Altima SE V-6 vs. Accord LX V-6

[snip]

No... you DO NOT want a FLAT POWER curve. Easy to show that

Acceleration = Power / (Mass x Speed)

So if power were constant (Flat) your acceleration would decrease as engine and
car speed increased. The whole idea of modern engine design with all the
variable valve timing etc. is to get a flat TORQUE curve. Because of the
physics of combustion, there is max value of torque you can get from an engine.
It's simply mean cylinder pressure x displacement. So the goal is to get the
torque up as fast as possible at low rpm (by achieving high volumetric
efficiency) and then *holding* it there at high rpm. Power dies when you reach
the rpm where you can no longer fill the cylinders fast enough to keep up
(torque falls)

Some of the previous replies are close - but I hope this makes it a bit more
clear.
Again, ideal IC engine would have a hp curve that is a straight line vs rpm, as
stated by someone previously.

Frank

 

[snip]

No... you DO NOT want a FLAT POWER curve. Easy to show that

Acceleration = Power / (Mass x Speed)

So if power were constant (Flat) your acceleration would decrease as engine and
car speed increased. The whole idea of modern engine design with all the
variable valve timing etc. is to get a flat TORQUE curve. Because of the
physics of combustion, there is max value of torque you can get from an engine.
It's simply mean cylinder pressure x displacement. So the goal is to get the
torque up as fast as possible at low rpm (by achieving high volumetric
efficiency) and then *holding* it there at high rpm. Power dies when you reach
the rpm where you can no longer fill the cylinders fast enough to keep up
(torque falls)

Some of the previous replies are close - but I hope this makes it a bit more
clear.
Again, ideal IC engine would have a hp curve that is a straight line vs rpm, as
stated by someone previously.

Frank

 

The main reason for my original post was to clarify the physical meaning of
torque vs. power. Torque is a force. Force alone doesn't provide energy or
power. It must be combined with movement (i.e. rotation) to make power.
Power, not torque, is the 'stuff' that accelerates the car. If I could have
an engine that produced maximum power over a wide range of speeds (without
other tradeoffs, like much greater maximum power over a narrower range of
speeds) then this is what I'd want.

Now, as I conceded in reply to Dave, I know nothing about the practical
aspects of engineering a car engine for best acceleration. I am learning
here as I read your replies. Sounds like physical laws governing heat
engines make a flat torque curve the best thing achievable.

My thinking (w/o much knowledge as I've confessed) was that, in real-world
engines of a given displacement and max h.p., if torque peaked at, say, 4000
r.p.m. instead of 3000 r.p.m., you'd get a rise in h.p. that exceeded the
3000 rpm peak-torque engine to that 4000 r.p.m. point, then a flatter h.p.
curve (that's higher than the hp curve for the 3000-peak torque engine) out
to the (say) 5800 peak-hp r.p.m. (Wish I could post a .jpg graph here...).
I figured this was the meaning of cars getting 6-speed xmissions these days
instead of 4-speed. Sounds like this is wrong -- that a real-world engine
that produces max. torque at 4000 r.p.m. would be out-powered by a
real-world engine (larger displacement, or same displacement but better
breathing?) that produces max torque at 3000 r.p.m. but the same max h.p.
at say 5800 r.p.m.

 

I would rather have a flat power curve -- equal to the MAXIMUM power output
achievable under ANY sort of valve timing... etc. However, you've made
clear that isn't achievable in the real world due to physical laws governing
combustion engines... so I'll accept that a flat torque curve is the best
that can be done.

 

The main reason for my original post was to clarify the physical meaning of
torque vs. power. Torque is a force. Force alone doesn't provide energy or
power. It must be combined with movement (i.e. rotation) to make power.
Power, not torque, is the 'stuff' that accelerates the car. If I could have
an engine that produced maximum power over a wide range of speeds (without
other tradeoffs, like much greater maximum power over a narrower range of
speeds) then this is what I'd want.

Now, as I conceded in reply to Dave, I know nothing about the practical
aspects of engineering a car engine for best acceleration. I am learning
here as I read your replies. Sounds like physical laws governing heat
engines make a flat torque curve the best thing achievable.

My thinking (w/o much knowledge as I've confessed) was that, in real-world
engines of a given displacement and max h.p., if torque peaked at, say, 4000
r.p.m. instead of 3000 r.p.m., you'd get a rise in h.p. that exceeded the
3000 rpm peak-torque engine to that 4000 r.p.m. point, then a flatter h.p.
curve (that's higher than the hp curve for the 3000-peak torque engine) out
to the (say) 5800 peak-hp r.p.m. (Wish I could post a .jpg graph here...).
I figured this was the meaning of cars getting 6-speed xmissions these days
instead of 4-speed. Sounds like this is wrong -- that a real-world engine
that produces max. torque at 4000 r.p.m. would be out-powered by a
real-world engine (larger displacement, or same displacement but better
breathing?) that produces max torque at 3000 r.p.m. but the same max h.p.
at say 5800 r.p.m.

 

I would rather have a flat power curve -- equal to the MAXIMUM power output
achievable under ANY sort of valve timing... etc. However, you've made
clear that isn't achievable in the real world due to physical laws governing
combustion engines... so I'll accept that a flat torque curve is the best
that can be done.

 

Torque is a term describing a force applying to an object perpendicularly. In
engine's term, torque is a twisting force at a particular rpm. Connecting these
points together give a torque curve. When referring to torque being applied over
time...during acceleration... the proper term is horsepower. A lot of people
don't realize the proper term.

1. Most gasoline burning street engines make the same ft*lb per liter at
1000-4000 rpm range. This means that they have equivalent horsepower per liter
in that range.
2. According to 1, doubling the engine means doubling the power, in that rpm
range. A 4.0 put out 2x the power of a 2.0 in that rpm range...

As for your question, due to usage of transmission with finite gearing, the
engine that put out more torque...power...at lower rpm accelerate the car faster
in lower rpm. This happens because we try to save gas by driving at a low rpm.
There is no difference at high rpm.
With continuously variable transmissions, they both would be the same since they
both be operating at max horse rpm during acceleration.

 

Torque is a term describing a force applying to an object perpendicularly. In
engine's term, torque is a twisting force at a particular rpm. Connecting these
points together give a torque curve. When referring to torque being applied over
time...during acceleration... the proper term is horsepower. A lot of people
don't realize the proper term.

1. Most gasoline burning street engines make the same ft*lb per liter at
1000-4000 rpm range. This means that they have equivalent horsepower per liter
in that range.
2. According to 1, doubling the engine means doubling the power, in that rpm
range. A 4.0 put out 2x the power of a 2.0 in that rpm range...

As for your question, due to usage of transmission with finite gearing, the
engine that put out more torque...power...at lower rpm accelerate the car faster
in lower rpm. This happens because we try to save gas by driving at a low rpm.
There is no difference at high rpm.
With continuously variable transmissions, they both would be the same since they
both be operating at max horse rpm during acceleration.

 

Well, if someone said they'd give me a 200 lb-ft peak torque
engine, I'd ask that it be constant at 200 out to infinity (okay,
as high as possible).

If someone said they'd give me a 200 hp engine, then I'd want it
to be a flat 200 hp at all rpm. So, yes, I'd want max power at
all revs.

So, it all depends on what you are offered

But seriously, as everyone is saying, ICE's by their nature are
much closer to the former (flat torque curve) than the latter
(flat hp curve). Electric motors are closer to the latter.

So which is better? For a given power, take the motor. In
theory, a CVT, or a whole lotta gears, will make the output of the
ICE approach that (if it is designed and controlled to sit near
the peak power rpm). But you'll need a near infinite gear ratio
to match the motor at launch..

 

Well, if someone said they'd give me a 200 lb-ft peak torque
engine, I'd ask that it be constant at 200 out to infinity (okay,
as high as possible).

If someone said they'd give me a 200 hp engine, then I'd want it
to be a flat 200 hp at all rpm. So, yes, I'd want max power at
all revs.

So, it all depends on what you are offered

But seriously, as everyone is saying, ICE's by their nature are
much closer to the former (flat torque curve) than the latter
(flat hp curve). Electric motors are closer to the latter.

So which is better? For a given power, take the motor. In
theory, a CVT, or a whole lotta gears, will make the output of the
ICE approach that (if it is designed and controlled to sit near
the peak power rpm). But you'll need a near infinite gear ratio
to match the motor at launch..