How much bhp required at a steady 70mph cruise?
Discussion
I'm just curious on this one.
My thinking is linked to hybrids which use electrical energy at low speeds and then petrol at higher road speeds.
Also I remember giving a lift to a passenger in a Jag XJ and with the cruise on at 70mph at 1,750 rpm he commented that the car was probably only using 70bhp - I genuinely would have no idea.
To keep it easy, the car is a modern 1,500kg saloon.
My thinking is linked to hybrids which use electrical energy at low speeds and then petrol at higher road speeds.
Also I remember giving a lift to a passenger in a Jag XJ and with the cruise on at 70mph at 1,750 rpm he commented that the car was probably only using 70bhp - I genuinely would have no idea.
To keep it easy, the car is a modern 1,500kg saloon.
He would be correct. The car only needs to sustain the speed, so will be resisting the air resistance and rolling resistance of the tyres. As speed climbs the required force(BHP) needed to maintain that speed rises exponentially. When you're at the speed you could maybe only need 40-50 BHP to propel a modern Aerodynamic car with low rolling resistance tyres. Hence why some cars are supposedly able to see such amazing fuel economy on a run.
Well given that cars with 50bhp can happily maintain 70mph with their engines not running at peak power output I would guess between 30-50bhp depending on the car. Really not much anyway.
In the Top gear episode where Clarkson raced the Bugatti Veyron across France he commented on the power-meter showing he was only using about (if I remember correctly) 50bhp, which left 950 to go.
In the Top gear episode where Clarkson raced the Bugatti Veyron across France he commented on the power-meter showing he was only using about (if I remember correctly) 50bhp, which left 950 to go.
I might be being dim, but for an approximation, can you take a car's BHP and maximum speed (assuming no limiter) and work back from there? Power required = velocity ^ 3.
Obviously your results are only good for a car of that weight and aero.
For example: my car can supposedly do 137mph with 165bhp (I should be so lucky on either count), so were that idea valid and my maths correct, it'd need a mere 22bhp to do 70mph.
Obviously your results are only good for a car of that weight and aero.
For example: my car can supposedly do 137mph with 165bhp (I should be so lucky on either count), so were that idea valid and my maths correct, it'd need a mere 22bhp to do 70mph.
Edited by trashbat on Monday 22 September 10:35
Prof Prolapse said:
Does any mass production car manage this?
I got 86 mph (verified with speeding ticket) in a 1.0 60bhp Micra once, and faster seems difficult.
There's a Smart diesel that has 53bhp and claims a top speed of 84mph. Slowest contemporary fossil fuelled car that I can think of. The aero can't be great though either.I got 86 mph (verified with speeding ticket) in a 1.0 60bhp Micra once, and faster seems difficult.
http://www.parkers.co.uk/cars/reviews/facts-and-fi...
39HP, 87MPH, impressive. Also scary I'd imagine. How badly was the Inbetweeners Hawaii special over-reading when they were driving to the caravan club and it managed 100MPH?
39HP, 87MPH, impressive. Also scary I'd imagine. How badly was the Inbetweeners Hawaii special over-reading when they were driving to the caravan club and it managed 100MPH?
Purely to overcome aerodynamic drag (using SI units for everything)...
P = (1/2)p(v^3)CdA
A 3-series has a CdA of 0.652
70mph is 31.3m/s
The density of air is 1.225kg/m^3
P = 0.5 * 1.225 * 31.3^3 * 0.652
= 12246 watts
= 12kw
= 16bhp
Obviously that doesn't include mechanical drag, which is rather harder to calculate.
P = (1/2)p(v^3)CdA
A 3-series has a CdA of 0.652
70mph is 31.3m/s
The density of air is 1.225kg/m^3
P = 0.5 * 1.225 * 31.3^3 * 0.652
= 12246 watts
= 12kw
= 16bhp
Obviously that doesn't include mechanical drag, which is rather harder to calculate.
Prof Prolapse said:
IanCress said:
Even a 45bhp car can do 90mph.
Does any mass production car manage this?I got 86 mph (verified with speeding ticket) in a 1.0 60bhp Micra once, and faster seems difficult.
I used to work as a driver's mate on HGVs when I was a student. One of our wagons was a 2000 Volvo FM12, 38 tons max weight, though probably more like 25-30 with the loads we carried, 380 hp. When it was plodding along the motorway, the boost gauge was usually reading fairly low, I reckon you'd be lucky if it was generating 100hp to keep a massive lump with the aerodynamics of a housebrick running at 56mph. When you hit a hill though it was up at 100% boost, probably generating well over 300hp & it just about managed to keep itself moving without losing speed. Modern wagons seem to be in the 450-550hp range, I suspect a lot of these will climb most motorway hills fully loaded without a noticeable slowing.
trashbat said:
I might be being dim, but for an approximation, can you take a car's BHP and maximum speed (assuming no limiter) and work back from there? Power required = velocity ^ 3.
Obviously your results are only good for a car of that weight and aero.
For example: my car can supposedly do 137mph with 165bhp (I should be so lucky on either count), so were that idea valid and my maths correct, it'd need a mere 22bhp to do 70mph.
Some are limited by gearing rather than power, though.Obviously your results are only good for a car of that weight and aero.
For example: my car can supposedly do 137mph with 165bhp (I should be so lucky on either count), so were that idea valid and my maths correct, it'd need a mere 22bhp to do 70mph.
Edited by trashbat on Monday 22 September 10:35
I assume the question is minimum power on flat ground, my old 105 1.5D used to lose speed on slight inclines on motorways.
Based on very simple maths I know a 40bhp fiat cinquecento would do about 80 mph. They say an average car needs 50% more power to do another 10 mph so I would say around 25 bhp is needed for 70 mph. Very rough maths with lots or roundings!
Edited by Sensibleboy on Monday 22 September 11:29
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