If they put you and your car on the GQ couple’s quiz, how well do you think you’d do? How well do you actually know your baby and what she brings to the table?
Most people view cars as mere steel boxes that facilitate movement. However, from a mechanical engineering perspective, a car is an engineering marvel that seamlessly combines principles of engineering thermodynamics, fluid mechanics, strength of materials and the theory of machines. At its core is the internal combustion engine, which converts the thermal energy in fuel into mechanical energy that propels the car forward. This conversion isn’t magic, and I’m going to take you through that journey.
Our story begins with fuel, the energy source that powers internal combustion engines. Petrol and diesel are the two most commonly used fuels. They have different chemical properties and energy contents, and these differences determine how air and fuel are mixed, the compression ratio, how the fuel is ignited, efficiency, and applications.
The engine’s job is to extract energy from fuel. It does this by literally burning it. In petrol engines, the fuel is ignited by a spark, while in diesel engines the fuel ignites spontaneously due to the heat generated during compression. The engine goes through four processes: intake, compression, power, and exhaust. In a four-stroke engine, each of these processes happens in a single stroke. A stroke is the movement of the piston from top to bottom or vice versa. In a two-stroke engine, the same four processes happen in two strokes; they occur simultaneously and are somewhat blurred together. Most engines are four-stroke engines, but very small ones (small enough to fit in your palm) and very large ones (big enough to fit in a two-storey building) are two-stroke engines.
The question now becomes: how do we measure what an engine produces? This is where torque and horsepower enter the chat. Torque is simply a force that causes rotation. In this case, it’s the force rotating the crankshaft, gearbox and eventually the wheels. For a car of any size to move any distance, whether on a flat surface, uphill, or at speed, it requires a
force to do so, and that force is the torque produced by the engine. Horsepower, on the other hand, is a unit of power; it tells us how fast that torque is produced. Horsepower is a function of torque and RPM (the speed of the engine).
We then proceed to the gearbox. Gears are examples of mechanical power transmission systems. Their job is simply to transmit the power (torque and RPM) produced by the engine. The idea of a “powerful” gear doesn’t really make sense because all gears transmit the same power from the engine. The entire purpose of the gearbox is to change the amount of torque and RPM delivered to the wheels while keeping the transmitted power the same. This allows the engine to run at low RPM for fuel economy or at specific RPMs to access peak power or torque.
Once the power reaches the wheels, it needs to act on the road. Friction between the tyres and the road makes this possible. The wheels move by rolling, which combines both rotational and linear motion. This rolling motion converts torque (rotational force) into linear force. If this force exceeds the available friction, the tyres start slipping or spinning, resulting in pure rotational motion. The same friction limit applies during braking: if the braking force exceeds the friction force, the tyres slide, resulting in pure linear motion. Anti-lock braking
systems ensure this doesn’t happen, because you can’t control a car with sliding tyres. This friction force can be increased by increasing the reaction on the wheels, either by increasing the weight of the car or through aerodynamic forces.
When a car moves through air, the air exerts a force on it. This force can be decomposed into a component parallel to the airflow, called drag and a component normal to the flow, called lift or downforce, depending on the direction. Drag pulls the car back and is therefore minimized. Lift is also undesirable in cars because it reduces the friction available at the tyres. Downforce, on the other hand, is desirable. It’s essentially a negative lift. Cars use rear wings to spoil lift (hence the name “spoilers”) or to actively increase downforce.
So now that you know what’s really going on beneath the surface, do you see the magic behind cars?