The best ‘box for you
Transmissions no longer come in only manual or auto. We investigate the pros and cons of the different types
TWO pedals, or three? Historically, European (and South African) drivers embraced the manual transmission and struggled to grasp Americans’ obsession with slow, torque-converter automatic ‘boxes that robbed performance and resulted in heavy fuel consumption.
Today, however, the picture is different, with the manual transmission on the automotive endangered-species list. The reason for the swing towards self-shifters is twofold: increased traf c volumes result in stop/start commuting, favouring the comfort of an auto; plus, modern self-shifters often surpass the performance and ef ciency characteristics of the best manual gearboxes.
However, not all transmissions are created equal. Which transmission type would suit your needs best? Read on to nd out.
WHY A CAR NEEDS A TRANSMISSION
An internal-combustion engine (ICE) has a relatively narrow speed range, which is unsuitable to propel a vehicle from standstill to maximum velocity. What’s more, torque is needed to pull away or scale a gradient. How, then, does a transmission help in getting you up that hill? A transmission is foremost a torque multiplier, where the torque output of the engine is multi-
plied by the gear ratio (and differential) ratio before it reaches the driven wheels (while power stays constant because it is a function of torque and speed). Just imagine trying to pull away in top gear…
The most common solution is to provide a set of xed gear ratios in a manual or automatic transmission to allow the engine to propel the vehicle over the entire operation speed range. We should add here that this topic was covered in more detail in the feature Why your car needs gears from the May 2014 issue (also available on Carmag.co.za).
THE QUEST FOR EFFICIENCY
In general, an ICE’S highest ef ciency in converting the chemical potential energy in fuel to motive force is achieved in a small speed and load range. Engineers call this region the area of lowest brake-speci c fuel consumption (BSFC) and it is determined on an engine dyno, where the fuel that enters and power that exits is measured over the entire engine and load range. Running an engine in the optimum BSFC region means it uses the least amount of fuel relative to its power output. This is the main reason why we see the number of gear ratios increase (up to 10 ratios in automatic transmissions), as this allows the engine to operate in this region more of the time.
Roughly a third of the energy in the fuel reaches the crankshaft of an ICE. The rest of the energy enters the cooling system, or is lost down the exhaust. The type of transmission also affects the ef ciency of the power transfer from the input to the output shaft.