Tire Size vs Economy

Where did he say that Prius has a manual transmission? I don't see it.

My household has had plenty of cars with fuel injection, but has never been offered a gasoline car with direct fuel injection. It has always been indirect, injected outside the combustion chamber.

Let me try another summary --

Fuel economy in most traditional cars will benefit from taller tires because:
(1) their engines suffer considerable 'pumping loss', which gets worse at lighter loads and higher RPM;
(2) their overall transmission / final drive ratios are geared for performance, not economy, causing their engines to spin at unnecessarily high RPM.
Taller tires reduce engine RPM, which reduces this fuel-robbing pumping loss.

Prius short-circuits this fuel waste both at the engine stage, and at the gear ratio stage. Therefore, changing tire diameter won't produce the same fuel economy change in a Prius that it could produce in a traditional non-hybrid.

Is this any more clear?

 

Direct fuel injection engines in a gasoline Otto cycle engine engine still have an intake manifold and throttle butterfly.

A gasoline direct injection engine can be run much leaner, so more air for the same amount of power requires less throttle opening. It still needs a throttle though.
HowStuffWorks "Direct Injection Engine Vehicle Makers"

Even diesels, which rarely have throttle plates still have intake manifolds.

 

You'll still loose energy on bumps. More so with stiffer tires/suspension. (this is seat of the pants impression from descending on mountain bike with locked suspension)

 

Your explanation is basically correct except for one little detail. Actually it's not an automatic transmission with infinite gear ratios. It is a summing differential that provides infinite output speeds but at the same gear ratio. The engine has a single gear ratio to the wheels and the electric motor has a single gear ratio to the wheels. However, the motor puts out it's peak torque at stall so it does a great job of providing torque at low speeds. The engine puts out it's max torque at around 3500 to 4000 rpm (I'm guessing here, but it's probably around there), so combine the two and it's a darn good drive system. The transmission is a summing differential. A summing differential takes two inputs (ICE & second motor/gen) and sums them to the output (wheels). The second motor/generator can be used to trim the speed of the output. It can either add speed to the output or it can subtract speed from the output (it rotates the opposite way to subtract speed). The summing differential can also take power from the ICE to drive the second motor/generator as a generator. They use the second motor/generator to trim the speed in order to keep the engine in the efficient speed range. The large traction motor drives in parallel with the output from the summing differential.

It's a very unusual drive system that works extremely well. I don't know who invented it, but that person was brilliant!

 

Because nothing has changed with regards to the ICE other than it takes more HP to turn the larger/heavier wheels.

 

I have a ScanGauge and one of the gauges I have it set to display is rpm. When I'm going at say 1000rpm and stomp on it, the rpm kicks up to 3000-4000. IOW, it acts like a CVT. At lower speeds, I can get it to display 0 rpm while the car is moving, or 1000rpm if I step on the gas so that the ICE is running.
I think it would be more accurate to say that the engine has a single gear ratio to the transmission, no??

 

It is a CVT, some people are hung up on what type of parts are in a transmission. What counts is what it does. One significant thing that it does is provides a continuously variable engine speed to output speed ratio at a high level of efficiency. The high efficiency qualifier is there to eliminate torque converters, slipping clutches, etc.

If it were a mysterious box sealed filled with pixie dust that accomplished the same thing, it would still be a CVT.

It also has the ability to add or subtract electrical power from the transmission.

 

Well, it produces a transmission function, with continuously variable effective ratio over a wider range than standard devices. And you still called it a transmission.

If it walks like a duck, quacks like a duck, and tastes like a duck, ...

 

I clearly stated that it is a fixed gear ratio single speed transmission. I said that the electrical path provides for essentially infinite gear ratios, which it does. To be clear, I should have said "provides for essentially infinite *effective* gear ratios".

Perhaps I should have written yet another exhaustive technical treatise on the PSD, but we have plenty of those, several of which I have written. In this particular case I was making a simple point, namely that the transmission is effectively a CVT, but has only one fixed set of gears. Additional detail is sometimes an impediment to understanding.

Tom

 

I agree with cycopathic that narrower tires tend to
give better MPGs. When a tire's contact surface
with the road is smaller it lowers the tire's overall
rolling resistance (for any given tire material
being used). Aptera takes this concept one step
further by reducing the number of wheels from four to
three inorder to lowers their tire's rolling resistance.
However, the effects of lower tire rolling resistance
only becomes signficant if the Prius is allow to coast/glide
frequently ( downhill, in a pulse-n-glide driving session,
or when slowing down to a stop). You can also increase
the tire pressure to its maximum sidewall pressure (which
increases its rigidity) to decrease a tire's rolling
resistance OR you can just get Low Rolling Resistance
Tires. However, narrower tire is an engineering compromise
since they don't give as much road grip as wider tires --
that road grip comes in handy in high speed turns...

An increase from 15 inch to 17 inch wheel will introduce
a slight difference in the odeometer readings if the car is
not recalibrated for the change but it shouldn't signficantly
affect the car's actual fuel efficiency .

However, the weight/mass difference between different
tire-wheel combinations could affect a car's fuel efficiency in
certain conditions; more specifically, a lighter weight/mass
tire-wheel combination should give better fuel efficiency
than heavier weight/mass tire-wheel combinations when at very
low acceleration rates. Why? Because the weight/mass of a
tire-wheel combination has a rotational mass which the
car's must exert force on. If both tires have the same
rolling resistance/road contact surface, then the force
required to turn a heavier tire-wheel combination should be
more than the lighter tire-wheel combination. If one
assumes (this is a big assumption) that the wheel is
frictionless then all the force required to turn the
wheel on an even flat road is caused by acceleration.
This force increases by the square of the acceleration
rate so the faster the acceleration rate the less
signficantly the rotational mass is to the turn the
wheel but on the flip side is that if one accelerates very
gently then the rotational mass starts to become more
of a signficant factor in fuel efficiency. How slow would
that have to be? I'm not sure - but I'm sure most
would run out patience or time before they could
figure it out.

hope this helps

Walter
2010 Toyota Prius, Blue Ribbon/Dark Grey, oem floormats
Yokohama Avid S33 (psi 50/psi 48)
100% grill blocking
ScangaugeII (FwT, SoC, RPM, GPH)
Odeometer=16500 miles/ overall 60.4 mpg
Last tank 727 miles/10.25 gallons/70mpg/E10, mainly logged via 16 mile stop-n-go urban commuter routes (+60min one way) in the Washington DC/MD/NoVa Area in the month of September driving average speed = 17mph

 

Walter, I also wanted to point out that I observe lower mpg at highway cruising speeds when running my 17" tires. The effect gets worse the faster I travel. I have not been able to nail down any specfic numbers but it appears that it takes an extra 3+ HP to maintain speed when compared to my lighter and narrower 15s.

I totally agree that the largest difference in mpg is observed by those who glide or coast often but even those who maintain long term highway speeds will see quite a drop. My commute is approx 98% freeway at 60mph-65mph unless I hit the backroads. Using the exact same techniques I acheive approx 47mpg-49mpg with the 17s and 55mpg-57mpg with the 15s. My fuelly records display a larger gap due to a couple really low tanks after high speed and agressive driving with the 17s. Eventually these numbers will settle out and I think my overall loss is around 7mpg. This jives with my earlier experience of a 6mpg loss using different 15" and 17" tires but the 15s (Integritys) were less efficient than the new dB tires.

 

Yes, it acts like a transmission, but it only has one gear ratio so there is not a varible effective ratio, just a variable effective ICE speed. You are 100% right, it still works like a tranny by allowing the engine to operate at a more efficient speed.

 

Just a misunderstanding. I guess I'm a purest and don't agree with the statement of infinite "effective" gear ratios, it just has 1 gear ratio, nothing more. No big deal. I have worked with a summing differential a few times in my line of work, it is a very interesting device.

 

Your explanation works for standard fixed gear transmissions. The Prius doesn't work that way and will change engine rpm as needed regardless of the tire size.

I've also noticed that it takes more horsepower to turn the heavier wheels. I have not tested this extensively but casual observation of the Scangauge shows that to maintain speeds of 60mph or more I am using over 15HP. In fact it is closer to 19 HP where the lighter 15" tires only require 12-16 HP to maintain the same speed.

U nfortunatelythe larger diameter tire does not reduce rpm and improve mpg like a regular non-CVT vehicle.

 

I don't understand your question.

In a traditional fixed-gear car, tire and engine RPMs are linked. And the engine spins much faster than necessary for steady cruising. And the engine has a strong RPM-related energy loss mechanism. So changing tire diameter / RPM also changes engine RPM, directly changing fuel economy.

In a Prius, this particular loss mechanism has been quashed at several points. Tire and engine RPM are decoupled. In a steady cruise, wheel RPM is set by speed and tire diameter, just like old cars. But engine is controlled to the minimum RPM necessary to produce the needed power. The eCVT is not fixed, but adjusts its effective ratio to match the engine output to the tires. If you change tire size in a way that doesn't change power needed for propulsion, then the eCVT merely changes its effective ratio. Engine speed won't change, and neither does fuel economy.

But there are also other tire-related loss mechanisms, such as rolling resistance and air drag. As F8L points out, his wider performance tires do change these factors -- and probably driver behavior too, due to the fun factor -- and fuel economy suffers.

 

Thanks you for explaining that better than I could, fuzzy.

In my particular situation I switched from a lighter, narrower, smaller diameter tire with less rolling resistance to a heavier, wider, larger diameter tire with higher rolling resistance. The net loss of mpg is due to each of the traits I listed. The only tricky part is the smaller tire causes my speedo to run approximately 2 mph too fast whereas my taller tires are nearly dead accurate. This means that I travel, on average, 2 mph slower with the smaller tires than the taller tires. In this case the tires did not affect mpg. It was my driving at a slower speed that affected mpg.

All of the other traits I listed reduced mpg with the taller tires because they simply require more energy to turn. Whether that energy is in the form of HP from the ICE or stolen kinetic energy which reduces the distance one can glide the end result is reduced mpg.

FWIW, if I drive the same route and with the same conservative techniques my 15" tires net 55-57mpg and my 17" tires net 47-49mpg.

 

So I recently relaxed the factory tires for my 2010 Prius v (bigger size) with fierce ZR. the mileage dropped about 5 mpg and when using EV mode the drag is noticeable. The owners manual recommends V rated tires but the dealer insists the resistance between V and Z is negligible. So any opinions on this speed rating issue?