Petrol ‘v’ Diesel

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’m often asked what the difference, in performance, is between two cars with an engine with a given power output, but one engine is a petrol and the other diesel.

In short, the performance difference between cars of a given equal power and weight will be little, subject to aerodynamic efficiency and gearing.  Generally speaking, the higher the power output, the more compromised the turbodiesel will feel when driven at one hundred percent.

There are several examples of cars with very similar power outputs but few with similar weights.  An easy example to make is the mark one Mondeo.  The 1·6 petrol produces 90 PS and the 1·8 TD produces 88 PS.  However, the TD is around 100 kg heavier, so the diesel has a materially lower power to weight ratio.  The diesel also has taller gearing, too, which reduces the effective output at the road.

Statistics

In terms of numbers, the 1·6 has a power to weight ratio of 71 PS per tonne and the 1·8 TD, just 65.

Looking at the manufacturers quoted figures, the 1·6 is quicker.  It reaches 62 miles per hour around a second and a half quicker than the TD, and despatches the important 30 to 70 dash a full three seconds quicker than the TD.  We’d expect there to be this difference, especially in the in-gear acceleration, given the greater power to weight and shorter gearing of the 1·6.

Power Delivery

On the road the two are different to drive.  The engines have different power characteristics.  The 1·6i-16 performs at its best from 4,000 to 6,000 rpm whereas the 1·8 TD performs at its best from 2,300 rpm to 4,000 rpm.  At a cruise of 60 Indicated the 1·6 is turning over at around 2,800 rpm compared with just 2,300 rpm for the TD.

Very Low Speed

By Very Low Speed, I also mean moving off.

Neither car has brilliant acceleration but the 1·6 is certainly an easier machine.  It’s easy to move off smartly but without wheelspinning.  This is much harder in the TD: too many revs and you get too much turbo boost, which is wheelspun away.  Too few revs and you get no boost pressure, so the car bogs down.  This makes the TD relatively difficult under certain circumstances, such as over a busy roundabout.  Advantage to the 1·6 petrol here.

Single Gear Acceleration

The TD has an immediate disadvantage here since there’s turbocharger lag to contend with.  You need to keep the engine operating between 2,000 and 4,000 rpm for meaningful acceleration.  The 1·6 by comparison, yes it doesn’t really get going until you have 4,000 rpm on the tachometer but this isn’t to say it cannot accelerate from lower engine speeds.  Gear selection is less critical.

Boot the accelerator from 60 in top gear and neither car exactly catapults forwards.  The 1·6 really wants a gear down and the TD has to get over the turbocharger lag.  Once the turbocharger is turbocharging, the TD has greater acceleration.

Try the same test from 40 in fourth, and the TD pulls very well (again, once the turbocharger is blowing) whereas the 1·6 feels sluggish in comparison.  On both cases, the 1·6 would prefer a lower gear or two but the TD doesn’t feel disadvantage.  The difference is most obvious the faster you are driving; comparing the two on the motorway, you’d take the TD.

Through The Gears

We need to consider through the gear acceleration, such as when overtaking.  As an example is when we are joining a dual carriageway from a slip road with a relatively tight curve, something I do most days now.  There’s a tight left hander, taken about thirty, which then flattens into a gentler right hander, forming the acceleration lane, then dumping you on to the dual carriageway after around 150 yards.  In both cars you’d use third, unless you were feeling particularly gung-ho in the 1·6 and you’d use second.  Accelerating from 30 in third, the TD has a small advantage until around 40 or so, when the 1·6 starts clawing back the distance.  Then you need to change up in the TD, somewhere around the 50 point.  Select fourth and wait for the lag.  Meanwhile, you’re still accelerating In the 1·6 until you reach the speed limit, when you can select top.

It’s the change that hinders the TD: advantage 1·6 petrol.

Airport Run

You can fit all sorts of stuff in the back of a Mondeo, from a fridge freezer to a stack of moving boxes, or luggage for four.  My final comparison is to compare the cars when laden with “stuff” such as four people plus their luggage.  Four people plus a suitcase each is reckoned to add somewhere around 300 kg to the car, which as a percentage, makes a greater impact on the 1·6 petrol.

Both cars are slower, that’s for sure, but you notice it less in the TD.  Whereas the 1·6 struggles up hills without changing down one or two gears.  The extra weight robs it of flexibility.  By comparison, the TD is slower but just as flexible.  Advantage, TD, especially on those long motorway gradients where the TD just keeps going but the 1·6 needs fourth.

Fuel Economy

Most people buy a turbodiesel because you get most of the performance of a mid-sized petrol engine combined with superior (lower) fuel consumption.  The same is true of the Mondeo.  Driven gently and with a good leaning towards fuel consumption, the 1·6 can certain return very respectable figures.  I have personally managed over forty to the gallon from a mark one Mondeo 1·6 on a 400 mile, mostly motorway drive at HGV pace.  However, from similar drives in the mark one Mondeo 1·8 TD, I could return upwards of fifty miles per gallon.  On average, the TD is some 25% more economical than the 1·6.

Summary

Driven hard through the gears or on the test track, the 1·6 petrol is going to be quicker than the TD.  With a greater rev range you often have a wider choice of gears to use for any one speed.  However when driven in a more sedate pace, especially when laden, you appreciate that the 1·6 needs that choice in ratio.  The 1·6 would be a better choice for the enthusiast, but really, enthusiasts wouldn’t be looking at the 1·6 or TD Mondeo.

For the motorway, the TD is the superior machine.  If you live in a hilly area, you face a difficult decision: the TD provides for a more relaxed hill climbing ability but if you get stuck behind something on a steep gradient, you’ll find the 1·6 better at overtaking!