Fuel Consumption/Economy Trends in las countries: The Tunisian Case Study Author



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Limitations


Tunisia has no indigenous driving cycle, an issue which is thoroughly discussed in the way forward section. Since the Tunisia market is in broad terms more streamlined with the European one, the study team obtained data for fuel economy/consumption based on the New European Driving Cycle (NEDC).

Because for some models the emissions figures were not available, the report eliminated those models from its analysis. Those models have made up a maximum of 1 % of all models all over the study years. Another limitation is the new LDVs sold through unauthorized dealers and parallel markets which are not to exceed 10% of total new LDVs sales. Therefore the studied new LDVS in the report comprise 89% of total new LDVS in Tunisia for the study years, at worst.


  1. Background information

    1. Fuel Economy


Fuel economy is a measure of the maximum distance that can be covered by a vehicle per unit of fuel. The most common metric of fuel economy is miles per gallon (mpg), which is especially, used in the United States. Kilometers per liter can also be used.

Fuel Consumption

Fuel consumption is the mathematical reciprocal of fuel economy. It is a measure of the amount of fuel consumed covering a given distance. It is measured in liters per 100 km in Europe and most of the world. In the United States it is measured in gallons per 100 miles. Being the reciprocal of fuel economy necessarily entails that for fuel consumption the relation. This in turn renders more instrumental in communicating the fuel savings, from improving fuel economy, in absolute terms to lay consumers. This is because the amount of fuel saved in improving fuel economy in the lower ranges of mpg is significantly higher than those at the higher ranges. Hence the benefits accrued from improving the fuel consumption of vehicles become more comprehensible to the average consumer.


    1. Factors Affecting Fuel/Consumption Economy


The report tackles two broad types of vehicles classified according to the fuel they utilize. Petrol powered engines (petrol fuelled vehicles), referred to as spark ignition engines, rely for the most part on a thermodynamic cycle, called Otto cycle. For petrol engines, a spark plug is used to ignite an air/fuel mixture exerting work on piston, which moves vertically inside a hollow cylinder, then mechanically transmitted to a crankshaft and through a clockwork of gears to the wheels. Diesel powered engines (Diesel fuelled vehicles) rely on heat generated from the compression of diesel/air mix for ignition and operating the pistons. For both types of internal combustion engines, 75% of the energy is wasted to coolants and exhaust with the rest doing the propelling work.

Vehicle Energy efficiency

  • Engine: The engine output power varies with its torque and speed. For each engine there is three-dimensional curves plotting the output power against both Torque and speed. From this curve an optimum zone is located where the engine’s energy efficiency is maximized. In reality the vehicles runs through various driving ranges and modes at points outside the energy efficient zone. Using turbo charging, smaller engines all drive engine towards operation at the maximum efficiency zone (Institute of Mechanical engineers, 2011).

    • Combustion interval: short combustion interval allows for more of the generated heat to be used in driving the pistons

    • Higher compression ratio and optimized exhaust valve opening: Compression ratio is the volume between the volumes of the combustion chamber when the cylinder is at the bottom stroke to that when the cylinder is at top stroke. Better control of exhaust valve opening improves the energy efficiency of engine (Institute of Mechanical Engineers, 2011).

  • Pump losses: The pump losses result from pressure gradients along the piston, so it is the extra work required to suck air in and out of inlets (Chiaberge, 2011).

  • Friction losses: Friction losses result from piston and crank shaft mechanical connections. Improving precision of cylinder dimensions minimizes piston friction losses. Crank shaft bearing design and features have a straightforward impact on the associated friction losses (Institute of Mechanical engineers, 2011).

  • Oil and coolant pumps: following the wide-spread recommendations for reducing energy consumption of pumps are applicable for automotive engines.

  • Power steering: using electric drives for power steering reduces fuel consumption

  • Aerodynamics: air resistance to a vehicle’s traction, termed drag force is dependent on a factor called the drag coefficient. Reducing drag coefficient reduces fuel consumption

  • Tire resistance: the mass of the car putting pressure on tires leads to energy losses. This resistance is a function of tire design and air pressure. Design options that reduce tire resistance may weigh on safety and levels of wear and tear. Optimum trade-offs must be reached.

  • Transmission terrain: increasing the number of gear ratios reduces the losses in the transmission terrain. Several transmission technologies, such as planetary (differential gearboxes) and dual-clutch transmission, are commercially available to date

  • Stroke-To-Bore Ratio: This is the ratio between the length of the stroke and the diameter of the cylinder. As the stroke to bore ratio increases, air into the cylinder travels a longer distance reducing losses. As stroke to bore ratio decreases the surface area of piston decreases which leads to lesser friction losses for the crankshaft bearings (Institute of mechanical Engineers, 2011)

  • Number of balancing shafts: Those are shafts used for countering the vibration effects of cylinders. They have weight and inertia which consume energy thus reducing efficiency. Different engine classes use different number of balancing shafts (Stone, 1999).

  • Vehicle Weight: Vehicle mass has a profound impact on vehicle’s fuel consumption. Replacing steel with the lighter aluminum in alternative body structures, such as space frame is an approach. Another is the use of composite and carbon fiber materials which can be introduced into the mainstream body design. A combination of material availability, cost consideration and a downgrade of structural performance in aluminum based structures limit these approaches. Another less radical approach involves using thinner steel, sandwiched steel (layers of aluminum and steel), or new steel designs. The downside of the said conventional approaches is jeopardizing stiffness, or increased costs (Institute of Mechanical Engineers, 2011).

  • Fuel: The energy content per liter of diesel is higher than petrol and accordingly has lower fuel consumption. Diesel’s carbon content is higher and so it emits more greenhouse gases on per liter basis. However, the lower fuel consumption leads to diesel-fuelled vehicles, generally, emitting less greenhouse gases than petrol fuelled ones on kilometer basis.

It remains to be said that different commercially available technologies, used by different automotive manufacturer, address the abovementioned points. From the fuel consumption perspective, those technologies synergize, influence or constrain each other. Accordingly, arriving at the right combination of technologies that have an impact on reducing fuel consumption requires trade-offs between fuel consumption and other performance parameters.

    1. Fuel Economy Standards


Climate change, and the associated urge to curtail the growth of greenhouse gas emissions by cutting down the consumption of fossil fuels, have combined with the uncertainties associated with volatile oil prices and the energy security challenges to bring the topic of reducing fuel consumption by vehicles to the fore of global environmental and energy agendas. Light duty vehicles have the most significant weightage of all vehicles’ total fuel consumption. In response, fuel economy standards have been on debate, being variably adopted by different nations and transnational bodies, since the oil crisis of the seventies.

The European Union has set its fuel consumption/economy standards where manufacturers have to meet average fuel economy levels for their entire fleets (GFEI, 2014). The assigned value to each manufacturer is calculated on the basis of the mass of a vehicle giving manufacturers a level of flexibility to increase and decrease the fuel economy of their different models. It also allows higher values for heavier vehicles through what is termed a limit curve (Automobile Fuel Economy standards, 2010). Penalties are applied using a sliding scale. The fuel economy limits continue to increase in response to regulation (Automobile Fuel Economy standards, 2010).



In a European context, the standards are realistic meeting lesser resistance from concerned civil society portions due to the predominance of small cars, efficient and widely spread public transportation and the proliferation of the more efficient diesel vehicles. Japan followed in the footsteps of the EU with its own stringent weight-based standards (IPCC, 2007). The USA has been adopting fuel economy standards since the seventies which have been slightly waxing and waning over time for light trucks, and constant for passenger cars since 1990 (GFEI, 2014). Light Duty Vehicles were regulated using different standards for passenger cars and light trucks. The US standards count on fuel economy, unlike which target fuel consumption. The same average fuel efficiency was required from each manufacturer regardless of vehicle attributes. It was calculated by the following formula

(Source: Centre for Climate and Energy solutions, 2014)

The downside of this approach is that the playfield is not level for large vehicle segments since compliance is easier for smaller ones. The standards were assessed by experts to have led to fuel savings of billions of barrels of oil over the years (Government Accountability Office, 2008).

With the support of the Obama administration, the US Environmental Protection Agency jointly with the National highway Traffic Safety administration has set fuel economy standards for 2017-2025 vehicles. Vehicles are classified on size basis for two broad categories: passenger cars and light trucks. Vehicle size (footprint) which is determined in a standardized way enters a formula that accounts as well for a manufacturer’s production or sales level. The standards are designed to accomplish a US fleet average fuel economy, by 2016, of at least 35.5 (GFEI, 2014). The target for 2025 is 54.5 mpg (New York Times, 2012). A shortcoming of those standards is restricting classifications of vehicles to size, which in light of the earlier discussion on the factors affecting energy efficiency of vehicles, is a factor among many.




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