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An engine burning conventional diesel fuel will produce approximately 2.73 kg of CO2 for every litre of fuel burned. There is no amount of catalysing or engine management that can reduce CO2 production.
4.1.8 Drag and Coefficient of Drag All vehicles have an inherent drag coefficient (CD). This is a unitless number that describes the amount of aerodynamic drag caused by fluid flow over any body. More streamlined bodies have lower CD, whereas more blunt bodies have higher CD. Figure 21, taken from Scania trucks, illustrates some examples of CD. It is estimated that every reduction of 0.02 in CD provides a 1% fuel consumption savings for highway transport vehicles .
4.2 Types of Add on Devices There are many devices that can be affixed to a straight truck, power unit (tractor) or a trailer (illustrated in Figure 22). Some of these are intended to reduce aerodynamic drag, some are intended to increase safety and reduce the severity of incidents and some can accomplish both tasks simultaneously. All such devices have been defined in this section to help the reader distinguish between the many types of safety and aerodynamic components that are currently available for attachment to a truck or a trailer. However, only devices attached to the sides of trucks and trailers are being considered in this study.
4.2.1 Devices Designed Primarily to Reduce Aerodynamic Drag 188.8.131.52 Aerodynamic Belly/Side Fairings Aerodynamic belly fairings are devices fitted to the longitudinal edges of a trailer and are intended to allow the air flow to pass alongside the trailer rather than underneath it. The fairings reduce vortices and prevent the air from contacting the underbelly, the spare tire, the rotating wheels and other running gear that are all relatively blunt and non-aerodynamic. The fairings typically clamp to the I-beam frame rails of the trailer and are relatively easy to install. They typically provide a clearance between 8 and 16 inches from the ground and may employ some form of angled lower edge to reduce the risk of damage to the fairing from impacting the ground.
The belly fairings (Figure 23) are often paired with gap fairings (4.3.2) as part of a complete trailer aerodynamic package. Results from field testing have shown that belly fairings can provide fuel savings of approximately 4% to 6.4% [30, 31]. Properly installed belly fairings do not alter the height, width or length of the trailer but do add approximately 114 kg (250 lbs) to the tare weight of the trailer. As shown in Figure 23, it is customary to integrate lights and reflectors directly into the fairing.
Until recently, belly fairings were not commonly found on Canadian trailers; however, various pilot projects have been introduced to determine the effectiveness of these devices on highway vehicles.
184.108.40.206 Gap Fairing Gap fairings are devices that are fitted to the front of van semi-trailers. These devices prevent air vortices from developing as air enters between the tractor and the trailer hence reducing aerodynamic drag. These devices have no impact on safety and are designed to minimize any interference with operations. Tests have shown that gap fairings can reduce fuel consumption by as much as 2% . When combined, gap fairing and belly fairings have been shown to reduce fuel consumption by as much as 9% . Gap fairings do not increase the height, length or width of the vehicle to which they are attached. An example is shown in Figure 24.
220.127.116.11 Boat Tails Boat tails are devices that are fitted to the rear of van semi-trailers (Figure 25). These devices shed vortices as they leave the trailing edge of the trailer hence reducing aerodynamic drag and are designed to minimize any interference with operations. Tests performed at Clarkson University have shown that boat tails can reduce trailer drag by as much as 9% and fuel consumption between 4% and 8% . Boat tails increase the effective length of a trailer therefore they are typically only mounted to 48 foot trailers. However, some jurisdictions do allow two foot boat tails on the more common 53 foot semi trailers.
4.2.2 Devices Designed Primarily to Improve Safety 18.104.22.168 Side Underride Guards Side underside guards are similar in appearance to belly fairings; however, their purpose is primarily to prevent other road users from slipping under the sides of the vehicles. Side under ride guards as used in the EU must comply with strength requirements in order to qualify as a side guard whereas belly fairings currently used in Canada do not. Side guards are generally classified as being either flush mounted (similar to belly fairings) or rail type which have no aerodynamic benefits. A rail style guard is shown on the Scania truck in Figure 4. Side underride guards are intended to protect vulnerable road users such as pedestrians and bicyclists and in some instances motorcycles but not passenger vehicles. It would not be practical to mandate that side guards be strong enough to prevent the ingress of passenger vehicles or trucks. There is simply not enough material or structure on the sides of trailers to attach a guard that could be made strong enough, or absorb enough energy, to prevent a passenger car from riding under the side of the trailer. Additionally, the amount of material required to fabricate such a guard would add so much weight to the tare weight of the trailer it would drastically reduce the amount of payload that could be carried on such a trailer. Front and rear impact guards have been purposely built to prevent a passenger vehicle from riding
under the truck or trailer because the width is so much less than the length of the trailer and because there is so much more structure to which the guards may be attached.
22.214.171.124 Rear Underride Guards The purpose of rear underride guards is to prevent other vehicles from driving underneath the rear of trailers. Although they can provide protection from bicycles and other road users, they are designed principally to prevent passenger vehicles from becoming pinned underneath the rear end of the trailer as a result of a rear end collision. The design, construction, testing and use of the rear underride guards in Canada are defined by regulations CMVSS 223. Rear underride guards (shown in Figure 26) do not provide any aerodynamic benefits and usually add approximately 44 kg (100 lbs) to the tare weight of a trailer, depending on the choice of material.
The guards are usually attached to the strongest structural sections of the trailer in order to provide maximum strength and energy absorption.
126.96.36.199 Front Under-ride Guards Front under-ride guards are similar to rear under ride guards except they protect other vehicles in the event of a head on collision. Front under-ride guards are not mandated in Canada.
4.3 Variations in Fairing/Side Guard Design and Construction There are a variety of side mounted options for heavy vehicles that may be mounted alone or in combination with other components as part of an aerodynamic package. The distinguishing
features for side guards are:
Inherent to vehicle design (Figure 8);
Flush/smooth type (Figures 27 and 28);
Rail type (Trailer in Figure 2);
Continuous (Figure 28); and
Additionally, the purpose and installation of side guards may vary between highway transport and inner city delivery trucks and indeed the method of installation will vary between different styles of trailers (e.g. tankers versus vans).
4.4 Implementation cost According to the study conducted by Freightwing and Transport Canada , the costs for conventional belly fairings was $1,825 and the cost for low rider belly fairings was $2,450.
These are costs that must be added to the price of a new trailer and borne by the consumer. It is estimated that the fairings will add between 0.2% and 5.0 % to the capital costs of a new trailer.
This results in a pay back period of 1.2 to 2.2 years under normal operating conditions and assuming a fuel price of approximately $1.00 per litre. Various calculation tools have already been established to determine the payback period for aerodynamic devices. The inputs to the
The cost to install side guards intended to provide under run protection on trucks and trailers varies from installation to installation. Table 18, taken from an Australia study , illustrates the range of costs from approximately $574 to nearly $2,500 AD. Guards intended to provide side under run protection are sufficiently rare in Canada that installation costs in Canadian dollars could not be found.
Table 18: Cost of rigid side guards for heavy commercial vehicles and articulated heavy commercial vehicles
4.4.1 Flush mount versus rail style The safety guard literature review was intended to update a previous review carried out by TRL , on behalf of the DfT, in 1995, and to attempt to identify any research to demonstrate any benefits of, or difficulties with, an integrated approach to underrun protection.
The review revealed that there had been little new research into the safety aspects of sideguards since the previous review in 1995 and the principles of good design remained the same, that is, low ground clearance and minimizing gaps.
More recent research showed that substantial additional benefits can be gained by replacing the traditional rail type sideguards with solid flat panels enclosing as much of the space between the wheels as possible. Research by De Coo et al (1994)  suggested that with the appropriate ground clearance, run over by the rear wheels could be eliminated in overtaking manoeuvres.
Research by Stöcker (1990)  suggested that other injury criteria can also be reduced by removing the possibility of entanglement with the guard, collision between the victim and any protruding structure and a more gentle collision with the ground.
4.5 Environmental benefits 4.5.1 How fuel is consumed in a heavy truck Fuel is consumed by the engine as it propels the vehicle down the road. There are five major factors that the engine must overcome that contribute to this fuel consumption. In general, these
can be categorized as follows:
Changes in grade or elevation;
Internal power train losses; and Accessory losses (e.g. air conditioning, alternator loads and air compressors etc) The percentage contribution to fuel burn for each of the five categories varies from vehicle to vehicle, and certainly the contribution from aerodynamics rises steeply with speed. The contribution to fuel burn from internal losses is generally modeled as a constant and the grade portion is obviously only present while the truck is ascending or descending a grade.
At 40 km/h, the power needed to overcome rolling resistance and accessory losses is nearly twice as great as the power needed to overcome aerodynamic drag. At 80 km/h, the power necessary to overcome aerodynamic drag is roughly equal to that of rolling resistance and accessories. At 121 km/h, the power necessary to overcome aerodynamic drag is approximately 2.5 times greater than rolling resistance and accessory losses. Table 19 illustrates the contributions to fuel burn at various speeds, assuming a zero grade and properly inflated tires etc and assuming that the internal power train losses can be modeled as a constant and independent of vehicle speed.
Since there is more than one form of energy drain, it stands to reason that reducing aerodynamic drag by, say, 10% will not result in a 10% reduction in overall fuel consumption.
Rather, it will be 10% multiplied by the percentage contribution of aero effects at that particular speed. For example, a 10% reduction of aerodynamic drag via the use of an aerodynamic package would have an overall effect of reducing fuel consumption by 4.7% at 80 km/h. These fuel savings would rise as speed increased to a maximum value of approximately 7.2% at 120 km/h.
4.6 Aerodynamics of Side Mounted Devices The addition of flush mount side fairings to highway trailers tends to smooth airflow and reduce cross-flow along and below the bottom edges of the trailer. Fairings entrain the air more efficiently under the trailer and keep crosswinds from causing turbulence under it.
Many tests have been conducted with the aim of quantifying the potential fuel savings from the addition of side guards or fairings.
A study  jointly performed by Technical University Delft in the Netherlands and TNT
transport concluded the following:
Initial driving tests with a trailer equipped with the aerodynamic side skirts over a straight stretch of public road revealed a cut in fuel consumption of between 5% and 15%. Subsequent research comprising long-term operational tests by TNT displayed a fuel reduction of 10%.
These results confirm the calculations and findings from the wind tunnel tests: these had already established that the observed 14 - 18% reduction in air resistance led to 7 - 9% less fuel consumption. In practice, the figures are in fact even better. Other tests have resulted in fuel savings in the 4% to 6% range based on the improved aerodynamic shape of the vehicles.
A similar study  was conducted jointly between Freightwing Inc, Transport Canada, the National Research Council and three major Canadian carriers. The aim of this project was to quantify any potential fuel savings as a result of installing belly fairings and low rider fairings (Figure 29) mounted on 53 foot van semi trailers. Although all three carriers used their vehicles differently, the overall average fuel savings was 6.4% using both types of fairings.