How our cars “waste” two-thirds of their fuel

These engines have the function of converting the thermal energy produced by the combustion of fuel into mechanical energy that will be used to move the vehicle. About 40-50% of the energy supplied by the fuel is converted into mechanical energy, and the rest is used as heat. The mechanical energy is not completely returned to the wheels of the car and about 30% will be lost due to friction.

Finally, the energy used to actually move the vehicle is only 30% of the total energy provided by the fuel. Where do these losses come from? Can we reduce them? What profit can be expected in car consumption?

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Heat engine operation

A heat engine consists of a combustion chamber where fuel is burned with air. This leads to an increase in the volume of gas in the combustion chamber, which pushes the piston down. The latter is connected to a connecting rod, which itself is connected to a crankshaft that will convert the vertical movement of the piston into rotation. This rotation is transmitted to the wheels of the car through a mechanical transmission (especially a gearbox).

Composition of a heat engine: movement of parts (red) and friction zones (yellow). Zephyris/Wikimedia, CC BY

The valves will open and close to admit air and fuel and allow burnt gases to exit the tailpipe. During combustion, only part of the thermal energy (40-50%) is converted into mechanical energy. The rest of this energy is lost and dissipated by the hot gases from the exhaust and the radiator that cools the engine. Combustion improvements combined with energy recovery systems can increase the percentage of energy converted and reduce fuel consumption by about 30%.

Friction losses

Now it is useful to define what is meant by friction. When two objects are in contact, the friction created 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, it will be easier to slip between our shoes and the ground, and it becomes very difficult to move while walking. On the other hand, you can choose cushions that will use low friction with the ground to allow for sliding motion. When two objects slide (or rub) against each other, there will therefore be resistance due to friction. This leads to energy loss in the form of heat, which is felt when, for example, rubbing hands. This is exactly what will happen between the moving parts in the engine and the manual transmission and what we will evaluate the effect of.

Tribology is the science of contact and friction problems and their management. Recent tribological studies have made it possible to estimate friction losses in heat engines and transmissions to automobile wheels. The figure above shows in yellow the areas of contact where frictional losses occur in the engine. The most significant losses occur around the piston (about 45% of losses), connecting rod, crankshaft and engine block (about 30% of losses) and around the valves and their actuation system (about 10%). losses). The remaining 10% corresponds to losses in engine accessories.

The mechanical energy from the engine is again reduced by losses in the mechanical transmission, especially due to friction in the gears of the gearbox. Due to all these losses, the mechanical energy provided by combustion in an internal combustion engine is ultimately reduced by about 30% under the conditions of average use of the vehicle.

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Can we reduce consumption by limiting friction losses?

Therefore, about 30% of the fuel is used to eliminate friction between moving mechanical parts. The reduction of these losses indicates a significant gain in consumption. Now it is necessary to focus on the elements of friction to discuss possible improvements. Engine and transmission parts are lubricated with oil, which is inserted between the surfaces and allows to limit the friction and wear of these surfaces.

To further reduce frictional losses, research in tribology covers two areas. The first is improved lubricants. The purpose of this work is to better control the change in lubricant properties, such as viscosity, with temperature. In fact, when the viscosity is low, friction is generally reduced, but the oil film can be very thin and contact with surface roughness and faster wear. For this purpose, the development of new additives that allow the creation of protective layers with low friction on the surfaces added to the lubricating oil is also a subject of research.

Solutions used to reduce friction and wear in mechanical contacts. Provided by the author

The second part refers to the improvement of the surfaces themselves, thanks to the production of carbon-based coatings in particular, which ensure protection of the surfaces in contact and less friction. Another way to limit friction involves the use of textured surfaces with an optimized mesh of voids to allow more effective lubrication of the dimensions.

Our recent work at the Pprime Institute in Poitiers (CNRS, University of Poitiers, ISAE Ensma) has shown that it is possible to reduce friction by 50% in certain contact types thanks to surface texturing.

As for combustion engine vehicles, various studies confirm that these new technologies can reduce friction losses by 50-60% for an average increase in fuel consumption of about 15%. This gain may seem small, but if it is combined with the improvement of engines and, above all, the reduction of the size and mass of the cars and, consequently, the width of the wheels, it is possible to save about 50% in fuel consumption. The growth of the SUV segment in the car market shows that, unfortunately, this is not the way car manufacturers have been going in recent years.

What solutions are there to reduce the bill in a very short time? If we exclude the purchase of a new car, the use of more efficient lubricants can reduce consumption by several percent, which remains low and does not compensate for the increase in the price of fuel at the pump. Moreover, the choice of a new lubricant remains complicated for the individual, since comparative studies are currently only available in the scientific and technical literature and are therefore intended for the informed public.

On the other hand, let’s not forget that vehicles are designed to carry several passengers. Carpooling allows you to divide the consumption by 2, 3, 4 or more if the consumption is related to the number of passengers. The rational use of vehicles remains the most effective and simple solution to reduce energy costs.

In the long term, is the electric car currently favored by the European Union and many manufacturers a more efficient solution in terms of frictional losses? The answer is yes. Since the number of mechanical parts in friction is very limited, these losses are estimated to be less than 5%. However, there are still many obstacles to make it an ideal solution: the weight and cost of batteries, the extraction and recycling of the materials needed to manufacture them.

Article from The Conversation

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