FUEL ECONOMY AND THE COST OF TRAVELLING – PART 1

Tom Olthoff looks at getting the most out of your dollar

Words and photography by Tom Olthoff

It has been said that a little knowledge can be dangerous but when it comes to operating a vehicle in an economic manner, a little knowledge may well save a few dollars and allow the budget to stretch a bit further.

With the ever-increasing cost of fuel, some tell us that it won’t be long before we can’t afford to turn the key. But is it really as bad as all that? Are people likely to stop travelling? Do we need to have shorter holidays? Should we revert back to swags and tents?  Is there something that we can do about it? Or do we simply adjust or get used to the circumstances?

The answers to these questions and many other is most likely to be – it depends. In this case it can depend on your point of view. Realistically the cost of fuel is almost the smallest expense, about 20 per cent, when it comes to owning and operating a motor vehicle. The reason that it may not seem this way is because we dig into our pockets every week to fill the tank whereas the loan repayments are taken out of the bank account automatically and items like insurance and registration come around annually. And maintenance and repairs can also demand a big outlay. The drop in the value of our car, referred to as depreciation, we don’t see at all. If you add it all up we pay a great deal, in addition to fuel costs, for the benefit of having the use of our own motor vehicle.

That doesn’t mean that we shouldn’t be concerned about fuel costs. Far from it, after all any money we spend buying fuel can’t be used for anything else.  And who wants to give up the occasional meal out just so we can keep the tank full.  So we’ll try to work out where caravanners may be able to reduce the running costs so that they can still continue to enjoy their holidays.

To do this we’ll have a look at new-car fuel consumption labels, choices in fuel, what happens to the power produced, effect of engine and vehicle speed on consumption and being a bit smarter about the way we drive our cars.

What Do The Numbers Mean?

In January 2001 Australian Design Rule (ADR) 81/00 – Fuel Consumption and Labelling for Light Vehicles – came in to effect. Currently ADR 81/01 requires all new vehicles up to 3.5 tonne GVM, including passenger vehicles, four wheel drives and light commercials sold in Australia to display a label on the vehicle’s windscreen that states its fuel consumption as per ADR 81.

This label was introduced to make motorists aware of the importance of fuel efficiency. The aim is to allow consumers to make a more informed decision when purchasing a vehicle by comparing the consumption of the vehicles on the shopping list. And you can’t argue with that.

Many consumers believe that these figures are realistic and come from driving the particular vehicle on the road under a variety of conditions. Unfortunately this is not the case. The figures on the label are purely a means for comparing one vehicle with another. The data is obtained under a given set of laboratory conditions. Actual consumption can vary greatly and depends on many different factors. Towing a trailer and additional wind resistance are two factors for which allowances are certainly not made.

Fuel consumption data provided prior to 2001 gave separate figures for Highway and City operating cycles. ADR 81/01 has combined these two. The current test is based on the internationally recognised United Nations ECE Regulation 101. This is the European fuel economy test and uses an identical drive cycle to determine both fuel consumption and noxious emissions.

The ADR 81.00 test cycle simulates an 11km journey with an average speed of 33.6km/h. About two-thirds of the test time simulates urban stop-go driving where the average speed is 18.8km/h. The other third of the test is similar to driving along a highway with the vehicle driven up to 100km/h. All this is achieved without the vehicle putting even one tyre on the road. While the method of measuring fuel consumption is obviously very accurate, it should be appreciated that all real life situations cannot be duplicated in a laboratory environment.
In an attempt to reflect real world fuel economy more accurately, the USA EPA has added three new tests for 2008 that will combine with the current city and highway cycles to determine fuel economy of new vehicles. A high speed/quick acceleration loop lasts 10 minutes, covers 8 miles, averages 48mph and reaches a top speed of 80mph. Four stops are included, and brisk acceleration maximizes at a rate of 8.46mph per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 20 to 30 degrees Celsius.

Fuel Consumption Factors

A study carried out by the Monash University in 2002 found that driver behaviours which affect fuel consumption include: choice of travel speed; smoothness of driving; choice of travel route; use of air conditioning and use of cruise control. The study also found that smoother driving has a greater potential for reducing fuel consumption in urban areas than for open road travel. In fact smoother driving can lead to greater reduction in overall fuel usage than lower travel speeds. Whether you have a trailer in tow doesn’t have any bearing on this.

Where Does The Power Go

As fuel needs to be burnt in order to produce power to get a vehicle moving, it is worth considering whether the power, or energy, is used efficiently. Although a diesel engine is considered to be more efficient in relation to the amount of power produced for a given amount of fuel, there is still a great deal of wastage.

Considering the potential energy in a litre of petrol, diesel or LPG, the amount of motive power ultimately obtained to move the vehicle is relatively small. It is in the region of 20-30 per cent. The remainder is lost or wasted to overcome the likes of friction between mechanical components, wind resistance and tyre rolling resistance. In round numbers, about 30 per cent of the heat developed from the burning of the fuel is lost into the cooling system, 30 per cent out of the exhaust, 10 per cent to overcome the friction of the engine’s moving components and 5 per cent to other thermal losses. This leaves about 25 per cent to actually turn the crankshaft.

Most internal combustion engines have an efficiency of about 25%-30% once thermal and mechanical losses are considered

But that is not the end of it. The useful power remaining then has to operate accessories like the alternator, air con compressor, water pump, etc.; rotate other components like the gearbox and drive shafts; overcome the rolling resistance of the tyres and push air out of the way. By this time there is only about 10-15 per cent left to move the passengers or pull a load.

In regard to wind resistance or aerodynamics, more than 80 per cent of the drag is caused by air flow around the vehicle, particularly in the case of a caravan which generally has a flat front. Another 10 per cent is the result of friction between the air and the surfaces of the vehicle and a further 10 per cent is the result of drag created by air flowing through the radiator, vents, around the wheels and under the vehicle.

Overcoming wind resistance requires a lot of power at higher vehicle speeds.

So an aerodynamically designed caravan would have a rounded sloping front, rounded rear, smooth surfaces and be low to the ground. As far as a caravan is concerned, that’s probably not very functional if we are looking to maximise interior space for a given length.

Fuel Types

Today in Australia we use four types of fuels. These are:

Diesel Fuel like petrol, is distilled from crude oil, and is used in an engine invented by German engineer Rudolf Diesel in 1894. Generally the term refers to fuel that is processed from petroleum but increasingly alternatives such as Biodiesel that are not derived from petroleum are being developed. Diesel is generally cheaper to refine and in many overseas countries costs less than petrol. In Australia, however, the opposite seems to be the case.
Petrol or Gasoline as it is known in North America, is a petroleum derived mixture produced in oil refinaries and consisting primarily of hydrocarbons, used as fuel in internal combustion engines. These days material that is simply separated from crude oil via distillation will not meet the required specifications for modern engines so special processes and additives are required to produce the fuel that is used in our vehicles.
Propane was identified as far back as 1910, is generally obtained from other petroleum products during oil or natural gas processing. When sold as a fuel it is also known as Liquified Petroleum Gas.  LPG or LP Gas as it is often referred to, is a mixture of propane with smaller amounts of butane and other products.
Bio-fuel or Biodiesel is the name given to a variety of fuels made from vegetable oils or animal fats.  The concept of using vegetable oil as a fuel dates back to 1895 when Dr. Rudolf Diesel developed the first diesel engine to run on vegetable oil. Diesel demonstrated his engine at the World Exhibition in Paris in 1900 using peanut oil as fuel. Today there are a number of companies experimenting with biodiesel fuels.

Ethanol Blends (or ethyl alcohol) are a clear, colourless liquid, generally manufactured from grain or sugar. (Currently around 90 per cent of Australia’s ethanol is produced from wheat). Blending ethanol and petrol in various proportions has been put forward as a means of reducing greenhouse gas emissions and alleviating adverse economic conditions in the sugar industry.

It is generally agreed that an ethanol/petrol mixture containing up to 10 per cent ethanol is acceptable for current engine design. For higher percentages of ethanol certain engine modifications are required.

Which Fuel?

The amount of power or energy that a particular fuel can produce depends on the amount of heat that is given off when the fuel is burnt. This is sometimes referred to as the calorific value and usually expressed in megajoules per litre (MJ/L). Most readers may be more familiar with the old term BTUs. The table below gives a comparison of different fuels.

Fuel Type                                           MJ/L

Petrol                                                   29.01
Diesel Fuel                                           32.19
LPG                                                     22.16
Heating Oil                                           34.74
Ethanol                                                 19.95
Methanol                                              14.57
90% petrol + 10% Ethanol mix 28.06

The table shows that diesel has the highest energy value while, based solely on the numbers, LPG has the lowest of the three commonly used fuels.

One reason that LPG has less energy content is that it has a lower density. While energy content per kilogram is higher for LPG than for petrol, you don’t get as much LPG for a given volume. The result is a lower power output. To some extent this can be overcome by tuning the engine for LPG. With modern computerised engine management systems that is easier to do. It is also possible for a fuel’s energy content to vary between summer and winter.

Cost Consideration

Many don’t believe that running an engine on diesel can be more economical. After all, pump prices show diesel to be more expensive. What is not always realised is that a diesel engine will consume about a third less than a petrol one operating under similar conditions. So the saving comes from needing to buy less diesel fuel to fill the tank.

A similar situation exists with petrol engines that require premium-unleaded fuel. Because the cost is often higher than regular unleaded, motorist will opt for the cheaper fuel. Yet the engine is likely to consume more trying to produce the same amount of power, negating any savings in the cents/litre. Using the more expensive fuel, as long as it is the right one for the engine, can actually improve the consumption.

When it comes to using LPG, the sums have to be done very carefully, even when rebates are offered. While there is no argument about the lower price of LPG we also need to consider the additional cost of converting an engine to operate on LPG or purchasing a dual-fuel engine in the first place. Generally savings are made in the long term rather immediate. If LPG sounds a desirable option and you are buying a new car, go for the factory fitted conversion as the performance and economy are usually optimised. There are also less hassles with warranty should problems arise.

Factors That Increase Fuel Consumption

NOTE: if  windows are open at higher speeds the additional air turbulence created and the subsequent increase in fuel usage can be greater than when the air conditioner is used with the windows closed.

Economy and Speed

Most motorists will probably agree that there is a relationship between speed and economy. After all, most divers know that if you drive faster the engine uses more fuel. But that is not always the case. At low road speeds the economy can actually be worse than if you drive faster. This doesn’t mean that you are better off speeding.

There is a condition, however, where the engine works most efficiently. A good analogy is a bicycle. When the pedals are rotating slowly, it can actually be physically harder to keep the bicycle moving, as well as more tiring, than when our legs are moving at a faster pace. There comes a time of course where if we try to pedal too fast we consume too much energy and quickly become puffed out. Figure 5 shows how fuel usage relates to speed. Note that this is an example only as the situation varies with engine and fuel type and many other factors.

There is an optimum speed for achieving good fuel consumption.

In part 2 we’ll discuss the affect of torque, why towing consumes more fuel, the best driving techniques and how to obtain accurate consumption data.