On basis of working
In petrol engines the mixture of air and petrol is drawn in by the falling piston
In diesel engines only air is drawn in by the falling piston
In petrol engine, the mixture is compressed upto about 1/7th to 1/11th of its original size.
In diesel engine, only air is compressed upto about 1/15th to 1/26th of its original size.
In petrol engine, the air and fuel mixture is ignited using a spark plug and burns expanding and forcing the piston down.
In diesel engine, fuel is injected at a high pressure into the hot, compressed air in the cylinder, causing it to burn and force the piston down. No spark is required.
In both petrol and diesel engines, the burned mixture of air and fuel is pushed out of the cylinder by the rising piston.
A diesel engine is also known as a "compression ignition" engine. Since the air is compressed to very high pressure raising its temperature and then diesel is injected in a very fine spray which causes the diesel to ignite and explode. Whereas a petrol engine is known as a "spark ignition" engine. Since a spark plug is required to ignite the mixture of petrol and air in the combustion chamber.
On basis of Power and fuel economy
Diesel engines are more efficient than gasoline (petrol) engines of the same power rating, resulting in lower fuel consumption. A common margin is 40 percent more miles per gallon for an efficient turbodiesel. For example, the current model Škoda Octavia, using Volkswagen Group engines, has a combined Euro rating of 6.2 L/100 km (38 miles per US gallon, 16 km/L) for the 102 bhp (76 kW) petrol engine and 4.4 L/100 km (54 mpg, 23 km/L) for the 105 bhp (78 kW) diesel engine.
Low speed diesel engines use 155 gram fuel per kWh for an overall energy conversion efficiency of 54.4 percent, which is the highest conversion of fuel into power by any internal or external combustion engine.
Although the calorific value of the fuel is slightly lower at 45.3 MJ/kg (megajoules per kilogram) than petrol at 45.8 MJ/kg, liquid diesel fuel is significantly denser than liquid petrol.
The average diesel engine has a poorer power-to-weight ratio than the petrol engine. This is because the diesel must operate at lower engine speeds and because it needs heavier, stronger parts to resist the operating pressure caused by the high compression ratio of the engine and the large amounts of torque generated to the crankshaft.
On basis of engineering application
A diesel engine is more easily turbocharged than a petrol engine. A petrol engine cannot be easily turbocharged due to the fact that if the compression ratio and the pressure in the cylinder is to high during the inlet stroke, the mixture starts to burn to soon, while the piston is on its way up. The diesel engine has no fuel in the cylinder, thus letting the turbocharger suck as much air as it can without creating any problems. (A turbo charger is a simple air compressor which compresses air in the combustion chamber for burning). Some diesel engines also have an intercooler which helps in blowing cold and oxygen rich air in the combustion chamber.
On basis of Power and torque
For commercial uses requiring towing, load carrying and other tractive tasks, diesel engines tend to have better torque characteristics. Diesel engines tend to have their torque peak quite low in their speed range (usually between 1600 and 2000 rpm for a small-capacity unit, lower for a larger engine used in a truck). This provides smoother control over heavy loads when starting from rest, and, crucially, allows the diesel engine to be given higher loads at low speeds than a petrol engine, making them much more economical for these applications. This characteristic is not so desirable in private cars, so most modern diesels used in such vehicles use electronic control, variable geometry turbochargers and shorter piston strokes to achieve a wider spread of torque over the engine's speed range, typically peaking at around 2500–3000 rpm.
While diesel engines tend to have more torque at lower engine speeds than petrol engines, diesel engines tend to have a narrower power band than petrol engines. Naturally aspirated diesels tend to lack power and torque at the top of their speed range. This narrow band is a reason why a vehicle such as a truck may have a gearbox with as many as 18 or more gears, to allow the engine's power to be used effectively at all speeds. Turbochargers tend to improve power at high engine speeds; superchargers improve power at lower speeds; and variable geometry turbochargers improve the engine's performance equally by flattening the torque curve.
On basis of weight of engine
Petrol engines are lighter than diesel engines.
On basis of complexity
Diesel engines dont need an ignition system, which reduces their complexity. But they are more noisy and may require frequent maintenance as compared to petrol engines. Also they are more durable.
Diesel engines may also need glow plugs in extreme cold conditions which heat up the cylinder so that a cold engine can start easily
On basis of Noise
In diesel engine there is more noise than petrol due to The characteristic noise of a diesel engine is variably called diesel clatter, diesel nailing, or diesel knock Diesel clatter is caused largely by the diesel combustion process; the sudden ignition of the diesel fuel when injected into the combustion chamber causes a pressure wave.
On basis of Reliability
The lack of an electrical ignition system greatly improves the reliability. The high durability of a diesel engine is also due to its overbuilt nature (see above), a benefit that is magnified by the lower rotating speeds in diesels. Diesel fuel is a better lubricant than petrol so is less harmful to the oil film on piston rings and cylinder bores; it is routine for diesel engines to cover 250,000 miles (400,000 km) or more without a rebuild.
Due to the greater compression force required and the increased weight of the stronger components, starting a diesel engine is harder. More torque is required to push the engine through compression.
On basis of Fuel flammability
Diesel fuel has low flammability, leading to a low risk of fire caused by fuel in a vehicle equipped with a diesel engine.
In yachts diesels are used because petrol engines generate combustible vapors, which can accumulate in the bottom of the vessel, sometimes causing explosions. Therefore ventilation systems on petrol powered vessels are required.