BX Power to Weight Ratios

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Post by Caffiend » Thu Feb 23, 2012 8:26 pm

Only one way to tell

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Post by BX Meteor » Thu Feb 23, 2012 8:43 pm

Philip Chidlow wrote:The point - if there ever was was one - that extrapolated from my original post was to see if there was a benchmark by which all cars could be judged with regards to their power to weight. The BX is a light car so initially I thought that a 2010 BMW - even though it had a more powerful engine might prove that newer cars are sacrificing efficiency (being forced to use bigger, more powerful engines) because of their ever-increasing bloatiness.

But the above will only mean anything if we can, a) compare the results to the BX's contemporaries and b) to modern 'equivalents.

Maybe

:?: :lol:

Either way it's a mental exercise - and who ever said there had to be a point?
Right, there is a simple answer to the above ..... well two .... one for motorway driving, one for town driving.

MOTORWAY: If you drive on the motorway in a BX, then regardless of the engine you have, you will need the same power at 70mph. That power will be about 20 BHP (I don't know the coefficient of drag for a BX so I cannot be exact). Whatever gear you are in, and whatever engine you have, and whatever weight your BX is (within limits) you will need 20 BHP to keep that car at 70 mph. The efficiency depends on the engine's gearing (it's engine speed) and its specific fuel consumption at that engine speed, whilst providing the 20 BHP to keep the car at 70mph. A weasel engine always has better specific fuel consumption than a petrol engine, because it has a higher compresssion ratio (the higher an IC engine's compression ratio, the more efficient its combustion cycle is).

With modern cars, they are likey to have a much better coefficient of drag, so they will require less power to drive at 70mph. Their engines are also likely to have a better specific fuel consumption for the correct gear (and hence engine speed) at 70mph.

TOWN DRIVING: ..... is a different matter, it depends more on car weight and the way the driver uses the throttle and gears.

EDIT: if someone has the coefficient of drag for a BX, in a post I will go through the calculation of HP required at 60, 70, 80, 90 mph. If you have the coefficient of drag (and body width and body height) for other cars, I will do the same. The calculation is easy. :wink:

(btw the coefficent of drag for the estate will be worse than for the saloon)

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Post by Mothman » Thu Feb 23, 2012 9:26 pm

Ok.
Point taken.

Andy

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Post by MULLEY » Thu Feb 23, 2012 10:57 pm

Modern cars usually have an extra gear & they can have a longer ratios due to the higher torque of their engines. So a BX n/a diesel is doing approx 3200 rpm @ 70, whereas a mondeo will be doing aprox 2000 rpm @ 70. Drag will also have an effect of course, however, because of modern engines having more torque, they can maintain the 70mph more easily, so the engine doesn't have to work quite so hard by having to press the throttle pedal harder to maintain any given speed. An example of this would be trying to maintain 70mph on a slight hill in a bx, i have to press the throttle pedal to pretty much flat to the floor, whereas in the mondeo, no change in throttle position.

This is what amazes me, my TXD is revving away, yet still returns comporable if not better mpg than most modern cars?? Perhaps we all need a 6 speed box with a very tall 6th gear to get the revs down to 2000 rpm. I wonder what the fuel economy would be at 70 if you only needed 2000 rpm in a BX?
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Post by BX Meteor » Thu Feb 23, 2012 11:11 pm

It's because in that example you are using a N/A weasel.

The air to fuel ratio in a weasel is not fixed. A weasel is always wide-open to the air-flow. With a turbo weasel, you have more air-flow. When you increase the throttle position in a weasel, you increase the amount of fuel injected, but with an N/A weasel you don't have an abundance of air like you do in a turbo weasel.

That's why you have to floor an N/A weasel but not a turbo weasel.

Note that in a petrol engine, the air to fuel ratio is fixed, and you control power by altering air-flow (and the carb or EFI adjusts the fuel to suit).

But the overall point about a weasel is that engine revs do not necessarily reflect fuel consumption, because the fuel injected is controlled by your foot, and you use your foot to control the speed. So you are adjusting fuel flow according to speed, regardless (within limits) of engine speed. This is not so true to the same extent for a petrol engine.

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Post by BX Meteor » Fri Feb 24, 2012 12:12 am

I wrote:EDIT: if someone has the coefficient of drag for a BX, in a post I will go through the calculation of HP required at 60, 70, 80, 90 mph.
Googling .... car comparison has been set up in Automobile-catalog here are BX comparisons.
Wikipedia has coefficents for loads of cars (Citroens too) but not BX.
Some BX coefficients in Citroenet, coefficent of drag for BX = 0.34.

The power to push a car through the air is given by .... P = ½ ρ C A V³
ρ is the air density (1.293 is usually used)
C is the car's coefficient of drag (C for BX = 0.34)
A is the car's cross-sectional area
V is the car's speed

The 1.6 Petrol has max 94 BHP out of the engine, and a top speed of 109 mph (fastest I ever had a 1.6 petrol at was 130 mph downhill with the wind behind me, but 130 on the speedo is a bit under 120). Anyway, if we say 10 HP is lost in the gearbox and tyres, then that means that the 1.6 BX at 110 mph has 84 HP at the tarmac.

I had to use A = 2.4 m³ to give 84 HP in the equation at 110 mph.

60 mph (27 m/s) .... P = ½ x 1.293 x 0.34 x 2.4 x 27³ = 10 kW = 14 HP
70 mph (31 m/s) .... P = ½ x 1.293 x 0.34 x 2.4 x 31³ = 16 kW = 22 HP
80 mph (36 m/s) .... P = ½ x 1.293 x 0.34 x 2.4 x 36³ = 24 kW = 33 HP
90 mph (40 m/s) .... P = ½ x 1.293 x 0.34 x 2.4 x 40³ = 34 kW = 47 HP
100 mph (44 m/s) .... P = ½ x 1.293 x 0.34 x 2.4 x 44³ = 47 kW = 64 HP
110 mph (49 m/s) .... P = ½ x 1.293 x 0.34 x 2.4 x 49³ = 63 kW = 85 HP

The HP figues above are directly proportional to the force required from the tyre on the tarmac to keep the BX at the given speed.

Note that according to the laws of aerodynamics you require 50% more HP at 80 mph than at 70 mph, and over 100% more HP at 90 mph than at 70 mph.

However you won't notice this in fuel consumption, because you are already losing power in engine pumping losses, in the gearbox and in the tyres. So if you add about 15 HP for all those, onto the above figures, the ratios of fuel efficiencies can be obtained.

Actual combustion work power on the pistons at 60 mph = 15 + 14 HP = 29 HP
Actual combustion work power on the pistons at 70 mph = 15 + 22 HP = 37 HP
Actual combustion work power on the pistons at 80 mph = 15 + 33 HP = 48 HP
Actual combustion work power on the pistons at 90 mph = 15 + 47 HP = 62 HP
Actual combustion work power on the pistons at 100 mph = 15 + 64 HP = 79 HP
Actual combustion work power on the pistons at 110 mph = 15 + 85 HP = 100 HP

i.e. you're likey to use 30% more fuel at 80 mph compared to 70 mph, and about 65% more fuel at 90 mph compared to 70mph.

These are at a constant crusing speed. If you continually decelerate and accelerate between 70 and 90 in motorway traffic, you will not use a lot more than you would at 70, depending on the total amounts of time spent at those higher speeds.

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Post by bxzx16v » Fri Feb 24, 2012 1:50 am

^^^ W T F ^^^

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Post by BX Meteor » Fri Feb 24, 2012 11:54 am

(^o^) . . . . . . . . . . . . . . . . . . . . (;|

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Post by Des Smith » Fri Feb 24, 2012 10:34 pm

How would this change if you drove your car off Beachy Head? Would the heavier one still accelerate faster, and how much impact would air resistance have on the downward trajectory?

Sounds like a potential item for Jeremy and his hairy friends at Top Gear...

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Post by BX Meteor » Fri Feb 24, 2012 10:41 pm

/\ I think they did something like that, a race to a point between a car dropped from a mile up (or such) and a car on the ground a mile away. I think they did it in the Utah salt flats (or such).

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Post by Willy » Fri Feb 24, 2012 10:56 pm

BX Meteor wrote:Actual combustion work power on the pistons at 60 mph = 15 + 14 HP = 29 HP
Actual combustion work power on the pistons at 70 mph = 15 + 22 HP = 37 HP
Actual combustion work power on the pistons at 80 mph = 15 + 33 HP = 48 HP
Actual combustion work power on the pistons at 90 mph = 15 + 47 HP = 62 HP
Actual combustion work power on the pistons at 100 mph = 15 + 64 HP = 79 HP
Actual combustion work power on the pistons at 110 mph = 15 + 85 HP = 100 HP
Bored, & maths is fun.

Let's calculate fuel consumption at a steady 60mph.

Assume power required at that speed is therefore 29hp.

29hp = 21KW

This means that the engine is producing 21KW of mechanical energy (some still gets lost as pumping losses but Brian has already seen to this), and in a gasoline heat engine thermal efficiency is abut 25%. Therefore, 80KW (why you need a radiator) of chemical energy is required to maintain this power output.

Gasoline contains roughly 10KWh of chemical energy per litre, meaning therefore that eight litres must be consumed per hour to sustain an 80KW power requirement.

So, that's 8 litres of fuel an hour, and as any coach/7.5T driver will tell you, 60mph = 100kph.

So that is 8l/100km for a BX travelling at 60mph, which is about 35mpg. A little tweaking of assumptions would make that figure a little more accurate, but still it's the right order of magnitude at least! :D
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Post by Mickey taker » Fri Feb 24, 2012 10:59 pm

didn' t the CX camper go over a cliff ?
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Post by BX Meteor » Fri Feb 24, 2012 11:22 pm

Willy wrote:
BX Meteor wrote:Actual combustion work power on the pistons at 60 mph = 15 + 14 HP = 29 HP
Actual combustion work power on the pistons at 70 mph = 15 + 22 HP = 37 HP
Actual combustion work power on the pistons at 80 mph = 15 + 33 HP = 48 HP
Actual combustion work power on the pistons at 90 mph = 15 + 47 HP = 62 HP
Actual combustion work power on the pistons at 100 mph = 15 + 64 HP = 79 HP
Actual combustion work power on the pistons at 110 mph = 15 + 85 HP = 100 HP
Bored, & maths is fun.

Let's calculate fuel consumption at a steady 60mph.

Assume power required at that speed is therefore 29hp.

29hp = 21KW

This means that the engine is producing 21KW of mechanical energy (some still gets lost as pumping losses but Brian has already seen to this), and in a gasoline heat engine thermal efficiency is abut 25%. Therefore, 80KW (why you need a radiator) of chemical energy is required to maintain this power output.

Gasoline contains roughly 10KWh of chemical energy per litre, meaning therefore that eight litres must be consumed per hour to sustain an 80KW power requirement.

So, that's 8 litres of fuel an hour, and as any coach/7.5T driver will tell you, 60mph = 100kph.

So that is 8l/100km for a BX travelling at 60mph, which is about 35mpg. A little tweaking of assumptions would make that figure a little more accurate, but still it's the right order of magnitude at least! :D
AWESOME \:D/

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Post by mds141 » Fri Feb 24, 2012 11:43 pm

The M.A.N. unit I drive for a living is rated at 440 bhp and as separate unit weighs approx 7.5 tonnes so = approx 58bhp per tonne. However if running at maximum plated weight with trailer which is 44,000 kgs the power to weight ratio drops to a paltry 10 bhp per tonne! Obviously Hgvs are primarily geared for max torque which makes all the difference.
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Post by Defender110 » Sat Feb 25, 2012 12:15 am

BX Meteor wrote:/\ I think they did something like that, a race to a point between a car dropped from a mile up (or such) and a car on the ground a mile away. I think they did it in the Utah salt flats (or such).
Indeed they did with a VV Beatle being dropped from one mile high and pip squeak trying to beat it to its landing point from one mile away in a Porche 911. The idea that the car would reach terminal velocity after being dropped from a mile high in the air, whereas Hammond could accelerate to cover the mile faster on the ground.
However, the South African salt flat the stunt was carried out on slowed his progress and he lost the race - though arguably the Beetle lost as it was smashed to smithereens.


it's not the only thing they dropped :(

http://www.youtube.com/watch?v=8_7fBjN0 ... re=related
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