Diesel and Petrol, also known as gasoline have been driving the automotive industry for the past few decades. Owing to their availability and price, they have become the dominant source of power in almost all vehicles powering over 95% of the total vehicles in the European Union.They are both sourced from crude oil or petroleum. Both are refined from petroleum using fractional distillation processes. Typically, diesel is easier to refine as compared to gasoline. However, these two fuels are fundamentally different-Energy supplied, efficiency, ignition temperature, emissions and so on.

Energy supply:

The calorific value of diesel is approximately 45.5 MJ/ kg which is lower than petrol at 45.8 MJ/kg. However, diesel is much denser producing nearly 36.9 MJ/litre compared to 33.7 MJ/litre for petrol. As a result, fuel efficiency of diesel engines are almost 15% higher. For example, A Suzuki Swift diesel will eat up just about 39.7 litre of fuel while a petrol variant will require at least 50 litres per 1000 km of travel. Every litre saved points at precious money saved meaning, a Diesel vehicle should have lower running costs. Now you know why the Olas and Ubers around are mostly always Diesel cars.

However we should note that Diesel vehicles in India are usually more expensive than their petrol variants.

Combustion and ignition:

Diesel and Petrol have similar auto ignition temperatures at 210 and 245°C. However, the in-cylinder temperatures of the 2 kind of engines are widely different. It is fair to note that the combustion temperatures of an engine depends on multiple factors such as engine design, compression ratio, cooling system, turbocharging apart from the type of fuel used.

In general, the average temperature for the combustion stroke for a petrol engine is nearly 850 °C, while flame temperatures exceed 1900 °C.

In case of a diesel engine, the air temperature towards the end of the compression stroke is nearly 700 °C. This might shoot above 2000 °C during combustion.

Emissions:

By virtue of its longer chains, diesel while producing more energy per unit also has more a higher Sulphur and Nitrogen content. As a result, Petrol which has smaller chains is cleaner. Over the past few decades, a lot of research has gone into improving diesel emission quality. BS IV compliant vehicles are almost 50 times less polluting as compared to BS I diesel vehicles. Today, while petrol and diesel emissions are comparable, diesel exhaust still has a higher NOx content which is extremely hazardous. Petrol on the other hand belches out a higher proportion of unburnt Hydrocarbons and CO.

 

*This article is a part of the series: “The Basics: Petrol vs Diesel”.