Noël Brunetiere, University of Poitiers
With the rise in the price of gasoline with no sign of abating, it seems the opportune moment to ask ourselves if our cars are inefficient. Europe has decided ban the production of combustion engine vehicles by 2035but most of the passenger cars currently on the roads around the world still belong to this category.
Their engines work by burning gasoline or diesel and converting the resulting heat energy into mechanical energy, which is used to propel the vehicle. Around the 40-50% of power supplied is transformed into mechanical energy, but the rest is dissipated as heat. Also, not all of the mechanical energy reaches the wheels; almost the 30% is lost due to friction.
In the end, the actual energy used to move the vehicle amounts to approximately 30% of the total energy supplied by the fuel. Where does this waste occur? Are we able to reduce it? How much could we save in fuel consumption?
How does a combustion engine work?
In a combustion engine, a mixture of fuel and air is burned inside the combustion chamber. This increases the volume of gas in the chamber, and the resulting pressure pushes the piston 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.
A series of engine valves open and close, letting out waste gases and entering a fresh dose of air and fuel. A limited part (40 to 50%) of the thermal energy resulting from combustion is transformed into mechanical energy. The rest is wasted, expelled through hot exhaust gases and through the radiator, which keeps the engine cool.
However, by improving combustion and installing energy recovery systems, we can increase the amount of useful energy converted and reduce fuel consumption by almost 30%.
Wasted fuel due to friction
Friction is understood as the force that acts as resistance to the sliding movement between two objects when they come into contact. For example, the friction between our shoes and the ground allows us to walk without slipping. In cases of low friction, such as when the ground is frozen, it is easier for our shoes to slip.
The resistance force due to friction causes a 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.
Tribology is a branch of science that deals with contact, friction, and how to mitigate their effects. Recent research have helped to estimate the energy losses due to friction that occur in the combustion engine of a car and in the transmission linked to its wheels.
In the diagram above, the contact areas where friction losses occur are shown in yellow. The largest energy losses occur around the piston (approximately 45% of the losses); followed by the joints between the connecting rod, crankshaft and cylinder block (approximately 30%); and around the valves and their actuation system (approximately 10%). The remaining 10% is lost through other engine accessories.
The useful mechanical energy of the motor is seen even more restricted by the losses in the mechanical transmission, caused in particular by the friction between the gears. In short, all these losses translate into a waste of around 30% of the mechanical energy supplied by the combustion engine under average operating conditions of the vehicle.
How to limit energy losses
Since around 30% of a car’s fuel is used to overcome the friction between its moving mechanical parts, reducing these losses could lead to substantial fuel savings. We must look at the elements exposed to friction to analyze possible improvements. Engine and transmission components are already lubricated with oil, which is pushed between the surfaces to prevent friction and wear.
With the aim of further reducing energy losses due to friction, research in tribology covers two main areas. The first, referring to the improvement of lubricants, aims to control how certain properties of the lubricant, 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 uneven surface contact and faster wear. To combat this, one branch of research aims to develop new lubricant additives that can coat surfaces with low-friction protective layers.
The second area of research is to create new coatings (especially carbon-based) that protect the surfaces that come into contact with each other and result in less friction. Another possibility is to texture the surfaces with a network of holes, which have the optimal dimensions for more effective lubrication.
We have recently carried out a investigation project at the Institut Pprime de Poitiers (run by the CNRS, the University of Poitiers and ISAE Ensma) who have shown that friction from some types of contact can be reduced by 50% through the use of surface texturing.
In addition, in the case of combustion engine vehicles, several studies have confirmed that this technology can reduce energy losses due to friction by 50-60% in the medium term, which is equivalent to 15% less fuel consumption. When combined with upgraded engines and smaller, lighter vehicles – and ultimately narrower tires – the amount of fuel saved could potentially reach about 50%.
However, the expansion of the SUV sector in the automobile market tells us that, unfortunately, car manufacturers have not adopted this fuel-saving path in recent years.
Drive less, the simplest solution
So what are our immediate solutions to reduce costs? Except in the case of the purchase of new vehicles, the use of more efficient lubricants You can reduce consumption by a few percentage points, a negligible amount given the rise in fuel prices. As if that were not enough, it can be difficult for individuals to know which lubricant to choose, since comparative studies are currently only available in the scientific literature.
However, one must not forget that cars are made to transport several passengers. When fuel consumption is divided among several passengers, car sharing can reduce consumption two, three, four times and more. But when it comes to lowering your fuel bill, driving less is still the easiest and most effective solution.
With far fewer mechanical components exposed to friction, energy losses in electric cars have been assessed at less than 5%. But before they can be hailed as a silver bullet, we must consider all other nuts and boltsincluding the weight of the car, the cost of its battery and the extraction and recycling of its manufacturing materials.
Noël BrunetiereResearch Director, University of Poitiers
This article was originally published on The Conversation. read the original.
