Should I Buy Petrol or Diesel?

Updated: Apr 24, 2017

Time to look at petrol versus diesel. Is a diesel car - or SUV - right for you? Especially if you’ve never owned one before? This report is, basically, diesel for dummies

DIESEL SALES

There has been a meteoric rise in diesel vehicle sales - and I don’t mean busses. In Australia, when you add diesel cars to diesel SUVs the sales growth is staggering: up from 41,538 new diesels in 2005 to more than 157,211 in 2015. That’s growth of almost 400 per cent, in just five years.

Part of that is simply that there’s more diesel engine alternatives on offer these days, but the main reason is because diesel simply grew up.

Driving a diesel today is nothing like driving grandad’s Massey-Fergusson tractor just after World War II. In fact, you’d be ticking the box for diesel every time, on so many new cars today - simply because the petrol powertrain often no longer keeps up.

DECADE OF DIESEL

Diesel Car & SUV Sales in Australia, 2007-2016

Petrol Car & SUV Sales in Australia, 2007-2016

Peak power is not all that accessible in most driving situations (outside of a drag strip, you can hardly ever get there). What really counts is mid-range power delivery - and this is where diesel rocks.

Peak power is not all that accessible in most driving situations (outside of a drag strip, you can hardly ever get there). What really counts is mid-range power delivery - and this is where diesel rocks.

PERFORMANCE

Petrol engines deliver more peak power - by virtue of spinning faster, but diesels deliver huge torque at low revs. And that means they produce more low-rpm power - maybe three or four times as much low-end power. So they just seem to storm ahead at 2000rpm - without very much fuss. And at those low revs, most petrol engines just can’t keep up, in terms of power production.

So, if your objective is to set the lap record somewhere you’d generally want to be using a petrol engine - and then you will need to rev it really hard to exploit the extra power it can deliver at those high revs. But if you just want to cruise effortlessly uphill at 2000rpm on the freeway - possibly with a moderately heavy trailer behind, you’d go for the diesel every day.

Diesel delivers 30 per cent better fuel efficiency, primarily as a consequence of its ability to tolerate more compression in the combustion chamber

Diesel delivers 30 per cent better fuel efficiency, primarily as a consequence of its ability to tolerate more compression in the combustion chamber

FUEL EFFICIENCY

Diesels are also more fuel efficient - about a third better.

This means you will get about 30 or 40 per cent more cruising range out of a diesel. If you’re going 700 kilometres on a tank of petrol, you’ll be going more than 1000 on a tank of diesel.

Even around the city, you don’t have to stop as often to fill up in a diesel - so you’ll have to consider all those future, missed opportunities to buy those two for one Kit Kats if you’re burning diesel. And we all know just how uplifting that can be...

See my 7 News investigation exclusive:
Which petrol drives your dollar further >>

Back to top >>

COST & COMPROMISE

Diesels cost more up front - generally two or three grand more for ordinary cars. More than that, sometimes. This is partly an economies of scale thing (more petrol engines are manufactured, therefore the unit price is lower) plus there are heavier-duty parts required in diesel engines, as well as the high-pressure fuel rail and the hi-tech injectors, and a turbocharger and intercooler, some or all of which components which many petrol engines lack.

The diesel is often packaged with an AWD drivetrain, with the petrol packaged with 2WD (usually front-wheel drive). Kia Sportage >>, Hyundai Tucson >> (2.0 petrol versions) and the Kia Sorento >> (petrol V6) are all front-drive, while the diesel equivalent is AWD, accounting for a significant chunk of the additional cost between the vehicles.

Above, L-R: Kia Sportage, Hyundai Tucson and Kia Sorento all offer petrol 2WD as the basic entry-level proposition and diesel AWD at the top of the range. (Sportage and Tucson also have higher-tech petrol AWD alternatives.)

It’s also harder to find the diesel pump at the servo, because petrol pumps outnumber diesel pumps about eight to one. And the diesel pump is typically filthy, and slippery underfoot, because petrol evaporates whereas diesel doesn’t, so it always leaves a smelly greasy residue on the concrete and on your hands. I’m pretty sure that’s more of an issue with the hands, if you’re a chick. Chanel Number 6 is unlikely to be inspired by the smell of diesel any time soon.

HIGH TECHNOLOGY

Diesel engines really grew up about a decade ago - maybe more. They added electronic injection control, hi-tech piezo-electric injectors and fuel rails that operate at brain-bending pressures - 2000 atmospheres or so. And turbocharging. All of that delivers very precise fuel control - it cleaned up the puffs of raw soot under heavy acceleration, it stopped the engine sounding like someone had thrown a handful on eighth-inch ball bearings in the crankcase, and it delivered unbelievable throttle response. And that’s why diesels are so good to drive today.

The fundamental difference between most petrol and diesel engines is where the fuel mixes with the air. In a petrol engine, it’s generally mixed outside the combustion chamber, in the inlet port. (This is called, typically, 'multi-point injection'.) In a diesel, the fuel is delivered right into the combustion chamber, under pressure. (Called 'direct injection' as in 'directly into the combustion chamber.)

It's ready to rip. It just goes off when it’s introduced. Combustion is - pretty much - spontaneous. There’s no spark plug. (Increasingly, new petrol engines feature this kind of direct injection, too, where the fuel goes directly into the chamber. They still need spark plugs though; they’re not auto-ignition engines. So, in a sense, petrol engines are currently playing catch-up on the technology front. CX-9 (right) is a good example of what happens when they actually catch up.)

The other big difference is compression. Diesel fuel can tolerate much more compression in the chamber - like, 17 or 18 to one, plus a turbocharger. Petrol can tolerate only about 10 or 12 to one, typically. And that’s where diesel’s added fuel efficiency comes from: when the fuel ignites in the chamber and produces a heap of heat and rapidly expanding gas, the expansion takes place through a greater range in the diesel engine, and that’s what delivers diesel’s intrinsically higher thermodynamic efficiency.

People think there's more energy in diesel. There's not. Per unit mass, they're about the same (ie, per kilo). Per litre, diesel is ahead thanks to its higher density. But compression tolerance is really what makes all the difference, thermodynamically.

That extra compression is why you get 30 per cent more range out of a tank of diesel.

Back to top >>

CASE STUDY: LATEST PETROL TECH

Above: Mazda CX-9 has one of the most advanced diesel-like petrol powertrains available today

Engine: Direct injection petrol inline four cylinder with spark ignition, turbocharging, intercooling, 16 valves, DOHC and variable valve timing. Runs 91RON unleaded minimum.

Peak outputs: 170kW @ 5000rpm & 420Nm @ 2000rpm

When you compare the CX-9's 2.5 petrol to the CX-5's 2.2-litre twin-turbo diesel (129kW @ 4500rpm and 420Nm @ 2000rpm) you realise just how close the two are in terms of performance.

Diesels cannot keep up on high-rpm performance, but petrols certainly benefit from diesel-like injection technology in the low and mid revs.

You can clearly see that, here, petrol has been (and with many brands, still is) playing a kind of thermodynamic catch-up with diesel, because this cutting-edge petrol engine is definitely the shape of things to come.

The real key to CX-9's outstanding engine performance is the adoption of direct injection plus a turbo and intercooler. For starters: 170kW from a 2.5-litre engine would have been considered insane a decade ago - and it would have involved 8000rpm. Accessible power is the key point here. It's a storm trooper all the way from just over 1500rpm to just over 5000rpm. Simply outstanding.

Above: Diesel i30's party trick is similar to most other diesel offerings - it delivers substantially more low- and mid-rpm power

Above: Diesel i30's party trick is similar to most other diesel offerings - it delivers substantially more low- and mid-rpm power

EXAMPLE: HYUNDAI i30

The Hyundai i30 is typical, as a petrol versus diesel car proposition. The 1.8 petrol i30 delivers peak power of 107kW at 6500rpm; the diesel delivers 94kW at 4000rpm. So the petrol wins ultimately on power. But for ordinary driving the diesel delivers 260Nm from 1900rpm to 2750rpm. The petrol’s cranking out a comparatively asthmatic 175Nm … and you have to rev it all the way to up 4700rpm to get it to deliver that torque. So the diesel’s delivering 50 per cent more torque, at half the revs or less - and that’s why they feel so unstoppable in ordinary driving.

And the jury’s in, on economy. 4.5 Litres per 100 kilometres for the i30 diesel, combined cycle, versus 7.1 for the petrol. The diesel i30 is 37 per cent more fuel efficient. It's pretty hard to make the case for the petrol i30, huh?

REVS & DRIVING

Here’s a common scenario: you’re driving along at 2000rpm. You get to a hill. It’s a steep one. So, you need more torque to climb to the top. In the petrol, the torque is bugger-all at 2000rpm, even when you mash the throttle wide open. So the transmission has to throw back one or even two gears, just to get the engine revs spooling up to the optimal delivery point for enough torque at that road speed. There’s a lot of rotational intertia shunting around in the engine and gearbox in those operating conditions, and that feels undignified. In the diesel, though, you just hit the accelerator. The torque ramps up automatically to the maximum available - because you’re already at the right revs. The car climbs the hill effortlessly. At least that’s how it seems, in comparison.

Above: Kia Carnival. There's no question the diesel is the premium performer in the market's best 8-seat people mover

Above: Kia Carnival. There's no question the diesel is the premium performer in the market's best 8-seat people mover

See also: One of my diesel favourites - the Kia Carnival >>
Diesel is not only a good performer in this vehicle (which is actually longer than a LandCruiser 200...) It is also the only known way to get decent economy from a vehicle with its own postcode.

Back to top >>

NO THROTTLE

That’s the other thing about diesels: there’s no throttle. Sure - there’s an accelerator pedal, but they’re connected to different things in petrols and diesels. Petrols have a throttle - that’s the butterfly gizmo in the inlet air plumbing that controls air going in. That’s how you vary the engine output in response to load. Diesels have no throttle - they run wide open the whole time, for air. The accelerator in a diesel is connected to the fuel delivery instead.

Above: Diesel i30 is clearly the pick - if the context of the assessment is superior real-world performance and tremendous fuel economy

Above: Diesel i30 is clearly the pick - if the context of the assessment is superior real-world performance and tremendous fuel economy

ECONOMICS OF OWNERSHIP

Back to the i30: The diesel i30 is clearly the pick - like so many ordinary cars with a diesel option. Unfortunately though, it’s also $2600 dearer, which is about 10 per cent more. A significant sum. It’s a lot to spend, to start saving money on fuel, right? Some people get absolutely wound up on this issue. It’s embarrassing. They get obsessed with calculating a break-even point - the distance you need to drive to save enough in fuel to offset the extra cash you paid up front, for the diesel engine.

That’s a fundamentally flawed analysis. For starters, the diesel just goes better. It’s therefore intrinsically worth more. And, when you sell the car down the track, the extra cost of the diesel is reflected in the value of the car. You always get a proportion of the extra cost back, at trade-in time. If the car sinks to half its new value when you sell it, you get half of the premium you paid for the diesel back as well.

The diesel engine costs more up front, for two reasons: First: economies of scale. Carmakers make more petrol engines, and that reduces the per-unit cost of each petrol engine. And, second, the diesel is more complex. It’s turbocharged. It’s got a 2000-atmosphere fuel pressure rail. There are peizo-electric injectors, and it’s got to be built to withstand higher internal pressures because of the higher compression. And, you need a beefier driveline to cope with the extra torque. So that extra cost is fair enough. (And you get a proportion of that price premium back when you sell the car.

People say diesels cost more to service. This is absolutely true, but lacks context. Modern diesels are not significantly more expensive to service - certainly not enough for servicing costs to be a determining factor in your decision to buy one

People say diesels cost more to service. This is absolutely true, but lacks context. Modern diesels are not significantly more expensive to service - certainly not enough for servicing costs to be a determining factor in your decision to buy one

SERVICING

It costs more to service a diesel, too … but not really that much more. The common claim that diesels are vastly more expensive to service than petrol is rubbish - for mainstream cars and SUVs.

According to Mazda Australia, the total cost for the first five Mazda CX-5 Akera petrol services will be $1561. The first five services on the Akera diesel will be $1717. 

The difference, if this takes place over three years (say) averages out to $1 a week. Less than 20 cents a day. So I wouldn’t be stressing too much about servicing costs in the context of deciding between the two powertrains.

Do the sums yourself at Mazda's service cost estimator >>

Over five years, the total cost of ownership - fuel, depreciation, servicing - it’s gunna be very similar. Too close for cost to be a factor in your decision to buy the petrol or the diesel.

See also:

EXHAUST FILTRATION

The final thing you need to consider is the particle filter. Some diesels have a filter in the exhaust pipe designed to catch microscopic particles, which are harmful to human health. (Some commentators have actually called diesel exhaust the asbestos of the 21st Century.)

I don’t know how much of that is media grandstanding and how much is a serious risk to health. The particle filters themselves hypothetically never need replacing but they do need a regular run on the highway to heat up enough to do their hi-tech voodoo and burn the trapped particles off, into a parallel universe. (Actually, they put it in slightly different terms in the owner’s manual - but that’s what really happens.)

How much highway driving? Not a clear-cut question - but a cumulative 60 minutes every fortnight should be enough. A good excuse to go for a quick burn on the freeway for coffee with a view, and back... You don't have to spend that much time cruising at 80km/h+ to remain in the clear on this.

Above: A diesel particulate filter is an expensive component to replace - but a regular trip up the highway and back each fortnight will generally keep yours in good health

Above: A diesel particulate filter is an expensive component to replace - but a regular trip up the highway and back each fortnight will generally keep yours in good health

If you don’t do that running on the highway regularly, the filter clogs up, potentially, and the engine enters ‘limp home’ mode (which is just what it sounds like - kinda the opposite of fun driving). When that happens you need to limp to the service centre to have the filter manually regenerated and the car’s computer reset. And if your diesel car actually has the particle filter equivalent of a bilateral tension pneumothorax, the filter itself might need to be replaced … at a significant cost. That’s bad.

This filter-clogging problem is substantial enough around the world - thanks to the proliferation of diesel and our increasing urbanisation in the first world - that one company - LiquiMoly - has developed a couple of products to deal with the clogged particle filter problem. One is called DPF Anti-Clog >> which is a preventative thing you can pour into the fuel tank if you don’t get out on the highway often enough, and the other is called Pro-Line DPF Cleaner >> which is designed to help mechanics resurrect particle filters that are on the cusp of having their life support switched off.

Not all diesel vehicles have these potentially problematic particle filters - but soon they will be required by Australian law. So, living with the regenerative requirements of the particle filter will soon be part and parcel of diesel ownership. If the orange 'check engine' light activates on the dashboard - do not ignore it. Visit the dealership ASAP, where they can put it in a service bay and manually regenerate it for you, which can save you a significant sum, compared to replacement of the whole unit.

Remember not to breathe the exhaust fumes in any case. (That’s actually good advice for pretty much all automotive exhaust.)

Diesel engine repairs can be quite expensive - but often this high cost is the carmaker's problem

Diesel engine repairs can be quite expensive - but often this high cost is the carmaker's problem

REPAIR COST

Modern diesels are expensive to repair. Typically, it's the common rail injection system that's expensive to repair, and this is purely a product of its complexity. Diesel particle filters (mentioned at length above) are also expensive things to replace (and removing them is both impractical and illegal).

The bottom line is that it is essential you maintain a good service history with your vehicle. You need to do this both during and after the factory warranty expires. Here's why:

Since 1 January 2011, when new laws came into force, Australian Consumer Law stipulates that carmakers are obliged to support you even after the factory warranty expires, and that they must do so for the reasonable life of the component or system that failed. So, let's say you buy a Mazda CX-5 Akera diesel >> It goes great for four years, then the diesel engine dies with the odometer on 75,000km (this is uncommon, just a hypothetical - it would be much more common for a Jeep or a Land Rover to shit itself spontaneously).

Mazda's warranty is three years or 100,000km - whichever comes first. It's common for owners to presume they're out in the cold in this situation, but the truth is, you're not. It's reasonable to expect a properly maintained engine to last longer than four years and 75,000km. Therefore, under the 2011 consumer law revisions, Mazda should pick up the tab on the repair. You might have to be a strenuous advocate for your own interest here, but that's what the law says.

Of course, if you didn't get your vehicle serviced properly, Mazda can successfully claim you have abused it, and the repair cost will be entirely your problem. (All of a sudden that $350 for a service every eight months looks like pretty good insurance, huh?)

The other common issue faced in practice is that the dealer will claim it's dirty fuel that is responsible for the failure, and that this is therefore an insurance problem. The insurer gets the fuel tested and they claim it's quite OK, and therefore it's the carmaker's problem under Australian Consumer Law. You can get stuck in limbo, with both sides claiming: Not my problem. This actually happened to a guy who contacted me - the problem was in fact a failure of the high-pressure fuel pump in his shitbox Jeep Grand Cherokee, which delivered metal fragments into the fuel rail, clogging the injectors: a $10,000+ problem.

A couple of points on this: Fuel systems have filters designed to trap contaminants (water, dirt, etc.) in the fuel. These are quite good at doing this, and they need to be replaced at the manufacturer's maintenance schedule. Stick to your guns here. It's usually not dirty fuel; it's usually component failure. Vehicle has a full service history, etc. You're not asking them to do you a favour - you're politely requiring them to honour their obligations under the law.

It's also essential to stack the deck in your favour with carmakers. Some carmakers are very good at customer support, and some are pathetic. If you buy a car from one of the pathetic ones, expect a fight. If you buy from a good one, expect support. Australia's most pathetic carmakers in this context are Holden, Ford, Volkswagen, Land Rover, Volvo, Chrysler, Jeep, Dodge, Fiat and Alfa Romeo (the last five are all owned by the one conglomerate: Fiat Chrysler). The ones that do a really good job at customer support are: Subaru, Hyundai, Kia, Mazda and Toyota. All the rest are somewhere in between.

See also:

CONCLUSION

If you haven’t driven a modern diesel car, maybe you should. Certainly don’t dismiss diesel, off the bat. It’s come a hell of a long way in the past decade. For ordinary driving, in many cars and SUVs, it’s way in front of petrol. Cost, despite the up-front premium, is generally not a deciding factor in the decision - at least, not an economically rational one.

Back to top >>

LATEST STORIES