Energy-efficient driving

Energy-Efficient Driving techniques are the most effective way a driver can reduce their fuel consumption. Vehicle labels provide fuel efficiency information based on standardized tests that report the fuel economy in automobiles. Fuel efficiency 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.

Simple fuel efficiency techniques can result in a dramatic reduction in fuel consumption without resorting to radical fuel saving techniques that can be unlawful and dangerous, such as tailgating larger vehicles on freeways to save fuel.

Techniques

Maintenance

Underinflated tires wear out faster and lose energy to rolling resistance because of tire deformation. The loss for a car is approximately 1.0% for every 2 psi (0.1 bar; 10 kPa) drop in pressure of all four tires. Wheel alignment, fuel evaporation while parked, and high engine oil kinematic viscosity, referred to as "weight" all reduce fuel efficiency.

Mass and Improving Aerodynamics

Drivers can also increase fuel efficiency by 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 efficiency by reducing both weight and aerodynamic drag. Some cars also use a half size spare tire, for weight/cost/space saving purposes. On a typical vehicle, every extra 100 pounds, increases fuel consumption by 2%. Roof Racks can increase fuel consumption by 20%.

Maintaining an Efficient Speed

Maintaining an efficient speed is an important factor in fuel efficiency. Optimal efficiency can be expected while cruising at a steady speed, 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 and 50  mph (80 km/h). 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).

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.

Choice of gear (manual transmissions)

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. In a gasoline engine, efficiency typically drops off more rapidly than in a diesel because of throttling losses. 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 efficiency 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).

Acceleration and deceleration (braking)

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 efficiency 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 sometimes 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 efficiency. Regardless of the speed you are traveling at, anticipating the traffic ahead will give you more time to react and that saves you fuel.

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 and coast, they will give time for the light to turn green before they arrive, preventing energy loss from having to stop. Coasting to a stop sign so that other vehicles ahead of you can pull away means you only have to stop once and does not take any extra time.

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.

Coasting or Gliding

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 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.

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.

Anticipating Traffic

A driver may improve their fuel efficiency 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.

Minimising Ancillary Losses

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 efficiency by up to 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. Using the passenger heating system slows the rise to operating temperature for the engine. Either the choke in a carburetor-equipped car (1970's or earlier) or the fuel injection computer in modern vehicles will add more fuel to the fuel-air mixture until normal operating temperature is reached, decreasing fuel efficiency.

Planing Ahead

Map out your route, especially if it’s long. Where possible and prudent, use four-lane rather than two-lane highways and avoid roads that cut through major cities and are dotted with stoplights, intersections and pedestrians. On shorter trips, listen to traffic reports and pick routes that—while perhaps longer—enable you to steer clear of accidents, road construction, steep hills and other trouble spots. Whatever your destination, always give yourself enough time to get there. Without the stress of running late, you’ll be more likely to brake gently, accelerate slowly and avoid high speeds.

Combine Trips

Run your errands one after the other, planning your route to avoid backtracking and rush-hour traffic. The longer excursion will enable your vehicle’s engine to warm up to the temperature at which it converts energy most efficiently.

Time of day

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 efficiency.

Fuel type

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, 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.

Energy losses

Example energy flows for a late-model midsize passenger car: (a) urban driving; (b) highway driving. Source: U.S. Department of Energy

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.

Safety

There is sometimes a tradeoff between saving fuel and preventing crashes.

In the US, the speed at which fuel efficiency is maximized often lies below the speed limit, typically 35 to 50 mph (56 to 80 km/h); however traffic flow is often faster than this. The speed differential between cars raises the risk of collision.

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

Coasting is another technique for increasing fuel efficiency. 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.

See also

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

External links

• [4]
• FuelEconomy.gov
• Lowest to Highest MPG Cars