Question 1

What is the mechanical difference between a petrol and a diesel engine?

Accepted Answer

Both are internal combustion engines, and the fundamental difference is **how the fuel is ignited** — which determines almost everything else about how they behave.

**Petrol — spark ignition.** Air and fuel are mixed and drawn into the cylinder, compressed at a **compression ratio typically around 10:1**, and ignited by a **spark plug** at a precisely controlled moment.

**Diesel — compression ignition.** Only **air** is drawn in and compressed far harder, typically **16:1 to 22:1**. Compressing air heats it substantially — well above the fuel's ignition temperature — and fuel is then injected directly into that hot air, where it **ignites spontaneously**. There is no spark plug.

**What follows from that difference:**

**Efficiency.** Higher compression ratios are thermodynamically more efficient, and diesel fuel contains more energy per litre. This is why diesels achieve better fuel economy, particularly at steady speeds.

**Torque versus power.** Diesels produce **more torque at low revs**, because the higher compression and longer piston stroke generate greater force earlier. This is why they suit towing, load carrying and motorway cruising. Petrol engines rev higher and produce more power at the top end.

**Weight and cost.** Diesel components must withstand far higher pressures, so engines are heavier and more expensive to build and repair.

**Noise and vibration**, from the abrupt pressure rise of compression ignition — much reduced in modern engines but still characteristic.

**Glow plugs**, not spark plugs, which pre-heat the cylinder for cold starting.

**Emissions differ in kind**, which is the crux of the modern picture. Diesels produce **less CO₂ per mile** but more **nitrogen oxides** and **particulates**, which are local air quality pollutants. That is why diesels were once promoted on climate grounds and are now restricted in cities, and why they need DPFs and AdBlue.

**Petrol is better suited** to short urban journeys, which is where diesel particulate filters fail to regenerate.

Question 2

What is the difference between torque and horsepower?

Accepted Answer

Torque is rotational force. Power is the rate at which work is done. They measure different things, and the relationship between them is fixed and simple. Torque is a twisting force, measured in

Question 3

Why does motorway speed cost so much more fuel?

Accepted Answer

Because aerodynamic drag rises with the square of speed, and the power needed to overcome it rises with the cube — so going faster costs far more than the speed difference suggests. The arithmetic.

Question 4

What is the difference between synthetic and mineral engine oil?

Accepted Answer

Mineral oil is refined directly from crude petroleum. Synthetic oil is engineered — either heavily processed from petroleum stock or built from chemically synthesised base molecules — so that the

Question 5

What is AdBlue and why does my diesel need it?

Accepted Answer

AdBlue is a solution of 32.5% urea in deionised water, used to reduce nitrogen oxide emissions from modern diesel engines. It is a consumable, not a fuel additive, and it goes in its own separate

Question 6

How do hybrid cars save fuel?

Accepted Answer

Hybrid cars save fuel through regenerative braking, electric-only modes, engine shut-off, optimized performance, and lightweight designs, achieving better fuel economy than traditional vehicles.

Question 7

How do hybrid cars work and what are their advantages?

Accepted Answer

Hybrid cars combine an internal combustion engine with electric motors for better fuel efficiency and lower emissions. They offer advantages like cost savings, reduced emissions, and tax incentives.

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