Best speed to engage range extender

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PeterHarvey

Member
Joined
Jun 25, 2014
Messages
18
Location
Durham, UK
The 'Range for speed' discussion showed that as speed increases, the range from the electric motor decreases. So, presumably, on a journey on which one intends to use both the electric motor and the range extender, there is a speed at which it will be more efficient to use the range extender. Any ideas on this? Perhaps it is the 55mph that is the maximum that the Eco Pro+ mode allows?

I realize that the relevant speed may vary a bit according to various conditions, and that driving over a certain speed also become inefficient for the range extender too.

Peter
 
If by 'efficient' you mean maximum range then for pure electric or for range extended the optimum speed is likely to be quite low, probably less than 30mph.

Do you have an i3 Peter? Haven't seen another 'in the wild' yet around Durham.
 
Every other Plug-in Hybrid EV (PHEV) currently for sale is a parallel hybrid, i.e. the gasoline engine in some mode powers or helps power the drivetrain directly, and your question would be more relevant to those vehicles, as there is most likely a (much debated) speed range at which the gasoline engine might be more efficient than at other speed ranges in comparison with electric power.

Even though CARB classifies the i3 REx separately from other PHEVs as a Range Extended Battery Electric Vehicle (BEVx), it is in fact a PHEV - but with a twist. It is the only series hybrid currently for sale, meaning that the gasoline engine does not power the drivetrain directly in any mode - it only powers a generator in order to maintain battery State of Charge (SOC). A gasoline engine in a series hybrid couldn't care less about speed, all it sees is load. Since, generally, gasoline engines are most efficient at high loads, and batteries decrease efficiency slightly under high loads, it would make sense to operate the REx during high load conditions like acceleration, climbing hills, and high speed freeway driving.
 
BrianStanier said:
If by 'efficient' you mean maximum range then for pure electric or for range extended the optimum speed is likely to be quite low, probably less than 30mph.

Do you have an i3 Peter? Haven't seen another 'in the wild' yet around Durham.

Yes, Brian, I meant maximum range. Of course I now realise that the range with the range extender running is still dependent on the range-according-to-speed dynamics of the electric motor. However, the range extender engine will be less efficient at lower speeds, so this remains a factor too. The electric motor has a decreasing range with speed, but the petrol engine is less efficient at lower speeds. There must be a 'sweet point' speed.

I don't yet have an i3 - mine is due in October. I currently drive a Pius plug-in.

Peter
 
ultraturtle said:
Every other Plug-in Hybrid EV (PHEV) currently for sale is a parallel hybrid, i.e. the gasoline engine in some mode powers or helps power the drivetrain directly, and your question would be more relevant to those vehicles, as there is most likely a (much debated) speed range at which the gasoline engine might be more efficient than at other speed ranges in comparison with electric power.

Even though CARB classifies the i3 REx separately from other PHEVs as a Range Extended Battery Electric Vehicle (BEVx), it is in fact a PHEV - but with a twist. It is the only series hybrid currently for sale, meaning that the gasoline engine does not power the drivetrain directly in any mode - it only powers a generator in order to maintain battery State of Charge (SOC). A gasoline engine in a series hybrid couldn't care less about speed, all it sees is load. Since, generally, gasoline engines are most efficient at high loads, and batteries decrease efficiency slightly under high loads, it would make sense to operate the REx during high load conditions like acceleration, climbing hills, and high speed freeway driving.

Yes, my apologies for overlooking the fact that the electric motor still drives the wheels when the range extender motor is running. Your point that the REx operates best under high load is the crucial point, but as this includes 'high speed freeway driving', it seems to me that there is still the question of the speed above which it is best to switch to REx. In my current Prius plug-in, I tend to drive at around 63mph on motorways/freeways, for the sake of a high mpg and low CO2. My guess is that 55 to 63 mph would generally be the best window for use of the REx, in terms of a good range but not driving too slowly.

Peter
(My year-round real average miles per UK gallon is 93- which I think is 74.4 miles per US gallon)
 
My i3 is a BEV, so I have no experience with the REx. What is probably more important concerning the use of the REx, is where and how you are going to be driving. The REx can keep up enough power to run the vehicle unless you are climbing a long hill at high speed, or doing an extended high-speed run. So, since it makes sense to have enough battery power left when you anticipate those high loads, if you turn on the REx earlier to maintain a fairly high battery voltage, you'll not be inconvenienced by the possibility you have not have enough power later on. It is also a good idea to consider using it prior to getting into the city, especially if they have a special fee structure for running electric...you want enough battery capacity left to handle that and not inadvertently engage the REx. The software in the vehicle tries to avoid using the REx at all...it is up to you to decide if it is prudent to engage it early. Keep in mind, that if you do, you may lose all of your cushion unless you can stop to refuel.
 
PeterHarvey said:
... it seems to me that there is still the question of the speed above which it is best to switch to REx. In my current Prius plug-in, I tend to drive at around 63mph on motorways/freeways, for the sake of a high mpg and low CO2. My guess is that 55 to 63 mph would generally be the best window for use of the REx, in terms of a good range but not driving too slowly.
My gas hog is a Plug-in Prius as well, so I get your point. If I were in your shoes, however, I would think more about making the switch to REx based on depleting the electric range. Since the PiP will only give you 11-12 miles of range, that is not usually an issue, and we both probably work harder than we need to in order to make the best of battery and gasoline power over the course of a 100 mile drive. In the i3, if you have a 100 mile drive, and know that you can get 70 miles of electric range on the easiest parts of it, I would recommend running the REx during the 30 continuous highest load miles of it (perhaps the fastest speed against known headwinds?). I'd further suggest you make the gasoline powered miles continuous, as their are efficiency and longevity issues associated with each engine warm-up and cool-down cycle.

As for speed, I would not recommend exceeding 75 mph, as that seems to be about the highest the REx can sustainably power (on average of level ground, zero wind, and normal temperatures) as reported by a few folks on this forum. It also jives with George Betak's Facebook posted measurements of 13.0 kW at 60 mph, 16.7 kW at 65 mph, and 20.6 kW at 70 mph. Extrapolating 25 kW or so for 75 mph, 80 mph would not be sustainable from a 28 kW gen-set.
 
BrianStanier said:
Do you have an i3 Peter? Haven't seen another 'in the wild' yet around Durham.
I was in Durham last weekend for a few hours (parked in Claypath) but didn't see any i3's although I did see a solar orange one in Cullercoats a few weeks ago.
 
Drove. 600km last weekend, seems to be many variables at work here. I noticed that under 90kph the car gained kms from after driving on the highway a long time and after driving on back roads then going to the highway the car lost kms faster. My swag or guess is that the cars possible distance is highly variable and a computer sets between the battery and the user so it will be almost impossible to figure it out without access or lots more data.
 
North American delivered i3s with REx don't have much flexibility on when they can engage it unlike the rest of the world where it can be turned on any time when the SOC is 75% or less. Ideally, you'd never turn the REx on. But, if your trip will be longer than your electric range, it is likely the best policy to turn it on when you are experiencing the highest load such as when cruising down the motorway at speed, or climbing long grades at speed and thus maintaining your total battery SOC up there for situations where you may need or want the full electrical capacity of the vehicle. Also, since those times are also when the tire and wind noise are greatest, you are much less likely to even notice that it is running, not that it is particularly noisy.

Whenever you 'need' the REx, you are defeating a good portion of the efficiency of the total package, and ideally, it should only be used for those unusual situations when you otherwise probably should have considered the use of a different vehicle...it is designed as a city car with limited range. The REx is a crutch for those wanting to make it into something it isn't. IMHO, if your normal use requires the REx, it's the wrong car for you.

Where I live, with my energy costs, I can go about 70-miles on $3.50 in electricity, and could likely go about the same augmented by the REx, but at a cost of double that. The fuel costs in Europe are considerably higher, and the electrical costs, depending on your rate structure and if you can use a 'free' EVSE station, may be more or less. There are some free EVSE units around where I live, but it is not worth the inconvenience of sitting around waiting for it to charge since they are not at places where I'd normally be stopping (and no DC fast chargers around here yet).

SO, since there are places where you can get free electricity, and no place where you can get free petrol, it seems pretty clear to me what your best solution is!
 
ultraturtle said:
PeterHarvey said:
... It also jives with George Betak's Facebook posted measurements of 13.0 kW at 60 mph, 16.7 kW at 65 mph, and 20.6 kW at 70 mph. Extrapolating 25 kW or so for 75 mph, 80 mph would not be sustainable from a 28 kW gen-set.

I don't think your extrapolation for 75 mph is quite correct, as the energy consumption does not increase linearly (isn't it the square of the speed?).
Anyway, 75 mph was not sustainable in my tests. 110 kph (69 mph) - actual, not indicated - was the best I could do while running on gas, flat roads, no load. (Other than my portly 180 lb. self). You can exceed 110, but you would deplete the battery.
 
Drag increases with the square of the velocity...it is not a linear function. Therefore, small increases in speed create significant increases in drag, and therefore the energy required to maintain those speeds. BMW did what it could to make the package efficient - narrow tires, smooth bottom, but it is tall. An artifact of needing to actually hold people comfortably while maintaining a tight turning radius and maneuverability while also maintaining a good sightline. Given that it is designed primarily as a city car, where high speeds aren't generally possible or legal, for those near enough to consider using one as designed, their high speed use won't be a huge proportion of the overall usage. Trying to stretch that for longer commutes or trips puts it outside of it's primary design considerations. That it CAN do it is one thing, that it's better at running around town is another.
 
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