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ENERGY CONTENT OF FUEL UNITS Fuel economy is usually expressed in one of two ways:
Converting from mpg or km/L to L/100 km (or vice versa) involves the use of the Reciprocal function, which is not Distributive . Therefore, the average of two fuel economy numbers gives different values if those units are used. If two people calculate the fuel economy average of two groups of cars with different units, the group with better fuel economy may be one or the other. Consider the following example: a Frenchman and an Englishman argue about whether English cars are more fuel efficient than French cars. To resolve the issue they obtain 2 French cars, (F1 and F2) and two English cars (E1 and E2). Each man tests the fuel economy of all four cars. The Englishman works in miles per Imperial gallon, while the Frenchman works in litres per 100 km. Note that 1 mile per Imperial gallon = 282.48 litres per 100 km. The Englishman obtains the following results. E1 has a fuel consumption of 94.2 mpg and E2 has a fuel consumption of 23.5 mpg. The average fuel consumption of the English cars is therefore (94.2 + 23.5)/2 = 58.9 mpg. For the French cars he obtains a fuel consumption of 47.1 mpg for each car. Thus he concludes that the British cars have better fuel consumption on average. The Frenchman obtains the same results, but expresses them in L/100 km. He measures the English cars E1 and E2 to consume 3 L/100 km and 12 L/100 km respectively. The average is therefore 7.5 L/(100 km). For the French cars he obtains 6 L/100 km for both. Thus he concludes that the French cars have lower fuel consumption. MEASUREMENT CYCLES Government-mandated fuel efficiency measurements generally have two regimens or driving cycle patterns: a city or urban cycle, and an highway or extra-urban cycle. In Europe, the two standard measuring cycles for "L/100 km" value are Motorway travel at 90 km/h and rush hour city traffic. A reasonably modern European Supermini may manage Motorway travel at 5 L/100 km (47 mpg US) or 6.5 L/100 km in city traffic (36 mpg US), with Carbon Dioxide emissions of around 140 g/km. An average North America n Mid-size Car produces circa 27 mpg (US) (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city; a Full-size SUV usually gets 13 mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway. Pickup Truck s vary considerably; whereas a 4 cylinder-engined light pickup with a produces circa 28 mpg (8 L/100 km), a V8 full-size pickup with extended cabin produces circa 13 mpg (US) (18 L/100 km) city and 15 mpg (US) (15 L/100 km) highway. An interesting example of fuel economy is the popular Microcar '' Smart ForTwo '', which can achieve up to 4.0 L/100 km (70.6 mpg) using a Turbocharged three-cylinder engine. The Smart is produced by DaimlerChrysler and is currently only sold by one company in the United States (see external link ZAP ). Diesel engines often produce greater fuel efficiency than petrol (gasoline) engines: above 50% of all cars sold in the European Union are now diesel vehicles. All these previously-cited fuel economy values are for operation on petrol, gasoline. New US light vehicles designated as Flexible Fuel Vehicle s (FFVs) running on E85 (85% Ethanol , 15% gasoline) will typically achieve from 5% to 15% less fuel economy in mpg on pure E85 than when operated on pure gasoline. Older non-turbo-charged fuel-injected FFVs running on E85 will typically achieve about 25% to 30% less fuel economy on E85. Over 4 million FFVs are currently operated on US roadways as of 2005; most tend to be light trucks or van vehicles, although newer "car-shaped" high performance cars are also being introduced in the 2006 model year (e.g., 2006 GM Chevrolet Impala). The driving interval tests described here test laboratory derived emissions and calculated fuel economy, but certainly not on-the-road fuel efficiency. In the United States, the Environmental Protection Agency (EPA) is the government body that makes the calculations that auto manufacturers use when advertising their vehicles. Separate numbers are given for City and Highway driving. The EPA tests do not directly measure fuel consumption, but rather calculate the amount of fuel used by measuring emissions from the Tailpipe based on a formula created in 1972. The cars are not actually driven around a course, but are cycled through specific profiles of starts, stops, and runs on a chassis Dynamometer in a Laboratory environment. As Emissions Standard s have become more strict due to Smog , most of the resulting numbers do not directly correspond to what people actually experience when driving. Most often, the EPA estimate of mileage is several percent higher than what the average driver manages to achieve in practice, although there are some cases where the difference is nearly 200% higher than what the average driver achieves. Correcting this discrepancy by means of an updated, more conservative, testing procedure which would understate rather than overstate MPGs, would help force automakers to improve fuel economy without changing the Corporate Average Fuel Economy (CAFE) standard. This tends to be fought vehemently by automakers and is politically unattractive. This is because the vehicles they produce would need to achieve better fuel economy just to meet the current standard. In the United Kingdom, the Vehicle Certification Agency [http://www.vcacarfueldata.org.uk/index.asp] has initiated a similar fuel economy rating system in accordance with European Community Directive [http://europa.eu.int/eur-lex/lex/LexUriServ/LexUriServ.do?uri=CELEX:31993L0116:EN:HTML 93/116/EC]. The ratings are based on an urban and extra-urban driving cycle. The urban cycle is a cold start followed by "a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31 mph (50 km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km)." The extra-urban cycle is conducted immediately following the urban cycle and consists of roughly half steady-speed driving and the remainder accelerations, decelerations, and some idling. Maximum speed is 75 mph (120 km/h), average speed is 39 mph (63 km/h) and the distance covered is 4.3 miles (7 km). The raw averages for all 2005 vehicles rated in the United Kingdom are: Urban cycle, 11.3, extra-urban 6.4 (L/100 km). This converts to 20.9 and 36.5 mpg, respectively, in United States measurements. CONSUMER ABILITY TO INCREASE FUEL EFFICIENCY ''See also:'' Fuel Efficient Driving Consumers are also able to adopt certain methods to increase fuel efficiency for their benefit, and for that of humanity. Reducing the vehicle's weight, especially in city driving, and frontal area, especially in highway driving are the two largest contributing factors. This is because in city driving, the vehicle stops more often, and Inertia plays a larger role. Weight increases the initial energy needed to get the vehicle moving. In contrast, in highway driving, the vehicle is generally constantly moving so Air Resistance plays a role. The type of engine a consumer chooses is a major contributing factor in fuel efficiency. The most efficient mass-production engines are petrol engines under a litre displacement, turbo-diesels under 1.4 litres, standard diesels and electric hybrids. Out of these, turbo-diesels tend to be the most efficient in terms of power per unit displacement. However, some consumers may not be interested in acceleration rates, so a standard diesel engine is the most efficient per litre displacement. Fuel efficiency can be hampered if the Air Pressure in the Tire s of Vehicle s is too low. Abrupt Acceleration and Deceleration can also decrease fuel efficiency. In order to avoid this, drivers generally accelerate as gradually as possible, especially uphill, and try to keep a stable Speed . They also time their movement to minimize slowing or complete stops, and coast whenever possible. Fuel efficiency can also be hampered by unnecessary Friction in the Engine due to drivers not taking advantages of situations where engine speed can be reduced. This tends to be especially pertinent to those driving with a Manual Transmission . Generally, conscious drivers use the highest reasonable gear, and shift up early and shift down late. Temporarily shifting to neutral on a sufficiently lengthy downhill grade will increase mileage. This is especially true for cars with few speeds or "close ratio" gear boxes, where the highest gear is lower. It makes slightly more difference for Carburetor cars, while cars with Fuel Injection - or carburetor cars with a fuel cut-off solenoid - benefit from the fuel cutoff when the car is left in gear. Fuel efficiency can also be increased by reducing the time when the engine operates but does not need to. For example, drivers may shut it down whenever it is unused for more than 10 seconds when not in traffic. (Turning off the key while the car is moving disables power steering and brakes, if fitted, and the steering wheel will lock if the driver follows habit by taking the key out.) Using air conditioning sparingly and relying on neutral settings where the outflow is neither hot or cold increases fuel efficiency . It is estimated that the air conditioning running at maximum setting can lower fuel efficiency by 5-25%. However, during traveling at high speeds, if the the windows are open, this creates a lot of Aerodynamic Drag . Thus this may consume more fuel than operating the air conditioner sensibly. High speed driving can drastically reduce fuel efficiency. Gasoline powered cars operate with maximum efficiency in the highest gear at the lowest speed in that highest gear without engine lugging. This effect is largely due to aerodynamic drag. In highway driving over 80-90 km/h (50-55 mph) the aerodynamic drag will rise sharply, thus increasing fuel consumption. Medium-sized motorcycles can get from 37-60 miles per US gallon (6.4–3.9 L/{100 km}) depending on how they are ridden. For example, the same 600 cm³ race bike which is capable of getting 50 mpg (4.7 L/{100 km}) will only get 30 mpg (7.8 L/{100 km}) if raced. 50 cm³ scooters, which do not require a motorcycle license to operate, can get upwards of 100 mpg (< 2.4 L/{100 km}). As fuel efficiency is related to the complete combustion per fuel molecule in order to get the most energy out of each unit of fuel, the air intake plays a significant role. Installing an intake that allows a greater and freer airflow will increase fuel efficiency. In addition, buying a cone-shaped air filter, which is generally affordable and relatively simple to install, eliminates the air-box and plate-shaped filter and allows a much larger amount of air into the engine, increasing efficiency as well as a boost of up to 5 horsepower. Installing a less restrictive exhaust in conjunction with increasing the intake increases the net rate of Diffusion , as it opens up bottlenecks in the air-flow. Dual- Spark Plug s also have some advantages in increasing fuel efficiency. Quad-spark plugs exist, but the benefit over dual-spark is negligible if it exists at all. The dual-spark design allows for a longer spark, thus more of the fuel is directly combusted. FUEL ECONOMY-BOOSTING TECHNOLOGIES
SEE ALSO
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