Opening Windows Versus Air Conditioning

When the weather gets hotter, it’s important to stay cool when you’re driving.  However, these days, it’s important to consider fuel consumption as well and get the most out of what you’ve paid for – and what we’re going to talk about in today’s article applies to electric vehicles as well!

The two best choices for keeping cool inside the car are using the air conditioning system and the old-fashioned method of opening the windows (if you’re over a certain age, you’ll always try to pantomime cranking a handle to indicate opening a car window.). However, you may have heard people tossing around the idea that opening the window is less fuel efficient. Or you’ve heard that using the air conditioning increases fuel consumption. Which of these is true?

It is certainly true that running the air-conditioning puts extra demands on the engine and consumes more energy when it runs (and this is true of internal combustion engines, hybrids and electric vehicles). This means that when you ask the system for some nice icy-cool air to flow through the cabin and keep you fresh rather than hot and bothered, you increase your fuel consumption.

However, opening the windows affects the drag and aerodynamics of your car. When they design them and test them, designers try to get the drag as low as possible, and they study the way that air flows around the vehicle at speed (usually using wind tunnels as well as computer modelling). This is done to reduce the amount of friction affecting the car, because the more friction that needs to be overcome, the more energy will be required, which requires more fuel, etc. etc. All these tests assume that the exterior of the car is rigid. However, when the windows open, all bets are off and the equations go out the window (almost literally). The open window affect the flow of air, which is how opening the windows cools you down, but it also increases turbulence.

The big question is which is worse in terms of fuel efficiency. Sweltering in the heat just isn’t an option – that’s downright dangerous, especially given some of the temperatures reached in some parts of Australia during summer. So what does the fuel-efficiency-minded person do?

The windows versus air conditioning debate has been going on for some time. In fact, the popular TV show Mythbusters had a go at it. They got both guys driving around a track in similar SUVs, one with the windows down and one with the air conditioning on to see which one ran out of fuel first.

The one with the air conditioning did, which looked like that case should be closed, but it’s not as simple as all that. Firstly, the Mythbusters test wasn’t a strictly controlled one. Even two vehicles of the same make and model will perform differently, depending on a range of factors, including the condition of the engine and the inflation of the tyres. Secondly, the two presenters have different builds and probably have different driving styles, simply because they’re different human beings. To be a more rigorous scientific test, the only thing different should have been the choice between air con and windows open. In other words, the test should have been conducted with the same vehicle driven by the same person with exactly the same conditions – which possibly wouldn’t be the case if you only drove the car once with the air con on and windows up, then with the A/C off and the windows down, as the operating temperature of the engine (cold start vs. hot start) also affects the fuel efficiency. Lastly, one test isn’t enough in the world of science – one result could be just a one-off exception. The ideal is to run test after test after test and see what the general tendency is.

It also gets more complex than that. It turns out that the more aerodynamic a vehicle is to start with, the bigger the effect of drag will be. In other words, in a smooth, sleek sedan, the effect of opening the windows will be greater in terms of percentage than opening the windows on a big chunky 4×4.

To cap things off, speed also has an effect. This is because the faster you go, the more air resistance your vehicle encounters, so the drag increases, and they increase exponentially. This means that if you’re driving at 100 km/h, the effects of drag are four times greater than what you experience at 50 km/h.

The problem was put to a team of actual engineers who ran a proper scientifically rigorous test* to solve the problem. They used two vehicles, a 2009 Ford Explorer to represent the big SUVs and a 2009 Toyota Corolla to represent the sedans. They were tested in the lab and on the road at a variety of speeds and at idle. Here’s what they found:

• At 40–70 miles per hour (that’s 64.4–113 km/h), in both vehicles, turning the air conditioning up to the maximum (which is how they ran the tests) used more fuel than opening the windows.
• Above 70 miles per hour (113 km/h), the two cars behaved very differently.
• At 75 mph (121 km/h), in the Toyota Corolla, there was no difference between having the air con on and having the windows down.
• In the Toyota Corolla, at 80 mph and above (that’s 129 km/h – did they test this legally on an actual motorway or did they have their own circuit somewhere?), having the air conditioning on was more fuel efficient than opening the windows.
• In the Ford Explorer, having the windows down continued to be more fuel-efficient than using the air conditioning.

The study also tested the air conditioning at different settings other than full blast, but you have to pay to see those results!

Of course, not all cars are Toyota Corollas and Ford Explorers, and each has its own drag coefficient and intrinsic fuel efficiency. However, a good general rule of thumb is that if you’re travelling around town, windows down is more fuel efficient. In small sporty vehicles, using the air conditioning is best at open road speeds, but having the windows down is more efficient for big chunky ones.

Here, I will have to add that there are some other advantages of using the air conditioning rather than opening the windows. Firstly, if the outside air is already hotter than comfortable, you’ll only feel a small drop in temperature if you open the windows. It might not be enough to drop temperatures of 40° or more to a nice comfortable room temperature of 18°C. However, the air conditioning will really drop the temperature to this ideal level.

The other problem is that it isn’t just air that can get through the window when its open. Having half a swarm of bees going through the window isn’t the best for safe driving. Nor is having a wasp fly through the window a good idea. Worse still are stones flying up. I’m not making this one up. Last summer, when we were towing a caravan with the windows down and had pulled over to let someone pass, a stone flicked up, glanced off the wing mirror and flew through the open rear window and hit my adult daughter in the face.  A freak accident, I know, but I know that from now on, both she and I will be using the air conditioning on the open road.

* Huff, S., West, B., and Thomas, J., “Effects of Air Conditioner Use on Real-World Fuel Economy,” SAE Technical Paper 2013-01-0551, 2013, https://doi.org/10.4271/2013-01-0551.

Terramechanics: The Science Of Off-Roading

A lot of us have purchased, or have considered purchasing, a 4×4 vehicle. This could be because we like the benefits of extra safety or the visibility of the higher body. Or it could be that we want to go off-road in the vehicle and do a spot of exploring. However, when we get behind the wheel of one of these vehicles and head for the nearest dirt road or river, we don’t often stop to think about all the science behind what we’re about to do. The most we might think about are things like the power and torque needed – and possibly the basic physics involved in getting over or around a gnarly bit of terrain.

However, there’s a whole branch of science related to off-road driving, known as terramechanics. In fact, there’s a specialised scientific journal on the topic. Terramechanics has been a scientific topic since the 1950s, and the concept was introduced by a Polish-born engineer by the name of Dr Bekker, who was one of the key developers of the lunar rover vehicle used on the moon. However, much of Dr Bekker’s work was more down to earth – literally.

Terramechanics, in a nutshell, is the science of how a vehicle operates off-road on rough, uneven and soft terrain. It mostly considers the interaction between the wheels and the ground, although the science also acknowledges that things like the length of the wheelbase, the torque and the ground clearance are all very important factors. However, it’s what happens where the rubber meets the (off) road that gets certain people in white coats (probably dirty white coats) very excited.

Now, if you get right into terramechanics, the maths gets pretty complicated. If you’re like me, you probably left quadratic equations behind once you left high school. However, engineers and designers in the field of terramechanics use them all the time. I won’t get into the heavy-duty maths, but here, we’ll have a little look at some of the things that get thought of when the designers are coming up not just with new 4×4 models but also with the tyres that go on them.

Vehicle-related factors

Load: the weight of the vehicle plus what’s in it – probably you, a friend, the dog and something to eat.

Contact area: Exactly how much of the tyre is touching the ground. This is affected by the design of the tyre, the width of the tyre, how much air you put in the tyre and the condition of the tread.

Rolling resistance: Also known as friction.

Torque: You knew this one was going to be important, didn’t you? That’s why the torque – the measure of rotational acceleration (rather than linear acceleration) is always given in the specs of any vehicle.

Wheel width: Put simply, more contact area means more grip.

Wheel radius: There’s a reason why 4x4s have bigger tyres, and it’s not just for better ground clearance.

Terrain-related factors

Designers have to consider these factors when they design tyres and the vehicles. As anybody who’s done any off-roading will know, not all types of terrain are created equal, and the techniques and tyres that work well with, say, snow won’t work with sand.

In fact, a lot of what goes on in terramechanics considers the properties of the soil or the other terrain (snow and sand). You might think of soil as just mud or good plain dirt, but it’s pretty complicated stuff. It’s a combination of solids (the actual particles of soil), liquids (water) and gases (air), and it’s constantly changing even in a single place, to say nothing of how soil varies from place to place. I won’t bore you with all the different factors, what they mean and how they affect each other, but some of the most important ones that researchers have to specify when they run tests of new tyre designs or even whole cars are the following:

Moisture content: How much water is in the soil at any point. This affects the shear strength of the soil, which is very important in a lot of the formulae used in terramechanics to work out whether a wheel will lose traction or not. The shear strength of anything is its ability to stand up to a force that will make it slip sideways.

Porosity: How much air is inside the soil – these pores are where the water goes when you water the garden.

Particle size and shape: How big the minute particles of soil are and what shape they are has a big influence on how the dirt sticks together, holds moisture and compacts under pressure. Most of us have known since childhood that sand and clay are very different, and this difference is mostly down to particle size and shape.

Specific gravity: How dense a substance is. Yes, this is related to the specific gravity known to home brewing enthusiasts.

After considering these basic factors, things start involving complicated equations that make my head ache.

You know, I’m kind of glad that when I go off-road driving, I don’t have to keep all these factors and the science in my head – otherwise, I’d overthink everything all the time and wouldn’t enjoy the experience. An experienced off-road driver will be able to do by feel and “instinct” (i.e. right-brain thinking) what the terramechanics expert would have to calculate. All the same, I’m glad that there are people working hard to make sure that our vehicles and the tyres on them are the safest and best they can be.

New Cars, New Year.

Happy New Year to you all!  2023 sees some brand new cars coming into view, and we’re set to observe a considerable increase in the number of EVs sold on our shores.  Here’s a brief look at some of the exciting cars and SUVs you can get your hands on in the near future!

Alfa Romeo Tonale

Here’s the first of the Alfas that take the special brand into EV mode.  The Alfa Romeo Tonale Hybrid kicks it off with its hybrid engine offering the new 48-volt hybrid propulsion system.  You’d expect an Alfa to be sporty, and the nicely designed compact SUV delivers on this front.  Comfortable interiors and decent technology make this a good way of upgrading into 2023.

BMW iX1

The smallest X model from BMW comes in as the X1, and the iX1 is the EV model with a battery range up over 400 km.  Nicely laid out interiors, an athletic driving style (typical Beemer), and a good dollop of performance make this a great new compact SUV.

BYD Atto 3

The BYD Atto 3 is a new kid on the block for Australia.  Being an electric medium-sized SUV with a decent range makes this an efficient EV for the new year.  The Atto 3 also offers a comfy interior and plenty of the latest technology and safety features.

CUPRA Born

CUPRA cars are exciting.  They have plenty of performance and are generally a well-priced product with high-end features.  This new model called the CUPRA Born is an EV with a handsome range of beyond 500 km.  The CUPRA Born is an exciting car to look at, and it comes with advanced technology and great connectivity.

Ford E-Transit

Ford’s lovable Transit has a new ticker with the latest vans now offering electric power.  This is a brand new, full-size, pure-electric E-Transit that features a 68 kWh battery and a driving range of up to 317 km.  Ford say that it is possible to fast-charge the E-Transit from 15% to 80% in a bit over 30 minutes.

GWM Tank 300

One of the best vehicles to come out of 2023 will be the impressive looking GWM Tank 300.  Doesn’t it make a statement!  The Tank is powered by a 2.0-litre petrol-electric hybrid system, and the internal-combustion engine offers a juicy 180 kW.  Nappa leather, all the tech, and very handy off-road makes this a hugely appealing.

Lexus RX

Toyota has their luxury Lexus brand offering their latest RX.  If you’re wanting a new hybrid, these are some of the best ones out there.  Toyota make a great hybrid powertrain with smooth performance and impressive efficiency.  The RX interior sees an updated luxury interior design with ever-impressive build quality.  The RX is roomy too and very comfortable.

Maserati Grecale

Maserati is becoming ever more affordable.  That’s a great thing because they make great cars.  The new Maserati Grecale is a front-engine, medium-sized luxury SUV that comes with the promise of plenty of performance.  Three engines are available: two 2.0-litre petrol motors and a 3.0-litre V6 petrol.  Generous on the equipment levels, the Grecale will be rewarding to drive.

MG4

MG offers the new MG4 hatchback this year.  It is an affordable electric hatchback that has been packed full of innovation, style, technology, and an impressive 440 km driving range.

Nissan X-Trail E-Power Hybrid

Nice to look at, and equally at home off-road as it is on, the Nissan X-Trail E-Power Hybrid SUV is very comfortable and very well-equipped.  Rear cargo capacity in the 5 seat version of this awesome SUV is 575 litres (super handy for a family).  A 7-seat option is also available.  The entry point model is a mild hybrid version and uses a 1.5-litre petrol most of the time.  The other powerplant uses the same 1.5-liter ICE, but it doesn’t connect to the wheels directly.  Instead, it becomes the electrical generator of the system that works in unison with a small battery operating as a buffer.  The wheels are fed power via electric motors.

Renault Kangoo

Renault’s new Kangoo definitely can do, especially with its brand new E-Tech EV versions becoming available for the Australian market.  This will be Australia’s cheapest electric van.  The E-Tech has a 90 kW/245 Nm electric motor that drives the front wheels via a single-speed transmission.  You should easily run about town for well over 250 km before needing to recharge.

Keep your eyes open for these new models travelling our roads and on showroom floors across Australia.  Also keep your ears open via Private Fleet, where we’ll keep you up to date with what other new models are coming our way shortly.  All the best for 2023!

Some More Exciting Mazda News

If you are anything like me, then you’ll be driving along the highway spotting the cars coming the other way.  One of the snazzier brands out on the roads would have to be those from out of Mazda’s showrooms.  Mazda’s great variety of models all look great and boast some striking modern designs.  Even the new Mazda BT-50 ute, a very reliable workhorse, is looking pretty slick, and so too the family-oriented new Mazda 6 Sedans and Wagons.  Mazda also offers a wide range of brand new SUVs with sharp looking exteriors and endearing interiors that are well-equipped.  The small MX-30 is one of these SUVs, and it also has some cool new materials used inside the cabin – like the cork inlays that offset nicely against the premium leather trim.

Toyota, Mazda, Kia, and Hyundai sell the most cars in Australia; Toyota being out in front by a decent margin, with Mazda coming in at second place.  For quite some time, in Australia, Mazda has been a popular vehicle to buy.  Mazda’s 2022 sales of 43,687 are down 9% on this time last year, though Mazda retains its 2nd placing to Toyota’s 1^st place for overall sales.  Sales drops have occurred right across the market, thanks to the shortages of components and current logistical issues.  Mazda had sold 101,119 vehicles in 2021, quite a large portion of the whole pie, considering all the brands that are available to buy new in Australia.

So, what’s some hot off the press Mazda news?

You may have noticed a tidy looking compact SUV running our roads the past couple of years.  Mazda’s MX-30 is one of Mazda’s newer creations in recent times – gaining in popularity too.  The little Mazda MX-30 has been a key model for Mazda in that this has been Mazda’s model of choice for bringing new technologies, hybrid motoring, and EV motoring into their modern fleet of vehicles that will also lead them in a new direction for future motoring.

As the direction of future transport trends toward cleaner engines and lower emissions, Mazda launched a series of mild-hybrid powertrains.  We saw these first being used in the Mazda 3 and Mazda CX-30.  The Mazda MX-30 provided a mild-hybrid engine, but Mazda also made available their brand new pure-electric drivetrain available for the MX-30.  Designed purely for an emissions-free city commute, the Mazda MX-30 Electric uses its 107 kW to whistle up to 0-100 km/h in 9.7 seconds and up to a top speed of 140 km/h.  The driving range proves to be over 175 km, a handy dollop of motoring before recharging needs to happen.

Some other new Mazda technology which will be implemented will be that of a new small rotary engine as a supplementary power source.  With a NEDC (New European Driving Cycle) accredited range of 224 km, this will provide Mazda-buyers with another means of excellent low-emission city transportation.

Mazda’s approach to electrification is based on the customers’ demand for EVs, and the regional infrastructure available to support them.  Within 5 years, Mazda hope to introduce a new hybrid system alongside more battery-electric models in their line-up of new vehicles.  This is in response to the newer European emissions rules that come into effect.

Towards the end of this decade, Mazda’s entire line-up will offer fully electrified versions.  One model that has been talked about as being another exciting Mazda EV will be the little MX-5’s progression into EV powering.  Now that’s a tasty thought!