Discussion
Chaps (etc),
As you will gather I'm a technical simpleton but something is bothering me and I hope someone can help shed some light on a simple technical conundrum for me.
As a keen private pilot I own/fly an aircraft that has an engine rated at 150hp (seems small by sportscar standards I know - and it's a 5 litre!). My car gumpf however rates the car's power as Bhp (ie Brake Horsepower) at a certain RPM.
So my questions:-
What is the difference between the hp of an aero engine and the Bhp of a car engine?
How does Bhp relate to RPM - ie why is Bhp quoted at a given RPM, what's the relationship betwen the two?
Thanks a mill....
As you will gather I'm a technical simpleton but something is bothering me and I hope someone can help shed some light on a simple technical conundrum for me.
As a keen private pilot I own/fly an aircraft that has an engine rated at 150hp (seems small by sportscar standards I know - and it's a 5 litre!). My car gumpf however rates the car's power as Bhp (ie Brake Horsepower) at a certain RPM.
So my questions:-
What is the difference between the hp of an aero engine and the Bhp of a car engine?
How does Bhp relate to RPM - ie why is Bhp quoted at a given RPM, what's the relationship betwen the two?
Thanks a mill....
Indicated HP Is the actual power the cylinders are producing, calculated by engine pressure,RMP and displacement.
BHP Is the power produced by the engine. Which is calculated by indicated HP minus Friction Loss.
Called brake horsepower because a brake is used (friction loss)
I think
>> Edited by cammers on Friday 16th August 19:36
BHP Is the power produced by the engine. Which is calculated by indicated HP minus Friction Loss.
Called brake horsepower because a brake is used (friction loss)
I think
>> Edited by cammers on Friday 16th August 19:36
quote:
In marine engines it's quoted as SHP - shaft horsepower, ie. the horse power available at the propellor shaft.
I'm surprised aero piston engines aren't measured in the same way. I'd have thought SHP at the propellor would be a good measure.
Strangely it's SHP for rotary wing (helicopters) but HP for fixed wing.....
Anorak mode off
>> Edited by dougclayton on Friday 16th August 17:25
way back in the early days of the industrial revolution horses were worked relentlessly. they would be asked to move 330 pounds through one foot in one second time and time again, and the unit of one horse power was born.
then the unions got involved, and insisted on a rest period every hour to prevent the horses being overstressed. hence the term *Break* Horse Power came into being ..
end of lesson.
then the unions got involved, and insisted on a rest period every hour to prevent the horses being overstressed. hence the term *Break* Horse Power came into being ..
end of lesson.
Thanks for all your replies gents - I've certainly learnt something here although I'm still a little confused as to why Hp is quoted at a given RPM - why is this not just a max RPM or is there a power peak where piston engines are most efficient?....sorry I'm such a numpty on this tech stuff.
Interesting that the marine and helicopter quotes are different again.
Thanks all, interesting stuff.
Interesting that the marine and helicopter quotes are different again.
Thanks all, interesting stuff.
quote:
way back in the early days of the industrial revolution horses were worked relentlessly. they would be asked to move 330 pounds through one foot in one second time and time again, and the unit of one horse power was born.
then the unions got involved, and insisted on a rest period every hour to prevent the horses being overstressed. hence the term *Break* Horse Power came into being ..
end of lesson.
That's by far the best explanation!quote:
way back in the early days of the industrial revolution horses were worked relentlessly. they would be asked to move 330 pounds through one foot in one second time and time again, and the unit of one horse power was born.
then the unions got involved, and insisted on a rest period every hour to prevent the horses being overstressed. hence the term *Break* Horse Power came into being ..
end of lesson.
Horsepower is related to engine speed because it is defined roughley as "RATE of doing work". The work in this case is producing effort at the crank i.e torque. Hence horsepower is defined simpley as a factor of torque and engine speed. Hence:
HP = torque (ft/lbs) x RPM
-------------------
5252
I might have the RPM and 5252 the wrong way round here!
Whenever you see a dyno chart of horsepower and torque (on the same axis) against engine speed, the lines will always cross at 5252 rpm no matter what the engine.
As for how "brake" horsepower relates to horsepower i'm not too sure!
HP = torque (ft/lbs) x RPM
-------------------
5252
I might have the RPM and 5252 the wrong way round here!
Whenever you see a dyno chart of horsepower and torque (on the same axis) against engine speed, the lines will always cross at 5252 rpm no matter what the engine.
As for how "brake" horsepower relates to horsepower i'm not too sure!
quote:
I might have the RPM and 5252 the wrong way round here!
Nope, they're the right way round. This also means that hp will always be higher than tourque above 5252 rpm. Which could be why the plane's engine is 150hp, Its probably doing 2000rpm and shoving out 400 Lbs of tourqe. Maybe ?? Do you know what it revs to Andy ?
As I understand it, the 'Brake' is quoted because of the way the power output is meassured. By the application of a brake. I thought it was 500Ib/ft/sec? But I'm preparred to bow to superior knowledge
And to answer your question, yes the RPM figure is quoted to show where max BHP is reached in the rev range. An engine becomes less efficient beyond a specific point. Helps to tell if an engine is going to be 'peaky' (F1 car ....850bhp @ 18,500rpm) or 'flexible' (My Ultima 450bhp at 5,800rpm) The F1 car has about 400ft/Ib of torque at about 9,000rpm, my Ultima has 500+ft/Ib at about 3,500rpm. One is suited to road use, the other to track use.
And to answer your question, yes the RPM figure is quoted to show where max BHP is reached in the rev range. An engine becomes less efficient beyond a specific point. Helps to tell if an engine is going to be 'peaky' (F1 car ....850bhp @ 18,500rpm) or 'flexible' (My Ultima 450bhp at 5,800rpm) The F1 car has about 400ft/Ib of torque at about 9,000rpm, my Ultima has 500+ft/Ib at about 3,500rpm. One is suited to road use, the other to track use.
I've got an engineering reference book handy with a whole section devoted to conversion between different measurements.
Anyway, it lists several different types of "horsepower" - mechanical, boiler, electric, metric and water. No references to either "brakes" or "breaks". ;->
1 Mechanical horsepower is equivalent to (among many, many other things), 550 ft-lb/sec. The other types of horsepower have their own conversions - some different and some that are similar.
This is an American reference book and, being an American, I have never heard the term "brake horsepower" in common use in the US while it is universally used in Britain.
My guess is that this is simply a matter of terminology.
Also, someone asked about their airplane's rather large engine putting out what seemed to be a relatively small amount of power as well as the practice of quoting RPMs when specifying the power of an engine.
Many factors will determine what sort of power an engine can or will produce. The engine designer has to balance conflicting issues like smoothness of operation, reliability, where peak power is produced, cost of materials for stressed components etc. If cost and user-friendliness were no object, we could all have cars with 2 liter engines putting out 500 hp although they'd be a total pain to drive around town.
Most engines, regardless of size and power, do their best work in a relatively narrow range of RPMs. Engines that produce power in lower ranges (typically those with more pistons and/or larger displacement) are sometimes referred to are being "torquey" (e.g. Dodge Viper's 8 liter V-10) and those who produce their power at high speeds (e.g. Honda S2000's 2.0 liter I-4) are referred to as "peakey". Those engines are actually a good comparison of this phenomena since both are normally aspirated (i.e. no turbo/super-charging).
The best way to get a true picture of the power of an engine is to look at a 2-dimensional chart of the hp and/or torque against engine RPM. You will quickly see what I'm talking about. Most companies quote their engine's peak power (why wouldn't they?) and include the RPMs as sort of "fine print". Most people don't really pay attention to that bit.
Put another way, which engine would you rather have, Honda's 2.0 liter I-4 that makes 240hp at 8900 RPM or Jaguar's 3.0 Liter V-6 that only makes 231hp but does it at 2800 RPM with less noise and less vibration?
Just my $.02
Anyway, it lists several different types of "horsepower" - mechanical, boiler, electric, metric and water. No references to either "brakes" or "breaks". ;->
1 Mechanical horsepower is equivalent to (among many, many other things), 550 ft-lb/sec. The other types of horsepower have their own conversions - some different and some that are similar.
This is an American reference book and, being an American, I have never heard the term "brake horsepower" in common use in the US while it is universally used in Britain.
My guess is that this is simply a matter of terminology.
Also, someone asked about their airplane's rather large engine putting out what seemed to be a relatively small amount of power as well as the practice of quoting RPMs when specifying the power of an engine.
Many factors will determine what sort of power an engine can or will produce. The engine designer has to balance conflicting issues like smoothness of operation, reliability, where peak power is produced, cost of materials for stressed components etc. If cost and user-friendliness were no object, we could all have cars with 2 liter engines putting out 500 hp although they'd be a total pain to drive around town.
Most engines, regardless of size and power, do their best work in a relatively narrow range of RPMs. Engines that produce power in lower ranges (typically those with more pistons and/or larger displacement) are sometimes referred to are being "torquey" (e.g. Dodge Viper's 8 liter V-10) and those who produce their power at high speeds (e.g. Honda S2000's 2.0 liter I-4) are referred to as "peakey". Those engines are actually a good comparison of this phenomena since both are normally aspirated (i.e. no turbo/super-charging).
The best way to get a true picture of the power of an engine is to look at a 2-dimensional chart of the hp and/or torque against engine RPM. You will quickly see what I'm talking about. Most companies quote their engine's peak power (why wouldn't they?) and include the RPMs as sort of "fine print". Most people don't really pay attention to that bit.
Put another way, which engine would you rather have, Honda's 2.0 liter I-4 that makes 240hp at 8900 RPM or Jaguar's 3.0 Liter V-6 that only makes 231hp but does it at 2800 RPM with less noise and less vibration?
Just my $.02
Ah, that would be a "brake" as in a dynamometer then, the device with applys the loading force against which the engine tested. Our test engineers refer to them as "brakes", or "water-brake". There are generally two different types, a hydraulic type using water and an electric type working on an eddy current principle.
For a car engine these can be either a chassis dyno/rolling road, measuring "at the wheel" horsepower, or as an engine dyno, measuring "at the flywheel" horsepower. These are both "braked" horsepower measurements as the engine was loaded or "braked" using the dyno. Not sure about straight HP, presume this is a theoretical or calculated value.
In general the flywheel figures given to you after a chassis dyno/rolling road session are a best guess of the losses from the drive train. Some dynos have a pretty clever run-down check which measures the losses on the run down from the power run, some dyno operators add a percentage depending on experience, some just guess !!
The largest dyno we have in our factory is good for over 20,000bhp..... but then we are testing gas turbines !!
For a car engine these can be either a chassis dyno/rolling road, measuring "at the wheel" horsepower, or as an engine dyno, measuring "at the flywheel" horsepower. These are both "braked" horsepower measurements as the engine was loaded or "braked" using the dyno. Not sure about straight HP, presume this is a theoretical or calculated value.
In general the flywheel figures given to you after a chassis dyno/rolling road session are a best guess of the losses from the drive train. Some dynos have a pretty clever run-down check which measures the losses on the run down from the power run, some dyno operators add a percentage depending on experience, some just guess !!
The largest dyno we have in our factory is good for over 20,000bhp..... but then we are testing gas turbines !!
Hi all,
I have wondered on this topic before too. How could my Range Rover 3.5litre V8 (this is before I bought my Chimaera
) have only 135bhp but my mates 2.0litre engine give 215bhp? Yet I know which would be quicker in an equal weight car.
And its to do with the equation that Jon showed. Bhp is the engine torque MULTIPLIED by the rpm. So if the peak torque occurs at a high rpm, then you can multiply by a BIG number and hence get a BIG Bhp figure (eg multiply by 18000 for a F1 car). Whereas a V8 produces peak torque at low rpm so you have to multiply by a low number so the Bhp figure doesn't look as impressive. This is also why my mate has to sit in 1st waiting for the revs to build before he can get some acceleration whereas the Chimaera should really come with a set of neck braces!
This is also why F1 cars stall on the grid, because at low rpm they've hardly got enough torque to get the weight of the car moving.
I think all manufacturers should be forced to print their peak torque figures, so that we can see the real engine performance.
Matt
I have wondered on this topic before too. How could my Range Rover 3.5litre V8 (this is before I bought my Chimaera
) have only 135bhp but my mates 2.0litre engine give 215bhp? Yet I know which would be quicker in an equal weight car. And its to do with the equation that Jon showed. Bhp is the engine torque MULTIPLIED by the rpm. So if the peak torque occurs at a high rpm, then you can multiply by a BIG number and hence get a BIG Bhp figure (eg multiply by 18000 for a F1 car). Whereas a V8 produces peak torque at low rpm so you have to multiply by a low number so the Bhp figure doesn't look as impressive. This is also why my mate has to sit in 1st waiting for the revs to build before he can get some acceleration whereas the Chimaera should really come with a set of neck braces!
This is also why F1 cars stall on the grid, because at low rpm they've hardly got enough torque to get the weight of the car moving. I think all manufacturers should be forced to print their peak torque figures, so that we can see the real engine performance.
Matt
I thought that they did? Even more useful would be to see the torque curves at the wheels for each gear - this allows you to compare each car far more accurately, so long as you know the weight of the car as well.
taylormj4 said:I think all manufacturers should be forced to print their peak torque figures, so that we can see the real engine performance.
Matt
[quoteI have wondered on this topic before too. How could my Range Rover 3.5litre V8 (this is before I bought my Chimaera ) have only 135bhp but my mates 2.0litre engine give 215bhp? Yet I know which would be quicker in an equal weight car.
Sorry, but unless it produces it's power in an extremely narrow band, the 215bhp engine should trounce the 135 one, given correct gearing. Or look at it another way, given correct gearing, the 215bhp engine can produce more torque at the wheels AT ANY SPEED compared to the 135 one (because it can use it's higher revs to translate the lower torque at a higher speed into higher torque at a lower speed). Of course, actual gearing will compromise this slightly, as will the width of the power bands from torque peak to power peak (the optimum gear change points).
) have only 135bhp but my mates 2.0litre engine give 215bhp? Yet I know which would be quicker in an equal weight car. Sorry, but unless it produces it's power in an extremely narrow band, the 215bhp engine should trounce the 135 one, given correct gearing. Or look at it another way, given correct gearing, the 215bhp engine can produce more torque at the wheels AT ANY SPEED compared to the 135 one (because it can use it's higher revs to translate the lower torque at a higher speed into higher torque at a lower speed). Of course, actual gearing will compromise this slightly, as will the width of the power bands from torque peak to power peak (the optimum gear change points).
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