Horsepower versus torque

[Frank]

And how is that not make it FWD untill the wheels slip?
Only the front wheels are getting power.

Because you have those three differentials that distribute the torque equally over all the wheels. One between the front wheels, one between the back wheels, and one in the middle between the back and front differentials. So, while none of the wheels slip, they all get the same amount of torque. Well, the inside ones a bit less when cornering, of course.

 

[darkblu]

And how is that not make it FWD untill the wheels slip?
Only the front wheels are getting power untill there is a difference in wheel speed between the front and rear wheels.
I really don't see why you're trying to argue about this.

radeonic, you've been seriously misled. what diguru is saying is that you have a nominal torque ratio that is a subject to redistribution at slipping, but in an AWD that nominal is not 100:0 front-to-rear. in rally/offroad cars it's usually 50:50 and in road AWDs the nominal's advantage is on the rear (usually in the range of 30:70 - 20:80)

 

[Moloch]

Because you have those three differentials that distribute the torque equally over all the wheels. One between the front wheels, one between the back wheels, and one in the middle between the back and front differentials. So, while none of the wheels slip, they all get the same amount of torque. Well, the inside ones a bit less when cornering, of course.

So are you trying to tell me that in an AWD car with a viscous center differntial the center diff is always sending equal power to the front and rear?

 

[Frank]

So are you trying to tell me that in an AWD car with a viscous center differntial the center diff is always sending equal power to the front and rear?

Exactly.

 

[nthd]

...
Is distance equal to acceleration times the square of time?
...

According to Newtonian mechanics: s = (ut) + (0.5a)t^2, where s is distance travelled, u is initial velocity, a is acceleration, t is time taken.

From a standing start, u = 0, and ut = 0, so s = (0.5a)t^2, or, distance travelled is proportional to the square of time.

Again, when u = 0, if a = 2, (0.5a) = 1, so s = t^2, i.e. then distance is equal to the acceleration multiplied by the square of time.

 

[KimB]

That's true for constant accelerations. But I don't buy that the acceleration of a car is close to constant.

 

[ShootMyMonkey]

No, it means higher efficiency. That's not the same thing as power.

Higher efficiency at *equal* fuel consumption rates? What I was giving was a very specific condition, so any differences show themselves in power. The RPM and other things are disregarded in this case.

In a total picture case, sure it would be higher efficiency, but for making high power and torque in a sane rpm range and remaining more efficient, how is this inherently a BAD thing on the road?

To make "serious" power you DO need rpms. Power = torque x revolution speed, remember? The torque is a function of displacement and, if present, forced induction overpressure.

I keep forgetting that tuning an engine to handle higher rpms doesn't affect the overall torque curve in any way.

If you can't increase rpm, you have to increase either displacement or boost pressure, which means either a stonking big engine(heavy) or a stonking big turbo/compressor(higher pressures which means more wear et.c.).

Which is more suitable outside the racetrack anyway.

It's not a benchmark if the playing field isn't level, hence the Wankel engined Mazda 787B reference...

My finger was pointed at this thread in that people feel free to use racetrack and drag strip results as benchmarks for gasoline and unarguable proof of their superiority under all conditions, but the moment Audi's R10 is mentioned the immediate response is that it proves nothing.

Which is especially funny since I thought the whole point of this thread was about consumer-level vehicles.

That does NOT mean that they are superior to petrol engines, which run smoother, have better power/weight ratio, are more "fun", sounds better et.c.

I don't know about the more fun part because it's also inherently fun to make more power immediately. And the power:weight ratio argument makes me scratch my head because it sounds as if people make the comparison as if the engine was the only thing in the entire vehicle that actually has weight. Adding 200 lbs to the engine can certainly throw off weight balance (as if the majority of vehicles were actually good on this point), but it's not doubling the weight of your vehicle.

It's a question of _compromise_.

I'd agree with this. I just don't see it as a *sacrifice* in the same way that you'd have to *sacrifice* something to get efficiency out of a hybrid or going to a smaller displacement gas engine to be more efficient. You can be more agressive with a diesel than you might be with a gas engine and still get better efficiency. "Sacrificing the high end" is a stupid point because people only go into the high end because they don't expect the car to move otherwise.

If these two engines output the same power, then engine A needs to generate less than half the torque at 5000 RPM of engine B at 2000RPM. Thus a smaller engine can be used.

You're assuming that the combustion characteristics are invariably the same, which is not true when comparing two different types of engines. If engine A needs to accelerate a vehicle at some rate k, it needs to be pushed into its higher rpm range. Engine B doesn't have to be pushed to produce the same acceleration, and if Engine B is a diesel, that also means less fuel consumption since a diesel can run lean quite safely, which a gas engine cannot.

So are you trying to tell me that in an AWD car with a viscous center differntial the center diff is always sending equal power to the front and rear?

For a true AWD, yes. Unfortunately, a lot of cars sold, particularly in the US, are mislabeled as AWD or full-time AWD/4WD when they're really not. The MS6, for instance is labeled an AWD vehicle when it's really FWD, but switches to *as much as* 50:50 front/rear when the front wheels slip. Subaru is about the only manufacturer who sells in the US whose AWD vehicles are always default 50:50 split.

 

[Moloch]

For a true AWD, yes. Unfortunately, a lot of cars sold, particularly in the US, are mislabeled as AWD or full-time AWD/4WD when they're really not. The MS6, for instance is labeled an AWD vehicle when it's really FWD, but switches to *as much as* 50:50 front/rear when the front wheels slip. Subaru is about the only manufacturer who sells in the US whose AWD vehicles are always default 50:50 split.

I've read the way Porsche does it is that it's RWD untill the wheels slip, which isn't a bad deal imo.
Btw Diguru, I was talking about AWD dsms if that helps, the eclipse gsx, talon TSI and laser AWDs.

 

[DuckThor Evil]

A car with twice the amount of torque and the same weigth will accelerate roughly twice as fast. And that's nowadays just about the only thing you notice or care about, power wise.

Man WAKE UP!
Your case and numbers are wayyyy! off.

 

[ShootMyMonkey]

I've read the way Porsche does it is that it's RWD untill the wheels slip, which isn't a bad deal imo.

Indeed. BMW, Infiniti, and Lexus all also offer AWD packages which are very rear-biased (don't know about completely RWD). And that's part of the attraction. Given the option, that's far better than anything that's FWD that can be as much as 50:50 Front:Rear. Unfortunately, the latter is often cheaper, because it's often done in the form of a Center Diff plopped onto an otherwise FWD chassis.

 

[Moloch]

Indeed. BMW, Infiniti, and Lexus all also offer AWD packages which are very rear-biased (don't know about completely RWD). And that's part of the attraction. Given the option, that's far better than anything that's FWD that can be as much as 50:50 Front:Rear. Unfortunately, the latter is often cheaper, because it's often done in the form of a Center Diff plopped onto an otherwise FWD chassis.

Well I've been looking around and most sites are saying that with a viscious coupling no torque is tranfered untill there is wheel spin.
For example, http://auto.howstuffworks.com/differential9.htm

 

[MPI]

Higher efficiency at *equal* fuel consumption rates? What I was giving was a very specific condition, so any differences show themselves in power. The RPM and other things are disregarded in this case.

In a total picture case, sure it would be higher efficiency, but for making high power and torque in a sane rpm range and remaining more efficient, how is this inherently a BAD thing on the road?

Well, I dont know what the hell you are talking about anymore, quite frankly... I responded to why putting a diesel in a rally car was a bad idea. And it still stands, too heavy and too little power.

Power for a given fuel consumption is what efficiency means obviously. But that's not really the primary factor in a rally car, now is it?

I keep forgetting that tuning an engine to handle higher rpms doesn't affect the overall torque curve in any way.

It sure does, but that is irrelevant. There's a physical limit to what torque an engine can nominally make. If you get 100% fuel-air fill into the cylinder(@1 bar) and ignite it, the gasses expand and pushes the piston which in turn turns the crankshaft. That is maximum theoretical torque.

In petrol engines, the practical limit is at about 130Nm per liter of displacement.

To make more torque, you have only two roads to go:

1. Increase displacement
2. Increase manifold pressure beyond 1 bar i.e. forced induction.

Both have their advantages and disadvantages, of course. The disadvantages of more displacement is obvious, the disadvantages of forced induction is higher temperatures and cylinder pressures, turbo lag if turbocharged et.c.

I think the flaw in your thinking has to do with seeing rpm as absolute, and the lower the better... The usable power band is _relative_ to the _entire_ rpm range. At normal power/liter figures of NA petrol engines(50-75bhp/liter) the usable power band is usually very wide. With low boost turbocharging(typically 80-100 bhp/litre) it's even wider. Add to that VVT, you get even more(as the range increases). Also, at comparable turbodiesel power(130-180bhp) for a car, we're not exactly talking about a Honda S2000(120bhp/liter, NA) or an F1(310bhp/liter)...

My finger was pointed at this thread in that people feel free to use racetrack and drag strip results as benchmarks for gasoline and unarguable proof of their superiority under all conditions, but the moment Audi's R10 is mentioned the immediate response is that it proves nothing.

Superiority in not all conditions, but most... The ones where diesels have a leg up is fuel economy and perhaps durability(although that's a bit diminished recently). Two major factors in endurance racing, which Le Mans is. Don't hold your breath seeing any diesels in Formula 1 or just about any other racing series...

Let's put it whis way; The acid test, if you will... Let's take fuel economy out of the picture: If running a diesel car did cost exactly as much as running a comparable petrol car, which one do you think most would choose?

I'd agree with this. I just don't see it as a *sacrifice* in the same way that you'd have to *sacrifice* something to get efficiency out of a hybrid or going to a smaller displacement gas engine to be more efficient. You can be more agressive with a diesel than you might be with a gas engine and still get better efficiency. "Sacrificing the high end" is a stupid point because people only go into the high end because they don't expect the car to move otherwise.

You're not "sacrificing high end", power is power, but all this hoopla about low-end is getting wildly out of hand. Yes, they peak their torque earlier, but that means you just have to put in the next gear earlier too. The reason that diesels have at least as many gears as petrol cars.

As I said, power is power, but this dismission of the inherent drawbacks of diesels is just a little bit silly.

 

[KimB]

Superiority in not all conditions, but most... The ones where diesels have a leg up is fuel economy and perhaps durability(although that's a bit diminished recently). Two major factors in endurance racing, which Le Mans is. Don't hold your breath seeing any diesels in Formula 1 or just about any other racing series...

Well, NASCAR has fairly long races (up to about 500 miles). If improved fuel economy could reduce the number of pit stops by enough to offset the reduced power/weight ratio, diesels might possibly be a win there.

 

[Frank]

Let's put it whis way; The acid test, if you will... Let's take fuel economy out of the picture: If running a diesel car did cost exactly as much as running a comparable petrol car, which one do you think most would choose?

In the US? Everyone would want a gasoline car, of course. Then again, how many have ever driven a car with a diesel engine?

In Europe, there are many people who like diesel engines better. Even the natually aspirated ones. People who demand that gasoline is always better, especially when big and/or trubocharged are generally younger males here in Europe.

But then again, 0-60 in six seconds with a car that weighs two tons isn't very average here. 0-60 in twelve seconds with a car that weighs one ton is more like our average.

 

[KimB]

But then again, 0-60 in six seconds with a car that weighs two tons isn't very average here. 0-60 in twelve seconds with a car that weighs one ton is more like our average.

Huh, well, that's kinda slow. My little Chevrolet Aveo with its 4-cylinder 1.6 liter gasoline engine makes 0-60 in just over 10 seconds.

 

[chavvdarrr]

Huh, well, that's kinda slow. My little Chevrolet Aveo with its 4-cylinder 1.6 liter gasoline engine makes 0-60 in just over 10 seconds.

even a 20yr old Lada can make 0-60 in 10 seconds, i guess DiGuru was exaggerating.

 

[DuckThor Evil]

Huh, well, that's kinda slow. My little Chevrolet Aveo with its 4-cylinder 1.6 liter gasoline engine makes 0-60 in just over 10 seconds.

Well after I read what diguru wrote on the first page I suggest that people don't take his words very seriously when he is talking about cars. Most cars weight more than 1 ton in europe too, however there are lot's of small cars in southern european countries like Italy and Spain, but still the average in Europe is much heavier than 1000kg.

I do agree that turbodiesel cars are very good, because of the torgue, you don't have to downshift to get good acceleration therefore making the power easily available, however there are many good turbocharged gasoline engines nowadays too that gives max torgue at very low reps.

even a 20yr old Lada can make 0-60 in 10 seconds, i guess DiGuru was exaggerating.

Which Lada would that be exactly, you do realise that we are talking about mph and not kmh?

 

[Mintmaster]

You're assuming that the combustion characteristics are invariably the same, which is not true when comparing two different types of engines. If engine A needs to accelerate a vehicle at some rate k, it needs to be pushed into its higher rpm range. Engine B doesn't have to be pushed to produce the same acceleration, and if Engine B is a diesel, that also means less fuel consumption since a diesel can run lean quite safely, which a gas engine cannot.

For the last time, there is no magic in diesel.

It has a higher energy density, but this is mostly due to a higher carbon content (and hence higher CO2 emissions, so you're only saving money and not the environment). That's where the lower fuel consumption comes from. It has little to do with the power curve.

A diesel outputting 100 Nm of torque at 2000RPM and a gasoline outputting 50Nm of torque at 4000RPM will output equal power and emit similar emissions. If a diesel is running leaner, it is not outputting as much power. Half the fuel per stroke will roughly halve the torque. This is true for gasoline also. That's how the gas pedal works. Power curves for an engine are given at maximum fuel:air ratio, i.e. full throttle. All internal combustion engines have a wide range of power that it can produce at a given RPM.

You're just stringing together a bunch of trite statements together that have no bearing on an apples to apples comparison of fuel consumption. If you want to look at efficiency at a given power output, you need to look at BSFC curves, like this one. It's hard to find this data, and I know the VW TDI has a low BSFC, but the higher carbon content of diesel reduces that advantage, and I don't think any other diesels are in that league.

Still, none of this has anything to do with the original post. Power matters for acceleration.

 

[Simon F]

Which Lada would that be exactly, you do realise that we are talking about mph and not kmh?

Maybe this one (see video at bottom of page)

 

[DuckThor Evil]

Maybe this one (see video at bottom of page)

Heh yeah I have seen that episode before, funny!
However that wasn't 20-years old and anyways I don't think the guy I quoted meant something like that.( not that I thought that you thought so either LOL!)