What is the difference between diesel and petrol cars?

Answer 1

Diesel can give much better fuel economy (miles per gallon or kilometers per liter) than petrol but the downside is that diesel engines cost more to manufacture than petrol engines.

So a petrol-engined vehicle usually costs less money to buy than a similar make and model that has a diesel engine.

The design of a petrol-burning engine is different from that of a diesel engine: a petrol engine burns petrol (called gasoline in the US) which is ignited by a spark plug. A diesel engine burns diesel fuel which self-ignites when compressed.

Answer 2

The main difference is the fuel that they burn, and more importantly, the way it is ignited. Apart from that diesels rev lower so they need another gear ratio and often aren't as fast off the start as petrol cars.

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The alternative name for the Diesel engine is the compression-ignition engine, since unlike the petrol engine which uses a spark to ignite the fuel / air mixture, it raises the air temperature in the cylinder by rapid compression sufficiently to ignite directly the injected spray of fuel oil. The compression ratio of a diesel engine is consequently significantly higher than that of a petrol engine; and the efficiency is often boosted by a turbo-compressor that on many large (e.g. lorry) engines is followed by an air cooler. These together increase the amount of air being compressed in the cylinder.

As for performance, I don't think there's that much difference now between them, not in cars anyway. A lower-revving engine would need a lower gear ratio (including that of the differential crown wheel & pinion) assuming same size wheels for the same speed; but to achieve that the engine needs to produce more torque at that speed. Most modern cars, petrol or diesel, have 1:1 4th gears and a 5th gear that is an overdrive (1: <1).