|Toyota Prius - Contents|
Efficient Prius driving entails avoiding inefficient modes, like using "B" or running the internal combustion engine (ICE) or excessive braking and speed, and maximizing efficient modes such as using electric vehicle (EV) modes. When possible, route planning allows both of these to be achieved, often with little or no change in elapsed time.
Above all, it's important to understand that a casual Prius driver needs to understand NONE of the tricks described below. "Just drive it and enjoy!"
Use "pulse and glide" driving technique for maximum efficiency.
Using "B" Shift PositionEdit
The Prius uses an electronic transmission mode selector, similar to a gear shift on a conventional car. Unlike a conventional vehicle with modes P R N D 2 1 (park, reverse, neutral, drive, 2nd gear, 1st gear) the Prius has a power button, a park button, and a selector for modes R N D and B (reverse, neutral, drive, and engine-Braking).
In the course of normal driving, the driver NEVER needs to use 'B' mode (engine Brake mode). The techniques below are optional. 'B' mode is appropriate when descending long grades (i.e. descending down a mountain road), where if the driver uses the brake pedal to control the vehicle's speed, the vehicle's brake pads will overheat and fail.
In a normal vehicle, the driver handles that condition by slowing to an appropriate speed and shifting to a lower gear (2nd or 1st). This forces the vehicle to spin the engine faster than needed, "wasting" energy through vacuum losses but helping maintain a safe vehicle speed without overheating the brake pads.
In a Prius, the driver handles that condition by selecting "B" mode. This tells the Prius to attempt to emulate "engine braking" in a conventional vehicle. Depending on the vehicle's speed and the battery SOC (state of charge), the Prius will do this using regenerative braking and/or high-RPM zero-fuel-flow engine braking.
Engine braking is inefficient because it uses the engine as an air pump, converting kinetic energy into heat. Unnecessary regenerative braking is also inefficient because of energy conversion losses. (Regenerative braking is good when it replaces friction braking, but it's not as efficient as coasting at a constant speed.) Because of this, "B" mode will not recharge your batteries more efficiently than just braking and is not recommended for general driving. Just as you might downshift into 2nd gear when going down a long, steep hill to prevent brake damage, you can "downshift" into "B" mode on a Prius. "Shifting" is all electronic, so the system won't let you shift in a harmful way. (Even if you try to shift into reverse when moving quickly forward, you'll just end up in neutral.)
However, in the case of the Prius "Plug-in" model (which provides an EV drive mode,) the "B" mode can be used to extend the range of the EV battery. If in hybrid mode, it is necessary to switch to EV mode and then engage "B". This is effective when approaching junctions as it is not then necessary to apply the brakes until actually arriving at the junction. Typically the range of the EV mode is extended by .1 - .2 miles each time this technique is used.
Unnecessary Engine IdlingEdit
Often you will pull up at a stop light that has just changed. In some suburban areas this may entail a wait of several minutes, as the lights cycle through various simultaneous or sequential left turns, cross traffic, pedestrians, etc. If the gasoline engine is running when you approached the stop it will take the better part of a minute before the computer recognizes the situation and shuts off the engine. There is a trick to getting the engine to shut off promptly. If you approached the stop in B (engine assist braking), you may then when stopped with the brake on, command D (drive) - the engine will stop immediately - at least in 2004 US versions.
Note: there is a consensus that extensive EV use decreases efficiency and puts strain on hybrid components. In the US one could install an EV-Mode Button which is a standard button on the dashboard in EU and JP Priuses, but uses the cruise control stalk when installed aftermarket. It allows the driver to force the engine to shut off (or not start) when driving below 35mph. The switch will not work above that speed or if the battery is lower than four bars, or if you are accelerating or during the first 60 sec of operation after a cold start (unless turned on prior to starting). It can run a battery to zero in a very short time, but saves anywhere from 1 to 5mpg depending on circumstances and expertise of use. EV mode is available as an aftermarket self-installed kit for ~$60 from Coastal Electronics, Inc. www.coastaletech.com/electric_only_mode.htm
Off throttle shift to electric modeEdit
(Transmission mode in D, cruising speed up to 45 mph (70 km/h), road level.)
When reaching cruising speed it is often possible to briefly go "off throttle", allowing the system to switch to electric only mode. If the road is nearly level a gentle re-application of throttle to just enough to maintain cruising speed in electric mode may enable electric cruising (depending upon road grade, wind conditions, and battery state).
Accelerate from stop to 35 to 45 mph (60 to 70 km/h)Edit
(Transmission mode in D, cruising speed up to 45 mph (70 km/h), road level.)
When running in traffic, a brisk (but not full throttle) acceleration, followed by an off throttle attempt (see previous) will be much more effective than gentle acceleration. This is a general technique applicable to non-hybrid cars. A gasoline ICE (Internal Combustion Engine) is more efficient (in terms of power produced/fuel consumption) when operating at higher effort due to smaller throttle losses.
Accelerate with no traffic to 25 mph (40 km/h)Edit
Transmission mode in D, cruising speed up to 25 mph (40 km/h), road level.
With substantial patience it is possible to accelerate to cruising speed using only electric power. Owing to the slow acceleration this should not be attempted with following traffic present. Although this will temporarily reduce fuel consumption since you're driving in electric mode, it will not improve the long term fuel consumption, since you have to recharge the battery at some point later (and the generation and use of electricity involve energy conversion losses). Though if the route contains descents the regeneration could reduce the losses. But as pointed out above, it is better to accelerate at a moderate pace.
Long uphill followed by long downhillEdit
(Transmission mode in D, cruising speed uphill to 65 mph (105 km/h), road uphill, followed by downhill at or below 50 mph (80 km/h) with transmission mode in B.)
The controller is blind to the future; it cannot tell that it would make sense to run the battery down on a long upgrade knowing that it can be quickly recharged on a subsequent downgrade. Nor is it possible for the driver to inform the system that this condition is coming. What happens is that the system will use the engine power when going uphill to charge the battery to normal state. Then, running down hill (with "B" selected) the battery is quickly charged to the maximum before the bottom of the hill. This full charge (battery state shown in green) will then cause the system to use the motor for compressive braking. It would be far more efficient to be able to use the battery capability to assist the motor (rather than taking charge from the motor). A run up the hill at sufficient throttle to demand electric motor assist (within speed limits, both statutory and by road and traffic conditions) may improve overall efficiency in this particular circumstance.
When using "B" on a long (1 to 2 km) downhill (e.g. over 60 mph, 100 km/h) you may find at higher speeds that the motor is used for braking in addition to the power recovery from the generator. Unless the battery is fully charged (showing green on the monitor), this is wasting excess energy which could be stored. By briefly braking to a somewhat slower speed, say 45 to 50 mph (70 to 80 km/h), the motor will not be used to retard the vehicle, all subsequent retarding energy will go to the battery, and a longer time will be spent recovering energy, and so more energy will be recovered. This is less useful on a long mountain downgrade where the battery will become fully charged regardless of the technique used.
Gentle rise and descentEdit
When driving up and down short hills do not use cruise control downhill. Using cruise control going downhill forces regenerative braking unnecessarily. Unless the final valley speed will be too great, shift to neutral. Regenerative braking does NOT recover anywhere near 100% of the energy going into the brakes. Switching to neutral for going down short hills is far more efficient ...unless you have an accident or get a ticket at the bottom of the hill!
Slow and SteadyEdit
When traveling on relatively flat surfaces at approx 0 to 25 km/h (15 mph), sometimes the engine will run seemingly unnecessarily. By applying the brakes it tells the car to regenerate and is usually enough to shut it down. Sometimes it is necessary to come to a complete stop. At speeds that low, there is no real need for the gas engine. Alternatively, if you have an EV switch, you can turn it once you are below 35mph.(See also "Engine Stopping" above)
Use slow speed to warm-upEdit
On the first trip of the day, both the NHW20, 04-current, models and earlier NHW11 will have relatively higher fuel consumption as the ICE and transmission warm-up. Driving the first mile or so at a low speed does little if anything to improve gas mileage and today's lubricants are designed to provide the necessary lubrication at their full range of operating temperature. The only exception where a more "gentle" operation would be in order is if you live in very cold climates, e.g., overnight temperature is below 32F. Then a short distance of easier acceleration and braking makes sense. You will save nothing during this time of fuel usage because the ICE controller is doing what is necessary to get the pollution control devices up to operating temperature as quickly as possible. Drive normally, let the computer do the work it's suppose to do.
In colder climates, our Japanese Prius owners report saving 100 ml of fuel from use of an engine block heater. They also report an additional 100 ml of fuel savings from warming the transaxle. ICE block heater savings have been widely reported.
42 MPH transition pointEdit
Below 42 MPH, the ICE and electric system will maintain speed cycling between EV and ICE-recharge power. But above 42 MPH, the ICE must spin all of the time even if power is not needed. As a general rule of thumb, try to stay comfortably below 42 mph, say 38 mph, or above 42 mph, 45 mph or higher to minimize having the ICE turn on and off unnecessarily.
Over filling the ICE oil level appears to drop mileage by about 3-5 MPG. Many Prius owners shoot for 3/4th full. Also, Toyota recently announced using 0W-20 as a recommended engine oil as an alternative to 5W-30. Since oil 'thins' in normal use, changing it more frequently is less likely to have a beneficial effect but keeping it clean with a new filter is highly recommended.
The new model NHW20 （2004-current） uses an advanced transaxle oil, Type WS, that oil testing reveals to be lasting well up to the 60,000 mile life. However, the NHW11 model （2001-2003） uses a thicker Type T-IV that appears to have less than half of the 60,000 mile recommended life. Based upon transaxle oil testing, older Prius should have their oil changed within 15,000-30,000 miles, more frequently in dusty areas. With the older model, be sure to have the pan dropped and all exposed parts wiped clean to remove any grit or dirt. The newer Prius does not have a transaxle pan that can be dropped.
Approaching home: what is the optimal battery level to leave for the next trip?Edit
Note: the following paragraph is totally inaccurate. There is little discharge overnight and no need to run batteries down, as they will have to replenished next day using energy from ICE. Self discharge of NMH batteries is negligible overnight only a fraction of a percent.
Some of the energy stored in the batteries overnight is wasted since all batteries will inevitably discharge to some extent even when not in use. If you have an EV switch (see "Unnecessary Engine Idling" above), it may be beneficial to run the batteries down when approaching home or when the vehicle is about to experience an extended rest, however, if batteries are left below 4 bars and you have to stop prior to one minute of driving the next trip, you will be unable to override the computer to stop the engine from idling at the first stop sign or stoplight.