Energy-efficient driving
Energy-efficient driving
Basic techniques
Maintenance
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.
Minimizing mass and improving aerodynamics
Removing common unnecessary accessories such as roof racks, brush guards, wind deflectors , 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. Another simple way to decrease the vehicle's mass is to drive with the fuel tank mostly empty and to fill up more frequently.
Maintaining an efficient speed
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 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).[11] 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.[13]
Choice of gear (manual transmissions)
The optimum efficiency point is around 1750 rpm, and 90% of maximum torque at that speed, for this turbo-diesel engine.
Acceleration and deceleration (braking)
Fuel efficiency during acceleration generally improves as RPM increases until a point somewhere near peak torque.
Generally, fuel economy is maximized when acceleration and braking are minimized. Maximize coasting time.
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. .
Anticipation
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.
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.
Rolling down the windows is often seen as the leading way to prevent this loss of energy.
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.[23]
Minimize Idling
By having the engine switched off, even for a short period, more fuel is saved than is lost from restarting the engine.
Fuel wastage can be minimised while idling by:
- Shifting the gear lever to neutral whenever possible, or
- Stopping the engine if the vehicle is held up for an extended period of time or at traffic jam.
Fuel type
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,[26] although Toyota has measured slight differences in efficiency due to octane number even when knock is not an issue.[27]
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.[28] Pinging is detrimental to an engine; it will decrease fuel economy and will damage the
engine over time.[29]
Pulse and Glide <= fuel cut
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.
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.
Causes of pulse-and-glide energy saving
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 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.
Coasting in neutral
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.
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Drafting
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 pneumatic 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;[34] however, there is less need for power steering at high speed.
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Energy losses
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.[37]