July 24, 2024

The rise in gasoline prices is not slowing down, so it’s time to ask: Are our cars too inefficient? Europe has decided to ban the production of new combustion engine-powered vehicles by 2035. However, most passenger cars currently on the road in France and around the world still fall into this category.

The engines convert the thermal energy from burning petrol or diesel into mechanical energy that is used to drive the vehicle. Only 50% of the energy supplied can be converted to mechanical energy. The remainder is lost as heat. The wheels do not receive all of the mechanical energy, as almost 30 % is lost to friction.

The energy required to move a vehicle is approximately 30% of all the energy provided by fuel. Where does this waste happen, can we reduce it, and how much power can we expect to save?

What is a combustion engine?

In a combustion motor, fuel and air are burned in the combustion chamber. The volume of the gas in the combustion chamber increases, and the pressure that results pushes the piston downward. A connecting rod connects the piston to the crankshaft, converting the vertical motion of the piston into rotational motion. The crankshaft then transfers this rotation to the mechanical transmission, including the gearbox, and finally to the wheels.

Diagram of a combustion motor: parts are moving (red) and areas of friction (yellow). Zephyris/Wikimedia/TCF, CC BY

The engine valves are then opened and closed, releasing waste gases and allowing in fresh air and fuel. Only a small portion (40-50%) of the thermal energy generated by combustion is converted to mechanical energy. The rest is lost and discharged via the hot exhaust gases, as well as through the radiator, which cools the engine. By improving combustion and installing energy-recovery systems, however, we could increase the amount of usefully converted energy and reduce fuel use by almost 30%.

Fuel waste due to friction

What is “friction”? This term is used to describe the force that acts to resist the sliding motion of two objects that come into contact. The friction between our shoe and the ground, for example, allows us to walk on the ground without slipping. When the floor is slippery, as it is when it is icy or snowy, our shoes will slide more easily, and walking is much harder. Skates, with their low friction, allow us to slide around the floor.

Friction is the main cause of the resistance that occurs when two objects slide (or rub) together. The friction causes energy to be lost through heat. We can see this by rubbing our hands together. The same thing happens in a car between moving engine parts and the mechanical transmission. We are researchers who want to understand the impact of such a phenomenon.

Tribology is a branch that deals with friction and contact and ways to minimize their effects. Recent research in this area has been able to estimate the energy loss due to conflict in the combustion engine of a car and the transmission connected to its wheels. The yellow areas in the diagram show the contact points where friction losses are experienced. Energy losses are greatest around the piston, at approximately 45%. Next comes the link between the connecting rod and the cylinder block (approximately 30%) and the valves with their actuation system (approximately 10%). The remaining 10% of energy is lost by other engine components.

The mechanical energy of the engine is restricted due to losses in the transmission system, mainly friction between gears. All these losses add up to a loss of about 30% of mechanical energy from the combustion engine under normal vehicle conditions.

Reduce fuel consumption by reducing frictional energy losses

Fuel savings could be substantial if these friction losses are reduced. Around 30% of the fuel used by a vehicle is spent to overcome friction. We must, therefore, examine the elements that are exposed to friction in order to identify possible improvements. Oil is used to lubricate the engine and transmission components. This oil is placed between surfaces in order to reduce friction and wear.

The goal of tribology is to reduce energy losses due to friction. It covers two major areas. The first area is improving lubricants. This research is aimed at managing how temperature affects certain properties of lubricant such as viscosity. The friction is reduced when the lubricant used is less viscous, but the oil film can be too thin. This leads to increased contact between surfaces and faster wear. In order to combat this problem, a branch of research is aimed at developing new additives that will coat surfaces with protective, low friction layers.

Reduce friction and wear on mechanical surfaces. Zephyris/Wikimedia/TCF, CC BY

Second, by improving the surface itself (using new coatings – primarily carbon-based ones), we can reduce friction and protect surfaces in contact. Surfaces can also be textured using a network with holes that are of the right size to provide better lubrication.

Recent research conducted at the Institut Pprime (led by CNRS University of Poitiers, ISAE Ensma and CNRS) in Poitiers has shown that surface texturing can reduce friction by up to 50% for certain types of contact.

This new technology, when combined with improved engines, smaller, lighter vehicles, and, ultimately, narrower tyres, could potentially save up to 50% of fuel. This seemingly small amount could reach figures as high as 50% when combined with smaller, lighter cars and improved engines. The growing SUV market in the automotive industry tells us, however, that car manufacturers have not adopted this fuel-saving option in recent years.

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What are the immediate cost-cutting solutions? More efficient Lubricants, except for the purchase of new vehicles, can reduce fuel consumption by only a few percentage points, which is a small amount when compared to the rising price of gasoline. It can also be difficult for consumers to decide which lubricant they should use since comparative studies only exist in scientific literature and are therefore limited to specialists.

We should not, however, forget that automobiles are designed to carry multiple passengers. Carpooling reduces fuel consumption when it is shared by multiple passengers. When it comes to cutting fuel costs, driving less is the most effective and straightforward solution.

Could the electric car, which is now highly praised, be more effective in the long term to reduce energy losses caused by friction? Electric cars are estimated to have energy losses of less than 5%, due to the fact that they have fewer mechanical parts exposed. Before it is hailed a miracle solution, we need to consider the rest of the nuts and bolts, including the weight of the car, the cost of the battery, and how the materials are extracted and recycled.


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