Short trips effect on efficiency

Short trips effect on efficiency

Hi all,

I make a lot of short city trips, 1-3 miles. Then park the car for more than a few hours. When I do this, I get mileage of under 20mpg. When I make longer trips (more than 5 minutes), I get 25+ mpg.

This is because it takes a while for an ICE car to warm up, and the initial part of any trip is much more inefficient than after the warm-up. Affecting my observations are other variables (longer trips tend to be more constant, higher speed travel, like highway versus city, etc), but in general ICE cars require a warmup.

Does this apply to the Model S? For example, would ten 3 mile trips separated by three hours in between, result in a much different efficiency than a single 30 mile trip? Assuming the same conditions (speed, stop and go profile, etc).
- Tony

stephen.kamichik | May 24, 2012

Probably not.

jbunn | May 24, 2012

Cabin temp control plays a factor the further the ambient is from desired. In the scenario, you adjust the cabin temp 20 times, vs once for the longer trip, so yes.

Unlike ICE, youll have good efficiency in stop and go traffic because you dont idle, and you have regen.

jerry3 | May 24, 2012

- Does this apply to the Model S

No. It really doesn't apply to any EV because the motors don't have to get warm to be efficient nor is there a catalytic converter which has to be warmed up.

Brian H | May 24, 2012

I think slow stop-and-go is almost the most efficient mode for an EV with good regen. Far better than highway, certainly.

EdG | May 25, 2012

If you look at you'll see you get the best range at about 20 mph. The "cost" of running the car stopped (HVAC, etc.) is higher per mile at lower speeds, and air resistance eats into efficiency at higher speeds.

That's why stop and go driving gives you the best range - because it's slow, not because of regen. The regen makes it so your variation in speed - stop and go - doesn't cost as much as in an ICE where you send off all your previously used accelerating energy as heat every time you stop. With regen you still lose, but only a fraction of the total energy put in for each "go".

So your 1-3 miles of driving will be great in an EV, and you'll really get fantastic range if you don't turn on the AC.

Brian H | May 25, 2012

comparing apples to apples, slow with AC is still better than fast with AC, and better than ICE with AC. So if you need AC to keep from melting and/or frying ...

thwang99 | May 26, 2012

Thanks for the comments, this is what I was thinking too. But yes, about the AC, for shorter trips the initial energy cost of the AC will be higher, there's no getting around that.

jerry3 | May 26, 2012

If you pre-cool the car before starting the AC use for short trips won't be different than for long trips. Yes, you'll pay for extra electricity used before you start, but you won't pay with range. Of course, range isn't much of a consideration on short trips :-)

Also depending upon how many hours you drive your car, it may make sense to have it kept at a constant temperature rather than letting it heat up and then cooling it. (Probably not, but it's something to consider)

thwang99 | May 27, 2012

Jerry3, yeah forgot about the pre-heating/cooling! Does anyone know if you can pre-heat/cool the car when it's in a parking lot not plugged into anything? I know it will reduce range, but on a hot day, it will make returning to the car much nicer.

Teoatawki | May 27, 2012

Yes, yes you can.

Volker.Berlin | May 28, 2012

thwang99, "mileage" of the Model S is in an entirely different category than any ICE. That's the upside of the "battery problem": Since it's so hard to store large amounts of electric energy in a form that can be carried around in a car, Tesla invested just as much into making the Model S a genuine hypermiler, as into the battery itself. Since range won't be an issue with your short trips, even if you have many of them throughout the day, the only thing to worry about is the "cost to fill'er up".

This cost is in a different ballpark than filling up an ICE. So much so, that a slightly reduced mileage due to short trips is of little importance. Actually, that's where electrics and hybrids excel. Driving a hybrid like the Prius or the Volt on a long trip does not gain much, it may actually be worse than, say, a VW TDI. It's the short trips where the electrical advantage kicks in with full force.

jerry3 | May 28, 2012

- Driving a hybrid like the Prius or the Volt on a long trip does not gain much, it may actually be worse than, say, a VW TDI

I've had both (Prius and TDI). No way will a Prius do worse than TDI on highway driving. I don't know where people get such wrong ideas from.

The last few trips.
--- Trip to NE starts here
06/13/09____95529____464____57.9 (4.1)
06/18/09____96021____492____54.7 (4.3)
06/21/09____96557____535____55.0 (4.3)
06/27/09____97073____516____57.2 (4.1)
--- Trip to NE ends here

--- Trip to NE starts here
--- Four hours of hard rain
10/09/09____100520____569____63.9 (3.7)
10/09/09____100967____446____51.3 (4.6)
--- It snowed here
--- Trip to MO starts here
--- Five people plus luggage
10/16/09____101312____344____51.4 (4.6)
10/17/09____101759____447____53.8 (4.4)
--- Trip to MO ends here
10/18/09____102219____460____54.7 (4.3)
10/28/09____102716____496____58.5 (4.0)
--- Trip to NE ends here

--- Trip to NE starts here
08/13/10____111690____625____59.8 (3.9)
08/14/10____112308____618____60.0 (3.9)
08/20/10____112972____663____64.2 (3.7)
08/22/10____113411____438____58.9 (4.0)
08/31/10____113922____510____61.8 (3.8)
--- Trip to NE ends here

--- Trip to NE starts here
01/07/12____128603____481____56.6 (4.2)
-- 13 F here
01/12/12____129042____438____52.7 (4.5)
01/15/12____129420____378____50.3 (4.7)
01/20/12____129094____481____56.2 (4.2)
--- Trip to NE ends here

Volker.Berlin | May 28, 2012

jerry3, very interesting. Thanks for setting that straight!

The question then becomes: Why/how does the Prius do better than the TDI on highway driving? In city traffic I can understand that the Prius can go short trips on electricity, and in particular, that acceleration from stand-still is handled by the electric motor, which is a massive gain b/c that's where an ICE gets particularly bad mileage. On top, regeneration helps with having the juice available when you need it (at the stop lights).

But on the highway (apart from stop-and-go traffic, that is)? If you go straight at higher speeds, there is not much of acceleration/deceleration going on that the electric motor/regeneration could be used for. Propulsion of the car relies entirely on the ICE in this situation. Why would a Prius perform better than any other (comparable) ICE on the highway?

Are you comparing apples to apples, i.e., latest generation Prius with latest generation TDI? Just wondering.

EdG | May 29, 2012

Isn't it true that hybrids get most of their benefit from using the smallest possible ICE? That way, when going at constant highway speeds, you're using your ICE at its peak performance, not wasting the cylinders as bigger ICEs do. My understanding is that the electric motor on a hybrid (not the Volt) is there for the acceleration to help get up to highway speeds.

The low speed use of the electric motor and the regen work to make the package as efficient as possible, but the main design criteria are the minimal ICE size to keep it going and the necessary electric motor size to make it a useful car.

So it doesn't surprise me that going at constant highway speeds a Prius does well.

jerry3 | May 29, 2012

Volker asked: Why/how does the Prius do better than the TDI on highway driving

The reason is that the engine is sized for steady state driving rather than for acceleration--that's what the motor/generators (MG) are for. In addition:

1. The Prius uses an Atkinson cycle engine which eliminates most pumping losses at part throttle.

2. The engine RPM is only loosely coupled to vehicle speed so the engine can run at the most efficient point in most instances (and shut off or just spin when the power demand is very low).

The Prius engine almost always runs between 34% to 38% efficiency compared to 20% on an old fashioned car.

- Are you comparing apples to apples, i.e., latest generation Prius with latest generation TDI?

I'm comparing the ones I had personal experience with: 2004 Prius (second generation) vs. 1998 TDI. Of course, it's possible to make up any number you want with cars that you haven't personally driven, just like EV-gibberish :-) However, I've never heard that the TDI in later years was all that much different than the one I drove.

- Propulsion of the car relies entirely on the ICE in this situation.

That is not actually true. The Prius is always using electric power to some extent (and if you "punch" it, MG2 will add its 50 hp or so). On a steady highway drive the small MG1 is powering and the large MG2 is generating. Some people call this heretical mode (and it took quite a while before this behaviour became accepted--at first a lot of people said "Noooooo". )

A curious fact is that the Prius only moves because of drag from one or the other of the electric motors. Think about the Prius power split device (PSD) like you would an open differential car with one drive wheel on ice and the other on pavement. When you rev the engine the wheel on ice will spin, the wheel on pavement won't, and the car won't move forward. So you get out and put some sand on the ice so that the other wheel will gain some traction. The power split device works the same way. One of the motors (and optionally the engine) spins and the other is the "sand". They always work in opposition to each other, but they can change personalities depending upon the circumstances.

For a much better explanation than I can give in a few sentences, go to the URL below. While it only describes the 2001 Prius, the principles are the same in every Prius--only the values and a few minor details change. The "Understanding the Prius" link in the upper left corner is where all the gory details are. There are even some diagrams.

- In city traffic I can understand that the Prius can go short trips on electricity

To get high mpg in the non-PIP, you want to be very frugal in your use of the battery. The best way is to think of the battery like a credit card: If you use a credit card sparingly then it can even out the cash flow over the month. If too much is charged, it can't be paid it off (or at least I can't pay it off) and then high interest charges are levied (the ICE runs a lot in the Prius to charge the battery). In the PiP, the additional battery is like having your rich uncle Bob pay off the balance.

Volker.Berlin | May 29, 2012

jerry3, thank you for taking the time. That's a very nice explanation, enlightening!

Of course, it's possible to make up any number you want with cars that you haven't personally driven, just like EV-gibberish :-)

I whole-heartedly agree. It's still interesting to know which cars precisely you were talking about/have experience with. Thanks again.

alexander.schechner | May 30, 2012

as you are THE Tesla Expert.
Do you have an idea when we are going to get the Model S in Germany ?
My reservation # is P375?

Norbert.Vienna | May 30, 2012

in late summer or early fall 2012 will be cars available for test drives for reservation holders
delivery of your car in about one year
since they start 2013 with europe first with the signatures etc

Volker.Berlin | May 30, 2012

Alexander, I agree with Norbert's estimate.

When I talked to Stephen Davies, Tesla Motors EU Inside Sales, at the Geneva Motor Show back in March, he was very careful not to be specific with regard to the estimated arrival of the Model S in Europe. All he would say was "early to mid 2013", with the Options & Pricing being released around half a year early, i.e., late summer/early fall 2012. I hope he is being overly defensive with his delivery estimate, but that's all he would say. In any case, since there are far fewer Signature reservations in Europe and production will already run at a considerable level (albeit still far from full blast), production models can be expected to follow only shortly after the first Signature deliveries.

You may want to start a thread in the Europe section of these forums to see if/when someone else has more information:

Ulm is my birth place, BTW.

Volker.Berlin | May 31, 2012

One more follow-up wrt delivery in Europe. It seems obvious to me, but may be worth mentioning: We will get more precise information as soon as the US Sigs are produced and Tesla gains some confidence in their production line and product quality. Currently, there are still many unknown on slightly risky factors, and if any problems pop up, delivery to outside the US will be delayed. The upside is that cars delivered to Europe can be expected to have grown out of their teething problems!