EV switch theory question

The EV button degrades system efficiency, and therefore fuel economy. Period.

In this car, all energy comes from gasoline.

The most efficient sequence is gas>engine>wheels. However, the Atkinson-cycle engine has unacceptably low end torque, so Toyota adds an electric motor (max torque at 0 rpm) to compensate.

The next most efficient is gas>engine>generator>battery>motor>wheels.

The least efficient is (anything to get to speed), then wheels>generator>battery>motor>wheels. We call that regen, and it's 25% at best.

If you accept my claim that engine alone is the most fuel-efficient drive system, then it's obvious that using the EV button (which requires that the engine replace energy lost while EV-ing) is less efficient.

The most efficient driving style appears to be brisk acceleration to speed, followed by long, l-o-n-g coasting stops - using kinetic energy to cover distance rather than to recharge the battery. Of course, it's a good idea to avoid road rage behind you...

 

In very specific circumstances, I've been able to raise my MPG using the EV mode. In less-than-ideal instances, it has lowered my MPG.
Cases in point:
best use is if u have a slower starting speed in the beginning of your AM commute, with a downhill portion coming AFTER the ICE is forced to operate due to faster speeds. This allows the ICE to charge the batteries you've just spent, but it's at a time when the gas engine is it's most effiecient (downhill and moderate speed 40-50 MPH).
Worst use is trying to drive slightly uphill on electric only. The car is just not able to do that very efficiently. If I do happen to try to drive uphill in EV mode, I do not get as far. If the gas engine has to kick on and charge the battery as well as drive the wheels uphill, then it is usually at 20 MPG!
So, it's really a judgement call. I don't deny that ultimately the gas engine has to charge the battery. Nothing in life is free. Love my Prius!

 

Eisenson is mistaken when he asserts that the EV switch always degrades system efficiency. What he says is generally correct. But there are a few limited situations when the EV is more efficient, and a few situations when it is desirable for reasons other than efficiency.

As for cost, a DIY installation costs only a few dollars. A pin, some wire, maybe a connector or two. If you are less-confident, or you prefer the cruise-control-stalk version over the light-flasher-stalk version, you can buy the Costal vesion for $45.

Okay, situations when Eisenson is wrong:

1. The gas engine does not propel the car during the first minute or so of operation. It is busy burning gas to warm itself up. If you are going to drive less than a minute you are better off using EV. Mileage continues low for the first 4 or 5 minutes, so if you are driving just 2 or 3 minutes, you're still better off in EV. Your battery will be recharged by the ICE the next time you drive, and your efficiency gain results from eliminating one (wasteful) warm-up cycle

2. If the SOC is high and you are approaching a downhill, you will gain very little regen. With EV you can go 1/4 or 1/2 mile on electric only, and recapture it all via regeneration on the downhill.

3. This may be disputed, but I have noticed that in winter, my SOC remains very high, so that little or no charging can take place during the warm-up. I believe that by using EV for the last 1/2 mile, and leaving the SOC low, all that energy produced during warm-up will have somewhere to go.

These are not big items. The improvement will be small. But they are exceptions to Eisenson's general rule.

There are also times when it's nice to have EV even if it's not efficient, such as gliding along in the parking lot to impress the pedestrians. But even that may be more efficient than running the ICE at 5 mph. Even the Prius engine is not efficient at very slow speeds. I think you are better off in EV, and then recharge when the engine is running in its efficient range.

However, it will greatly lower your mileage to be constantly and indiscriminately using EV.

 

Ok I have a question. When I am at work some days I have to very quickly go from one location to another. It is between a half mile and one mile. Getting from one place to the other fast, allows me to simulate being in two places at once. It is too far to walk and I can't run bad knee and back. I have to drive to do it fast. Would the EV switch be appropriate here? It is much too short a distance to get the car warmed up so no matter what if I use the ICE it will be inefficient. Some of you who know more on the theory and practice help me figure this out. Does this make sense? Will it harm the car?

 

hdrygas:
i dispute your claim that one mile is much too short to warm up the car.

i track this nearly every morning because of my short distance to the freeway.

in the past week or so, the morning temp has averaged in the mid to low 60's and my car is warmed up in slightly over ½ a mile.

i was told by someone who got their car last nov that the warmup in the winter isnt really much longer (realize that we do not get severe weather here. the average overnight low in the winter is about 40-45º and 50º is more common)

as for ev mode for a mile. if you battery is charged, it is advisable because although your car is warmed up, unless you plan to drive the car again right away, you are wasting the warmup cycle as Daniel said. but realize that EV mode is only good for a very limited distance.

it would be conveinent in places where large hills are present. but to be honest with ya,

you are familiar with 4th ave in downtown Olympia. that hill is no where max'ing my SOC most of the time. in fact only a few times have i filled all the bars.

i question whether trying to outthink the computer as far as EV mode goes is wise, but for short trips, it definitely does increase the overall mileage of the tank.

and Evan... if you lived in the Pacific Northwest, you never would have suggested a segway!!

 

really... i was in Delta Junction... (Ft Greeley) for 2 years. loved the place!! wish it was a bit cheaper to live up there.

as far as the weather goes here... i ignore it. i ride my bike for all trips (as long as its not snowing or dark) under a mile no matter what the weather... and despite the fact that i live in WA St, i do not own a rain coat or umbrella.

 

Driving a mile in EV will probably take 4 bars off your SOC. At 2 bars the ICE will kick in. So you'd probably need 7 bars to start, which seldom happens. If the drive is half a mile and you have 5 bars, you can probably do it. Of course, terrain makes a big difference.

Whether or not you should is another matter:

Driving with the ICE is inherently more efficient than driving in EV mode. If your engine is cold, and you have enough charge, I believe using EV is beneficial because it avoids the warm-up. But if your engine is warm, then I'd advise against using EV.

Also worth noting: you cannot accelerate at all hard in EV. If you are in a big hurry you might prefer giving up any very small benefit in order to be able to drive normally.

 

Great replies! I will give it some thought. I might install a EV switch and experiment. Sometimes this happens many times sometimes once a day, sometimes never.

 

The more often you make the drive, the warmer your engine is likely to be, and the less benefit you are likely to see from using EV.

However, the EV switch is really cool, for those times when you are telling a passerby about the car, and then you get in and drive away in near-total silence. Then you disengage EV as soon as you are out of earshot. Also impresses the pedestrians in parking lots.

Maybe the biggest reason for the switch is the geek factor rather than the efficiency.