Beyond Gasoline: Going Electric-Only in the Prius II
Prius II w/ an EV Button and how its use points us towards PHEV's.
Tim Smith -
CleanMPG.com - June 25th, 2007
General
One of the more commonly discussed topics regarding the current iteration of the Toyota Prius (model years 2004 and on) is the "EV Switch" that allows the driver to operate the vehicle on battery power alone under certain conditions. This is standard equipment on European and Asian market Prius whereas US and Canadian market Prius lack this feature from the factory. There is one caveat, however: the Prius we get in North America may not have the hardware but it does retain the programming. This allows those of us who feel slighted to install our own switch and use EV mode just like the rest of the world. There are several ways to accomplish this:
-Install the Toyota EV switch, available through online sources such as
Sigma Automotive. This consists of the factory button that fits in one of the blanks in the dash as well as the wiring required to hook it up.
-For those who want a less conspicuous installation,
Coastal Electronic Technologies makes a device that allows the cruise control stalk to activate EV mode. Once installed, the driver simply pulls the cruise control stalk for 2-3 seconds to request EV. Hold it for another 2-3 seconds to cancel EV mode.
-For those who want the best bang for their buck, it is also possible to rig up a regular momentary switch to the right computer pins and it will behave just like the factory switch.
Which route is "best" depends entirely on personal preference.
Installation
The installation procedure is fairly straight-forward but depends on which option you have chosen. I happened to choose the Coastal mod, and posted a thread on the topic
here. The long and short of it is that someone comfortable with removing trim panels and tapping a few wires can probably get the job done without too much fuss.
How it works
Perhaps the best known trick of the Prius is its ability to shut down the engine and run on battery power alone at low speeds. (Below 41mph, to be exact.) Operation is limited in this state due to the relatively low energy capacity of the battery and the relatively low power that the electric motor can provide by itself. Under the best of circumstances it takes a rather delicate touch on the accelerator to maintain electric-only operation (often refered to as "stealth mode") and cruising range is limited to no more than a few city blocks on level ground before the battery becomes too depleted. But running without the engine can come in very handy while slogging through very slow traffic, parking lots, or other scenarios where very little power is needed to travel a short distance.
Normally the Prius will only enter "stealth mode" after a full warm-up cycle which generally takes a few miles worth of travel. But the EV button overrides this programming by sending a signal to the hybrid system. Under the right conditions the switch can shut down the running engine or even prevent the engine from starting at all. But this signal is really no more than a polite request, as Toyota's programming will not allow EV operation if conditions are such that the hybrid system or emissions controls would be at risk. The following are a few examples of things that would prevent EV mode from engaging:
-The hybrid battery is too cold or too hot.
-The vehicle is travelling at speeds greater than 34mph.
-The engine has already begun but not completed its initial warmup process.
-The battery is at a very low or very high state of charge.
In addition, EV mode may be cancelled by conditions including the following:
-The vehicle exceeds 34mph
-The driver requests too much power via the accelerator pedal (basically full-throttle)
-Battery state of charge drops below acceptable levels.
In either case, a failure or disengagement of EV mode is indicated by three rapid beeps and a message on the multi-function display. This may be irritating but rest assured that these safeguards are in the best interests of vehicle longevity and minimizing tailpipe emissions.
Having discussed what happens when EV mode doesn't engage, the next logical question is how do we know when EV mode
does engage? The answer depends on where the vehicle was sold. For those European and Asian vehicles that had the EV switch straight from the factory, an indicator illuminates to tell the driver that the vehicle is in EV mode. But North American Prius lack this light. The only real indicator for those vehicles (other than a motionless engine) is that the engagement of EV mode automatically switches to the Energy display so that the driver can see battery state of charge. Of course this doesn't help if you are already watching that screen, so starting off on the Consumption display might be a good habit for frequent EV mode users.
Usage
Using an EV button is a more complicated subject than the uninitiated might suppose. After all, if the engine isn't running you aren't using any gas, so you should do that as often as possible, right? WRONG! Overuse of the EV button can be a very bad thing when it comes to fuel economy. The reason is that the energy in the battery is not free, but rather came from gasoline at some point in time. It may have been recaptured during braking or it may have been generated directly from the engine. But either way going from engine to battery and back to wheels is a very round-about path for energy to take and losses are incurred at every step along the way. All of those steps and losses mean that running electrically actually may be less efficient than just starting up the engine and letting it drive the wheels. So it is not in the best interests of fuel economy to engage EV and run down the battery at every opportunity. But with that in mind there are a few situations where engaging EV mode does serve a purpose.
Perhaps the best use of the EV button is to prevent a cold start, which is the time during which the most smog-forming emissions are produced. (Nitrogen oxides and hydrocarbon levels are highest right after starting a cold engine since the catalytic convertor has yet to reach operating temperature.) So let's say you just want to back the Prius out of the garage and move it a few feet into driveway to give it a wash. Use the EV button! This does not require enough energy for significant battery depletion and you can cut out a session of nasty emissions production. One thing to keep in mind in this situation is that EV mode must be engaged before the engine starts, which is roughly 8 seconds after pushing the "Power" button.
Missing that 8 second window and allowing the engine to start puts the Prius into its warmup phase. For the purposes of this article only a few details about warmup are particularly important. First, it is impossible to engage EV mode during the very first stage of engine warmup, which lasts about a minute unless ambient temperatures are very cold. (That's why it is important to be on the ball if you only intend to maneuver around the driveway without running the engine whatsoever.) Second, warmup continues after that first minute and involves relatively high fuel consumption (for a Prius) due to the fact that the engine won't shut itself down until it has reached operating temperature. Third, the EV switch can be used to mitigate the negative fuel economy impact of engine warmup after that first minute.
The basic principle behind this is to force the Prius to act as if it were fully warmed up, which is to say that you use the engine to accelerate and shut it down while you are coasting or braking. Pretty simple, really. The main difference between techniques is what to do about that first stage. Some drivers, and particularly our Japanese counterparts, swear by allowing the engine to pass through that first stage before moving at all. Battery state of charge (SoC) will wind up somewhat higher due to charging and EV mode will be active immediately, meaning no more unnecessary idling after that point. The other school of thought is that one should go ahead and start driving during initial warmup and just use the EV switch to kill the engine (not to drive on battery power!) as soon as it becomes available. What works best? Beats me, but I have a feeling that a few Prius pilots amongst the CleanMPG community will be looking for that answer in the near future.
Up to this point I have been harping on the fact that the last thing you want to do with the EV switch is to run electrically over any distance more than a few feet. Of course there may be a situation on your daily commute where that is exactly what needs to be done and you can't be entirely sure that a very light touch on the accelerator is sufficient to keep the engine off. (Keep in mind that the accelerator pressure needed to start the engine actually changes with battery SoC. The less charge you have in the battery the more willing the Prius is to start the engine.) The scenario could be extremely slow creeping traffic, for example, or a pair of very closely-spaced stop signs. This is one last situation where the EV switch might be put to use in order to ensure that the engine doesn't come online when you know that starting it would be a waste. Just don't forget to disengage EV when it's time to get back up to speed!
Application to PHEVs
Having discussed the relatively limited circumstances under which EV mode should be used, you might find yourself asking "What's the point?" Indeed, this modification probably appeals to many of us for its "cool factor" more than anything else. But what if it could do more? What if you didn't have the limitation of a battery that only lets you drive a half mile? And what if running electrically helped your gas mileage rather than hurt it? Enter the Plug-in Hybrid Electric Vehicle (or PHEV for short.)
In many ways the PHEV is the next logical evolution of the hybrid vehicle. The architecture is essentially the same with two modifications. One, the battery is much larger with enough capacity to run the vehicle anywhere from 10-40 miles all by itself. Two, the energy in that big battery doesn't come from gasoline but instead from the regular 110V outlet in your garage. Just plug it in overnight and it will be ready to go by morning. Neat, huh?
But it's more than neat. Given the current state of increasing fuel prices, potential supply disruptions, and the political ramifications of burning Middle Eastern oil, PHEVs are downright smart. Electricity in the US is generated primarily from domestic energy sources, occasionally from renewables such as wind, and is cheaper than gas on a cost per mile basis. More and more people realize this and many want to put a PHEV in your driveway today. Even better, options are starting to pop up that would allow you to convert your existig Prius II (or other hybrid, for that matter) into a PHEV.
CalCars.org is one such group of forward-thinking individuals out get your Prius running as a PHEV. Their
"Prius Plus" concept is an ongoing home-brew effort to provide kits that would allow up to 60mi of EV-only range depending on the battery technology used. Other operations such as
HyMotion and
A123 Systems have entered the race as well. Makes that EV switch sound better, doesn't it?
Unfortunately there are still limitations associated with adding more batteries to your existing hybrid. According to CalCars, the 34mph EV speed limit still applies to the Prius. You also have to consider the reduced power available from the electric motor alone vs. the higher output of the gasoline powerplant. Still, the battery's power can be used to augment, rather than replace, the gasoline engine for dramatically improved fuel economy if higher speeds are needed.
Conclusion
So how does this all add up? Speaking as a current Hybrid owner, I can only hope that mass-market PHEVs and PHEV conversions come sooner rather than later. The cost of fuel is only going up and the pain associated with getting that oil isn't getting any easier to bear. The value of driving gasoline-free around town at the push of a button may increase dramatically if these trends continue.
Huge thanks to Marc (mparrish) and Jerad (locutus) for their technical input to this article, not to mention Hobbit whose work has long been the driving force behind much of our knowledge of the Prius II's inner secrets. Another thanks to Wayne (Xcel), editor extraordinaire. 
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