FUEL ECONOMY AND THE COST OF TRAVELLING – PART 2

Tom Olthoff looks at getting the most out of your dollar when towing a van

Words and photography by Tom Olthoff

Last month we looked at what the fuel consumption figures tell us, where the power goes, types of fuels and the factors that control the amount of fuel consumed. This month we’ll discuss the affect of torque, why towing consumes more fuel, the best driving techniques and how to obtain accurate consumption data.

What Is All This Talk About Torque?

When it comes to towing, obviously there are vehicles that do the job better than others. But is that necessarily reflected in the specifications? The answer is – not always.

Take for example a Mitsubishi 380 and a Pajero, both with a 3.8L petrol engine. The engine specifications are almost identical. In fact on paper the 380 has more power and torque. But most would agree that a Pajero could pull a particular load better than a 380.

What is important is not so much the specifications but the power that’s available to turn the wheels. And because of the gearing built in to the car, this can vary a great deal.  In the case of the Pajero the gear ratios are designed to provide good pulling power while the Magna offers superior economy when touring. Sadly we can’t win on both fronts.

Unless the size and type of engine is taken into account, power output figures can be misleading. Have a look at the figure below which shows a Pajero’s petrol and diesel engine’s power output and torque characteristics. To understand this better it helps if to think about Torque as representing pulling power.

You’ll notice that the maximum power output for the petrol engine is higher than that for the diesel (184kW versus 121kW), but it occurs at 6000rpm. As we normally don’t have the engine turning this fast when towing, it doesn’t help us very much. Similarly the maximum power of the diesel occurs at 3800rpm, again higher than we would normally see on the tachometer.

The Torque data on the other hand presents a more interesting picture. Although the petrol engine produces its maximum pulling power (329Nm) at 2750rpm, there is 300Nm or more available between about 1750rpm and 4500rpm. This is great for towing. But have a look at the diesel. Not only does its maximum torque (381nM) occurs at a nice low 2000rpm, between 1200 and 3750 rpm there is more pulling power than the larger capacity petrol engine can produce. This means that at normal towing speeds the turbo diesel always has lots of grunt on tap.

You are probably asking what all this has to do with fuel consumption. Well, the spot where maximum torque is developed, in this example 2000rpm for the diesel and 2750rpm for the petrol, is about where the engine works most efficiently. In other words, it extracts the most energy out of a given amount of fuel. This doesn’t mean that you should always be driving along with the tachometer pointing at these numbers. But when the going gets tough, like scaling a steep incline, the engine should be kept around about the maximum torque mark. But as we mentioned above, there is some latitude either side of that.

The diesel will provide useful power between 1500 and 3000 rpm while the petrol prefers 2000 to 4000 rpm. Please note that this only applies to the Pajero engines used for this example. Other makes and models can be very different to this. But knowing where your engine works happily not only improves performance but also improves fuel consumption.

The point we are trying to make is that if you keep the revs low thinking that the engine would use less fuel, could well put you on the wrong track. Often the engine is happier at higher revs and as a result will use less petrol. This is one reason why towing in top gear is often not recommended as the engine is really labouring and efficiency is way down. Consequently the accelerator is pushed down further and more fuel is consumed.

When confronted with a hill it is better to change down a gear early so as to keep the engine operating where it’s most efficient. Going back to the bicycle, most would be aware that having the legs moving quicker when going up hill, or in to a head wind, is in fact less tiring then selecting a gear where the pedals rotate slowly but a lot of energy is required to keep the movement going.

How Much More?

Often first time caravanners are surprised at the increase in fuel usage when towing a van. Interestingly the difference between towing and not towing is greater these days than say twenty years ago. One reason is the improvement in solo fuel consumption due to technology. But this has affected suburban motoring more than being on the highway.

So why aren’t there greater fuel reductions in highway motoring? The only way this can be improved is by using lighter and more aerodynamic cars. When it comes to towing a caravan, wind resistance, rolling resistance, weight and towing speeds haven’t changed much. In fact today we are probably towing at higher speeds than 20 years ago.

Power is needed to overcome the wind and the weight. As the power can only be obtained by burning fuel, we can’t expect worthwhile reductions unless we tow light and aerodynamically shaped caravans at lower speeds. Simply trading-in the full height van for a pop-top won’t do it.

With the bedroom at the front the shape of the van can be more aerodynamic providing better economy than a front kitchen pop-top with a near vertical front.

For the average size pop-top or caravan, budget for an increase in consumption over solo figures of about 30 per cent to 50 per cent. If this sound a lot, remember that the car is not any less fuel efficient today than it was yesteryear. It’s just that these days when we’re not towing the car is so much more frugal.

Note that the power needed to push an object through the air increases with the cube of the increase in speed. For example, a car cruising at 60 km/h may need only 10kW of power to overcome air drag. The same car at 120 km/h requires 80kW. With a doubling of speed the drag or wind resistance is eight times as great. And where do you think that additional power is coming from? Yes, burning fuel!

A low profile unit like a EcoTourer is more economical to tow due to less wind resistance.

Good economy and safe towing is all about compatibility. A large 4WD towing a big van may see less increase in fuel usage than a passenger sedan towing a pop-top.

The Best Techniques

Obtaining the best economy starts the moment you turn the key. Assuming of course that you purchased the right vehicle in the first place. Don’t start the engine ten minutes or more before departure to let it warm up. As well as wasting fuel and create lots of undesirable emissions, you are being unkind to the power plant. Not to mention the annoyance to the neighbours. Yes, I know the truckies do it but they need to run the air compressor to fill the air reservoir so the brakes can be released in order to get moving.

If possible avoid early morning or late afternoon traffic. A lot of stop-start and idling wastes a great deal of fuel. Rainy and windy days will increase consumption so if you are not in a hurry, stay put another day. 

With a manual transmission don’t be too eager to get into top gear. Operate the engine at a speed where it is happy and not where it is working slowly but hard. If you drive a car with a turbo-diesel engine, don’t just flatten the accelerator when you want some get up and go. It is still prudent to use the gears.

Keep speeds below 90km/h as fuel usage increases sharply above this speed. Don’t drive close to other traffic so it is easier to maintain a steady speed. As mentioned earlier, it is all about smooth driving. A 10 per cent increase in road speed will not see you arrive at your destination 10 per cent quicker. The faster the speed, the more often you need to slow down and then accelerate again to reach cruising speed.

For long distant travellers there is also the old debate about driving clockwise or anti-clockwise around Oz. If you were on a fuel economy drive then this may be a consideration.  To see the country at its best and obtain the most enjoyment, I believe it is more important to be in a certain spot at a certain time. If that means travelling in a particular direction, so be it.

Calculating Consumption

We have all met the caravanner who boasts about achieving fantastic fuel consumption while towing heavy loads. Unless you travel downhill with a tailwind, it doesn’t happen. Calculating consumption from one tank of fuel is not very accurate. Reliable figures are only obtained after several fills and a couple of thousand kilometres.

If you wanted to impress fellow caravanners you wouldn’t quote figures obtained between Sydney and Lithgow. If you included these in the overall consumption picture, it takes a long time to get the initial average down to an acceptable figure. Those who have a trip computer will attest to this.

If you do want to keep track of the consumption over a particular journey, start by filling the tank at the outset. Then at each fill record the number of litres. It doesn’t matter if the tank is not filled to the brim. At the end of the journey once again fill the tank to the top. Calculate the total number of litres used and divide by number of ‘hundreds’ of kilometres travelled. For example, if you used 400L to cover 2560km, divide 400 by 25.6.  In this case the answer would be 15.6L/100km.

For those who like to do things right, kilometres per litre or miles per gallon do not represent consumption. But you can debate that around the campfire.

Summary

Like everything in life, caravanning is a compromise. Anyone too hung-up about the extra fuel used when towing our accommodation behind probably should consider motels. There is no doubt that the bigger and heavier the trailer, the more fuel the engine is going to consume. As such, it may be better to settle for the smallest van that will do the job rather than the biggest that can be towed. But can you place a price on comfort and space?

By all means, care about the fuel consumption. With a bit of practice and concentration most of us can probably save ten percent quite easily. But in the overall picture, selecting the right vehicle may well provide a bigger saving then driving shorter distances. And sitting home vegetating is not considered conducive to a long and happy life.

P.S. In case you have ever wondered why the Americans are so concerned about oil supplies, consider the fact that they consume 1.4 billion litres of petrol a day (yes, billion). Hopefully people will realise, before it is too late, that every time they turn the key we are using up more non-renewable energy.

PPS: And another thing – apparently dieting can also help according to this piece from some US research; “Obesity has become a major public health problem in the United States. There are numerous health implications and risks associated with obesity. One socio-economic implication of obesity is that it reduces passenger vehicle fuel economy (i.e., the miles per gallon achieved by automobiles, which include cars and light trucks driven for non-commercial purposes).”