How much does Jeremy Clarkson really know about cars?

Hey guys, I have another one for you. This might not be a perfect example of what I'm on about, but it shows that JC twists reality for jumping on a bad car. We're talking about the review of the Chrysler Crossfire in episode 3x09. I don't like this particular car very much, so don't get the idea that I'm just defending it or something.
After the actual test, Jeremy talks about it with James and quotes the brochure saying:"the Crossfire has an aerodynamically limited topspeed of 155mph". He laughs about it and states that every car is limited by its aerodynamics, and that a Mini Metro would go a million miles if there was no air.

Now, if Chrysler would have left "aerodynamically" out of that sentence, what do you think how many people would think that it's limited electronically? There's a gentlemans agreement in Germany that all the major car companies have to limit their cars to 155mph. As the Crossfire is based on Mercedes SLK, it would've been the obvious thought that this is an electronic limit, which isn't true. That's why they said "aerodynamically limited".
Additionally, a Mini Metro wouldn't go a million miles or even come close to that without air, cause it would hit the rev limiter in top gear at a certain speed.

Now, I understand that this is hairsplitting, but still, it is an example of how little JC cares about some things, or maybe even doesn't know. If you pick on something, you should be damn sure you're right about your statements. At least that's what I'm thinking.

Regards
the Interceptor
 
"a mini metro would go a million miles an hour" sounds alot better than "a mini metro would go a lot faster until it hit the rev limiter in top gear" doesnt quite get the point across well enough

its more an entertainment show than a science or mechanics show. You tried Fifth Gear on for size yet?
 
I agree, but then, I'd rather forgo that joke. I like Top Gear for what it is, and I like JCs way of seeing and saying things. And he does make a lot of true statements. If he'd just leave the false ones out, I'd be even happier.

Regards
the Interceptor
 
Supercharger: Uses compressed air to RAM fuel into the cylinders. This leads to a bigger bang and a nice increase of power.

Turbo: Catches hot air from the exhaust, redirects it to a chamber with propellors in it. The chamber is divided in 2, each section has 1 prop which is mounted to the same axle as the other.

The exhaust fumes enter the upper chamber, which spins the propellor. The propellor in the lower chamber spins at the exact same amount of RPM and forces more air into the cylinders. More fuel isn't needed. That's why turbo's only work at a high level of RPM from the engine.

Turbo anti-lag: Props are kept spinning by a small motor and ergo, the turbo doesn't need a high amount of airflow. This results in performance boost at lower revs.

That's why supercharged cars drink more petrol per mile then a turbo charged one.


Am i right? Please correct me if i'm wrong.
 
PieceOfTat said:
Supercharger: Uses compressed air to RAM fuel into the cylinders. This leads to a bigger bang and a nice increase of power.

Turbo: Catches hot air from the exhaust, redirects it to a chamber with propellors in it. The chamber is divided in 2, each section has 1 prop which is mounted to the same axle as the other.

The exhaust fumes enter the upper chamber, which spins the propellor. The propellor in the lower chamber spins at the exact same amount of RPM and forces more air into the cylinders. More fuel isn't needed. That's why turbo's only work at a high level of RPM from the engine.

Turbo anti-lag: Props are kept spinning by a small motor and ergo, the turbo doesn't need a high amount of airflow. This results in performance boost at lower revs.

That's why supercharged cars drink more petrol per mile then a turbo charged one.


Am i right? Please correct me if i'm wrong.

Not exactly right. You explained the method of compressing correctly, but once that air is compressed, they are identical. The reason that they both want to get more air into the cylinde is so that they can use it to burn more fuel and therefore get more heat and energy out for every detonation.
The reason that supercharged cars are less efficient is that the turbines of the supercharger are mechanically driven by the engine as opposed to driven by exhaust gas. This causes the supercharger to take more horsepower away from the engine than the turbocharger does (don't get me wrong, the amount taken away is far less than the extra amount allowed by compressing the air, but it is a drag on power nonetheless), because the supercharger method creates more friction and all that.
The end result is that the engine with a supercharger has to make more power than that with the turbocharger to create the same net horsepower once the chargers soak up the horsepower they need to run, and so has to burn more petrol to do so.

Your anti-lag explanation is correct, but it might be a bit misleading to state that a small motor keeps is spinning. What keeps it spinning is excess fuel dumped into the exhaust. When the motor isn't running full throttle (and so there isn't enough exhaust to keep the turbos spinning full force), there is extra oxygen making it to the exhaust that can burn with the fuel dumped. This causes the exhaust to burn and continue to expand while in the exhaust pipe, and so provides more air flow for the turbo to work off of. It's like the afterburner in a jet. I think James May's explanation is very good, if mine sucks. (http://www.bbc.co.uk/topgear/series_8/prog_5/index.shtml)
I think your explanation shows understanding, but I wouldn't want anybody else reading to get the wrong impression.
 
geoff_a_pult said:
PieceOfTat said:
Supercharger: Uses compressed air to RAM fuel into the cylinders. This leads to a bigger bang and a nice increase of power.

Turbo: Catches hot air from the exhaust, redirects it to a chamber with propellors in it. The chamber is divided in 2, each section has 1 prop which is mounted to the same axle as the other.

The exhaust fumes enter the upper chamber, which spins the propellor. The propellor in the lower chamber spins at the exact same amount of RPM and forces more air into the cylinders. More fuel isn't needed. That's why turbo's only work at a high level of RPM from the engine.

Turbo anti-lag: Props are kept spinning by a small motor and ergo, the turbo doesn't need a high amount of airflow. This results in performance boost at lower revs.

That's why supercharged cars drink more petrol per mile then a turbo charged one.


Am i right? Please correct me if i'm wrong.

Not exactly right. You explained the method of compressing correctly, but once that air is compressed, they are identical. The reason that they both want to get more air into the cylinde is so that they can use it to burn more fuel and therefore get more heat and energy out for every detonation.
The reason that supercharged cars are less efficient is that the turbines of the supercharger are mechanically driven by the engine as opposed to driven by exhaust gas. This causes the supercharger to take more horsepower away from the engine than the turbocharger does (don't get me wrong, the amount taken away is far less than the extra amount allowed by compressing the air, but it is a drag on power nonetheless), because the supercharger method creates more friction and all that.
The end result is that the engine with a supercharger has to make more power than that with the turbocharger to create the same net horsepower once the chargers soak up the horsepower they need to run, and so has to burn more petrol to do so.

Your anti-lag explanation is correct, but it might be a bit misleading to state that a small motor keeps is spinning. What keeps it spinning is excess fuel dumped into the exhaust. When the motor isn't running full throttle (and so there isn't enough exhaust to keep the turbos spinning full force), there is extra oxygen making it to the exhaust that can burn with the fuel dumped. This causes the exhaust to burn and continue to expand while in the exhaust pipe, and so provides more air flow for the turbo to work off of. It's like the afterburner in a jet. I think James May's explanation is very good, if mine sucks. (http://www.bbc.co.uk/topgear/series_8/prog_5/index.shtml)
I think your explanation shows understanding, but I wouldn't want anybody else reading to get the wrong impression.
:oops:

You're a real anorak! :lol:
 
Nah, just a mechanical engineering student.
It comes with the territory.
(I am, afterall, helping to design a race cart from scratch. http://www.fsae.com/ I'll try to start a thread and post some pic's when we get the frame built and start assembling.)
 
the Interceptor said:
Additionally, a Mini Metro wouldn't go a million miles or even come close to that without air, cause it would hit the rev limiter in top gear at a certain speed.

As a physicist I can tell you that Clarkson is bang on in this case.

WARNING: SCIENCE CONTENT
What happens when you're cruising in your car at say 50mph at 2000rpm. That means that the net-force on your car is zero... that is the power output of the engine minus the friction of the tires minus the air resistance is zero. Well, in order to accelerate that total must be greater than zero. So, you increase the power output and accelerate. However, since air resistance follows the square of velocity, you soon reach a new equilibrium and cruise at a new, more powerful engine speed.

Without air, in order to accelerate power output - friction must be greater than zero. Since friction with the road does not increase with velocity, you could simply rev the engine to the optimal point in the power curve and accelerate forever.
 
Friction with the road does not increase with velocity, but internal friction of the car does. At some point (a ridiculously fast point, but shy of a million an hour), the friction of the post-transmission parts (the wheel bearings, the differential, etc) would equal the horsepower output of the engine. You also have to consider the fact that the tires would shred themselves at probably anything above 200, and that many of the stresses limiting cars come from things other than air resistance. And I reiterate what others have said in declaring Clarkson's comment uncalled for in the firstplace- many cars are limited by things other than aerodynamics. Some Porsches and Ferraris are redline limited (as the Metro would be if the final drive ratio were not adjusted), and all the powerful Germans, as well as many cars that are unstable at speed, are electronically limited.

That said, I of course never watch Top Gear for science content, because I have classes for that.
I watch Top Gear for amusing one-liners about cars, and in that criteria, Clarkson was right on.
 
you dont get a billion people to watch your show worldwide if you talk like a know-it-all car nerd. he presents car information in an interesting and entertaining way. he uses very helpful metaphors and analogue visual aids to get everyone up to speed. the evo 8 mr fq-400 comes to mind as an example. here's a fellow who brought a milk bottle to show how a much smaller a 2L evo is as compared to a 2.5L milk bottle. 400bhp from a so small a space. now that's interesting!

he also knows enough about cars to say what's wrong with them. who wants a car with a cheap plastic interior?
 
geoff_a_pult said:
Friction with the road does not increase with velocity, but internal friction of the car does. At some point (a ridiculously fast point, but shy of a million an hour), the friction of the post-transmission parts (the wheel bearings, the differential, etc) would equal the horsepower output of the engine. You also have to consider the fact that the tires would shred themselves at probably anything above 200, and that many of the stresses limiting cars come from things other than air resistance.

The mechanical engineers always make things difficult.

But still... the car would continue accelerating until it completely vapourised itself, rather than reaching some arbitrary speed barrier.
 
optimusprime said:
geoff_a_pult said:
Friction with the road does not increase with velocity, but internal friction of the car does. At some point (a ridiculously fast point, but shy of a million an hour), the friction of the post-transmission parts (the wheel bearings, the differential, etc) would equal the horsepower output of the engine. You also have to consider the fact that the tires would shred themselves at probably anything above 200, and that many of the stresses limiting cars come from things other than air resistance.

The mechanical engineers always make things difficult.

But still... the car would continue accelerating until it completely vapourised itself, rather than reaching some arbitrary speed barrier.

Conceded.
(It's not that we make things difficult, it's just that if we looked at our problems the way you can look at yours, we'd have a lot of devices that should work, but don't)
 
optimusprime said:
geoff_a_pult said:
Friction with the road does not increase with velocity, but internal friction of the car does. At some point (a ridiculously fast point, but shy of a million an hour), the friction of the post-transmission parts (the wheel bearings, the differential, etc) would equal the horsepower output of the engine. You also have to consider the fact that the tires would shred themselves at probably anything above 200, and that many of the stresses limiting cars come from things other than air resistance.

The mechanical engineers always make things difficult.

But still... the car would continue accelerating until it completely vapourised itself, rather than reaching some arbitrary speed barrier.
But wouldn't at some point the rotation of the tires be greater then the maximum RPM of the gears and motor?

Like when you ride a mountain bike very fast down a step hill. You can be in the highest gear, but when you try to peddle faster, you just spin out the crank without accelerating, since you can no longer peddle faster then the current speed.
 
TomCat said:
But wouldn't at some point the rotation of the tires be greater then the maximum RPM of the gears and motor?

You'd have to significantly modify the gearing.
 
^ Yes, but we're assuming that the car has been properly geared to reach whatever its ridiculous top speed is, otherwise it wouldn't have been aerodynamically limited in the first place.
(that is why I specified friction in the post-transmission drivetrain, because the engine and transmission will be geared to a million miles an hour, and so won't produce any more friction than normal)

Got beaten to it. :oops:
 
TomCat said:
optimusprime said:
geoff_a_pult said:
Friction with the road does not increase with velocity, but internal friction of the car does. At some point (a ridiculously fast point, but shy of a million an hour), the friction of the post-transmission parts (the wheel bearings, the differential, etc) would equal the horsepower output of the engine. You also have to consider the fact that the tires would shred themselves at probably anything above 200, and that many of the stresses limiting cars come from things other than air resistance.

The mechanical engineers always make things difficult.

But still... the car would continue accelerating until it completely vapourised itself, rather than reaching some arbitrary speed barrier.
But wouldn't at some point the rotation of the tires be greater then the maximum RPM of the gears and motor?

That's what I would've thought - unless the transmission has an infinitely variable ratio (so road speed would constantly increase while rpm remains constant), the motor's only going to hang together up to a certain rpm, and then it'll tear itself apart.

Edit: Beaten :(

But - let's say the car has been geared astronomically long to reach this theoretical ludicrous speed - surely it's going to struggle to even begin to accelerate with that sort of gear (or is there where the IVT (Infinitely variable transmission) kicks in...)
 
geoff_a_pult said:
^ Yes, but we're assuming that the car has been properly geared to reach whatever its ridiculous top speed is, otherwise it wouldn't have been aerodynamically limited in the first place.
(that is why I specified friction in the post-transmission drivetrain, because the engine and transmission will be geared to a million miles an hour, and so won't produce any more friction than normal)
Well in that case, it's possible. It would take forever, unless you came up with a 50 speed manual transmission or the worlds craziest CVT. But with no air, the motor would overheat and explode. lol
 
^We'll just have to try with a prius then, but I think at this point we just might be over analyzing the statement.
 
Better start building that highway on the moon and convert a Prius to electric power.
 
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