Why do diesel cars give more mileage and less power than the petrol ones?

Diesel cars give more mileage and less power than petrol cars due to following reasons-

1. Diesel as a fuel has higher energy content than Petrol. Diesel contains 38.6 Mega Joules per litre of energy as compared to 34.8 MJ per litre of petrol. So, each litre of diesel produces more energy than an equivalent litre of petrol. This would mean you need to use less diesel to achieve a certain amount of power than you would with petrol. This amount is typically about 25% lesser diesel than equivalent petrol.
2. Diesel engines can operate at higher compression ratio around 17-21 . While petrol engines can operate only at compression ratio around 8-12. The efficiency of engine is directly proportional to compression ratio. The high compression ratio in a diesel engine means that it needs heavier parts and longer pistons and connecting rods to compress air better. It has a higher thermal efficiency than a petrol engine. This increased length of pistons also means it has a better mechanical advantage than a petrol engine and develops higher torque. Torque is the turning force that the engine generates – and since more of this comes at a lower rpm in a diesel engine, it is easier to move away without revving the engine too much and this also saves fuel.
3. Diesel can auto ignite at high temperatures and this is the principle on which a diesel engine works. When this temperature inside the cylinder in an engine goes higher than 210 degrees centigrade (the auto ignition temperature of diesel), a small spray of diesel is injected into the cylinder which then ignites spontaneously. As in diesel engines, diesel is minutelydirectly sprayed into a cylinder, lesser fuel is used. Petrol GDi (Gasoline Direct Injection) engines also have direct fuel injection in the combustion chamber, but most petrol engines are MPFi (Multi Point Fuel Injection).
4. Diesel has cetane (six-sided) rings in its makeup instead of octane (eight-sided hydrocarbon rings), so its burning characteristics are better – it is more slow burning, allowing for a longer burn time and better efficiency. This is also the reason why diesel engines don’t reach high rpm ranges.

The power of an engine= rpm* torque. Petrol engines have low compression ratio and lighter mechanical parts as compared to diesel engines, so they reach higher rpms. This results in more power than the diesel engines.

Source Quora-Question answered by Manvi Arora.