Time over & under 40 mph?
 

Im curious about something, it goes to how to "spec" my hybrid drive system.

How much time to people spend driving over 40mph and how much time under 40mph.

The gauge in the Beemer says my average speed overall is 46.6mphId imagine that that factors all "car running but not moving" time.

If you'd like to answer, it would be a huge help if you said the normal use of the car IE "I drive 15 miles to work with this car, and thats about it" or I'm on the highway on an overnight trip at least 3 times a month".

Thanks!

46.6 is a fairly high average speed. My last 2k miles has been about 38-39 MPH. I am fairly sure it used distance versus time the engine is running to make the calculation, so if you are stopped the clock is still running.

In winter here this car is my sole means of transportation. My trips are usually 20 or more miles average up to a max of 100 locally. Occasionally I will take a road trip of 300 to 400 miles total.

Higher average speeds tend to reduce the effectiveness of hybrid power trains as far as mileage.

regards
Mech

You actually want to compare DISTANCE traveled over and under 40mph for your calculations.

If you grab the spreadsheet linked here, you'll notice that for the LA92 + EPA FTP drive cycles that it uses, 7% of the distance traveled is over 60mph, 25% is over 50mph, and 40% is over 40mph. The data is from epa.gov.

My own commute? It's 2mi in an urbanized 35mph zone, 4mi in a PSL 45 where I can go 50mph, then 3 mi in an awful village with PSL 30. Then there's time I spend coasting down to red lights and stop signs. The ScanGauge says it's 22mph average, but I'll characterize it as 38mph distance-weighted average.

Thanks guys, that confirmed my suspicion that im the highway bound holdout lol.
(outside of 2 mile trips locally, everything is down the interstate for me)

People are far more receptive to data that confirms their suspicions than that which contradicts. People usually make bad scientists and engineers.

I didn't mention the other half of the miles I put on my car a year. Some of it is running errands around town at low speeds, and much of it is rural interstate at 70mph.

Perhaps telling why I wanted to know would help ;-)

Im playing with the configuration of my hybrid.

The Diesel's main role is to serve as a highway speed only motor.

I have some decisions to make with regard to the rest:

1 option would be to build a 10kw pack and "trust" that all of my non highway speed running would be covered on a single charge (Because the gearing of the diesel would render it of little help for speeds under say 50)

option 2 would be to have a seperate gen to recharge the pack while driving (based on the answers above, seems like a needed thing)

One option which I am quite in the dark on is to use a regen scenario on the main drive motor while its being "driven" by the diesel.

Need to do more research on option 3 :-)

Ask 10 people that question and you'll get 10 different answers. You have to look at your own route(s).

In that case, in order to get a 46mph average, you'll be spending a lot of time well over 40mph.
If I hit 46mph average, I usually have spent around 3/4 to 4/5 of the distance on the motorway.

Is the hybrid system worth it when it's only going to be used on short local trips that don't make up too much of your distance ?
You'll have to carry it around all the while.

For me the use of a hybrid system is as much about optimizing the 2 power sources as it is being an EV

The EV drive line will weigh about 200lb in the trike and should yeild an electric only range of about 50 miles. It will be capable of the full range of speed between 0mph and 70mph

The diesel alone will not. It will have only a centrifigual clutch and one gear rendering it useful only for speeds of 50 to about 75

So on the one hand I think of the ev stuff as a "transmission" for the diesel (that happens to be able to drive the car for 50 miles at a clip ;-)

Average speed in my wifes car (She has the gauges) we are lucky to break 30mph over the coarse of a full tank of gas.

So the electric drive is more like a first stage booster which gets you through the most inefficient ICE phase (initial acceleration), then the diesel comes on for the cruise phase where it's rather efficient.

That could work out well overall.

Exactly Euro, well put :-)

What? I thought the acceleration phase was where ICEs have MAX efficiency :confused:

Generally yeah, provided engine speed is high enough. Below ~1k rpms it's pretty bad even at full load, so something like a new Prius will use the motor up to ~5+mph no matter what IME, but even then that depends on gearing. A parallel hybrid like Honda's stuff still has conventional gearing so 1k rpm for the engine is pretty much right off the line.

They certainly don't have max fuel efficiency during acceleration.

I beg to differ.

You see, when accelerating, what- 10x? more work is being done per unit of time vs steady state cruising.

It has been well established that ICEs are at max efficiency i.e. doing the most work per unit of fuel burned when they are at or near WOT... NOT when the throttle is nearly closed and they are loafing along.

They are at or near WOT when accelerating, and the throttle is nearly closed in comparison at steady state cruise.

So it stands to reason that in spite of the fe gauge showing the lowest mpgs during accel, that is because the amount of work being done per unit of fuel is not displayed. And it's lower because of being in lower gears.

Then when you look at electric propulsion you see that acceleration almost disproportionately draws the batteries down, while if the vehicle was to be accelerated by other means and the electric drive used only for steady state cruise, the range would be extended enormously. This premise has been affirmed to me just about every time I use my electric bicycle- when I pedal a bit whenever acceleration is desired it MAJORLY saves on the batteries.

So.... for max efficiency I think ICEs should be used for acceleration and electric should be used for cruise.

The reason P&G works is because most engines are too lightly loaded for optimal efficiency at cruise. So, yes, engines do have their peak efficiency at low RPM, high throttle conditions.

However, this just means that most ICE are drastically oversized (and overgeared) for cruising. If you have a reliable source of electric assist, you can equip the vehicle with an ICE that's just large enough to cruise with, and use the electric (which has a very flat efficiency curve) to accelerate. This is how hybrids work, and they do work.

This thing about "electricharger" motor assist seems exactly backwards to me, and doomed to not provide optimal results.

We measure fuel efficiency as amount of fuel used to cover a fixed distance.
(Or as you do, the distance you can cover with a fixed amount of fuel.)
No-one is really looking for or even interested in work done per unit of fuel when driving a vehicle.

Any OBC or ultragauge / scangauge / mpguino will show you that during the acceleration phase , FE is really suffering.
But then, you know all that ...

You haven't picked up what I'm putting down....

ICEs are most efficiently converting fuel to work during acceleration EVEN THOUGH that is when MILES per gallon are the worst.

Prius has a 78 HP ICE. If the cruise hp requirement is, say, 15 hp, why doesn't it have about a 25 HP ICE (more than 15 to account for some of the smaller transient load increases like winds and hills, and adding in a duty cycle of less than 100%)?????

Insight I has a 67 hp ICE and a 6-13 hp electric. Shouldn't those power values be reversed?

I think both Frank AND some of the others are right.

If an ICE is "specked" to provide acceleration hp as well as cruise hp, cruise mode will, by design be quite restrictive and lower bsfc.

Also, HP for HP The hybrids available are weighted more toward the ICE side of the equation, going back to the needing to be "specked" for the ICE to both accelerate and provide cruise power.

Theres one thing I'm sure of, Electric motors shine at providing low end torque and suck at providing torque at the top of their curves.

The tweak Im kind of excited about is in the way that someone suggested I set mine up, avoiding a LOT of inneficiencies:

Use 2 drive sprockets on the electric motor, one leading to the wheel, one leading to a centrifigual clutch on the diesel.

for low speed (45 and under) its an electric car.
above 45, the deisel comes on and stays on at WOT. Actually about 80% of full output, its best bsfc sweet spot.

Here is the fun part, because the diesel is specked at 125% of the highway requirement, when the car is being "pushed' by the diesel which is running at a constant speed and output, one can modulate regen on the controller to controll speed.

so rough numbers:

50mph flat ground hp requirement to move car 4hp , about 5hp goes into pack via regen.

70mph flat ground hp requirement to move car 5.5 hp about 3.5 hp goes into pack via regen,

60 going up a steep hill requirement to move car 9hp. All engine power (and all electric power for that matter) goes to moving car, no regen.

Naturally, one could use electric power to cruise on highway as well and I know I would till the batteries were sufficiently discharged, say a nice battery preserving 70% soc.

No this is essentially dialing in regen to control speed. it would be super cool if someone could help figure out how to use a gps's signal for example to talk to a cycle analyst or other multi gauge to basically take over the regen dialing upping and downing as a sort of cruise control :-)

Specked, like eggs? :confused:

No no no no no... you do NOT want to govern speed with "regen"! It is not efficient.

specked, perhaps im not spelling it right, as in specifications meant for.

Id love to learn more on your opinions of regens efficiency, I am aware of the regens function of putting amps into a pack based on occasional braking, not too much upside there.

But I have the Sevcon people telling me that their brushless motor controller is as efficient on the regen side as it is on the drive side (High 90s)

That the Sevcon has 90% eff for regen may be, but that is not the efficiency of the whole system. Brake regen is reclaiming some portion of energy already expended in getting up to speed that would otherwise be completely wasted- a good thing generally, but some of us have habits and conditions that allow such gentle braking that the penalty for paying for and carrying around the regen equipment is greater than the payback.

I think it is safe to say that your best bet to keep the batt pack charged is via plugging it into a wall outlet- money-wise, fuel efficiency-wise, emissions-wise, all that.

Oh- "spec'd" is short for "specified".

Agreed on Regen, and how to best keep the pack charged.
The ICE is for when I need to go further then that ;-)

I know a loaded ICE gets more efficient when you look at the specific power output.
But as it is, it's the mpg - the fuel used - that really counts.

Look at the EPA car guide.
Highway mpg are about 50% higher than city mpg (except for hybrids).
So it makes sense not to use a big ICE for the most fuel consuming phase of the ride.

Surely the acceleration phase is also a real drain on a battery in an EV.

Yes, though I doubt Toyota or Honda dared to do that when they launched their hybrids.
Instead they opted for a concept with rather minimal changes to normal car designs.

If they had dared, then essentially they'd have been building an EV with a tiny ICE cruise engine / range extender that never powers the car directly.
Buyers may not have been ready for that.
Probably still aren't.


"What about the engine ?"
"Oh, I went for the sports version, it has the 0.35 L engine !"
"Dude, every cilinder on my car is bigger than your entire engine !"

Yes, but the Insight's battery is too small to get it over a mountain. That being the case, the ICE has to be sized to handle a continuous hill climb, which requires around 50HP.

Honda's proposing to put a 160HP electric motor and a 2.0L Atkinson engine in the Accord PHEV. I'm sure it'll sell about as well as the first Accord Hybrid, because its design philosophy appears to be the same: add electric drive, go faster than before. Too bad go-fast enthusiasts don't care about fuel economy, and vice versa.

Should have been a 60HP electric motor and a 1.0L Atkinson engine in a Fit or second-gen Insight.

No, the specific power output is what counts. Specific power output times road load equals fuel consumption, and road load isn't going to change unless you change tires, vehicle mass, or aero.

I'm at a loss as to how to rephrase it again so we can cross this gulch. I know what you are saying but I don't think you're hearing me.

Acceleration requires multiple times more energy than cruising, we can agree on that.

What Frank is trying to say is the IC engine is operating at higher BSFC when accelerating.

It may be better understood by adjusting the road conditions to the ideal state.
You climb a grade using the highest gear and then coast downhill with the engine off.

This is essentially pulse&glide (engine off glide). You can see this when you have an instant fuel economy reading. Slight grades reduce instant mileage fractionally, while the potential energy state is increased by elevating the vehicle. Then that same energy is used to coast downhill with no fuel consumed.

In climbing a grade, your ideal would be at best BSFC in highest gear. The increased efficiency translates into increased elevation, which is stored energy. Like the roller coaster which travels most of its track without power the benefit is distance travelled without any additional energy requirements.

Understanding this, it should be easy to understand that constant speeds where lower power requirements are normal, is precisely where the throttled IC engine is least efficient, even though the mileage may seem to be better (and it is) the load on the engine is so low that efficiency is terrible.

Build a vehicle with 4 separate engines that are each 25% of the size of the original engine. Use all 4 for acceleration and only one for cruising and you mileage would be much better, because you could optimize the BSFC for the conditions by using 1,2,3 or 4 depending on the loads applied.

If your 60 MPH sustained load is 15 HP, you need a max 25 HP engine for best BSFC, but then you would not have any reserve power for sustained grades or strong acceleration.

Car engines are designed for the worst case scenario so they have the power reserves to provide acceleration or grade climbing capabilities, but that same over sized engine operates at less than half its maximum efficiency when it only needs to produce 10% of its maximum power, because the air intake is restricted and the actual compression before combustion is less than half of the same compression when there is no manifold vacuum.

regards
Mech

I understand, there needs to be power to accellerate and climb hills.

On a highway however the power demand hovers around x HP, my plan, and others can do what they see as best, is to optimize a 1 speed diesel to cover cruise needs and electric to accellerate from a start and cover increased cruise needs (like going up hills)

as well as use regen more as a "gen" to recapture diesel hp back into the pack.