Should I choose petrol, turbo-diesel or hybrid?

There are hundreds of reasons to buy a more efficient car, and they all start with a dollar sign.

But an individual’s motivation for more efficient motoring can vary according to a myriad of personal factors.

Some buyers are clearly concerned for the environment, while others are eager to adopt – and be advocates for – the latest in technology.

But many are just looking for a car that’s cheap to run.

The Australian car market is one of the most diverse in the world, and consequently we have a huge variety of cars to choose from.

While family SUVs and dual-cab utes are the most popular models at the moment, showrooms also offer everything from small urban runabouts to high-powered sports cars and traditional large sedans to rugged 4x4 off-roaders with an ever-growing league of electrified hybrid and battery-powered options sprinkled among them.

Some brands offer a choice of powertrain options in the same model – petrol or hybrid, for example, or (particularly in large SUVs) petrol or diesel engines – while consumer demands and sensitive price points can sometimes dictate the option according to the size or category of vehicle.

Pint-sized city cars, for example, are predominantly offered with small-capacity petrol engines these days and the majority of dual-cab utes and dedicated 4x4s have diesel engines for high torque outputs.

Differing powertrain types and technologies provide varying results in specific driving scenarios.

So, what is the right powertrain and fuel type for you?

Despite rapid advances in battery technology – and some countries planning to ban the sale of combustion engines by the end of the decade – the development of conventional petrol-powered engines hasn’t come to a grinding halt.

Tightening emissions regulations have forced car companies to constantly search for new solutions in order to reduce fuel consumption and produce less hazardous emissions.

The most common consequence is to replace larger-capacity, naturally-aspirated engines with downsized turbocharged alternatives that offer similar levels of – and in some cases, better – performance while consuming less fuel.

Most have advanced computer-controlled valve systems and electronic throttle bodies to ensure a smooth and efficient delivery of power, while some even have a mild-hybrid system that shuts the engine down when the vehicle is stopped at the traffic lights to further save fuel.

Commonly called an idle-stop system – or stop/start – it uses stored power in the 12V battery to keep running auxiliary systems, such as the air-conditioning, headlights, audio system etc, while the engine is turned off and, with a higher-voltage alternator or a belt-driven generator, the engine can instantly be reignited when the driver releases the brake pedal to get going again.

Obviously, this system ensures the engine is more efficient in heavy traffic and urban commuting rather than out on the highway.

But, in general, small cars with small engines – because of their low mass and low-capacity engines – are among the most efficient options for vehicles on the road.

However, it’s worth noting that modern turbocharged variants often require premium unleaded fuel to run properly, so any improvement in fuel consumption could be eroded at the pump by the higher-priced fuel.

Daily use guidelines:
• Minimum driving distance of 5km or more for optimal efficiency
• Suitable for heavily congested routes with idle-stop system enabled
• Ideal for flowing arterial roads in intermediate and outer suburbs (60-80km/h)
• Also a good choice for longer journeys into rural areas at speeds above 60km/h

By their very nature, larger cars often require larger engines. And while they bring obvious improvements in performance, they also bring with them a penalty in fuel consumption – especially when driven every day in heavy traffic.

But, because the majority are now equipped with as many fuel-saving measures as possible, a big engine can be surprisingly efficient in certain driving situations.

If your regular commute is mainly on the freeways or open highways, a powerful car like the BMW Z4 M40i sports car, with a 3.0-litre turbocharged six-cylinder engine, will run at a constant speed of 100km/h using less than 6.0L/100km, as we can attest.

A large-capacity engine can maintain a steady speed at low revs (below 2000rpm) in a very high gear. In essence then, the one certain way you’ll witness economical figures from an engine larger than 3.0 litres will be by confining your travel to freeways, away from peak-hour traffic.

Daily use guidelines:
• Minimum driving distance of 5km or more for optimal efficiency
• Not suitable for heavily congested routes and inner-urban areas
• Acceptable for flowing arterial roads in intermediate and outer suburbs (60-80km/h)
• Also suitable for freeways and longer journeys into rural areas at speeds above 60km/h

Due to its thermodynamic efficiency, the turbo-diesel engine can be very economical. It’s also an appropriate choice for drivers who need torque (for towing and heavy loads) rather than power.

The turbo-diesel has fallen out of favour in conventional passenger cars over the past few years following Volkswagen’s highly-publicised emissions-cheating scandal and further improvements in the development of more efficient turbo-petrol engines.

But turbo-diesel power is still a mainstay in categories such as a 4x4 off-roaders and dual-cab utes, and is a common option in large family SUVs.

The general scientific consensus seems to hold that diesels are on the way out. They won’t last as long as petrol engines in the European market, as more manufacturers move to hybrids, plug-ins, battery-electric vehicles and fuel-cell vehicles. Australia is not Europe, of course…

For the time being, turbo-diesels do provide a viable alternative for some buyers.

In the Australian context, turbo-diesels work very well over longer journeys. Once they’re up to operating temperature they’re very frugal, but most of them come with diesel particulate filters (DPF) to meet mandated emissions standards. These filters don’t take kindly to regular and frequent short trips from cold starts.

If you own a turbo-diesel that spends much of its time traversing the route between home and shops, it’s important that you take the vehicle out once a month for an extended run at open-road speeds to burn the soot from the DPF.

Turbo-diesels can be fine in town, powering courier vehicles and taxis – vehicles that work for a living and rack up a lot of kilometres each day without the engine switched off for an extended period.

In short, the place for a turbo-diesel is in the outer suburbs and rural areas. Drive a turbo-diesel SUV or passenger car (or light commercial vehicle even) for at least half an hour and it will produce economical fuel consumption figures.

But it’s not the type of powertrain to suit inner-urban types who drive no more than five or 10 minutes each way.

Daily use guidelines:
• Minimum driving distance of 10km or more for optimal efficiency
• Suitable in a workhorse capacity for heavily congested routes
• Competitive with hybrids on flowing arterial roads in intermediate and outer suburbs (60-80km/h)
• Ideal for freeways and longer journeys into rural areas at speeds above 60km/h

A hybrid vehicle links a conventional petrol engine with an electric motor that is powered by an on-board battery pack that regenerates electricity while the vehicle decelerates.

Toyota is a major proponent of hybrid technology, having first introduced the pioneering Toyota Prius hatchback more than 25 years ago.

Essentially, in a series/parallel hybrid like the Prius, the electric motor provides additional assistance to the petrol engine while accelerating and, in most cases, can move the vehicle away from a standstill on its own, allowing the petrol engine to chime in at efficient road speeds.

Because of that, hybrid vehicles are optimised to perform at their best in populated metropolitan areas.

Over a decade ago, driving a third-generation Toyota Prius between Sydney and Melbourne, and then running it back and forth between work and home for a month was instructional. The period revealed that the Prius was less economical (4.9L/100km) at a steady speed of 110km/h than it was (4.4L/100km) negotiating urban traffic commuting back and forth to work.

Where the Prius and this type of hybrid excels is in a very specific scenario. Let’s say you commute to an inner-urban location from suburbs further out. Most of the roads along the route are free-flowing arterials, but there are always a couple of bottlenecks at certain points.

The Toyota Prius will crawl along below 20km/h for a kilometre or more of that bumper-to-bumper traffic without the petrol engine restarting. This is where the hybrid can really save money and conserve fuel, measured against a conventional petrol-engined car.

If you do need a car that will undertake the occasional country run, the hybrid will do that as well, but its fuel economy on that run won’t necessarily match its around-town efficiency.

Daily use guidelines:
• Minimum driving distance of 5km or more for optimal efficiency
• Ideal for heavily congested routes and inner-urban commuting
• Acceptable on flowing arterial roads in intermediate and outer suburbs (60-80km/h)
• Adequate for freeways and longer journeys into rural areas at speeds above 60km/h

Plug-in hybrids currently occupy the position as a perfect transitional vehicle between yesterday’s petrol-powered technology and tomorrow’s fully-electrified future.

As an extension of a hybrid, they still have both a petrol and electric engine but have a more powerful electric motor and a larger battery pack that provides between 30-80km of zero-emissions motoring at speeds up to 100km/h.

While the battery can be topped-up on the run using regenerative braking, unlike a series hybrid it can also be recharged at public charging stations or at home.

This not only gives a plug-in hybrid the ability to be an electric car around town but also the flexibility and performance of a conventional vehicle with long-range driving capabilities.

The biggest issue with PHEVs at the moment is the premium cost they come with because of their complex powertrains.

They are at their best in the suburbs, especially in densely populated communities closer to city centres, and for owners who would be travelling only 5-10km a day.

Since the electric motor doesn’t have to warm up for peak efficiency, the PHEV (run in electric mode only) is as clean and green from the moment the car leaves the driveway as it is at the moment it pulls up outside the school to pick up the kids.

Typical owners are not the sort of people who trade up to a new car every three years, so resale value is not an issue for them. Nor is the question of well-to-wheel emissions.

These buyers likely buy the more expensive car with every intention of keeping it for longer and thus amortise the premium purchase price by saving on running costs – swapping petrol from the pump for power from the plug.

At a pinch, if they need a larger family car for infrequent weekends away in the country, they may choose a vehicle like the Mitsubishi Outlander PHEV, which remains a reasonably economical vehicle to run at open-road speeds and can recover some electrical energy from braking and downhill runs.

Daily use guidelines:
• No minimum driving distance for optimal efficiency
• Ideal for inner-urban working
• Also ideal for flowing arterial roads in intermediate and outer suburbs
• Acceptable for freeways, but may be limited by range for longer journeys into rural areas

There is absolutely no doubt that most of us will be driving a fully-electric car in the future. But not quite yet.

Battery-electric vehicles offer zero-emissions motoring with plenty of performance and lower running costs, and what’s not to like about that?

However, there is only a handful of EVs on sale in Australia at the moment, all of which command a significant price premium as car companies attempt to recoup the investment in emerging technologies.

Battery-electric vehicles – as the name suggests – only have electric motors and battery packs, which offer similar levels of performance to petrol engines but, in most cases, have limited driving range. And, obviously, need to be recharged rather than refuelled.

The price alone restricts electric cars to those reasonably wealthy, while so-called ‘range anxiety’ and the inconvenience of public charging through a perceived lack of infrastructure has scared traditionalists from even considering an electric vehicle.

Even though an EV will happily cruise at highway speeds, they are most efficient dashing through the urban jungle where heavy regenerative braking power is essentially topping up the battery with free power.

Daily use guidelines:
• No minimum driving distance for optimal efficiency
• Ideal for inner-urban working
• Also ideal for flowing arterial roads in intermediate and outer suburbs
• Acceptable for freeways, but may be limited by range for longer journeys into rural areas