Energy consumption with different size of tire in a general vehicle (combustion engine)

[AndreaBianchi1984]

Hi everybody,
I'm an electronic engineer and i thinking about this issue:
In a vehicle you can select different kind of tire/wheel rime. Most of the time people say that if the overall diameter is reduced...the consumption increaseas...let's phisycally analyze the situation:
Here below, when speaking about BEFORE i refere to the situation where the original tire/wheel were mounted.

1) If the overall diameter is reduced, the rolling circunferenc is reduced as well.

2) Since the rolling circunference is reduced, for each single revolution of the wheel hub the car will travel for a reduced value of meter than before.

3) to travel the same distance, the wheel (and consequently the wheel hub) will have to perform more revolution than before.

4) perform more revolution than before does not mean necessary to have more fuel(energy) consumption, that's because for each revolution the distance performed (hence the energy needed) will be lower than before. In the end, theorically, the work performed should be the same as before, since L=force*space is not changed in any way.

5) Finally, the speedmeter value will not be real since it will measure a speed higher than the real one of the wheel.

6) From my point of view, there is no doubt up to now...but....Considering the point 5, to mantain the same speed as before, i will have to press deeper the accelerator, that's why it may seems that I should have to keep the engine RPM higher, and consequently the consumption should be higher...

For sure there is something wrong in the point here above, since the 2 conclusions (4 and 6) are completely in contradiction.
Obviously, i am not taking into consideraiton the possible differences between the type of wheel...let's suppose everything is the same...

May somebody explain to me where the error (or maybe errorS) is/are?

Thanks to everybody.

Andrea

 

[rb1957]

i think that the fact that the speedo is off (with the reduced circumference tire) is irrelevent; unless you're comparing consumption at 50km/hr (or some such number).

assuming the car is in the same gear (with both tires) then the smaller tire will need higher engine revs to go the same distance. if you set the same accelerator position for both then tire revolutions will be the same for both and the smaller tire will have travelled less distance (and so will take longer to get there). BUT this means that the smaller tire is travelling slower than the larger one, which means less drag force; which means (if you have the same accelerator position, producing the same engine power) the engine is creating more power than it needs to overcome drag, which means it'll go faster. BUT that doesn't make sense ... two identical cars (except for tire diameter) will have the same drag and so should travel the same speed for the same accelerator position.

if two engines are producing the same power then power transmitted to the ground (T*w) is the same. if the ground force is the same ('cause drag is the same) then T will be smaller for the smaller tire, and so w will be higher to achieve the same ground speed (w*r). that seems to say that rpm will be higher (for the same accelerator position, for the same engine power) but to achieve higher rpm (and presumably lower efficiency) you need a different accelerator position ... ??

 

[1gibson]

4) is incorrect. Faster wheelspeed = faster engine speed = more fuel used to travel the same distance. If you are talking a lot of stop and go driving, then the gearing difference may come into play with slight economy advantage to the smaller tire, but that is a stretch.

5) is irrelevant to the question

6) seems to draw a conclusion from the irrelevant information from 5. If you are comparing "70mph" on the speedometer with two different tire sizes, what value does that have? You know the conditions are different.

 

[TheTick]

1gibson's analysis of #4 is also incorrect. It assumes that the engine efficiency w.r.t. RPM is either constant or unfavorable. It also assumes that engine speed and power output are the same. Anyone who has ever driven a man's car (one without those inane P-R-N-D-2-1 sissy levers) would know this instinctively.

 

[drawoh]

AndreaBianchi1984,

There is a forum108 here.

You are trying to reduce a complex problem to some simple theory. Who is it claims that smaller tires result in less efficiency?

Smaller tires will cause you to go slower at a given speedometer reading, resulting in fuel savings, I should think. Otherwise, the behaviour of the smaller diameter tires versus the originals, is complex. The manufacturer had some reason to select them. You will improve your efficiency by driving closer to the most efficient speed of your engine, whatever that is.



--
JHG

 

[TheTick]

My 1995 Corolla got better mileage at 75 MPH than at 55 MPH.

If you are talking about TRUE speed (not just speedometer reading) then it's nearly all about the engine efficiency. Drag is the same. At constant speed, rotational KE of the wheels is constant, thus no difference in power consumed.

If the smaller tire causes the engine to run at a speed that is more efficient, then miles/gallon (or kilometers/liter) improves.

Not to be too harsh, but it does strike me that this query is coming from a sparky. I don't know why so many EE's have so much trouble with conservation of energy problems, but they do.

 

[AndreaBianchi1984]

Obviously i am refering to a constant speed travel (let's speak about an highway where i travel constantly 80km/h=1333meters/minute for 60 seconds), without changing the gear (let's say always the 4th).

Supposing i will go at the same speed, with the two different tires, the Work need to move a car of 1 tons for 1333 meters is the same.
Tyre can perform 10,000 revolution, each one with a lenght of 0.1333 meters or 5,000 revolution with a lenght of 0.2666.
Hence, no matter the tyre dimension, the work performed should be the same.

But most of the time, car builder, mechanical people repairing car and changing tyres indicates that a more litlle diameter tyre increases the engine consumption (fuel consumption). I do not believe it is like this but something give me some doubt...

I do not want to make something complex, easier...the reason is a bet (without money )with a my friend who is strongly convinced that little gear produce higher consumption (i am not intendeed to change my gear for a lot of other reasons, so i am not trying to convince myself for some specific reason). It's only something that i's like to understand...That's all

 

[AndreaBianchi1984]

Sorry to everybody, since i am not an american, neither english and i cannot understand all the abbreviations, (sorry for this)...may you please indicate the complete word?
Thank you very much
Andrea

 

[AndreaBianchi1984]

The tick...can you explain to me what sparky stands for?
Anyway, i am just curious to know, that's all... as explained before in my last topic...

 

[redpicker]

Simple observation will show that the vehicles with the smallest tires get the best fuel efficiency.

This may be a much more simplistic than the OP was wanting, but sometimes it takes a simplistic approach to illustrate just how complex the situation can be.

rp

 

[GregLocock]

Everyone is ignoring, perhaps for good reason, the contribution of the tire itself to the rolling resistance of the car. Another factor is that a small tire will improve the aerodynamics of the car.

Basically the model is too simple.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[drawoh]

GregLocock,

I was about to post and point out about rolling resistance.

A bigger tire would have a larger footprint on the road. Is tire slippage a factor in normal driving?

A skinnier tire would improve aerodynamics. A smaller diameter tire of the same width would sit the car lower to the road, possibly affecting the aerodynamics.

--
JHG

 

[TheTick]

"Sparky" = electrical engineer (one who makes things that make sparks)

Your assumptions about work done is correct. The possible difference in work is reduced drag due to car being lower (as the rightfully esteemed GregLocock noted).

Again, I think the single biggest factor is the engine efficiency versus RPM.

 

[rb1957]

"sparky" = electrical engineer ... who works with electricity, which produces sparks

personally i think it makes more sense to compare with the same accelerator position, so you "know" the engine is using the same amount of fuel (no?),
and i think the difference in drag (due to shallower tires) is small since the bulk of the car is the same. of course the smaller tires mean the car body is closer to the ground which would probably reduce the drag coefficient enough to upset all the hand waving; i mean we're talking about <5% diameter, right? and not 50% smaller ??

if the point of the question is tire size (as originally posed, smaller tires travel less distance ...) then you're saying that you'd re-rig the smaller tires so that car body is the original height

 

[TheTick]

Accelerator position only governs the opening through which fuel flows, not the rate at which fuel flows through the opening.

 

[Screwman1]

What about engine pumping efficiency? Since an engine is a big air pump isn't it usually true that for maximum efficiency you want to have the widest throttle opening (lowest restriction to air flow) combined with the lowest RPM. As I remember, that is the way that most of the super efficient cars get run: Wide Open Throttle (in a very tall gear) up to speed and then shut off the engine and coast.
Smaller diameter tires will lower the height of the vehicle and reduce drag and they may or may not have less rolling resistance (I think that things like tread pattern and rubber composition may out weigh the effect of the diameter)> Certainly inflation pressure will have huge impact on rolling resistance.

 

[Tmoose]

Hi Andrea,

i think you need to consider the effect of throttle position on Brake Specific Fuel Consumption.


More open throttle at any rpm will mean more torque which is directly related to the Brake Mean Effective Pressure (BMEP) which is the Y axis on this chart. Barely open throttle = not much torque = low BMEP .
Typical looking BSFC chart here. It also has lines for constant HP.

http://img208.imageshack.us/img208/1358/19210mrmopartorque.png

If I want to use the least amount of fuel when driving my old Volvo at 70 mph, and it takes 20 kW to go 70 mph, and I drive an hour to get to work, then what I want is a lowest number of grams of fuel for each of those kW. The islands are probably grams of fuel/HP/hour.
If I drove to work in 4th gear my gearing/tire size would result in 3500 rpm @ 70 mph, then the engine would burn about 240 grams of fuel on my 70 mile 1 hour trip to work.
If I shift into 5th for the trip home, and my 70 mph rpm was 2600, I'd use about 220 grams of fuel.

At small throttle openings (low torque or BMEP , Y axis) gas engines simply use more grams of fuel every hour for each kW.
Often you'll hear about closed throttle causing "pumping losses" that are behind the greater relative fuel use. I think the cruising manifold vacuum of 14 inches of Hg/ 7 psi/ is no more important than the lower "compression/expansion ratio" of compressing a "vacuum" is directly worth 5 or 10 % thermal efficiency, and the early ignition timing (more advance) that is required by the slow combustion at part throttle probably hurts thermal efficiency some more.

There are a few instances when combinations of gearing, road load, and tuning can produce a little better MPG cruising at a higher rpm and mph, but the general behavior is most likely to be better fuel economy with "taller" gearing.

 

[rb1957]

ok, how about this ...

the engine has a power curve, power produced vs rpm, for different throttle positions.
assuming a final drive ratio then you have power vs road speed.

the car has a drag curve, power required vs road speed.

assuming that drag doesn't change with the smaller tire, then the smaller tire will modify the engine power vs road speed curve ... the engine power curve is unchanged, but the relationship between rpm and road speed has changed. The same engine power point will be related to a smaller road speed value.

compared to the fixed drag curve, the power available for a given road speed has been reduced. this means the smaller tires will need a higher throttle setting to achieve the same road speed to produce the power required to overcome drag.

well, TTWISI

 

[SnTMan]

Given all the above, we can arrive at an answer: It depends. As usual

Regards,

Mike

 

[EnergyMix]

And here in America, I see cars where people have replaced the original tires with these huge jacked tires so that they can take their cares off-road. Ruin the environment and waste gas at the same time (Realize I live very close to the Great Lakes, where there are a number of sand dunes that are in danger of being destroyed by off-road vehicles. I'm not an avid environmentalist, but I do advocate for common sense.)