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Practice is clearly trying to diverge from theory: automotive textbooks claim that an all-wheel drive car is more economical. This is true primarily for models whose transmission is built according to the classical scheme: permanent all-wheel drive, coaxial arrangement of shafts going to the front and rear axles, center and inter-wheel differentials, moreover, they are conical, symmetrical and free. Thus, the torque is distributed evenly over four wheels. However, schemes with other types of differentials are also possible. An indispensable condition for the theory to work is that additional friction should not arise in them.
For example, the Chevrolet Niva is perfect. True, she does not have a single-drive version. It’s not a problem, we’ll do it ourselves: it is enough to disconnect the front universal joint and block the center differential. In this case, twice as much torque will go to each rear wheel. This means that the rolling resistance force is higher, but not twice, but in proportion to the square of the input torque. Why? Due to greater deformation of the rear tires and additional losses in the increased contact patch. That is why, theoretically, the appetite of single-drive cars can increase.
The algorithm for calculating fuel consumption for a car with a drive on one axis and on both:
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1 - calculating the torques on the wheels, we take into account the efficiency of the transmission; for rear-wheel drive, it is slightly higher due to the fact that the center differential does not work (locked)
2 - when calculating the rolling resistance force, we take into account the size, the coefficient of rolling resistance and the coefficient of elasticity of tires
3 - from the external speed characteristics we learn the distribution of power and engine torque, as well as the amount of fuel consumed
4 - knowing the torques on the wheels and tire parameters, we calculate the rolling resistance forces
5 - work on overcoming rolling resistance is recalculated into fuel consumption per 100 kilometers
But really - how much more is it? Let's make a calculation for the first three Niva gears: there’s no point in driving higher because of the small torque on the wheels. Having counted fuel consumption at each point, we build curves. They show that, ceteris paribus, a car with permanent all-wheel drive, albeit not much (a maximum of about 0.3 l / 100 km), but still more economical than a single-drive. Check in practice? To the landfill!
Theoretical graphs of fuel consumption to overcome the rolling resistance of a Chevrolet Niva all-wheel drive (blue) and single-wheel drive:
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Day test "Niva" went in the usual all-wheel drive version, the other - with the front cardan removed and the drive only to the rear axle. To minimize the measurement error, we set the daily mileage of 250 km. Drivers changed every hour.
Honestly, none of us fully believed in the victory of theory over practice. Therefore, we gave science a head start - we set such modes of movement in which it is most likely to get a sensational result. Ideally, you need to constantly drive in the moment zone (for example, smoothly accelerate or storm the climb) and in lower gears. But this is unrealistic even in a landfill, not to mention public roads. Acceptable compromise: alternate vigorous, but without slipping acceleration to 70 km / h (sorting through the first, second, third gears) with smooth braking almost to a complete stop. It resembles a city ride, but without stops at traffic lights. The flow rate was determined by the topping method: they started with a full tank and at the end of the day replenished the supply.
According to the results of the first day, the average gasoline consumption of an all-wheel drive car was 14.8 l / 100 km. The next day, having lost one cardan, "Niva" ate an average of 14.2 liters per hundred.
The rear-wheel drive car has a higher rolling resistance force, therefore, theoretically, fuel consumption is greater. In fact, the situation is the opposite:
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Do not rush to throw stones at scientists who allegedly want to attract attention in any way. The theory works, but in different conditions. In our case, the engine torque and the mass of the car are not large enough so that the tire loss exceeds the transmission loss. Having loaded the car with ballast and blew out a good atmosphere from each tire, we may bring the theoretical result closer to the practical one, but the result will not change.
Therefore, for the average driver, the axiom that a single-drive is more economical than a full drive is still true. And to confirm the inverse theory there is another, more specific technique.
WHERE THE THEORY WORKS?
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Here is a classic example of the complete coincidence of theory with practice. The triaxial Ural-375 until 1965 had an asymmetric lockable center differential with the ability to connect the front axle. Then the design was simplified by applying a permanent four-wheel drive. The car immediately became more economical. This was noticeable already on the asphalt, and when driving on the ground, the difference in flow rate increased even more. Yes, with a decrease in the number of drive axles, losses in the transmission are reduced, but at the same time, losses in the wheels increase and traction decreases.
