reduced, under the average conditions of use of the vehicle, by approximately 30% due to all of these losses.

Can we reduce consumption by limiting friction losses?

About 30% of the fuel is therefore used to overcome friction between moving mechanical parts. A reduction in these losses suggests a substantial gain in consumption. It is now necessary to focus on the elements in 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 the wear of these surfaces.

To further reduce friction losses, research in tribology concerns two areas. The first is the improvement of lubricants. This work aims at a better control of the variation of the properties of the lubricant such as the 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. For this, the development of new additives added to the lubricant which allow 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 thanks to the production of coatings, in particular based on carbon which ensure the protection of the surfaces in contact and a lower friction. Another way of limiting friction is through the use of surfaces textured by a network of cavities whose dimensions are optimized to allow more effective lubrication.

Works that 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 certain types of contact thanks to surface texturing.

In the case of combustion engine vehicles, various studies confirm that these new technologies can in the medium term 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 above all a reduction in the size and the mass vehicles and therefore the width of the tiresfuel consumption savings of the order of 50% are attainable. The increase in the segment of SUV on the market automotiveshows that this is unfortunately not a path that has been taken by motor vehicle manufacturers in recent years.

In the very short term, what are the solutions to reduce the bill? Excluding the purchase of a new vehicle, the use of 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 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. the carpooling authorizes, if the consumption is related to the number of passengers, to divide the consumption by 2, 3, 4 or more. A rational use of vehicles remains the most effective and simplest solution to reduce the energy bill.

In the longer term, the electric car, which is now acclaimed 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 parts in friction being very limited, these losses are evaluated at less than 5%. There remain, however, many locks to raise to make it the ideal solution: the weight and the price of the batteries, the extraction of the materials necessary for their manufacture and their recycling.