How much bhp required at a steady 70mph cruise?
Discussion
This is an apparentyl simple question, but the answer is harder to come by in general terms, since everything is vehicle and engine power measurement specific.
Engine horse power is measured in one of two aways!
- SAE (Soceity of Amercican "Auto Engineers) which was used fro most vehicles in UK at least until about 1980. Briefly, his is the power of the engine as measured when said engine has a free inlet and exhaust stystem, does not have to frive its own water pump or alternator, and so a real figure
- DIN (Deutsche Engineuer Norm) which is a more real ilfe figure because the engine does have to deal with its own ancilliaries and it so represents a value more in line the engine installed in your car.
I do no know exactly, but for the same engine, DIN is about 15 bhp less than SAE.
Driiving at 70 mph requires a given BHP to overcome rolling resistance, Wind resistance, and the power consumed by the drive train, mainly gearbox and back axle.
If you measure the power at the rear wheels, you have accounted for all the above except the Rolling rsistance of the two non-driven wheels and a ballpark figure is about 18-20% of the DIN engine power. bhp lost (although it will vary with speed of the vehicle.
My practival experience is that!
a) My 1938 Morris 8 had 28 SAE (18 DIN) bhp and aerodynamics of brick wall. On a good day would make 50-55 mph. From the above, this was achieved on about 12-14 hp at the wheels
b) my mate's Austin A35 35 bhp SAE (About 22 DIN)would make about 65 mph . Achieved on maybe 20 bhp at the rear wheels
c) My Scimlitar Coupe (nicely aerodynamic) would make 120 mph on 138 bhp DIN and so about 110 bhp at the rear wheels
d) MG Magnette 72 bhp SAE (60 DIN) would make about 80 mph on about 550DIN at rear wheels
A very good article in Wikipédia provides formulae that for a given vehicle and all other things equal, the BHP required for a given speed, is proportional to the cube of the velocity. They refer to engine power without definition, but I think we should talk about BHP at the rear wheels.
Applying the average of my real life examples the ôwer for 70 mph ranged from 27 tp 46 bhp DIN, with an average of 40.5.
I will be the first to admint that these numbers are very subjective and open to question, but at least this gives a directional guide.
I would only comment that whatever the real value, the power consumption for hubrids will be about the same, after adjustment for power losses during electricty generation, and use, including heat losses in the battery . Just remember that the electriciy has to be produced by a power station with a carbon footprint of around 0.5 kg carbon per Horse power, or by the engine in a hybrid. Maybe others would like to estimated grms CO2 km for 100% gasoline and 100% electrcity from a power station at 70 mph ?
I have written enough for now !!!
Engine horse power is measured in one of two aways!
- SAE (Soceity of Amercican "Auto Engineers) which was used fro most vehicles in UK at least until about 1980. Briefly, his is the power of the engine as measured when said engine has a free inlet and exhaust stystem, does not have to frive its own water pump or alternator, and so a real figure
- DIN (Deutsche Engineuer Norm) which is a more real ilfe figure because the engine does have to deal with its own ancilliaries and it so represents a value more in line the engine installed in your car.
I do no know exactly, but for the same engine, DIN is about 15 bhp less than SAE.
Driiving at 70 mph requires a given BHP to overcome rolling resistance, Wind resistance, and the power consumed by the drive train, mainly gearbox and back axle.
If you measure the power at the rear wheels, you have accounted for all the above except the Rolling rsistance of the two non-driven wheels and a ballpark figure is about 18-20% of the DIN engine power. bhp lost (although it will vary with speed of the vehicle.
My practival experience is that!
a) My 1938 Morris 8 had 28 SAE (18 DIN) bhp and aerodynamics of brick wall. On a good day would make 50-55 mph. From the above, this was achieved on about 12-14 hp at the wheels
b) my mate's Austin A35 35 bhp SAE (About 22 DIN)would make about 65 mph . Achieved on maybe 20 bhp at the rear wheels
c) My Scimlitar Coupe (nicely aerodynamic) would make 120 mph on 138 bhp DIN and so about 110 bhp at the rear wheels
d) MG Magnette 72 bhp SAE (60 DIN) would make about 80 mph on about 550DIN at rear wheels
A very good article in Wikipédia provides formulae that for a given vehicle and all other things equal, the BHP required for a given speed, is proportional to the cube of the velocity. They refer to engine power without definition, but I think we should talk about BHP at the rear wheels.
Applying the average of my real life examples the ôwer for 70 mph ranged from 27 tp 46 bhp DIN, with an average of 40.5.
I will be the first to admint that these numbers are very subjective and open to question, but at least this gives a directional guide.
I would only comment that whatever the real value, the power consumption for hubrids will be about the same, after adjustment for power losses during electricty generation, and use, including heat losses in the battery . Just remember that the electriciy has to be produced by a power station with a carbon footprint of around 0.5 kg carbon per Horse power, or by the engine in a hybrid. Maybe others would like to estimated grms CO2 km for 100% gasoline and 100% electrcity from a power station at 70 mph ?
I have written enough for now !!!
To comment on this 4 year old thread, I'd like to present a alternative and simple calculation. The DOE states that one gallon of gasoline has 33.7 kW.hrs of energy content. Additionally, modern gasoline engines are 25 - 30% efficient, maybe a bit more. If your car does 25 miles per gallon while cruising on the highway, then at 70 mph fuel efficiency will be somewhat less, so let's assume it does 22 mpg.
One hour at 70 mph will then use 70/22 = 3.18 gallons which have an energy content of 3.18 x 33.7 = 107 kW. hr.
If only 25% of that that energy is useful then the power required would be 107x0.25 = 27 kWs or 36 hp, which agrees roughly with the previous (more rigorous) calculations. Obviously better mpg would reduce the figure.
One hour at 70 mph will then use 70/22 = 3.18 gallons which have an energy content of 3.18 x 33.7 = 107 kW. hr.
If only 25% of that that energy is useful then the power required would be 107x0.25 = 27 kWs or 36 hp, which agrees roughly with the previous (more rigorous) calculations. Obviously better mpg would reduce the figure.
Edited by Powerguy on Sunday 14th October 16:54
Powerguy said:
To comment on this 4 year old thread, I'd like to present a alternative and simple calculation. The DOE states that one gallon of gasoline has 33.7 kW.hrs of energy content. Additionally, modern gasoline engines are 25 - 30% efficient, maybe a bit more. If your car does 25 miles per gallon while cruising on the highway, then at 70 mph fuel efficiency will be somewhat less, so let's assume it does 22 mpg.
One hour at 70 mph will then use 70/22 = 3.18 gallons which have an energy content of 3.18 x 33.7 = 107 kW. hr.
If only 25% of that that energy is useful then the power required would be 107x0.25 = 27 kWs or 36 hp, which agrees roughly with the previous (more rigorous) calculations. Obviously better mpg would reduce the figure.
That doesn't really work for me. It would be a fairly inefficient car that only does 22mpg at a steady 70mph. I would imagine your average 1.6 diesel is capable of 3 times that - somewhere in the region of 60-70mpg at a steady 70mph. It would therefore require just 12hp to drive at 70, which is nowhere near correct. I feel you've fiddled the figures until you've come with a reasonable sounding answer.One hour at 70 mph will then use 70/22 = 3.18 gallons which have an energy content of 3.18 x 33.7 = 107 kW. hr.
If only 25% of that that energy is useful then the power required would be 107x0.25 = 27 kWs or 36 hp, which agrees roughly with the previous (more rigorous) calculations. Obviously better mpg would reduce the figure.
Edited by Powerguy on Sunday 14th October 16:54
IanCress said:
Powerguy said:
To comment on this 4 year old thread, I'd like to present a alternative and simple calculation. The DOE states that one gallon of gasoline has 33.7 kW.hrs of energy content. Additionally, modern gasoline engines are 25 - 30% efficient, maybe a bit more. If your car does 25 miles per gallon while cruising on the highway, then at 70 mph fuel efficiency will be somewhat less, so let's assume it does 22 mpg.
One hour at 70 mph will then use 70/22 = 3.18 gallons which have an energy content of 3.18 x 33.7 = 107 kW. hr.
If only 25% of that that energy is useful then the power required would be 107x0.25 = 27 kWs or 36 hp, which agrees roughly with the previous (more rigorous) calculations. Obviously better mpg would reduce the figure.
That doesn't really work for me. It would be a fairly inefficient car that only does 22mpg at a steady 70mph. I would imagine your average 1.6 diesel is capable of 3 times that - somewhere in the region of 60-70mpg at a steady 70mph. It would therefore require just 12hp to drive at 70, which is nowhere near correct. I feel you've fiddled the figures until you've come with a reasonable sounding answer.One hour at 70 mph will then use 70/22 = 3.18 gallons which have an energy content of 3.18 x 33.7 = 107 kW. hr.
If only 25% of that that energy is useful then the power required would be 107x0.25 = 27 kWs or 36 hp, which agrees roughly with the previous (more rigorous) calculations. Obviously better mpg would reduce the figure.
Edited by Powerguy on Sunday 14th October 16:54
But diesel has a slightly higher energy density, plus the efficiency figures are considerably higher, balancing things out in the other direction, meaning that more bhp needed will result from the equation.
In short, you can not use the unmodified petrol equation above for a diesel, as then the output will be suspiciously incorrect, as you did correctly note.
IanCress said:
That doesn't really work for me. It would be a fairly inefficient car that only does 22mpg at a steady 70mph. I would imagine your average 1.6 diesel is capable of 3 times that - somewhere in the region of 60-70mpg at a steady 70mph. It would therefore require just 12hp to drive at 70, which is nowhere near correct. I feel you've fiddled the figures until you've come with a reasonable sounding answer.
Not sure we’re on the same page here. Far from fiddling, if you just googled “mpg at 70 mph” you’d get a lot of information - for example a 2018 Toyota Highlander has an advertised 28 mpg on the highway; and that mpg is correct at 55 mph. At 70 mph it would be about 17% less efficient, bringing the figure close to the 22 mpg that I stated.And - why did you bring up diesel anyway? Calculation was done on the basis of energy content for gasoline - diesel energy content and engine efficiency is an entirely different calculation. You can do it if you want, my purpose is served by pointing out the possibility of an alternative theoretical approach based on energy content and engine efficiency.
Powerguy said:
Not sure we’re on the same page here. Far from fiddling, if you just googled “mpg at 70 mph” you’d get a lot of information - for example a 2018 Toyota Highlander has an advertised 28 mpg on the highway; and that mpg is correct at 55 mph. At 70 mph it would be about 17% less efficient, bringing the figure close to the 22 mpg that I stated.
And - why did you bring up diesel anyway? Calculation was done on the basis of energy content for gasoline - diesel energy content and engine efficiency is an entirely different calculation. You can do it if you want, my purpose is served by pointing out the possibility of an alternative theoretical approach based on energy content and engine efficiency.
Fair enough, forget diesel. My old 1.0 Ecoboost Fiesta would do around 60mpg at 70mph.And - why did you bring up diesel anyway? Calculation was done on the basis of energy content for gasoline - diesel energy content and engine efficiency is an entirely different calculation. You can do it if you want, my purpose is served by pointing out the possibility of an alternative theoretical approach based on energy content and engine efficiency.
IanCress said:
Fair enough, forget diesel. My old 1.0 Ecoboost Fiesta would do around 60mpg at 70mph.
The trip computer might say that but I very strongly doubt that's anywhere close to being accurate.I could believe 60mpg for a diesel Fiesta but to achieve that in a petrol Fiesta you'd have to stay in L1 with the lorries and I'd make an educated guess that the true figure for a petrol Fiesta is probably closer to 45 mpg at a steady 70mph.
IanCress said:
Fair enough, forget diesel. My old 1.0 Ecoboost Fiesta would do around 60mpg at 70mph.
Good for you, Ian .. or for your Fiesta! It just goes to show that every car is different, but the calculation remains correct - although maybe too simplistic for some.As a disclaimer, I mentioned that the approach is not terribly rigorous. However if you know your overall thermal efficiency accurately and your mpg, it’s a surprisingly quick and dirty calculation, especially for people not well-versed in rolling friction dynamics like myself
Ron99 said:
IanCress said:
Fair enough, forget diesel. My old 1.0 Ecoboost Fiesta would do around 60mpg at 70mph.
The trip computer might say that but I very strongly doubt that's anywhere close to being accurate.I could believe 60mpg for a diesel Fiesta but to achieve that in a petrol Fiesta you'd have to stay in L1 with the lorries and I'd make an educated guess that the true figure for a petrol Fiesta is probably closer to 45 mpg at a steady 70mph.
IanCress said:
Not sure why you think the trip computer would be that far out.
Certainly when worked out at the pumps the car was quite capable of 60mpg on a relaxed run, sticking to 70mph. Sticking with the lorries I saw up to 74mpg indicated, over 30 miles, but never had the patience to do that over a whole tank.
You probably had favourable wind and weather conditions. If you only measured over a short distance your fuel gauge was also probably lying since most gauges are not linear and not accurate.Certainly when worked out at the pumps the car was quite capable of 60mpg on a relaxed run, sticking to 70mph. Sticking with the lorries I saw up to 74mpg indicated, over 30 miles, but never had the patience to do that over a whole tank.
Also, over at Honest John's 'real mpg' pages the reported mpg of the previous generation Fiesta 1.0T is averaging in the mid-40s.
I would like to see photographic evidence to show that you - or anyone else - can consistently achieve 60mpg at an average speed of 70mph in any petrol car over hundreds of miles.
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