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bhp/Torque





Found this very informative piece of info for those who want to learn!



Originally horsepower was considered to be one unit when a horse could raise, by single pulley, one hundred fifty pounds at a rate of 2.5 mph, or 550 foot pounds per second.

This in energy terms turns out to be 746NM per second, or in electrical terms is 746Watts.
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Mechanical horsepower = torque (in foot-pounds) * RPM / 5252.

The torque output of an internal combustion engine varies with RPM, being zero at very low RPM, rising to a peak level, then falling to zero again at the engines maximum speed. Some engines produce high torque only in a narrow band of output speeds, others have a wider power band. Maximum horsepower is determined by looking at the point on the speed-torque chart where torque*rpm is greatest. This gives you an idea of the maximum power the engine can produce, but to get the full picture you need to look at the power/speed curve to see how the engine will behave in reality.

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***MAD POST ALERT BELOW***

First, note that there is no such thing as a "torque" engine. Any engine that produces 300 ft-lbf of torque at 3000 rpm has 171 hp at that speed. Regardless of its stroke, bore, displacement, supercharger, etc. Also, any engine that makes 300 ft-lbf of torque at 5000 rpm will produce 286 hp. This is due to the simple relationship between power and torque:

power = (torque * rpm) / 5250

where power is in horsepower, and torque is in ft-lbf. Also note - at 5250 rpm, hp is equal to torque numerically.

Racing engines are designed to produce a lot of torque at high engine speeds, because this will lead to increased horsepower. This engine can then be geared for whatever range of speeds it will normally be driven in.

The biggest problem with production cars is they spend the vast majority of their time under 4000 rpm, and larger engines (like my Mustangs 4.6 l) spend most of their time below 2500 rpm. Another related problem is that due to the direct relationship between power and rpm, the power produced at these low engine speeds is low. So to get more power at common driving engine speeds, you must therefore increase the torque at these lower engine speeds.

Engine power increases with rpm because power is a function of time. Torque, on the other hand, is relative to the force exerted on the piston by the expanding gases in the cylinder. If you had perfect, equal cylinder filling on each stroke, then this force would be proportional to the bore, or displacement of the cylinder, and this force would be equal per stroke regardless of rpm. You can think of power as being determined therefore by how often this cylinder fills and fires in a given amount of time. One of the reasons two-stroke engines produce so much more power than a comparable four-stroke engine - although it has a poorer volumetric efficiency (the efficiency of filling the cylinder with fuel-air mixture), it fires twice as many times in the same amount of time.

So to increase low-end torque, we must actually also see what increases low-end power. And vice-versa. Engine torque will derive from the pressure exerted on the top of the piston by the exhaust gases. The average pressure exerted on the piston through the length of the stroke is known as the mean effective pressure (mep), and is given in terms of psi. Another measure, imep or indicated mean effective pressure, is determined by using some fancy equipment to actually measure on a running engine what the pressure is. Engineers commonly use mep as an indication of power plant performance in internal combustion (IC) engines.

Because its pretty hard to measure the imep, another measure, brake mean effective pressure, or bmep, is often calculated in its stead. This equation is:

bmep = (hp * 33,000) / (Length * Area * N)

where hp is power in horsepower, Length is the length of the stroke in ft, Area is the piston top surface area in square inches, and N is the power strokes per minute (or rpm / 2 for a four-stroke engine).

Now, what are the things that affect this bmep? Well, the first is volumetric efficiency. This of course is the efficiency of filling the displacement of the engine with fuel and air mixture on each stroke. You could also look at it this way - if a cylinder only fills with 85% efficiency on each stroke, this is similar to reducing your engine displacement (size) by 15%. Due to frictional losses, valve overlap, etc. this will not ever be 100% in a naturally aspirated engine. In fact, this efficiency falls off steadily as engine speed increases. One way to get back these losses is to use a turbo or supercharger, which will compress air into the cylinder, and thus give a volumetric efficiency relative to a naturally aspirated engine of more than 100% (but its not actually more than 100%, because you cant look at it that way…)

What else increases volumetric efficiency? Well, tuning of the exhaust and intake runners will achieve some level of "pressure wave supercharging", to help move the intake gases in and the exhaust gases out. A lower restriction air cleaner, fuel injection system, intake manifold, larger and more valves, etc. will also increase airflow into the engine. Likewise, lower restriction exhaust, removal of the catalyst, larger and more exhaust valves, etc. will allow the exhaust gases to flow out of the cylinder more easily. Also note that for both the intake and exhaust cycles, the valve timing and lift are also key to not only achieving greater volumetric efficiency, but also to tuning where the maximum possible flow through the valves will occur.

And when youve looked at what you can do with respect to volumetric efficiency, there is now friction horsepower loss to contend with. This tends to increase as a function of the square of the rpm of the engine, and can be very substantial at high speeds. One reference I have here gives the friction horsepower of a "stock" 350 Chevy as being 12 hp at 2000 rpm, 25 hp at 3000 rpm, 47 hp at 4000 rpm, 72 hp at 5000 rpm, and 110 hp at 6000 rpm. This friction loss can be reduced by careful engine building and lower-friction bearings and components, and by synthetic oils. But the trend of increase remains the same shape.

Now lets look at bore and stroke. Increasing the stroke of an engine by increasing the crankshaft throw not only increases the displacement of the engine, but also increases the mechanical advantage on the crankshaft. So increasing the stroke increases the torque both from increased displacement and from greater mechanical advantage. Thus, even for two engines of the same displacement, the one with the greater stroke should have the greater torque. However, there are some other things to consider here:

1) As stroke length increases, thus does the piston speed at the same rpm, and therefore the frictional power loss will increase as well.

2) As this piston speed increases, the volumetric efficiency will fall off as well. A larger bore engine will also allow for larger and/or more valves, with less shrouding needed. Thus, if the two engines have the same displacement, then the shorter stroke one will have the greater volumetric efficiency.

3) You also have to consider inertia effects of the longer connecting rod, and increased side-forces on the rings and piston due to the increased rod angularity with respect to the bore centerline.

Of course, there are those that will argue that a larger bore engine may result in an increased chance of detonation at high compression ratios than a smaller bore engine, due to an increased distance that the flame front must travel across. But this will vary greatly due to combustion chamber design, and I dont know if it can be analyzed so generally. Overall, many claim that the small bore/long stroke engines are much better suited to the higher compression ratios, and if in our example of the two equal sized engines we also have a higher compression ratio than the larger bore one, then the larger stroke engine will see even more torque improvement.

Another theory related to this is the "long rod" theory, which says that the longer the connecting rod is, the more time or "dwell" the piston will have at top dead center (TDC). And therefore, there will be more time allowed for the gases to burn completely at the highest engine pressure. Thus, you will get even more force out of each firing with the same amount of fuel as the rod descends through its stroke. However, what really happens is that the long rod has a very small mechanical advantage due to its angularity at these high pressures, and thus this effect is negated. A short rod, attached to a crankshaft with a long throw, will move towards a higher angularity faster than a long rod with a short crank throw.

Another advantage of a long throw/short rod engine is that cam timing is not nearly so important as in a short throw/long rod engine. This is due to the fact that the piston spends less time at or near TDC, and thus allows for an earlier valve lift start and longer duration. Also, the exhaust valve can stay open longer as the piston ascends, increasing the amount of exhaust gas that is pushed out of the cylinder, and thus increasing the volumetric efficiency.

And so on, and so on. There are a great many things I have left out or glossed over, because I am tired of typing. Hope this helps somewhat more.
 


nice one m8 !..

I am being a wee bit pedantic here lol.. but just a slight clarification..

2) As this piston speed increases, the volumetric efficiency will fall off as well. A larger bore engine will also allow for larger and/or more valves, with less shrouding needed. Thus, if the two engines have the same displacement, then the shorter stroke one will have the greater volumetric efficiency.

The VE will not alter directly in relation to bore / stroke combinations, ie, it is not directly related to piston speed. (AFAIK !)

And yes, he is saying the right thing but in a roundabout way.. ie - a shorter stroke engine will have a higher VE ....IF the larger valves that the larger bore size allows are fitted in a properly designed head, with a properly designed cam profile, and, the induction system as a whole is designed around this..

Ultimately it can be easily proven that ultimate power output is directly proportional to valve throat area. !

Bloody good article though m8.. respect !

Joe.:)
 


Quote: Originally posted by CUPSIZE? on 15 October 2002

Any engine that produces 300 ft-lbf of torque at 3000 rpm has 171 hp at that speed. Regardless of its stroke, bore, displacement, supercharger, etc. Also, any engine that makes 300 ft-lbf of torque at 5000 rpm will produce 286 hp. This is due to the simple relationship between power and torque:

power = (torque * rpm) / 5250

where power is in horsepower, and torque is in ft-lbf. Also note - at 5250 rpm, hp is equal to torque numerically.


Does this mean that the 172 which has roughly half this figure 147 ft-lb will have approx 76.5BHP at 3000RPM and 143BHP at 5000rpm then at 5250 the torque will equal BHP numerically? ie 147BHP and 147 ft-lb - nice one - this must be at the flywheel though because Max BHP comes in after torque has reached its max - ie past 5250RPM we go up to 172BHP appox at x RPM?
 


Thats a good little article. I think there should be a place where every one can add their knowledge / theories to build up a knowledge database on cliosport. A little extra know-how is always appreciated.

A couple of things about that I cant get my head round though. First, how does the length of the con-rod affect the time taken at TDC? And secondly, if small bore / large stroke is such a good thing then why do all performance engines (look at any bike engine) use large bore / small stroke?
 


Ok Steve, its about angles.. if the stroke is the same, a shorter rod will go through a faster arc at the top of the stroke, a longer rod will form a shorter arc. Try to imagine a rotating pivot (crank), a rod ir differing length - short and long, and a top pivot (little end or piston pin) - draw it on a pice of paper even. You will see that the there is an angle formed by the rod to piston pin, first one way, and then the other as it passes tdc. on the shorter rod the angle is greater, therefore the time at tdc varies with rod length. (Hope that makes sense !)

a small bore large stroke or vica versa is not an indicator of suitability for any particular application. The piston speed on a long stroke engine reaches greater acceleration than on a shorter stroke for the same rpm, hence, loading on the rod is increased (and on the crankpin!) - this may be of no consequence to a normal car, but, on a bike the engines tend to rev far higher and the shorter stroke means less load (hence less material costs) - also, the maximum valve area can be increased for the same cc due to the size of the head chamber / bore.

On a car - such as the clio, the longer stroke is chosen as rpm is not really a problem (ie - not too high relatively).. the key thing to modern engine design is often making it FIT in the engine bay, if you go for a short stroke, larger bore, then the engine will be longer as the diameter of the 4 pots is greater. Longer stroke usually means more torque at low revs too which is better suited to a road car.

there is no ideal, horses for courses...

Joe.:)
 
  2005 Audi A3 3.2 Quattro


Okay, Im trying to get my head around all this now

My 1.4 is rated at 99 lb-ft torque at 3750 rpm, 95 bhp @ 5750

do I then use the formula hp = torque * RPM / 5250 like in the following.

hp = 99lb-ft * 3750 / 5250

hp = 70.71hp @ 3750 RPM

How would I then go on to so something like Captain spreadsheet where he has power and torque figures throughout the RPM range?
 


Hi M8, yes, thats correct...

and also 86 foot pounds of torque at max rpm...

to do what I have done you NEED an accurate torque plot from a rolling road for your car.

from that you can calculate VE, BHP etc.

Joe.
 


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