|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.
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.
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.
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.