The Chemistry of Fuel: Petrol vs Diesel #3

Diesel ignites in your motor by compression ignition, and petrol motors ignite by a spark.

This obviously means that diesel motors must run at higher compression than petrol motors.  One implication of this is that diesel motors require stronger batteries to start, as they must do more work in compressing the air in the cylinders.

Traditionally, diesel motors are slower revving and produce their power low in the rev range, whereas petrol motors are higher revving and produce their power higher in the rev  range.  This resulted in diesels only being used in vehicles where a slow revving, torquey motor was an asset, like trucks, trains, and large 4WDs.

In recent times however, the Europeans – notably the Germans – have led the way with revolutionising the diesel motor.  Foremost among these is Audi which it is essentially now unbeatable in the Le Mans 24 hour endurance race.

This revolution in design has resulted in two things.

Turbochargers, although not a new concept, are relatively new on diesel motors.  What they do is to provide high pressure air to the motor, thereby providing an abundance of oxygen to allow the motor to run efficiently at higher revs.

Secondly, and more importantly, common rail fuel injection technology has been developed.  Essentially, this is where the diesel is injected under much higher pressure than the older diesels – up to 20 or 30,000 PSI. The result of this is very fine atomisation of the fuel.  So when it ignites the size of the droplet that must burn is very small and generally is able to burn with high efficiency.

The implication of this is very little black smoke from unburnt fuel, and no smell of diesel.

So modern diesels – turbocharged common rail designs – have an efficiency that the older diesel engines could only dream of.

So now these cars produce lots of mid range torque, and they are able to rev well because of the turbocharger, and they produce extraordinary fuel efficiency.

Let’s now will do a comparison of diesel versus petrol in the same car, let’s say a Hyundai i30.

So here are the important numbers in the table.  The diesel motor is smaller, uses less fuel, and produces its maximum torque (255Nm) almost off idle at 1900 rpm.  The petrol motor doesn’t produce its maximum torque (which isn’t as much anyway) until the motor is screaming at 4000 rpm.

Anyone who has owned or driven one of these vehicles will know what it’s like – from the moment you take off the motor pulls strongly.  Because it pulls so strongly so well so easily, you don’t have to put your foot down much.  And this is the point – the fuel consumption figures quoted above are delivered under controlled driving conditions.

In reality however, the difference between the diesel and petrol fuel consumption is much greater.  The reason simply is the ease with which diesel motors pull in normal driving, and the fact that you don’t have to floor the accelerator to get them moving.  So in reality a petrol car uses a lot more fuel than its diesel counterparts, even on these figures.

But what of hybrids, I hear you ask.

That’s tomorrow’s topic.