Tech Speak: Understanding the Numbers

BMWMdials

Car brochures or information pamphlets often include specification sheets that are meant to tell you all you need to know about the car. Some of this information is easily digestible, but other items need further explanation. Here are some terms you should know. 

The engine

Number of cylinders

The ideal number of cylinders for any particular engine size has become an important statistic. More cylinders with smaller bores make an engine smoother, but increases manufacturing costs, servicing expense and fuel consumption. A smaller number of cylinders with bigger bores increase engine roughness but are cheaper to manufacture and service. In the latter case, the smaller piston areas in contact with the bores will also reduce frictional losses significantly and decrease fuel consumption. Many manufacturers have recently changed from 4 to 3 cylinders on their budget market models and some Fiats only have 2 cylinders (see our review of the Fiat 500C 0.9 TwinAir Lounge).

 
A contemporary Mercedes-Benz 4-cylinder engine.

Cubic capacity

This is the volume of the cylindrical shape the piston moves through when it’s going from the top of its travel to the bottom. It’s calculated from the bore and the stroke times the number of cylinders. This number tells you how much air the engine is (theoretically) capable of sucking in over two revolutions. Fuel is added to this volume in a more-or-less fixed ratio so that it is related to the maximum torque an engine can develop. For marketing purposes, an engine's cubic capacity is usually expressed in litres eg. 1.4- and 2.8-litre engines.

A video that explains cubic capacity:


Compression ratio

This is the volume of mixture above the piston at the bottom of its travel divided by the volume above the piston at the top of its travel. It is a measure of the extent to which the mixture has been compressed just before combustion occurs. An engine’s efficiency increases as the compression ratio increases, but petrol engine compression ratios are limited to approximately 12:1. Above this value, harmful detonation (non-uniform combustion) is likely to occur. Turbodiesel engines compress air only, and require ratios that are sometimes as high as 22:1. These high ratios generate enough heat to ignite the fuel a fraction of a second after it is injected.

Engine output

An engine’s output is usually given in Newton metres as well as kilowatt. The former is a measure of torque and is related to the size of the force that a car’s wheels can transmit to the road while the latter is a measure of power, which is essentially the speed of force delivery. This means that a high torque value can get you slowly up a steep hill, but you need a high power output to go fast.

Engine output graphs

Some publications will publish power and torque output graphs. These are measured at the factory by means of an engine dynamometer that is coupled to the engine at the flywheel. The readings are always taken at full throttle and the important points to notice are the maximum values of the power and torque delivery as well as the engine speeds where this occurs. The shape of the torque curve is particularly important; if it rises to a high value quickly and stays there the vehicle will not need frequent gear-changing, but if it struggles to get to a maximum value and then falls rapidly the driver will have to change gear frequently when driving in traffic.

Gearbox/transmission and the rest

Gearbox

A gearbox adjusts the ratio between engine speed and driven-wheel speed to suit the driving conditions. Down changes increases engine speed as well as the torque being transmitted and up changes reduces engine speed as well the torque being transmitted.

This occurs not only because the torque changes with the engine speed, but also because a gearbox is a torque multiplier. The ratios are chosen by the manufacturer to work well with the shape of the torque curve in order to optimise fuel consumption without hurting the performance too much.


A cross-section of a Mercedes-Benz 7G-tronic automatic transmission.

Engine efficiency improves at large throttle openings. At low engine speed, a lot of energy is lost when air flows past a semi-closed throttle valve. However, high-speed fuel consumption is higher because more energy is being delivered. The more gearbox ratios there are, the closer the engine can get to the most economical speed/power output combination. This explains why some automatics have as much as ten ratios, and they employ electronic control to search for the ratio that best approaches the sweet spot under all driving conditions.

A video that explains how a manual transmission works:


Tyre size

The code on the side of the tyre can best be explained by picking a particular tyre. For example, a 185/65 R14 85T tyre has a width, measured across the widest part, of 185 mm, and a height measured from the ground to the base of the wheel rim, of 65% of 185, which is 120.25 mm. 65 is called the aspect ratio or profile. Percentages above 60 are known as high profile whereas the lower percentages are called low-profile.

Some modern tyres have profiles as low as 30, because this increases road grip at the expense of ride comfort. The letter R implies radial-ply, 14 is the wheel diameter in inches, 85 is a load rating and T is a maximum speed rating. An interpretation of the latter two codes is given on a chart that may be consulted at tyre dealers.

Fuel tank

The quoted fuel tank capacity is usually less than the maximum that the tank can take. Most tanks have an air gap equal to about 13% of the total volume to prevent the tank bursting in an accident, and also prevent spilling due to fuel expanding on a hot day. If a pump attendant keeps trickling more fuel in after the automatic stop has kicked in, this extra fuel will fill the air gap and make the tank unsafe.

Performance claims

The variation in the power output of vehicles leaving a factory is plus or minus 4%. This is due to a slight variation in the clearance between parts. A claimed output of 100 kW could be anything from 96 to 104 kW. This means that car buyers have no idea what the exact outputs of their vehicle's engines are.

Changes in atmospheric pressure and temperature have an effect on the power being delivered with the result that most performance figures are only estimates. In addition, naturally aspirated engines lose about 16% of their power output at Gauteng altitudes, but turbocharged engines lose only about 4%.

Factory performance figures are usually measured using the fastest production car they can find under very favourable weather conditions. Most factory fuel consumption figures are measured according to the New European Driving Cycle (NEDC) inside an atmosphere-controlled room on a chassis dynamometer.

The speeds chosen exceed 70 kph only for a few seconds. The test conditions guarantee repeatability, but the results have been criticised for delivering fuel consumption figures that are unachievable in the real world.

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