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Energy-efficient driving is a driving practice intended to improve fuel economy in automobiles. Fuel economy can be improved in many ways, including: increasing engine efficiency, reducing aerodynamic drag, rolling friction, and energy lost to braking (and to a lesser extent by regenerative braking). Techniques used to improve fuel economy range from simple coasting to complex pulse and glide with the extreme case being drafting.
Terms for driving techniques to maximize fuel efficiency include hypermiling.
Key parameters to maintain are proper tire pressure, wheel alignment, and engine oil with low-kinematic viscosity, referred to as low "weight" motor oil. Inflating tires to the maximum recommended air pressure means that less energy is lost to rolling resistance due to tire deformation, leaving more energy to available to move the vehicle. Under-inflated tires can increase rolling resistance by approximately 1.4% for every 1 psi (0.1 bar) drop in pressure of all four tires. Equally important is the scheduled maintenance of the engine (e.g. air filter, spark plug), and addressing any on-board diagnostics codes/malfunctions in the Engine Control Module and related sensors, especially the oxygen sensor. Another factor related to maintenance is fuel evaporation. This can be minimized by always closing the fuel-tank lid tightly and by parking in the shade.
Drivers can also increase fuel economy by driving lighter and/or lower-drag vehicles and minimizing the amount of people, cargo, tools, and equipment carried in the vehicle. Removing common unnecessary accessories such as roof racks, brush guards, wind deflectors (or "spoilers", when designed for downforce and not enhanced flow separation), running boards, push bars, and narrow and lower profile tires will improve fuel economy by reducing both weight and aerodynamic drag. Some cars also use a half size spare tire, for weight/cost/space saving purposes. Another simple way to decrease the vehicle's mass is to drive with the fuel tank mostly empty and to tank more frequently.
Maintaining an efficient speed is an important factor in fuel efficiency. Optimal efficiency can be expected while cruising with no stops, at minimal throttle and with the transmission in the highest gear (see Choice of gear, below). The optimum speed varies with the type of vehicle, although it is usually reported to be 35 mph (56 km/h) or higher. For instance a 2004 Chevrolet Impala had an optimum at 42 mph (70 km/h), and was within 15% of that from 29 to 57 mph (45 to 95 km/h). The US government 2005 Fuel Economy Guide includes a plot showing the optimum between 50 and 55 mph (80 and 89 km/h) for an unspecified vehicle.
Hybrids typically get their best fuel efficiency below this model-dependent threshold speed. The car will automatically switch between either battery-powered mode or engine power with battery recharge.
Road capacity affects speed and therefore fuel efficiency as well. Studies have shown speeds just above 45 mph (72 km/h) allow greatest throughput when roads are congested. Individual drivers can improve their fuel efficiency and that of others by avoiding roads and times where traffic slows to below 45 mph (72 km/h). Communities can improve fuel efficiency by adopting speed limits  or policies to prevent or discourage drivers from entering traffic that is approaching the point where speeds are slowed below 45 mph (72 km/h). Congestion pricing is based on this principle; it raises the price of road access at times of higher usage, to prevent cars from entering traffic and lowering speeds below efficient levels.
It has been researched that driving practices and vehicles can be modified to improve their energy efficiency by about 5%, or so.
Engine efficiency varies with speed and torque, as can be seen in a plot of brake specific fuel consumption. The optimum efficiency point is around 1750 rpm, and 90% of maximum torque at that speed, for this turbo-diesel engine. For driving at a steady speed one cannot choose any operating point for the engine—rather there is a specific amount of power needed to maintain the chosen speed. A manual transmission lets the driver choose between several points along the curve. In the turbo diesel example, one can see that too low a gear will move the engine into a high-rpm, low-torque region in which the efficiency drops off rapidly, and thus best efficiency is achieved near the higher gear. In a gasoline engine, efficiency typically drops off more rapidly than in a diesel because of throttling losses, and the trend discussed here is even more dramatic. Because cruising at an efficient speed uses much less than the maximum power of the engine, the optimum operating point for cruising at low power is typically at very low engine speed, around or below 1000 rpm. This is far lower than the above mentioned 1750 rpm. This explains the usefulness of very high "overdrive" gears for highway cruising. For instance, a small car might need only 10–15 horsepower (7.5–11 kW) to cruise at 60 mph (97 km/h). It is likely to be geared for 2500 rpm or so at that speed, yet for maximum economy the engine should be running at about 1000 rpm to generate that power as efficiently as possible for that engine (although the actual figures will vary by engine and vehicle).
Fuel efficiency varies with the vehicle. Fuel efficiency during acceleration generally improves as RPM increases until a point somewhere near peak torque (brake specific fuel consumption.) However, accelerating too quickly without paying attention to what is ahead may require braking and then after that, additional acceleration. Experts recommend accelerating quickly, but smoothly.
Generally, fuel economy is maximized when acceleration and braking are minimized. So a fuel-efficient strategy is to anticipate what is happening ahead, and drive in such a way so as to minimize acceleration and braking, and maximize coasting time.
The need to brake in a given situation is in some cases based on unpredictable events which require the driver to slow or stop the vehicle at a fixed distance ahead. Traveling at higher speeds results in less time available to let up on the accelerator and coast. Also the kinetic energy is higher, so more energy is lost in braking. At medium speeds, the driver has more freedom and can elect to accelerate, coast or decelerate depending on whichever is expected to maximize overall fuel economy. Traveling at posted speeds allows for best civil planning and should allow drivers to best take advantage of traffic signal timing.
While approaching a red signal, drivers may choose to "time a traffic light" by easing off the throttle, or braking early if necessary, far before the signal. For example, a driver who is approaching a red light should adjust vehicle speed in advance, such that the vehicle arrives at the intersection when the light is green. It is also important to account for the time it takes for the stopped traffic at the light to start moving again. In theory, the ideal situation is the driver slowing immediately to the calculated speed that allows the car to be barely behind the car in front as that vehicle is accelerating from the light. If the driver does this the instant the red light is recognized, this will result in the vehicle having maximum speed, and kinetic energy, as it reaches the intersection. This means that energy lost to braking is as little as possible. Instead of coasting up to the light and stopping, the driver will now be traveling at a slower speed for a longer time, allowing the light to turn green before he arrives. The driver will never have to fully stop, as accelerating from just a few mph is much more efficient than from a full stop. Using this practice during periods of traffic congestion may affect other drivers and the overall effect is not obvious.
Another problem with this technique is that some traffic lights (usually on minor roads where they intersect major roads) are not timed but triggered. They will stay red until a car arrives at the intersection. In this situation, the optimum strategy may be difficult to determine.
Conventional brakes dissipate kinetic energy as heat, which is irrecoverable. Regenerative braking, used by hybrid/electric vehicles, recovers some of the kinetic energy, but some energy is lost in the conversion, and the braking power is limited by the battery's maximum charge rate and efficiency.
An alternative to acceleration or braking is coasting, i.e. gliding along without propulsion. Coasting dissipates stored energy (kinetic energy and gravitational potential energy) against aerodynamic drag and rolling resistance which must always be overcome by the vehicle during travel. If coasting uphill, stored energy is also expended by grade resistance, but this energy is not dissipated since it becomes stored as gravitational potential energy which might be used later on. Using stored energy (via coasting) for these purposes is obviously more efficient than dissipating it in friction braking.
When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required.
It should be noted that Coasting with a vehicle not in gear is prohibited by law in most US states. An example is Maine Revised Statues Title 29-A, Chapter 19, §2064 "An operator, when traveling on a downgrade, may not coast with the gears of the vehicle in neutral".
A driver may further improve economy by anticipating the movement of other traffic users. For example, a driver who stops quickly, or turns without signaling, reduces the options another driver has for maximizing his performance. By always giving road users as much information about their intentions as possible, a driver can help other road users reduce their fuel usage. Similarly, anticipation of road features such as traffic lights can reduce the need for excessive braking and acceleration.
Using air conditioning requires the generation of up to 5 hp (3.7 kW) of extra power to maintain a given speed. The National Renewable Energy Laboratory in a 2000 report suggest that a 400-W load on a conventional engine can decrease the fuel economy by about 0.4 km/L (1 mpg). A/C systems cycle on and off, or vary their output, as required by the occupants so they rarely run at full power continuously. Rolling down the windows is often seen as the leading way to prevent this loss of energy. This technique, however, causes increased drag in the form of air resistance and the cost savings is less than is generally anticipated. Using the passenger heating system slows the rise to operating temperature for the engine. Either the choke in a carburetor-equipped car or the fuel injection computer in newer vehicles will add more fuel to the fuel-air mixture until normal operating temperature is reached, decreasing fuel economy.
Octane rating is only a measure of the fuel's propensity to cause an engine to ping or knock; this pinging is due to pre-combustion, which occurs when the fuel burns too rapidly (before the piston reaches top dead center). Higher-octane fuels burn more slowly at high pressures. For the vast majority of vehicles (i.e. vehicles with standard compression ratios), standard-octane fuel will work fine and not cause pinging. Using high-octane fuel in a vehicle that does not need it is generally considered an unnecessary expense, although Toyota has measured slight differences in efficiency due to octane number even when knock is not an issue. All vehicles in the United States built since 1996 are equipped with OBD2 and most will have knock sensors that will automatically adjust the timing if and when pinging is detected, so low-octane fuel can be used in an engine designed for high octane, with some reduction in efficiency and performance. If the engine is designed for high octane then higher-octane fuel will result in higher efficiency and performance under certain load and mixture conditions. For other vehicles that have problems with pinging, it may be due to a maintenance problem, such as carbon buildup inside the cylinder, using spark plugs with the improper heat range or ignition timing problems. In such cases, higher-octane fuel may help, but this is an expensive fix; proper repair might make more long-term sense. There is slightly less energy in a gallon of high-octane fuel than low-octane. Pinging is detrimental to an engine; it will decrease fuel economy and will damage the engine over time.
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These are less broadly applicable, and some may compromise safety or possibly be illegal in some territories.
Pulse and Glide(PnG) is also known as Burn and coast. This method consists of rapid acceleration to a given speed (the "burn" or "pulse"), followed by a period of coasting down to a lower speed, at which point the burn-coast sequence is repeated. Coasting is most efficient when the engine is not running, although some gains can be realized with the engine on (to maintain power to brakes, steering and ancillaries) and the vehicle in neutral, or even with the vehicle remaining in gear. Most modern petrol vehicles cut off the fuel supply completely when coasting (over-running) in gear, although the moving engine adds considerable frictional drag and speed is lost more quickly than with the engine declutched from the drivetrain.
Some hybrid vehicles are well-suited to performing the burn and coast. In a series-parallel hybrid (see Hybrid vehicle drivetrain), the internal combustion engine and charging system can be shut off for the glide by simply manipulating the accelerator. However based on simulation, more gains in economy are obtained in non-hybrid vehicles.
Much of the time, automobile engines operate at only a fraction of their maximal efficiency, resulting in lower fuel economy (or what is the same thing, higher specific fuel consumption (SFC)). Charts that show the SFC for every feasible combination of torque (or Brake Mean Effective Pressure) and RPM are called Brake specific fuel consumption maps. Using such a map, one can find the efficiency of the engine at various rpms, torques, etc.
During the pulse (acceleration) phase of pulse and glide, the efficiency is near maximal due to the high torque and much of this energy is stored as kinetic energy of the moving vehicle. This efficiently-obtained kinetic energy is then used in the glide phase to overcome rolling resistance and aerodynamic drag. In other words, going between periods of very efficient acceleration and gliding gives an overall efficiency that is usually significantly higher than just cruising at a constant speed. Computer calculations have predicted that in rare cases (at low speeds where the torque required for cruising at steady speed is low) it's possible to double (or even triple) fuel economy.
These two- or three-fold improvements in fuel economy are possible only at city driving speeds of say 25 or 35 miles/hour. This is because cruising (steady speed) at such low speeds is very inefficient since the torque needed is so low that the efficiency read on a BSFC map is very poor. Pulse and glide significantly improves this. Unfortunately, city driving often involves many stops at signals and stop signs which were absent in the computer simulation which showed such multiple fold improvements. In other words, in the real world one is unlikely to see fuel efficiency double or triple. Such a failure is due to signals, stop signs, and considerations for other traffic; all of these factors interfering with the pulse and glide technique. But improvements in fuel economy of 20% or so are still feasible.
It is possible to coast in neutral with either a manual or automatic transmission. Modern automatic transmissions/transaxles depend on an engine driven fluid pump for lubrication and coasting with the engine off may lead to damage or failure of the transmission. To perform the maneuver, the driver shifts into neutral, and then keys the ignition back to turn off the engine (this position may be either "off", or "accessories" which will keep accessories such as audio electronics and ventilation blowers running)"Ignition switch" in CDX Online eTextbook. The driver should be careful not to key it to the "lock" position since this could lock the steering wheel, but most cars will not let one go to the lock position if the car is being driven. "Shift Interlock System" in Autoshop-101. The driver recovers normal operation by starting the engine in the normal way, by turning the key to "start" to crank the starter motor, and then releasing the key back to "run". Before putting the transmission in gear, if necessary, the driver may rev the engine to match the vehicle's gear and speed. The fuel economy from this advanced technique is increased noticeably over any short distance trip, largely because there are no engine idling losses (see figure below). Most modern automatics' computer systems do a very good job at putting the transmission in the proper gear after coasting in neutral, and the driver should press partway down on the accelerator when re-engaging.
Some, but not all, hypermilers use this maneuver, and some may use it more safely than others. The technique is used for general coasting, or as part of the pulse-and-glide maneuver, or when going down hills or in other situations when potential energy or momentum will propel the vehicle without engine power. Some hypermilers may use this maneuver while going downhill, around a corner, and without braking; however, that practice is in all likelihood more dangerous than coasting to a stop on a level and straight road, where stopping distance is shorter and visibility is greater. Vehicle control may be compromised. Turning the engine off will cause the power brake assist to be lost after a few applications of the brake pedal. Power steering is instantly lost; Steering is still possible, but can often require considerably more arm strength to turn the wheel.
For safety reasons, the maneuver is not recommended for use in traffic, since the driver will want the car to be in gear if sudden acceleration is needed as an evasive maneuver. The driver should first look for traffic behind the vehicle before attempting the maneuver. It can be considered more courteous to not coast if another vehicle is closely following. The proper etiquette and acceptable driving practices are controversial, and is worsened by a lack of communication between drivers.
Despite the potential risks, it does in fact save fuel to turn the engine off instead of idling. Traffic lights are in most cases predictable, and it is often possible to anticipate when a light will turn green. Some traffic lights (in Europe and Asia) have timers on them, which assist the driver in using this tactic.
Some hybrids must keep the engine running whenever the vehicle is in motion and the transmission engaged, although they still have an auto-stop feature which engages when the vehicle stops, avoiding waste. Maximizing use of auto-stop on these vehicles is critical because idling causes a severe drop in instantaneous fuel-mileage efficiency to zero miles per gallon, and this lowers the average (or accumulated) fuel-mileage efficiency.
Drafting occurs where a smaller vehicle drives (or coasts) close behind a vehicle ahead of it so that the vehicle in front shields the vehicle behind from the headwind. Wind tunnel tests of a car (model) ten feet behind a semi-truck (model) showed a reduction of over 90% for the wind force (aerodynamic drag). The gain in miles/gallon however is only 20–40%.
Drafting involves turning off the engine and gliding in neutral while drafting a larger vehicle, in order to take advantage of the reduced wind resistance in its immediate wake (this practice is illegal in some areas due to its danger); while tailgating itself is inherently risky, the danger of collision is increased as hydraulic power for power brakes is used up after a few applications of the brake pedal, and there is a loss of hydraulic pressure that provides power steering; however, there is less need for power steering at high speed.
There are a few anecdotal reports on the internet of persons who claim to have coasted for a long distance (without using any engine power most of the time) behind a larger vehicle. If these are true, it must mean that in some cases the aerodynamic drag is no longer drag but pushes the behind vehicle forward so as to overcome its rolling resistance.
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The US television show Mythbusters (Discovery Channel), in their June 6, 2007, episode, drove a Dodge Magnum station wagon at 55 mph (89 km/h) behind a Freightliner tractor trailer and measured the station wagon's fuel economy. As they got closer their results ranged from a baseline (no truck) figure of 32 to 35.5 mpg-US (7.4 to 6.63 L/100 km, a 11% greater distance covered per unit volume of fuel or a 10% reduction in fuel consumption) at 100 feet (30 m), and then progressively up to 44.5 mpg-US (5.29 L/100 km, a 39% greater distance covered per unit volume of fuel or a 28% reduction in fuel consumption) at 10 feet (3 m), as a result of decreased drag consequent of drafting. However, the show's hosts had serious safety concerns about drafting behind tractor trailers which prompted them to advise against using the technique.
Understanding the distribution of energy losses in a vehicle can help drivers travel more efficiently. Most of the fuel energy loss occurs in the thermodynamic losses of the engine. The second largest loss is from idling, or when the engine is in standby, which explains the large gains available from shutting off the engine.
In this respect, the data for fuel energy wasted in braking, rolling resistance, and aerodynamic drag are all somewhat misleading, because they do not reflect all the energy that was wasted up to that point in the process of delivering energy to the wheels. The image reports that on non-highway (urban) driving, 6% of the fuel's energy is dissipated in braking; however, by dividing this figure by the energy that actually reaches the axle (13%), one can find that 46% of the energy reaching the axle goes to the brakes. Also, additional energy can potentially be recovered when going down hills, which may not be reflected in these figures.
Geoff Sundstrom, director of AAA Public Affairs, notes that "saving fuel and conserving energy are important, but so is safety, and preventing crashes." In the US, optimal highway speed for fuel-efficiency often lies between the legal minimum speed and the legal speed limit, typically 45 to 70 mph (72 to 110 km/h). However, these legal speeds may actually be slower than average traffic speed. The hypermiler thus avoids the danger of higher speeds; however, the speed differential created between cars can be problematic in some cases. Driving at speeds much lower than other vehicles may promote other problems; namely, drafting of vehicles failing to yield right of way. Coasting in neutral with or without the engine off may lead to reduced control in some situations, and drafting at any closer than 3 seconds to the vehicle in front is a recognised risk.
Coasting in neutral is another fuel economy reduction method. A driver legally[who?] needs to have the ability to bring the vehicle to a stop under any circumstances, including when the engine stalls during normal driving. In the event that there is a loss of engine power, decelerating to a stop is recommended[by whom?] as the safest action. As a safety feature, vehicles are designed to retain some limited ability to steer and brake even when all engine power is lost.
It should be noted that a lot of the fuel savings are gained through better road awareness and anticipation. Improving these skills inevitably makes for a safer driver. For example someone relying on the brakes to slow for a junction may have little or no additional braking capacity if something untoward occurs. By contrast someone relying on engine braking has all the vehicle's mechanical braking in reserve. In either case an efficient driver is likely to be more aware of what's going on (or about to start) in front of them so will probably not be caught out to the same extent.
Hypermiling contests have been held on selected courses.
The Maximum Fuel Economy contest was held in Elkhart, Indiana, where "world records" for the Honda Insight (213 miles per US gallon (1.10 L/100 km; 256 mpg-imp) round trip), Toyota Prius (136 miles per US gallon (1.73 L/100 km; 163 mpg-imp) round trip) and the Ford Escape Hybrid (76 miles per US gallon (3.1 L/100 km; 91 mpg-imp) mpg round trip) were set.
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