Torque and Horsepower - A Primer

[engineer_roy]

Para 3 (must be an edit) can you beat 1660 psi on the drawing pin?

[machin]

Para 1, you need the power curves

Yes indeed. We've covered that many times over. please read the thread entirely. A single power figure will only allow you to determine the acceleration at one road speed. to determine the acceleration from one speed to another requires the power at all engine speeds used in that acceleration.

Para2, different displacements is changing the goalposts.

Next thing you'll be saying we should keep the engine speed the same?!!! How does that allow you to compare a motorbike engine with a diesel engine? It doesn't!!!

[machin]

Para 3 (must be an edit) can you beat 1660 psi on the drawing pin?

My case in point:- How do you know how sharp my drawing pin is???!!! You don't!

[engineer_roy]

Machin, it doesn't matter how big or small psi was the figure mentioned and 1660 psi on aa piston of 2 inches diameter creates a great deal more power than the same pressure on your pinhead.

[machin]

Machin, it doesn't matter how big or small psi was the figure mentioned and 1660 psi on aa piston of 2 inches diameter creates a great deal more power than the same pressure on your pinhead.

It makes no power at all if the engine speed is zero! That was my point.

Also, 1660psi BMEP?! Are you sure about that figure?

[xxChrisxx]

Machin, it doesn't matter how big or small psi was the figure mentioned and 1660 psi on aa piston of 2 inches diameter creates a great deal more force than the same pressure on your pinhead.

Fixed your post.

When it's moving it'll have a power associated with it.

[beelsebob]

Machin, it doesn't matter how big or small psi was the figure mentioned and 1660 psi on aa piston of 2 inches diameter creates a great deal more force than the same pressure on your pinhead.

Fixed your post.

When it's moving it'll have a power associated with it.

Or when it's stationary and producing noise/heat/magnatism(that would be weird)/...

[machin]

Well that was fun. A bit off topic. The point I think we've all agreed on is that BMEP is NOT an easy way of comparing the ability of two completely different engines to accelerate a car. can we move on?

[strad]

I admit I'm a little confused. I thought BMEP was more of a tuning tool.

[xxChrisxx]

I admit I'm a little confused. I thought BMEP was more of a tuning tool.

It's just a convenient way of expressing the output of the engine. For ease of comparison between dissimilar engines. As it's linked to both power and work output when tuning a specific engine you can use any metric you like.

The MEP curve is the same shape as the torque curve. Becuase:

Pmep = Pn / Vd*N
Where: P is pressure, n is to distingush between 2 and 4 stroke. The N is rngine speed. As engine speed is a multiplier and it changes, the MEP curve is a different shape to the power curve.

It can be expressed as torque:
Pmep = 2pi*Tn / V.
As you can see the 2 pi and n are constant, and the Volume displaced by the engine is constant. So

Pmep ~ T. (MEP is propoertional to torque).

When not comparing engines, it's more straight forward to talk about torque output as they physical output. Or power output if you want to know what the engine is capable of.


Before you said, why don't be stick with the real world. Engineering is all about making assumptions, that still reflect reality but reduce complexity. So as much as we deal with real issues, we try to make them as close to the theoretical ideal as we can (makes the analysis and maths easier).


The bottom line for tuning (ignoring things like mixtue quality), is simply to cram as much air in as possible. It doesn't matter what figure you use to measure the output, the goal is always the same.

[machin]

BMEP is very useful for Engineers who are developing an engine, and it is right that you want as much BMEP as possible across a wide a rev band as possible.

In order to work out the acceleration that an engine provides from its BMEP value you also need to know the bore/stroke and number of cylinders (or displacement) and the engine speed at which that BMEP is available.

For example a 0.5 litre single cylinder engine running at 500rpm may have the same BMEP as a 3 litre V12 engine running at 15,000rpm, but it won't make anywhere near the power, and therefore won't be able to accelerate your car very well. For that reason, it is nowhere near as convenient for determing if an engine will accelerate your car quicker or not; you need to know a lot more info before you can make that decision.

Power is the far more convenient figure for this purpose as you don't need any other engine details to work out what the maximum acceleration /top speed of your car will be (given gearing freedom); a single power figure will allow you to work out a single acceleration or top speed. A power curve (it only needs to show % of max speed on the X-axis -an actual engine speed is not necessary) will allow you to work out acceleration from one speed to another, given freedom of gearing.

[engineer_roy]

Maybe my article PLAN 2 will help,I wrote it especially to move the debate concerning Torque and Power further ahead, and I trust you will find it useful in that respect. Get right to the end for the power curve (probably start there if you wish). You will find it in articles on this site, just search "PLAN 2".
I look forward to further discussion with you all, machin, chris, beelsebob, strad and uncle Tom too. Hopefully it will encourage some new contributors to this great Forum for I have written in simple terms. Best wishes Roy

[xxChrisxx]

Maybe my article PLAN 2 will help,I wrote it especially to move the debate concerning Torque and Power further ahead, and I trust you will find it useful in that respect. Get right to the end for the power curve (probably start there if you wish). You will find it in articles on this site, just search "PLAN 2".
I look forward to further discussion with you all, machin, chris, beelsebob, strad and uncle Tom too. Hopefully it will encourage some new contributors to this great Forum for I have written in simple terms. Best wishes Roy

Old school manually drawn graphs, nice They are a bit difficult to see the pictures. So I'll link these. Which show the same thing, but maybe slightly easier to see.

This graph shows axial piston forces, starting from TDC on the power stroke.


This one shows the resulting torque output for a single, 4 cyl and 8 cylinder engine over the full cycle.



This comes from a low RPM WOT cycle of the engine. I don't have the high RPM graphs to hand, but they show how the torque/load case alters when inertia becomes the dominant loading.

[engineer_roy]

Nice charts Chris,sorry about the quality of mine, my HRDC mates think they're retro and fit the timeframe. Tomba asked for improvement, I'm working on it.
Have you accounted for the mass of the big end of the conn-rod being centrifuged? Seems a bit low but maybe the materials are that much lighter now. I know it's a constant but it does add to the bearing load and half of it stresses the conn rod bolts too.

[machin]

Roy,
I've read your article. It is a very good description of the intricacies and all the little things that need to be considered when designing an engine. And for that I commend you. I also recommend anyone with an interest in the details of engine design to take a read.

My main bugbear is people thinking that high flywheel torque figures (without any appreciation of the importance of speed) gives more acceleration. Hopefully people have learnt that you need to understand the importance of speed when using torque to decide if one engine will provide more acceleration than another engine. The beauty with Power is that you don't need to know the speed. I realise you'll understand this, but I think you muddied the waters a little bit by bringing BMEP into this discussion. BMEP is an interesting and useful quantity for engine designers, but is not convenient when selecting an engine or comparing completely different engines.

My backgruond is marine propulsion system design, ranging from 200kW installations to 15,000kW. In the engine selection process for these projects I only ever looked at power curves; these were naval vessels that were performance critical -the power curves give you all the information you need to know to calculate a vehicle's performance and check they would meet the performance specs of acceleration and top speed that were contractual requirements (I did a few calcs to see what the torque was, but only out of curiosity!). I certainly never felt the need to look at BMEP figures!

Personally I don't care for the whole "does an engine produce power or torque" debate that has blighted a few of these pages. An engine's output can either be described in terms of power or torque, one doesn't come before the other. But Power is the more useful figure for engine comparisons.