Through your comments and feedback, we’ve found a lot of people were unsure of the pros and cons of each engine type and were often curious as to the difference in the economy and drivability offered between the differing power plants.
With that in mind, I’ve set out to tackle a few makes and models of car (including a Minivan, Small Hatch and Mid Sized Sedan) against each other over the next two months to try and advise buyers which engine best suits their needs.
Perhaps by way of explanation it’s prudent to start with the obvious question of what exactly the power and torque figures mean in relation to an engine’s output and how these figures apply to real world driving when selecting a petrol or diesel fueled car.
To understand the figures, I’ll first try to simplify the often baffling difference between power and torque with a quick explanation of each term.
Power, to us of the metric age, is measured by the unit of kW (kilowatts) and essentially refers to the rate at which torque is produced. In the early days, an engine was measured in relative terms - how much work it could do as compared to a horse - thus the term horsepower (hp) came in to being.
The definition of horsepower is simply this, one unit of horsepower is the force required to elevate 33,000 pounds the distance of one foot in one minute. Translated in to metric terms it becomes a little messy, but in very basic terms more horsepower means more speed.
Torque on the other hand is an often over looked part of the equation, but is nonetheless very important, especially in the case of a diesel engine that more often than not will produce less power than its petrol siblings. Measured in Nm (newton metres) or ft lbs (foot pounds) in the old days, torque is a unit of measure used to describe the turning or twisting force an engine produces.
When the piston (linear motion) is applied in the case of an engine through the crank shaft (rotational motion) the torque or rotational force figure can then be calculated by use of a fulcrum from the centre point of the crank.
Think of it as a lever placed on the engine’s crankshaft measuring the force produced (note that time is not a factor – torque can be constant). In basic terms again, more torque means greater pulling power.
So with the physics lesson over, it’s time to see how all this is relative to selecting an engine, and what it means in real world terms so far as driving one, and indeed what running one will cost you.
My first guinea pig was the petrol variant, in this case the Roomster Style 1.6 litre four-cylinder. The double overhead cam sixteen valve engine revs rather freely and like most petrol engines, develops the majority of its motivation higher in the rev range.
The multi-point injection set up helps develop a rather smooth 77kW which is on board at 5,600 rpm, just shy of the red line. Coupled with a somewhat lack luster torque figure of 153Nm @ 3,800rpm the figures don’t really do a lot to inspire a would-be buyer, at least on paper.
Fortunately for the petrol Roomster the transmission is its saving grace, which is why I chose to review the automatic. In my opinion it does a better job than the manual when mated to this particular petrol engine, and made for a fairer comparison to the TDI model.
The six-speed automatic (with sport mode and tiptronic style shifter – floor mounted) adapts itself to your driving style and very quickly learns how best to react to throttle input selecting the required gear to keep Roomster moving at a respectable pace.
At first drive it seems a little slow, almost frustrating, but after a half-hour or so you very quickly notice the dynamics of the vehicle’s performance have changed - for the better - and that it is now moving with a lot more fluidity.
It’s not fast, it’s not meant to be, but it does get along rather well considering what’s under the bonnet would be more at home in a small hatch than a 1,210kg people minivan.
Around town it’s quite friendly, and although it is a tad slow for my likings, will keep with traffic easily enough. You’ll notice a lot more throttle input is required to maintain pace, especially in stop-start traffic or hilly areas but in all the car moves well enough not to present any issues.
The climate control has little bearing on performance or fuel consumption and does a great job considering the area it needs to cool/heat and the panoramic glass roof. With only two people on board Roomster manages hills rather confidently, but load in a few more people and their kit, and the petrol engine soon becomes a little asthmatic.
Moderate to larger hills, especially at highway speeds, requiring a lot more throttle input to maintain speed, and steep grades knock the stuffing out of it requiring low gears and a lot of revs to keep things moving along. I should imagine towing would also be an issue here though the test vehicle was not fitted with the optional 1,200kg (braked) tow kit.
Given the extra right foot required to keep things moving, the fuel economy soon starts to suffer and with the best intentions of getting the most from our test candidate, I could still only manage 9.9 litres / 100km for my week of combined city and highway driving (1,000km for each car), 2.2 litres above the claimed ADR results.
Moving on to the diesel now, the Roomster Style TDI comes in one guise only, the slightly higher capacity 1.9 litre five-speed manual. The single overhead cam eight valve turbo charged oiler manages 77kW, just like its petrol brother, though the power comes on sooner - 4,000rpm due to the low revving nature of a diesel engine.
The upshot of this is a tidy 240Nm of torque from just 1,800rpm which means minimal turbo lag, and linear, consistent pull in every gear – even fifth!
A little noisier at normal running speeds than the petrol, the diesel manages itself more confidently across a wider range of driving and can keep up to fast paced city traffic with very little effort. The extra torque on hand also means additional weight isn’t an issue and even when fully loaded I found very little need to down shift on all but the most precipitous inclines.
The pace carried is better than the petrol too. Maintaining speed is easier, highway cruising is more manageable, and the manual gearbox is simple, light to use, and is well geared for any situation you could possibly be faced with. Overtaking is easy enough and in most cases can be managed in top gear.
Perhaps most importantly the diesel came out trumps in the fuel consumption race (as you’d no doubt expect) returning 6.0 litres / 100km for the week. Although this was again over the ADR figure (of 5.5 litres / 100km) the difference is far better than that of the petrol, especially when you consider that the PULP fuel required for the petrol model and the diesel fuel for this model are roughly the same price, at least here in Victoria.
For my money, the TDI (diesel) is the pick of the litter. You’d really have a tough time pushing the petrol argument for a small capacity engine in a car of this size, and given the overall usability of the diesel engine – not to mention the economy gains – the choice is clear. Diesel wins this round.
Scoreboard - Diesel 1 : Petrol 0
Next month’s comparo will feature the Skoda Octavia.
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Impressive performance, spacious interior, price.
Bland appearance, understeer at the limits.
(3.75)
(4.25)
