Guest Author - William Charles Vetter

From my previous article on piston engines you know that the pistons drive the main crank in your engine so that it spins. Idling, it spins around 900rpm. At speed it can be anything up to 7,500rpm. You can't simply connect a set of wheels to the end of the crankshaft because the speed is too high and too variable, and you'd need to stall the engine every time you wanted to stand still. Instead you need to reduce the revolutions of the crankshaft down to a usable value. This is known as gearing down—the mechanical process of using interlocking gears or a combination of hydraulics and planetary gears in the case of an automatic transmission to reduce the number of revolutions of that spinning crankshaft.

Unlike an electric motor, internal combustion engines create very little actual horsepower at lower spinning speeds commonly known as low rpm, as the engine speed increases so does horsepower, in what could be graphed as a curve, with rpm at the bottom and horsepower on the vertical axis. Eventually the engine will reach maximum rpm, the power decreases as this maximum speed nears. It would be very uneconomical to drive with the engine at its maximum speed not to mention that this causes undue strain on the entire engine. When you step on the accelerator sharply you will notice that an automatic transmission will down shift for an instant increase in horsepower and then upshift again to reduce engine speed as necessary for comfort, economy and engine longevity. If your car has a manual transmission you will have to manually down shift to achieve maximum acceleration. This downshift moves the engine to a point in its power curve where maximum horsepower is achieved.
The reason that we shift gears in a car is so that the engines speed under the hood matches the speed of the road beneath us. While you drive the sound of the engine only sounds normal if the car is in drive as you accelerate to highway speeds. Cars equipped with automatic transmissions use a dash mounted tachometer to allow the driver to monitor engine rpm. Tachometers disappeared from most cars’ instrument clusters during the 1960s; thankfully they have returned in the last ten years most likely due to the fact that aftermarket tachometers were one of the most added accessories to cars of this era, ditto for the much maligned red “idiot” lights.

Transmissions have gone through a steady stream of improvements since the first cars started down the road 100 years ago. Most of these past improvements were aimed strictly at improving performances in speed and horsepower but the next 10 years will see even more advancements as engineers work overtime to make engines and their transmissions ever more economical without losing any performance gains of the past.

Next week what is a rear end differential, CV joints and why do I need these things.