4 Ways To Determine The Correct Fuel Consumption

You need to know your correct fuel consumption for many purposes. Certainly, planning and optimizing fuel costs are among the most important ones.

In order to come to the right conclusions and decisions, the values must reflect real-world use in day-to-day business.

Determining the correct fuel consumption is not easy at all. Believe me!

Quite in contrast to the pure task of calculating the fuel consumption value itself, which is pretty simple. You can find out all about the types of fuel consumption values and their calculation in the article: 4 Ways To Calculate Fuel Consumption (incl. calculator).

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But why should it be difficult?

Modern trucks show one or more consumption values on the display.

You can look them up at any time. What’s complicated about that?

Well, before you start working with fuel economy figures, you should ask yourself the following questions:

  • How big is the scatter and the accuracy of your value?
  • Under what conditions was the value measured?
  • If you want to compare several fuel consumption values, are they comparable?
  • If you want to compare your fuel consumption value with information from other sources, such as journals or magazines, do you know enough details of how they have been derived?

The difficulty lies in the quality and availability of the input data for the calculation. The input values are typically the root cause for the scatter and the non-comparability of calculation results.

This article will introduce you to different ways how to find out the correct fuel consumption. Each method has its advantages and disadvantages, which you will learn about here.

Knowing this, you will be able to choose the right method for your particular use case. You will learn how to assess the consumption values and how to work with them.

In my blog, I am going into detail about this problem in many articles. You can find all of them in the category: Determine consumption.

So let’s jump right in:

How much fuel does a truck use?

This question has no definite answer. The right answer is: It depends on the circumstances.

I’m going to give you the range of heavy truck fuel consumption because when you make a measurement, you need to know the magnitude of the expected result.

The average fuel consumption of European trucks on the road is in the order of 25 to 35 l/100 km, depending on use case, payload and driving conditions. 
The instant consumption varies between 0 (downhill) and well over 100 l/100 km (severe uphill).
When transporting extra heavy loads and in demanding off-road applications, an average diesel consumption of over 50 l/100 km is to be expected.

As you can see, the scattering of the values is quite big. It is therefore not correct to calculate just one single value and assume that it is representative of the whole operation of the truck. More data must be considered to come to a reliable conclusion.

Each consumption value is affected by unique boundary conditions, which must be taken into account when conclusions are drawn.

If you continuously work with your consumption values, you will develop a feeling for the numbers. This will help you to spot anomalies and errors in your data.

The quality of the input determines the quality of the output.

For vehicles, we use relative fuel consumption.

For calculating the correct relative fuel consumption, two input values (fuel volume consumed and distance driven) need to be available in the right quality and the right format. If this is not the case, then the values must be measured.

The quality of the input data determines the accuracy of the result.

When I talk about measuring diesel consumption, I’m actually talking about two different measurements. One is a volume measurement and the other is a measurement of a distance.

They each require very different measuring tools and procedures.

The input data are measured, the fuel consumption is then calculated using the input data. That’s the way it works.

Both measurements are not exact by nature. Every measurement has errors. This is inevitable.

It is therefore important to know how big the errors could be and where they occur. Then you can decide whether they are acceptable or not.

There will be an article on the hidden errors during measurement available very soon.

Every measurement method has its own peculiarities that need to be known and mastered.

You will read some of them below, but much more in the following articles.

If I describe the exact procedure in further articles, then it will mainly be about avoiding, recognizing, compensating for, or taking into account errors.

How influencing factors affect the fuel consumption value

When working with fuel consumption, it is not enough to know the fuel consumption value alone. In addition, the influencing factors under which the respective consumption value was achieved are required.

You will confirm that the fuel consumption is always different.

It is practically impossible to achieve exactly the same consumption every time. There are always influencing factors that make fuel consumption sometimes higher or sometimes lower.

The 4 options for determining fuel consumption are actually 4 different alternatives for dealing with these influencing factors.

I distinguish these influencing factors into two groups:

Systematic influence factors

The systematic influencing factors always have a predictable effect.

We look for them because they offer the way to reproducible improvement potentials.

Systematic factors can be influenced. Changes in these factors always have the same effect on fuel consumption.

Although they can be influenced, we cannot control them all and not at all times.

This results in two tasks:

  1. First, we need to examine the impact of these factors on fuel economy in our particular use case.
  2. Second, we need to find out which of the factors we can influence and which we can not.

The factors, which we can influence, are levers for improvement. We can reinforce the positive factors and suppress the negative factors. By doing so, we achieve a sustainable fuel consumption improvement.

At the same time, we need to deal with the factors we can not influence. We need to exclude their effect. The different measurement methods use different approaches.

Working with the influence factors involves the recording of information on the systematic influencing factors during driving. You have to remember that. I will keep coming back to this.

Random influence factors

Random influencing factors also have an effect on fuel consumption, but they are beyond our control.

Weather and traffic are examples of random factors. Since they dilute the result, they have to be eliminated somehow.

Random influencing factors do not offer any starting points for improvements, but they still have to be taken into account because they affect fuel consumption and cause scatter.

In this respect, too, the various measuring methods deal with it very differently. I’ll go into more detail about that as well.

4 Ways to measure fuel consumption

1. Fleet monitoring

With fleet monitoring, the fuel consumption value is derived from many individual values of the vehicles in a fleet.

The most important requirement for this method is the availability of a large number of individual fuel consumption values. Therefore, this method fits people running high mileage under normal operating conditions.

Anyone can use fleet monitoring, that's the beauty of it.

If you’re one of those people who write down the mileage every time you fill up your truck, then you’re already a user of the fleet management method.

Fleet monitoring is the "mass method" and requires a sufficiently large number of individual values as input for the calculation of a final representative consumption value. The higher the number of input values, the more accurate the overall result. 

You may not be surprised if I tell you that this representative result is called “average consumption”. You can find more about the average consumption in a separate article as well.

In the article “How To Find Out The Fuel Consumption Using Fleet Monitoring”, I explain how exactly to carry out the fleet monitoring method, and how to acquire a representative result, that you can work with.

A number of important aspects concerning the influencing factors must be taken into account when using fleet monitoring. If it is done correctly, the value can be used to make correct statements.

Dealing with influencing factors using fleet monitoring

The basic assumption of fleet monitoring is that the influencing factors average out and neutralize each other over time and with a large number of consumption values.

But that only works to a limited extent.

In fleet monitoring, we use chance to deal with the random influence factors. Therefore, we need many input values to give chance a chance. There will be a dedicated article on the scatter, which is all about normal distribution of random values.

Chance can only affect the random influences, not the systematic influences.

The systematic factors need to be addressed differently. We have to study them very carefully. Because they lead the way to fuel improvement measures, they are of high value to us.

Professional IT systems offer the possibility to automate the calculation and evaluation.

You can find all articles about fleet monitoring in the fleet monitoring category.

2. Fuel consumption measurement run

During a fuel consumption measurement drive, the fuel consumption of a specific vehicle is measured.

A consumption measurement drive is the "method of the class". This method requires extreme accuracy in execution to get an accurate result.

The method of consumption measurement runs is typically used to check or determine the effectiveness of fuel consumption optimization measures.

If you have attended a training course for consumption-optimized driving, then you have probably already experienced it yourself:

  • At the beginning of the training, you drove a set route and the trainer recorded the fuel consumption of this journey.
  • At the end of the training, you drove the same route again and applied everything you have learned.
  • In the evaluation, the trainer beamed with joy and told you how much you had learned from him because the consumption value of the second trip was significantly lower than that of the first trip.

Without being aware of it, you carried out a consumption measurement drive.

Of course, not only the influence of the driving style is measured in this way.

The main area of usage is the evaluation and confirmation of vehicle improvement measures.

If you read in a magazine about a test of a vehicle, you will usually find a statement on the fuel efficiency of the vehicle presented. With a high degree of probability, this information was determined using the “consumption measurement drive” method.

Can you trust what it says?

Generally yes, because these measurements are done by professionals. However, your fuel consumption would still be different, if you would repeat the measurement.

I’m going to explain in detail what to look out for when making up your mind about whether you can trust such a statement.

Do you want to carry out a fuel consumption measurement drive yourself? I will write another article that will help you with this project.

Dealing with influencing factors during a fuel efficiency measurement run

Since only very few measured values are created during the consumption measurement drive, chance doesn’t stand a chance.

The trick, that works for fleet monitoring, cannot be used here.

For this reason, it is necessary to meticulously consider each individual influencing factor.

I’ll go into detail on how to treat each influence factor during a measurement drive in a separate article.

A consumption measurement drive is associated with a considerable amount of effort for preparation, execution and evaluation. It is therefore only used if specific questions have to be answered.

For this reason, you will consider using the results of other people’s measurements. Be it magazines, manufacturer information, or other institutions that do such tests and publish the results.

It is all the more important that you know what to look out for when you trust the results of other people’s measurements.

3. Fuel consumption simulation by computer

The fuel consumption simulation method uses mathematical models to calculate the fuel consumption of a virtual vehicle in a computer.

The virtual vehicle is a virtual twin of the real vehicle for which we want to know the fuel consumption. So the value we get corresponds to the consumption value of the real vehicle.

A consumption simulation is the "method of analytics". A validated mathematical vehicle and driver model is required for this method.

This method is very popular with vehicle manufacturers. They have the information needed for the calculation, the necessary know-how, and the equipment. (Another group are the companies and research institutions that research and develop such tools.)

For a vehicle developer, this method offers a unique opportunity. He can determine the fuel consumption of a vehicle or the impact of improvement measures before the vehicle even exists in reality.

This saves a lot of time and money. Therefore, the know-how and the equipment for these calculations belong in the repertoire of every vehicle manufacturer.

The quality of the result depends on the quality of the mathematical/physical model and of course also on the quality of the input data.

Mathematical models always have simplifications that reduce accuracy. For this reason, they are continuously improved.

The input values, in turn, often have to be determined or at least confirmed by real measurements.

To do so, consumption measurement runs and bench tests with real vehicles or vehicle systems need to be carried out. The calculation method always must be calibrated by reality.

Influencing factors in consumption simulation

By using the consumption simulation, all influencing factors can be consciously controlled. This is a unique advantage and disadvantage at the same time. As a consequence, the result is very repeatable. But on the other hand, it is synthetic and doesn't reflect real live to the full extent.

There is no randomness in a computer unless you consciously build in a random number generator.

All fuel consumption influencing factors can be set in a controlled manner without exception. This even applies to the behavior of the virtual driver.

This means that even small improvement measures can be isolated and evaluated very nicely.

Due to the controllability of the influencing factors, the results can be compared with each other without restrictions.

However, transferring the results into daily practice is not easy. Chance rules the real world, and the dice roll just the way they want. It requires some effort and some compromises to translate the simulation results into real-world applications.

VECTO is a consumption simulation tool

An example of this method is the VECTO Tool of the European Union (Vehicle Energy Consumption Tool).

The legislator has set it up to evaluate the efficiency of the vehicles of all manufacturers.

You receive these calculated values from the manufacturer together with the papers for your vehicle. That’s a law.

I will go into detail about VECTO in a separate article.

It is important to understand what is and is not reflected in these values.

Why did the legislator in the case of trucks decide in favor of the consumption simulation method?

Well, because the vehicle variance in commercial vehicles is extremely large, it is impossible to do measurements with each vehicle variant sold. Doing a simulation for every variant is feasible.

The simulation offers the opportunity for a fair comparison of vehicles from different manufacturers.

Both sides, legislators and vehicle manufacturers, have agreed on simplifications in the model, which reduce accuracy but increase fairness.

If everything is done right, the real fuel consumption should actually be better than the VECTO value, because some consumption-saving factors are not taken into account. But this tool is also constantly being developed and should become more and more accurate over time.

Simulation light for the common people

As you read my articles, you will come across mathematical formulas. These formulas are small mathematical models.

By using these formulas in Excel spreadsheets, I can do calculations and create very simple simulation tools.

The result might not be very accurate, but by trying different input values, I can get information about the magnitude of change in the result. This gives me a feel if a measure or an influencing factor has a big or a small impact.

I highly recommend this exercise. Using the formulas to build such small and simple self-service estimation tools gives you a very good insight into the physical correlation of input and result.

Having pointed this out to you, you will recognize these little guessing tools as you read the articles. Watch out for it! In some articles, I am going to provide some for download.

4. Test bench measurement run

During a test bench measurement, the fuel consumption of a real vehicle is measured on a test bench under controlled conditions.

A real vehicle and a roller dynamometer test rig equipped with a sophisticated control system, installed in a building with weather simulation capabilities, are required for a test bench drive.

If you have access to such a very expensive test stand, measurements can be carried out quickly and with a very high level of repeatability.

A test bench measurement combines the advantages of the consumption simulation and the measurement drive method. 

To carry out such a measurement takes significantly more effort than the consumption simulation, but less than doing a measurement run on the road.

The vehicle needs to be converted into the various states that are to be measured. Therefore, prototype parts need to be manufactured and installed in the vehicle.

Since this is a stationary situation, data logging and measurement sensors in the vehicle can be hooked up to the test stand.

Because it is a measurement with a real vehicle, there is no compromise of a simplified mathematical vehicle model.

Influencing factors on test benches

The influencing factors can be controlled during test bench measurements. As a consequence, the results are repeatable. The systematic influencing factors are real and can be isolated and investigated very well.

The environmental influences are consciously controlled on the test bench.

  • The winds are generated by fans.
  • The outside temperature in the test room is kept at the desired level.
  • The road profile is mapped via the performance rollers of the test bench.
  • There are no traffic jams or road obstacles.

However, the transfer of these measured values into practice has the same difficulty as with the simulation. Typically, an artificial setting of parameters is used on the test rig, while in practice, chance picks the influencing factors and different consumption values set in.

You might have recognized the discussion on NEFZ cycle (Neuer Europäischer Fahrzyklus, English: New European Driving Cycle) and WLTP cycle (Worldwide harmonized Light vehicles Test Procedure) for passenger car emission and fuel consumption measurement.

Those cycles are these kinds of artificial settings of parameters, I am talking about.

Since the environmental influences are defined by the legislators, the result is reproducible but not very close to the real world’s consumption.

This is helpful for the engineers, who get a clear understanding, of what the legislators are looking for when they write their laws and regulations. That is at least the intention. As I’m sure you’ve read or heard, some have pushed it beyond legality, which has destroyed trust in the entire industry.

If the specifications are chosen in a way that the test consumption is significantly lower than the real world consumption, the customers get confused and angry.

Nonetheless, for development purposes, this method offers a great possibility to measure fuel consumption optimization measures, because the results have little scatter.

Road to Rig

Road to Rig is a special way to do the test bench measurement. The parameters of the test cycle are not set artificially, but are recorded during a fuel consumption measurement drive and then applied to the test rig. By doing so, similarly realistic results can be achieved.  

Road to Rig essentially shifts a test drive from the road to the test bench. It combines the fuel consumption measurement driving test procedure with the test bench measurement procedure.

This is a very interesting method. It provides realistic yet reproducible results. Using this method, a recorded fuel efficiency measurement run can be repeated many times under the same conditions.

However, acquiring the data requires a significant one-time effort. Therefore, most of the time the focus is on the powertrain data only.

Summary

  • Calculating fuel consumption is easy, the difficulty lies in the availability and quality of the input data.
  • Influencing factors are the root cause of scatter, but provide also potential for optimization.
  • Fleet monitoring is a simple method that anyone can use. It provides the most realistic fuel consumption values.
  • The consumption measurement drive must be well planned and carried out carefully in order to obtain meaningful results.
  • Computer fuel consumption simulation is very cost- and time-efficient, but compromises accuracy.
  • Test bench measurement runs combine advantages from the measurement drive on-road and the controlled conditions of a simulation.
  • The fleet monitoring method is the best method for drivers and entrepreneurs because it does not require expensive equipment.

After you have read this article, I would recommend making yourself familiar with the different types of fuel consumption values and how you can calculate them.

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  1. Hi there! I could have sworn I’ve visited this website before but after browsing through a few of the posts I realized it’s new to me. Regardless, I’m certainly happy I came across it and I’ll be book-marking it and checking back regularly!

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