Motoring – Petrol vs Diesel

The difference between a petrol and diesel engine is not just the loud, rough noise of a diesel car or the price of fuel at the filling station. Let’s take a look under the bonnet before you decide which one you prefer.

Terrence Maseko*, master technician at one of the major American car brands, says because they both use different fuel, the operating principles are slightly different.

“The operating system of a petrol engine needs air that is channelled to the combustion chamber through the intake manifold. The air is then compressed during the compression stage. The injectors inject petrol, and the spark plugs ignite to combust the air-fuel mixture.” The compression ratio of a petrol engine is 9:1. “With diesel, the air goes through the same process as petrol. But, during the compression stage, it is more intense. It’s compressed twice as much as petrol. This will form heat in the cylinders to ignite the compressed air and fuel,” he adds. The compression ratio for a diesel car is 18:1.

The difference is essentially the compression ratio. Both diesel and petrol four-stroke engines have:

– air going into the engine.

– air compressed in the cylinders.

– fuel burning with the compressed air.

– exhaust gases taken out of the engine after the power stroke.

Because they use different operating principles, most diesel engines have more of an advantage over petrol ones due to the difference in compression ratio. “Most diesel engines are turbocharged while most petrol engines are naturally/ normally aspirated. But, you can get a diesel engine that does not have turbochargers and petrol with turbochargers,” Maseko says.

Motoring journalist Vuyi Mpofu says diesel engines require specialised maintenance, and typically consume less fuel than petrol engines. They are designed to last much longer than petrol engines. Maseko says buying a petrol-based car is more convenient than diesel because they are less expensive. This is because the materials used to produce the engines are cheaper. Diesel vehicles are always more expensive because their engines are technically more complicated.

According to Maseko, “diesel engines have torque (turning