CVT Question

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JJW - 09 May 2007 15:08 GMT

I have a basic question about general torque power and CVT
transmissions. Does having a CVT mean that the vehicle can have a
smaller engine and maintain the same power? For example if car had an
automatic transmission and 3.5 liter V6 engine, and the same model car
had a CVT and 3.0 V6, would the CVT compensate for the smaller engine at
all? If so, what percentage?

Thanks in advance!
JJW

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Rui Pedro Mendes Salgueiro - 09 May 2007 18:13 GMT

> I have a basic question about general torque power and CVT
> transmissions. Does having a CVT mean that the vehicle can have a
> smaller engine and maintain the same power?

Sort of. When a manufacturer says that an engine has 200 HP they mean
200 HP at a certain RPM, for instance 6000 RPM. In a non-CVT gearbox,
the engine can't be kept at 6000 RPM all the time.

So, for instance, suppose that going from 2nd to 3rd, the RPM would drop
from 6000 to 4000. At 4000 the engine would probably produce about 133 HP
(to simplify I am assuming the torque would be more or less constant. That
won't be generally true, but this is an approximation). When accelerating
in that 3rd gear from 4000 to 6000 the engine would produce an average of
166 HP (more simplifications/approximations: (133 + 200)/2 ).

OTOH, a CVT car with the same engine, accelerating (full throttle) at
the same speeds would keep the engine at 6000 RPM (this is usually what
such gearboxes are programmed to do) so producing always the full 200 HP.
If the losses in the transmission are not higher than the conventional
gearbox this would mean, in this example, an advantage of about 20%.

The amount of advantage depends a lot on the characteristics of the
engine and the non-CVT gearbox. If the engine was very peaky and the
gears very separated the CVT advantage would be higher, if the engine
was very "torquey" (*) and/or the non-CVT gearbox was "close-ratio",
the advantage would be less (a "close-ratio" gearbox loses a bit due
to needing more gear changes, but on the whole should be a win since
it keeps the engine at higher RPMs).

(* another way of saying that the engine is weak in the top-end)

> For example if car had an automatic transmission and 3.5 liter V6
> engine, and the same model car had a CVT and 3.0 V6,

What are the other characteristics of those engines ? 3.5l is only 16%
more displacement than 3.0l, but the increase in power might be much
more (or less).

> would the CVT compensate for the smaller engine at all?
> If so, what percentage?

As written above, that percentage would depend on the gear ratios and
the torque curve of the engines.

An important point: the above refers to an objective comparision of
the CVT versus non-CVT gearbox. In a subjective comparision the CVTs
might have a number of inconvenients (since these days their control
is computerized, the presence of these problems is a function of the
cleverness of the programming):

- one of the reasons why one likes a powerful engine is the ability
to cruise at a low rpm with an instant response from the engine
when one presses the throttle. CVTs tend to, first, let the engine
rev to high RPM and then accelerate. Even if this is faster overall,
it might feel slower than the traditional way.

- sometimes one wants to use full throttle at low RPMs. CVTs tend to
rev the engine quite high if one uses more than a bit of throttle.

Signature

http://www.mat.uc.pt/~rps/

.pt is Portugal| `Whom the gods love die young'-Menander (342-292 BC)
Europe | Villeneuve 50-82, Toivonen 56-86, Senna 60-94

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JJW - 11 May 2007 15:35 GMT

Thanks to everyone who responded to my questions. I was trying to get a
general sense about how effectively CVT's use the HP of a given engine
performance-wise versus a traditional non-CVT transmission.

I can see there are a lot of factors to consider.

Best,
JJW

>>I have a basic question about general torque power and CVT
>>transmissions. Does having a CVT mean that the vehicle can have a
[quoted text clipped - 54 lines]
> - sometimes one wants to use full throttle at low RPMs. CVTs tend to
> rev the engine quite high if one uses more than a bit of throttle.

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Rui Pedro Mendes Salgueiro - 14 May 2007 19:18 GMT

> Thanks to everyone who responded to my questions. I was trying to get a
> general sense about how effectively CVT's use the HP of a given engine
> performance-wise versus a traditional non-CVT transmission.

> I can see there are a lot of factors to consider.

Yes, if you want to predict the performance based on the characteristics
it is a bit hard. But there is an easier way: just measure it (or for the
ordinary costumer, check manufacturer's figures or magazine tests).

There are a number of acceleration figures that are usually published
(in Europe, 0-100 km/h, 0-400m and 0-1000m. In the USA I think it is
0-60 (or 62) mph and 1/4 mile.

Also there are figures published for "reprises". That is less standardized
but it used to be 40 km/h to 100 km/h, IIRC (I see that Car & Driver tests
30-to-50-mph and 50-to-70-mph). Those are not comparable between automatic
(any type) and manual gearboxes, because the downshift done by the auto
gearbox is a big advantage, but since you were interested in CVTs versus
conventional automatics...

Those figures and a test drive to check if you like the way the CVT works
should be enough to form an opinion.

Signature

http://www.mat.uc.pt/~rps/

.pt is Portugal| `Whom the gods love die young'-Menander (342-292 BC)
Europe | Villeneuve 50-82, Toivonen 56-86, Senna 60-94

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Don Stauffer in Minnesota - 10 May 2007 14:39 GMT

> I have a basic question about general torque power and CVT
> transmissions. Does having a CVT mean that the vehicle can have a
[quoted text clipped - 5 lines]
> Thanks in advance!
> JJW

Depends on whether you truly mean engine horsepower or performance.
Engine horsepower does not change with type of transmission. However,
a CVT could allow you to use a somewhat less powerful engine and get
same average performance.

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Harry Smith - 10 May 2007 14:43 GMT

> I have a basic question about general torque power and CVT
> transmissions. Does having a CVT mean that the vehicle can have a
> smaller engine and maintain the same power? For example if car had an
> automatic transmission and 3.5 liter V6 engine, and the same model car
> had a CVT and 3.0 V6, would the CVT compensate for the smaller engine at
> all? If so, what percentage?

To get picky about the wording, it is not entirely correct to say
"torque power", as torque is force (measured in pounds if SAE or
Newtons if metric) multiplied by a lever arm into rotational force
(measured in pound-feet if SAE or Newton-meters if metric). Power is
work (horsepower if standard or kilowatts if metric), more like
measurements of energy. Its like the difference between measuring how
hard you hit something (force or torque) and how much energy is given
to something by hitting it over and over again for a period of time
(work). One is an instantaneous measurement (torque) and the other is
calculated from that and tells how much energy will be produced by
that torque over time...

Hopefully that came out halfway comprehensible...

Harry

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Ashton Crusher - 11 May 2007 06:46 GMT

>> I have a basic question about general torque power and CVT
>> transmissions. Does having a CVT mean that the vehicle can have a
[quoted text clipped - 18 lines]
>
>Harry

Another point to ponder in the torque vs HP mindmeld is that you can
have torque, such as on the rear axle, even when there is no movement
of the axle, but to have power you must have movement.

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Ivan Jager - 11 May 2007 19:34 GMT

> Power is
> work (horsepower if standard or kilowatts if metric), more like
> measurements of energy.

Actually, power is work over time. (Or energy over time.) Energy is
generally measured in Joules (I don't know if there's any non-metric
unit for it.) A Joule is the energy of one Newton applied through a
distance of one meter. (Same as one pound through 8.85 inches.)
In case you didn't notice, energy and torque have the same units, but
very different meanings. (Also, torque is a vector, while energy is a
scalar, but let's not go into that.)

A Watt is one Joule per second. One HP is about 746 Watts. This measures
how quickly you can produce energy.

Now, for acceleration, what matters is the mass of the vehicle and the
net force excerted on it. If we assume zero air resistance, and infinite
friction at the wheels, net force is the same as torque at the wheel
divided by wheel radius. Let's say A is acceleration, M is mass, F is
force, P is power, and S is speed. Then we have A=F/M. Since
P=F*distance/time=F*S, we have F=P/S and A=P/(M*S). So the faster you
go, the more power you need to maintain the same torque at the wheels
and the same acceleration.

So back to the OP's question: I know next to nothing about CVTs in
practice, but in theory they should allow you to run the engine at peak
power the whole time, which would give you more acceleration than if you
could only run at peak power for an instant or 5. Whether a 3.0L with a
CVT will accelerate better than a 3.5L with a traditional transmission
depends entirely on the characteristics of the engines and
transmissions. Basically, for whatever time period you're interested in,
take the average HP produced for the car with the normal transmission,
and that is how much peak power you'd need from the CVT to get the same
average acceleration. (If masses and efficiencies vary, you'd need to
account for that too.)

Keep in mind that I said the same *average* acceleration. Whichever car
has that acceleration happen earlier will be ahead of the other one,
even though they'd both reach the same speed at the same time.

Ivan

Ivan

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