Energy-efficient driving

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Energy-efficient driving is a driving practice intended to improve fuel economy in automobiles.^[1] 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).

Terms for driving techniques to maximize fuel efficiency include hypermiling.^[2]

Some fuel economy techniques have raised concerns from several quarters as they can be unlawful and dangerous, such as tailgating larger vehicles on freeways to save fuel.^[3]

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Key parameters to maintain are proper tire pressure, wheel alignment,^[4] and engine oil with low-kinematic viscosity,^[5] 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 available to move the vehicle. Under inflated tires can increase rolling resistance by approximately 1.4% for every 1 psi (0.07 bar; 7 kPa) drop in pressure of all four tires.^[6] Equally important is the scheduled maintenance of the engine (e.g. air filter,^[7] 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.^{[citation needed]}

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.^[8] 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 fill up more frequently.^{[citation needed]}

Maintaining an efficient speed is an important factor in fuel efficiency.^[5] 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.^{[9][10][11][12]} 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).^[11] The U.S. 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.^[13]

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.^[14] 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 ^[15] 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.^[16]

Engine efficiency varies with speed and torque. 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 powerband. For a turbo diesel 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.^[17] In a gasoline engine, efficiency typically drops off more rapidly than in a diesel because of throttling losses.^[18] 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 (7.5–11.2 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.^[17]) 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.^[19]

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 is somtimes caused by unpredictable events. At higher speeds, there is less time to allow vehicles to slow down by coasting. Kinetic energy is higher, so more energy is lost in braking. At medium speeds, the driver has more time to choose whether to accelerate, coast or decelerate in order to maximize overall fuel economy.

While approaching a red signal, drivers may choose to "time a traffic light" by easing off the throttle before the signal. By allowing their vehicle to slow down early, they will give time for the light to turn green before they arrive, preventing energy loss from having to stop.

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.^{[citation needed]}

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.

Turning the engine off instead of idling does save fuel. 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.

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 .^{[citation needed]} 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 (0.94 mpg_‑US) — at a baseline fuel economy of 20 km/L or 5 L/100 km this is equivalent to a change of approximately 0.1 L/100 km.^[20] 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.^[21] 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.^[22]

Stop/start driving is much less efficient and much more polluting than driving at a constant speed. Driving at an earlier or later time, or using an alternative route to avoid heavy traffic may provide better fuel economy.

Octane rating is only a measure of the fuel's likelihood to cause an engine to ping or knock; this pinging is due to precombustion, 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,^[23] although Toyota has measured slight differences in efficiency due to octane number even when knock is not an issue.^[24] 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.

Pulse and Glide (PnG) is also known as Burn and coast and periodic driving. 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.^[25][26] 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.^[26] 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.^[25][26]

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)).^[27] 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 combinations of rpm, torque, etc.^[26]

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.^[25][26]

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.^[25][26][28]

Drafting occurs where a smaller vehicle drives (or coasts) close behind a vehicle ahead of it so that it is shielded from wind. Scale-model wind tunnel and Real-World tests of a car ten feet behind a semi-truck showed a reduction of over 90% for the wind force (aerodynamic drag). The gain in efficiency is 20–40%. Driving close behind another vehicle is inherently risky, and the danger of collision is increased because power-assisted braking and steering are lost.^[29][30]

Most of the fuel energy loss in cars occurs in the thermodynamic losses of the engine. The next biggest 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.^[32]

There is sometimes a tradeoff between saving fuel and preventing crashes.^[5]

In the US, the speed at which fuel efficiency is maximized often lies below the speed limit, typically 45 to 70 mph (72 to 113 km/h); however traffic flow is often faster than this. The speed differential between cars raises the risk of collision.^[5]

Drafting increases risk of collision^[33] when there is a separation of fewer than three seconds from the preceding vehicle.

Coasting is another technique for increasing fuel economy. Shifting gears and/or restarting the engine increase the time required for an avoidance maneuver that requires acceleration. Therefore some believe the reduction of control associated with coasting is an unacceptable risk.

The term "hypermiling" gained popularity as a result of the rise in gasoline prices during the 2000s.^[34]

In 2008, the word hypermiling was selected as the best new word of the year by New Oxford American Dictionary.^[35]

• Alternative propulsion
• Carpool
• Fuel economy in automobiles
• Fuel saving devices
• Rat running
• Plug-in hybrid
• Start-stop system
• Vehicle efficiency

• FuelEconomy.gov
• Lowest to Highest MPG Cars