With rising gas prices showing no signs of abating, it seems like a good time to ask: Are our cars not efficient enough? Europe has decided to ban the production of new vehicles powered by combustion engines by 2035, but the majority of passenger vehicles currently on the roads in France and around the world still fall into this category.
Their engines work by burning gasoline or diesel and converting the resulting thermal energy into mechanical energy, which is used to propel the vehicle. At most 50% of the supplied energy is converted into mechanical energy, but the rest is dissipated as heat. In addition, not all mechanical energy is delivered to the wheels, with almost 30% lost through friction.
In the end, the actual energy used to move the vehicle represents approximately 30% of the total energy supplied by the fuel. So where is all this waste produced, can we reduce it and how much can we reasonably expect to save on fuel consumption?
How a combustion engine works
In a combustion engine, a mixture of fuel and air is burned inside the part called the combustion chamber. This increases the volume of gas in the chamber and the resulting pressure pushes the piston component down. The piston is connected to the crankshaft by a connecting rod, which converts the vertical movement of the piston into a rotational movement. This rotation is then transferred by the crankshaft to the mechanical transmission (including the gearbox) and then to the wheels.
Several valves in the engine are then opened and closed, letting the waste gases out and then a fresh dose of air and fuel entering. A limited part (from 40 to 50%) of the thermal energy resulting from combustion is converted into mechanical energy. The rest is wasted, discharged through the hot exhaust gases and through the radiator, which keeps the engine cool. However, by improving combustion and installing energy recovery systems, we will be able to increase the amount of usefully converted energy and reduce fuel consumption by almost 30%.
Fuel lost due to friction
It is worth mentioning what is meant by “friction”. The term refers to the force that acts as resistance to sliding motion between two objects when they come into contact with each other. For example, the friction between our shoes and the floor allows us to walk without slipping. In cases of low friction, such as when the ground is icy, it is easier for our shoes to slide against the ground and walking becomes much more difficult. However, we could choose to wear skates, which use their low friction with the ground to allow us to glide.
Essentially, when two objects slide (or rub) together, the resulting drag force occurs due to friction. This leads to the loss of energy through heat, which we can observe by rubbing our hands, for example. In a car, exactly the same phenomenon occurs between the moving parts of the engine and the mechanical transmission. As researchers, we aim to assess the impact of this phenomenon.
“Tribology” is a branch of science concerned with contact, friction and how to mitigate its impacts. Recent research in this field has helped to estimate the energy losses due to the friction that occurs in a car’s combustion engine and in the transmission linked to its wheels. In the diagram above, the contact areas where frictional losses occur are shown in yellow. The most important energy losses occur around the piston (approximately 45% of the losses); followed by the links between the connecting rod, crankshaft and cylinder block (about 30%); and around the valves and their drive system (approximately 10%). The remaining 10% is lost through other engine accessories.
The useful mechanical energy of the engine is further restricted by the losses in the mechanical transmission, caused in particular by the friction between the gears. Ultimately, all these losses lead to a waste of around 30% of the mechanical energy supplied by the combustion engine in average vehicle operating conditions.
Could we reduce fuel consumption by limiting frictional energy losses?
Since about 30% of a car’s fuel is used to overcome friction between its moving mechanical parts, reducing these losses could mean substantial fuel savings. As such, we need to look at the elements exposed to friction to discuss possible improvements. The engine and transmission components are already lubricated with oil, which is inserted between the surfaces to prevent friction and wear.
With the aim of further reducing frictional energy losses, tribological research covers two main areas. The first concerns the improvement of lubricants. This research aims to manage how certain lubricant properties, such as viscosity, are affected by temperature. In general, friction tends to decrease when a less viscous lubricant is used, but its oil film may be too thin, causing more contact between uneven surfaces and faster wear. To combat this, a branch of research aims to develop new additives for lubricants that can coat surfaces with protective, low-friction layers.
The second area of research involves improving the surfaces themselves by creating new coatings (mainly carbon-based), which protect the surfaces that come into contact with each other and result in less friction. Alternatively, the surfaces can be textured with a network of holes, which have the optimal dimensions for more effective lubrication.
We have recently carried out a research project at the Institut Pprime de Poitiers (directed by the CNRS, the University of Poitiers and ISAE Ensma), which has shown that the friction of some types of contact can be reduced by a 50% by using the surface texture.
In addition, in the case of vehicles with a combustion engine, several studies have already confirmed that this new technology can reduce frictional energy losses by between 50 and 60% in the medium term, assuming a fuel consumption of around 15% less When combined with improved engines and smaller, lighter vehicles and, ultimately, narrower tires, this seemingly small amount of fuel savings could reach figures of around 50%. The expanding SUV sector in the automotive market, however, tells us that this fuel-saving route has unfortunately not been adopted by car manufacturers in recent years.
So what are our immediate solutions to reduce costs? Except for new vehicle purchases, the use of more efficient lubricants can reduce consumption by a few percent, a negligible amount in the face of rising fuel prices. In addition, it can be difficult for people to know which lubricant to choose, as comparative studies are currently only available in the scientific literature and are therefore restricted to a specialized reader.
However, we must not forget that cars are made to transport several passengers. When fuel consumption is divided among several passengers, car sharing can reduce consumption by two, three, four times more and more. But when it comes to reducing fuel bills, driving less is still the most efficient and simple solution.
As for the longer term, could the electric car — now much praised — be a more effective solution to reduce frictional energy losses? With far fewer mechanical components exposed to friction, energy losses in electric cars have been assessed at less than 5%. But before it can be considered a miracle solution, we have to consider all the other nuts and bolts, including the weight of the car, the cost of the battery and the extraction and recycling of its manufacturing materials.
