What is the Purpose of a Transmission? Why is a Transmission Necessary?

A transmission in a car or other vehicle or application interfaces the input (for example, an engine) to an output (for example, wheels).

First, the output speed of the engine needs to be matched to the speed required by the wheels.

Take for example a typical car traveling at 65 mph (miles per hour).  With 28 inch diameter tires, the wheel speed is roughly 780 rpm (revolutions per minute).  A typical gasoline engine in a passenger car is simply not going to able to produce the required torque at 780 rpm to keep the vehicle at that speed (at least not without lugging the engine). 

So matching the input speed from the engine to the required output speed for the wheels is the first function of a vehicle transmission.

Alright, so let's suppose we make our simple transmission be a reduction gearbox in the required range.  Suppose after analysis we decide the engine in our example car should operate at 2600rpm to keep the vehicle moving smoothly at 65 mph.  So some simple math would tell us the required ratio for our gearbox would be 2600/780 or 3.33.  That is, for every 3.33 turns of the input (engine), the output (wheels) would turn once.  So that's great and we're finished with our job right!  Not quite...

What about when our example car is moving at 10 mph in a school zone?  The wheel speed would be roughly 120 rpm.  With out 3.33 ratio gearbox, the engine speed would be about 400 rpm.  That's not going to work so well for our engine, as most gasoline engines require a speed of at least 550rpm to run smoothly.  The engine might even stall out at 400 rpm.  So now what can we do?

The concept of multiple gear ratios 

Well what we need is to make the ratio of our gearbox variable.  So let's say at 10mph we decide our engine should operate at 700 rpm.  So our desired gearbox ratio for this operating condition is 700/120 or 5.83.  So for every 5.83 turns of the input (engine), the output (wheels) turn 1 time.  Now let's name our gears.  We'll call this lower gear (the 5.83 ratio) 'first' gear, and the higher gear ratio we worked out earlier (3.33) 'second' gear. 

Changing between different gear ratios

Now we need a way to change between first and second gear.  In a manual transmission, the gear changes (shifting gears) is done manually via the gearshift and use of a clutch to disengage the input (engine) from the transmission during the shifting.  In an automatic transmission, a similar thing takes place, but is controlled by a computer (or a mechanical hydraulic circuit in older designs).

Decoupling the input from the output 

But now let's suppose we want to stop at a traffic light with our example car.  We are faced with a few problems.  How can we allow the engine to keep turning while the wheels are stopped?  And more importantly, how do we get the vehicle moving again with the engine rotating but the wheels not rotating?  We must devise a system to deal with these issues.  In a manual transmission, this issue is handled with a clutch.  The clutch is comprised of a pressure plate and a friction material.  The clutch allows slippage between the input and the gearbox while still transfering torque.

Automatic transmission torque converter

In an automatic transmission, this issue is handled with a torque converter.  The torque converter is typically composed of two turbines coupled by a fluid (automatic transmission fluid).  The torque converter allowsthe input to rotate without any rotation of the output (up to a certain input rpm).  It is still transfering torque even if the output is not turning.  Most modern automatic transmissions also incorporate a torque converter lockup so that after there is little to no difference between the input and output speeds of the torque converter, the input and output of the torque converter can be locked together to increase efficiency.

Alright, so now we have all our problems solved.  We can match the desired input speed of the engine to the required output speed of the wheels.  We can switch between our gear ratios to select the best gear ratio available for our operating conditions.  Our two speed transmission is ready for production! 

Number of gear ratios

Modern automatic and manual transmissions typically use at least four selectable gear ratios.  Some transmissions have more.  Jatco produces automatic transmissions with seven gears.  Manual transmissions often have five or six gears on even modest passenger cars.  Why so many gear ratios, when we needed only two per our example above?  There are many reasons, primarily cost, efficiency and driver/operator comfort/perception.  We will discuss these reasons further in a future article.

Continuously variable ratio

We would be remiss not to mention the continuously variable transmission (CVT).  This type of transmission allows the gear ratio to be varied continuously within a range, rather than changing in steps like a conventional transmission.  (However, CVT's can also emulate step changes).  We will discuss CVT's in detail in a future article.