Energy-efficient driving

Fuel economy-maximizing behaviors describe techniques that drivers can use to optimize their automobile fuel economy. The energy in fuel consumed in driving is lost in many ways, including engine inefficiency, aerodynamic drag, rolling friction, potential energy required to climb hills, and kinetic energy lost to braking (absent regenerative braking). Driver behavior can influence all of these. The city mileage of conventional cars is much lower than highway mileage due to: 1) a high proportion of idling time, 2) operation mostly at very inefficient low-output engine operating points, and 3) more frequent braking.

Terminology

Various terms describe drivers using unusual driving techniques to maximize fuel efficiency. A few of these are:

• Hypermilers are drivers who exceed the United States Environmental Protection Agency (EPA) estimated fuel efficiency on their vehicles by modifying their driving habits. The term 'hypermiler' originated from hybrid vehicle driving clubs and Wayne Gerdes in particular. As people began comparing fuel efficiency, they noticed that by using certain driving techniques, they could greatly improve their mileage. With the aid of real time mileage displays, drivers were able to refine these driving techniques and greatly exceed the EPA rating for their vehicle. Decades before the word 'hypermiler' was used, the techniques were used in events such as Mobil Economy Run dating to 1936. Gas rationing during World War II forced some drivers to adopt these techniques, but they largely fell out of favor with the population after the war. Hypermiler Wayne Gerdes can get 59 MPG in a Honda Accord and 30 MPG in an Acura MDX.

• Nempimania (also Nenpimania) is an obsession with getting the best fuel economy possible from a hybrid car. It is derived from the Japanese "nempi" (燃費)--a contraction of nenryōshōhiryō （燃料消費量） meaning fuel economy, and "mania". Nempimania is exhibited by owners of the Toyota Prius and other hybrid owners by various habits aimed at maximizing fuel economy: slow starts, "Pulse and Glide", timing stoplights, driving barefoot, etc.

• Ecodriving is a term used in Europe to name initiatives which support energy efficient use of vehicles. The campaigns include training courses with hands on training - fuel gauges etc.

Techniques used to maximize fuel economy

Techniques used to improve fuel economy include basic techniques that can be used by most drivers, and advanced techniques that are more specialized, but can be used to achieve extremely high mileage.

Basic techniques

Maintenance

Key parameters to maintain are high tire pressure, tire balance and wheel alignment, and engine oil with low-kinematic viscosity referred to as low "weight" motor oil, which is filled just to the low-level mark. Inflating tires to the maximum recommended air pressure means that less energy is required to move the vehicle. Under-inflated tires can lower fuel efficiency by approximately 1.4 percent for every 1 psi drop in pressure of all four tires. Equally important is the proper maintenance of the Engine Control Module and the sensors it relies on to control engine operation, particularly the oxygen sensor.

Minimizing mass

Drivers can also increase fuel economy by driving lighter-weight vehicles and minimizing the amount of luggage, tools, and equipment carried in the vehicle.

Efficient Speeds

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. The optimum speed varies with the type of vehicle, although it is usually reported to be in the range of 35 to 55 mph . For instance a 2004 Chevrolet Impala had an optimum at 42 mph (70 kph), and was within 15% of that from 29 to 57 mph (45 to 95 kph). Drivers of vehicles with fuel-economy displays can check their own vehicles by cruising at different speeds and monitoring the readout.

Acceleration and deceleration (braking)

Fuel efficiency varies with the vehicle, but generally acceleration is most efficient at moderate throttle openings It is also important to keep the engine RPM in an efficient range, so acceleration is more fuel-efficient when up-shifting occurs at a lower RPM. Low-RPM up-shifting is easily implemented with a manual transmission.

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. Gentle acceleration and deceleration is helpful in avoiding unnecessary acceleration. 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 "degrees of freedom", and can elect to accelerate, coast or decelerate depending on whichever is expected to maximize overall fuel economy.

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

The alternative to acceleration and braking is coasting. Coasting is an efficient means of slowing down, because kinetic energy is dissipated as aerodynamic drag and rolling resistance, which always must be overcome by the vehicle during travel. Coasting normally entails losses in the engine as well, which may be idling, consuming fuel, and/or adding friction. For a discussion of the controversial technique of coasting with the engine off, see the section on advanced techniques below.

Fuel type

It is commonly believed that efficiency of a gasoline engine is related to the fuel's octane level; however, this is not true in most situations. Octane rating is only a measure of the fuel's propensity to cause an engine to "ping", this ping 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 . Most vehicles equipped with emissions systems have sensors that will automatically adjust the timing, if and when ping is detected, so low octane fuel can be used even if the engine is designed for high octane, at some reduction in efficiency. If the engine is designed for high octane then higher octane fuel will result in higher performance (with full-open throttle), but not necessarily fuel cost savings, since the high-octane is only needed with the throttle fully open. For other vehicles that have problems with ping, it may be due to a maintenance problem, such as carbon buildup inside the cylinder or incorrect spark plug tip length. 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. Ping is detrimental to an engine; it will decrease fuel economy and will damage the engine over time.

Trip computer

Modern hybrids come with built-in trip computers which display real-time fuel economy (MPG), which helps the driver adjust driving habits. However, most gasoline powered vehicles do not have this as a standard option (although some luxury vehicles do). However, most vehicles produced after 1996, have one of three standardized interfaces for "on-board diagnostics", which provides information including the rate of fuel consumption, and the vehicle speed. This streaming data is sufficient to calculate the real-time fuel economy.

Generic aftermarket or "add-on" products are available, such as the "ScanGauge", which will connect to a vehicle's onboard computer, read the real-time information, and calculate and display the instantaneous fuel economy. This information assists the driver by displaying the fuel consumption. This provides a general indicator to the driver who can then infer in real-time how driving techniques affect gas mileage. This can help the astute driver to learn how to drive more efficiently, However, such a device does not do all the work for the driver. The device only measures fuel consumption, and fuel economy. It does not indicate braking statistics, for example, nor does it teach a driver how to "time a traffic light" by adjusting the vehicle speed, such that the vehicle arrives at the intersection when the light is green, and braking is minimized.

Advanced techniques

These are less broadly applicable, and some may compromise safety.

Pulse and glide

This method consists of accelerating to a given speed (the "pulse"), followed by a period of coasting (the "glide"), and then repeating the process. The glide is most efficient when the engine is not running. Because some cars inject extra fuel when the starter is activated, this was originally best accomplished with a manual transmission. Hybrid vehicles, such as the Toyota Prius, are ideally suited to performing this technique as well: the internal combustion engine as well as the charging system can, be shut off for the glide by simply manipulating the accelerator.

Auto-stop, forced stop, and draft-assisted forced stop

In the auto-stop maneuver, the vehicle's transmission is put in neutral, the engine is turned off (a "forced stop"), and the vehicle coasts to a stop. It is possible to coast in neutral with either a manual or automatic transmission. To perform the maneuver, the driver shifts into neutral, and lets the tachometer stabilize, then keys the ignition back to the first position, referred to as "IG-I", to shut off the engine and electronics. The driver then keys forward to IG-II to start the electronics and continue coasting. The key should remain in the ignition in the IG-II position, and not the IG-I position , in order to avoid engaging the steering wheel lock. The driver recovers from "stealth mode" by starting the engine in the normal way, by turning the key to IG-III to crank the starter motor, and then releasing the key back to IG-II. 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).

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 breaking ; however, that practice is in all likelihood more dangerous than an auto-stop on a level and straight road, where stopping distance is shorter and visibility is greater. Vehicle control may be somewhat compromised, and this can be more-or-less dangerous or safe depending on the situation. Turning the engine off will cause the power brake assist to be lost after a few applications of the brake pedal. Power steering is lost is quickly lost, although it is not needed at high speed, only at low speed. Steering is still possible at low speed, 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. Both sides of the debate are often argued passionately, yet sometimes neither of the proposed driving methods is in complete accordance with the rules of the road. Both hypermilers and regular drivers may at different times violate the same rule yet blame the other type of driver.

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) have timers on them, which assists the driver in using this tactic.

Draft-assisted forced stop, a variation of the forced (auto)stop (sometimes abbreviated as D-FAS), 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 with D-FAS 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.

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, or tailgating a large vehicle such as a semi-tractor trailer on the highway, will substantially increase fuel mileage, but at great risk. Should the truck suddenly apply the brakes, a rear-end collision could ensue. Furthermore, any savings in gas mileage would likely be spent on repairing windshield damage from stones or other debris kicked up by the truck. The drafting driver's visibility is also greatly diminished, thus the driver is unable to avoid small road obstacles such as tire re-treadings.

Hybrid and electric engines

Main articles: Hybrid vehicle, Plug-in hybrid, and Electric vehicle

The most effective commonly available hybrid vehicles in the hypermilage marathons are the Honda Insight Hybrid, the Toyota Prius Hybrid, and the Honda Civic Hybrid. Other hybrids have also done very well. Some historical non-hybrid vehicles such as the Honda Civic CR-X HF and the Smart Fortwo have also done remarkably well on mileage. The Toyota and Ford hybrids use two motor generators called a series-parallel hybrid with unique characteristics different from the single motor generators of the Honda and GM hybrids (as of January 2007). The Honda motor generator is integrated with the engine, the Integrated Motor Assist (IMA) that enhances the low-end torque of the engine. The current GM hybrids turn-off the engine at a stop and restart it when ready to leave.

The Toyota and Ford hybrids have a threshold speed—around 42 mph in the case of the Prius—above which the engine must run to protect the transmission system. Below this model-dependent speed, the car will automatically switch between either battery-powered mode or engine power with battery recharge. These hybrids typically get their best fuel efficiency below this model dependent threshold speed. Coasting can be achieved by using Neutral transmission range. The Honda IMA vehicles have a limited, battery-only, powered capability, although after-market modding has made the Insight capable of running in electric only-mode [7]. They achieve higher fuel economy [8]. Another way to save fuel includes turning off the engine on manual transmission vehicles when coasting.

The GM hybrids have an engine auto-stop when halted. As of January 2007, they have no battery-only, powered capability. In late 2007, GM will introduce two two-mode hybrid, full-size SUVs, which can be powered by electric motors, V8 engines, or a combination of both.

Energy Losses

Example energy flows for a late-model midsize passenger car: (a) urban driving; (b) highway driving. Source: U.S. Department of Energy [6]

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. Very little fuel energy actually reaches the axle. However, any mechanical energy that doesn't go to the axle is energy that doesn't have to be created by the engine, and thus reduces loss in the inefficiency of 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 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. Any statistic such as this must be based on averages of certain driving behaviors and/or protocols, which are known to vary widely, and these are precisely the behaviors which hypermilers leverage to the full extent possible.

Safety

"Saving fuel and conserving energy are important, but so is safety, and preventing crashes." On the highway, the most fuel-efficient driving is often done between the legal minimum speed and the speed limit, often around 55 mph, which coincides with the vehicles maximum fuel efficiency. This speed is very often much slower than the average highway vehicle. This practice is safe, because one avoids dangerous high speeds. However, driving at speeds much lower than other vehicles can pose other significant risks, such as causing aggressive drivers to tailgate the slower vehicle. Coasting in neutral and/or with the engine off may lead to reduced control in some situations.

Tailgating

On some roads, the norm is to drive above the speed limit, and a driver traveling at a legal speed can easily and inadvertently incite road rage in another driver. In particular, slower driving may lead to faster drivers tailgating the slow vehicle, which is a dangerous situation, particularly at high speeds.

There are many reported accounts of road rage and tailgating by aggressive drivers, when hypermilers drive in a manner that other drivers are unaccustomed to, such as coasting to a stop.

The risk of tailgating is largely caused by the accident avoidance time being reduced to much less than the driver reaction time. For maximum safety, driving instructors advocate using the "3 second rule", regardless of speed. In the US, if an accident occurs due to tailgating, the tailgater is liable for injury and damages.

The risk of severe road rage may be lessened by permitting aggressive drivers the opportunity to pass when it is safe to do so.

Coasting in neutral

Those who warn than coasting can be dangerous claim that the driver has less control of the vehicle, and will take longer to react in an emergency. Some advocates of the method claim that there are few if any formal driving practices which state that instantaneous evasive acceleration is more safe than not, or that the lack of acceleration is categorically unsafe. For instance, under no circumstances is it advisable to run a red light, thus it is difficult to envisage a scenario when acceleration to a red light would be the best option or otherwise essential for an emergency evasive maneuver, or when coasting to a red light would be categorically unsafe.

In a collision-avoidance emergency, the safe technique focuses entirely on controlled braking, and not at all on acceleration. The proper technique is to use threshold braking (maximum deceleration without skidding), then to wait one second for the weight to shift onto the front wheels in order to increase vehicle cornering stability and to increase the maximum lateral acceleration that is possible without skidding, and then to turn the vehicle rather quickly and sharply to avoid the object. If the lead vehicle initiates an emergency stop, the trailing vehicle is likely to need 3 seconds to avoid a collision.

A prominent adherent to this technique is Wayne Gerdes, who rolls with the engine off.

Driving in neutral or coasting without engine power is not limited to the "auto-stop" maneuver used in hypermiling. During normal driving, situations may occur where there is a similar loss of engine power.

One function of the driving laws is to help increase safety. However, the safety issues are not always clear cut, and often neither are the laws. One thing is certain: a driver legally needs to know how control the vehicle safely when the car is in neutral. The general practice of coasting in neutral is against the law in many states, yet there are exceptions to this law, and some states advocate its use in certain circumstances, for example: "If you are on ice and skidding in a straight line, step on the clutch or shift to neutral." Also, in a stuck throttle emergency, the safe procedure is to put the transmission in neutral, and if that is ineffective, to turn-off the engine. Also, a driver legally 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 as the safest action. As a safety feature, vehicles are designed to have some ability to steer and brake even when all engine power is lost.

See also

• Alternative propulsion
• Carpool
• Fuel economy in automobiles
• Low-energy vehicle
• Plug-in hybrid
• Vehicle efficiency

External links

• A leading website with much information
• 2007 Hybridfest.com winner Bill Kinney tells how to Get 89 MPG in a Honda Insight

External links

• Saving Gas with Extreme Driving — KOTV report on hypermiling
• Ecodriving for Buses
• European initiative
• Track and compare your hypermiling over time
• Microtrends: Hypermiling Times Online September 14, 2007