Diesel and petrol engines are both types of internal combustion engines, but they differ in how they ignite the fuel-air mixture. Petrol engines use spark plugs to ignite the mixture, while diesel engines rely on the heat generated by compressing air to ignite the fuel. Diesel engines are generally known for their fuel efficiency and high torque, making them suitable for heavy-duty applications, while petrol engines often offer smoother performance and are more common in smaller vehicles.