This document discusses the operational aspects of fuel economy and suggests how a driver can reduce his/her fuel consumption. While we put emphasis on diesel engines and their operation in highway transportation, it is believed that much of the techniques discussed here are applicable to a common car driving as well. Thus every visitor to these pages may benefit from reading their lines. 

Document summary

Engine characteristics are designed and optimized for the most optimal operation and are determined by the engine's characteristic torque curve. This torque curve provides the driver with information on the most fuel efficient range of the engine RPM to use. The driver should shift in the narrow range of RPM within the peak of the engine torque and the valley of the engine's BSFC (Brake Specific Fuel Consumption). Overloading the engine (too high gear), together with addition of excess fuel (pedal on the floor) which the engine can not utilize should be avoided. Using slow, gentle acceleration, careful fuel pedal management techniques, and inertia of the vehicle(s) I was able to increase my fuel economy by 6-10%. To the contrary of popular claims, the use of automatic transmissions do not contribute to fuel economy. In fact, the opposite may be true due to energy losses in automatic torque converters, and because automatics allow unskilled and inexperienced drivers to drive trucks. This may become a safety issue as well and inevitably leads to reduced fuel economy as an increased fuel consumption is correlated with driver's inexperience and the lack of skills.


1) Internal combustion engine
2) The chemistry of combustion
3) Driver and fuel economy
4) The effect of automatics

Related pages:

Cummins ISX435ST profile
Automatics: a safety issues
Gerhard Plattner's tricks

1) Internal combustion engine

From our college physics we recall that an internal combustion engine serves to convert chemical potential energy harnessed inside the chemical structure of fossil fuels into useful mechanical work. This is true for both, gasoline and diesel engines. In this review however, I will focus on diesel engines since they are primarily used in highway transportation.

The diagram below depicts the function of a typical four stroke diesel engine. The engine pictured here, is an indirectly injected engine with a pre-combustion chamber. These engines were common in the 60s, 70s and even 80s. They would hardly be found in trucks these days. Today, separate fuel injection pump connected to fuel injectors through high pressure fuel lines is replaced with a fuel pump and the fuel injector contained in one unit. This arrangement allows for much higher fuel injection pressures and better atomization of fuel injected directly inside the fuel combustion chamber.

To better understand the fundamentals, we may now review the individual parts of the diesel engine cycle:

1) During the Intake stroke the piston is in TDC (Top Dead Centre) and intake valve is open. As the piston travels downward, the space above the piston is filling up with clean air. At the piston BDC (Bottom Dead Centre) the cylinder above the piston is filled with air and the intake valve begins to close. In reality, the intake valve still remains open for a short period of time when piston starts heading upward, for air still continues flowing inside the cylinder by its inertia. This ensures better filling.

2) Both valves are closed, and the air inside the cylinder is being compressed as the piston heads upward toward its TDC. During this Compression stroke, due to the high air pressures inside the cylinder the air heats up to the temperature above fuel flash point.

3) Before the piston reaches its TDC, the fuel injection begins. The first injected fuel ignites rapidly and the pressure and temperature of gases in the combustion chamber (the space above the piston) increase rapidly.

4) the fuel injection continues so as to maintain the pressure of gases in the combustion chamber, and the pressure on the piston as constant as possible. This pressure of gases pushes the piston back down towards its BDC. It is this Power stroke that performs the engine's useful work.

5) Before the piston BDC the exhaust valve opens, and during the next Exhaust stroke,  the exhaust gases are pushed out of the cylinder while the piston travels back upward. Next, the entire process is repeated again.

During the engine development stage, it is the engineers' task to optimize the shape and bore of intake and exhaust manifolds, exhaust back pressure, the shape of the combustion chamber, the angle of the injection nozzle, the valve and fuel injection timing as well as the pressure and the length of the fuel injection to strike the balance between the optimal engine performance, the fuel consumption, and acceptable exhaust emissions. The result is an engine with its performance characteristics recorded as an engine torque curve.

On the left is the characteristic torque curve of a naturally aspirated, air-cooled diesel engine Deutz F8L610. This engine was developed in St-Laurent, QC., Canada for automotive applications in the 80s. It was intended for pickup trucks, SUVs,  and light straight body delivery vehicles. The torque curve depicts the engine power (blue line), engine torque (red line), and brake specific fuel consumption BSFC (orange line) as a function of engine RPM.

2) The chemistry of fuel combustion

The combustion of diesel fuel within the engine combustion chamber is in fact an oxidation reaction where long chains of hydrocarbons (diesel fuel) under high temperatures react with the oxygen present within the compressed air above the piston to form CO2 (carbon dioxide) and H2O (water). For this to happen efficiently, every single fuel molecule would have to be completely accessible to the excess concentration of surrounding oxygen molecules. In this case, a complete combustion would result. Unfortunately, in reality this may not be the case. Often times, poorly atomized fuel or excess of fuel in the combustion chamber does not permit a complete access of oxygen to the reacting fuel. This results in an incomplete combustion. Incomplete combustion is then accompanied with the production of pollutants such as CO (carbon monoxide), hydrocarbons, particulates (soot, or black smoke) and NOx (nitrogen oxides). Poor combustion is also characterized by an incomplete energy conversion resulting in reduced engine performance and an increased fuel consumption.

3) The driver's effect on fuel economy

The vehicle operator can not influence the engine's inherent characteristics built in during the engine development process. What the driver can influence however, is the way in which the engine's performance is utilized. Fuel economy goes hand in hand with the most efficient utilization of the engine power. The timing of shifting gears, and careful fuel management so that the excess fuel addition to the combustion chamber is avoided are the two most important aspects of engine power management.

For example, let us now look at the above torque curve of the Deutz F8L610 engine to suggest some rules for the most economical engine operation. The goal is to obtain the most power at the lowest fuel consumption. A quick glance at the engine torque curve diagram suggests that the most torque can be obtained at around 2200 RPM, while the BSFC curve bottoms out at about 2000 RPM This means that the driver should operate the engine within the range of these two values. The F8L610 engine was quite forgiving for shifting at the wrong RPM. Not all engines are. Notice that the BSFC curve is relatively flat between 1400 and 2200 RPM. This means that the driver shifting anywhere in this range of RPM would do all right. However, at 1400 RPM the engine torque would be substantially reduced. Compensating for the lack of engine torque at 1400 RPM by excess fuel on a loaded truck would be a poor technique. However, if a lightly loaded vehicle was rolling on level road, or slightly downhill so that the engine torque was sufficient for the needed power, shifting at 1400 RPM on the F8L610 engine would have been quite beneficial. Not only we would get by at relatively low BSFC, but also at reduced RPM.

It is important to realize that how the driver steps on the fuel pedal, and how much fuel is delivered to the cylinders at any particular moment is crucial determinant of fuel economy. If we supplied excess fuel to the cylinders which the engine can not use, the extra fuel would be wasted. It would not combust completely and the NOx emissions would skyrocket. Externally, on diesel engines this incomplete combustion would be exhibited by black smoke leaving the exhaust as it is often seen during heavy acceleration. An experienced operator can recognize the symptoms of an excess fuel addition as the engine sound changes without any appreciable change in acceleration. Newer trucks come with fuel flow gauges normally reading either instantaneous fuel consumption in litres per 100 Km or expressed as mileage in MPG. These are excellent reference tools for the driver to use. Other units might have turbo boost pressure gauges which can be used instead, since the instantaneous turbo boost pressure is correlated to the engine fuel flow. The goal is to add as little fuel as possible (the lowest turbo boost pressure possible) in order to obtain the desired engine power.

In relation to fuel economy, the use of cruise control is perhaps the most detrimental. This is because cruise control is designed to maintain vehicles' speed literally "at all costs". Therefore use cruise control sparingly and only on level stretches of  the highway. Sure, there would be someone now standing up and waving a red flag, hello! cruise control saves fuel! Well, it depends. On my girlfriend's car it does. On a loaded tractor/trailer it does not. Just compare the gasoline engine's 200 Hp in relation to the car's gross weight of 1.5 tones with a typical 435 Hp diesel engine pulling 40 tones! If the cruise control on your car added just a little fuel, the engine would easily accelerate with the 1.5-tone car at minimal fuel addition. Now try this on a 40-tone tractor/trailer! When the cruise control wants to maintain the same speed into a slightest hill on a loaded truck, it must add much excess of fuel to accomplish this task.

As implied above, the idea is to apply as little power (fuel) as is necessary to maintain the desired speed or acceleration. Accelerate slowly, and if possible only on top of the hill or downhill so that the vehicles' mass would help to attain the desired speed at reduced engine output. This inevitably brings me to the conclusion that the effect of reduced speeds on improved fuel economy might be debatable. This is particularly true in the hills. The vehicles' mass and their inertia are quite important determining factors in fuel economy (see Cummins ISX435ST fuel consumption profile). If we slowed the truck coming downhill to say 80 Km/h behind a slow car which could not have been passed earlier because of installed speed limiter, the extra inertia which could be used on the next hill would have been lost and it would have to be compensated for by supplying an extra engine power uphill. So for example instead of making the hill at a realized fuel consumption of 75 litres per 100 Km, we would end up burning perhaps 120 or 130 litres per 100 Km! Secondly, the effect of reduced speeds on fuel economy is engine torque curve specific. Slowing one truck with a particular engine and transaxle ratios might be beneficial, whereas slowing another truck with a different engine and/or different transaxle ratios may not be.

4) The effect of automatic transmissions on fuel economy

The fact that vehicles with automatic transmissions are generally less fuel efficient than vehicles with the standard transmissions has been well established. This happens because of high energy losses in hydraulic torque converters of automatic transmissions. From the data collected for 1993 models and published in the 1993 Canadian fuel consumption guide, out of the 125 cars marketed with both, standard and automatic transmissions, 96 models with manual transmission had lower fuel consumption (by an average of 8.2%) over their automatic counterparts, in three models there was no difference, and 26 models with automatic transmission showed some increase of fuel economy (by an average of 3.6%) as opposed to manual transmission equipped models. It must be born in mind however, that even though these fuel consumption tests were run in vehicle manufacturer's facilities using standard protocols, the test driver's skills and driving habits may not reflect an average driver and thus may not be representative of the reality. It is also important to note that all these tests are conducted on brand new vehicles, and therefore do not take into consideration the increasing losses in automatic torque converters due to ageing automatic transmission fluid (ATF). Interestingly, when fuel consumption for vans and SUVs was analysed, there was very little if any advantage to automatic transmission. In case of pick up trucks there was a similar, but less clearly defined relationship as observed in cars. Trucks with smaller engines strongly favoured standard transmissions. Trucks with larger engines (larger torque converters) favoured automatics by a slight margin. This was not the case for the only diesel engine (GM 6.2 litre) listed in the guide whose fuel consumption always fared better with a manual transmission.
The following graph shows the relationship of the fuel consumption for cars with both standard and automatic transmissions tested on a laboratory chassis dynamometer and expressed as a function of engine size. It is interesting to note the existing relationship between the engine displacement and automatic transmission fuel efficiency. As the engine (torque converter) size and therefore the engine to vehicle mass ratio increases so does the torque converter efficiency and the energy losses are minimized. This in turn results in somewhat improved fuel economy of automatic transmissions in vehicles with high engine/mass ratio. This is why we observe somewhat more favourable fuel economy with automatic cars with large engines, and to smaller extent in some pick up trucks, but not in vans and SUVs as they have not reached the optimal displacement/G.V.W. ratio.

For the above mentioned reasons the construction of highway tractor automated transmissions is different. Automatic torque converters can not be used because energy losses would be insupportably high. Truck automated transmissions such as the Eaton UltraShift automated transmission are de facto automatic shift manual transmissions. Technically speaking, they possess all serviceable components like, gears, and clutch as manual transmissions do except that a simple shift stick is replaced with a rather complex electronically controlled clutch and shifting mechanism.

Lately, automated transmissions became a popular choice for some transportation carriers replacing their fleet. Frequently heard argument for the use of the automated trannies is that they save fuel through consistent shifting, and reduce maintenance costs. So far, none of these arguments have been validated by actual fuel consumption measurements. As it was pointed out during our line haul fuel consumption measurements, the assumption that automatics would shift more consistently than drivers on standard transmissions would, is unfounded. Most drivers pick up the right shifting technique within a couple of weeks through their practice. As far as maintenance costs are concerned, from the previous discussion it follows that having a simple shift stick replaced by a complex electronically controlled clutch and shifting mechanism would hardly result in appreciable reduction of maintenance costs. So it follows, that carriers opt for automated transmission equipped trucks only in order to minimize the expenditure of their resources required for training of new drivers. Such practice, though very popular at the moment, might prove counter productive in the long run. Automatic transmissions do enable drivers with minimal skills and minimal training to drive trucks. However, if we disregard the issue of safety for the moment, this will affect the carriers overall fuel consumption since driver's fuel economy is inevitably tied to his/her skills and experience.