Understanding Power,torque And Engine Size

[rion]

Hi guys, I made a research on the web if I can understand Power,Torques and Engine Size and it's relationship to each other. and I'm still scratching my head. :P

can anyone care to explain me in the most simpliest form? :P

also, I don't get it why some Engines have bigger size like a 2.5 Diesel can have different Output and Tourqes compared to another a different Diesel Engine with the same Size...

[T600]

If you really want to know, then I'll try; the concept is dead easy, but explaining it isn't such a piece of cake.

Torque is the rotational counterpart of force. i.e. the engine's ability to turn the drivetrain, and ultimately the wheels.

Relationship between torque and power is very simple. Torque x rotational speed = power.

Power is the ability to 'do work'; in this case accellerate the car.

Engines with lots of torque produce more power at low revs; so you get power without having to rev the nuts off them. Alternatively some engines don't produce so much torque, but still achieve reasonably high power by using lots of revs - e.g. the Toyota VVTLi engine. That engine is a great example; Toyota/Yamaha took the basic 1.8 engine and gave it the ability to rev to much higher speeds - lots more power without really increasing the torque. Traditionally more power was achieved by increasing torque - e.g. stick a turbo or supercharger on - more torque, hence more power without increasing the rpm.

As to engine design - I guess the ideal is a reliable, low weight, simple engine that can produce lots of torque across a wide rev range and as a result lots of power, and at the same time use as little fuel as possible. (Electric motor then?)

But that's not so simple to achieve, so different manufacturers take different design decisions. This leads to huge differences between apparently similar engines.

Diesel engines tend to produce lots of torque and are efficient, but don't rev to the same limits as a petrol engine; they therefore tend to have lower power relative to the torque that they produce.

Torque is worth considering if you want power off the line without lots of revs. But power to weight ratio is the key performance factor.

Fortunately, you don't really need to worry about all this - it's just detail really. The key factors are are the bottom line - mpg and 0-60.

Hope that was reasonably simple.

[PaulT]

You want more power out of your petrol engine. A few possibilities:

1. Increase the cylinder capacity/add more cylinders.

2. Add a compressor to stuff more mixture into the cylinders.

3. Invent some valve trickery which allows high power outputs at the top end.

In these times of EU and US emission regulations, forget about doing 1 or 2 if you are producing a mass-selling car. Your best bet is to add valve trickery, go for high end power and low enough torque that you can make the engine and transmission light and fuel efficient. As a bonus, get better handling to go with it

Paul.

ps. power is really the *rate* of doing work, T600 :bookworm:

[T600]

ps. power is really the *rate* of doing work, T600  :bookworm:

←

You're right. That's what I meant, but 'rate' would have been a better choice of word.

[nrgizerbunny]

THe only thing that actually matters is power.

as the replies above have indicated power is the product of torque and RPM (not strictly true units - RPM is in rad/s)

Power = Torque x RPM

It doesn't actually matter if a particular engine has naff all torque as it can be geared with a short ratio (like the ratio used to pedal up a hill on a bike).

If an engine has loads of torque but no RPM (such as a diesel.) then its typically geared very long (like a big gear ona push bike) and that allows slow engine speed but high road speed.

--------Engine design-------------

Most engine configurations are usually square or slightly off square - (note this is a big assumption and some radical engines don't follow this)

square means the bore (the diameter of a piston) and the stroke (the distance the piston moves up and down) are almost the same.

Engine power should really be measured in BMEP (brake mean effective pressure) that takes into account the engine capacity of the engine when calculating efficiency - therefore its possible to compare a 1.0l metro with a 6.0l hummer.

I would recommend you go down your local libraray and get "introduction to internal combustion engiens" by Richard stone - well worth the read.

have a nice day

sanj

[T600]

Or 'Physics for Game Programmers' by Grant Palmer 'cos it's a great book and it's got a picture of my old car on page 215. Fame at last!

Tony

[Karma Supra]

I agree with everything that has been said above...

However I would also like to add that a wide power band, althought not measuable in raw figures, makesa huge difference.. a nice flat torque curve....

the 1JZ-GTE is an awesome engine because although as stock it only makes 280 bhp, it has a ridiculously wide powerband, the torque curve is very flat. It also scores well on the BMEP front ;) I can;t remember waht the figure was, but I once worked out how many times more effecitve it was then the horrid viper V10.....

[rion]

Thanks Guys! It's becoming clear to me. but I'd like to know nrgizerbunny's said about

"THe only thing that actually matters is power"

since Power is the product Torque but how come different Engines can have same torque but different HorsePower. is Power mentioned different from a HorsePower?

and also

Engine power should really be measured in BMEP (brake mean effective pressure)

what does that exacly mean? is this where I measure power by how my Vehicle makes a full brake or full stop?

I appologize if I asked something funny. :P

[T600]

Torque is important rion.

I don't think you need worry about BMEP.

[Karma Supra]

Torque is very important... this is why:

1, pub talk BHP figures don't impress me...

2, I have a 3 litre inline six turbo

Now then... where are the Honda mob

[Black Knight]

Torque is essentially leverage (or work, helps with the definition below). When you tighten a bolt with a spanner you are applying torque to it, force at a distance. The SI unit is Newton Metres (Nm) but lb/ft is more commonly quoted

Power is the rate of doing work (as someone mentioned above). The SI units of power are Watts but horse power is more commonly quoted. 1Kilowatt = 1.35 hp

Power is a function of torque with the equation being

b.p = (2 x Pi x T xN)/60000

b.p = brake power in kW

T = Torque in Nm

N = Crankshaft speed in RPM

Simply what this means is that to get power you need Torque and Engine RPM's but you can use one to compensate for a lack of the other.

Most people get too tied up in the engine technology to understand actually what they mean and do. It's far easier if you use a pedal bike as an annalogy since most people know how they work and essentially works the same way (legs providing the linear force of the piston forcing the crank to rotate and then being transmitted through a gearing system, chain in the bike, actual gears in a car)

If for example you take me and my mate cat. Cat is far smaller than I am (she's probably about half the weight I am if I'm honest). If we go for a ride and ride side by side we need the same amount of power to stay with one another. Because my legs are stronger I can push down harder on the pedals and create more torque at the crank (how it's quoted by manufacturers) than cat can. She has to pick a lower gear and pedal faster to get the same power output.

Now imagine we stop the bikes and want to have a drag race. I put the bike in top gear because I've the strength and weight to apply a massive load to the pedal and give me great levels of torque at the crank. This is transmitted through the gearing system to give me Torque at the wheel and the acceleration I want. Cat being a lot lighter can't stand on the pedal to give her torque at the crank (she'd just keel over sideways :) ). Instead she has to pick a lower gear and pedal faster to give her the same torque at the wheel as I get.

To convert it into engines Imagine I'm something like a stonking great diesel and cat is a VVTL-i engine.

You can get away with very little torque so long as your gearing and rev range can make up for it. Normally when someone complains that an engine lacks torque actually what they mean is the power band is too narrow to allow you make good use of it (i.e. you have to change down a gear to get the torque at the wheels to give you the acceleration you want to overtake)

Hope that's an easy enough explaination :)

[Black Knight]

Sorry for got about the engine size bit.

Power/Torque (see above) is dependant on how much air you can get in the engine.

If you burn methane in an engine (essentially the simplest combustion calculation) at a stochiometric mixture (i.e. exactly the right amount of oxygen to burn with the methane) it works as:

Methane+oxygen => Carbon Dioxide+Water+Heat

or

CH4 + 2 x O2 => CO2 + 2 x H2O + Heat

That heat causes the gasses to expand and produces the force on the piston which makes the power.

Ok, so we don't run our cars on methane we run them on petrol or diesel. Far more convenient because it's a liquid. This means it's very easy to push through the injection system into the engine since it's essentially incompressible.

What limits the power is how much air (i.e. oxygen) you can get into the engine to get that stoichiometric mixture (although practically you tend to run engines slightly richer than stochiometric but I'm not going to get into why here).

The tricks to getting more air into an engine are:

1. Simply make it larger so you can get more air into the cylinder

2. Compress the gas so it takes up less room before it gets to the cylinder by super/turbo-charging

3. Make the air more oxygen rich than normal by using nitrous oxide

[nrgizerbunny]

Thanks Guys!   It's becoming clear to me. but I'd like to know nrgizerbunny's said about

"THe only thing that actually matters is power"

since Power is the product Torque but how come different Engines can have same torque but different HorsePower. is Power mentioned different from a HorsePower?

and also

Engine power should really be measured in BMEP (brake mean effective pressure)

what does that exacly mean? is this where I measure power by how my Vehicle makes a full brake or full stop?

I appologize if I asked something funny. :P

←

Black Knight pretty much covered everything but to pick up specfically on your questions

Power is the turning force multiplied by the speed at which your turning. A bit of algebra

P = RPM x T

Lets assume P is constant, you could have

P = Very Small RPM x Massive Torque

or

P = Massive RPM x Very small Torque.

As you see above, P is still the same value.

If I use a real world example. The queen Mary 1/2 (can't remember which) boat had Diesel engines, these produced massive torque (can't remember the actual figure) but spun at something like 200RPM, so extremely slowly.

These were replaced with Rolls-Royce Marine Trent Engines, which are effectively jet engines, these spin at around 30,000RPM but have very small torque. The actual power output of the two engines was the same but the way they went about getting the power was different.

Hope this helps

Sanj

[Black Knight]

It's off at a bit of a tangent but it might clarify what the "Break power" is.

The way you measure engine output is on a dynomometer. How they operate can vary but a simple one is just a big motor (electric normally).

Now the motor can rotate of it's own accord clockwise or anti-clockwise. The thing that stops it is a balance of some sort (weighing scale/load-cell/spring balance etc.)

You bolt the engine to the motor and then spin the motor. The dyno motor actually spins the engine even when the engine is turned off (like cranking it around) however because the engine being powered it tries to push the motor the other way (for this lets say anti-clockwise) the balance on the motor registers this, it knows how far away from the centre line it is so you can work out the torque required to turn the engine.

Now when you start the engine it now tries to push the dyno around insted (clockwise in this case) but the dyno won't let it move any faster so it tells you how much torque is produced for a given speed.

It's telling you how much of a braking effect the dyno is having to carry out

[rion]

Thank you very much guys! this information is very helpful to me. I saved this page so I can read it again and again!