Machine Design

Questions to embarrass know-it-all engineers

Many engineers do not understand the technical principles behind commonly encountered mechanisms and physical phenomena. If you happen to be at a social gathering where engineers are present and the conversation flags a bit, here are topics that can liven up things considerably.

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Khol editorials

  1. How does an automotive differential work? (Note, I did not ask you to explain what it does. Explain how it functions internally.)

  2. What is the difference between full-time and part-time four-wheel drive?

  3. In the worst case, and if the differentials are not limited slip, what is the minimum number of wheels that will be driven with full-time four-wheel drive?

  4. How does all-wheel drive differ from four-wheel drive, and what piece of equipment makes a vehicle all-wheel drive?

  5. Explain the process by which a torque converter, the coupling device used in virtually every automotive automatic transmission being manufactured today, delivers more output torque than input torque.

  6. In a single-joint Cardan coupling (commonly called a "universal joint" in automotive parlance), under what condition does the speed of the output not track exactly with the speed of the input?

  7. Explain the process by which an unsymmetrical airfoil at zero angle of attack produces aerodynamic lift. (Explain the process in Newtonian terms without referring to Bernoulli's Theorem or utilizing mathematics of any form.)

  8. Sketch a sectioned schematic of a turbojet engine and explain where the thrust comes from, then trace its path through the engine case to the engine mounts or pylon. (The schematic must be of a pure turbojet with no fan bypass.)

The answers can be found below.

Warning: I have no answer for Question No. 8. Initially I assumed that thrust was generated on the forward end of the burner cans by pressure generated when fuel burns in the cans in the same manner that thrust is generated in a rocket nozzle. But after observing an engine cut in cross section for display purposes, I saw that the cans are made of metal too thin to sustain a meaningful structural load.

Also, I am pretty much at a loss to answer Question 7. I have an idea of what might explain the decrease in pressure of faster moving air, but it sounds so hokey I am embarrassed to offer it.

-- Ronald Khol, Editor
Send feedback to MDeditor @


1. The gear at the end of the drive shaft engages a ring gear carrying a number of planet pinions that, in turn, engage bevel gears attached to the axles. If the axles, and thus the bevel gears, are constrained to turn in the same direction and at the same speed, as they are on dry pavement, the pinions do not rotate but merely act as keys to lock the bevel gears to the ring gear. When a wheel loses traction, it spins faster than the opposing wheel, allowing the planet pinions to rotate, thereby causing the other wheel to turn more slowly or, in the worst case, stop completely. The pinions thus act as fulcrums for a force (torque) balance between the bevel gears.

2. Full-time four-wheel drive incorporates a differential between front and rear drive shafts, preventing potentially damaging axle wind-up when the vehicle is driven on dry pavement.

3. In the worst case, without limited-slip differentials, full-time four-wheel drive becomes one-wheel drive.

4. All-wheel drive incorporates a differential with limited slip capabilities (or slip-detecting lockup) between front and rear driveshafts.

5. A torque converter is conceptually similar to a fluid coupling in that both have an input impeller that drives hydraulic fluid against an output turbine. The torque converter differs in that it has an intermediate set of vanes called a stator, which changes the direction of the oil coming off the turbine so that it flows in the same direction the impeller rotates. This change lowers the drag on the impeller allowing it turn appreciably faster than the impeller in a fluid coupling. The process produces something akin to a hydraulic mechanical advantage resulting in torque being greater at the output than at the input.

6. If the input shaft is at an angle to the output shaft, with a constant input speed the output speed varies sinusoidally with each rotation of the coupling. The speed variation is proportional to the angular offset of the input and output.

7. I don't know the answer, but here is a half-baked explanation. If two adjacent air molecules are separated by the airfoil as it passes, they supposedly meet and become adjacent again at the rear of the airfoil. The molecule traveling over the top surface moves faster over it because it traverses a longer path than a molecule on the bottom surface. Since air pressure is the impingement of a vibrating molecule against a surface, the molecule moving faster impinges fewer times on the airfoil, producing less pressure, resulting in lift. Readers with a better explanation are invited to submit it to mdeditor @

8. I have never been able to figure out where the thrust comes from, and people who claim to know can't offer an explanation that clarifies it for me. Again, readers who know the answer can forward it to mdeditor @

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