Petrol or Diesel, Which Engine Will Serve You Best?

Planning to buy your first car?, Do you know the difference between a diesel engine and a petro engine? What about the Advantages and Disadvantages of both engines, and with this in mind, which engine should you go for?

The principal difference between diesel and petrol engines (other than the fuel they use) is their ignition.

Spark ignition: Petrol engines suck or inject a mixture of air and petrol (aka gasoline or motor spirit) into the cylinders, squash the mixture into the combustion chamber (a compression ratio of about 10:1) and ignite it with a spark. Bang! That releases the energy of the fuel which is then harnessed as the engine’s power. The engine is turned “off” by cutting the electricity supply that provides a spark.

Compression ignition: Diesel engines inject a mixture of air and diesel (technically “gasoil”, a distillate), and compress it to about double the pressure of a petrol engine (circa 20:1) That is such a squeeze that, even without a spark, it goes Bang! Fuel energy released, engine powered. The engine is turned off by cutting the fuel supply.

In all other methods of function they are much the same.

Each type of system has its pros and cons. The differences used to be quite stark, but have been progressively “levelled up” by advanced technology and design.

Traditionally, and still residually, petrol engines were relatively lighter, a bit cheaper, speeded up more flexibly to higher revs, delivered more horsepower (accelerative force, like a sprinter), were quieter, and were a bit less fussy about the purity of their fuel.

Diesel engines, while coming second in all those respects, delivered more torque power (working force, like a wrestler or a weightlifter) and they were intrinsically more fuel efficient – especially at bigger engine sizes on things like trucks, buses, and agricultural and construction equipment, and in static chug-chug-chug engines used on big generators. Being lower revving and more robustly built to cope with their compression ratio and heavy torque levels they lasted longer. And not having a complex electric ignition system of coils, distributors and spark plugs, they were and are less prone to spluttering in wet conditions. Indeed, given a watertight snorkel on the air intake and exhaust tailpipe, they can (presumably) keep running even when completely submerged.

At this stage of technical development, almost all cars and light utility vehicles were petrol-engine, and almost all big commercial vehicles used diesel engines. Entirely rational. It was almost infradig to drive a diesel-engine car, unless it was a taxi.

This clear dividing line has changed, perhaps led by large 4WD vehicles, for which diesel engine characteristics were more suitable…except for their lack of acceleration and speed in modern usage as “luxury cars”. Enter the turbocharger which solved that downside while maintaining a dramatic advantage in fuel economy over petrol equivalents amid rapidly rising fuel prices. Modern turbo diesel 4WDs use much (!) less fuel than petrol-engine 4WDs delivering the same power and pace. The difference is most significant in bigger engines (circa above 3 litres).

The legendary 3.5 litre V8 petrol engine of the Range-Rover, which was the first mass produced 4WD that could do 160 kph (“100 miles per hour”) and drive over rocks the size of armchairs, is no longer out on its own. There are now numerous 4WDs of many makes that can go even faster and have equal or better off-road ability. Including the latest petrol and diesel-powered Range Rovers.

The technology and trends that started with 4WDs then spread down the size-chain. Fuel price rises made diesel efficiency more interesting, diesel engine longevity was a plus, and combined with quieter engines, easier starting, more flexible engine revs and optional turbocharging, the downsides were neutralised (and even reversed in some respects).

That trend has hit a bit of a speed bump, first through concerns that diesel engine exhausts emit more particulate matter and then in the overall swing from fossil fuel engines of any kind towards electric and hybrid engine power systems. More about those facts and myths some other time.

Meanwhile what car you buy must always start and end with your budget. Whether a petrol or a diesel engine is more suitable for you depends primarily on the size and type of car you drive, whether it is brand new or out-of-date, and on what you use it for, where, and how.

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