Petrol vs Diesel #4: Hybrids

We have seen that for a given engine size a modern turbodiesel produces more torque lower in the rev range than a petrol motor, and uses a lot less fuel.

Whenever this is raised with the petrol fraternity, the comeback is always “well, get a hybrid then.”

Let’s have a look at it – how does a state of the art petrol hybrid motor stack up against a modern turbodiesel motor.  We will compare the Toyota Prius with the Ford Fiesta diesel.

But first, let’s look at the concept of the hybrid –  what are they, and what are they supposed to do.

It seems to me that most people think that a hybrid motor is somehow able to pull energy out of the air, and deliver spectacular fuel economy as a result.

Let’s get one thing clear – the ultimate source of energy in your car is the very molecules in your fuel, and the chemical energy they contain, whether it be a petrol hybrid or a regular motor.

The hybrid improves economy by one thing and one thing alone – it minimises the energy losses that result in city driving with many stops and starts.  That’s all a hybrid is – its ultimate goal is to make the fuel economy as close as possible to the economy you would get with country driving.  We all know that for most cars there is a significant difference between the fuel economy you get in the city and the country.

The reason simply is that when you brake you are taking energy out of the system, and you must put it back in by accelerating, therefore using extra fuel – more than you would if you hadn’t had to brake.  When you brake the mechanical energy of the car is converted into mechanical action (scraping away your disks and brake pads) and heat.

With the hybrid, however, when you brake a server kicks in which acts to charge the batteries.  That is, the energy is not lost – it is put back into your batteries, to be used again at a later time.  Those of us who are a little older will remember the old pushbike generators.  When you turned the generator on so that you had lights, your bike slowed down, and you had to do extra work to turn the generator to provide the light.

The brakes in hybrids work the same way. Although they contain regular disks and pads, these are only used in more abrupt stopping, and for less abrupt stopping a dynamo generates power that goes back into your batteries.

So the idea is that the fuel consumption of a hybrid is pretty much the same in the country as the city.  And this is certainly the case with the Prius.  Country driving uses 3.7 L/100km  and city driving is 3.9 – hardly any difference at all.  This therefore results in a lower average (quoted at 3.9).

Let’s now look at the Fiesta diesel.  Its country economy  is 3.2 L/100km and its city economy is 4.6.  This kind of variation is normal in non-hybrid vehicles.  The quoted average however is 3.7, lower than the hybrid.

Also the Fiesta producers 200 Nm of torque at only 1750 rpm, whereas the hybrid has to spin up to 4000 rpm to produce its maximum of 142 Nm.

And then there’s the cost – the Fiesta comes in at 22K, whereas the Prius starts at 33K, going up to 50 K.

On top of this, the batteries in hybrid vehicles do not last as long as the motors, and must be replaced at certain intervals.

And in case you aren’t convinced yet, there is one last advantage of diesel over petrols.  Service intervals.  One of the reasons that the oil in your motor must be changed is because unburnt fuel gets into the oil and dilutes it, so it loses its ability to lubricate your engine.  In years gone past this has been a bigger problem in diesels then petrol, as diesel is less volatile than petrol, and will not evaporate as quickly.

But now the situation is the opposite.  Modern turbodiesels are so incredibly efficient that there is virtually no oil pollution happening.  Therefore, they have extraordinarily long service intervals – up to 30,000 km for some European vehicles.

So let’s summarise.

Diesel motors last longer, have longer service intervals, produce more talk at lower revs, and use less fuel.  Also diesel as a chemical is safer than petrol.

Diesel or petrol?  It’s a no-brainer.

Leave a Reply

Your email address will not be published. Required fields are marked *